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Fast delayed coincidence technique: The XP1020 photomultiplier and limits of resolving times due to scintillator characteristics
NUCLEAR
INSTRUMENTS
AND
METHODS
31
(1964)
71-76;
0
NORTH-HOLLAND
PUBLISHING
FAST DELAYED COINCIDENCE TECHNIQUE : THE XP1020 PHOTOMULTIPLIER OF RESOLVING TIMES DUE TO SCINTILLATOR CHARACTERISTICS? G. PRESENT,
A. SCHWARZSCHILD*,
Department
AND
CO.
LIMITS
I. SPIRN and N. WOTHERSPOON
of Physics, New York University,
New York
Received 18 June 1964
Tests of the new XPlO20 photomultipliers in a delayed coincidence apparatus using plastic scintillators are described. Prompt resolution curves were measured as a function of energy loss in the scintillators and triggering levels of the fast discriminators used to process the multiplier outputs. The best resolution obtained with ~1 MeV loss in each scintillator was 2.1 x lo-10 sec. (fwhm). The effect of finite growth and decay
times of the light output from commonly used the ultimate resolving time is considered. It is growth time, due to finite transfer times in the a drastic effect upon the available time resolution the limiting factor in obtainable resolution with multipliers.
1. Introduction
For these reasons it seemed valuable to perform measurements with these photomultipliers in a working coincidence system. In determining the optimum working conditions for the multipliers, we have performed measurements suggested by the theoretical work of Gatti and Sveltoa) and identical to those described in detail in (S). The results of these tests suggest that the theory of Gatti and Svelto does not properly describe the coincidence system. The source of the discrepancy is that the theory assumes a simple exponential decay for the scintillator light output. In recent years the mechanism of energy transfer and light emission from the scintillators has been studied in detail. The theoretical work of Kallmann and Brucker3) and the recent measurements of Koechlin4) have shown that there is an appreciable time required for energy transfer from the ionizing radiation to the light emitting centers of the scintillator. Thus the light emitted from organic scintillators does not follow a pure exponential decay. The finite transfer time, which produces an effective rise time in the scintillator light output function, has a profound effect upon the ultimate time resolutions obtainable in coincidence systems. We have combined the experimentally determined light output time functions of Koechlin and the theoretical considerations of El-Wahab and Kanes) regarding the statistical processes involved in the coincidence arrangement, to calculate theoretical resolving times neglecting any effects of the photomultiplier. The results of this
The ultimate resolving time of a fast coincidence system is determined mainly by the time constants of the light output from the scintillator and the transit time variation of pulses in the photomultiplier. The technology of circuitry design is sufficiently advanced so that at present the electronics external to the photomultiplier contributes negligible width to the ultimate resolving times obtainable. A survey of the pertinent theoretical treatments of fast concidence systems and the experimental results obtained under various conditions were given by Schwarzschildl) (hereafter referred to as (S)). Since the original writing of that paper (1961) there has been very little improvement of the techniques. Recently, however, a new photomultiplier has become available which is appreciably faster than the one used in tests described in (S). This paper will report on tests made of the time resolution of a fast delayed coincidence system, utilizing the XP1020 Photomultiplier manufactured by La Radiotechnique of Paris. It is common to attempt to study those characteristics of the photomultiplier operation which are contained in the “single electron response”, such as the full width at half maximum of such pulses, the jitter time and the variation of transit time for light incident upon different parts of the photocathode surface. In principle, knowledge of these properties should enable one to predict the possible resolving time of a coincidence system using these multipliers. However, in practice the measurements are extremely difficult to perform, especially as photomultiplier design improves. In addition, the existing theories of the effects of multiplier time dispersion and phosphor properties upon available time resolution give only qualitative agreement with experiments.
scintillators on shown that the scintillator, has and is probably the new photo-
This work partially supported by the U.S. Atomic Energy Commission and the U.S. Army Signal Corps Engineering Laboratories, Fort Monmouth, New Jersey. Permanent address : Brookhaven National Laboratory, Upton, Long Island, New York. 71
72
G. PRESENT et al. slow aynode pulse XPIO2O p ate photomultiplier _
variable 1~7 delay /
_
"\ / scintillators
X'PIO20 slow dynode pulse photomultipfier ~plate
L~
i //clip
j
ping ~
~
~-me topulse height converter ~.
