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Comparison of plastic scintillators with nanosecond lifetimes
NUCLEAR
INSTRUMENTS
AND
METHODS
Io9 (I973) 233-235;
©
NORTH-HOLLAND
PUBLISHING
CO.
C O M P A R I S O N OF P L A S T I C S C I N T I L L A T O R S W I T H N A N O S E C O N D L I F E T I M E S * T. M. K E L L Y a n d J . A .
MERRIGAN
Research Laboratories, Eastman Kodak Company, Rochester, N. Y. 14650, U.S.A. R. M. L A M B R E C H I "
Chemistry Department, Brookhaven National Laboratory, Upton, N.Y. 11973, U.S.A. Received 8 J a n u a r y 1973 T h e scintillation lifetimes o f Pilot U, M a n d B, N a t o n 136, N E 111 and an experimental scintillator are c o m p a r e d . T h e
new Pilot U scintillator exhibits the fastest decay time (r 1 = 1.36 ns) yet obtained for a plastic scintillator.
1. Introduction Nuclear or positron lifetime measurements in the nanosecond region often require the use of plastic scintillators to detect the timing signalst). Previous publications have shown the importance of the shape of the light pulse from the plastic scintillator in determining the ultimate resolution of nanosecond timing spectrometerst-3). Naton 136 (also known as KL 236) has in the past often been the plastic scintillator of choice. Although a small number of liquid scintillators*) have been found to have very short scintillation lifetimes, a solid scintillator is more desirable for timing measurements particularly if it is characterized by subnanosecond rise and decay times and a narrow full width at one-half maximum. We have measured the scintillation lifetimes of the recently available Pilot U, and an experimental plastic scintillator whose composition was based on the results from liquid scintillators, and compared them to the scintillation lifetimes of Naton 136, Pilot B, Pilot M and NE I11.
The general shape of the scintillation decay curves is illustrated in fig. 1. The decay of the light signal can be described as the sum of two exponential components,
2. Experimental The mean scintillation lifetimes (1.44 /1/2) w e r e measured by the fast-slow coincidence method of start-stop timing coupled with single photon detection. The subnanosecond decay spectrometer has been fully described elsewhereS'6). The start signal was derived from the scintillator under study, which was optically coupled to an RCA 8575 photomultiplier tube (PM). The detection of a single photon by an RCA 8850 PM was used as the stop signal. The scintillators were excited by 6°Co g a m m a radiation with a 20% window on the start side. The instrumental response function, measured by the Cherenkov radiation techniqueT), was 1.0 ns full width at one-hlaf maximum. * Research performed in part under the auspices of the U.S. Atomic Energy Commission. 233
f tL = "~ e x p ( - ~i'q) + " - - e x p ( - t / T 2 ) .
I(t)
"1~1
(1)
"C2
We have analyzed the curves by a nonlinear regression technique to obtain the values for the mean lifetimes, r 1 and r 2, and the fraction of the decay curve, J~, which has the mean lifetime rz 8). Only f2 is reported since f l +f2 = 1. We also report the mean lifetime of the decay from 90% to 10% of the maximum light intensity, Z~o.9° This parameter was derived from a simple weighted least-squares fitto the data in the region from 90% to 10% of the maximum of the decay spectrum. Although this parameter may be of interest to some in
I04~
1
:
:
!
Pdol
"
e~'~'g~ili~i~
o P,lot B Noton-136
olm
103'
'~
. . . . .
U
•
I
--i
°~-" %~°
IP
t, ~o
,..
~'~%,~ • ao
,
8
~ ~
! 0
1 2
4- . . . . .
#,
1 8
1_ 10
.'%..
i . . . . ~2
14
Time (n sec)
Fig. 1. Scintillation decay spectra for Pilot U, Pilot B and Naton 136 scintillators excited by 6°Co gamma rays as measured by the single-photon technique (background subtracted).
234
T.M.
K E L L Y et al.
