nuclear student

7/30/2019 Nuclear Student

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Inside the center of the atom, far below the electrons, lies the atoms tiny and extremely dense core.For the entire year, we have focused on the chemistry associated with an atoms electrons. Wecompletely ignored the nucleus, taking the positive core for granted as it holds the electrons in placethrough electrostatic attractions between the charged particles (protons attracted to the electrons).The history of science, over the past 1000 years, has focused on technology, medicine, and a longtime ago alchemy. The goal of all alchemists was to turn other species into gold to have Midastouch so to speak. This is not possible through chemical processes, as that only involves themovement of the electrons. No species can be turned into gold by losing or gaining electrons. Gold

is gold because of the numbers of protons and neutrons. However, we now know that it is possible totransmutate (change) one element into another through nuclear reactions which is NOT the sameas a chemical reaction. The goal is no longer to change species into gold (personally I would shoot

for platinum ), as there are far more important and valuable products of nuclear reactions.

Society as a whole has many concerns about the applications of nuclear chemistry in their own lives.Many of their concerns stem from mis-information. The promise of an abundant energy source andtreatment for diseases comes hand in hand with the threat of nuclear waste contamination, nuclearmelt-downs, and nuclear war/terrorism. Can we, as fallible humans, harness the power of thenucleus without destroying ourselves or others? Do we have the moral strength to use our powersonly for good? Or are the risks just too great?

The changes that occur in the nucleus are completely different from all that we have studied to this

point. In chemical reactions, electrons are shared, lost or gained, in order to form new compounds.In these processes, the nuclei just sit there are watch the show, passively sitting by and neverchanging their identities. In nuclear reactions, the roles of the subatomic particles are reversed. Theelectrons do not participate in the reactions, instead they stay in their orbitals while the protons andneutrons undergo changes. In fact, during these changes, in nearly every case, the change results inthe formation of a different element! Nuclear reactions are accompanied by energy changes that area million times greater than those in chemical reactions. Energy changes that are so great thatchanges in mass are detectable. Also, nuclear reaction yields and rates are not affected by the samefactors (e.g. pressure, temperature, and catalysts) that influence chemical reactions.

First, it is important to understand nuclear stability. Why are some nuclei stable where others arenot? When nuclei are unstable, they are termed radioactive. All matter is composed of atoms andmany atoms are unstable. In fact, over half of the elements in the periodic table including uranium,are in a constant process of rearranging themselves. This is not something that humanity can

control.

When the nucleus of an atom attempts to become more stable, it releases energy, known asradiation. Once this happens the original atom changes into a new atom. In some instances, a newelement is formed and in other cases, a new form of the original element, called an isotope, appears.The spontaneous change in the nucleus of an unstable atom that results in the emission of radiationis called radioactivity and this process of change is often referred to as the decayof atoms.

A stable nucleus will remain intact indefinitely. An unstable nucleus will not, and a great majority ofnuclei from atoms on the periodic table are unstable! The unstable nucleus exhibits radioactivity:the nucleus will spontaneously disintegrate (fall apart) or decay by emitting radiation. Each type of

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unstable nucleus has a characteristic rate of decay. Some decay very quickly, e.g. in a fraction of asecond, others can take billions of years.Why Radioactivity:Radioactivity comes out of the nucleus of atoms. The nucleus is radioactive because it is unstable.Like electrons in an excited state dropping back down to ground state and releasing a photon, nucleineed an outlet for their excited state. This outlet is radiation or nuclear reactions. Nuclear reactions,radioactivity, are spontaneous decays, there is no way to tell when it will occur, but eventually theywill decay. These radioactive decays occur in any atom with more than 83 protons. Also, in any

atom with an exceptionally small or large proton to neutron ratio will be radioactive.

Recall that theatom is an electrically neutral, spherical species that contains a positively chargednucleus surrounded by one or more negatively charged electrons. The electrons move rapidlyaround the nucleus and are held there in space by attraction with the positively charged nucleus.The nucleus takes up 1 ten-trillionth of the volume but makes up 99.97% of the atoms mass and istherefore incredibly dense. The atoms total diameter is about 10,000 times the diameter of thenucleus!The nucleus is composed of neutrons and protons. The protons are positively charged and theneutrons have no charge at all, they just contribute to the overall mass of the atom. The magnitudeof the charge of a proton is equal to that of an electron, but the electron is negatively charged. Anatom is neutral because the number of protons ALWAYS equals the number of electrons.

Protons and neutrons are collectively termed nucleons. Most elements exist in nature as a mixtureof isotopes, which are species that are the same atom (they have the same number of protons andelectrons if neutral!) but different numbers of neutrons. A complimentary term to the isotope is thenuclide, which represents the isotopes of an element. It refers to the variety of nuclei with aparticular composition of nucleons (the differing numbers of protons and neutrons). Each isotope is anuclide. This means that 16O which has 8 protons and 8 neutrons is a nuclide, and 17O which has 8protons and 9 neutrons, is a nuclide of oxygen.

Recall that we can write the element or a particular isotope from the periodic table using a variety ofnotations.

Atomic Notation: 3 TypesA = Atomic Mass = protons + neutronsZ = Atomic Number = protonsX = Atomic Symbol = the elements letter designation

The same type of notation can and will be applied to the subatomic particles in the nucleus. Thus, a

neutron that has a mass of 1 (due to the neutron itself) and a charge of zero will be written as n10 .

A proton, that has a mass of 1 (due to the proton itself) and a charge of +1 will be written as p11 .

And an electron, that has a mass of zero (due to the electron itself) and a charge of -1 will be written

as e0

1-.

n10 (neutron) p11 (proton) e

0

1-(electron)

Type 1 XA

ZC

12

6This type came first and gives the most information

Type 2 XA

C12

This type came second as the first type is redundant.You do not need to tell me carbon has an atomicnumber of 6, all carbons atomic numbers are 6.

Type 3 X A C -12This type came last and is the easiest to type, and stillrelays all the info you need. This symbol is spoken,

carbon twelve.

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It is important to be very familiar with this notation, as we will use it in nuclear reactions. It isimportant to remember how to determine the numbers of protons, neutrons, and electrons inparticular nuclides (or isotopes) in order to understand what type of nuclear reaction is taking place.

Concept Test:

Determine the number of protons and neutrons in lC3517

Write the atomic notation for the nuclide of chlorine that has 20 neutrons:

In 1896, Antoine-Henri Bacquerel discovered that uranium minerals emitted a penetrating radiationthat produced images on photographic plates, even though the plates were covered with paper toprevent them from developing in the presence of light but somehow, the radiation penetrated the

paper and developed the plate! Several years later, Marie Curie began a search for other mineralthat emitted radiation.

Marie Curie found that thorium minerals emit radiation. She also showed that the intensity ofradiation depended on the concentration of the radioactive element in the mineral, not on the natureof the compound. Marie Curie and her husband, Pierre, through their examination of uraniumminerals, discovered two other radioactive elements, one was named Polonium (after her nativePoland), and the other was named Radium.

