temperature effects in photomultipliers and astronomical photometry

Temperature Effects in Photomultipliers and

Astronomical Photometry

Andrew T. Young

Temperature coefficients are reported for photomultiplier tube types used in astronomy. For 1% sta-bility of gain and color response, temperature regulation of 10 C or better is generally required. This isnearly an order of magnitude better than what is usually achieved at the telescope, but careful use of awell-designed cold box should make 1C temperature stability possible. For maximum stability andreproducibility, ordinary blue-sensitive tubes should be avoided at wavelengths longer than 5000 A,and trialkali cathodes should not be used beyond 6500 A.

Introduction

Users of photomultipliers are universally familiarwith the fact that cooling is very effective in reducinga tube's thermionic dark current and the accompanyingundesirable shot noise, but some of them are not awarethat the sensitivity of the photomultiplier has a large,wavelength-dependent temperature coefficient. How-ever, one can predict from published data in the physicsliterature' that temperature variations of one to fourdegrees Celsius can cause systematic errors ec ceedingone percent ( 0.01 mag.) in most astronomicalphotometry. While photoelectric observers commonlyclaim an accuracy of 0.01 mag. or better, they rarelymention the existence of temperature effects, and nonumerical values for temperature coefficients have everbeen published in the astronomical literature. Fur-thermore, there is a widespread belief among observersthat the use of dry ice as a refrigerant is so effective instabilizing the behavior of the tube that temperatureeffects can be ignored in a refrigerated tube.

The purpose of this paper is to estimate the degree oftemperature regulation actually achieved at thetelescope, to present new data on tube types used in as-tronomy, and thus to assess the importance of temper-ature effects in astronomical photoelectric photometry.

Temperatures of Refrigerated Tubes

Average Cathode Temperatures

Kron2 has published measurements of the darkcurrents of several photomultipliers with Ag-O-Cs

The author is at Harvard College Observatory, Cambridge,Mass.

Received 10 August 1962.This work was supported by a grant from the National Aero-

nautics and Space Administration.

(S-1) cathodes. Because of the low work function, thedark currents of these cathodes are extremely tem-perature-dependent; therefore the dark current meas-urements can be inverted to estimate temperatures.

Kron's tubes were "encased in well-built, hermeticallysealed containers, filled with dry air, and mountedinside a thermally insulating box." Two of thetubes were cooled with "a stiff slurry of pulverized dryice in isopropyl alcohol" as well as with plain dry ice.When dry ice alone was used as a refrigerant, the darkcurrents fell below the value at room temperature by afactor of about 107. When the alcohol was also used,the dark currents were about six times lower. Thelogarithm of the dark current is roughly a linearfunction of the temperature, although the slope isgreater at the higher temperatures. The additionalcooling produced by adding the alcohol to the icemust therefore be at least

(log 6) X (room temperature - dry ice temperature),(log 10')

or 11C. Because of the curvature of the log I vs Trelation, 15 or 20 degrees would be a better estimate.

The writer has repeated this experiment with twoFarnsworth FW-118 tubes (which also have S-icathodes) mounted in an astronomical photometer,very similar to those used by H. L. Johnson.3 Thecold box of this photometer will be considered typicalof cold boxes in general astronomical use, and will bereferred to as a "Johnson-type cold box" in the re-mainder of this article, although similar cold boxeshave been used by others. Because alcohols becomevery syrupy at low temperatures, ethyl acetate wasused as the heat-transfer liquid. This ester remainsquite fluid at -78 0C. In these tubes, which seem

January 1963 / Vol. 2, No. 1 / APPLIED OPTICS 51

to be slightly less temperature-sensitive, the darkcurrent fell by about 05 on cooling with dry ice alone,and by an additional factor of 5 when the ethyl acetatewas added; again a decrease of about 15'C is indicated.

These data show clearly that the use of dry ice alonein the usual cold box fails to cool the cathode todry-ice temperature. They also raise the questionof how much above - 780 C is the temperature of thecathode, even when a heat-transfer liquid is used.This question can be roughly answered by measuring thechange in the rate of cooling (based on the slope of thedark current vs time relation) in the two cases. Theincrease in cooling rate when the liquid is used is about50%. Let us assume that the differences between dryice and the final cathode temperatures Td and Tw in thedry and wet cases, respectively, are inversely propor-tional to the rates of cooling. Then

I'd - 7CO,Id - Tco, = 1.5.7w- T'CO2

Since we have already determined that Td - T = 15°,we have

Id - Co 2 = 450C and T - Tco2 = 300C.

This means that the average cold box cools thephotomultiplier cathode only about two-thirds of theway from ambient temperature toward the - 780Cof the dry ice. This conclusion is supported by theexperiments of Hogg,4 who has measured the cathodetemperature directly with a thermocouple.

