Cosmic Ray Induced Dark Current in PhotomultipliersAndrew T. Young Citation: Review of Scientific Instruments 37, 1472 (1966); doi: 10.1063/1.1720022 View online: http://dx.doi.org/10.1063/1.1720022 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/37/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Silicon photomultiplier device architecture with dark current improved to the ultimate physical limit Appl. Phys. Lett. 102, 183502 (2013); 10.1063/1.4804192 Photomultiplier Camera for Fluorescence Detection of CosmicRay Induced Showers AIP Conf. Proc. 674, 296 (2003); 10.1063/1.1604087 Reduction of Dark Current in Photomultiplier Tubes Rev. Sci. Instrum. 43, 556 (1972); 10.1063/1.1685687 ``CosmicRay'' Effect in Photomultiplier Tubes Rev. Sci. Instrum. 25, 1218 (1954); 10.1063/1.1770987 Cosmic rays from the dark Phys. Today

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Sun, 21 Dec 2014 08:57:06

THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 37, NUMBER 11 NOVEMBER 1966

Cosmic Ray Induced Dark Current in Photomultipliers

ANDREW T. YOUNG

Department of Astronomy, The University of Texas, Austin, Texas 78712

(Received 9 May 1966)

Cerenkov light flashes produced by cosmic rays traversing the window of an end-on photomultiplier create a fundamental noise limitation that cannot be reduced by cooling the tube. Although the cosmic ray pulse rate is only 1 or 2 cm-2/min, the individual pulses may be hundreds of times larger than single electron pulses. Thus in dc or charge integration photometry, cosmic ray noise can exceed thermionic dark current noise at temperatures as high as + lOoC. These assertions are based on the observed behavior of EMl 6256 and 9558 tubes. The implications for photometry of faint astronomical sources are discussed.

INTRODUCTION

THE dark current of refrigerated end window photo-mulitplier tubes contains large bursts (Fig. 1). The

large irregularities look like leakage currents, but occur even in well insulated enclosures where electrical leakage should not be a problem. The purpose of this paper is to demonstrate that much of the "dark current" and most of the associated noise are produced by Cerenkov pulses from cosmic rays. Cerenkov pulses in photomultipliers have been observed by Amaldi1 and by Chodil et al.2

Amaldi's paper has given the impression that cosmic ray pulses are difficult to observe at sea level, and are there­fore unimportant. On the contrary, it is shown below that this can be the basic noise limitation in certain types of work. We first present qualitative evidence that cosmic rays induce appreciable Cerenkov radiation in photomulti­plier windows, and then discuss the problem quantitatively.

INSTRUMENTATION

An EMI 6256B photomultiplier (13-stage Venetian blind type with Sb-Cs dynodes and a 1 cm S-13 cathode)

TIME

was mounted in a TE-102-TS thermoelectrically cooled chamber made by Products for Research (West Acton, Massachusetts). The inner wall of the chamber, which is a Mu-metal shield at cathode potential, is kept within half a degree of a preset temperature by means of a tempera­ture sensor and servo system. The light weight of the photomultiplier chamber, and the absence of liquid re­frigerants, made it very easy to change from one orienta­tion to another. A General Radio 1230-A electrometer amplified the photomultiplier anode current, and was also used to measure electrical leakage in the photomulti­plier chamber. The output of the GR amplifier was con­nected to a Brown recorder with a t sec pen speed through a suitable ammeter shunt (usually 1.0 n to match the 5 mV full scale of the recorder to the 5 rnA output of the amplifier). The high voltage for the photomultiplier was supplied by a very stable Fluke 412B regulated supply. Leakage resistance between the Mu-metal shield (cathode) and ground was 1011 n. Leakage resistance between anode and ground was on the order of 5 X 1014 n, near the limit of measurement. With 2000 V applied to the photomulti-

FIG. 1. Dark current of an EM! 6265B refrigerated to -8°C. Time constant-14 sec; tube axis-horizontal.

1 U. Amaldi, Jr., Nuovo Cimento 6, 946 (1957). 2 G. Chodil, D. Hearn, R.-,C. Jopson, H. Mark, and C. D. Swift, Rev. Sci. lnstr. 36, 394 (1965).

1472

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Sun, 21 Dec 2014 08:57:06

DARK CURRENT 1473

plier voltage divider, a leakage current less than 10-11 A appeared at the anode at all temperatures.

Because of the small size and low thermal emission of the 6256 cathode, dark current pulses are readily resolved on the Brown recorder when the tube is cooled 20 to 30°C below room temperature. However, a precaution must be taken to record the very largest pulses without distortion. If the amplifier has a short time constant, a large pulse is passed over before the recorder has time to respond fully, owing to the limited speed of the pen. The time {:onstant of the electronics must therefore be large enough to hold the pulse amplitude until the pen can respond. This was achieved by increasing the time constant of the input circuit of the amplifier, which has the additional advantage of preventing any overloading of the amplifier. The total circuit capacitance was about 135 pF; use of the 1010 n input resistor gave about a 1.4 sec time con­stant. At 1500 V across the photomultiplier voltage di­vider, all pulses could be kept less than 5 cm high on the chart, so that the deflections represented the pulse sizes accurately within a few per cent.

