or NaI(Tl) scintillators for simultaneous counting of α, β and γ rays

Nuclear Instruments and Methods in Physics Research A 340 (1994) 540-545 North-Holland

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

Phoswich detectors combining doubly or triply ZnS(Ag), NE102A, BGO and/or NAM) scintillators for simultaneous counting of a, ß and y rays Shigekazu Usuda *, Hitoshi Abe, Akira Mihara

Japan Atomic Energy Research Institute, Tokat-mura, Naka-gun Ibaraki-ken, 319-11, Japan (Received 13 September 1993) Phoswich detectors for simultaneous counting of a, R and y rays have been developed: ZnS(Ag)/Au Mylar/NE102A, ZnS(Ag)/Au Mylar/BGO and ZnS(Ag)/Nal(TI) for a and R(y) rays and ZnS(Ag)/Au Mylar/NE102A/BGO and ZnS(Ag)/NE102A/Nal(TI) for a, 0 and y rays . They were prepared by coupling a ZnS(Ag) film scmtillator for a counting with a scintillator(s) for (3 and y counting having different rise time . In order to adjust each component of pulse height within a given dynamic range, a sheet of Au-coated Mylar (Au Mylar) was used, if necessary, as an optical ND filter for lowering transmittance of scintillation of the ZnS(Ag). Characteristics of these phoswiches were examined by a technique of pulse-shape discrimination . Excellent discrimination among the radiations was attained and small tailings from each other peak were obtained for the prepared phoswiches . 1 . Introduction For radiation monitoring, it is useful to detect simultaneously and separately a, ß and y rays . The authors have directed toward the development of a detector suitable for flow monitoring of various solutions containing actinides [t-4]. A thin film of ZnS(Ag) is one of the most sensitive scintillators to a rays but insensitive to R and y rays and the decay time of the scintillator is relatively slow [5]. Organic scintillators, such as NE102A plastic and trans-stilbene crystal, are especially sensitive to ß rays and have generally a fast decay time . Therefore, the combination of the ZnS(Ag) and one of the organic scintillators optically coupled to a single photomultiplier tube (PMT), viz. ZnS(Ag)/ NE102A or ZnS(Ag)/stilbene phoswich detector, was prepared for simultaneous counting of a and ß (including y) [1,2]. The phoswiches had excellent properties of pulseshape discrimination (PSD) between a and R (y) rays (a figure of merit (FOM): 8-11) compared with single scintillators with good PSD properties such as CsI(TI) and stilbene scintillators (FOM : 1-4) [3,6-8]. However, the phoswiches had a disadvantage, i.e . was difficult to adjust the pulse height within a given dynamic range, especially when detecting low energy-(3 (y) rays, be-

* Corresponding author .

cause of too large a difference in pulse height between two scintillators composing the phoswiches . On the other hand, inorganic scintillators, such as NaI(TI) and BGO, arc highly sensitive to hard y rays due to their high density. If the rise time of amplified pulses from the inorganic scintillators differs to some extent from that of the ZnS(Ag) and organic scintillators, alternative combinations are designable : duplex phoswiches for simultaneous a and y (including ß) counting which are prepared by coupling two scintillators, the ZnS(Ag) and one of the inorganic scintillators and triplex phoswiches for simultaneous a, ß and y counting which are prepared by coupling three scintillators, the ZnS(Ag), one of the organic scintillators and one of the inorganic scintillators. The present paper deals with the PSD properties of the prepared phoswiches after examining the rise-time properties of four scintillators of interest under the same PSD conditions . A description is also given on how to adjust the pulse height within a given dynamic range in the case when too large a difference exists in pulse height ascribed to the respective scintillators . 2. Experimental 2.1 . Sctntillators Table I shows the dimensions and the physical constants [5] of scintillators used ; ZnS(Ag) film,

0168-9002/94/$07.00 © 1994 - Elsevier Science B.V . All rights reserved SSDI0168-9002(93)E1190-9

541

S. Usuda et al. /Nucl. Instr. and Meth in Phys . Res. A 340 (1994) 540-545 Table 1 Dimensions of scintillators used and their physical constants [1] Scintillator

Dimension

Decay time [ns]

Light output, % of Nal(Tl)

Peak wavelength [nm]

Density [g cm -3 ]

ZnS(Ag) NE102A BGO Nal(TI)

0 2 in ., 10 mg cm -2 O 2 in . x 5 mmt QJ 2 m. x 5 mm t O 2 in . x 5 mm t

200 2.4 300 230

130 28 8 100

450 423 480 410

4.09 1 .032 7.13 3.67

NE102A plastic, BGO (Bi,Ge 3012 ) crystal and Nal(Tl) crystal. For the ZnS(Ag), ZnS(Ag) powder was coated

on an acetylcellulose sheet. The Nal(TI) was sand-

wiched between two quartz glasses of 02 in . x 3 mm thickness to avoid deliquescence and to transmit scintillation light from the ZnS(Ag).

the scintillators covered with a thin film of aluminum-

coated Mylar (0.25 mg/cm 2 ), 0 and y rays from the other sources were measured after passing through an aluminum window of 2 mm thickness.