Why is the photomultiplier cathode some 40'Chotter than the metal walls of the cold box a few milli-meters away? To answer this question, one must lookat the heat budget and thermal conductivities of thetube and its container. The details of the calculationdepend on the particular tube type. For an end-onEMI tube, the thermal conductivity to the box walls islimited primarily by heat conduction across the endwindow from the center to the edge. This window ismade of fused quartz 3 mm thick and 25 mm in radius,which corresponds to a heat flow of about 6 mW/ 0 Cof temperature difference between center and edge.Heat transfer across a 1-mm air space between the sidesof the tube and the walls of the box amounts to about28 mW 'C -/cm of length along the tube, and conductionalong the 1-mm-thick glass side walls is about 13mW0C-1/cm temperature gradient along the length ofthe tube. The resultant heat flow from cathode tobox walls is a few milliwatts per degree Celsius oftemperature difference between cathode and box.Even for the RCA P21 this is the order of magnitudeof the heat conductivity, because the metallic connec-tion between the cathode and the base has a very smallcross section.

The amount of heat conducted in from the baseshould be small if the voltage-divider chain is kept away

from the tube socket. Since the thermal conductivitybetween the dynodes and the cold box is of the order of1 mW0 C-' per stage (the nickel alloy wires betweendynodes and base are about 1 mm in diam and 5 cmlong) and the maximum power dissipated in the dynodesis about 1 mW/stage, the internal tube currents are anegligible source of heating, and the dynodes should benearly at dry-ice temperature.

This leaves only the entrance window of the coldbox as a source of heat. This window is generally ator near room temperature; it is often heated slightlyto prevent dewing. At 300'K a blackbody radiatesabout 45 mW of thermal energy per square centimeter;a typical window has an area of about 3 cm2 . Evi-dently the thermal radiation alone is enough to ac-count for most of the cathode heating, even if con-vection currents between the tube and the window areneglected.

If the above explanation of the cathode heating iscorrect, then the cathode should be brought muchcloser to -78'C by placing a transparent radiationshield in front of the photomultiplier. Experimentsshow that placing even a single layer of Saran Wrapinside the cold chamber ahead of the tube produces asignificant improvement in cathode cooling. However,an ideal heat shield should not only absorb the incidentthermal radiation, but should be thick enough toconduct it to the dry ice instead of reradiating it to thecathode. A 1-cm slab of fused quartz serves thispurpose very well, if it is no larger in diameter thannecessary and is mounted in a metal ring in goodthermal contact with the walls of the cold box. Testsmade with such an arrangement indicate that thecathode temperature will fall to within 5C of therefrigerant temperature after several hours of cooling.

The heat shield need not be a simple window, butcan be an active part of the optical system. In somecases, it may be convenient to use a field lens as theheat shield inside the cold box. However, one shouldremember that a thick lens will conduct more of theheat flux to the box walls than a thin one, and thatfused quartz is a better heat conductor than glass.

Temperature Variations at the Telescope

At first sight, it might appear that the temperatureregulation achieved in the ordinary cold box would beadequate; if the cathode is cooled two-thirds of theway to dry ice temperature, then two-thirds of theambient temperature variation should be eliminated.However, since the cathode is heated by radiationand cooled by conduction, the problem is not so simple.If the cathode is 40'C warmer than the dry ice, a 1Cchange in cathode temperature requires a 2.5% changein heating; but a 1° change in window temperature at300'K causes a 1.3% change in heat radiated to thecathode. Thus the cathode suffers about 53% of the

52 APPLIED OPTICS / Vol. 2, No. 1 / January 1963

change in ambient temperature. This figure is directlyproportional to the temperature difference between thecathode and the refrigerant; in a properly designedcold box with an internal cold window, this differenceshould be 5°C or less and the cathode should show lessthan 7% of the fluctuation in window temperature.

In addition to this variation, however, one must addthe variation in cathode temperature which resultsfrom allowing insufficient time to establish thermalequilibrium in the box between the initial addition ofdry ice and the beginning of observations. Theauthor's experience with Johnson-type cold boxes showsthat this time interval should be at least two to threehours. Also, for a 1P21, Engstrom 5 says that "Whenaccurate temperature of the cathode has to be known,it is necessary to hold the tube at a constant tem-perature for several hours to allow the parts insidethe tube itself to reach equilibrium." However, inone of the most precise and careful photometric in-vestigations ever carried out, Serkowski6 reports thatthe dry ice was put on the P21 only 1V/2 hr beforebeginning observations. From this cause alone onecan expect the cathode temperature to have been atleast 50C warmer at the beginning than at the end of thenight.