During the next few years, Bacquerel, the Curies, and Rutherford began to study the nature ofradioactive emissions. Rutherford observed that elements other than radium were formed as radiumdecayed. In 1902, they proposed that radioactive emission results as an element changes from oneelement into another, completely different element. To many, at this time, this sounded like the

revival of alchemy. And it was met with ridicule. Now, however, we know this to be true! Undermost circumstances when a nuclide (isotope) of a one element decays, it changes into a nuclide(isotope) of a different element.

Their work led to an understanding of the three most common types of radioactive emission.

3 Main Types of Radioactive Particles:

alpha particles (symbolized as a helium nucleus He4

2) are dense, positively charged

particles identical to helium nuclei. They consist of two protons and two neutrons, identical tothe nucleus of a helium atom. A sheet of paper or a person's surface layer of skin will stopthem. Alpha particles are only considered hazardous to a person's health if they are ingestedor inhaled and thus come into contact with sensitive cells such as in the lungs, liver andbones. A source of alpha particles is radon gas, a colorless and odorless gas formed from theradioactive decay of radium which in turn is one of the products of the uranium decay chain.It is not radon itself that is a health concern, but the radioactive products into which it decays.Radon, being a gas, is simply the vehicle by which members of the uranium decay chain canenter the lungs. Outside the body, radon is not a concern since the alpha particles it emitscannot penetrate the skin.

beta particles (symbolized as , -1, or0

1- ) are negatively charged particles identified

as high speed electronswhich are emitted from the nuclei of many fission products. They cantravel a few feet in air but can usually be stopped by clothing or a few centimeters of wood.

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They are considered hazardous mainly if ingested or inhaled, but can cause radiation damageto the skin if the exposure is large enough.

gamma particles (symbolized as or sometimes0

0) are high energy photons

(electromagnetic radiation) about 105 times as energetic as visible light. They penetratematter easily and are best stopped by water or thick layers of lead or concrete. Gammaradiation is hazardous to people inside and outside of the body.

The three types of particles that are emitted behave very differently in the presence of an electricfield. Alpha particles are positively charged and thus bend towards the negative plate. Betaparticles are negatively charged and thus bend towards the positive plate. Gamma particles have nocharge and are thus not affected by the charged plates. The degree of bending is related to themass of the particle. Alpha particles are heavier than beta particles and thus are less easily movedin space.

When a particular nuclide decays, it forms a nuclide (the product) that is of lower energy and theenergy that is lost, is emitted radiation. Remember that nature always wants to form a lower energyspecies and as such, nuclear decay is no different.

Nuclides can decay in several ways, but they all share some things in common. First, the reactantspecies/nuclide is called the parent species while the product is called the daughter. Second, the full

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The use of a magnetic field todirect beta particles is whatallows your TV to work. Your TVhas a phosphorescent screenthat is bombarded with preciselydirected beta particles. If youlook very closely at your TV whenit is turned on you can see smallblocks of color.Every second your TV shoots 60beta particles a second at eachone of those little blocks. Itstarts shooting in one corner andworks it way across the screenthen drops down to the next row.It hits every block in every rowthen begins again. It fills thescreen 60 times a second with

colored blocks. These blocks, incon unction with each other


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atomic notation of the nuclides is used in writing the nuclear reactions. Writing the equations in thismanner allows us to indicate the type of nuclear reaction the nuclide participated in.

When a nuclide changes from one isotope to another or from one element to another, the atomicmasses will change (for species that change isotopes), and if the species changes from one elementto another, both the atomic mass and the atomic number will change. In order to balance theequation, we must account for all species that are gained or lost in the change.

Lets examine the types of nuclear reactions that we will see:

Alpha Decay: Since an alpha particle represents a helium nucleus, we will be losing two protonsand two neutrons from our parent nuclide. A general reaction is seen below, followed by an actualexample of alpha decay. Again, we must make sure that we account for all species!

eHXX4

2

4-massatomic

2-numberatomic

massatomic

numberatomic +=

HeRnRa4

2

222

86

226

88 +=

Notice that when you add the atomic masses of the daughter 222-Rn with 4-He you get the sameatomic mass that appears in the parent 226-Ra (222+4 = 226). And when you add the atomic

numbers of the daughter 222-Rn with 4-He you get the same atomic number that appears in theparent 226-Ra (86+2 = 88). Since alpha decay involved the loss of a helium nucleus, you are losingprotons. The product in an alpha decay will be a different element it will be the element that is 2atomic numbers away! Also, the mass difference will be 4 amu different between the parent and thedaughter species.

Beta Decay: A beta particle represents the loss of an electron. It might seem odd that an electronis leaving the nucleus, but that is exactly what happens in beta decay. How is this possible? Aneutron is located in the nucleus. A neutron is a neutral particle. Why is a neutron neutral? It isneutral because a neutron is the combination of a proton and an electron:

p11 + e0

1- n10

for decay

n10 p11 + e

0

1-

Remember that a beta particle is an electron, so the more common representation of the neutronlooks like this:

n10 p11 +

0

1-

In essence, for beta decay, the electron is ejected from the nucleus, leaving behind the proton.Since the neutron no longer exists as a neutron, but now as a proton, the overall mass of the speciesdoes not change (remember that the mass is due to number of protons and neutrons, and while we

lost a neutron, we kept it as a proton, so no net change in mass!). BUT, by losing the neutralparticle, we gained a positive particle, which means that the total number of protons in the nucleushas changed and it changed by one.

iN63

28 uC6329 +

0

1-

Beta decay will result in a species that has the same atomic mass, but contains oneMORE proton than itself, thus its daughter will be found one atomic number higher than itself.

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Note: when a neutron decays a neutral particle called a neutrino (: the little neutral one) is also

emitted. It is emitted in other nuclear reactions as well. They will not be discussed further except tomention that experiments in Japan have shown that they do have mass and they may account for asignificant portion of the missing matter in the universe (remember that matter cannot be createdor destroyed . . .) For simplicitys sake, they will not be included in any of the nuclear reactions.

Positron decay: A positron is the antiparticle to the beta particle. A key idea of modern physics isthat every fundamental particle has a corresponding antiparticle, or a particle with the same massbut opposite charge. The positron has the same mass as a beta particle but opposite charge,

therefore it has a +1 charge. It is symbolized as0

1. Sometimes positron decay is referred to as

positive beta decay. In 1932, Carl D. Anderson found positrons created by cosmic-ray collisions in acloud chamber, in which moving electrons (or positrons) leave behind trails as they move throughthe gas. The electric charge-to-mass ratio of a particle can be measured by observing the curling of

its cloud-chamber track in a magnetic field. Originally, positrons, because of the direction that theirpaths curled, were mistaken for electrons traveling in the opposite direction.

Positron decay occurs through a process whereby a proton in the nucleus is converted into a neutronand a positron is expelled. This process is called pair production, which involves energy turning intomatter as a high energy photon becomes an electron and a positron simultaneously. The electronand proton bind and form a neutron, while the positron is expelled.

Because the proton becomes a neutron and stays in the nucleus, the overall mass will not change,but the charge will. In essence, the atom just lost a proton. Therefore the new species will havethe same mass but will have one fewer proton, so its atomic number will decrease by 1.

