IRE TRANSACTIONS ON NUCLEAR SCIENCE

Design of a Gamma- Ray Spectrometer Using the PhoswitchTechnique for Rejection of Charged Particles*

FRANK C. JONESt

Summary-The "phoswitch" technique may be used to con-struct a gamma-ray spectrometer that will not respond tocharged particles. The theory and design of such a deviceis discussed and some preliminary data on its performanceare presented.

During the past two years the cosmic ray laboratoryof the Enrico Fermi Institute for Nuclear Studies hasundertaken an experiment designed to detect andmeasure the energy distribution of gamma rays in theatmosphere at balloon altitudes (approximately 117,000feet). One of the first problems encountered was thelarge background of charged cosmic ray particles ofboth primary and secondary origin. To keep thischarged radiation from being included in the measure-ment, some form of guard counter arrangement isneeded. A separate guard counter that would giveprotection in all directions would take up a con-siderable amount of space and would require severaladditional photomultiplier tubes to obtain sufficientlight collection. Space and weight would be saved ifthis guard counter could be incorporated as a part ofthe basic gamma-ray detector. By using the "pho-switch" technique suggested by Wilkinson' it ispossible to construct a compact detector using onlyone photomultiplier that combines the functions of agamma-ray spectrometer and a charged particle guardcounter.

The basic technique consists of a photomultipliersimultaneously viewing a fast and a slow phosphor.When an ionizing event takes place in one or both ofthe phosphors, the shape of the pulse out of the photo-multiplier will depend on what fraction of the ionizingenergy was deposited in each phosphor. Observationof the output pulse shape can then give added in-formation about the ionizing event. If, for example,a slow phosphor is completely surrounded by a fastphosphor and only those pulses are accepted which donot contain the characteristic pulse of the fast phos-phor, one has a detector that will respond only toneutral radiation and not to charged particles.

*This work was supported in part by the U. S. Air ForceOffice of Scientific Research, Contract No. AF 18(600)}666.

tEnrico Fermi Inst. for Nuclear Studies and Dept. ofPhysics, Univ. of Chicago, Chicago, Ill.

ID. H. Wilkinson, "The Phoswitch-a multiple phosphor,"Rev. Sci. Instr., vol. 23, pp. 414-417; August, 1952.

To simplify the determination of the pulse shape,it is desirable to use two phosphors whose decaytimes differ as widely as possible. Plastic scin-tillator and CsI (Tl) meet this requirement very welland were the materials used in the phoswitch gammaspectrometer. The fact that CsI has a rather high Z,and therefore has a high photoelectric cross section,makes it efficient for use in a gamma detector.

Fig. 1 shows the physical construction of the pho-switch. The CsI (TI) is a single cylindrical crystalsupplied by Harshaw Chemicals and the plastic jacketis Pilot Scintillator B. The whole assembly iswrapped in aluminum foil and operated at the potentialof the photomultiplier cathode. The RCA 6810-Aphotomultiplier was chosen chiefly for its high gainwhich minimized the amplifier requirements.

Fig. 2 shows the fourteenth dynode and anode circuits. The slow pulse is used for the integrated pulseheight measurement and the fast pulse is analyzed forthe shape determination.

There are several methods by which the pulseshape may be electronically determined. A simplemethod, and the one used in this system, is an anti-coincidence gate circuit that is "closed" by any fastoutput pulse that exceeds a specified level. For agiven amount of energy E deposited in a phosphor byan ionizing event we will assume that a given fractione is emitted in the form of a light pulse that decaysexponentially with a time constant I where e andT are characteristics of the phosphor.

The initial value of the light pulse will be equalto EV/T. If the gate circuit is set to close when an

CsI(TI) Optically JoinedTo Plastic With cSilicone Oil

Lucite Light Pipe

Silicone Greos

Fig. I-Construction of a phoswitch gamma spectrometer.

175

IRE TRANSACTIONS ON NUCLEAR SCIENCE

6810-A PHOTOMULTIPLIER

1DY13

IDY14 220K

p

loon1

FAST SLOUTPUT OUT

lay line and the anticoincidence gate is set to re-spond to the polarity of the reflected signal, the ratioof the critical energies becomes

Es'IE/ =-cfT f (T),f Cs= U

= .OI1jd

'PWTPUT

Fig. 2-Fast and slow output circuits of photomultiplier.

energy Es (pulse height = Ef c/Tf) is deposited inthe fast phosphor it will require an energy E5' de-posited in the slow phosphor to close the gate where

Es fefTE EA

esi

For plastic scintillator and CsI (TI) this gives Es' =120 E,' using the following values2,3

ef=0.02 T1= 3 x 10 9 sec

cs = 0.06 TS= 1.1 x 10 6 sec.