3. Results and discussion

2.2. Preparation of phoswiches Fig. 1 shows a schematical arrangement and the principle of a typical phoswich, ZnS(Ag)/Au

3.1 . Rise-time properties of single scintillators

Mylar/NE102A/BGO, for simultaneous counting of

Rise time distributions of a and 0 (y) rays from the 244Cm, 137Cs sources and background (BG) were mea-

Mylar) of 02 in . and 0.88 mg/cm2 density commonly

setting (Delay: 0.7 ws, Range: 1 Rs). Table 2 summa-

a, 0 and y rays . A sheet of gold-coated Mylar (Au

used for a proportional-counter window was placed, if

necessary, behind the ZnS(Ag) to adjust the intensity of scintillation light.

and counting rate in the peak area . The data for the ZnS(Ag) and NE102A scintillators differed a little from

of the scintillators and the different measurement con-

The simultaneous counting circuit was the same as

reported before [3], by which rise-time and pulse-height distributions were obtained at the same time . Pulse-

shape discrimination was performed with a rise-timeto-height converter (RHC) [9]. It should be noted that

the PSD properties of phoswiches depended on the setting of the RHC, especially on the delay time (De-

lay) and the conversion ratio of rise time to pulse The following radiation sources were used :

rizes the measured results of the rise-time properties

the previous data [2,3] due to the different dimensions

2.3 . Measurement system

height (Range).

sured with four single scintillators at the same PSD

244Cm

137

for a counting, Cs and "'Co for 0 and y counting, 24 'Am for soft--y counting and 90 Sr for 0 counting . While a rays from 244Cm were measured directly with

ditions but the same tendency was obtained . It was

confirmed that the ZnS(Ag) was quite insensitive to 0 and y rays, its rise time (the maximum-peak channel:

T) was the slowest among the scintillators in spite of a

decay time faster than that of BGO and Nal(TI) (see

Table 1) and its background level was very low; the NE102A with high sensitivity to 0 rays had the fastest rise time among them and its peak width (the full-

Table 2 Measured results with single scintillators under the same PSD conditions . Range: 1 ws (1 ns/ch) Scintillator

Source

Rise time [ch] T

ZnS(Ag)

NE102A

BGO Source ZnS (Ag) Fig. 1. Schematical arrangement of a ZnS(Ag)/Au Mylar/NE102A/BGO phoswich for simultaneous counting of a, ß and y rays .

Nal(TI)

244 Cm 137Cs

BG 244 Cm

137 Cs BG 244 Cm 137Cs BG 137Cs

BG

W1/

429

19

194 186 186 326 338 347 296

5 .4 5 .4 4.6 22 33 40 11 14

-

298

-

Counting rate [cps] 815 0.002 0.002 818 817 5.3 862 5282 42 1893 30

S. Usuda et al. /Nucl Instr. and Meth . i n Phys Res A 340 (1994) 540-545

542

On basis of the above results, the following combinations can be designed : ZnS(Ag)/BGO and ZnS(Ag)/ NaI(TI) for a and ß (y) counting as duplex phoswiches, in addition to ZnS(Ag)/NE102A reported before [1,2]; ZnS(Ag)/NE102A/BGO and ZnS(Ag)/ NE102A/NaI(TI) for a, ß and y counting as triplex phoswiches .