1.36 ns, has a significantly shorter decay time than any of the other commercially available materials. In fact, a close inspection of fig. 1 reveals a full width at onehalf maximum about 15% less, and the rise (or energy Scintillator 171(ns) T2 (ns) f2 r9o0 (ns) transfer) time is faster for the Pilot U than for the other plastic scintillators. Because of these properties, Pilot Pilot U 1.36 12.5 0.12 1.54 Experimental 1.87 I 1.5 0.12 1.96 U should be considered and evaluated as a scintillator N a t o n 136 1.70 12.5 0.12 1.90 for fast timing applications. Pilot U contains a wavePilot B 1.69 11.5 0.13 1.92 length shifter and the pulse height is stated by the Pilot M 2.26 14.7 0.16 2.64 manufacturer to be 68% of anthracene. The 12% N E 111 (A) 2.27 17.1 0.12 2.32 intensity, and the 12.5 ns lifetime of the longer-lived N E I l l (B) 1.66 12.5 0.12 1.85 component on Pilot U is comparable to that observed with the other scintillators. evaluating scintillator performance, care must be taken The fluor in our experimental poly(vinyl toluene) since this lifetime is affected greatly by the shape of the scintillator, 4,4"-bis(2-butyloctyloxy)op-quaterphenyl, instrumental response curve. In fact, the lifetimes Zxo90 showed a considerably longer decay time (r t = !.87 ns) for Naton 136 in this paper are about 10% less than than it did when dissolved in toluene, in which case previously reported5), owing to the greater accuracy rl = 1.27 ns4). Hence, according to our preparation afforded by the RCA 8850 PM tube for detecting the procedure, it does not compete effectively with several single photons. However, the lifetimes z~ and z2 are of the commercially available scintillators. unaffected because the shape of the instrument resoThe surprising discrepancies between the two lution function is taken into account in computing samples of NE 111 are outside of any experimental these parameters. error. The values obtained from the (B) sample of Care was taken to ensure that the measurements NE 111 are in good agreement with those given by were taken for truly single-photon response. Only after Birks9), whereas the values obtained from the (A) additional light filtering with neutral density optical sample of NE I 11 agree closely with those reported by filters before the stop PM yielded no change in the Kunze and Langkaul°). Consistent with the discrecalculated values were the lifetimes reported. Increases pancies, Binkert et al. I1) reported a decay lifetime of of up to 1 ns for r~ were observed for a 100-fold 1.75+0.08 ns for NE 111 which is to be contrasted decrease in the light intensity reaching the stop PM, with the data listed for the scintillator in table 1. The even though a start-stop ratio greater than l0 was origin of these large variations between samples is not maintained in all cases. Also, except where noted, the clear. We did not observe appreciable differences in the reported lifetimes represent the average of several light output with either sample of NE 11 I. The manudeterminations on at least two different samples of facturer states the light output to be 55% of anthracene. scintillator material. The results are reproducible to It is of interest that the value of ~9o for the (A) sample 5%. o f N E 111 is close to that for NE 2115). We recommend The experimental plastic scintillator was prepared as that an independent "in-house" experimental detera right circular cylinder. One gram of 4,4"-bis(2- mination of the decay time of plastic scintillators is butyioctyloxy)-p-quaterphenyl was dissolved in 0.3 mol desirable if they are to be used for start-stop timing with of distilled vinyl toluene. The solution was deaerated high resolution. It is to be stressed that we observed by the freeze-vacuum-thaw technique, held at 100°C excellent reproducibility between different samples of for 72h, and then heated to 170°C for 24 h. The the other commercial scintillators reported in table 1. resulting poly(vinyl toluene) scintillator was cooled and removed from the glass container. Although slightly References 1) A. Z. Schwarzschild a n d E. K. W a r b u r t o n , Ann. Rev. Nucl. cloudy in appearance, it was clear enough to allow Sci. 18 (1968) 265. accurate scintillator decay measurements. All other "~) B. Bengtson and M. Moszynski, Nucl. Instr. and Meth. 81 samples were obtained commercially. (1970) 169. TABLE I
Scintillation decay parameters o f several plastic scintillators.
3. Results and discussion
The results of the lifetime measurements on the scintillators studied are summarized in table 1. It is clear that the new scintillator, Pilot U, with a z t of
a) M. Cocchi a n d A. Rotz, Nucl. Instr. and Meth. 29 (1964) 45. 4) F. Lynch, I E E E T r a n s . Nucl, Sci. NS-15 (1968) 102. ~) R. M. Lambrecht, T. M. Kelly a n d J. A. Merrigan, C h e m . Instr. 2 (1970) 2. 6) R. M. Lambrecht, T. M. Kelly a n d J. A. Merrigan, J. Phys. C h e m . 74 (1970) 2222.
COMPARISON OF PLASTIC S C I N T I L L A T O R S 7) j. Kirkbride, E. C. Yates and D. G. Crandall, Nucl. Instr. and Meth. 52 0967) 293. a) R. Scott and T. M. Kelly, unpublished program (1971), based on the method of W. H. Lawton and E. A. Sylvestre, Technometrics 13 (1971) 461.
235
9) j. B. Birks, J. Phys. B, Ser. 2, 1 (1968) 946. 10) R. Kunze and R. Langkau, Nucl. Instr. and Meth. 91 0971) 667. 11) T. Binkert, H. P. Tschang and P. E. Zinsli, J. Luminescence 5 (1972) 187.
