SCINTILLATION SPECTROMETRY The State of the Art - Analytical

REPORT

FOR ANALYTICAL

CHEMISTS

SCINTILLATION SPECTROMETRY The State of the Art by William C. Kaiser Electronics Division Argonne National Laboratory Argonne, Illinois

This Report describes the present uses of, and instrumentation involved with, scintillation spectrometry. Special attention is given to new scintillator materials, advances in photomultiplier tubes, and improvements in resolution. Applications of organic scintillators and future developments are also discussed.

T^HE

SCINTILLATION

PROCESS

Was

•*- first observed by Crookes in 1903 when he saw light produced by a particles impinged on a ZnS screen. In 1908 Crookes and Regener first used such a system for counting a particles. The observations were made in a darkened room by use of a microscope. It is said that one of the qualifications of a good physicist of this time was his ability to see the scintillations. Marsden has recounted that on train journeys his colleague Geiger would urge him not to put his head out the window lest a cinder should impair his ability as a human scintillation counter. As J. B. Birks says in his book "The Theory and Practice of Scintillation Counting," "Truly the

early nuclear physicists needed to be men of vision" (3). As crude as this method was, several historical feats were achieved with it: (1) The determination of the charge 2e on the a particle which was in excellent agreement with the charge e obtained by Millikan's oil drop experiment. (2) Led to the discovery of the atomic nucleus by Rutherford in 1911. (3) The detection and measurement of the range and energy of the pair of a particles in the 7Li (p, 2») reaction by Cockcroft and Walton in 1932. For this work coinci-

dence counting was used, employing two observers and two ZnS screens, to show that the two particles were of equal energy and traveled in opposite directions. With the development of gas ionization chambers in the 1930's, scintillation counting fell into disuse, but the development of the photomultiplier tube during World War II gave it rebirth. The first application of the photomultiplier tube to scintillation counting was by Curran and Baker in 1944 (7). The experiment was described in a classified report and was not disclosed until 1948. By this time several other experimenters had published papers describing measureVOL. 38, NO. 11, OCTOBER 1966

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27 A

REPORT FOR ANALYTICAL CHEMISTS

ments on scintillations produced by α, β, and γ rays. In 1947 Kallman (12) discovered that the scintillations from large transparent blocks of naphthalene produced by β and γ rays could be detected with photomultiplier tubes. Bell, in 1948 (2), discovered that crystalline anthracene gives five times the pulse height of naph thalene. A major breakthrough in scintillation spectrometry came when Hofstadter (10) discovered, that same year, that sodium iodide crystals activated with thallium give even larger pulses than anthra cene. In 1950 Hofstadter and Mclntyre (14) showed that Nal (Tl) crystals could be used as gamma-ray spec trometers, and since that time, NaI (Tl) has been used as a detector for various types of nuclear radiations, extending from x-rays and gamma rays to all kinds of ionizing parti cles. There were other developments in organic scintillators in 1949-50. Several experimenters reported ob serving scintillation pulses in cer tain organic liquid solutions. Schorr and Torney (1950) (18) found that organic plastics also exhibited scintillation properties. The organic liquids and plastics were found to have shorter scintil lation decay times of the order of 2-3 nsec. as compared to 200 nsec. for Nal (Tl) and ZnS.

Materials The most important scintillators and their physical properties are shown in Table I. There are a vast number of plastic and liquid scintil lators (A coverage of these would be the subject for a paper in itself). Three materials which show promise for the future but which have not been available long enough for a complete evaluation areCaI 2 (Eu) (.Π), CaF 2 (Eu) (16), and CsI(Na) (δ). (1) CaI 2 (Eu) has 50% more light output than Nal(Tl), but to date has proved to be impossible to grow in large crystal sizes. Also, it is highly hygroscopic and is very difficult to handle or package. (2) CaF 2 (Eu) seems to have a bright future in particle de tection where corrosive liq uids are involved because of its glass-like properties. (3) CsI(Na) would seem to have a very bright future. It has 85% of the light output of Nal(Tl) and is not hygro scopic to any great extent. This permits the rigid can ning techniques needed for Nal(Tl) to be relaxed. CsI(Na) has another interesting property. Its maximum scintilla tion efficiency is at 80° C. This is

Table I. Material

in contrast to Nal(Tl) which has its maximum scintillation efficiency at just below room temperature. Thus for high temperature work CsI(Na) might well be better than Nal (Tl). Too, it has the built-in advantages of a higher Z, which makes it more efficient to photo electric events. Photomultiplier Tubes

