apollo-soyuz pamphlet no. 2 x-rays, gamma-rays

8/8/2019 Apollo-Soyuz Pamphlet No. 2 X-Rays, Gamma-rays

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Apollo-SoyuzPamphlet No.2~'

X-Rays, Gamma-Rays(NASA-EP-134) APOLLO- SOYOZ PAM PHL ET NO. 2:X - H AY S , G A M M A - R AY S(National Aeronautics a ndSpace Adminis t ra t ion) 74 p MF A 0 1 ; SOD HCset of 9 volumes CSCL 22A

G3/1

N78-27148

Unclas2 24990


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Apollo-SoyuzExperimentsInSpace

This is one of a series of ninecurriculum-related pamphletsfor Teachers and Studentsof Space ScienceTitles in this series ofpamphlets include:EP-133 Apol lo-Soyu z Pam phlet No. 1: The F l ightEP-13 4 Apol lo-Soyuz Pamphlet No. 2:X-Rays ,G a m m a - R a y sEP-135 Apol lo-Soyuz PamphletNo 3: Sun. Stars . In BetweenEP-13 6 Apol lo-Soyuz PamphletNo 4:Grav i ta t ional FieldEP-13 7 Apo l lo-Soyu z Pamphlet N o 5: The Earth f rom Orbi tEP-13 8 Ap ol lo-S oyu z Pamphlet No 6: Cosm ic R ay DosageEP-139 Apol lo-S oyu z Pamphlet No. 7: Biolog y in Zero-GEP-140 Apol lo-Soyuz PamphletNo. 8:Zero-G TechnologyEP-141 Apol lo -Soyu z Pamphle tNo. 9:General Science

O n The C over The Crab Nebula, a S trong X -R ay Source (Tau X -1 )Lick Observatory Photograph


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Apollo-SoyuzPamphlet No. 2:

X-Rays,Gamma-RaysPrepared by LouW illiam s Pageand Tho rnton Page FromInvestigators' Repor ts of Experimental R esultsand Withth e Help of Adv ising Teachers

NASANational AeronauticsandSpace Ad m inistrat ion

W ashington, D.C.20546October 1977


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For sale by the Superintendent of Documents,U.S. Government Printing Office, Washington, D.C. 20402(9-Part Set; Sold in Sets Only)Stock Number 033-800-00688-8


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Preface

The Apollo-Soyuz Test Project (ASTP), which flew inJuly 1975, arousedconsiderablepublic interest;first, because the space r ivals of the late1950'san d 1960's were wo rking together in a joi nt endeavor, and second, becausetheir mutualefforts include d develop ing a space rescue system . TheASTPalso included significantscientific experiments ,th e results of w h ichcan beused in teaching biology, physics,an d mathematicsin schools an d colleges.

This series of pamph lets discussing the Apollo-Soyuz mission and experi-men ts is a set of curriculum supplem ents designed for teachers, supervisors,curriculum specialists, an d textbook writersas well as for thegeneral public.Neither textbooks nor courses of study, these pamphlets are intended to

provide a rich source of ideas, examples of the scientific method, pertinentreferences to standard textbook s, and clear descriptions of space exp erim en ts.In a sense, they may be regarded as a pionee ring form of teac hin g aid. Seldomhas there been such a forthright effort to provide, directlyto teachers,cur r icu lum-re levan treports of current scientific research. High schoolteachers w ho reviewed th e tex ts suggested th at advanced stude ntswho areinterested mightbe assigned to studyone pamphletan d report on it to therestof the class. After class discussion, students mightbe assigned (withoutaccess to the pamphlet)one or more of the "Questions fo r Discussion" fo rformal or informal answers, thus stressingth e application of what w aspreviously covered in the pamphle t s .

The autho rs of these pamp hlets are Dr. L ou W illiam s Page, a geologist, andDr. Thornton Page, an astronomer. Both have taught scienceat severaluniversitiesan d have published14 bookson sciencefo r schools, colleges,an dthe general reader, including a recent one on space science.

Technical assistance to thePages was provided by the Apollo-SoyuzProgram Scientist,Dr. R. Thomas Giul i , and by Richard R . Baldwin,W . Wilson Lauderdale,and Susan N . Montgomery, membersof the group atth e NAS A LyndonB . Johnson Space Centerin Houstonwhich organized th escientists' participationin the ASTP an d published their reportsofexper imen-ta l results.

Selected teachersfrom highschools an d universi t ies throughou tth e UnitedStates reviewed th e pamphlets in draft form. They suggested changesinwording, the addition of a glossary of terms unfamiliar to students , an dimprovementsin diagrams. A list of the teachers and of the scientific inve s-tigators who reviewedth e texts fo r accuracy follows this Preface.

This set of Apollo-Soyuz pamphletsw as initiated an d coordinatedby Dr.Frederick B . Tu ttle, Directorof Educ ational Programs,and was supportedbyth e NASA Apollo-Soyuz Program Office,by Leland J . Casey, AerospaceEngineer fo r ASTP, and by Wil l iam D . Nixon , Educational ProgramsOfficer, all ofNASA Headquartersin Washington , D.C.


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TeachersAnd Scientific InvestigatorsWho Reviewed the Text

Harold L. Adair, Oak Ridge NationalLaboratory, Oak Ridge, Tenn.Lynette Aey, Norwich Free Academy, Norwich, Conn.J . Vemon B ailey, NA SA Lyndon B. JohnsonSpace Center, Houston,Tex.Stuart Bowyer, U niversi tyof Californiaat Berkeley, Berkeley, Calif.Bill Wesley B row n, California State U nivers ity at Chico, Chico, C alif.Ronald J . Bruno, Creighton PreparatorySchool, Omaha, Nebr.T. F. Budinger, Universi tyof Californiaat Berkeley, Berkeley, Calif.Robert F. Collins, Western States Chiropractic College, Portland, Oreg.B. Sue Criswell, Baylor Collegeof Medicine, Houston,Tex.T. M. Donahue, U niversi ty of Michigan, Ann Arbor, Mic h.David W. Eckert, Greater Latrobe Senior High School, Latrobe, Pa.Lyle N. Edge, B lanco High School, Blanco,Tex.Victor B. Eichler, Wichita State University, Wichita, Kans.Farouk El-Baz, Smithsonian Inst i tut ion, W ashington,D.C.D. Jerome Fisher, Emeritus,University of Chicago, Phoenix, Ariz.R. T. Giul i , NASA Lyndon B. JohnsonSpace Center, Houston,Tex.M . D. G rossi, Sm ithsonian Astrophysical Observatory, Cambridge, M ass.Wendy Hindin, North Shore Hebrew Academy, Great Neck, N.Y.Tim C. Ingoldsby, Westside High School, Omaha, Nebr.Robert H . Johns, Academyof the New Church, BrynAthyn , Pa.D. J . Larson, Jr., Grumman Aerospace, Bethpage, N.Y.M. D. Lind, Rockwell International Science Center, Thousand Oaks, Calif.R. N. Little, U niv ersity of Texas,Austin,Tex.Sarah M anly, Wade Ham pton High School, G reenvil le ,S.C.Katherine Mays , B ay City Independent School District,B ay City, Tex.Jane M. Oppenheimer, Bryn Maw r College,Bryn M a w r, P a.T. J . Pepin, U niversi ty of Wyom ing, Laramie, W yo.H. W. Scheld , N ASA LyndonB. Johnson Space Center, Houston,Tex.Seth Shulman, Naval Research Laboratory, Washington,D.C.James W . Skehan, Boston College, Weston, Mass.B. T. Slater, Jr., Texas Educat ion Agen cy,Austin,Tex.Robert S. Snyder, N AS A George C. M arshall Space Flig ht Ce nter,Huntsvi l le ,Ala.Jacquel ineD. Spears, Port Jefferson Hig h School, PortJefferson Station, N.Y.Robert L. Stewart , Monticel lo High School , M onticel lo, N.Y .Aletha Stone, Fulm ore Jun ior High School,Aust in ,Tex.G . R . Taylor, NA SA Lyndon B. Johnson Space C enter, Houston,Tex.Jacob I. Trom bka, N AS A Robert H. G oddard SpaceFlight Center, Greenbelt , Md.F. O. Vo nbu n, NA SA Robert H. Goddard Space Flight Cen ter, G reenbelt, Md.Douglas Wink ler, Wade Hamp tonHigh School , Greenvil le ,S.C.


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PageIntentionally

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Contents

Section 1 Int roduction '.". . ' ' . ' . . . . ' . : . .v 1

High-Ene.rgy.-R9d>iattbn,. .'.,. . .". . '.!*"... .h 3A. The Electromagnetic Spectrum awcJ.What Gets Through

Our Atmosphere .-' 3B. Photon Energy 4C. X-Rays , Gamma Rays, and Cosmic Rays 7D. X-RayTelesco pes Collimators and Angu lar Resolution 8E. X-Ray Detectors 11

F. Gamma-Ray Detectors 14

Section 3 X-Ray and Gamma-Ray Sources in Space 17A. The Beginning of X - R a y Astronomy 17B. The NASA Uhuru X - R a y Satellite 20C. X-Ray Sources 21D. Explanations of IntrinsicX-Ray Power Output 25E. Questions fo r Discussion (Atmosphere, Planck Law, Energy) 31

Section 4 X-R ay Spectra of Cosmic Sources 33A. Differences in Penetration of Hard and Soft X-R ays 33B. The Soft X-RayExperiment, MA-048 34C. MA-048 Voltage Breakdown 38D. MA-048 Experiment ResultsAnX - R a y Pulsar 39E. Questions for Discussion (Interstellar Matter, Instrumental

Calibration, B ackground, Black Holes, DopplerEffect) 42

Section 5 Gamma-Ray Detectorsand Nuclear Reactions in a Spacecraf t 43A. Gamm a-Ray Sources and Back ground 43B. Gamm a-Ray Detectors 43C. The Crystal Activation Experiment,MA-151 44

Section 2

Section 6 Conclusions . 49

V H


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Appendix A Discussion Topics (Answersto Questions) 50

Appendix B SI Units an d Powers of 10 53

Appendix C Glossary 56

Appendix D Further Reading 61

V I M


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Figures

Figure 2.1 The Spectrum of White Light 32.2 The Electromagnetic Spectrum and What Gets Through

th e Earth 's Atmosphere 52.3 Schem atic Diagram of a De ntist 'sX-Ray Machine 62.4 High-Energy RadiationComing in on the Earth 82.5 How Optical Telescopes Form Images of Stars 92.6 X-Ray Collimator and its Angular Resolution 102.7 lonization Chamber 122.8 Proportional Counter 13