102

3.2. Optical properties of Au Mylar for pulse-height adjustment

When too large a difference exists in pulse height of signals from the respective scintillators composing a phoswich, it is necessary to set each component of pulse height within a given dynamic range. In this case, an optical neutral density (ND) filter is useful to lower the transmittance of either scintillation and to control the relative intensity of the respective scintillators. Fig. 2 shows a transmission spectrum of a sheet of Au Mylar, measured with an absorption spectrometer . While the ultraviolet light of wavelengths shorter than about 310 nm was steeply absorbed with the An Mylar,

500 600 400 Wavelength, nm Fig. 2 . Transmission spectrum of a sheet of Au Mylar . 300

width-at half-maximum : W1 , 2 ) was narrow ; the BGO and NAM) had an intermediate rise time between the ZnS(Ag) and NE102A and the sensitivity to y rays was very high . Table 3 Measured results with duplex phoswiches Phoswlch

Sources

Rise time [ch] a

ZnS(Ag)/NE102A a

ZnS(Ag)/Au Myiar/ NE102A a

ZnS(Ag)/BGO a

244Cm + 137Cs 244 Cm 137C s BG 244 Cm + 137CS 244 Cm 137C s 244 Cm+ 241Am (y) 24 'Am ( -y)

ZnS(Ag)/Au Myiar/ BGO b

ZnS(Ag)/Nal(T1) a

T 457 456

W1/2 19 l9

468 469

43 43 -

427 427

BG 244 Cm + 137CS 244Cm

644 642

55 54

422 423

20 20

137CS

6° Co BG 244Cm + 137CS 244 Cm 137CS

6° Co y° Sr BG Range : 1 ws (1 ns/ch) . n Range : 0 .5 Ws (0.5 ns/ch)

470

-

BG 244 Cm + 137CS 244 Cm

24 'Am (y)

d

R (y)

43 19 19

137C S

FOM

-

-

-

-

T 184 185 183 187 186 186 186 185 185 187 334 341 331 344 403 393 398 412 399 295 293 295 272 296 294 298

W1/2 8 .0 8 .2 6 .3 8 .4 5 .7 5 .5 56 12 12 5 .4 40 42 21 42 52 41 35 45 11 17 11 28 8 .7 17 16

10 .0

5 .7 5 .2 1 .6

2 .5

4 .0

Counting rate [cps] a

ß (1')

818 814 0 .11 0 .002 815 815 0 .16 813 0 .017 0 .002 822 814 4 .3 0 .003 823 814 8 .9 17 007 817 817 0 .7 25 21 0 .007 0 .006

327 2 .9 365 3 .6 656 6 .2 654 38 40 6 .3 1867 5 .0 1866 5 .1 3929 53 3901 5488, 28 1453 28 1453 741 2477 107 29

543

S. Usuda et al. /Nucl. Instr. and Meth. in Phys . Res. A 340 (1994) 540-545

106 105 g

U

101 10' 10 1 10°

106 105

0

200 400 600 800 Channel Number (Energy)

1000

Rise-time distributions

FOM value between the (Y-ray peak and ß (y)-ray peak of 137CS was diminished to 5.7 from 10, the degree of real separation between two peaks was apparently similar as shown in Fig. 3. The tailing corresponding to the degree of overlap of two peaks was almost the same for both phoswiches without and with Au Mylar (less than 0.03%). The ZnS(Ag)/Au Mylar/NE102A phoswich can be useful for similtaneous counting of a rays and soft ß (y) rays . Fig. 4 shows pulse-height and rise-time distributions of a and (3 (y) rays with the ZnS (Ag)/BGO and ZnS(Ag)/Au Mylar/BGO phoswiches. The results are also shown in Table 3. The pulse height of the ZnS(Ag) was so different from that of BGO for the former phoswich that another sheet of the Au Mylar was also placed in the latter phoswich . Since the PSD properties of the former under the same PSD conditions were not so good (FOM : 1.6), the rise-time distribution was measured with the latter under another setting (Range : . The amplitude of signal from the latter was 0.5 Ls) enlarged about 5-fold compared with the former . Although the PSD properties of ZnS(Ag)/Au Mylar/BGO (FOM : 2.5) were inferior to those of the above phoswiches (cf. Fig. 3), the counting efficiency

Channel Number (Time: ns)

Fig. 3. Pulse-height and rise-time distributions of a and p (y) rays with ZnS(Ag)/NE102A and ZnS(Ag)/Au Mylar/ NE102A duplex phoswiches . (Sources : 244 Cm and i37 Cs .) a transmittance of 10-20% for the visual light was roughly constant . Since the wavelength of the maximum emission of ZnS(Ag) was 450 nm [5] (see Table 1), the Au Mylar seemed suitable for an optical ND filter for the scintillation light from the ZnS(Ag). By interposing the Au Mylar behind the ZnS(Ag) as shown in Fig. 1, it can be expected that the pulse height ascribed to the ZnS(Ag) only will be lowered by a factor of 5-10 . 3.3. PSD properties of duplex phoswiches