Finally, the temperature of the cathode can dependon the orientation of the photometer, both because ofthe presence of convection currents in the air aroundthe tube, and because of the shifting of the dry icein the box. The amount of this effect can be large, atleast 10'C; however, a more typical value at thetelescope might be 50 C because the telescope is gener-ally moved at frequent intervals from one position toanother. This is a particularly serious problem,because it can produce systematic errors in the ob-servations that depend directly on zenith distanceand/or declination. These errors therefore will appearundiminished in the final results, while other tempera-ture effects may be partly eliminated in the correctionfor extinction and reduction to the standard system.Fortunately, the use of a cold inner window shouldpractically eliminate any convection currents, and theuse of ethyl acetate on the dry ice drastically reducesthe effect of the shifting of the dry-ice load in differentpositions.

On the whole, then, one can expect cathode tem-perature variations of 5-10'C and somewhat smallerdynode temperature variations during a few hours whenusing conventional equipment and observing tech-niques. It should be possible to reduce these variations

Table I. Listing of Tubes Subjected to Measurement

Tube type Cathode Dynodes

EMI 6256 A Sb-Cs (S-13) 13 stage Venetian blind Sb-CsRCA 1P21 Sb-Cs (S-4) 9 stage Focused Sb-CsEMI 9558 C "Trialkali" 11 stage Venetian blind Sb-Cs

(S-20)Farnsworth Cs-Ag-O (S-1) 16 stage Box-and-grid Mg-Ag

FW-118

to about one degree by observing the following pre-cautions:

1. Use a cold inner window in the cold box toblock the heat flow from the heated outer window.

2. Always use ethyl acetate or some similar fluid(e.g., acetone or methylene dichloride) on the dry iceto insure good thermal contact with the box.

3. Fill the box with dry ice at least 3 hr beforebeginning observations, and keep it full during thenight. It would be best to keep the box loaded withdry ice 24 hr a day.

If these precautions are not observed, the tempera-ture variations of a refrigerated cell may easily exceedthose of an unrefrigerated cell. In other words, theuse of dry ice as it is usually employed probably doesnot significantly reduce the temperature fluctuations ofthe photomultiplier cathode.

Magnitudes of the Temperature EffectsThe first data on temperature effects in the commonly

used Sb-Cs cathode surface were published over adecade ago.7 In 1957, Kinard 8 discovered the negativetemperature coefficient of Sb-Cs dynode surfaces.In 1959, Lontie-Bailliez and Meessen' added their ownexperimental results to all the previously publisheddata in a comprehensive review of the subject. Somedata have also been published on the important new"trialkali" cathodes.' 0 However, some of the pub-lished data are rather conflicting, and none of the tubetypes used by astronomers has ever been measured.Therefore, the temperature effects for a number oftubes of astronomical interest were measured in thisstudy.

Experimental Procedure

Table I gives the tube types which were tested;two of each type were used. The EMI 6256 is identicalto the more widely used type 9502, except that it hasa quartz window.

Table II. Characteristics of Filters Used in Experimentation

Central wavelength () 3200 3570 3933 4170 4550 5000 6250 7300 8600 >lgBandwidth at half-peak

transmission 200 70 60 200 100 250 300 350 170 -

Blocked to at least X 8500 8500 1 . 2 l.2 pu 1. 2 1A 1.2 1.2 1. 2 1. 2-


1.01

0 +0.4

Z+02C.)a:

-0.2

- - --3000 4000 5000 6CA, A

Fig. 1. Temperature coefficient of Sb-Csfunction of wavelength (after Lontie-BailliezSolid curve: T = +20'C. Dashed urve: T =

cathodes asand Meessen- 80'C.

The optical part of the experimental apparatusconsisted essentially of a voltage-regulated 100-Wlight bulb, the set of interference filters listed in TableII, and a Johnson-type cold box with a heated quartzouter window, containing the end-on photomultiplierunder test. The voltage-regulating transformer usedreduced the short-term fluctuations in the light outputof the bulb well below 1%, even at X 3200. Thephotomultiplier voltage divider consisted of precisiondeposited-carbon resistors, all of the same type,mounted outside the cold chamber.

For the P21's, the window was a layer of SaranWrap, and a voltage divider of ordinary carbon re-sistors was contained in the cold chamber. The trans-mission of the Saran Wrap was measured and provedto vary by only 2%0 over the wavelength range from3200 to 7300 A.

The power supply for all the photomultipliers was aNortheast Electronics type RE-2003, which has a long-term stability of about 1 part in 104 and a short-termstability, under the actual conditions of operation, ofabout 1 part in 105. The anode currents were amplifiedby a General Radio type 1230-A electrometer and re-corded on a Bristol strip chart recorder. The measureddeviations from linearity in the amplifier-recordercombination are less than 0.1%. The inherent ac-curacy of measurements made with this system is afraction of a percent, and is chiefly due to errors inreading the chart paper. Errors caused by long-termchanges in the light bulb may amount to a few percent,but are surely less than 1% for periods of the orderof a day or two. Thus, data for a single tube should beaccurate to about 1%, and comparisons between differ-ent tubes should not be worse than a few percent.

All the tubes were measured with the box at roomtemperature and at dry-ice temperature (-78C) .