2.9 Crystal Scintillator Gamma-Ray Detector WithAnticoincidence Counter 15

Figure 3.1 Diagram of Aerobee Rocket Flight 183.2 Photograph of the Crab Nebula 193.3 Location of an X-Ray Source in the Sky by Uhuru 203.4 Celestial Coordinates 223.5 Map of theX-RaySources Located by Uhuru 233.6 Top View and Cross Section of the Milky W ay Galaxy 243.7 Impacts of Infalling Electronson Stars 283.8 Synchrotron Radiation From Free Electrons Moving

in a Magnetic Field 293.9 The Compton Effect 303.10 The Inverse Compton Effect 30

Figure 4.1 The MA-048 SoftX-Ray Detector 344.2 Ope ration of the MA-048 SoftX - R a y Proportional Cou nter 364.3 Sim plified Block Diagram of MA -048 De tector Electronics System 374.4 Spectrum of Cygnus X-2 Observed by the MA-048 Experiment 384.5 MA-04 8 SoftX-Ray Count R ates as a Function of Time 404.6 Explanation of the SMC X-1X-Ray Pulsar 41

Figure 5.1 Diagram of the Sodium Iodide Crystal and Cell for theMA-151Experiment 45

5.2 Diagram of MA -151 C ontainerHolding the GermaniumCrystal and Metals 46

5.3 G raph of Postflight M A-1 51 Counts in the SodiumIodide CrystalCom pared W ith Apo llo 17 Postflight Counts 47

5.4 Grap h of Postflight Counts in the Germanium Crystal 48

IX


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1 Introduction

After 4 years of preparation by the U.S. National Aeronauticsan d SpaceAdminis t ra t ion(NA SA ) and the U .S.S.R . Academy ofSciences, the Apolloan d Soyuz spacecraft were launc hed onJ u l y 15, 1975. Two days later, at16:09 Greenwich mean time onJu ly 17, the two spacecraft were docked.Then th e astronautsan d cosmonautsmet for thefirst international handshakein space, an d each crew entertainedth e other crew (oneat a t ime)at a meal oftypical Americanor Russian food. These activitiesand the physicsof reactionmotors, orbits aroundth e Earth, an d weightlessness (zero-g)ar e describedmore fully in Pam phle t I, "The Spacecraft, Their Orbits, andDock ing"(EP-133).

Thir ty-fourexperim ents were performed wh ile Apolloan d Soyuz wereinorbit: 23 by astronauts, 6 by cosm onauts, and 5 joi ntl y. These experim ents inspace were selected from161 proposals from scientistsin nine differentcountr ies .They ar e listed by n u m b e r in Pamphle tI, and groupsof two ormoremore ar e described in detail in Pamphlets II th rough IX (EP-134 throughEP-141, respectively). Each experiment was directed by a Principal Inves-tigator, assistedby several C o-Investigators,and thedetailed scien tific resultshave been publishedby N A S A in two reports:The Apollo-SoyuzTest ProjectPreliminaryScience Report (N AS A TM X -58173 ) and the Apollo-Soy uz TestProject Sum mary Science Report (NA SA SP-412). The simp lified accountsgiven in these pam phlets have been reviewedby the Principal Investigatorsorone of the Co-Investigators.

The x-ray an d gamma-ray experiments describedin this pamphletare ofconsiderable inter est because they representthe new field of high-energyastrophysicsthat came into being wh en instrum ents couldbe carried aboveth eEarth's atmosphereby rockets. These instru me nts have detected x -raysan dgamma rays comingfrom objects an d regions of space that werenot evensuspected of being x-ray sourcesin 1960. The measurem ents show superhotstars, collapsed stars. Black Holes, and magneticfields between the stars. Awhole new view of the universe has been opened in x-ray an d gamma-rayastronomy.

The Soft X -Ray Exp erim ent, MA -048, w as designed to locate andstudythe sources of x-rays coming toward Earth from deepspace. Under thedirection of Herbert Friedmanat the Nav al Research Laboratory (N R L)inWashington, D.C. ,a group of seven scie ntists, headedby Seth Shulman ,builtth e e q u i p m e n tan d analyzedth e data. For background in unders tand ingx-raytelescopesand their"count rates," Sections 2 and 3 outli ne the operation ofx-ray detectors and show how x-ray data have led astronomers to the conc eptof "collapsed" starsvery high density Neutro n Stars and Black Holesthatcurve space backinto itself so that no particlesor radiationcan get in orout .


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The Crystal A ct ivat ion Experiment , MA -151 , made no m easurements ofstars but tested the detectors used in measuring gamma rays outside theEarth's atmosphere. The detectors were tested before andafter the Apollo-Soyuz flight at four major laboratories:th e Lawrence Berkeley LaboratoryinCalifornia, the Los AlamosScientific Laboratory in New Mexico, the OakRidge Nation al Laboratoryin Tennessee, and the NA SA RobertH . GoddardSpace Flight Center(GSFC) in Maryland. These tests were coordinated by J.I. Trombka of GSFC. The differences between preflightan d postflight testresults show how cosmic rays, other high-speed protons, and neutronschanged th e detectors during217 hours in orbit.

Both x-rays an d gamma rays ar e high-energy radiationan d come fromregions of space whereth e condit ionsar e extreme ( low den si ty, high tempera-ture, and high-speed particles). Cosmic rays are high-speed particles, notelectromagnetic w aves. The nature and behavior of both high-speed particlesan d high-energy photonsar e described in the next section.


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2 High-Energy Radiation

A The Electromagnetic Spectrum andWhat Gets Through Our AtmosphereLight carries energy in the form of waves. Visiblel ight can be spread out by aprism or adiffraction grating into a spectrum from short violet waves to longred waves (Fig. 2.1) . Du ring the last century ortwo, physicists have learnedhow to measure invisible ultraviolet(UV) waves, which are shorter thanviolet, and invisib le infrared (IR) wave s, which are longerth anred. Later theydiscovered even shorterx-rays' an d even longer radio waves.These waves

ar e generated by oscillations (back-and-forth motions)of electric chargesfast oscillations for the short waves and slow oscillations for the long waves.Although they are like water waves, light waves are not waves in anymaterial. Theyar e waves of electric an d magnet ic fields. Altogether, thesewaves make up the electromagnetic spectrum(Fig. 2.2), ranging from veryshort gamma rays through visiblel ight waves with a wavelength of about5000 angstroms(500 nanometers)to radio waves several kilometersin length.

Any hot body, such as the Sun or astar, "broadcasts" al l these waves,which move at the veloci tyof l ight ,3 x 108 m/sec (186 000 miles/sec).A hotstar gives out mostly short ultraviolet waves, a cool star mostly longerre dwa ves , and the Sun (atintermediate temperature) m ost ly vis ible l ight agood

Infrared

Ultraviolet

The spectrum of white light. Figure 2.1

'Project Physics,Sec. 18.6; PSSC, Sec. 23-9. (Throughoutthis pamphlet , referenceswill begiven to key topics covered in two standard textbooks:"Project Ph ysics ,"second edi t ion .Hol t ,Rinehartan d Wins ton , 1975,an d "Physical Science StudyCom mi t t ee , " (PSSC),fourth edi t ion ,D. C . Heath, 1976.)


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reason why h u m a n eyes evolved to be sensitive to wavelengths near5000angstroms. Lucki ly,th e ozone layer of the Earth's atmosphere blocks gammarays, x-rays, an d most of the ultraviolet wave s. (These high -energy raysdestroy livin g cells by ionizing mo lecules in side the cell.) Most of the infr aredwaves are absorbed by water, carbon dioxide,an d other molecules in theatmosphere. Some radio wavesare blocked by ions near the top of theatmosphere.

The transparency of the atmosphere for the electromagnetic spectrum isshown at the top of Figure 2.2. The scale of wavelength is given in bothangstromsan d meters (1 angstrom = 0.1 nanometer) .The numberof oscilla-

tions per second is called the frequ enc y and is measu red in hertz. Th e scale forfrequency goes the opposite direction from thescale for wavelen gth: the longradio waves have low frequencies (3 X 103 hertz) and the short x-rays havehigh frequencies (3 x 1018 hertz). Note that frequency/ multipliedbywavelength X is equal to velocity v, or (3 x 1018 hertz) (1 angstrom) =3 x 1018 A/sec or 3 x 108m/sec(186 000miles/sec), whichis the velocityoflight.

Photon EnergyAbout a century ago, it was shown that lightalso has particle characteristics.Light w aves come in packets called qu anta or pho tons,w h i c h cannot besubdivided. Theq u a n t u m theory showsthat the energy E of an indiv isiblequan tum or photon is proportional to the freq uen cy; that is, E = hf, w here h isth e Planck constant . The short-wavelength, high-frequen cy gamm a rays andx-rays thus are high-energy photons.

X-rays are generated on Earth in the laboratory, the dentist's office, or thehospitalby us ing a hi gh voltage to shoot electrons at a target in a vac uu m tube(Fig. 2.3).The higherth e voltage, th e faster th e speed of the electron and thelarger it s kinet ic energyon impact. This energyis measured in uni ts calledelectronvo lts. If the potential on the x-ray tube is 10 000 volt s, the electronhas a kinetic energyof 10 000 electronvolts,or 10 kiloelectronvol ts . Wh ensuch an electron hitsth e target, it s energy is converted intoan x-ray photonwi th an energy E of 10 kiloelectronv olts.

For th is reason, x-rays and gamma rays are usually described (as on thelow est scalein Fig. 2.2)by their photon energy insteadof by the waveleng thXor f requency/ . The relation betwee nE (in electronvol ts)and X (in angstroms)is easy to remember:E= 12 345/X. Therefore, x-rayswi th a wave leng thof 1angstrom are photonswi th energy of 12 345 electronvolts, or 12.3 kiloelec-tronvolts. Visible- l ight photons (X = 5000 angstroms) have much lowerenergy:E = 12 345/5000 = 2.47 electronvolts.In general, fo r electromag-netic radiation,/ = c/X, wherec is the velocity of electromagnetic waves,3 x 10 8 m/sec.


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The electromagnetic spectrum and what gets through the Earth 's atmosphere. Figure 2.2

Atmospheric transparency

1.0

0.5 Ozoneabsorption

I I I I I i\\

Molecularabsorption

I I

Ionospherereflection

J ' I I I

10-5A 10- 4 A 10- 3 A 10' 2 A 0.1A 1A 10A 100A 1000A 10 4 A 10 5 A 10 6 A 10 7 A lOM 10 9 A 10 10 A 10"A 10 12 A 10' 3 A 10 14A10 15 A10 "15m 10" 14 m I0~13rn 10" l2m 10 -11 m 0.1nm 1nm 10nm 100nm l m 10/nm 100jum 1mm 10mm 100mm 1m 10m 100m 1km 10km 100km

3 x 1 0

1 1

1 23 MeV 1 .23 MeV

I

9 Hz 3 x 1 01 7 Hz

1

3 x 1 01 5 HziI

11

1 2.3 keV 1 23 eV 1 .23

Photon energy ti i

Wavelength X

3 x 1 01 3 H z 3 x 1 C

Freque

I

eV 0.01 eV

I I I !