At the same time various scintil lators were being discovered and improved, new and improved de signs of photomultiplier tubes were being developed. Two recent de velopments in photomultiplier tubes which have contributed greatly to the advancement of scintillation de tection are CuBe dynodes and the bialkali cathode (13). Both have improved the stability of photomul tiplier tube gain and the bialkali cathode has improved the cathode sensitivity. A recent photomultiplier tube de veloped by RCA is the 8575, which has a 50 Ω output impedance for fast pulse work incorporated with the high efficiency bialkali cathode, BeCu dynodes, and an electron op tical system which is fast and has a high photoelectron collection effi ciency. Resolution Before discussing scintillation spectrometry further, it is impor-

Physical Constants of Scintillators

Wavelength of Max. Emission

Decay Const, μsec.

4100 5650 4350 4350 4100 4400

0.25 1.1 0.65 1.0 1.0 1.4

3 Density,g./cm.

Relative Pulse Height

Applications

Inorganic Crystals ΝαΚΤΙ) Csl (Tl) CsKNa) CaF2(Eu) KKTI) Lil(Eu)

3.67 4.51 4.51 3.17 3.13 4.06

210 100 175 100 50 74

a, x-rays α, β, y et, β, y, x-rays ot, β, y, x-rays Seldom used

1.25 1.16

100 60

°ίι βι y, fast η α, β, y, fast η

28-48 27-^9 25

α, β, y, fast n α, β, 7, fast n

η

Organic Crystals Anthracene Stilbene

Plastics Liquids Glass Scintillators

28 Α

·

4400 4100 3500-4500 3550-^500 3950

ANALYTICAL CHEMISTRY

0.032 0.006

Organic Solutions 1.06 0.002 to 0.005 0.86 0.0015 to 0.008 0.005 to 0.1 2.6

η


Csi37 27 yr.2 580 mg/cm Be Geometry 40% 1 Div. = 14.4 kev Relative Line Width, „ = F"" Width at Half Maximum AE Average Energy Ε Relative Variance, V 0 = •£—z ο in

MAX

Figure 1 . Output of scintillation spectrometer for monochromatic gamma ray

tant to define resolution. In most fields such as optical spectrometry or semiconductor spectrometry the unit for describing resolution is the same as that for describing the spectral line; for instance, Ang stroms for optical spectrometry or keV of energy for semiconductor spectrometry. This is true because the resolution is relatively constant over a wide range of measurement. However, the resolution achieved by a scintillation spectrometer is dependent on several items which are all governed by statistics. Therefore, the output of a scintilla tion spectrometer for a monochro matic gamma ray is in the form of a pulse distribution such as that shown in Figure 1. The resolution of such a spectral line is described in terms of the ratio of its relative full-width at half-maximum (FWHM) and its average energy expressed in per cent. For example, a 662 keV gamma ray having a FWHM of 53 keV would be de

scribed as having a resolution of 8 per cent. When the quality of two scintillation spectrometers is com pared it is accepted practice to compare their resolution of the 662 keV 137Cs gamma ray. It should be noted that besides the monoenergetic 662 keV spec trum produced by photoelectric ef fect, there is also a lower energy continuum. This continuum is due to Compton events and pair pro duction. In each of the processes a portion of the scattered and annihi lation gammas escape the scintilla tor during a large portion of time, thus producing pulses of lower height than the photopeaks. The resolution of scintillation spectrometers has improved steadi ly over the years. For example, in 1956, at the 5th Scintillation Coun ter Symposium in Washington, D. C , Dr. C. J. Borkowski of Oak Ridge National Laboratory stated, "Rarely does one obtain an energy resolution below 10% on spectrome-