Fig. 3 shows pulse-height and rise-time distributions of a and 0 (y) rays, which were measured with ZnS(Ag)/NE102A and ZnS(Ag)/Au Mylar/NE102A without and with the ND filter of Au Mylar, respectively . The signals from the latter phoswich were amplified by a factor of about 7 compared with those from the former . The PSD properties of the duplex phoswiches and the counting rate of each peak are summarized in Table 3. The latter phoswich enabled to detect both a rays and 60-keV y rays of 24'Am but the former did not. Although the Wi12 value for the a.-ray peak of 244Cm was enlarged by the Au Mylar and the

U C C CJ 5

U

Channel Number (Energy) 106 105

i

û 10' . â U

Rise-time distributions ß(Y) Au Mylar free ~\ (1 ns/ch) \ i

10'-1 10' 100 1 0

200

400 600 Channel Number (Time)

800

1000

Fig. 4. Pulse-height and rise-time distributions of a and 0 (y) rays with ZnS(Ag)/BGO and ZnS(Ag)/Au Mylar/BGO duplex phoswiches . (Sources : 244 Cm and i37 Cs .)

S. Usuda et at. /Nucl. Instr. and Meth in Phys Res. A 340 (1994) 540-545

544

50000

137Cs (Solid)

2°4Cm+137Cs(Dotted)

U G C

L

U 0

U

241Am

40000 30000 20000

'Sr

(x 10 -r)

Z44Cm+ 137C s

10000 200

400

600

fq~. .lr A

Channel Number (Energy)

200

ç

37Cs (Solid)

104

CJ

10 3

Ô U

101 10, 1 0°

,~

0

: ,., . .,. 200

R(Y)

a

Z44Cm+13_1Cs (Dotted)

"- ?`Cm (Slid) o

~,...,~., ....., .:. +r i 400

600

Channel Number (Tune ns)

Boo

1000

Fig. 5. Pulse-height and rise-time distributions of a and ß ly) rays from Z44C m+ 137C s, Z44Cm, 137Cs and BG with a ZnS(Ag)/Nal(TI) duplex phoswich .

for y rays was greatly improved because of the use of BGO. Fig. 5 shows pulse-height and rise-time distributions of a and (3 (y) rays from Z44Cm+137Cs, Z44Cm, 137CS and BG with the ZnS(Ag)/NaI(TI). The results are also tabulated in Table 3. Because the light-output

20000

Y

16000

U

h 0

U

12000 8000 4000

200

400

600

800

Channel Number (Tune ns x 1/2)

600

800

I 1000

Channel Number (Time: ns) Fig. 7. Rise-time distributions of a, ß and y rays from Z44Cm+137Cs, 244Cm, 137Cs, °°CO, 24 'Am and 90 Sr with a ZnS(Ag)/NE102A/Nal(TI) triplex phoswich

106 10 5

. / 1 ,

400

1000

Fig. 6. Rise-time distributions of a, p and y rays from 137 244CM + 137C 3 +9o Sr, Z44Cm, Cs and 9° Sr with a ZnS(Ag)/Au Mylar/NE102A/BGO triplex phoswich .

intensity of Nal(T1) is comparable to that of ZnS(Ag) as shown in Table 1, it was possible to count simultaneously a rays from 244 Cm and not only hard y rays from 60 Co but also soft y rays from 241Àm without the Au Mylar. In addition, this phoswich exhibited a good resolution (FOM : 4.0) and a small tailing in its rise-time distributions. 3.4. PSD properties of triplex phoswiches Rise-time distributions of a, R and y rays from 244 Cm + 137 Cs +yoSr, Z44 Cm, 137 Cs and "() Sr with ZnS(Ag)/Au Mylar/NE102A/BGO and those from Z44Cm+ 137Cs, Z44 Cm, 137CS, 6°Co, 24'Ain and 9o Sr with the ZnS(Ag)/NE102A/NaI(Tl) are shown in Figs . 6 and 7, respectively . The measured results are summarized in Table 4. It was found that a, ß and ^l rays except soft y rays from 241Am were detected reasonably with both triplex phoswiches . For the soft y rays, the detection was difficult with the former phoswich but efficient with the latter phoswich, in spite of the dependence of -y-ray energy on the rise time ascribed to the Nal(Tl). The similar dependence was also observed for a CsI(Tl) scintillator [4]. The properties of the ZnS(Ag)/NE102A/NaI(T1) were superior to those of ZnS(Ag)/Au Mylar/NE102A/BGO in the resolution among the radiations and the tailing. An essential difficulty arises in separation between ß and y rays because of the similar interaction of the radiations with the scintillators and the formation of bremsstrahlung X-rays due to hard ß rays . However, the triplex phoswiches are practically effective as a simultaneous counter for a, ß and y rays because the counting efficiency of ß rays for NE102A and that of y rays for BGO or NaI(Tl) is considerably different.