"H z 3 x 1 09 H z 3 x 1 0r H z 3x10* H z 3 x 1 03 H z

ncy f

Gamma rays X-rays Ultraviolet \Visiblelight

Infrared Radio waves


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X-Rays ,Gamma Rays, and Cosmic RaysManmade x-rays are used in dentists' offices and in hospitals to penetratelow-densi ty material an d make "shadow photographs" of higher densityteeth and bones. The highe r the x-ray energy, the more pe netrating they are.You can't focus x-rays with ordinary lensesor mirrors like you can visiblel ight; most of the x-rays ar e absorbed or go straight throu gh. Dense materialslike lead (i n sufficient thick ness ) stop eventh e high-energy("hard") x-raysan d th us are used as shields to protect dental assistants and others w ho w orkwith x-ray machines every d ay. (The ionization in cells where x-ray photons

are absorbed damages h um an tissueif one person accum ulatesa large dose byrepeated exposure.) Thin layers of lower density materials stop the low-energy ("soft") x-rays but let most of the hard x-rays through. This is thebasis for x-ray filters that allow the me asurem ent of soft and hard componen tsin a beam of x-rays from a cosmic source like the Sun and its corona (seePamphlet III).

Anoth er form of"radiation" in space is the cosmic ray, w hic h is actually ahigh-speed particle (atomic nucleus), not an electromagnetic wave. Fiftyyears ago, cosmic rays were thoughtto be waves. Thenit was discovered thatth e high-speed (high-e nergy ) particles produced gam ma rays (photons) wh enthey hit the Earth's upper atmosphere, as shown in Figure 2.4. Some of theparticles and gamma rays get through, but most of them are stopped by theatmospheric shield, which alsostops x-rays and most of the ultravioletwaves as shown in Figure 2.2. In spacecraft above the atmosp here, there isno shield except that providedby the spacecraft wallsor by filters in theexper imente q u i p m e n t .Space scientists wi shi ng to measure x-rays or gamm arays must disting uis h betw een gamma rays and high-en ergy cosmic-rayparticles that behave very much like gamma rays. This distinctionis usuallymade by using an anticoin cidenc e coun ter that automatic ally subtracts theparticle cou nts from all the counts recorded, leavin g only the x-ray orgamma-raycounts. A "count" is recorded whenone photonor particle passesthrough the detector. T he intensity is the num ber of count s per second.

Cosmic rayscome in toward the Earthfrom the nearby Sun (solar cosmic

rays). Other rays of high er energy come more or less uni form ly from alldirections and aret h o u g h tto originate in space betweenth e stars in theM i l k yWay Galaxy (galactic cosmic rays). These solarand galactic cosmic rays (Fig.2.4) pose a hazard to crewmembers on long space missions aboveth eatmosphere. Some protectionis providedby the pressure hull of the spacecraftcabin.


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. / X-r.

/// ^F7 r a c

X-rays and gammarays from

' I r a cosmic source

n

Solarcosmic rays

Figure 2.4 High-energy radiation coming in on the Earth.

X-Ray TelescopesCollimators and AngularResolutionIf an x-ray detectoron a spacecraft counts x-ray photons,how can wetell fromwhich direction theyar e coming? A telescopeforms imagesof stars in visiblelight; th e central image is the star toward whichth e telescope is pointed(Fig.2.5). In general, x-rays cannot be imaged this way, so the x-ray astronomermust build a tube of lead or other x-ray-absorbing materialto l imit th e field ofview of his detector. If he uses a tube of small diameter, so as to define th edirection accurately, veryfe w counts willbe recorded because th e area of thetube wil l let on ly a few x- rays in. Therefore, hepacks a num ber of tubes sideby side in front of a detector (Fig. 2.6). These tubes al l point in the samedirect ion, thus forminga "honeycomb" or "eggcrate" having an area ofseveral square centimeters.(You can get an idea of th is arrangement by


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How optical telescopes form images of stars. (X-rays go on through, forming Figure 2.5no image.)

(a) Refractor (lens) telescope. (b) Reflector (mirror) telescope.


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X-rays

Connection toelectronics

Shield forx- rays from ,bottom an d sides

Figure 2.6 X-ray collimator and its angular resolution.

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looking at a distant light througha handful of soda straw s.) Suc h "col-limators" accept x-ra ys froma circle of about 5 in thesky, and it is thereforeimpossible to disting uish between two x-ray sources less than 5 apart. In thiscase, the "angular resolution" is 5.

There are several waysto narrow dow nth e area in the skywhere an x-raysource m ay be. One m ethod is to sweep the x-ray telescope across the source(up anddow n, r ightan d left) several timesan d note whereth e counts cease atthe edgesof the 5collimatorfield. Inthis way ,th e locationof an x-ray sourcein the sky can be limited to an ' 'error box" of a few arc-minuteson each side,and the x-ray sourcecan be identified w ith some kn ow n visible star, ne bula,or galaxy in that box.

[I X - R a y DetectorsThe simplest x-ray detectoris photographicfilm as used in thedentist's officeor in a hospital; how ever,film doesn't work wellin a sweep acrossan x-raysource. It is also difficult to convert th e blackeningof the developed filmtothe number of x-ray photons that passed throughit . Therefore, x-rayas -tronomershave constructed several types of x-ray and gamma-raydetectorsthat are quantitative and more sensitive. The earliest detector w as an ioniza-tion chamber a g as-filled tube with metal electrodes charged to a few volts'potentialdifference (Fig. 2 .7). W hen x-rays pass throu gh the tube, they ionizesome of the gas by kno ckin g electrons out of the atom s. The gas then cond uctsa sm all curre nt between the electrodes that is propo rtional to the intensity (thenumber of x-ray photons passing through the tube each second).

Later m odifications of the ionization chamber g reatly improved its sensitiv-ity by increasing the voltage between the electrodes and reducing the gaspressure. These n ew detectors (Geiger coun ters) simply coun ted each x-rayphoton passing throughby amp lifying each pulseof electric current.

The latest detector, the proportional counter shownin Figure 2.8, is filledwith gases such as argon and m ethane at about atmospheric pressu re. W hen anx-ray passes through,it ionizes th e gas, and the high voltage (approximately2000 volts) causesthe ions and electrons to' 'cascade" toward the electrodes,

producingmore ions and electrons on the w ay. Wh en these ions and electronshi t th e electrodes, they producea pulse of current thatis proportional to theoriginal numbe rof ions formedby the x-ray. Higher energy x-rays form m oreions in the tube and thereby p roduce larger pulses. The electronic circuit sortsthe pulses and stores them in"bins" (different sections of the electronicmem ory) according to pulse size. Because the pulse size is proportional to thex-ray energy, th e first bin gets th e countof x-ray photons with energy between12 0 and 140 electronvolts (low ener gy, sm all pulses). The second bin gets the

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Figure 2.7 lonization chamber. The current reading at A measures the number of x-rayscrossing the tube per second from all directions.

count of 140- to 160-electronvolt photons;th e third bin, 160- to 240-electronvoltphoton s; and so on up to bin 128,which gets the count of 9.5- to10.5-kiloelectronvolt photons (high energ y, large pulses). Thu s, the propor-tional counter measuresth e x-ray spectrumthe numberof photons in eachenergy range or in each group of wavelengths.

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Special gas at1 atmosphere pressure

Cathode I oyage IIIUUII^Proportional counter. An x-ray photon ionizes a gas atom, releasing one ormore electrons that are accelerated toward the high-potential anode, ionizingmore atoms and releasing more electronsmost of them close to the anode.The large electric pulse of this cascade is proportional to the number of ionsformed by the x-ray and therefore to the energy of the x-ray photon.

Figure 2.8

Investigators can,of course, choose the numberof bins to be used and thepulse sizes to be stored in each bin. On a space mission like Apollo-Soyuz,they can also select the interval o f time the x-rays are counte d before the binsare emptied by radioing the total count to Earth(the NASA Lyndon B.Johnson Space Center (JSC)in Houston). To detect short-term changesinx-ray intensity and to keep the bin size (numberof counts) reasonably small,investigatorsmake th is counting-time interval small.The Apollo-Soyuzin -vestigators useda counting intervalof 3 milliseconds.

This 3-millisecond interval showshow fast th e proportional counteris .Each electron pulse lasts about1 microsecond. Evenin a high-inte nsity x-ray

beam, th e chance of two photons arrivingwithin 1 microsecond is extremelysmall. (The two would then be counted as one.) Statistical corrections can bemade later if the count rate ever gets that high.

For lower energy photons (ultraviolet photons of energy from 4 to 200electronvolts),a channeltron can be used as a detector.

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Gamma-Ray DetectorsFor higher energy photons (gamma rays), crystal scintillatorsare used asdetectors. In some transparent crystals, suchas sodium iodide (Nal), gammarays produce flashes of l ight ("scintillations") as they are absorbed. Eachgamma ray is converted intoan electron-positron pair.The l ight flashes areconverted to electric p ulses by a pho tom ultip lier (a sensitive light detector)looking into the crystal, and the pulses are counte d. The only problem isthatcosmic-ray particlesalso produce scintillations in the crystal, so an anticoin-cidence detector must surround the crystal, as shown in Figure 2.9, andsubtract the particle counts. Fortunately, a transparent plastic, such as lucite,flashes when a high-energy particlepasses through bu t ignores gamma rays.Therefore, flashes in theplastic that coincidewith flashes in thecrystal can becanceled in the anticoincidence detector.

A honeycomb or eggcrate collimator doesn't work well as a gamma-raytelescopebecause th e honeyc omb material emits secondary gamm a rays wh eni t absorbs an incom ing one. The detector wou ld thus count more gamm a raysthan were actually received. One way todefine the gamma-ray direction is touse the electron-positron pair created by the gam ma -ray absorption. T his pair,with kinetic energy equ al to the energy of the gamm a ray, contin ues in thesame direction (a result of the conservation of m om entu m ) and can bedetected farther down the telescope axis by another particle detector (theCerenkov detector in Fig. 2 .9). As you can see, these high-en ergy-ph otondetectors mustbe complex in order to'' sort out' ' photonsof various energiescoming from one direction and to avoid cosmic rays comingfrom all direc-tions.