Table II. Crystal Sizes and Resolutions" Speaker

Represented Institution

Branson Managan Carlson Managan Brooks Stewart

HEW ANL Harshaw ANL GE ANP Harshaw

° Reported at the Total Absorption Gatlinburg, Tenn., May 1960

Crystal Size in. 4X4 4X4 5X5 8X6 8X8 9 X 12

% Resolation for ™Cs 0.662 MeV 7.5 6.8 6.8-7.2 8.6 9.5 10.0

Gamma-Ray Spectrometer

Symposium,

VOL 38, NO. 11, OCTOBER 1966

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29 A


ters with crystals 5 X 5 in. or lar ger" (4). He also stated that even though 9 X 10-in. crystals were in use, only one out of five hundred melts produced a good crystal. Four years later at the Total Ab sorption Gamma-Ray Spectrometer Symposium at Gatlinburg, Tenn., the resolution and crystal sizes shown in Table II were reported. Since 1960 the best reported reso lutions for Nal(Tl) crystal spec trometers have not improved ap preciably. However, in 1960, a res olution of 6.8-7.5% for 4 X 4-in. crystals was achieved by careful se lection of both photomultiplier tubes and Nal(Tl) crystals. To day, because of improvements in the manufacturing processes of both photomultiplier tubes and Nal(Tl) crystals, this same resolu tion is more commonplace. Also, Nal(Tl) crystals as large as 16-in. are available in limited numbers. Improved packaging techniques have also played an important role. The high index of refraction of NaI(T1) (~1.85) and its hygroscopic property have made canning tech niques difficult. Improved reflec tors and surface conditioning and light coupling techniques have played an important role in increas ing the efficiency of light transfer from the crystal to the photomulti plier tube. One problem in the transfer of light from the crystal to the pho tomultiplier tube does not involve efficiency of transfer as much as distribution of light across the tube cathode—this is due to the cathode nonuniformity. Figure 2 is the plot of cathode uniformity for a pho tomultiplier tube. Even though this tube was selected as having a relatively uniform cathode, it can be seen that there is still a factor of 5 difference in sensitivity across the tube face. Events occurring near different points on the tube face will experience different conversion efficiencies in the photomultiplier tube, giving an additional spread in pulse height. This situation can be corrected somewhat by use of a lightpipe to spread the light more uniformly across the cathode sur face. It should be remembered that there is an optimum length of light pipe for each individual situation. This is true because of light losses 30 A

·


KEY Figure 2. Relative sensitivities for an RCA two-inch diameter photomultiplier, Type 6342A, Serial No. 1-60-438

in the lightpipe. When this effect becomes more dominant than the lack of uniform illumination of the photocathode, any further increase in lightpipe length is harmful. Fig ure 3 shows this relationship for a 4 X 4-in. crystal mounted on a 5-in. photomultiplier tube. Note that the peak-to-valley of the 60Co 1.17 and 1.33 MeV peaks shows a de cided optimum value of lightpipe of about 1 inch (Figure 4 shows this spectrum.)

Compton Background Reduction

Three methods of achieving a re duction of Compton background using small crystals are the twocrystal spectrometer, the pair spec trometer, and the anticoincidence spectrometer. A two-crystal spec trometer requires coincidence between the Compton recoil elec tron and the scattered gamma ray at a fixed angle. This eliminates all other pulses.

4" χ 4" NaKTI ) Crystal 5" Dla. E.M.I Photomultiplier Type 9579A Serial Number 5074

Source Above Crystal © Cobalt 60 8 4.43 Mev (Pu-Be)

Height of Lightpipe in Inches Figure 3. Plot for determination of optimum length of lightpipe

REPORT

The pair spectrometer employs three crystals, each having its own associated photomultiplier tube. The output of the center crystal goes to a pulse-height analyzer provided there is a triple coincidence with the other two crystals. The triple coincidence occurs when pair production takes place in the central crystal and the two adjacent crystals intercept the annihilation photons accompanying the destruction of the positron. The energy deposited in the center crystal is the total kinetic energy of the pair. Obviously, it cannot be used to detect gamma rays below 1.02 MeV in energy. The anticoincidence spectrometer consists of a small crystal surrounded by a larger one. Figure 5 illustrates such a unit used by Raboy and Trail of Argonne National Laboratory (17). The pulses from the center crystal which are not accompanied by a coincidence pulse from the surrounding crystal are accepted by the pulse-height analyzer. Figure 6 shows the recorded spectrum with and without the anti-

coincidence crystal. It is easy to see how much better the 1360 keV line is defined when the anticoincidence circuits are used. The anticoincidence spectrometer is more efficient than either the pair spectrometer or the two-crystal spectrometer. However, it is still limited by the size of the center crystal. Total Absorption Spectrometer