All of the equipment was set up about 180 m above sea level in an office two floors below the roof of The University of Texas Physics Building. The structure of the building is probably adequate to shield against the electron com­ponent of cosmic rays, but the much more penetrating rou mesons must come through practically unhindered.

QUALITATIVE OBSERVATIONS

Since the cosmic rays are directed groundwards, and since Cerenkov radiation is emitted in a cone centered on the direction of motion of the radiating particle, the Cerenkov radiation produced by cosmic rays traversing the tube envelope must be primarily directed downward. The total number of photons produced per centimeter of

FACE DOWN

path length in quartz by a relativistic charged particle is about 103 between the absorption edge of quartz at 1900 A and the photocathode cutoff at roughly 5500 A. The CsaSb cathode quantum efficiency is about 15% over this region, so that one can expect about 30 photoelectrons to be produced per particle normally incident on the 2 mm thick tube window. Thus, when the tube faces upward, considerable numbers of rather large pulses should appear at the anode. These should not appear when the tube faces downward, because the Cerenkov radiation is then directed away from the cathode.

In order to investigate such pulses, the tube was oper­ated as described above, and the large pulses were recorded on the strip chart: Fig. 2 shows typical records. Four hour runs were made in each of three positions-tube face up, face down, and horizontal. The pulses larger than 6.35 mm on the chart were then hand counted and classified into successive 6.35 rom intervals of height. Thus a crude pulse height distribution was constructed for each orientation.

The three cumulative distributions are shown in Fig. 3; a statistically significant excess number of pulses appears when the detector looks upward or horizontally, as was expected. Since the window presents a larger cross section to the sky when it lies in a horizontal plane (tube axis vertical) than when it stands in a vertical plane (tube axis horizontal), the total number of pulses is greater in the former case than in the latter. However, since the cosmic ray trajectories are primarily vertical, greater path lengths and hence larger individual pulses are more likely with the window vertical (tube axis horizontal) (see Fig. 4). Figure 3 shows these features dearly: The greatest number of pulses occurs with the tube face up, but the largest pulses are more common with the tube axis horizontal.

Calculation of the tube gain based on the manufacturer's cathode and anode sensitivities, together with the observed dependence of gain on voltage, allowed a rough estimation

FACE UP

IA. .,..\ .•. LJ\. ,_. --Lw._ , •. I\..~_ "-A "'."""t .•. , ,[---A_._A~A~L_JL.......L.. ~ll_"-<.,,,--,,--tlL.<.-\~ __ .L. .. L,",,-,~,",LA,,_ ... l,..ALj~L BARE WINDOW

.. it .... _, . ' •. 1, l.,\-.L~.lLll.l'--k ___ ._l._\,~",---l~-u' __ "k-,-_.JL~ ""'\~LL"-,~LLlLAL'-'LL ... ",-Lt,-,,',,L_,_J, I ... c J MIRROR

FIG. 2. Da;k current pulses from the tubt; of Fig. 1 at _8°~ with tube axis verticaL Left hand traces-tube face down; right hand traces­tu,be face,up, top row-tube face covered WIth black tape; mIddle row-tube face bare; and bottom row-tube face covered with first surface mIrror. TIme constant-l.4 sec.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Sun, 21 Dec 2014 08:57:06

1474 ANDREW T. YOUNG

500 ,....--,----,----,--.,.----,--,----,------,

100

50

20

10

• Face up o Foce down

t\ \

\} *~ \ \-1 EMI 6256 B -11252

1.5 ltV -6-C

, , h~

, I, !i"chu 01'1 ship charn

pulses/minute

1.0

0.5

0.2

0.1

0.05

0.02

0.01

0.005

FIG. 3. Integral pulse height distributions (total number of pulses N greater than a given height h in 4 h) for the 6256B in different orientations. Error flags extend .y N above and below each point. T=-8°C.

of the pulse heights in terms of photoelectrons per pulse. It turned out that the smallest pulses counted correspond to about 50 electrons at the cathode. Considering the un­certainities of the calculation, this is in reasonable agree­ment with the expected 30 per minimum Cerenkov pulse. The total pulse rate (about 1.2 min-I, face up) also agrees well with the total expected cosmic ray rate (about It min-I) for a 1 em cathode.

The large pulses all originate at the cathode; with cathode connected to the first dynode, the largest anode pulse observed in 2 h was only about 3.8 mm. In fact, such small pulses are probably due to Cerenkov induced photo­emission from the first dynode. This experiment shows that direct expulsion of electrons from dynodes by cosmic rays is relatively unimportant.