It should be noted that cosmic-ray particlesare also recorded by "tracks"in the glassy particles of the lunar soil. When etchedwi th hydrofluoric acid,these soil particles show mic roscopic line s caused by cosm ic-ray particlesthatpenetrated them: thedeeper the penetration, the longer the track and thehigher the cosmic-ray en ergy. G eophysicists coun t the num ber of tracks persquare mil l imeterin many different particles that have been exposedon thelunar surface . They can es timate wh en these soil particles were exposed by

noting th e depth at w h i c hth e soil w as collectedby the astronauts.In thisw a y,th e lunar soil, which has recorded cosmic-rayintensity dur ing th e last fewbillion years, is used by geophysicistsas a cosmic-ray detector.

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3 X-Ray and Gamma-RaySources in Space

PAGE BLANK WT

The Beginning of X-Ray AstronomyAlthough cosmic rays were well knownin the 1950's, no astronomer ex-pected to find x-ray sources in the sky. Because x-rays are blocked by theatmosphere (Fig. 2.2), x-ray telescopes had to be carried outsidethe atmos-phere by soundingrockets. These rockets have been used since1946 fo rfar-ultraviolet observations of theSun. As shown in Figure3.1, NASA ' ssmall Aerobee rocketscan be shot up 320 or 480kilometers(200 or 300miles)fo r a few minutesof observations,and the instrumentscan be recovered w henthey fall back to Earth. Instead of point ing th e rocket in a preplanned

direction, th e foundersof x-ray astronomy, Herbert Friedmanof the NRL inWashington, D.C., and Riccardo Giacconi of the American Science andEngineering Com pany (AS&E ) in Cambridge, Massachusetts, arranged tospin th e rocket an d survey a large part of the sky bysweeps withan x-raytelescope pointed out the side of the rocket.

It was in this way that rocket flightsin 1963 first discovered a strong x-raysource in the constellation Scorpiusan d another in Taurus. Astronomersnamed these new objects by the constellation name follow edby "X-l ." Thedesignations "X-2," "X-3," and so on were used fo r x-ray sources laterdiscovered in the same constellation. Each constellation covers anarea of thesky about 10 or 20 on eachside. Because ofpoor angular resolution, it wasdifficult to identify these x-ray sourceswith astronomical objects. T aurusX -lw as located accurately whenit wasocculted (eclipsed)by theMoon. A n x-raytelescope on a rocket recordedth e t ime whenth e Moon passed in front of thesource by t iming th e x-ray cutoff. The Moon's location w as accuratelyk n o w n , an d Taurus X -l had to be in asmall stripof sky that w as along th eMoon's edge at that time. TaurusX-l was thus identified withth e CrabNebula , and its size could be estimated from th e t ime taken by the Moon tocover it .

The fantastic Crab Ne bula (Fig.3.2) is a glowing massof gas knownto beth e rem nant of a supernova explosion in A.D .1054, when a very bright"newstar" was seen by Chinese and Japanese astronomers. In the 900 years sincethen , th e explodinggas has expanded to the 3-arc-minute nebula show ninFigure 3.2. This makes sense; the violence of such a giant star explodingcould leave a source of high-energy x-raysthat might last fo r centuries. TheSun w as also found to be an x-ray source,th e most intensein our skybecauseit is so close. However,if our Sun were at the distanceof the nex t nearest star(4 light-years), it would be barely detectableas an x-ray source.

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Peak of trajectory


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Photograph of the Crab Nebula. Figure 3.2

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The NASA Uhuru X-Ray SatelliteDur ing th e 1960's, x-ray astronomers discovereda few dozen x-ray sourcesby using x-raytelescopes on rocket fl ights and on some un man ned satellites.Two of these x-ray sources were identified with supernova remnants andseveral with th e Large Magellanic Cloud (LMC) outside our Milky WayGalaxy. Then,in 1970, th e NASA Explorer42 satellite, whichw as devotedentirely tox-ray astronomy,w as launched from Keny ain eastern Africa.Thelaunch day, December 12, 1970, was the seventh anniversaryof Kenyanindependence, so Explorer 42 was named Uh uru , which means"freedom" in

Swah il i . I tcarried tw o x-ray telescopes that were pointedout opposite sidesan d scanned the sky as thesatellite rotated slowly (once every12 minutes ) .The x-ray telescopes were built by Giacconi and his collaborators at AS&E.These m en analyzed th e x-ray cou nts (energyof 2 to lOkiloelectronvolts) thatwere transmittedby radio to the ground receiversof the NASA SpacecraftTracking and Data Network (STDN).

Figure 3.3 Location of an x-ray source in the sky by the Uhuru satellite. Two of Uhuru's 5sweeps gave maximum count rates for the heavily outlined strips. The x-raysource must be in the shaded "box" where the two strips cross.

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Uhuru circled th e Earth every96 minutes in an orbit 540 kilometers (335miles) above the E quator at an orbital inc lination of 3. Its spin axis slow lychanged so that th e x-ray telescope swept across each source from differentdirections (Fig. 3.3).The telescope collimators eachhad a fan-shaped beam5 wide by 0.5 thick (alongth e sweep direction).As these fan-shaped beamsswept across a source in the sk y, the x-ray coun t rateincreased to a maxim um,then decreased. The time of the m ax im um count rate show ed the 5 by 0.5strip of sky where th e source was.On a later sweep across th e same source,another strip was plotted thatcrossed the first one. The x-ray source waslocated in the "box" where the two strips crossed. This techniquecan

determine the location of an x-raysource to within a few arc-minutes.

Q X-Ray SourcesU h u r u had located an d measured almost200 x-ray sources beforeit s elec-tronic circui tsfailed after 3 years of operation. Although the early sourcenames (Scorpius X -l , etc.)ar e being retained, numerousnew sources ar ek n o w nby the ir celestial c oordinate s,2 right ascension (wh ich is like longitudeon Earth) and declination (like latitude)see Figure 3.4.Thus,"3U 0930-40'' designatesa source in theThird (last) U huru C atalogat right ascension09hours and 30 minutes(on a scale increasing eastward from0 to 24hours)an ddeclination 40.

The 3 U sources are plotted in Figure 3.5, a map of the sky wh ere the usualright-ascensionan d dec linati on coordinates have been replacedby galacticlongitudeand latit ude . These coo rdinates are based on our positionwithin th eM i l k y Way Galaxya huge disk of more than 100billion stars plus gas anddust (Fig. 3.6). Our S un is located about tw o-t hird s of the wayfrom the centerto the edge of the disk. From our position inside this disk, we see thefamiliarM i l k y W ay band of stars aroundth e sky, representinga view alongth e disk.The concentrationof stars in the M i l k yWay is highest in the directionof theGalaxy center, towardth e cons tellation Sagittarius. G alactic longitudeisO atthis point and increases eastward. (In Fig. 3.5, wh ich is a map of the sphere ofthe sky as seen from theinside, east is to theleft .) G alactic la titud e begins at 0along th e Galactic Equatorthe middleline of the M i l k yW ay band seen inth e sky.

It is clear that strong x-ray sources (sho wn by the large dots on Fig. 3.5)tend to be concentrated near theMilky Way, which indicatesthat they areprobably objects in the disk of our Galax y. How ever, two groups at the lower

2Project Physics, Sees.5.1 to 5.3.

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right (galactic long itudes 270 and 300?, l atitude s 30 and 35 S) are identifiedwith th e Magellanic Clouds,tw o small galaxiesdefinitely outsideour Galaxy(extragalactic). The Magellanic Cloudsare visible only fromthe SouthernHemisphereof the Earth; theyare not visible from th e United States.

Other sources nearth e Galactic North Pole (latitude90 N) areprobablyextragalactic. Thefaint ones near the Galactic Equator may be extragalactic,or theymay beobjects in the Milky W ay disk that are faint because of the largedistance between themand the Earth.

South Celestial Pole

Figure 3.4 Celestial coordinates. The observer is looking outward from the center of animaginary sphere. Each point in the sky (on the celestial sphere) has its owncoordinates, right ascension and declination.

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These x-rays have traveled enormous distances,as muc h as 200 000light-years from the M agellanic Clouds and 100 000 light-yearsacross ourGalaxy. Thisfact brings us back to the penetrating power of x-rays. Some ofth e ordinary light from a star is absorbed by interstellardust as it trave ls acrossth e Milky W ay disk. Even with th e largest telescopes, optical astronomerscannot see as far as thecenter of our G alaxy. However, hig h-energy (hard)x-rays come through the dust as easily as through a piece of paper (which isabout the equivalentof the dust in our line of sight across the 100 000-light-year galactic disk).The faint source in Sagittarius couldbe at the faredge ofth e diskor even farther awayin another galaxy.

180' 150" 120 90Auriga Cassiopeia

/ // / Large Magellanic

Small Magellanic/ Ccud/Cloud

ae r

90'Galactic South Pole

A map of 2- to 10-kiloelectronvolt x-ray sources located in the sky by theUhuru satellite. Constellation names indicate their approximate areas. UrsaMajor is the "Big Dipper." Sagittarius is in the direction of the center of ourMilky Way Galaxy. The Magellanic Clouds are outside galaxies.

Figure 3.5

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To p view

Cross section1 0 X 3 000 light-years

Galactic plane

(galactic latitude = 0

Figure 3.6 Top view and cross section of the Milky Way Galaxy, a gigantic rotating disk ofstars, gas, and dust, about 1 00 000 light-years in diameter and 5000 light-years thick, with a bulge or halo around the center of mass.

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You m i g h t th ink that the bright x-raysources (large dots on Fig. 3.5) ar eth e nearer ones, but the brightness thatw e observe depends on two factors:distance an d intrinsic power. A bright x-ray source may be a distant one ofvery high intr insic power; that is, high power ou tpu t. Thefaint ones may berelatively nearby stars wi th very low intrinsic x-ray out put . Ident i f icat ion ofth e x-ray sources wi th optical objects can help solve this problem.Forinstance, we know how far away theCrab Nebula is and, from the x-raybrightness, we can compute itsintrinsic x-ray power outpu t . This poweroutput is probably sim ilar to theintrinsicpower of other supernova remnants.However, thisdoes not help us to determ ine thein t r ins icx-ray powerof x-raystars like Scorpius X-l , because they m ay generate x-raysin a completelydifferentway.