INSTRUMENTS
AND
METHODS
Io9 (I973) 233-235;
©
NORTH-HOLLAND
PUBLISHING
CO.
C O M P A R I S O N OF P L A S T I C S C I N T I L L A T O R S W I T H N A N O S E C O N D L I F E T I M E S * T. M. K E L L Y a n d J . A .
MERRIGAN
Research Laboratories, Eastman Kodak Company, Rochester, N. Y. 14650, U.S.A. R. M. L A M B R E C H I "
Chemistry Department, Brookhaven National Laboratory, Upton, N.Y. 11973, U.S.A. Received 8 J a n u a r y 1973 T h e scintillation lifetimes o f Pilot U, M a n d B, N a t o n 136, N E 111 and an experimental scintillator are c o m p a r e d . T h e
new Pilot U scintillator exhibits the fastest decay time (r 1 = 1.36 ns) yet obtained for a plastic scintillator.
1. Introduction Nuclear or positron lifetime measurements in the nanosecond region often require the use of plastic scintillators to detect the timing signalst). Previous publications have shown the importance of the shape of the light pulse from the plastic scintillator in determining the ultimate resolution of nanosecond timing spectrometerst-3). Naton 136 (also known as KL 236) has in the past often been the plastic scintillator of choice. Although a small number of liquid scintillators*) have been found to have very short scintillation lifetimes, a solid scintillator is more desirable for timing measurements particularly if it is characterized by subnanosecond rise and decay times and a narrow full width at one-half maximum. We have measured the scintillation lifetimes of the recently available Pilot U, and an experimental plastic scintillator whose composition was based on the results from liquid scintillators, and compared them to the scintillation lifetimes of Naton 136, Pilot B, Pilot M and NE I11.
The general shape of the scintillation decay curves is illustrated in fig. 1. The decay of the light signal can be described as the sum of two exponential components,
2. Experimental The mean scintillation lifetimes (1.44 /1/2) w e r e measured by the fast-slow coincidence method of start-stop timing coupled with single photon detection. The subnanosecond decay spectrometer has been fully described elsewhereS'6). The start signal was derived from the scintillator under study, which was optically coupled to an RCA 8575 photomultiplier tube (PM). The detection of a single photon by an RCA 8850 PM was used as the stop signal. The scintillators were excited by 6°Co g a m m a radiation with a 20% window on the start side. The instrumental response function, measured by the Cherenkov radiation techniqueT), was 1.0 ns full width at one-hlaf maximum. * Research performed in part under the auspices of the U.S. Atomic Energy Commission. 233
f tL = "~ e x p ( - ~i'q) + " - - e x p ( - t / T 2 ) .
I(t)
"1~1
(1)
"C2
We have analyzed the curves by a nonlinear regression technique to obtain the values for the mean lifetimes, r 1 and r 2, and the fraction of the decay curve, J~, which has the mean lifetime rz 8). Only f2 is reported since f l +f2 = 1. We also report the mean lifetime of the decay from 90% to 10% of the maximum light intensity, Z~o.9° This parameter was derived from a simple weighted least-squares fitto the data in the region from 90% to 10% of the maximum of the decay spectrum. Although this parameter may be of interest to some in
I04~
1
:
:
!
Pdol
"
e~'~'g~ili~i~
o P,lot B Noton-136
olm
103'
'~
. . . . .
U
•
I
--i
°~-" %~°
IP
t, ~o
,..
~'~%,~ • ao
,
8
~ ~
! 0
1 2
4- . . . . .
#,
1 8
1_ 10
.'%..
i . . . . ~2
14
Time (n sec)
Fig. 1. Scintillation decay spectra for Pilot U, Pilot B and Naton 136 scintillators excited by 6°Co gamma rays as measured by the single-photon technique (background subtracted).
234
T.M.
K E L L Y et al.