C116 B115 +

0

1

Positron emission will result in the daughter having the same atomic mass, butwill be one atomic number LOWER than the parent species

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http://en.wikipedia.org/wiki/1932http://en.wikipedia.org/wiki/Carl_D._Andersonhttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Masshttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Carl_D._Andersonhttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Masshttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/1932


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Electron capture: Electron capture occurs when the nucleus of an atom draws in an electron froman orbital of the lowest energy level, the 1s orbital. As the electron comes into the nucleus, it will beattracted to and bind with a proton. This will neutralize the protons positive charge and create aneutron. This is an electron that is taken INTO the nucleus, not an electron that leaves, as such, itshould not be confused with the beta particle mentioned previously. In order to distinguish betweenthe two types of electrons, the symbol for this extranuclear electron that enters the nucleus is :0

1-e. The loss of this inner electron from the first shell is a vacancy. This vacancy will be quickly

filled by an electron that resided in a higher energy level. When the electron from the higher n value

falls to the lower energy level, a photon is released in the x-ray region of the EM spectrum.

Fe5526 +0

1-e Mn55

25+ h (x-ray)

Electron capture causes the loss of a proton as it becomes a neutron. As such,the atomic mass will stay constant but the atomic number will DECREASE

by one. The daughter will be the species one atomic number LOWERthan the parent species.

Electron capture results in the same product that would result from positron decay but the processesare entirely different and should not be confused!

Gamma Emission: Gamma emission involves the radiation of high energy or gamma () photonsbeing emitted from an excited nucleus. Recall that an atom in an excited electronic state willpromote electrons to higher energy levels. Those electrons cannot stay in the higher levelindefinitely, the atom releases the energy absorbed, the electron falls, and the energy is released as

a photon (h) which is of a specific energy usually in the UV or visible region, but also the IR. A

nucleus that is excited will need to release that energy also, and it does so by releasing a photon inthe gamma region. The gamma photon is of MUCH higher energy (shorter wavelength) than a UV orvisible photon. Many nuclear processes leave the nucleus in an excited state, so gamma emissionaccompanies most other types of decay. Because gamma rays have no mass or charge, gammaemission will not change the atomic number or the atomic mass of the species. Gamma rays willalso result when a particle and its antiparticle meet and annihilate one another.

U23892 00

42

23490 HeTh ++

For example, when uranium-238 undergoes alpha decay, a gamma ray is also emitted.

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Other Types of Nuclear Reactions:

Neutron Emission a neutron is emitted from the nucleus cause the mass to drop by 1.

Nuclear Equations:Nuclear equations are similar to chemical equations in that the total mass must be conserved. Thedifference is the components in the nucleus change. For instance, in the first example with C-14 themass did not change but the number of protons did giving a total mass on each side of 14 and a totalnumber of protons as 6. 6 = 7 + -1

Examples:

01

14

7

14

6NC

+

n1

0

14

7 ++ NHC1

1

14

6

n3KrBaUUn1

0

92

36

141

56

236

93

235

92

1

0+++

Electromagnetic Radiation EM Radiation:Electromagnetic radiation is most simply defined as light, and as you know, not all light is visible tothe human eye. Electromagnetic radiation is broken up in to regions based on the frequency of thelight, the full range of radiation is called the electromagnetic spectrum, EMS.

The major regions are radio, micro, infrared, visible, ultra violet, x-rays and gamma rays.

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Usage of Different Wavelengths of light:Radio waves are used to transmit radio and television signals. Radio waves have wavelengths thatrange from less than a centimeter to tens or even hundreds of meters. FM radio waves are shorterthan AM radio waves. For example, an FM radio station at 100 on the radio dial (100 megahertz)would have a wavelength of about three meters. An AM station at 750 on the dial (750 kilohertz)uses a wavelength of about 400 meters. Radio waves can also be used to create images. Radio

waves with wavelengths of a few centimeters can be transmitted from a satellite or airplaneantenna. The reflected waves can be used to form an image of the ground in complete darkness orthrough clouds.

Microwave wavelengths range from approximately one millimeter (the thickness of a pencil lead) tothirty centimeters (about twelve inches). In a microwave oven, the radio waves generated are tunedto frequencies that can be absorbed by the food. The food absorbs the energy and gets warmer. Thedish holding the food doesn't absorb a significant amount of energy and stays much cooler.Microwaves are emitted from the Earth, from objects such as cars and planes, and from theatmosphere. These microwaves can be detected to give information, such as the temperature of theobject that emitted the microwaves.

Infrared is the region of the electromagnetic spectrum that extends from the visible region to about

one millimeter (in wavelength). Infrared waves include thermal radiation. For example, burningcharcoal may not give off light, but it does emit infrared radiation which is felt as heat. Infraredradiation can be measured using electronic detectors and has applications in medicine and in findingheat leaks from houses. Infrared images obtained by sensors in satellites and airplanes can yieldimportant information on the health of crops and can help us see forest fires even when they areenveloped in an opaque curtain of smoke.

The rainbow of colors we know as visible light is the portion of the electromagnetic spectrum withwavelengths between 400 and 700 billionths of a meter (400 to 700 nanometers). It is the part of theelectromagnetic spectrum that we see, and coincides with the wavelength of greatest intensity of

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sunlight. Visible waves have great utility for the remote sensing of vegetation and for theidentification of different objects by their visible colors.

Ultraviolet radiation has a range of wavelengths from 400 billionths of a meter to about 10billionths of a meter. Sunlight contains ultraviolet waves which can burn your skin. Most of these areblocked by ozone in the Earth's upper atmosphere. A small dose of ultraviolet radiation is beneficialto humans, but larger doses cause skin cancer and cataracts. Ultraviolet wavelengths are usedextensively in astronomical observatories. Some remote sensing observations of the Earth are also

concerned with the measurement of ozone.

X-rays are high energy waves which have great penetrating power and are used extensively inmedical applications and in inspecting welds. X-ray images of our Sun can yield important clues tosolar flares and other changes on our Sun that can affect space weather. The wavelength range isfrom about ten billionths of a meter to about 10 trillionths of a meter.

Gamma rays have wavelengths of less than about ten trillionths of a meter. They are morepenetrating than X-rays. Gamma rays are generated by radioactive atoms and in nuclear explosions,and are used in many medical applications. Images of our universe taken in gamma rays haveyielded important information on the life and death of stars, and other violent processes in theuniverse

Given all the reactions that we examined, there are several ways that an unstable nuclide mightdecay, but can we predict how it will decay? Can we predict if a nuclide will decay at all? Ourknowledge of the nucleus is still very limited compared to our knowledge of the atom as a whole.But there are some patterns that do emerge.

The Stability Band: The neutron to proton ratio (N/Z)Remember that the number of neutrons (N) is determined by taking the atomic mass (A) andsubtracting the number of protons:

#no = atomic mass atomic number

or #no = A-Z

A key factor that determines the stability of a particular nuclide is its ratio of neutrons to its number

of protons, or the N/Z ratio. For lighter nuclides, where the N/Z ratio 1, this provides stability. For

heavier nuclides to be stable, the number of neutrons must exceed the number of protons. As youincrease the positive charges in the nucleus (meaning the number of protons) you need more neutralbuffers which are the neutrons in order to increase its stability. Stability can be thought of as theamount of time that the nuclide will exist as that isotope and not undergo some sort of decay.However, if the N/Z ratio is too high or too low, the nuclide will be unstable and will decay. Thereare some generalities that can be made.