In the gamma spectrometer the gate was set torespond to a fast phosphor pulse with an integratedheight = E,'e, which was equal to the integrated heightof a slow phosphor pulse with E. = 0.125 mev.

Therefore(0.125) x (0.06)

(002'= =0.375 mevf (0.02)

and

E P- (0.125) x (1.1) x 103E'= ~~~~=45.8 mev.s ~~~~3

Under certain circumstances the ratio ES'Ef' canbe greatly increased by making use of a shorted delayline. Assuming, once again, ideal exponentiallyshaped pulses it may be shown that the height of theinverted pulse reflected back from the shorted end ofthe line is

[ x(e)]x'where T is the delay time of the line. If the fast out-put of the photomultiplier is fed to such a shorted de-

2J. B. Birks, "Scintillation Counters," Pergamon Press,London, Eng., pp. 57, 79, and 88; 1954.

3From technical literature of Pilot Chemicals, Inc., 36Pleasant St., Watertown 72, Mass.

(a)

(b)

(c)Fig. 3-Pulse shapes obtained from fast output of pho-

switch. (a) CsI (TI) pulse, (b) plastic scintillator pulse,(c) combination of plastic scintillator and weak Csl(TI) pulse. (Scale-horizontal: 0.2 Isec per division;vertical: 0.05 volt per division; taken on Tektronix type533 oscilloscope).

176

GAMMA-RAY SPECTROMETER USING THE PHOSWITCH TECHNIQUE

PUSH-PULLOUTPUtTO CRT

BRKiHTENING*PULSETO CRT

Fig. 4-Block diagram of pulse shape determination and re-cording circuit.

where

f(T) has a maximum [/(T) = 5/T1] for T = 0, butthis is a finite ratio of two vanishing quantities sincethe pulses are of zero height. A practical value for Tis 'T1, since for this value the pulse from the fastphosphor has nearly its maximum value and f(T) isstill (0.43) TS/TI. For CsI (TI) and plastic scin-tillator, this gives

Es (1-85 x 104) Ef'.,

If the pulse from the slow phosphor contains anyhigh-frequency components with frequencies of order

i/,,, the delay line technique cannot distinguishthem from fast phosphor pulses and thus loses muchor all of its advantage.

As can be seen in Fig. 3, this condition exists inthe gamma spectrometer and since the fluctuationswere of the same order of magnitude as the pulse it-

l4

z

~~ --- ~~~Blank Off B

40 60 B0 100 120 140CHANNEL NUMBER

Fig. 5-Pulse height spectrum of Cs137 gamma ray-blank

circuit turned on and off.

self, the delay line technique offered no essential im-provement over the system used. Because thesefluctuations are statistical in nature and are propor-tional to the square root of the pulse height, improv-ing the light collection at the photocathode shouldreduce the relative importance of these high-frequencycomponents to the point where using the delay linetechnique would offer some advantage.

Fig. 4 is a block diagram of the recording systemused in the high-altitude gamma-ray experiment. Thecathode-ray tube face is continuously photographedto give a record of the heights of those pulses thatdo not contain a fast pulse from the plastic scin-tillator. The delay line shown in the diagram is usedonly for pulse shaping and is not used for the dis-crimination technique just described.

Fig. 5 shows the spectrum of pulse heights ob-tained when the phoswitch gamma spectrometer is ex-

zzz

I

0

c 03

,lwa.

40 60 80 100 12U 140CHANNEL NUMBER

Fig. 6-Pulse height spectrum of Cs137 gamma ray and p32beta ray-blank circuit turned on and off.

posed to the 0.663 mev gamma ray of CsI37 with theblanking circuit turned on and off. The difference be-tween the two curves is due to the photons that inter-act in the plastic jacket of the phoswitch. Fig. 6shows the more dramatic difference that is obtainedwhen the above spectrum is measured in the presence

of P32 beta rays. Preliminary measurements usingcosmic-ray tz-mesons give an efficiency of 98.9 per

cent ± 0.2 per cent for rejection of charged particles,and using p32 beta rays an efficiency 99.3 per cent

±0.1 per cent was obtained.The author wishes to thank Professor J. A. Simp-

son for suggesting this problem and for many helpfuldiscussions, and Edward Stone, who helped with thedesign and construction of much of the equipment thathas been described. The author also gratefully ac-

knowledges an Eastman Kodak Fellowship held duringpart of the time this work was in progress.

177

j\N~~~~~~N

Blank On \1-- Blonk Off

IO'l

.11-

2 T1 - exp

f(T) =- 'Cf

-- 0

2 TI - exp

ICs