The large pulses observed with the tube face down are

I

not to be attributed to intrinsic tube background or phos­phorescence; they may also be due to Cerenkov light. The reason is that although the cosmic rays all move downward, some of their Cerenkov radiation moves upward. Two effects contribute to this-first, since the Cerenkov radia­tion is emitted in a cone of vertex semi-angle 48°, part of that cone of radiation is directed above the horizon for cosmic rays incident from altitudes less than 48° (see Fig. 5). This mechanism was overlooked by Amaldi,l Secondly, some of the downward directed radiation is re­flected at the lower face of the window and returned to the cathode. Thus, most of the incident cosmic rays produce some illumination of the cathode, but the pulses are gen­erally smaller when the tube is face down. In fact the number of pulses observed in the smallest height interval counted is roughly the same for all orientations of the tube.

THEORY OF CERENKOV PULSE HEIGHTS

If we ignore for the time being the complication intro­duced by reflection and refraction at the outer face of the tube window, we can construct a simple theoretical model to describe the pulse height distribution in a horizontal window. We suppose that all Cerenkov photons strike the cathode, so that the pulse height is just proportional to the cosmic ray path length 1 in the window. If the window is of unit thickness, l=secfJ (see Fig. 6). If the angular distribution of the cosmic ray intensity is

n (e)drl 0:: cos20drl,

which is approximately true,3 we can convert from de to dl and integrate over all azimuths to find

n (l)dl 0:: 1-4dl. (1)

Setting the proportionality factor equal to 1, we see that the number of ligh t pulses larger than h is

N(h)= i co

n(l)dl=th-3, (2)

where h?1 because the minimum pulse size corresponds to a normally incident particle. From now on, we use h to

/ I / I window \.1 "-

I

"I

/ i~ :~ ~----~'--/----t~~ ___ -------- I

I ----cathode I

I" I " I "-t

I ( a) f

\. \

( b)

3 A. E. Sandstrom, Cosmic-Ray Physics (North-Holland Publishing Company, Amsterdam, 1965), p. 64.

FIG. 4. Comparison of cosmic ray paths through tube window in different positions; (a) tube axis vertical, window plane horizontal; (b) tube axis horizontal, window plane vertical.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Sun, 21 Dec 2014 08:57:06

DARK CURRENT

Cathode

Window

FIG. 5. Geometry of upward moving Cerenkov radiation. The Cerenkov wave front (electromagnetic shock wave) is shown by the light line.

denote the normalized pulse height; it is the ratio of the cosmic ray path length in the window to the window thickness.

QUANTITATIVE OBSERVATIONS

In practice, this distribution is not observed, because not all the photons are detected; some are refracted out of the window, mostly at the outer surface. As a crude improvement in the experiment, we can attach a first surface mirror to the outside of the window in order to reflect escaping photons to the cathode. Then, if detection were perfect, we should find the same N(h) with the tube either face up or face down (see Fig. 2).

200

N(h)

100

50

20

10

5

2

EMt 6256 B -11252 1.5 kV "'S·C

1475

• Face up o Face down

FIG. 7. Differential pulse height distributions ob­served in a 6256B with mirror over tube face. Error flags extended -oJ N on either side of data points. Solid line-fit to h-4 law. Count­ing interval-4 h.

\

Relative Pulse Height --

The results of such an experiment are shown in Fig. 7. o ~ ____ ~ ______ -L ____ ~ __ -+ __ ~ __ ~

The tube face was covered with an aluminized microscope cover glass. The pulses are larger and more numerous than without the mirror, but there remains some asymmetry between the face up and face down orientation, due to losses in the mirror, etc. Nevertheless, the h-4 law [Eq. (1)J fits the data reasonably well. Since a minimum pulse height exists, the distribution must fall to zero for suffi­ciently small pulses; this lower cutoff probably accounts for the deviation of the lowest channel counts below the h-4 line.

An alternate approach is to make the outer surface of the window completely absorbing. Then each Cerenkov pulse is separated into two parts, of which one is absorbed and the other strikes the cathode. By measuring pulse height distributions facing upward and downward, the two halves can be measured separately. Then, because each distribution is a monotonic function of the angle 0,

Window

FIG. 6. Geometry relating cosmic ray path length and angle of incidence on a horizontal window.

200

N(h)

100

50

20

10

5

EMI 6256 8 #11252 1.5 kV

0

2

5 10

\

5 10

20 40

Face up Face down aest h- 4 fil Reconstructed

original distribution

FIG. 8. Differential pulse height distribu­tions 0 bserved with black tape over tube face. All other condi­tions identical to those of Fig. 7.