In addition, the trialkali tubes and one of the infrared-, sensitive tubes were measured with the cold box at

some intermediate temperatures and at liquid-nitrogentemperature (about - 190C). Ethyl acetate wasgenerally used on the dry ice to promote cooling, butmost tests were made before the importance of using aninner cold window was discovered. Therefore thecathode temperatures were warmer than the boxtemperatures.

Sb-Cs Cathodes

In their survey article, Lontie-Bailliez and Meessen'put together a large number of observations on different

IO Sb-Cs cathodes. Their results can be summarizedas follows: (1) In the blue, the temperature coefficient

a of the Sb-Cs cathode is about -0.25%/C at room1'). temperature and is roughly independent of the wave-

length. (2) For X > 5000 A, the temperature coeffi-cient rapidly becomes more positive with increasingwavelength, at a rate of about 6 X 10-6 0 C-1 A-i.

rrax~~~~~ I * I I r _ A 1XLt

'Ihat is, or every IUU A we move to the red, the tem-

2.0

Scold

S hot

I .0 h

0.8 _

0.5

0.3

0.2

0. I

I I I

3000 4000 5000 6000 7000 A

Fig. 2. Ratio of cooled to room-temperature anode sensitivityas a function of wavelength for EMI 6256 (crosses) and RCA1P21 (dots) photomultipliers. The solid curve is the predictedratio for a cathode temperature of -40'C and a dynode tem-perature of -70'C. The 1P21 points are anomalously loweredby condensation on the window. Cold box temperature =

-78 0C (dry ice).


+ +.

xx x x x

x

+

x

_0

-o I

+

x

STS2 7

6000 7000 8000 9000 A

(a)

2.

1.0

.5

.2

.05

3000 4000 5000 6000 7000 8000 9000 A

Xb-

(b)

Fig. 3. Ratio of cooled to room-temperature anodea

sensitivity fortures. Cf. Fig

two EMI 9558 photomultipliers and various cold box temper-.2.

perature coefficient increases by 0.06%/ 0C. (3) Asthe temperature is lowered, the plot of temperaturecoefficient against wavelength retains practically thesame shape, but becomes more positive at every point(Fig. 1). This means that for any wavelength shorterthan 5500 A, there is some temperature below roomtemperature at which the Sb-Cs cathode has maximumquantum efficiency. At X 4500 this is very nearly dry-ice temperature; however, the fractional increase inquantum efficiency from room temperature to dry ice isonly about 15%. The efficiency falls off much moresteeply on the cold side of the maximum than on thehot side. Thus, at X 4500, about 40% of the maximumsensitivity is lost at liquid nitrogen temperature.(4) The temperature coefficient of the dynodes is- 0.04o/°C -1 per stage on the average, but the valuesfor individual tubes vary from zero to -0. 1%/ 0C - perstage. For a given tube, the dynode temperature co-efficient is practically independent of both temperatureand operating voltages.

With the assumption that the temperature coeffi-cients for Sb-Cs cathodes and dynodes given by Lontie-Bailliez and Meessen are applicable to the EMI 6256,

the expected ratio of refrigerated to room-temperatureresponse has been calculated as a function of wave-length. In accordance with the discussion of cold-boxproperties given above, the cathode was assumed to becooled 60'C, and the dynodes, 90'C. The resultingfunction is plotted as the solid line in Fig. 2, and theexperimental data for the two 6256's are represented bycrosses. The agreement is quite satisfactory, especiallyconsidering that the dynode temperature coefficient(which affects the height of all points regardless ofwavelength) is known to be widely variable from tubeto tube. Data are also given in the figure for twoRCA 1P21's; the curves are lowered because con-densation on the unheated Saran Wrap window of the1P21 cold box blocked part of the light to the re-frigerated tubes. The shape of the 1P21 curves showsthat the thermal properties of P21 cathodes aresimilar to those of other Sb-Cs cathodes.

Trialkali CathodesFigure 3 shows the effects of cooling two EMI 9558

C photomultipliers to various temperatures. As inFig. 2, the ratio of anode response at a given box


ST

S29 2.

1.0

-"I

T --

* Ts -34*

A TV-78g

* T-190-

EMI 55 c

* 5647

.5

.2

.1

.05

3000

+ T - 18A T - 7-

o T= -190-

EMI 9558 C

# 5099

4000 5000

I

.1

2.0

I .5

1.0

40 )0 5000 6000 7000 8000 9000 10,000 AX -

TUBE T, T2

* 9611

O 9611

8611

-190' + 30'

- 7' + 30'

-78' +25'

Fig. 4. Ratio of cooled to room-temperature anode sensitivityfor two Farnsworth FW-118 photomultipliers cooled with dry iceand with liquid nitrogen.

sidered here. The effect of the window frost musttherefore be primarily a neutral geometrical shadowing.While the figures probably show correctly the effectsof nitrogen cooling on the relative spectral response,the measurements need to be repeated with an ex-perimental setup especially designed for work attemperatures below - 100°C, with particular attentiongiven to the problem of measuring the true cathodetemperature.