Explanations of Intrinsic X-Ray Power OutputNormal electromagnetic radiation from a very hot star cannot accountfo r x-ray sources. Astronomers find many red and yellow starswi thtem-peratures from 3000 to 6000 K, fewer stars from 7000 to 20 000 K, andvery few stars as hot as 50 000 K.(One star w itha temperatureof 100 000 Kw as observed from Apollo-Soyuz;see Pamphlet I I I . ) To radiate mostlyx-rays, a star's surface would haveto be 10 000 000 K orhotter. (Thedominant wavelength in angstroms radiated by a"perfect black body" attemperature T is \ m = 28 800 000/7.) The surface of such a superhot starwould cool very rapidly, however, unless thestar had an inexhaust iblefuelsupply. The total energy radiatedper second is proportional to T4, and asizable fraction of this energy appears in visible wavelengths. If x-ray sourceswere superhotstars, they would be visually bright and would probablydis-appear after a few months or a year, w h i c h is not the case.

Physicists have studied the generation of x-rays and suggest threepos-sibili t ies for the sources in space: (1) bremss t rah lung , (2 ) synchro t ronradiation, and (3) the inverse Comptoneffect.

1. Bremsstrahlung. Bremsstrahlungis a German word meaning"brakeradiat ion" and is the source used in dentists ' an d doctors' x-ray m achines .The "braking" occurs when a high-speed electronor ion strikes some othermaterial and is decelerated, stopped,or bounced off in a new direction. Thetheory of electromagnetic waves predicts such radiation when an electriccharge experien ces a large acceleration (change in velo city) . The only ques -t ion is how theelectrons or ions got their high velocityout in space in the firstplace.

There ar e several possibilities.W e know that therear e extreme condi t io nsin th e atmospheres of starshigh-speed particles (suchas the solar w i n dcoming from the Sun), electric fields, magnetic fields, and very high

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temperaturesallof wh ich affect th e motions of electrons an d ions. How-ever, none of these conditions explainth e acceleration of electrons to speedshigh enough to produce x-ray bremsstrahlungof the energy measured inScorpius X -l . A nother po ssibility is gravitational force. If matter falls intoward a hot, massive star,it would be ionized into electronsan d ions, wh ichwould then impacton the surface of the star.

Figure 3.7 shows th e calculation of impact velocity (whichis equal to thevelocity of escape from th e star's surface; see Pamphlet I) and the resultingphoton energy in electronvolts (1 electronvolt= 1.60 x 10"19 joules). Theinfalling protons wi th a mass mp of 1.7 x 10

27 kilograms have mu ch higherkinetic energy than electrons w itha mass m of 9.1 X 10~31 kilograms. (Thekinetic energy of the electrons is 1840 times smaller.)The proton impactstherefore give the soft x-ray bremsstrahlung from an ordinarystar. Eachelectron impactreleases approximately 1 electronvolt of energy andproducesinfrared photons (see Sec. 2B).To get hard x-raysof energy 50 to 100kilo-electro nvo lts (Fig. 2.2), proton impact velocitiesof 3000 to 4000 km/sec arerequired. Such velocitiesare possible onlyif the star is m uch more com pactthan the Sun (about one thirty-fifth of the Sun's radius), that is, havingm u c h higher density and much higher surface gravity.

Normal stars are about th e same size as the Sun, approximately 106

kilometers in diameter and 2 x 1030 kilograms in mass. Giantstars are 100times largerand 10t imes more massive.A ll these stars are kept "puffed up"by the huge release of energy from nuclear reactions in theircores (seePamp hlet II I). In these nuclear reactions, hydrogen is converted to hel iumunder th e extremely high pressurean d high temperaturein the core of a star.After most of the hydrogen is used, th e nuclear-energy outputdecreases, and,generally afteran explosion, the old star collapses. Its gas is nolonger keptinflated by radiation pressure and hotgases. Gravitational force pulls thecooling gas into a White Dwarf. Other changes occurin the compressed gas,which becomes "degenerate"that is, it no longer followsth e normal gaslaws. G ravitation al forcehas less an d less opposition an d pulls th e materialofthe star into sucha compressed state that mostof the atoms are broken intoneutrons an d jammed close together. The densi ty becomes extremely high.

W h i t e Dw arf stars are about 100 time s smallerthan the Sun. A Neutron Starshould be less than 100 kilometers in diameter, or lessthan one-hundred th o fthe diameter of the Earth.

N e w t o n ' sLaw of G ravitation statesthat gravitatio nal forceFg = G m M l r2

(see PamphletI), where G is a constant ,m and M are themasses attractingeach other, and r is thedistance between them. Becauser2 is in the denom-inator.Fa has very large valuesin a collapsed star.These huge forces furthercompress the material of the star and ove rwh elm nuclear forces. W hite D warfsand N eu tro n Stars are stable stages inth is process of collapse, when the

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material resistsfurther com pression for a time (perhaps 100 million years).When the density reaches values like 108 gm/cm3 in a White Dwarf , itchanges the geometry ofspace, as stated in Einstein's Theory ofGeneralRelativity. If the star's mass M is large enough and its radius Rgets smalleno ugh , space is curved so sharply arou nd the collapsed star that light canno tge t out . A t this stage, th e star, now less than 10 kilometers in diameter,becomes a "Black Hole." (The Earth wou ld haveto be squeezed to the size ofa Ping-pong ballto become a Black Hole.) X-ray observations have recentlygiven new evidence of NeutronStars an d Black Holes. White Dwarfs havebeen observed with optical telescopes for the past 50 years.

Figure 3.7 givesa simplified pictureof electronsand protonsfalling into stars.The electrons an d protons are accelerated to the velocityof escape ve beforethey strike th e surface of the star. Actually, thereis gas above th e visiblesurface of an ordinary star, like the corona above the Sun's surface (seePamphlet III).So the electrons an d protons seldom reachth e full value ofve = 615 km/sec. Whenth e electrons strikegas atoms at speeds of 500 to600 km/sec, their kinetic energyis equivalentto about 1 electronvolt.Theprotons have about 1840 timesas much energy and give soft x-raysasbremsstrahlung.However, if the star is a Neutron Star with much smallerradius (/J^y), th e impact velocitiesare more than 80 times larger, and theproton brem sstrahlung comesoff as gamma rays with 14 megaelectron-volts of energy. Even the electronsfalling on a Neutron Star produce7-kiloelectronvolt x -ray brem sstrahlun g.

Where does th e infalling matter come from? Astronomers guessthat itcomes from anothe r starclose bya less compact companionin orbit aroundthe compact star, as described in Section 4. This scientific sleuthingalsofinds other meansof generating x-raysin space. (The good detective-scientisttries to fit all theavailable clue san d then get more evidenceto "prove hiscase in court.")

2. Synchrotron radiation. If there is little materialon whichto impac t(thatis, if there is no possibilityof bremsstrahlung), high-speed electronsin nearlyempty space ca n generate x-raysby spiraling in a magneticfield. Thiseffect iswell demonstrated in modern physics laboratorieswith a synchrotron, amachine inwhich groups of electrons or protons ar e guided around circularpaths by a magnetic field. Their speed is increased by an alternating electricfield pulsed to pull them aroundin their orbits. S ucha mach ine producesl ightan d x-ray s becauseth e electrons o scillatein spirals aroundth e magn etic linesof force. The same situation couldarise in space (Fig. 3.8).

Each o scillating electron em its synchrotron radiation electromagn eticwaves of frequency/that dependon the electron's speed v and thestrengthHof the magnetic field (f is proportional to H m2v4) . Even in a small magn eticfield (m uch less than existson Earth), an electron movingat very high speed

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Infallingfree electrons and protons Impact velocity

v e = ^IGMJR /= 615 km/sec ^/

dPhoton energy = 1/2 rripV e

2 = 2keV^o

(soft x-ray) f

v e escape velocity

G gravitational c onstant

M star's massF t star's radiusE photon energy

/V photon energy of a Neutron Star

rrip proton mass

R/y radius of a Neutron Star

Vgft escape velocity from N eutronStar

Infallingfree electrons and protons Impact velocity

Photon energy w = 1 /2 m p v e ^ = 14 MeV

(hard x-ray)

= 50 000 km/sec Jf M

Radius

Figure 3.7 Impacts of infalling electrons on stars. Only very dense stars have enough gra-vitational potential to generate hard x-rays by impact (bremsstrahlung).

will radiate x-rays. Again, you mayask, where do the high-speed electronscome from? And whatcaused the magnetic field in space? Astronomersanswer thatth e strong m agneticfield inside a large star wouldbe trapped inth e "plasma" of ions an d electrons an d carried outward, a l though weakened,

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High-speed electron spiralsin magnetic field H

Circularlypolarized synchrotronx-ray radiated forward

Electron

Plane polarized synchrotronx-ray radiated sideways

Synchrotron radiation from free electrons moving in a magnetic field H. The Figure 3.8field forces the electrons to "spiral" around lines of force.

to great distancesby a supernova explosion.The Crab Nebula (Fig. 3.2)m aythus have a magneticfield throughout its large volume. Electrons might be

shot into this magneticfield from nearby stars. In any case, cosmic rays arepassing through it and emitting synchrotron radiation of higher energy thanthe electrons.

As seen from the side view (Fig. 3.8), synchrotron radiation should beplane polarized. 3 (It is circularlypolarized if viewed along the magneticfield.) Plane polarizationw as found in light an d x-rays from th e Crab Nebula(Fig. 3.2), wh ich confirmed that synchrotron radiation is com ingfrom thatnebula. Synchrotron x-rays are to be expectedfrom fairly large volumes suchas nebulae, whereas bremsstrahlung x-rays are more likelyfrom individualstars. In addition, there is good evidence of a small magnetic field throug houtthe disk of our Galaxy, and synchrotron radiation can occur wherever elec-trons or other charged particles are shot into it.

3 . Inverse Compton effect. The inverse Comptoneffect is a third possiblesource of x-rays inspace. The Comptoneffect4 (Fig. 3.9) was discovered inlaboratory experiments by the American physicist Arthur Compton. Hefoundthat a photon passingan electron and an ion inlow-densitygas may throw ou tthe electron (thatis , increase th e electron's energy fromE, to 2 ) by losing

'Project Physics, Sec. 13.7.'Project Physics, Sec. 20.2; PSSC, Sec. 25-4.

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Incomingx-ray ofenergyM,

Ionbefore

i

E lectronenergyE, =

Ionafter

Electron leaveswith higherenergy E 2 = ^

X-ray

leaves withlower energy hf2

Figure 3.9 The Compton effect. In this interaction between an x-ray and an electron neara positive ion, energy and monentum are conserved.

some of its own energy (hft). That is, 2 - , = fc/, -hf2, wherethe reducedphoton frequency/2 is less than the original/i. U nder certain con ditions,theinverse occurs;fo r instance, whena photon overtakesan electron movinginthe same direction. The energy andfrequencyof the photon are then increasedand the energy of the electron is reduced (Fig.3.10). After several suchencou nters, an ultraviolet photon can be "beefed up" to anx-ray photon bystealing the energy of electrons and ions.