Relative Pulse Hei;ht --

20 40

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Sun, 21 Dec 2014 08:57:06

1476 ANDREW T. YOUNG

FIG. 9. Geometry relating cosmic ray path length and angle of in­cidence on a vertical window.

the corresponding pieces can be recombined. The pulse heigh ts hI and h2 corresponding to the same frequency n in each distribution must refer to the two parts of pulses corresponding to the same angle O. Therefore,

n(h1+h2) =n(h1) =n(h2)

is the distribution function of the total light. This approach was tested experimentally by pressing a

piece of black vinyl tape (Scotch No. 33) into optical contact with the tube face (see Fig. 2). The technique is imperfect because small areas remain separated from the silica, and because areas near the edge of the useful cathode may not be properly blocked out. Nevertheless, a large face up-face down asymmetry is observed; the total counts in 4 hare 283 face up and 109 face down at -8°C.

To demonstrate that the source of the large pulses is not thermal emission, the experiment was repeated at +3°C. The total counts in 4 h at the higher temperature were slightly smaller (257 face up and 105 face down), although the thermionic emission (as judged by the aver­age current) more than doubled. In fact, a slight decrease

TABLE I. Number of light pulses, Cerenkov current, and Cerenkov noise for various values of the normalized pulse height h [Eqs. (1), (6), and (5), respectively.]

Relative mean pulse height h 3 5 7 9 11 13 15 Total

A. Face up

pulses N(h) 158 92 26 6 4 0 1 287 current hN(h) 474 460 182 54 44 0 15 1229 noise h2N(h) 1442 2300 1274 486 484 0 225 6191

B. Horizontal

N(h) 124 50 21 20 0 3 0 223 hN(h) 372 250 147 180 55 39 0 1043 h2N(h) 1116 1250 1029 1620 605 507 0 6127

C. Face down

N(h) 132 17 6 1 0 0 0 156 hN(h) 396 85 42 9 0 0 0 532 h2N(h) 1188 425 294 81 0 0 0 1988

is to be expected if the pulses are due to Cerenkov radia­tion, because the cathode and dynode efficiencies fall with temperature. The indicated decrease in Cerenkov pulse height in 11 °C is about 5.5% based on the observed slopes of the integral count curves. The indicated temperature coefficient of -0.5%;oC is quite typical for photomulti­plier tubes.

When allowance is made for this slight temperature effect, the pulse height distributions at -8 and +3°C appear indistinguishable. Data from both runs have there­fore been combined in Fig. 8, where the heights measured at +3°C have all been increased by 5.5% to make them correspond to the -8°C data. The reconstructed original distribution fits a line of slope -4 [d. Eq. (l)J remark­ably well, considering the crudeness of the experiment and the neglect of edge effects.

The pulse height distribution measured with the tube horizontal can also be accounted for quantitatively. The analysis with the tube face vertical is similar to that with the window plane horizontal, except that the path length for a cosmic ray from zenith angle 0 becomes (sinO)-! instead of (COSO)-1 (Fig. 9). However, a considerable sim­plification can be made if we consider only large pulses. In this case the cosmic rays mostly come from a region near the zenith where () is small and cos2()::d. We then find that

n (h )dh = canst· dO ex h-2dh, (3) for h»l.

As a test of this inverse square distribution, the data

zoo

N(h)

100

50

20

10

5

2

\ ?-\

6256 B #11252 1.5 kV

FIG. 10. Differential pulse height distribution ob­served in a 6256B with vertical window. Counting interval-4 h.

h--

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Sun, 21 Dec 2014 08:57:06

DARK CURRENT 1477

from Table I for a vertical tube face (axis horizontal) are plotted in Fig. 10. Strictly speaking, such data should be obtained with a mirror over the tube face. However, for small 8, most of the Cerenkov photons are totally reflected to the cathode anyway. Rather surprisingly, even the smaller pulses fit the h-2 law quite well. However, there appears to be a deficiency of very large pulses. The reason is that because of the finite size of the cathode, the maxi­mum possible value of h is the diameter-to-thickness ratio .of the cathode window, which is 5 for the 6256B.

In order to investigate this problem further, and to demonstrate that large pulses are not restricted to a parti­cular tube or cathode type, an EMI 9558B was operated in the same cooling chamber. This tube has a 5 cm S-20 (trialkali) cathode on a 2 mm glass window. The maximum value of h is thus about 25. Because of the much larger cathode area, the Cerenkov pulse rate is much higher in this tube. Under the recommended operating conditions, the gain is about 2X 106 and a 6.35 mm pulse on the Brown recorder corresponds to about 90 electrons at the cathode. With the tube axis horizontal, such pulses occur about once a minute. The expected minimum Cerenkov pulse height is about 15 photoelectrons at the cathode, so that the smallest counted pulses correspond to h,....,6. The ap­proximations made in deriving Eq. (3) should be very good in this case. However, the observed pulse height distribution, plotted in Fig. 11, is much steeper than an inverse square law. The reason seems to be again the effect of the cutoff at h = 25.