When weak or extended sources of gamma rays are encountered, more sensitivity is required. A simple method of increasing sensitivity and reducing the Compton background, at the same time, is to use large crystals. A scintillation spectrometer using such a crystal is known as a "total absorption spectrometer." Figure 7 shows the reduction in the Compton background produced by using a 4 X 4-in. in place of a 2 X 2-in. Nal(Tl) crystal. Figure 8 is a photograph of an 8 X 6-in. Nal(Tl) crystal mounted on 12-in. photomultiplier tube. In Table III the qualities of two 8 X 6-in. crystal packages and one 10 X 8-in. package are compared. All three are of identical construction


4" x4" Nal (Tl) Crystal 5" Diameter E.M.I. Photomultiplier Tub· Type 9S79A, Serial Number 5074 I" Polished Light Pipe

201

1.33 Mev Peak Resolution 5.8%

6000-

11.6! I Peak-to-Valley Ratio Source 33" Above the Center of Crystal

3000 —

190

ο Channel Number

Figure 4. Spectrum for 60Co gamma-ray incidents

except for crystal size (in the 10 X 8-in. package no lightpipe was used). When the 10 X 8-in. crystal was packaged it was felt that the crystal was sufficiently thick so that no lightpipe would be neces sary. A comparison of the peak-tovalley ratios for the e0Co 1.17 and 1.33 MeV peaks and the resolution of the sum peak show that this as sumption was not true. One trick for improving the reso lution of large crystals is to deliber ately make the crystal efficiency nonuniform. For instance, if a large crystal has some impurity which causes self-absorption of the scintillation light, then events oc curring far from the photomultiplier tube will produce smaller pulses than those occurring near the tube.

This condition can be improved by polishing the sides of the crystal near the photomultiplier tube, thus forcing the light occurring in this region to experience more reflec tions before entering the tube. This, in turn, means a more uniform path length, and thus the overall spectral line width is reduced. Background Radiation Reduction

As the Nal(Tl) crystals get lar ger and the gamma ray sources to be measured become weaker, the problem of background radiation becomes more acute. There are several sources of this background and several methods of reducing it. One source of background is cos mic rays. The effect of these can BOTTOM PLATE TO RING HOUSING

RING HOUSING

RING

END PLATE-

ANTICOINCIDENCE RING OF Να I ( T D -

, END CAP FOR INNER CRYSTAL HOUSING ~

SIX PHOTOMULTIPLIER TUBE ASSEMBLIES REQUIRED FOR RING END P L A T E QUARTZ LIGHT PI P E N

μ METAL SHIELD >

INNER CRYSTAL HOUSING

PHOTOMULTIPLIER - " 0 " RING LIGHT SEALS ^PHOTOMULTIPLIER

INNER CRYSTAL Να I

Figure 5. Anticoincidence spectrometer 32 A

·


(Tl)

be reduced in one of two ways or a combination of the two. The first method is to use an anticoincidence scintillator ring around the detector. This unit works in the same way as the unit used to reduce the Compton scatter spectrum. Here, however, because high energy charged particles are involved, a plastic scintillator can be used. Figure 9 shows a plastic scintillation ring built to act as an anticoincidence shield for the 10 X 8-in. Nal(Tl) crystal discussed earlier. A unique method of using the cosmic-ray anticoincidence shield is to detect both the shield and the gamma-ray detector scintillations with the same photomultiplier tube, and to use pulse-shape discrimina tion to reject the unwanted pulses. Such a unit is known as a phoswich. This detector is used in the instru ment shown in Figure 10 to deter mine the radioactive isotopes in an atomic cloud. The unit is flown be neath a wheather balloon and tele meters the spectrum to the ground where it is stored in a multichannel analyzer. Figure 11 shows a spec trum obtained with this unit. A second method for reducing cosmic rays is to prevent the events from reaching the crystal. The most common method of doing this is to place the detector under-


Ο

30

60

90

120 PULSE

Figure 6. Spectrum of anticoincidence crystal

24

180

210

240

270

Na showing effects produced with and without the

ground, and then provide further shielding such as an iron room to reduce the shower particles. The iron helps shield against a further source of background found in the soil, concrete, etc. Also, the radon content of the air is a source of background. This can be elimi nated by a filtering process. An installation using these tech niques is the iron room for whole body counting, used by H. M a y of the Radiological Physics Division of Argonne National Laboratory. Figure 12 is an exterior view of the iron room showing the door open and a patient inside.

Figure 7.