One would expect the extreme infrared response tofall off with cooling, but the data taken do not extendto long enough wavelengths to show this. The changesin spectral response and in anode sensitivity are of theorder of 30% for a 100 0C temperature change. Thesetubes have Ag-Mg dynodes, which have been found'to have rather more variable temperature coefficientsthan Sb-Cs dynode surfaces.

temperature to that at room temperature is plotted.Tests of the two tubes operated as diode photocellsshow that the cathode sensitivity is unaffected bydry-ice cooling in the range 5000-6000 A; in the bluethe quantum efficiency is raised by cooling, and in thered it is sharply reduced. The dynode temperaturecoefficients for these tubes are about twice as large asthe average reported by Lontie-Bailliez and Meessen.

The anode dark currents of these photomultipliersincreased by about a factor of two when liquid nitrogenwas used instead of dry ice. At such low temperaturesthe large negative temperature coefficient of the multi-plier section overwhelms the diminishing positivetemperature coefficient of the cathode dark current.The curves taken with liquid nitrogen as refrigerantprobably have been affected by window icing and aretoo low; nevertheless, there is a genuine drop in cathoderesponse at temperatures below about - 100'C, justas in the case of the Sb-Cs cathode.

Ag-O-Cs Cathodes

The infrared-sensitive FW-118's were tested in thesame way as the Sb-alkali-metal tubes. Figure 4shows the results. Since the dark current noisemasked the weak signals for XX 4550 and 5000 attemperatures above 0C, an extrapolation was made toestimate the room-temperature response at thesewavelengths; the error made is probably less than 5%.The one FW-1 18 tested at a box temperature of - 190'Cwas strongly influenced by window icing, and the datataken at this temperature have arbitrarily been multi-plied by 50 at all wavelengths to make them comparableto the data taken at - 780C. Some error may havebeen introduced if the iced window did not transmitall wavelengths equally; however, even a layer ofwater 0.2 mm thick would cause less than 1% of actualabsorption anywhere in the wavelength range con-

Very-Low-Temperature Effects

The drop in sensitivity below -100 0C which isshown by semitransparent antimony-alkali cathodeshas been blamed on their low electrical conductivity atsuch temperatures." As a corollary, some workershave claimed that the 1P21 cathode, being plated on ametallic base, does not lose sensitivity when cooledwith liquid nitrogen.

Suppose that the resistance of the cathode is sufficientto cause an appreciable drop in cathode-to-first-dynodepotential; the effect should be roughly equivalent toconnecting a large resistor in series with the cathode.If the voltage drop in this resistor is large enough toaffect the photoelectron collection efficiency by afactor of two, it must amount to some tens of volts.Then if we double the light input to the tube, the in-creased cathode current must cause a further largedrop in K-D1 potential and collection efficiency, parti-ally offsetting the increase in photoelectric emission.Additional increase in cathode current will soon use upall the available K-D1 supply voltage in the seriesresistance, and a saturation current must exist. Inthis regime, the cooled cathode must be very nonlinearin its apparent response to light. On the other hand, ifwe decrease the illumination by a factor of ten, thepotential drop in the series resistance falls to a fewvolts, and no appreciable loss in collection efficiency orapparent cathode sensitivity is produced. Below thislevel the tube's response to light will be linear. Nosuch nonlinearity has ever been observed, however;the data complied by Lontie-Bailliez and Meessen'from a wide variety of sources agree reasonably well atthe lowest temperatures, and show that at 5000 Athe Sb-Cs cathode drops to about 30% of its peaksensitivity when cooled with liquid nitrogen. Likewise,the sensitivity loss of the EMI 9558 tubes at the lowesttemperatures reached does not show any dependence on


- *. - ----------F -

FW-118

O

cathode current. Finally, published data on the re-sistivity of cooled cathodes" show that cathode resistivevoltage drops in photomultipliers-where cathodecurrents are generally below 10-12 A-can be neglected.Therefore the effect is inherent in the cathode materialand should also be present in the case of the P21.

There is, in fact, direct evidence that the P21cathode also falls to about one-third of its normalsensitivity at X 5000 when cooled by liquid nitrogen.Engstrom's data5 show a loss in anode response of about2 dB in going from room temperature to liquid nitrogen;if his P21 had a normal dynode temperature coefficient,this indicates a reduction to 39%,, of the room tempera-ture cathode sensitivity.

The fact that P21's and other Sb-Cs cathodes havebeen operated with liquid nitrogen cooling," ' withoutshowing any loss in cathode sensitivity, simply demon-strates the ineffectiveness of the usual astronomical coldbox; if the cathode is not cooled below - 130'C, itshould show very nearly its room temperature sensitiv-ity to blue light. However, it is better to use a properlydesigned cold box with dry-ice cooling, not only becausethe temperature stability is much improved and be-cause the maximum quantum efficiency is reached,but also because the dark current is so insensitive totemperature near - 1000C that the difference intemperature will not increase the dark current by morethan about 50%.