100

!N (h)

50

20

10

5

2

+ Observed

? Corrected \ l f\

\ FIG. 11. Differential pulse \\t height distribution ob-

9558 B "'8592 1540 V

2

served in a 9558B with

\

window vertical. Dots­raw data; circles-data cor­rected for edge effects (see

t 1

text). Counting interval-

\ 1 h.

j\

10 20

h-

\ \ , ~

,),

\ 'I/'

\

\

\ \

\ \

\

\

\ \

\ \ e ~

J'" \ 0

\ \

\ \

\ \

\ \

\ \

\ \

\ \

\ \ It -'-

FIG. 12. Geometry of edge loss for large Cerenkov pulses.

Figure 11 shows only 1 h of data, but no pulses larger than 3.18 cm could be found in several hours of data. We therfore assume that the maximum value of h corresponds to this height, about 450 photoelectrons per pulse; the minimum Cerenkov pulse height is then about 18 photo­electrons, which seems reasonable.

To simplify the analysis of the edge effects, we suppose the cathode to be square instead of circular. As seen from the zenith angle 8, the projected width of the cathode is a=Dsin8, where D is the true cathode diameter (see Fig. 12). However, those mesons which pass through the edge of the window have greatly reduced path lengths and therefore abnormally small Cerenkov pulses. We ignore their contribution. Since the projected width of the edge is b=tcos8, where t is the window thickness, the number of pulses of height h= cosec8 is reduced by the factor

(a-b)/ a= 1- (b/a) = 1- (t/D)cot8~ 1- (t/D)h, (4)

since cot8~frl=h for small 8. Equation (4) thus provides an estimate of the attenuation of the upper end of the pulse height distribution due to the edge cutoff. If we divide the observed counting rates by this attenuation factor, we can correct for the edge effect. The corrected data are included in Fig. 11, and fit the expected h-2 law very well.

To summarize, we have the following evidence that the large dark current pulses are in fact photoresponses to Cerenkov flashes in the tube window:

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Sun, 21 Dec 2014 08:57:06

1478 ANDREW T. YOUNG

(a) Size. The pulses are of about the right amplitude (2::30 electrons at the cathode per pulse for the 6256B) to be Cerenkov photoresponses.

(b) Total frequency. The total counting rate is in agree­ment with the known muon flux near sea level (",1-2 cm-2/min).

(c) Pulse height distributions. Experiments designed to show the true amplitude distribution of the total Cerenkov emission yield the pulse height spectra expected from the angular distribution of cosmic ray muons near sea level.

(d) Orientation effect. The pulse height distributions observed with the tube face up, face down, and horizontal are accounted for by the directional nature of the Cerenkov light (Figs. 7, 8, 10, and 11).

(e) Lack of gravitational effect. If the face up-face down asymmetry were due to a gravitational effect within the tube, it should not depend on the placement of reflecting or absorbing surfaces against the outside of the window. The dependence of the asymmetry on optical conditions is qualitatively in agreement with the Cerenkov mechanism (Fig. 2).

(f) Temperature effect. The slight temperature depen­dence observed is typical of the photoresponse of photo­mulitpliers, but is of different sign and magnitude from the temperature dependence of thermionic emission (Fig. 13).

FACE DOWN

~~;'V';v\./l."tJJ~~U SIftIT __

.all 10 110 ... -------------------------------------

NOISE ANALYSIS

Although the rate of production of Cerenkov pulses is. low ('" 1.2 min-I), their large size makes them a significant noise source in dc or charge integration photometry. The reason is that the mean square noise fluctuation is just the mean square deflection minus the square of the mean; because these pulses are so large, their squares are very large, and they raise the mean square fluctuation sub­stantially in spite of their infrequent occurrence.

For a tube facing upward, Eq. (1) is approximately true, and we can calculate the contributions of pulses of a given height or larger to the total Cerenkov noise and current, taking the noise to be

Z(h)= In'" 12n (l)dl=h-l, (5)

and the current to be

i(h)= foo In(1)dl=!h-2, (6)

since each pulse contributes to the current in proportion to its height. From Eq. (5) we see that half the noise comes from pulses over twice the minimum height, although from Eq. (2) we find that these make up only t of the total number of Cerenkov pulses. The contributions to the noise and current of the pulses shown in Fig. 3 are given in

FACE UP

I .' Ill, I ,1 ,j f~ .'l i\ !i J ~i\~ ;-., ,I ;'1 .," i :, ·,1 ~ 1\".,

/.'l,""v_r'\, ___ "" ..... ·'c.,~--,I' . .I"'l"-,,.,.,....,'\.\"',,~I\oL-"'l!""~ (\/Vt~! ,Iv' .,j ,;y Vf ~"L.vv\.'; Vl;~",,",/,' ~'[,..:'VI,.",_ ... n'VLr/ _____________________________________ L, _______________________________________ _

T·-e"c

FIG. 13. Dark current of 6256B at different temperatures, tube face covered with black tape; left side-tube face down; right side-tube face up. Time constant-14 sec.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Sun, 21 Dec 2014 08:57:06

DARK CURRENT 1479

Table 1. The noise contribution of large pulses in the face up condition is considerable.