S. Usuda et al. /Nucl. Instr. and Meth. in Phys. Res. A 340 (1994) 540-545

545

Table 4 Measured results with triplex phoswiches Phoswlch

Sources

y ZnS(Ag)/Au Myiar/ NE102A/BGO n

244 Cm+137Cs+ 9° Sr 244Cm+ 137Cs 244Cm 137C

s

9° Sr BG ZnS(Ag)/NE102A/ Nal(Tl) a

244Cm + 137Cs 244Cm 137

Cs

241Am (y) a° Co 90 Sr

BG

FOM

Rise time [ch]

'v

0

T 698 698 697

W1 12 65 67 66

-

-

-

-

455 454

20 20

-

-

-

-

-

-

-

-

T

W112

T

W1/2

219 219 219 219 220 219

7.5 8.4 6.8 7.8 5.5 6.6

399 396 392 401 394 404

42 41 51 41 57 62

191 187 191 (185) 183 184

2.1 8.8 7.1 7.5 8.1

295 12 295 16 294 11 271 25 298 8.9 295 17 298 16

184

11

Counting rate [cps]

R/a

ß/y

y/a

oa

R

y

6.6 6.4

3.7 3.6 (3 .0) 3 .7 2.8 (2 .7)

2.8 2.8

826 824 813 6.7 0.2 0.2

542 401 2.8 367 178 27

2735 2278 65 2767 99 66

9.4

5 .2 (4 .4) 5.7 7.0 4.4 (4 .2)

5.0

819 817 1.0

124 0.9 115

1197 32 1383

18 2.8 0.037 0 004

3.8 823 151 0.7

3600 2200 106 32

Range: 1 Ws (1 ns/ch) . n Range: 0.5 Vs (0 .5 ns/ch) . a

4. Conclusions

Acknowledgement

Structurally duplex and triplex phoswich detectors were prepared for simultaneous counting of gross a, ß and y rays by combining doubly or triply the following

of the Institute of Radiation Measurements for his

scintillators :

a thin

ZnS(Ag) film for a counting, NE102A plastic for ß (including y) counting, and BGO or Nal(TI) crystal for y (including ß) counting . Pulseshape discrimination of each radiation was successfully carried out because of a proper difference in rise time of amplified signals from the respective scintillators .

When the pulse height of the ZnS(Ag) is far larger

than that of the other scintillators, only the pulse height of the former was conveniently lowered by interposing a sheet of Au Mylar as an optical ND filter behind

the ZnS(Ag) . Consequently, simultaneous counting of a rays and not only hard but also soft ß and y rays became possible . Among the prepared phoswiches, the authors re-

commend the ZnS(Ag)/NE102A/Nal(TI) as a detec-

tor for simultaneous a, ß and 1' counting for radiation monitoring purposes . If other combinations of scintillators with different rise time and selective sensitivity to

each radiation will be found, other types of phoswich detectors may be developed.

The authors are grateful to Dr . Hiroshi Tominaga

helpful comments and valuable discussions concerning the manuscript .

References [1] S. Usuda, J. Nucl . Sci. Technol. 29 (1992) 927. [2] S. Usuda and H. Abe, J. Nucl . Sci. Technol. 31 (1994) 73 . [3] S. Usuda, A. Mihara and H. Abe, Nucl . Instr and Meth . A 321 (1992) 247. [4] S. Usuda and H. Abe, Nucl . Instr. and Meth . A 321 (1992) 242. [5] G.F. Knoll, Radiation Detection and Measurements (Wiley, New York, 1989). [6] P. Harihar, W.R. Stapor and A.B . Cambell, Nucl . Instr. and Meth . A 272 (1988) 763. [7] P. Harihar, A.R . Knudson, W.R . Stapor and A.B . Cambell, Nucl . Instr. and Meth . A 283 (1989) 62. [8] R.A . Winyard, J.E . Lutkin and G.W. McBeth, Nucl . Instr. and Meth . 95 (1971) 141 . [9] S. Korbara and T. Kumahara, Nucl . Instr. and Meth . 70 (1969) 173.