150 HEIGHT

Elimination of cosmic ray and natural radioactivity interactions reduce the background appreciably. However, to be a large order of im provement, this method must be ac companied by a reduction in the ra dioactivity of the detector package. This radioactivity is mainly due to the presence of 4 0 K and radium in the crystal and the glass of the photomultiplier tube. In the past five years the potassium content of N a I(T1) crystals has been reduced until it is negligible. M a n y glasses contain 0 . 1 % K 2 0 . This, along with the radium content of the sand used in glass production, makes up

Effects on spectrum produced by use of crystals of two different sizes VOL 38, NO. 11, OCTOBER 1966

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33 A


Figure 8. An 8-in. χ crystal package

6-in. Nal(TI)

the radioactivity of the photomultiplier tube (15). Photomultiplier tube manufac turers have taken three separate approaches for reduction of this background: (1) The use of quartz faced tubes (2) The use of low potassium content glass (3) The development of an all ceramic and metal tube All these approaches are very ex pensive and thus have not been too popular. A cheaper approach, especially when one considers the cost of replacement tubes, is to use a lightpipe to separate the tubes

Table III.

8 X 6 in. #1

PA

8.7

8 X 6 in. #2

)l/


glass stem. Note also t h a t the tubes have been debased, because the bases have proved to be a source of radium. A unit which uses nearly all the techniques described to improve spectral determination and effi ciency is the total absorption coincidence spectrometer of Greiner and Samuelson (8). The unit consists of two well crystals face to face for &π geometry. The two wells match up and contain a beta detector operated in t h e Geiger re gion. The source to be analyzed splits the β detector into 2 portions. Around the well crystals there is a 3-in. layer of plastic scintillator on five sides which acts as a cosmic

Resolution of Nal (77) Crystal Packages for Gamma Sources % Resolution of ur Cs {0.662 MeV)

10 X 8 in.

·

from the crystal and to absorb some of the radiation. If an appreciable absorption of the radiation is de sired one can use a pure N a l lightpipe. The high Ζ of N a l makes it an excellent absorber. Although this material scintillates when bom barded b y gamma rays or radioac tive particles, the scintillations are in the ultraviolet region and the glass faced photomultiplier tubes will not respond to them. Figure 13 shows the detector used in the Argonne National Labora tory iron room. I t consists of an 11X 4-in. N a l ( T l ) crystal and four 3-in. R C A photomultiplier tubes. These tubes are all ceramic and metal construction except for the

Length of LighP pipe, in.

Crystal

34 A

Figure 9. Plastic scintillation ring used as an anti coincidence shield

t

7.8

None

8.8

Peak/Valley Ratio for mCo (1.33 MeV) 5.3 1 6.9 1 3.9 1

Resolution of Co Sum Peak (2.50 MeV)

60

4.8 4.3 5.7


Figure 10. Instrument for use in weather balloons to determine radioactive isotopes in an atomic cloud

ray shield. The entire unit is enclosed in an 8-in. thick iron shield to reduce the natural background. When all the circuits are in use, the unit is used for the determination of absolute disintegration rates. In the region of gamma-ray energies from 70 to 2800 keV, the well crystals have an efficiency of 76.5 to 64.5%, respectively, with a maximum efficiency at 530 keV of 89.5%.

Spectral Analysis

The identification of the isotopes making up the gamma r a y spectra is primarily dependent on experience. Two important tools to aid in this work are the catalogues of gamma ray spectra compiled by R. L. Heath (9) and C. E. Crouthamel (6). However, each scintillation spectrometer has its own characteristics and the catalogues can serve

Figure 1 1 . Spectrum obtained with instrument shown in Figure 10

VOL 38, NO. 11, OCTOBER 1966 ·

35 A


Figure 12.

Iron room for whole body counting

only as guides. It is therefore de sirable to calibrate the scintillator with sources of known energies and intensities. Figure 14 shows the spectrum of a patient from the Argonnc National Laboratory iron room. Figure 15 shows how such a spectrum is ana lyzed. This latter spectrum is of a Marshallese who was exposed to the fallout of a hydrogen bomb blast. Applications of O r g a n i c Scintilla tors

The fast decay time of organic scintillators enables them to be used for such studies as the lifetimes of

excited states of nuclei and short lived elementary particles. By op erating two or more such counters in coincidence it is possible to ob serve sources of low intensities in the presence of a large background of other sources. Also by using two photomultiplier tubes to observe the same scintillator and demanding coincidence it is possible to observe low energy radiations which pro duce pulses of the order of the ran dom tube noises. Liquid scintillators lend them selves to 4-7Γ counting geometry by incorporating the radioactive speci men into the scintillator. Both liq-