The minimum N/Z value for stability is 1. Two exceptions are1

1H and

3

2He. For lighter

stable nuclides, N/Z 1:4

2He,

12

6C,

16

8O,

20

10Ne

The N/Z ratio which indicates stable nuclides increases gradually as Z increases. The ratio will

be larger than 1 but not much greater than 1.5. Bismuth-209 is the heaviest stable nuclide,which has an N/Z value of 1.52.

All nuclides with Z > 83 are unstable, regardless of their N/Z ratio. Therefore the largest

members of Groups 1A, 2A, 6A, 7A, and 8A are radioactive, as are all actinides and theelements in the 4th transition series (Period 7).

Why are neutrons necessary? What does it mean for the neutron to be this buffer inside thenucleus? Given that the protons are positively charged and the neutrons have no charge, whatexactly holds the nucleus together? Nuclear scientists answer this question and explain theimportance of this N/Z ratio in terms of two opposing forces. Electrostatic repulsive forces between

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the positively charged protons (remember +/+ do not want to be anywhere near one another!!)would rip the nucleus apart if it were not for something called the strong force. The strong force isan attractive force that exists between the neutrons and protons (termed nucleons) in a nucleus.This force is about 100 times stronger than the proton-proton repulsive forces but it only operatesover very short distances. It is the competition between the repulsive forces and attractive forcesthat ultimately determines nuclear stability.

Interestingly, the oddness or the evenness of the N and Z value is related to some important patternsof nuclear stability. Two interesting points appear when stable nuclides are examined:

Elements with an even number of protons (even Z) usually have a larger number of stable

nuclides (isotopes) than elements with odd numbers of protons.

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Over half of the known stable nuclides (isotopes) have BOTH even N and Z values. Only 7

isotopes with odd N and odd Z are either stable:2

1H,

6

3Li,

10

5B,

14

7N or they decay so

slowly that their amounts have changed very little since the Earth was formed50

23V,

138

57La,

17671Lu.

One model of nuclear structure that attempts to explain these findings theorizes that the protons and

neutrons lie in shells, nucleon shells, or energy levels where the protons and neutrons reside, andthat the stability results from the pairing of the nucleons. This arrangement leads to the stability ofeven numbers of species, such that all species are paired. Just like noble gases (elements 2, 10, 18,36, 54, and 86) are extremely stable because of their filled energy levels (s 2p6) for electrons, nuclideswith N or Z values of 2, 8, 20, 28, 50, 82, and 126 are also exceptionally stable. These are so calledmagic numbers and are thought to possibly correspond to the number of protons or neutrons thatwould exist in a filled nucleon shell. A few examples are given below:

12NPb

20NCa8NO

2NHe

50NSr

28NTi

208

82

40

20

16

8

4

2

88

38

50

22

=

=

=

=

=

=

Predicting the Mode of Decay:Remember that nature always moves towards a more stable product. As such, an unstable nucleusmust undergo some sort of change in order to become stable. An unstable nucleus will decay in amanner that brings its ratio of N/Z into the band of stability. Generally speaking, nuclides will decayas follows:

Neutron rich nuclides: Isotopes/nuclides that have too many neutrons are unstable. Thesespecies have a large N/Z ratio. In order to achieve stability, these species will undergo betadecay, which converts a neutron into a proton. Thus, the overall number of neutronsdecreases, the number of protons increase, and the N/Z value is lowered or reduced as well.

Neutron-poor nuclides: Isotopes/nuclides that have too few neutrons are unstable. These

species have a small N/Z ratio. In order to achieve stability, these species undergo positrondecay or electron capture. Both of these processes convert a proton into a neutron. Thus,the number of neutrons is increased and the number of protons decreased, thus the N/Z valueis increased as well.

Heavy nuclides: Isotopes/nuclides with atomic numbers (Z) > 83 are too heavy and do not lie

in the band of stability. These species must reduce their mass thus their number of protons

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and neutrons thus they undergo alpha decay. This will reduce their atomic mass by 4 andthe atomic number by 2 (remember an alpha particle is represented by helium).

A parent nuclide may undergo several decays, in succession, before reaching a stable form. This iscalled undergoing a decay series, where each decay step happens, one after the other until thenuclide is stable. Sometimes this is also referred to as a disintegration series. Typically a decayseries is depicted using a grid-like display to show the species as it changes from one unstablespecies to another, until finally becoming stable.

Or, the decay might be shown in a simple manner as shown below.

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Finally, the decay could be shown by writing out the balanced nuclear reactions that the speciesundergoes in a step by step manner (only the first four steps in the decay are shown below):

HeThU

UPa

PaTh

HeThU

42

23090

23492

01-

23492

23491

01-

23491

23490

42

23490

23892

+

+

+

+

Remember that alpha decay changes the mass of the nuclide and the number of protons (N and Zboth change), while beta decay turns neutrons into protons so the mass number stays the same, butthe number of protons increases. (thus N decreases by 1, but Z increases by 1 so the mass staysconstant)

Rate of Radioactive Decay:We know that systems will change to reach the minimum energy state. As such, radioactive decay isall about turning an unstable nuclide into a stable one. However, there is no mention of how longthis process will take. Radioactive nuclei will decay at a characteristic rate, regardless of thechemical species in which they occur. This means that regardless of the source of the unstablenuclide, it will decay at the same rate. The decay rate, or activity, of a radioactive sample is thechange in the number of nuclei divided by the change in time. Because the number of nuclei aredecreasing, and rates are inherently positive, we must take the negative of the change in order tokeep the rate positive!

Decay rate = - nuclei

time

The SI unit for radioactivity is the becquerel (Bq) and is defined as one disintegration per second.1Bq = 1d/s. A more common unit for radiation is the curie (Ci). 1 Ci = the number of nucleidisintegrating each second in 1 gram of radium-226. Because the curie is so large, millicuries (mCi)

or microcuries (Ci) are often used. Several other units of radiation are given below:

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Units of Radiation:Becquerel: 1 nuclear disintegration per secondRoentgen: 2.1 x 109 ion pairs per cm3 of dry airRoentgen: 1.8 x 1012 ion pairs per 1 g of tissue

rad:The absorption of 0.01 Joules of energy per kilogram of material, radiation absorbed dose.gray(Gy): The absorption of 1 Joules of energy per kilogram of material. The official SI Unit.

rem: (roentgen equivalent man) specific to human beings. this unit is found by multiplying the amount of radiation in rads by the QF.

rem = rad x QF

the QF (Quality Factor) is determined by which radiation you are exposed to:

QF = 1 and QF = 10 QF = 20

Curie(Ci): 37 billion radioactive decays in one second.

equivalent to the number of decays in one gram of radium in one second.