The correction for edge effects given by Eq. (4) should have been included in the foregoing analysis, but the in­frequency of extremely large pulses in a horizontal window makes this effect unimportant. However, for a vertical window, the large pulses are much more common; in fact, if we substitute Eq. (3) for n(l)dl in Eqs. (5) and (6) both the current and noise integrals diverge. This divergence does not occur in reality because of the cutoff at h=D/t.

Since Fig. 10 suggests that Eq. (3) represents the ob­served pulse spectrum well even for small h, we adopt

n(l)dl= [1- (tl/D)JZ-2dl (7)

for the vertical window. Writing H for D/t=hmax , Eq. (5) becomes

If H»1, the total noise (for h=1) is approximately tHo Thus half the total noise is contributed by pulses for which h~H(1-2-!)~0.3H, which make up somewhat more than H-I of the total pulse count. For the 9558 (H ~ 25), half the noise is contributed by the largest 5% of the pulses, corresponding to a rate of about 2 min-I.

Since one Cerenkov pulse of k electrons at the cathode contributes as much noise as k2 individual electron pulses, the Cerenkov noise is comparable to the thermionic noise when

(9)

where nc and nth are the rates of occurrence of Cerenkov and thermal (single electron) pulses, respectively. When the Cerenkov pulses contribute half the total noise, they contribute only a small fraction of the total current, since then

Because of this, the dark current noise reaches a nearly constant value at temperatures where the current itself is still falling rapidly on cooling. This effect is demonstra,ted in Fig. 13, which shows dark current traces at different temperatures. In the 6256B k is about 30, so that the Cerenkov noise equals the thermal noise when about 1000 thermal electrons are emitted per minute (15-20 seel). Since this occurs slightly below room temperature, the total noise does not decrease much when the tube is cooled below about 10°C, although the total dark current con­tinues to decrease markedly below this point.

The Cerenkov pulses in the dark current of the 6256B are unusually large, because the window is moderately thick (2 mm) and is made of fused silica, which transmits in the far ultraviolet (S-13 response). The glass windows of S-l1 tubes, which absorb below 3000 A, should produce

Cerenkov pulses about half as high (i.e., about 15 electrons at the cathode per pulse). This represents a factor of 4 reduction in noise, which means that one can profitably cool S-l1 cathodes by about another 20°C to about -10°C before the Cerenkov noise limit is reached.

In order to reduce the Cerenkov noise, one can

(a) make the window thinner; (b) make the cathode window combination less sensitive (c) make the cathode smaller; or (d) decouple the cathode optically from the window.

In the ITT tube FW-118, with a 2 mm diameter S-1 cathode on a 1 mm window, the effect of (a) is to reduce the pulses by a factor of 2, the effect of (b) is to reduce them by an additional factor of about 20, and of (c) limits the value of H to about 2. Thus each Cerenkov pulse should produce about 1 photoelectron, or less, on the average. In fact, this is exactly what is observed; no pulses can be found in the dark current larger than those produced by light. In Amaldi's experimentl with a DuMont 6291, a low (6%) cathode efficiency also seems primarily respon­sible for the small (",5 electrons) cathode pulses.

All end window tubes have the cathode strongly coupled to the window optically. However, the metal backed cath­odes of the RCA 931-A (lP21) and similar types are de­coupled in two ways. First, the cathode is not in contact with the glass, so that photons striking the inner window surface at less than the critical angle are totally reflected and never reach the cathode. Second, the cathode sub tends a limited solid angle from any point on the window, so that many of the transmitted photons miss the cathode entirely. A rough calculation indicates that the apparent solid angle subtended by the cathode as seen from a point inside the glass is about 0.07 sr. We may take about half of this as a reasonable estimate of the pulse height reduction factor due to effect (d) since a single Cerenkov flash spreads into about 2 sr. Because the thin glass en­velope would reduce the minimum Cerenkov pulse to about 10 photoelectrons if the cathode were on the glass, we can expect less than 1 photoelectron per cosmic rayon the average. This practical absence of large dark current pulses explains why the IP21 has been such a good tube for dc photometry at very low light levels; the tube may usefully be cooled until the thermionic dark pulse rate is '" 1 min-I. No sensitive end window tube can be expected to give such low noise in dc work, no matter how much it is cooled, unless the window is made very thin. We might expect similar performance from a quartz end window tube like the 6256B only if the window were made about 100 times thinner-20 jJ, thick instead of 2 mm.