Figure 13. Detector used in Argonne National Laboratory iron room Circle No. 233 on Readers' Service Card36 A

·



Potient EA

K 40 (of I30g K)subtracted

.48^0 Z n 6 5 subtracted

κ40 r1"

Cs ^„

.1 0.5 Energy (mev)

1.0 »

.2

.3 .4

.5

1.5

.6 .7 .8 .9 Energy MEV

1.0 I.I

1.2 1.3 1.4 1.5

Figure 15. Analysis of gamma ray spectrum of Marshallese patient

Figure 14. Typical spectrum of patient

uid and plastic scintillators are of fered in a wide variety of shapes and sizes. Plastic scintillators can be purchased as thin foils, fila ments, tubes, beads, large sheets, and cartwheels. Some of today's cosmic ray work is being performed with plastic scintillator cartwheels on the order of two meters diameter and 10 cm. thick. The linear scintillation response of organic scintillators to β particles of energies greater than 125 keV makes them suitable for /3-ray spec trometry. Their low density and low atomic number of their individ ual elements make them rather in sensitive to γ-rays. Thus they can operate effectively as /3-spectrom-

subtractea-

eters in the presence of moder ately intense γ-ray fields. The high hydrogen content of or ganic scintillators permits them to be used for fast neutron spectrome try, by observing the scintillations produced by the recoil protons. This use is further enhanced by the dependence of the pulse shape of the scintillation on the density of the ionization track. Thus, the use of pulse shape discrimination tech niques can be used to eliminate pulses due to γ-rays. The low densities and low atomic numbers of the individual elements of organic scintillators makes them poor choices for x-ray or γ-ray spectrometry. The edge of their

broad Compton distribution is a poor substitute for the photopeak of NalfTl) or CsI(Tl). However, because of the advantage of a 4ir counting geometry, liquid scintilla tors along with a large battery of photomultiplier tubes are used for whole body counting. An example of this type unit is the HUMCO II whole body counter used by Anderson at the Los Ala mos National Laboratory (1). The patient is introduced into the coun ter well by a trough and sling ar rangement. Shielding is provided by 7 Va in. of naval armour plate in the form of a room 8 X 10 X 7 feet high. Use of room type shielding provides easy access to the counter

3 6 0 kev

8 0 0 kevHUMCOQ CHANNEL 3

70 KILOGRAM SUBJECT ~ \

Figure 16. Three 137Cs spectra with three dis tinct distributions of the source

_ POINT SOURCE

5 GALLON JUG OF WATER

ο

38 A

•


10

20

30

40 50 60 PULSE HEIGHT

70

80

90

100

REPORT

for installation and repair. Figure 16 shows three 137Cs spec tra with three distinct distributions of the source. In one case the source is in a five-gallon jug of water. In the second case it is dis tributed throughout a 70 Kg. sub ject. In the third case it is a point source. Notice the geometry effect on resolution, and the broader peak as compared to the H. May spec trum shown in Figure 15. What about the future of scintil lation spectrometry? In the appli cations where energy resolution is of primary concern, there is no doubt that semiconductor spectrom eters are far superior to scintillation spectrometers. In fact, they have already taken over in many appli cations. However, where extended sources such as the human body are encountered, scintillation spectrom etry will still play a major role. Whole body counting, as present ly practiced, is not too practical for the really ill patient because of the long time required for the collection of data. Two installations are now under study—one at Walter Reed Hospital using eighteen 9-in. crys tals, and one at Brookhaven Na tional Laboratory using fifty-six 6in. crystals to reduce the counting time of a patient. It is hoped that these units will localize the position of a radioactive isotope in the human body without the tedious scanning techniques now employed. Future Scintillation Detectors

What about the scintillator of the future? It will: (1) have 5 times the conversion efficiency of NaI (Tl), (2) have a very high Ζ num ber such as Pb, (3) be nonhygroscopic, (4) be easily obtained in large sizes, (5) be easily machined, and (6) have a fast response time ( < 1 nsec). This scintillator should then be coupled to a photomultiplier tube having a cathode quantum ef ficiency of at least 80%, a cathode uniformity of < 1 % , a time spread

SCINTILLATION SPECTROMETRY The State of the Art - Analytical

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