An activity is meaningful only when one considers a large number of nuclei in a macroscopic sample.For example, suppose that you have 1 x 10 15 radioactive nuclei of a particular type and it decays at a

rate of 10% per hour. What this means is that that average of all the decays results in 10% of theentire sample of radioactive nuclei will disintegrate in that hour. Thus, after 1 hour, 10% of theoriginal number of nuclei, thus 1 x 1014 nuclei will decay. This leaves behind, 9 x 1014 nuclei thathave not yet decayed. During the next hour, 10% of the remaining 9 x 1014 nuclei will decay, whichis 9 x 1013 nuclei. This process will continue to occur as the nuclei continue to decay. Radioactivedecay is a first order process, it will depend on the number of nuclei present.Half-Life:Decay rates are commonly expressed in terms of the fraction of nuclei that decay over a given timeperiod. The half-life (t1/2) is the time required for one half of a number of an isotope to decay into anew isotope. Half-lives differ greatly from isotope to isotope, they can range from picoseconds tobillions of years. The number of nuclei that are left after the set time is the original number thatwere present.

For example,

14

C has a half-life of 5,730 years. What that means, is that if you had a 50.0 gramsample of14C, after 5,730 years you would have 25.0 grams left. Then, after another 5,730 yearsyou would have 12.5 grams left and so on.

There are several mathematical equations that can be used to calculate amounts or the half-life ofspecies:

ln ktN

N

t

0 =

ln 2/12/1t

t ktN

N=

t1/2 = ln 2k

Thus, the half-life is not dependent on the amount of nuclei that are present! A 50.0 gram sample of14C will have the same half-life as a 150.0 gram sample of14C!

Concept Test:How long would it take for 500 grams for protactinium-234 to decay to 10 grams?

The half life of Pa-234 is 72 seconds.

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Given the following table of information, we can see that our time is very reasonable!

Mass Time500 0

250 72125 144

62.5 216

31.25 28815.6 3607.8 432

The answer should be longer than 360 sec but shorter than 432 sec and it is!

The nuclear processes that we have been considering so far have involved radioactive decay, wherea nucleus emits or absorbs (electrons capture) a few small particles or photons. Eventually, throughthe decay processes, the product is a lighter nucleus. Two other processes cause changes in/withthe nucleus as well. They are fission and fusion. In nuclear fission, a heavy nucleus splits into twolight nuclei. In nuclear fusion, lighter nuclei combine (think fuse together) and form a heavierproduct. Both fission and fusion release enormous quantities of energy.

The Mass Defect:Throughout the 20th century, it has become known that mass and energy are interconvertible. Thetraditional mass and energy conservation laws that we discussed in term 1 have been combined tostate that the total quantity of mass-energy in the universe is constant. Therefore, when anyreacting system releases or absorbs energy, there must be an accompanying loss or gain of mass.

This relationship really did not concern us (was negligible) when we were examining chemicalreactions. The energy changes involved in breaking or forming chemical bonds are so small that the

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mass changes are therefore negligible as well. For example, when 1 mole of water breaks up into itsatoms, heat must be put into the system:

H2O (g) 2H (g) + O (g) H = +934 kJ

Mass is equivalent or related to energy through Einsteins equation E = mc 2 where m is the mass andc is the speed of light. We can use this equation to examine changes in reactions. Remember that

H values are changes the difference in energy between the products and the reactants ( H =

npHfp - nrHfr). So the equation becomes:

E = mc2

or

m = E/c2

Using Einsteins equation, we can examine the change in mass that occurs during the chemicalreaction:

m = 9.34 x 105 J/mol

(2.9979 x 108m/sec)2

m = 1.04 x 10-11 kilograms/mole = 1.04 x 10-8 grams/mole

The change in mass, while a number that can be calculated, is 10 ng which is a change that is too

small to measure with any balance that we have today. (m = mass products mass reactants).

Such small mass changes that occur when chemical bonds break or chemical bonds form allows us tosay that mass IS conserved in chemical reactions.

Nuclear processes are accompanied by a much larger measurable mass change. This mass changeis related to the enormous energy required to bind the nucleus together or to break it apart. Forexample, what happens if we try and rip apart the nucleus of a carbon atom into its protons andneutrons? A carbon-12 isotope has 6 protons and 6 neutrons. A proton is essentially a hydrogenatom (remember that the mass of an electron is negligible). So lets combine 6 H atoms with 6neutrons and see what we get:

Mass of 6 1H atoms = 6(1.007825 amu)Mass of 6 neutrons = 6(1.008665 amu)

H atoms = 6.046950 amu+Neutrons = 6.051990 amu

total mass = 12.098940 amu

BUT!! The mass of one 12C atom is exactly 12 amu!The difference between the calculated (predicted or theoretical mass) and the actual mass is

0.098940 amuNote that the mass of the real nucleus is LESS than the calculated or the combined mass of itsnucleons (protons and neutrons). The mass decrease occurs when the nucleons are united into thenucleus and is called the mass defect. The size of this mass 9.8940 x 10-2 g/mole IS measurable onany laboratory balance.

If we know the m (the mass defect) we can, using Einsteins equation, calculate the energy

equivalent associated with the defect. For 12C, after converting the mass to kilograms, we can

calculate E!

E = mc2

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E = (9.8940 x 10-5 kg/mol)(2.9989 x 108m/sec)2

E = 8.8921 x 1012 kgm2/sec2mol = 8.8921 x 1012 J/mol

E = 8.8921 x 109 kJ/mole

This quantity of energy, the energy associated with the loss of mass, is called the nuclear bindingenergy for 12C. In general, the nuclear binding energy can be calculated for any nucleus, and it is the

quantity of energy required to break up the nucleus into its component protons and neutrons(nucleons). Such that:

1 mole Nucleus + Nuclear Binding Energy nucleons (protons and neutrons separated)

Binding energies are typically expressed in electron volts, specifically the megaelectron volt (MeV).

1 amu = 931.5 x 106 ev = 931.5 MeV

We can compare the stabilities of nuclides by determining the binding energy per nucleon. Theequation is:

Binding EnergyTotal # nucleons

Fission or Fusion: The Means of Increasing the Binding Energy per Nucleon

Binding energies for different species vary greatly. And it is known that the greater the bindingenergy per nucleon, the more stable the species, meaning the harder it is to rip the nucleus apart.Nuclides with atomic masses less than 10 (less than 10 total nucleons) have rather small bindingenergies, with the exception of helium, which has an unexpectedly large binding energy, which is a

reason why it is emitted intact as an alpha particle from nuclei. When species have more than 12nucleons, binding energies vary between 7.6 to 8.8 MeV.

The binding energy in the table above peaks for elements that have a total number of nucleonsaround 60. In other words, as the binding energy increases up to this point, the stability of thenuclides increases as well. Then the binding energy begins to decrease after masses = 60, so thenuclides become less stable. This point, mass number = 60 is important. It represents the place

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where the atom can be most stable. Other nuclides would like to get to this place on the plot.Species with lower numbers of nucleons need to INCREASE their total number of nucleons whilespecies with too many nucleons need to lose some. Two nuclear processes can help a nuclidebecome more stable.