1.36 ns, has a significantly shorter decay time than any of the other commercially available materials. In fact, a close inspection of fig. 1 reveals a full width at onehalf maximum about 15% less, and the rise (or energy Scintillator 171(ns) T2 (ns) f2 r9o0 (ns) transfer) time is faster for the Pilot U than for the other plastic scintillators. Because of these properties, Pilot Pilot U 1.36 12.5 0.12 1.54 Experimental 1.87 I 1.5 0.12 1.96 U should be considered and evaluated as a scintillator N a t o n 136 1.70 12.5 0.12 1.90 for fast timing applications. Pilot U contains a wavePilot B 1.69 11.5 0.13 1.92 length shifter and the pulse height is stated by the Pilot M 2.26 14.7 0.16 2.64 manufacturer to be 68% of anthracene. The 12% N E 111 (A) 2.27 17.1 0.12 2.32 intensity, and the 12.5 ns lifetime of the longer-lived N E I l l (B) 1.66 12.5 0.12 1.85 component on Pilot U is comparable to that observed with the other scintillators. evaluating scintillator performance, care must be taken The fluor in our experimental poly(vinyl toluene) since this lifetime is affected greatly by the shape of the scintillator, 4,4"-bis(2-butyloctyloxy)op-quaterphenyl, instrumental response curve. In fact, the lifetimes Zxo90 showed a considerably longer decay time (r t = !.87 ns) for Naton 136 in this paper are about 10% less than than it did when dissolved in toluene, in which case previously reported5), owing to the greater accuracy rl = 1.27 ns4). Hence, according to our preparation afforded by the RCA 8850 PM tube for detecting the procedure, it does not compete effectively with several single photons. However, the lifetimes z~ and z2 are of the commercially available scintillators. unaffected because the shape of the instrument resoThe surprising discrepancies between the two lution function is taken into account in computing samples of NE 111 are outside of any experimental these parameters. error. The values obtained from the (B) sample of Care was taken to ensure that the measurements NE 111 are in good agreement with those given by were taken for truly single-photon response. Only after Birks9), whereas the values obtained from the (A) additional light filtering with neutral density optical sample of NE I 11 agree closely with those reported by filters before the stop PM yielded no change in the Kunze and Langkaul°). Consistent with the discrecalculated values were the lifetimes reported. Increases pancies, Binkert et al. I1) reported a decay lifetime of of up to 1 ns for r~ were observed for a 100-fold 1.75+0.08 ns for NE 111 which is to be contrasted decrease in the light intensity reaching the stop PM, with the data listed for the scintillator in table 1. The even though a start-stop ratio greater than l0 was origin of these large variations between samples is not maintained in all cases. Also, except where noted, the clear. We did not observe appreciable differences in the reported lifetimes represent the average of several light output with either sample of NE 11 I. The manudeterminations on at least two different samples of facturer states the light output to be 55% of anthracene. scintillator material. The results are reproducible to It is of interest that the value of ~9o for the (A) sample 5%. o f N E 111 is close to that for NE 2115). We recommend The experimental plastic scintillator was prepared as that an independent "in-house" experimental detera right circular cylinder. One gram of 4,4"-bis(2- mination of the decay time of plastic scintillators is butyioctyloxy)-p-quaterphenyl was dissolved in 0.3 mol desirable if they are to be used for start-stop timing with of distilled vinyl toluene. The solution was deaerated high resolution. It is to be stressed that we observed by the freeze-vacuum-thaw technique, held at 100°C excellent reproducibility between different samples of for 72h, and then heated to 170°C for 24 h. The the other commercial scintillators reported in table 1. resulting poly(vinyl toluene) scintillator was cooled and removed from the glass container. Although slightly References 1) A. Z. Schwarzschild a n d E. K. W a r b u r t o n , Ann. Rev. Nucl. cloudy in appearance, it was clear enough to allow Sci. 18 (1968) 265. accurate scintillator decay measurements. All other "~) B. Bengtson and M. Moszynski, Nucl. Instr. and Meth. 81 samples were obtained commercially. (1970) 169. TABLE I
Scintillation decay parameters o f several plastic scintillators.
3. Results and discussion
The results of the lifetime measurements on the scintillators studied are summarized in table 1. It is clear that the new scintillator, Pilot U, with a z t of
a) M. Cocchi a n d A. Rotz, Nucl. Instr. and Meth. 29 (1964) 45. 4) F. Lynch, I E E E T r a n s . Nucl, Sci. NS-15 (1968) 102. ~) R. M. Lambrecht, T. M. Kelly a n d J. A. Merrigan, C h e m . Instr. 2 (1970) 2. 6) R. M. Lambrecht, T. M. Kelly a n d J. A. Merrigan, J. Phys. C h e m . 74 (1970) 2222.
COMPARISON OF PLASTIC S C I N T I L L A T O R S 7) j. Kirkbride, E. C. Yates and D. G. Crandall, Nucl. Instr. and Meth. 52 0967) 293. a) R. Scott and T. M. Kelly, unpublished program (1971), based on the method of W. H. Lawton and E. A. Sylvestre, Technometrics 13 (1971) 461.
235
9) j. B. Birks, J. Phys. B, Ser. 2, 1 (1968) 946. 10) R. Kunze and R. Langkau, Nucl. Instr. and Meth. 91 0971) 667. 11) T. Binkert, H. P. Tschang and P. E. Zinsli, J. Luminescence 5 (1972) 187.