Individual Differences

It is well known that different tubes of the same typeshow different spectral sensitivities, but there is verylittle in the published literature to indicate how muchvariation the user can expect. To give some idea ofthe tube-to-tube variation, we have plotted the ratioof the responses of the two tubes of each type, expressedin stellar magnitudes, in Fig. 5. Because some ob-servers have been reluctant to use the end-on EMItubes for U,B,V photometry, the ratio of the meanEMI 6256 response to the mean P21 response is alsoplotted. It appears that systematic differences be-tween EMI and RCA tubes are no larger than theindividual differences between P21's. In this respectit is worth recalling that Mikesell'5 tested 38 1P21'sand found that the ratio of blue response to response atX 6100 varied by a factor of 16. Considering the lowerleakage, larger gain, and higher quantum efficiencieswhich are available in tubes such as the 6256 it seemspreferable to use end-on tubes in place of the P21.

Two comments can be made about the curves shownin Fig. 5. First, the ratio of the -1 cathodes shows anextremum at about 8000 A. Since the -1 surfaceitself has a bump in its spectral sensitivity at thiswavelength, it appears that one of these tubes has amuch larger bump than the other.

Second, the antimony-alkali-metal cathodes all show

large variations in the red, but are generally well-behaved in the blue. The 9558 (S-20) tubes show aphenomenon which is also present in all four Sb-Cstubes: the cathodes with the largest relative far-redsensitivity also show the smallest temperaturecoefficients at long wavelengths. This correlation isunderstandable in terms of Spicer's description ofsuch cathodes: the far-red sensitivity is due to looselyfilled acceptors (impurities) in these p-type semi-conductors, and the distribution of impurity statesincreases "in something like an exponential mannerabove the top of the valence band. If such were thecase, the slope of the yield curve [logarithm of photon re-sponse plotted against photon energy] should increaseby about a factor of 4 on cooling from 3000 to 770 K.Such an increase was found.. . ." In other words, tubeswith sharp red cutoffs become even sharper when cooled,while those with long red tails of low slope-and hencehigher relative far-red sensitivity--lose a smaller frac-tion of their long-wavelength response on cooling.

Finally, it is evident that Sb-Cs cathodes havesimilar spectral response and small temperaturecoefficients for X 5000 A, but become unreliable atlonger wavelengths; similarly, S-20 cathodes are well-behaved out to only 6000 or 6500. Photometricsystems intended to have high accuracy and reproduci-bility should not push to the red of these limits. Thus,for the highest precision, a wavelength band at 7500 Ashould be measured with an S-1 cathode, even thoughthe S-20 cathode has ( on the average) five times as much

Am

-1.5

-1.0

.5

.5

+ 1.0

+1 .5 I

+ 2.0

3000 4000 5000 6000 7000 8000 9000 10000

As-Fig. 5. Ratios of responses of two tubes of the same type, as a

function of wavelength. For the EMI 9558's, the ratio is in thesense (tube No. 5647)/(tube No. 5099); cf. Fig. 3. The curvemarked EMI/RCA is the ratio of the mean of the 6256's to themean of the 1P21's.


a L

65f 5 I* S6°.

\\

sensitivity at this wavelength. Stability and repro-ducibility must be considered when selecting a photo-cathode; sensitivity alone is an insufficient criterion.

Astronomical Effects

General Considerations

Since photoelectric observers commonly claim anaccuracy of 0.01 mag. or better, temperature effectsshould be kept below 1% in all photoelectric investiga-tions. In fact, it would be desirable to keep photocellchanges below 0.1%, since observers sometimes reportobservations to this accuracy. In the following dis-cussion, % stability is adopted as a minimum standard.For comparison, typical cathode temperature fluctua-tions of 7C and dynode variations of 3C are hereassumed for a conventional cold box.

Gain Stability

The trialkali tubes tested had over-all temperaturecoefficients larger than - 1%/ 0 C in the blue, and Sb-Cstubes typically have temperature coefficients of about-0.7%/C. Evidently a minimum requirement for1% stability of response is a temperature regulation of

the order of 1 0 C-a value which is certainly not realizedin most photometers. owever, most of the gainvariations arise in the dynode section of the tube.Since the dynodes probably suffer somewhat smallertemperature variations than the cathode, one canexpect over-all sensitivity fluctuations of about 3% inordinary circumstances. If a standard light sourceis not used, these 0.03 mag. errors can enter directlyinto the determination of extinction and will appear asboth systematic and random errors in the results.Several photoelectric observers, both in this countryand abroad, have abandoned the use of standard lightsources because they show both temperature andhysteresis effects. However, according to data fur-nished by the U.S. Radium Corporation, the tem-perature coefficients of most radioactively excitedphosphor light sources are about five times smallerthan those of photomultipliers. The use of suchsources to check gain variations in the photomultiplieris therefore recommended. For the highest accuracy,the source should be carefully thermostated.