Fission: A heavier nucleus can split into lighter ones. This means that a species with toomany nucleons will now split into to nuclei that have less nucleons. The product nuclei willhave a greater binding energy per nucleon and thus be more stable. Energy will be released

as the product is more stable (at a lower energy state) than the reactant. Nuclear powerplants use fission as do atomic bombs

Fusion: A lighter nucleus needs to gain more nucleons. It can do so by combining with other

nuclei. Again the product is more stable than the reactant, so it is at a lower energy state andthat energy is released. The sun and stars generate energy through fusion as do hydrogenbombs. Current research is focusing on the fusion of hydrogen nuclei to form a stable heliumnucleus as a useful source of energy.

Of the many beneficial applications of nuclear reactions the greatest is the potential for limitlessamounts of energy. In the mid-1930s, Enrico Fermi bombarded uranium with neutrons. What they

observed, were, what they believed, were smaller particles. Subsequent experiments proved theseresults. In fact, when uranium is bombarded with neutrons, it breaks apart into 92Kr and 141 Ba andreleases a lot of energy.

The uranium-235 nucleus can split in many different ways, but what is most important is that ithappens quickly (10-14 seconds) and it releases extraordinary amounts of energy. In fact, pound of

coal releases 2 x 104

J when it is burned while pound of uranium releases 2.1 x 1013

J of energy that is a billion times more!

One of the other products of a fission reaction is the release of neutrons. These neutrons will bemoving, pretty fast, and they will begin colliding with the species that are present, which will causemore splitting and more neutrons to be produced . . . and you can see where this is going! This iscalled a chain reaction, where splitting and collisions propagates more splitting.

The occurrence of a chain reaction depends on the amount of substance. This is termed criticalmass. There must be enough of a species present or the neutrons will miss the species and fly outof the sample. Think about firing a gun at a group of animals. If you are 50 feet away and there is 1animal, you have to be a pretty good shot right? What if you are 50 feet away from 100 animals.

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Chances are, just firing in the general direction of the group will land you a hit. The same applies tothe neutrons. A direct hit is likely if there is enough stuff there! The minimum amount of nuclidenecessary for a chain reaction to occur is called the critical mass. If the mass of the sample is lowerthan the critical mass, then the neutrons will leave the sample and a chain reaction will not occur.

Uncontrolled Fission: The Atomic BombWhen the fissionable species is present in abundance, meaning they exceed this critical mass, theensuing chain reaction brings about an explosion. In order to by some time subcritical masses offissionable material are kept separate and then brought together by some explosions which thenallows the chain reaction to take over.

Controlled Fission: The Nuclear ReactorIt is not too difficult to understand some peoples fears with regard to nuclear energy compared tonuclear bombs. After all, they are the same process, just used or handled differently. What if thepower plant goes haywire out of control what if it explodes like the atomic bomb. What preventsthe fission inside the nuclear reactor from experiencing the uncontrolled chain reaction?

Nuclear Reactors:

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Nuclear reactors are not the most popular source of power in this country. There are many peoplewho wish that these power plants would be shut down. They have good reasons and bad reasons forwanting them to be shut down. One good reason is that we do not have a truly viable method forgetting rid of the reactor fuel once it has been used. This material is called spent fuel. One badreason is the fear of the plant becoming unstable and causing a nuclear explosion. The fuel used ina nuclear reactor is not concentrated enough for an explosion, like that in an atomic bomb, to occur.This is simply not possible. So, put the thought of mushroom clouds or melt downs ending in Chinaout of your mind!

You may be asking yourself, with all the controversy why build them at all? Well, we are an electricsociety. All methods for producing electricity will be developed in some capacity. A tidal power plantwas built in the US many years ago, at great taxpayer expense, unfortunately it failed miserably.The flow of the water in tides is just too slow to produce electricity.

Speaking of electricity, how is it generated? To produce voltage, you need 3 things. A magneticfield, a current carrying conductor (a wire) and relative motion between the two. By relative motionbetween the two it is meant that either the wire or the magnetic field must move so that themagnetic lines of flex cut across the wire. So, if a wire is waved it back and forth between a magnet,a voltage will be generated. Seems simple enough, so why build a nuclear reactor or build a dam ifall that is need is to move wire? Well the wire used in electric power plants is actually a very longwire wrapped thousands of times around an axle, then large magnets are placed around the wire.

With this amount of wire the axle can weigh quite a lot. By a lot I mean hundreds of tons. Lookbelow, these are BIG!

That is a man standing on the generator. The other picture is of a row of generators like those foundin most dams.

The problem is how do you turn this axle? Humans have come up with a few means. First a turbineis attached to the axle. A turbine is really a large fan, but instead of these fan blades blowing air,water or steam or wind is used to push the blades. Since the turbine is attached to the axle wrappedin wire the wire turns in the magnetic field, abracadabra electricity is being generated. When wateris used, this is called a hydroelectric plant, or dam. When wind is used, this is called a windmill. Butthese methods can only be used if there is water or wind available. The most common method used

to turn the generator is with a steam turbine.

Where does the steam come from? Boiled water. It is as simple as that, if you can boil water youcan generate electricity. In order to boil the water you must have a heat source, for example, fire.The typical heat sources are coal, natural gas or oil, and garbage is now being used as well. It isunclear who lives down wind of the garbage burning electrical power plant but I am sure they are notthrilled with their power source! There is another method, nuclear power.

There are two types of nuclear reactions that can be used to generate power. Nuclear fusion andfission. The sun uses nuclear fusion, which is the combining of two smaller nuclei to make one largenuclei. It might not seem like much energy would be given off by this reaction, but sit outside on a

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the first sunny day of spring with in a bathing suit and tell me the sun does not have a lot of power.This is the method we would like to use, the problem is there is too much energy given off. Thefusion reaction gives off so much heat it melts every container known to man. Humans are currentlyattempting to create a magnetic field which will hold the fusion reaction but this research is movingahead very slowly. The best part of this reaction is the waste product is helium, certainly not athreat to the environment as it is a noble gas.

Fission is the nuclear reaction of choice for nuclear power plants. In this reaction a neutron hits and

enters a large nucleus. This nucleus is split into two smaller nuclei, a few neutrons and releases a lotof heat. Not as much heat as in fusion but plenty enough to boil water. The main problem with thisreaction is that the two smaller nuclei produced in the splitting are very radioactive. And their halflives are very long, ranging from thousand to millions of years. It is very difficult to build a containerwhich will hold something for a million years. Other ideas of disposal have been considered such asshooting the waste into space or into the sun, but this is not really a practical solution.

Regardless of politics, these power plants exist. Here are the basics on how a nuclear reactor worksand why the odds are stacked against a leak of any radioactive material reaching the environment.

The fuel used in nuclear power plants is U-235. This in itself is a problem as uranium is not acommon element, and the most common isotope of uranium is U-238. Unfortunately U-238 does notfission every time a neutron hits it, normally it simply absorbs the neutron and becomes U-239. So,U-235 must be purified out of large sample of all the uranium isotopes. This is very costly. Anaircraft carrier, such as the USS Abraham Lincoln costs 2 billion dollars, the nuclear reactor cores U-

235 fuel that is used to run the Lincoln is 1 billion of that total cost.