Electron leaveswith lower energy

1

2

Ionbefore

X-ray leaves withhigher energy hf4

Figure 3.10 The inverse Compton effect. When the angle between an incomng photon andthe electron velocity v is correct, the electron loses energy to the photon.

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E Questions for Discussion(Atmosphere, PlanckLaw, Energy)This new field of x-ray astronom y has raisedseveral questions that need morethoughtand research. The fo llowin g go beyond the con tent of this pamphletbut are interesting topicsfo r discussion.

1. If the Earth's atmosphere were suddenly made transparentto allwavelengths of electromagnetic radiation(Fig. 2.2), what major changeswould you expect on Earth? If it had always been thisway, what furtherdifferences would be expected?

2. In a color televisionset, there is a transformer giving a potential of20 000 volts for the scanning electron beam. Every0.05 second, the beamscans the picture in 525 lines, each line consistingof 525 dots (pictureelements or "pixels"). What bremsstrahlung would be expectedfrom th etelevision tube?

3. In Section 2A, the statement is made that hot bodies radiate allwaveleng ths in theelectromagnetic spectrum. Wh at about zero wave length ?

4. Cosmic rays comefrom all directions. Some are absorbed each secondby the Earth, other planets,the Sun, and stars. Does this mean that cosmicrays are decreasing in intensity?

5. How can we measure or estimate the distance to an x-ray source?

6. What further measurements shouldbe made on x-ray sourcesby a newhigh-energy satellite to replaceUhuru?

7. Where does the energy come from that is radiated in synchrotronx-rays?

8. If cosmic-ray electrons are passing through a magnetic field through outthe disk of ourGalaxy,what wouldyou expect to be added to the map ofx-raysources in the sky (Fig. 3.5)?

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4 X-Ray Spectra ofCosmic Sources

A Differences in Penetration of Hard andSoft X-RaysBecause of thepenetratingpowerof x-rays, we can "see" x-ray sources(Fig.3.5) throughth e interstellar smogof our Milky W ay Galaxy, but we have noeasy way toestimate how faraway these sourcesare. The optical astronomercan estimate distances ofstars in the Milk y Way by interstellar reddening andby the strength of interstellar absorption lines in their spectra. The redden ingoccurs because red light penetrates dus t or smog better than bluel ightdoes. Afairly distant star is thus redder than it ought to be (as indicated by otherfeatures in itsspectrum, suchas the strengthsof spectral lines) because mostof its blue light has been absorbed by smog en ro ute to the Earth. In the samemanner, soft x-rays(0.1 to 1 kiloelectronvolt)ar e less penetrating than hardx-rays (2 to 20 kiloelectronvolts). Thus, we expectthat measurements of adistant source would showless than "normal" soft x-rays, whereas a nearbysource would have the"normal" ratio of soft-to-hard x-rays. (Sofar, anactual standard fo r "normal" has not been established.)

A partial surveyof soft x-rays fromsourcesin the M i l k yWay was made byF. D . Seward on two Aerobee rocketfl ights in 1972, and the results tend toconf i rm this distance effe ct. Soft x-rayi n t ens i ty in the 0.3- to 0.5-kiloelectronvolt bandw as compared to the hard x-ray intensity(2 to 10kiloelectronvolts) measured by Uhuru. Two sources stand out as beingnearby: Vela X-l and CygnusX - l . Both are large nebulae (supernovarem-nants) several degreesacross an d about 3000 l ight-years from Earth. Anothersupernova remnant, CassiopeiaX -l (Fig. 3.5), could not be detected in softx-rays of energy 0.3 to 0.5 kiloelectronvolt, although it showed w eakly in the0.5 to 1-kiloelectronvolt range.It is estimated to be 10000 l ight-yearsdistant. However, other soft x-ray sources are not nebulae, and the ratio oftheir soft-to-hard x-ray strengthis possibly different because of the waythe i rx-rays ar e generated. Seward estimates that sevenof these sourcesar e about40 000 light-years fromus , near th e center of the G a l a x y,an d that another10are about 15 000 light-years away.

Optical spectra can he lp specify w hat the"normal" color of a star should

be, and hence the degree of astar's reddening can be used to estimate itsdistanc e. H owe ver, furth er stud y of x-ray spectra is needed before dist anc escan be estimated from the ir characteristics.Unt i l that is done, reliabledis-tances of x-ray sourcescome only from optical studies of the objects withwhich they ar e ident i f ied.

PRECEDING PAGE LANK NOT FILMED

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g The Son X - R a yExperiment, MA-048The MA-048 Experimentwas a study of spectra in the range from 0.1 to 10kiloelectronvolts and a survey of the soft x-ray background overa largefraction of the sky. Althoughthe x-ray detector, a 30- by 75-centimeter (12-by 30-inch) proportional counter (Fig.4.1), had periodic high-voltage break-downs betweenthe electrodes (at 2700 volts), it detected 12 x-ray sources,including CygnusX-2, Hercules X-l, Vela X-l , SMC X-l in the SmallMagellanic C loud,and theW hite Dw arf star calledHZ 43(number43 in alistprepared by Milton Hum asonand Fritz Zwick yin 1950).The skybackground

in 0.25-kiloelectronvolt x-rays was measured on a separate rocket flight, andtwo new emission regions were discoveredfrom Apollo-Soyuz.

Proportionalcount ef

Detectorfront face(plastic membrane andhoneycomb collimatof)

Calibrationsource

Figure 4.1 The MA-048 soft x-ray detector. The structure of the detector front acts as acollimator.

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The MA-048 proportional counter (Fig. 4.1) consistedof a rigid box, 5centimeters (2 inches)deep, with a thin plastic front win dow that facedoutward on the side of the Apollo Service Module nearth e back. This plasticmembrane (Kimfol polycarbonate)w as only 2 micrometers thick.It was thinin order to admitsoft x-rays, w hich are easilyabsorbed. The membrane wassupported on the outside (againsta 1.1 -atmosphere pressure inside)by analuminum honeycomb collimatorwith a 4 field of vie w. When tested beforeflight in a vacuum chamber,th e membranedid not break but it leaked gas at 3cm3/m in. T he gas had to be replaced durin goperations from a tank (gas bottle)on the back. This procedure requ ired accurate valves. T he gas was a m ixtu re

of 90-percent argon and 10-percent m ethane that was chosen for its absorptionof soft x-rays. (The x-rays ionizethe argon.) The pressureand temperatureofth e gas were measured continuouslyan d transmittedby radio to the MissionControl Centerat JSC in Houston.

M a n y cathode wires, grounded (at zero potential), were stretchedacrossth e box as shown in Figure 4. 1, and a set of seven anode w ires (at2700 volts)w as located nearth e front membrane .A soft x-ray photon wo uld ionizeth eargon-methanegas near th e front membrane; it could no t penetrateto the backof the box. The freed electrons"cascaded" (Fig. 2.8) an d produced moreelectrons. All the electrons collected on the anode wires,which recorded acurrent pulse proportional to the photon ene rgy. W hen a cosmic ray ionizedth e gas near the back of the box, the current pulse was also recorded by asecond set of seven anode wires (veto anodes) nearth e back (Fig. 4.2). Thiscount was subtracted as in an anticoincidence detector (see Sec. 2F). Theelectronic c oun ting circuitis shown in Figure 4.3. (The data anodesare nearth e front membraneand the veto anodesare near th e back wallof thecounter. )The anticoincidence gate cancelsout simultaneous pulsesfrom th e data an dveto anodes. The analog-digital (A-D) converter converts each pulse intobinary digits and sorts pulsesinto 128 bins according to pulse size, therebyrecord ing the c oun t of 0.13-, 0.15-, 0.20- th roug h 9 .0- , and 10.0-kiloelectronvolt photons. The m easured spectrum of Cygn us X -2 ( thestrongest source detected) is shown in Figure 4.4.

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Honeycomb collimator

X - r a y

Entrance foil (membrane)

Ga s molecule Primary detectoranode wire (data)

Cathodewires

Backgrounddetectoranode wire(veto)

Back plate

Figure 4.2 Operation of the MA-048 soft x-ray proportional counter.

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Simplified block diagram of the MA-048 detector electronics system. Figure 4.3

^S Anticoincidencegate

Telemetry(radiotransmissionto Earth)

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> 2

.5 1 2Energy, ke V

10

Figure 4.4 Spectrum of Cygnus X-2 observed by the MA-048 Soft X - R a y Experiment.

Q MA-048 Voltage BreakdownThe MA-048 equ ipm ent wascarefully calibrated in a vacuum chamber atNRL before the Apollo-Soyuzflight and checked out (voltage regulators,

pressure meters, thermom eters, electronics) at the NA SA Joh n F. Ken nedySpace Center (KSC) just beforeth e Apollo launch.The experiment operatedsuccessfully for 25 minutes af terit was first turned on, but then a voltagebreakdown occurred. A discharge (electric current) betweenth e 2700-voltanode w ires and the cathode w ires or wa lls took place. It was pro bably causedby dust or sm all irregularities in the wa lls or w ires wh ere the electricfield w ash i g h . The gas became cont in uou sly ionized(n o pulses, thereforeno counts) .Wi t h the astronauts ' help , the coun ter boxes were evacuated to hard vacu uman d refilled wi th th e argon-methane mixture ,but the voltage breakdownoccurred again 1 or 2 minu tesafter the 2700-volt potential was turned on.

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It looked as if the Soft X-Ray E xperiment had failed. H owe ver, because itworked for 1 or 2 m inut es after being turned on, the NR L investigators at theMission Control Ce nter suggested tur nin g the voltage on and off every 2minutes. TheMA-048 equipm ent subsequently m easuredsoft x-rays in 1- or2-minute intervals for an additional 30 min utes , mak ing a total of 55 minute sof successful operation for the experiment.

MA-048 Experiment ResultsAn X-RayPulsarCounts were transmittedto theMission Control Centeran d recorded thereas afunction of time. A sample set for 14 minutes 51 seconds, show ing a scanacross Cygnus X-2, is reproduced in Figure4.5. The full 55 m inutes of suchdata required man y months of study to determine the voltage breakdowns,discrete sources, an d soft x-ray background emission.Two broad emissionregions in the background standout. One, in the constellation Centaurus,coincides wi th a supernova remnant estimatedby optical astronomersto beabout 8000 l ight-years away .It s temperatureis about 3 000 000 K. Theotherregion coincideswith C y g n u s X-6 and is anarea about 1 by 8,estimated tobe about 6600 l ight-years away and therefore about 110 by1000 l ight-years inextent. Because a supernova remnant shouldbe a more or less sphericalnebula, th is elongated x-ray source is thou ght to be a result of severalgiant-star ex plosions,al l adding up to a 3 000 000 Ktemperatureof the ionsand electrons in an elongatedregion.