The effects of dynode gain variations can be partiallyeliminated by the use of pulse-counting techniques.However, the quantum efficiency of the cathode is stilltemperature dependent, and sensitivity variations ofnearly 2% may still occur with the conventional coldbox. Careful operation with a properly designedbox can reduce this figure by a factor of 5 or 10.

Pulse-counting should also reduce the influence of the"fatigue" effects which appear when anode currentslarger than one microampere are drawn from the tube.These effects occur in the last few dynode stages, and

R

4

3

2

5000 6000 7000 A

Fig. 6. Response curves for V band of U,B,V photometry,using a cooled (dashed curve) or uncooled (solid line) photomulti-plier. The refrigerated cathode is assumed to be cooled 60'Cbelow room temperature (cf. Fig. 2).

are independent of the light intensity at the cathode-i.e., they depend only on the electron currents in thelast few stages. Engstrom' has found a 20% decreasein sensitivity after drawing 5MA from a 931-A for 40min, and both positive and negative sensitivity changeshave been reported for 1P21's. All of the end-windowtubes in the present study showed a gain in sensitivityof the order of 5 or 10% in ten minutes for 5,uA anodecurrent; the change was rapid at first and then leveledoff. When the anode current was reduced, the tubesvery slowly returned to their original sensitivity.These high anode currents correspond to very highcounting rates, of the order of 107 pulses/see, butequipment is beginning to become available for theseranges.

Color Stability

In all photometric investigations, it is assumed thatthe color zero point of the photometer remains fixedduring a night's observations; indeed, it is oftenassumed that the instrumental color system is constantfor days or weeks at a time. Thus, Weaver' 6 saysthat "the zero-atmosphere colors and magnitudes ofthe extinction stars are constants and should have thesame numerical values each night," and Hardie17

bases his method of determining extinction on theassumption that "the zero-point term [in the colors]was constant over many nights." However, tempera-ture variations at the cathode can seriously changethe color systems.

For the U, B, V system the temperature effects appearmost strongly in the visual band. Figure 6 shows theV response curves for Johnson's P21 measured atroom temperature and calculated for the same tubecooled with dry ice in a conventional cold box, on the


assumption that the tube behaves according to thecurves of Fig. 2; the response curves have been normal-ized for X < 5000 A. The effect of cooling is to reducethe relative V response by 0.18 mag. and to reduce theeffective wavelength by about 50 A., which reduces theB-V color baseline by about 4%. Since the standardstars of the U, B, V system were measured with thephotocell refrigerated, " we should expect that the B-Vcolors of stars calculated from the published responsecurves and theoretical stellar energy distributionswould be 0.18 mag. redder than the observed colors.The actual discrepancy20 is slightly less, and amounts to0.13 mag. on the average. The difference, if significant,may mean that Johnson's P21 was unusually red-sensitive, or that his cold box was unusually inefficient.

In any case, we can say that the temperature coeffi-cient of the B-V zero point is 0.003 mag./ 0 C, and thatthe ordinary cold box will allow this zero point towander by at least 0.02 mag. A cathode temperatureregulation of 3VC or better is required to hold the U, B,V color system fixed to an accuracy of 0.01 mag.,even if a standard source is used to measure the dynodegain variations. Even in the blue and ultraviolet, oneshould never reduce together observations made with acooled and an uncooled photomultiplier-as at leastone spectrophotometric observer has done!

I.0

0.1

.01

.001

Fig. 7fiectionsrefrigerat

The temperature-color effect also enters into U, B, Vmeasurements in another way. Johnson' 9 says thatthe ultraviolet deflection of DS Peg required a correc-tion of 0.25 mag. because of the red leak in the ultra-violet filter. Since the effective wavelength of theleak is about 7000 A, it must be very temperature-dependent. If we make the rather poor assumptionthat these red stars radiate as blackbodies, we obtainthe correction curves of Fig. 7 for cooled and uncooledphotomultipliers. The curves are not to be takenliterally, but serve to illustrate the magnitude of thered leak and its temperature dependence. Cooling thephotocathode has the desirable effect of reducing theleak, but one can expect very large variations-perhapsa factor of 30-from tube to tube.

A serious temperature effect is also present in therecent photometry of Borgman, 2 1 who used a dry-icecooled P21 at 5200 A and 5800 A. His X 5200-X5800 colors show a scatter of about 0.03 mag. Thecathode temperature coefficients at these two wave-lengths differ by nearly 0.4%/'C, so that a temperaturevariation of about 80 C should suffice to account for thescatter. This value is about what one can expect forthe usual cold box. Borgman's difficulties serve toemphasize the unreliability of Sb-Cs cathodes to thered of 5000 A.