The U-235 and all the radioactive products of the fission reaction are contained in fuel assemblies.The fuel assemblies are basically bars or rods composed of very strong very corrosion resistantmetal. All the radioactivity of a nuclear reactor is trapped in these assemblies. There would have tobe a serious problem for the fuel assembly to break and release radioactive material.

A nuclear power plant is divided up in to loops. These loops prevent water that has touched thereactor fuel assembles from reaching the environment. This water, called primary coolant, is notactually radioactive, but it is isolated in case there is a problem. The primary coolant is contained inthe primary loop. Again, this loop is wholly separated from the next loop of water called the

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secondary loop. The secondary loop contains the water that is actually boiled to steam and is usedto turn the steam turbine. Remember that the entire purpose of the reactor is to turn the generator.The next loop is the condensing loop. It cools the spent steam from the turbine back to into liquidwater which is looped back to be boiled again. The final cooling loop begins at a lake or river. Thislake or river water cools the condensing loop so it can cool the spent steam.

For a reactor to release radioactive material to the environment the fuel assemblies would have tobreak, at the same time that the primary loop ruptures into the secondary loop which must itself leak

into the condensing loop whereupon it must burst into the cooling loop. This is just not going tohappen unless someone bombs the plant. Regardless, if any of these systems where to break andleak into any of the other systems the reactor would instantly and automatically shut down.

Primary Coolant:

normally composed of water

very high pressure, 2000 lb.

very high temperature, 500F

water performs 2 functions, heat exchanger and as a moderator to slow neutrons to aid in fission

of U-235

Primary Loop:1. begins cycle at pump

2. flows to reactor core and is heated by U-235 chain reaction3. is piped to steam generator through U-tubes and heats secondary water

Secondary Loop:1. begins at the pump under the condenser2. flows to bottom of steam generator3. hits U-tubes and is heated to steam4. is piped to steam turbine where it strikes the turbine blades and is spins the turbine5. exits the turbine and enters the condenser6. hits condenser U-tubes and is condensed to water

Basic Diagram of a Nuclear Reactor:

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Radioactive Dating:Radioactive dating uses half-lives to determine how long something has been around. All objectstake in radioactive materials over the course of their lives. When they die they stop taking in theseradioactive materials. The radioactive materials decay over time. If you know how much of aparticular radioactive material is in a live organism you have the No value. You can use a detector todetermine the amount of this radioactive material in the dead organism, which give you N. If youknow the half-life you can calculate how long the organism has been dead.

C-14 dating is most famous isotopes used for dating but it has limitations. The accuracy of half-lifedating is very poor after the isotope has undergone 5 half-life cycles. So, C-14 with a half-life of 5730years is not very accurate after 40,000 years. Potassium-40 is used to date farther back in history.K-40 has a half-life of 1.28 x 109 years this allows scientist to date much older objects, meteors,dinosaurs, crater, etc. A table of useful isotopic decays follows.

Useful Isotopic Decay

System Material Half-life/years Age range/years

C-14 organic remains 5730 200 - 50,000

U-238 to Pb-206 ratio Minerals 4510 million 10-4500 million

U-235 to Pb-207 ratio Minerals 704 million 10-4500 million

Rb-87 to Sr-87 ratio Minerals 48,800 million 60-4500 million

K-40 to Ar-40 ratio Minerals 1250 million 0.1-3000 million

Sm-147 to Nd-143 ratio Minerals 110,000 million 1000-4500 million

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Transuranium Elements:Humans have created new elements by bombarding atoms with other particles such as, alphaparticles, neutrons, or even with other nuclei. Most of these can be found past uranium on theperiodic table and therefore are named transuranium elements. Uranium is the element with themost protons that occurs in nature. As of today there are not a lot of uses for transuraniumelements. Here is a short list of their uses today: in smoke detectors, nuclear power and nuclearbombs. Most of these isotopes do not last long, having half-lives in the seconds.

Acute Dose-Response Effects in Humans:

Dose (mrem) Effect

10,000,000Immediate prostration, coma, followed by death within 1 or 2 days from severecentral nervous system damage.

1,000,000Immediate nausea, vomiting, diarrhea. Death within 1 or 2 weeks from blistering ofsmall intestine. Complications from depressed bone marrow activity.

100,000No overt effects. Some depression of white cell count. Statistical increase inprobability of radiogenic leukemia and life shortening (1 to 5 days/rem).

10,000Effects are difficult to measure. In early embryo, developmental defects are possible.Subtle abnormalities of brain structure and perhaps also function mayoccur above 10 rem.

1,000No measurable effects except a statistical increase of tumor incidence before

age of 10 in infants exposed in-utero.

Inverse Square Law of Radiation:This equation calculates the intensity of radiation for a given distance from the source of theradiation and is called the inverse square law of radiation. The equation used to calculate the effectsof gravity is in the same format, simply replace I, intensity with G, gravity.

I = intensity of radiationd= distance from the intensity

2

x

2

y

y

x

d

d

I

I=

Radiation Detection

25

and radiation are really not athreat outside the body. The deadlayer of skin that covers your bodyis enough to shield you from them. and radiation are really onlyharmful if the radiation isgenerated from within the body. Aradioactive isotope can be eaten,

drank or breathed in. When thisoccurs the isotope can decay. Thetissue in the body has noprotective layer and is easilyionized, killing the surroundingcells. Or worse, mutating them.

Shielding will also reduce the intensity. The calculation forshielding works on 10th thickness. This is the amount of shieldingto reduce the intensity of the radiation to 1/10 th of what it was.

This is specific for the type of radiation and the shielding used.For example, the 10th thickness for gamma is 2 inches of lead.The 10th thickness for neutrons in 10 inches of concrete or water.


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We detect radiation by using its high energy to cause a reaction to occur. Detection is necessary asradiation has the potential to harm us. Radiation that harms living organisms is called ionizingradiation. If radiation is strong enough to cause atoms and molecules in the body to be ionized badthings can happen to the organism. Cells die, or worse cells live in a mutated state, which is onecause of cancer.The detector is normally gas filled and when ionizing radiation enters this area, the interior gas isionized. Thus, the neutral gas is now charged. A large electrical probe runs through the middle of the

detector and when these ions are formed in the detector, they will now be attracted to this probe inthe middle of the detector as opposite charges attract. The center probe is positively charged so thenewly released electrons are drawn to the probe, then travel through the detectors circuits wherethey are counted. They travel back to the chamber holding the gas and return to the ionized gasfrom which they came. More ions mean more ionizations which means more electrons andultimately, more radiation. The Geiger counter is such a device also known as an ion-chamberdetector.

Another method used to detect radiation is a scintillation counter. When radiation hits a material inthe detector the material gives off a photon, which is read by a photon detector and counted. Thesedetectors use a sodium iodine crystal, which when it is exposed to high energy radiation, will releaselight, which is detected by the photo detector. Hand held scintillation counters are still in use by theNavy. Others do use the scintillation detectors but ironically they have grown while the gas filleddetectors have shrunk. With the exception of the Navy, all other scintillation counters are now largecabinet-sized pieces of equipment and the gas filled counters are very small.