Another exciting discoverywas an x-ray pulsar in the Small MagellanicCloud (SMC ),a galaxy of stars, gas, an d dust about200 000l ight-years fromus (100 000 l ight-years outsideour Galaxy).The x-ray sourceSMC X-l hadbeen discovered in1971, and U huru observations show that it is aclose pair ofstars in a 3.9-day orbit around each oth er. O ne of these is a hot blue gia nt star.The x-rays are cut off by an eclipse asthis blue star comes between Earth andth e companion star that emits x-rays(Fig. 4.6).

The soft x-ray counts from SMC X -l measured by the MA -048 Ex perime ntan d by an earlier rocketflight withth e same typeof proportional cou nter showthat SMC X -1 is not constant between eclipses. I n photon energies from 1.6 to7 kiloele ctronvolts, the inte nsity fluctuates about 15 percent every0.716

second. No variatio n was detected in the softest x-ray band (photon energyfrom 0.18 to 0.28 kiloelectronv olt), probably because these verysoft x-raysare mostlyabsorbed by interstellar matter between the E arth and theSMC.

Measuring a period as short as 0.716 second was possible because th eMA -048 x -ray coun ts were telemeteredfo r each 3-millisecond interval.SM CX-l is undou btedly a pulsar, giv ing regular pulses of x-rays. It has a shorterperiod than a dozen other pulsarsthat have been observed. These pulsars arethought to be collapsed stars (NeutronStars, Fig. 4.6) that are rapidly rotatingand have "hotspots" on them . The two other x-ray pulsars in the M ilk y WayGalaxy, Centaurus X-3 and HerculesX- l , are s imilar.

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8 5000

o

to -X

o 5 o" ex, 25003 o >

O OJ

0

4000

0200

A :

,Cygnus X-2

80

JuL^^-^OU^^-JJHigh-voltage dischargeprecursor Discharge

160 240Bin number (2.784 sec/bin)

320

Figure 4.5 MA-048 soft x-ray count rates as a function of time for 14 minutes 51 sec-onds during the flight, in four energy bands.

If this theoretical modelis correct, th e rapidly spinnin g Ne utron Staris asource of bremsstrahlung x-rays, probablyfrom material falling in from a"giant tide" on the companion star.For some reason, probably its magneticfield, th e Neutron Star has a hotspot where the x-ray output is well above

average. As it rotates, this hotspot faces toward the Earth , then awa y, thu scausing theperiodic variationsin the x-ray counts m easuredby the MA-048Experimen t. At 200 0 00 light-years, SM C X-l has the largest intrinsic outpu t(3 x 1031 J/sec or 3 x 1038 ergs/sec) of any kno wn pulsar.

W hen the N eutron Star m oves behind the blue giant star, the x-raysfromSMC X -l are eclipsed. The Dopplereffect in the 0.716-sec ond pulses ofx-rays due to the orbital motion of the Neutron Star should shortenthe pulsesas the N eu tron Star approaches the Earth and lengthen th em as it recedes about2 days later. If this Dopplereffect (see PamphletIV) can bemeasured,it willconfirm the model sketched in Figure 4.6 and determine the orbit sizeaccurately . From the orbit size and period, the combine d double-star mass canbe calculated. (This has been done for Hercules X -l .) T he"giant tide" in

Figure 4.6 is less certain. If the materialfrom the blue giant star extendsbeyond th e neutral point betweenthe twostars, whereth e gravitational forcescancel, thenthe gas in thegiant star is spewed intoth e Ne utro n Star,as shownin Figure 4.6, providingth e infall fo r bremsstrahlung x-rays.

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Orbital speedin 3.89-day period1

About 10 million km

\

Enlargement ofNeutron Star(about 10-kmdiameter)

\\

\

\

To Earth

X. Oft.

Explanation of the SMC X-1 x-ray pusar. Figure 4.6

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Questions for Discussion(Interstellar M atter, Inst rum ental Calibration , Backgrou nd, Black Holes,Doppler Effect)

This is jus t one detective stud y inspace science,and it is by no me ans solved.For instance:

9. Why should th e material of the blue giant star extend beyondth eneutral point in Figure 4.6 w h e n th e gravitational forcecloser to the star ispulling it back in ?

10. If youcould observe C y g n u sX -2 from a place muchcloser to it thanwe are, how would the spectrumdiffer from that in Figure 4.4?

11. The "calibration source" in Figure4.1 contained radioactive iron-55,which emits 5.9-kiloelectronvolt x-rays.How could this have been usedtocheck th e MA-048 sensi t ivi ty duringth e Apollo-Soyuz mission?

12. What ca n account for the backgroundof soft x-rays com ingin from al ldirection s and detected in all parts of the sky?

13. Black Holes have no material surface.There is a boundary throug hwhich no waves or particles canpass. W hat provides the impacts forinfall ing

electrons generating bremsstrahlung x-rays?14. If the Sun(wi th a present radiusof 695 0 00kilo me ters) were collapsed

to a White Dw arf, i t would have a radius of7000 kilom eters; if collapsed to aNeutron Star, a 10-kilometer radius. W hat happens to the m aterial of the Sun(or any other star) in such a collapse? W hat about the mate rial in a Black Hole?

15. If a Neut ronStar rotating oncepe r second is mo ving towardus wi th avelocity of 107 m/sec, what period of rotation w ould we m easure from pulsesof its x-ray emissio n (Fig 4.6)?

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5 Gamma-Ray Detectors andNuclear Reactions in aSpacecraft

Gamma-Ray Sources and BackgroundGamma-ray photons have the highestenergy from 100 kiloelectron volts to1000 megaelectronvolts and higher. They can be generated in thesamemanner as x-rays are generated. They could be bremsstrahlungfrom cosmic-ray impacts, or synchrotron radiationfrom cosmic rays passing throughmagnetic fields, or the result of the inverse Comptoneffect on hard x-rays.Specific gamma-ray energies comefrom the decay of pi mesons("heavyelectrons" produced by cosmic rays); from nuclear reactions; andfrommatter-antimatterannihilation such as proton-ant iprotonannihilations,w h i c h

produce 10 0-megaelectronvo lt photons,an d electron-positronann ih i l a t ions ,which produce 0.5-megaelectronvolt photons. Specific gamma-ray energiesfrom nuclear reactionsar e used in physics laboratoriesto identify th e nuclearreactions thatar e taking place in an experiment .

The penetratin g power of gam ma rays is so greatthat they pass throughinterstellar material vir tual ly without loss. Strangely enoug h, theory showsthat very high energy (morethan 105 megaelectronvolts) gamm a rays experi-ence a loss in space by interact ingwi th th e residualmill imeter-waveradiation(a leftover from the"Big-Bang" origin of the unive rse) to form electron-positron pairs. Because th ey are generated by cosmic rays, gam ma rays w ereexpected to come mainly from a general backgroundal l over the sky ratherthan from discrete (isolated) sources. How ever, several strong x-ray sources

(such as the Crab N ebu la) have beenfound to be gam ma-ray sources also.

Gamma-Ray DetectorsThe detection of gamma-ray background and sources depends on crystalscintillators (Sec. 2F) large crystals of germ anium or thal l iu m-d ope dsodium iodide. ("Thal l ium-doped"means a small amount of thal l ium w asadded during th e manufac tureof the crystal.) O ne difficulty with thesedetectors is the "ins trum enta l background," f lashesthat are not due togamma raysfrom outside.The rate of these flashes mu stbe de te rmineddur ingpreflight cal ibrat ion. Experim entson previous Apollofl ights showedthat thisbackgroundcount increased duri ngth e fl ight, probably because cosmic raysan d secondary rays wereabsorbed in the crystal an d created radioactiveisotopes there. (The secondariesinclude the neu trons released w he n cosmicrays are absorbed in nearby materialsin the spacecraf t . ) Gamma raysar ereleased as the radioac tive isotopes decayinside th e crystal and are countedth e same as if they came from outs ide .

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The Crystal Activation Experiment, MA-151To m easure the background change, two standard gam ma-ray detector crys-tals were carriedin Apollo-Soyuzan d checked fo r backgroundflashesshortlyafter th e 9-day flight. In the Crystal Activation Experim ent, M A- 15 1, threeother laboratoriescooperated with GSFC in testing the crystals and othermaterials after th e flight. These tests were coordinatedby J. I. Trombka atGSFC.

Figure 5.1 is adiagram of the sodium iodide crystalin its steel cylinde rwitha glass en d-windo w through which a photom ultiplier tube could"look" fo r

background flashes inprefl ightan d postflight tests. S ucha crystal scintillatorcould be used to detect0.2- to 10-megaelectronvolt gamm a rays, but it wo uldneed some sort of anticoincidence plastic scintillator around it, as in Figure2.9.

Figure 5.2 shows how materials were packedin the other container:asmaller germanium detectoran d 100-gram sheetsof scandium, y t t r ium,an duranium, wh ichar e used as shieldsan d filters in gamma-ray counters . Bothpackages were mounted on the inside wall of the Apollo Command Moduleand were recovered 80 minutesafter splashdown on Ju ly 24, 1975. In thisinside position, the samples were exposed to both solar and galactic cosmicrays, to the gamma-ray an d neutron secondaries from nearby spacecraftmaterials (most ly aluminum,steel, an d plastics), and to Van Allen beltprotons (see Glossary)for a total of 217hours. A similar experimenthad beenperformed on theApollo 17 mission to the Moon.

Several tests were completedon the recovery shipwithin a few hours aftersplashdown. Theinstrumental backgroundflashes in thesodium iodidean dgermanium crystals increased from those of the preflight tests, and thegamma-ray spectrum showe d whic h radioactive isotopes were createdby theexposure. Figure 5.3 is a plot of the gamma-ray spectrumfrom inside thethallium-doped sodium iodide crystal. The Apollo-Soyuz results are aboutone-third th e levelof the Apollo 17counts,with each energy ch annel countedfo r 20 min utes. The energy ch annels are each 2.5 ki loelectronvolts wide,ranging from 25 kiloelectronvoltsto 1.25 megaelectronvolts.The peaks aredue to the spontaneous decayof three radioactive isotopesof iodine,one ofindium, and one of te l lur ium.Sodium -24 (sodium isotopeof atomic weigh t24) was also detected. The lower energy x-ray spectrumfrom inside th egermaniumcrystal is plotted in Figure5.4, wh ich show s 10-ki loelectronvoltx-rays from gall ium-67an d germanium-71 .Zinc-65an d cobalt-56 were alsodetected.