Finally, the data for S-1 tubes (Fig. 4) show that atemperature regulation of about 3C is required tostabilize the response of these tubes to 1%. Thisfigure is very similar to the temperature regulationrequired for U, B, V photometry.

Conclusion

/ If 1% stability is taken to be a minimum require-/ / ment for an astronomical photoelectric photometer,

/ / ~ then most existing photometers fail, by a factor of about/ / 3, to achieve an adequate degree of temperature regula-

/ / tion. Careful use of dry ice as a refrigerant in a properly/ / designed cold box can reduce temperature effects well

/ / below 1%, but it is doubtful that a stability of 0.1%/ / can be reached without the use of a closed-loop thermo-

stat system, in which the cathode temperature ismeasured and used to control the refrigeration.

With existing photometers, the use of a standardsource to monitor instrumental gain and color changes is

/ / almost mandatory if 0.01 mag. accuracy is to be reached.Pulse-counting techniques can help reduce the effectof temperature-induced gain variations, but thetemperature-color effect remains as a fundamentalproperty of the photocathode.

+ .5 1.0 1.5 2.0 2.5 3.0 Even the most carefully regulated photomultiplier,B-V however, can display significant sensitivity variations

Calculated red-leak corrections to ultraviolet de- as a result of "fatigue," aging, and short-term fluctua-for Johnson's U,B,V photometer. Solid line: 1P21 tions of unknown origin. The need for a really reliableted withdryice. Dashedline: lP21notrefrigerated. standard light source is likely to become even more


pressing as we try to go from hundredth of a magnitudeaccuracy to 0.001 mag., and temperature control of thelight source will be as important as temperature controlof the photomultiplier.

The author wishes to thank M. S. Roberts andH. C. Ingrao of Harvard College Observatory forproviding the 1P21's and a cold box to fit them. Healso wishes to thank Mr. Ingrao for providing referencesto the physics literature.

References1. M. Lontie-Bailliez and A. Meessen, Ann. soc. sci. Bruxelles

73, 390 (1959).2. G. E. Kron, Publ. Astron. Soc. Pacific 70, 285 (1958).3. H. L. Johnson, Astronomical Techniques, ed. W. A. Hiltner

(Univ. of Chicago Press, 1962), Chap. 7.4. A. R. Hogg, private communication.5. R. W. Engstrom, J. Opt. Soc. Am. 37, 420 (1947).6. K. Serkowski, Lowell Obs. Bull. No. 116.7. N. Schaetti and W. Baumgartner, Helv. Phys. Acta 24, 614

(1951).

8. F. E. Kinard, Nucleonics 15, 92 (1957).9. W. E. Spicer, Phys. Rev. 112, 114 (1958).

10. G. Frischmuth-Hoffmann, P. Grlich, and H. Hora, Z.Naturforsch. 15a, 1014 (1960).

11. W. E. Mott and R. B. Sutton, Handbuch der Physik, S.Flilgge, ed. (Springer-Verlag, Berlin, 1958), Vol. 45, p. 93.

12. W. J. Harper and W. J. Choyke, J. Appl. Phys. 27, 1358(1956).

13. H. L. Johnson, Sky and Telescope 17, 558 (1958).14. W. J. Harper and W. J. Choyke, Rev. Sci. Instr. 27, 966

(1956). (The Sb-Cs curve labeled -175 0 C in this paperactually corresponds to a temperature of only about-110 0 C.)

15. A. H. Mikesell, Astron. J. 54, 191 (1949).16. H. Weaver, Astrophys. J. 116, 638 (1952).17. R. Hardie, Astrophys. J. 130, 663 (1959).18. H. L. Johnson and W. Morgan, Astrophys. J. 114, 522

(1951).19. H. L. Johnson and W. Morgan, Astrophys. J. 117, 313

(1952).20. A. D. Code, Stellar Atmospheres, J. L. Greenstein, ed. (Univ.

of Chicago Press, 1960), Chap. 2.21. J. Borgman, Bull. Astron. Inst. Netherlands 15, 255 (1960);

ibid. 16, 99 (1961).

Future IssuesEach issue will specially feature one area of applied optics in ad-dition to the usual contributed papers, shop and technical notes,reports of meetings, Letters to the Editor, book reviews, Meet-ings Calendar, and special columns. The feature subject ofeach issue is the responsibility of the named editor.

1963

Astronomy II

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CoblentzCommemoration

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Photographic Optics

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Visibility

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Optics in Japan

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James G. Baker

W. L. Hyde

F. Dow Smith l

R. A. Woodson

Mary E. Wargaannounced

D. P. Feder

S. Rosin

E. K. Plyler

M. Kent Wilson

George C. Higgins

D. Q. WarkS. Q. Duntley

C. W. Peters

R. A. OetjenG. Newkirk Jr.

L. M. Branscomb


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1965

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temperature effects in photomultipliers and astronomical photometry

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