Medical ApplicationsRadioactive tracers are used to make a part of the body temporarily opaque to radioactivity to checkfor a problem, like a leak in your digestive system, or to check if an organ has increased in size. Theisotopes absorb the radiation and do not allow it to reach the x-ray film or detector.

X-rays can diagnose bone breaks and the CAT scan or CT (computed tomography scanning,) which ismany x-rays taken with a computer used to generate an image, can diagnose internal injuries.

Radiation therapy for cancer victims is another important use of radiation. Directing gammaradiation at tumor can kill the tumor. This is sometimes the only hope for those persons with atumor in an inoperable location, i.e. the brain.

As a side note, MRI, magnetic resonance imaging, is a big improvement over x-rays in diagnosingproblems in soft tissue, muscles, organs, tendons and the like. X-rays dont show soft tissue well MRIdoes, the damage tissue is affected differently than healthy tissue, all based on charges, but on thecomputer screen it shows up as a different color.

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Scientific Applications

Atom Tracking:Using isotopes allows chemists to track the flow of a molecule or atom through a biological process.For example a scientist can use heavy water, water which has its normal hydrogen, H-1 replaced withH-2 called deuterium. It is twice as heavy as a normal hydrogen but it chemically behaves the sameas normal hydrogen. The water will react normally and the hydrogen will be tracked through seriesof chemical reactions. The deuterium acts as a tag. The substance which has taken up this heavy

hydrogen has a different spectrum or appearance when analyzed by certain instruments.

A Further Look at the Limits of Nuclear StabilityOne of the basic questions of nuclear science remains unanswered: What combinations of neutronsand protons does nature allow to form a nucleus?

It is possible that there are isotopes of aluminum with twice the mass of a normal aluminum nucleus,but no one has produced such isotopes. We cannot predict what the mass limit might be, yet.

The Rare Isotope Accelerator, RIA scientific program will allow dramatic steps toward understandingnuclear binding and the limits of nuclear stability. Working with a variety of very fast ions frompowerful accelerators, scientists have already made several thousand different nuclei that can beshown on a chart of nuclei.

The figure shows three different kinds of nuclei. The combinations of neutrons and protons thatmake up the stable nuclei, those found on the earth are shown by the black squares. The unstableknown nuclei that have been produced at one time or another in the laboratory are shown in yellow.The larger region of unknown but predicted nuclei is shown in green. RIA will allow scientists toexplore this region.

The unexplored green region, terra incognita, of the chart of nuclides is very large with literallythousands of unknown nuclei. The limit on the neutron-rich side is called the neutron drip line. Theaddition of another neutron would lead to its immediate re-emissionits like trying to add a drop ofwater to a bucket filled to the rim.

Experiments with RIA will extend our present knowledge of heavy isotopes and the neutron drip-linefar beyond the present limit, our present limit is oxygen, not too far down this long road of research.Challenging experiments are planned that will detect and identify individual rare nuclei as they flyfrom the production target.

Additional Information:

What is PET?The name "PET" comes from Positron Emission Tomography. It is a new scanning technique inmedical research. PET allows us, for the first time, to measure in detail the functioning of distinctareas of the human brain while the patient is comfortable, conscious and alert. We can now study the

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chemical process involved in the working of healthy or diseased human brains in a way previouslyimpossible. Before the advent of the PET scanner, we could only infer what went on within the brainfrom post-mortems (dissections after death) or animal studies.

PET represents a new step forward in the way scientists and doctors look at the brain and how itfunctions. An X-ray or a CT scan shows only structural details within the brain. The PET scanner givesus a picture of the brain at work.

Positron: Antimatter equivalent of the electronA positron is an anti-electron. Positrons are given off during the decay of the nuclei of specificradioisotopes. A type of radioactive fluorine produced at TRIUMF for the PET program is a positronemitter. When matter collides with its corresponding antimatter, both are annihilated. When apositron meets an electron, the collision produces two gamma rays having the same energy, butgoing in opposite directions. The gamma rays leave the patients body and are detected by the PET

scanner. The information is then fed into a computer to be converted into a complex picture of thepatients working brain.

How does it work?A conventional "X-ray" is taken by firing X-rays through aperson and onto a film. This "shadow" image showssome structures in the body, such as cartilage and bone. ACT scanner uses fine streams of X- rays. By firing them

through the body from several directions, the CT scanneris able to build up a composite picture of anatomical

details within a "slice" through the person. MagneticResonance Imaging (MRI) does much the same thing, butusing magnetic and radiowave fields. In contrast, the PETscanner utilizes radiation emitted from the patient todevelop images. Each patient is given a minute amount of a radioactive pharmaceutical that closelyresembles a natural substance used by the body. One example of such a pharmaceutical produced atTRIUMF is 2-fluoro-2-deoxy-D-glucose (FDG), which is similar to a naturally occurring sugar, glucose,with the addition of a radioactive fluorine atom. Gamma radiation produced from the positron-

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emitting fluorine is detected by the PET scanner and shows in fine detail the metabolism of glucosein the brain.

What does a PET

scan show?The brain functionbeing studiedduring a PET scan

determines which radiopharmaceutical is used.

Oxygen-15 can be used to label oxygen gas for the study of oxygen metabolism, carbon monoxidefor the study of blood volume, or water for the study of blood flow in the brain. Similarly, fluorine-18is attached to a glucose molecule to produce FDG for use in the observation of the brains sugarmetabolism. Many more PET radiopharmaceuticals exist, and research is under way to develop stillmore to assist in the exploration of the working human brain. For example, dopa, a chemical activein brain cells, is labeled with positron-emitting fluorine or carbon and applied in research on thecommunication between certain brain cells which are diseased, as in dystonia, Parkinsons disease,or schizophrenia.

How much radiation does a patient get?PET scans using radioactive fluorine in FDG would result in patients receiving exposures comparable

to (or less than) those from other medical procedures, such as the taking of X-rays. Other scanningagents - for instance, 6-F-dopa or radioactive water - normally cause even less exposure.

PET radioisotopes produced atTRIUMF

Labeling agent Half-life

carbon-11 20.3 minutesoxygen-15 2.03 minutesfluorine-18 109.8 minutesbromine-75 98.0 minutes

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PET: Research tool of the futureThe PET program at TRIUMF and UBC is in aunique position among world medical researchcenters in having many of the expensive majorfacilities required to mount a powerful PETprogram. These include the cyclotrons andradiochemistry laboratories at the TRIUMF projectthat are the source of the PET scanning agents.

The laboratories are linked to the UBC HealthSciences Centre Hospital by the worlds longest"pea shooter", a 2.4 km pneumatic pipeline usedfor the delivery of PET scanning agents in theshortest possible time. These facilities have a capacity that has not been equaled by any otheruniversity/health sciences center in the world.

http://www.triumf.ca/welcome/petscan.html

More information on the PETTVI scanner
http://www.triumf.ca/welcome/petscan_pettvi.html

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