/The metal foils were also activatedby the exposure,giving scandium-46,n e p t u n i u m - 2 3 9 (from u r a n i u m ) , y t t r i u m - 8 7 ,an d z i rconium-89 ( f romy t t r i u m ) . T he last isotope, producedby neu t ron cap tu r e , show eda

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backgroundof 0.71 thermal neutron/cm2 sec in the Apollo spacecraft. Thesedata, togetherwith m uch other information collectedon earlier SkylabandApollo missions, will makepossible the proper design and operation of futurehigh-energyexperimentsin space.

6.985 cmSodiumiodide 9.334 cm

Diagram of the sodium-iodide crystal and cell for the MA-151 Experiment. Figure 5.1

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Resilientpacking

0 .5

0.2

0.25 cm

I ,Intrinsic

5 cm germaniumdetector

1.0 cmi I

0.86 cm {

5 cm ~]Resilient

F i ~" -

High-puritygermaniuman d packing |

^---j J

;[_-; ... i1 Scandium \_\_ 1C

\\

I4.80 cm

* 0.14cm

I

'

8.4! 3 cm

| \\. 0.58cmpacking0.25 cm Yttrium

Figure 5.2 Diagram of the MA -1 5 1 container holding the germanium cry stals and metals.

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Graph of postflight M A - 151 counts in the sodium-iodide crysta l compared with Figure 5.3Apollo 1 7 postflight counts.

1000 i

100

oO

10

Tellurium-121,iodine-124. iodine

i i i i i i i i i i i i i i i i i i i

40 80 120 160 200 240 280 320

Channel (~2.5 k e V / c h a n n e l )

360 400 440 480 520

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10 2 r-

10 1

3o

10

10 keV

\

H

V

1

80 '00 120 140 160

Channel number

180 20C 220

Figure 5.4 Graph of postflight counts in the germanium crystal showing the 10-kiloelectronvolt peak contributed by gallium-67 and g e r m a n i u m - 7 1 (440 -minute count time at 0.066 keV/channel).


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6 Conclusions

Du ring the last 10years, high-energy astrophysics hasbecome one of the m ostinterestingresearch studies inspace science. X -ray and gamm a-ray observa-t ions have shown that th ings ar e going on in our Milky W ay Galaxy, an dbeyond it , that werenot even suspectedin 1960. Such observations,includingth e MA-048 Soft X-Ray results from Apollo-Soyuz, have almost confirmedth e existence of Neutron Stars and Black Holes, wh ich were purely specula-t ive theories as late as1965. As space techniques and instrum ents sensitivedetectors ofsoft x-rays and accurate detectors of gamma rays are impro ved,w e will learn more about the strange sources scattered th roug hou t our Milk yW ay and other galaxies.


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Appendix A

Discussion Topics (Answers to Questions)1. (Sec. 3E) Ultraviolet sun l igh t , solar x-rays, an d cosmic rays are

harmful to most l iv ing organisms and cause genetic mutations (refer to abiology text). If they were notabsorbed, the atmosphere would be heatedentirely from the bottom, causin g more turbulence and chan ging w ind pat-terns. Surface temperatures wouldbe lowered because of the lack of the"greenhouse effect" (holdingin infrared "heat waves" because th e atmos-phere is opaque to the m) . If this had always been thecase, animal life wouldprobably have remained in theseas.

2. (Sec. 3E) The 20 000volts produce electronsin the scanning beamwi th energies of 20 kilo electro nvo lts (alittle less because of the work neededto pull each electronout of thecathode). Because several electronsare neededfor each of three colors in each of 276 000 pixels, at least 5 520 000 x-rayphotonsof 20-kiloelectronvolt energyare produced each secondat the back ofthe television screen. Most of these brem sstrahlun g x-rays areabsorbed in theback and sides of the television set, but about 20 percent come out thefront.

3. (Sec. 3E) If thewaveleng this zero, /= c/k = infinity, and thephotonenergy E = h f i s infinite. Therefore, evena superhot body cannot radiateatzero wavelen gth (referto the Planck Law).

4. (Sec. 3E) Cosmic rays ar e generated by the Sun andother stars invaryingamounts . (Studiesof the lunar soil m ay indicate such chang esin solarcosmic rays dur in g the past.) New galactic cosmic rays are probably acceler-ated to very high energies by"collisions" wi th magnetic fields in the hugeinterstellarclouds of the Galaxy. Thus, therear e many sourcesof new cosmicrays to replace th e rays that ar e absorbed.

5. (Sec. 3E) Identificationof an x-ray sourcewi th an optical object w hosedistance can be estimated is the best method to date. In depen dent estimatesmay be possible w hen comp lete x-ray spectracan be measured ,from w h i c h(a ) intrinsic x-ray outputcan be inferredor (b) soft x-ray absorption alon gth el ine of sight can be measured.

6. (Sec. 3E) More complete measurements shouldbe made of x-rayspectra, incl udin g gam ma rays; rapid time variation sin x-ray in tens i ty an dspectrum; polarization of the x-rays; and greater an gu lar resolution andaccuracy of the source position.

7. (Sec. 3E) As a high-speed electron spirals around mag neti c line sofforce and radiates photons, itskinet icenergy is reduced by an amo un t equ al tothe photon energy radiated.

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8. (Sec. 3E) Synchrotron radiation fromcosmic rays moving throughth eGalaxy 's magnet icfield should produce a background of x-rays an d gammarays close to the Galactic Equator; thatis, in thedisk of the M i l k yW ay Galaxy(see Fig. 3.5).

9. (Sec. 4E) InSection 4D and inFigure 4.6, th e escape of gas from th egravitational fieldof the blue giant star can be caused by convection an dradiation pressure in the atmosphere of that star. In fact, as stars age, they tendto increase in size.

10. (Sec. 4E) Because th e interstellar materialabsorbs more of the verysoft x-rays thanof the harder x-rays (higher photon energy),th e spectrum ofCygnus X-2 ,as recorded m u c h closer to it, would be higher in the low-energy(left) side than is shown in Figure 4.4.

11. (Sec. 4E) W h e n th e calibration source w as pushed in front of the leftside of the MA-048 detector, it s known intensi tyof 5.9-ki loelectronvol tx-rays (about 500 photons/sec)would enter the detector through th e thinplastic front w indo w just l ike x-rays fromacosmic source.If the proportionalcounter recorded 500 more counts/sec aboveth e background count in the5.9-kiloelectronvolt bin, thenit s sensi t iv i tyw as correct. If not, a correctionwould need to be made to the ins t rume nt readings.

12. (Sec. 4E) The backgroundof soft x-rays com ingin from al l direct ionsmight be synchrotron radiation from electronsan d protons in the solar windpassing throughth e Earth's magnetic fieldan d synchrotron radiationof otherprotons an d electrons farther away that have been fired intoth e magnetic fieldof the M i l k y W ay Galaxy. Or it could be many more dis tant sources,overlapping, al l over th e sky.

13 . (Sec. 4E ) Reason ing abou t cond i t ionsat the " S c h w a r z s c h i l dDiscon t inu i ty, "th e "edge" of a Black Hole at distance /?y from it s center ofmass, is t r icky. The equa t ion /?v = 2GMlc can be derived from th e fo rmulafor escape velocity, m ve

2 = 2GmMlR fo r test mass m at distance R from alarge massM, by lett ingve approach th e velocity of l ightc. In thet ime frameof the fall ing m a s s w, it takes infinite t ime to cross the /? = Rs discon t inu i ty.However, it is unl ike ly that infall ing material wil l fall directly toward M \most of it will go into an orbit that is slowly circularized by collisions. X-raybremsstrahlung wil l come from impacts of new m aterial with material inorbit outside R s

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14. (Sec. 4E) If the solar radius, wh ich encloses about2 x 1030

kilograms at a mean den sity of 1.4 gm/cm3, is shru nk to a 7000-kilometer-radius W hite Dwarf star with the same mass, the dens ity mu st increase by afactor of 1 mill ion. This high-density materialis no longer a normal ga sand is called degenerate,wi th al l electrons compressed in toth e lowest atomicorbits. If the collapse starts at a higher rate, grav itation al force modifiesatomic nuclei into neutrons with even higher density:(7000/10) 3(1 400 000) =4.8 x 1014 gm/cm3. Further compression formsa Black Hole witha radiusRsof 3 ki lometers .

15. (Sec. 4E) The Doppler effect (see Pamphlet IV) wil l decrease th eobserved period of rotation or increase the frequ en cy /of x-ray pulses from1.0 per second to 1.1 per second. Whena source is approaching Earthatvelocity v, the frequencyof its output (l ightor pulses) is increased by A/,where//(/+ A/) = vie, and c is thevelocityof the lightor pulses,3 X 108m/sec,which is the velocityof x-ray pulseson the wayfrom th e pulsarto Earth. Thesame Dopplerformula applies to sound waves, which have a veloci ty mu chless tha ne . The observed period of the rotating Ne utron Star isdecreased from1.00 to 0.91 second.

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Appendix BSI UnitsPowers of 10

InternationalSystem (SI)UnitsNames, symbols ,an d conversion factorsof SI units used in these pamphlets:

Quantity Name of unit

Distance meter

Mass kilogram

Time second

Temperature kelvin

Area square me ter

Volume cubic meter

Frequency hertz

Density kilogrampercubic meter

Speed, veloc ity me ter per second

Symbol Conversion factor

m km = 0.621 milem = 3.28 ftcm = 0.394 in.mm = 0.039 in.

/urn = 3.9 o x 10~5 in. = 10" A1 nm = 10 A

kg 1 tonne = 1.102 tons1 kg = 2.20 Ib1 gm = 0.0022 Ib = 0.035 oz1 m g = 2.20 x lO "6 Ib = 3.5 x 10~5 oz

sec 1 yr = 3.156 x 107 sec1 day = 8.64 x 104 sec1 hr = 3600 sec

K 273 K = 0 C = 32 F373 K = 100 C = 212 F

m- 1 m2 = 104 cm2 = 10.8 ft2

m3 1 m3 = 10s cm3 = 35 ft3

Hz 1 Hz = 1cycle/sec1 kHz = 1000 cycles/sec1 MHz = 106 cycles/sec

kg/m3 1 kg/m3 = 0.001 gm/cm3

1 gm/cm3 = densi tyof water

m/sec 1 m/sec = 3 . 2 8 ft/sec1 km/sec = 2240 mi/hr

Force newton N 1 N = 105 dynes = 0.224 Ib f

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