To avoid the large noise problem of the Cerenkov pulses, we must go to pulse counting; for then all pulses are counted equally regardless of height. A common mis­conception of the advantage of pulse counting is that the

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Sun, 21 Dec 2014 08:57:06

1480 ANDREW T. YOUNG

numerous very small pulses originating at the dynodes are not counted but, because of their small size, they contribute little to the current and negligibly to the dc noise anyway. The real advantage of pulse counting is that the very big pulses are discriminated against (relative to dc work), and the noise level is thereby reduced. Clearly, any tube used as a pulse counter can be usefully cooled until the rate of thermionic emission is below the rate of Cerenkov pulses-about 1 min-l/cm2• However, one must be sure that the giant pulses do not overload the electronics or cause spurious echo pulses or ringing.

If one wanted to push below even this noise limitation, one could try making the window so thin-say a few mi­crons-that essentially no Cerenkov photoelectrons are ever produced. This approach may be limited by direct expulsion of electrons from the cathode by the cosmic rays themselves. A second approach is to use the large size of the Cerenkov pulses to separate them from bona fide single electron pulses at the cathode. By using a second counter with about a 10 electron threshold in anticoincidence with the normal counting channel, the Cerenkov pulses can be ignored. Unfortunately there are always a few small Cerenkov pulses due to edge effects, but the majority can probably be removed. Since a cooled tube always faces a window of some sort, Cerenkov pulses in the chamber window can produce additional dark counts. However, if the window is kept thin, the 1P21 argument applies and the problem can be kept small.

In pulse counting, selection of the "best" tube depends primarily on dark count rate rather than on pulse height distribution. In this case, the EMI end window tubes are preferable to the IP21, because they typically have an order of magnitude smaller dark count at a given tem­perature. Part of the reason for this is that the prefabri­cated cathode of the IP21 must be partly processed before assembly; some contamination inevitably results, which leads to lower sensitivity and higher dark current. There may be an additional effect due to band bending by the metallic substrate. Thus, for pulse counting, the IP21 can­not be expected to give as low dark noise as a good end window tube. This is even more true if the cosmic ray pulses can be discriminated against according to height, which is not possible in the case of the IP21.

SIGNIFICANCE FOR ASTRONOMY

Even at sea level, the minimum noise level of many cooled photomultipliers is set by cosmic rays. At the major U.S. observatories, which are at altitudes of 1.6 km or more, the number of cosmic ray pulses is about twice the value at sea level, or roughly 3 or 4 cm-2/min.

The cosmic ray noise in an end window tube used for dc photometry is then similar to that of a few thousand thermionic or night sky photoelectrons per minute, or

about 30 seci . This is the photocurrent corresponding to a star of about 17.5 magnitUde observed with a typical broadband photometer on a telescope with 1 m2 effective collecting area (about a 1.27 m reflector). This is also the photon noise contributed by about 25 sec2 of sky of bright­ness equal to one 21 magnitude star per square second. Since one generally uses a measuring area of 100 sec2 or more, the Cerenkov noise is only marginally important in broadband photometry, except on small telescopes «60 cm aperture). However, it is very significant in spectrophotometry, where bandpasses 10 to 100 times smaller are commonly used; G. de Vaucouleurs has found the cosmic ray noise to be a serious problem in scanning galaxy spectra at McDonald Observatory. Faint star spec­trophotometry should therefore be done only with pulse counting equipment if end window alkali-antimonide tubes are used.

At balloon or satellite altitudes, the counting rate is so high-about t count/sec cm-L-that even pulse counting is seriously affected.2 A dc system at balloon altitudes can be severely affected by cosmic ray noise. For example, the automatic tracking system of Stratoscope II, using RCA 7265 tubes (5 cm, S-20 types), became progressively worse with height. " ... The most troublesome difficulty was that a number of times tracking was lost, particularly for some of the fainter objects, by occasional disturbances in the automatic tracking mechanism. However, the re­acquisition was generally quite fast. The only consequence of this difficulty, therefore, was that a number of the spectrometer tracings are not complete .... "4 As these "disturbances" were rather severe for 8th magnitude stars observed with a 91 cm telescope, which should pro­duce about 106 photoelectrons/sec in the tracking pho­tomultipliers, the size of the disturbing pulses must have been quite large. However, since the bandwidth of the electronics was several hundred cycles per second," it is reasonable that large cosmic ray pulses of several hundred photoelectrons could have caused major disturbances. Furthermore, the large amount of glass placed in front of the tubes may have added substantially to the Cerenkov illumination.

The best way to reduce such problems in analog (dc) systems is to reduce the amount of glass near the cathode as much as possible. At any altitude, shielding against electrons is practical, but heavy particles are too penetrat­ing for any reasonable amount of shielding to be effective. Anticoincidence discrimination against cosmic rays may be necessary if the dark sky above the atmosphere is to be fully exploited.

If dc techniques are used, one should be careful not to use a more ultraviolet sensitive tube than is required. A glass

4N. J. Woolf, M. Schwarzschild, and W. K. Rose, Astrophys. J. 140, 833 (1964).

6 E. R. Schesinger, Electronics, 47 (8 February 1963).

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Sun, 21 Dec 2014 08:57:06

DARK CURRENT 1481

window tube may give more dark counts than one with a quartz window, owing to radioactivity of the glass; but: it: gives smaller cosmic ray noise and therefore is preferable for dc work, although the quartz tube may be better for pulse counting. In fixed equipment, such as at a coude focus, a face down mounting position is several times better than face up or horizontal mounting (d. Table I).

Finally, a good end-on tube need not' be cooled below the ice point to give optimum signal-to-noise ratio in dc

THE REVIEW OF SCIENTIFIC INSTRUMENTS

work (see Fig. 13). This greatly simplifies the problem of temperature stabilization, compared to dry ice cooling.

ACKNOWLEDGMENTS

I wish to thank X J. Woolf for a discussion of the cosmic ray problems experienced in the Stratoscope II experiment, and G. de Vaucouleurs and R. G. Tull for discussion of their experiences with noise in refrigerated photomultipliers at McDonald Observatory.

VOLUME 37. NCMBER 11 NOVEMBER 1966

New Method of Displaying Disturbance and Wave Propagation

O. STIRAND, V. KREJCi, AND L. LASKA

Institute of Physics, Czechoslovak Academy of Sciences, Prague, Czechoslovakia

(Received 6 June 1966; and in final form, 26 July 1966)

The measured phenomenon modulates in intensity a horizontal line on a CRO screen. The vertical position of this line is electrically linked with the position of the detector (e.g., photomultiplier). If the phenomenon is re­peated many times while the detector slowly moves, the individual lines fuse into an intensity modulated space­time diagram, similar to a TV picture. The method quickly provides accurate and full information of even very complicated and fast wave phenomena, and increases the signal-to-noise ratio by summarizing many independent measurements into a single picture. Examples of the use of the method for measurements of ionization waves in a discharge plasma are given.

1. INTRODUCTION

WHEN examining complex physical phenomena that depend on two independent variables, it is usually

very desirable to obtain the result of the measurement in the form of a diagram (e.g., on an oscilloscope screen) with the coordinates of each point being given by the values of the independent variables, and the quantity being measured converted to the brightness of the point. A diagram of this sort supplies information on the meas­ured relationships far more clearly than if they were represented by oscillograms or systems of curves. An evalu­ation of the characteristic constants is more straightfor­ward and faster from such a diagram; it is possible to achieve a higher signal-to-noise ratio by repeating the whole measurement several times and recording the re­sultant diagram on a charge storage tube or exposing it continually on one film.

A similar "television" method has been employed suc­cessfully for, e.g., displaying the magnetic domain struc­ture on the surface of a ferromagnetic sample, l or the distribution of the spectrum modes radiated by a plasma relative to the length of the discharge column.2 The paper describes a new display method based on the above prin­ciple, intended for investigations of the phenomenon of the wave of stratification in a discharge plasma. The

1 J. Kaczer, Czech. J. Phys. 5, 239 (1955). 2 H. Lashinski, Phys. Rev. Letters 13, 47 (1964).

phenomenon proper has been described in detail earlier.t

It is a wave process that can be induced under certain conditions by an impulse disturbance at any point of the positive column. It propagates in the direction from the cathode to the anode with a velocity of the order of 1()2 to 105 m/sec, and is manifested by, e.g., changes in the in­tensity of light radiated by the discharge at the given point.

Wave processes of this sort have hitherto been studied by various optical or electronic methods. The earliest method used a rotating mirror for the time resolution of the discharge. The rotating camera method4 and the method using a camera with rotating film employed by Pfau and Rutscher5 are based on a similar principle. Other stroboscopic methods for direct visual examination of the wave processes6•7 have also been worked out. In modern measurements, however, an ever increasing use is made of the electronic method applied first by PUppB and re­fined by Donahue and Dieke.9 The method consists in measuring the changes of light radiated by the discharge, successively at several points by a phototube or a photo-

31.. Pekarek, Czech. J. Phys. 4, 221 (1954). 4 A. H. v. Gorcum, Physica 2, 535 (1935). & S. Piau and A. Rutscher, 7th Intern. Conf. on Phenomena in

Ionized Gases, Belgrade (1965). 6 A. A. Zaytsev,Yestnik Moskovskogo Universiteta No. 10 (1952). 7 J. StudniCka, Cs. Cas. Fys. 14, 325 (1964). 8 W. Pupp, Z. Physik. 33, 844 (1932). 9 T. M. Donahue and G. H. Dieke, Phys. Rev. 81, 248 (1951).

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Sun, 21 Dec 2014 08:57:06