(model ~ IO5} Ioutput
j
1
channel
[.,~t..... triple co,ne. ]
~
__l
J slowC3~.... )triplecoine[ f l
GATE
_DISPLAY
MULTI- CHANNEL
analyzer
j
Fig. 1. Block diagram of delayed coincidence system.
calculation show that the resolving time obtained with the XPI020 photomultiplier is close to the limit determined by the scintillator properties alone.
frequency compensated attenuators (50 g~) at the discriminator input. Time calibrations of the converter system are obtained with the use of General Radio air-dielectric fixed lines.
2. The coincidence system 2.2. PHOTOMULTIPLIER AND SCINTILLATOR
2.1. THE ELECTRONICS Our electronic system is shown in fig. 1. The components are all commercially available 6) and are completely transistorized. The slow circuitry such as linear amplifiers, single channel analyzers and slow coincidence system are all of the Chase 7) design. The fast discriminators and the time to pulse height converter are similar in design to those described by Sugarman et al.S). The tunnel diode discriminator triggers on current pulses of 2 m A and its output is a constant height pulse which is delay line clipped to about 10 nsec. before entering the time to pulse height converter. Tests were made of the overall electronic resolution. If both discriminators are triggered (at equal levels) by the output of a single multiplier, the resulting time spectrum exhibits a width of less than 65 picosec. Since the level of triggering of the fast discriminators is fixed, variations in the effective signal triggering level are effected by insertion of fixed,
The photomultiplier dynode voltage division was performed according to the manufacturers specifications. A negative high voltage supply was used, thus enabling relatively high bypass capacity between the last few dynodes and ground. The multiplier and phosphor were wrapped in aluminum foil which was connected to the photocathode potential. The voltage divider carried a static current of 4 mA at 2000 V. The focussing electrodes were adjusted for all measurements to yield maximum signal output but extensive measurements were not performed to determine their effect upon the time or pulse height resolution. The slow signal used for pulse height analysis was derived from the eighth dynode. (There are a total of twelve dynodes in the XP1020.) The derivation of the pulse from the multiplier plate was performed in an unconventional manner for this phototube. The anode signal leaves this multiplier in a coaxial connector. The shield of this connector is
THE
XPI020 P H O T O M U L T I P L I E R
t DYNODE DIVIDER f _ TOGRID
Ii
-
_~ ~50K~ [I ~>
o.i/~r
T ~~ ~ /'L~ MULTIPLIER_ /
I
. I PLATE 0 05~u.F ~
DYNODE ~12 f\
'
/-
PHOTOMULT, PLIE~ COAXIAL CONNECTER
T27.0~ O
50,0, COAX.
DISCRIMINATOR
<
~509. ._.~ov~
Fig. 2. O u t p u t circuit for XP1020 photomultiplier.
connected to the last dynode and must be at a negative potential with respect to the anode. The manufacturer suggests that the anode voltage supply therefore be an independent positive voltage source and that the coaxial shield be grounded. The coaxial output is to be fed directly into a 100 ~2 transmission line. It is claimed that this arrangement produces less ringing than conventional tube design. Since we are using the anode signal to trigger a discriminator which is a regenerative device and is unaffected by the pulse shape beyond the initial triggering time, it was felt that this "coaxial h o o k u p " was overly cumbersome for our application. A diagram of our anode circuitry is shown in fig. 2. The phosphor used in all our measurements was Naton 136. The scintillator was machined, polished and optically coupled to the photomultiplier with silicone grease. 3. Measurements The measurements consist basically of a determination of the full width at half maximum of time spectra obtained as functions of the fraction of the integrated p.m. output pulse required to trigger the fast discriminators. The measurements were performed for several different total energy losses in the scintillator. In all cases narrow pulse height windows (--~20~) in both counters were used to minimize the effects of pulse amplitude variation and to define the energy loss in the scintillator. In all cases the mean energy loss in both scintillators was the same, and the fraction of the pulse output used for discriminator triggering was equal for both multipliers.
AND
LIMITS
OF R E S O L V I N G
TIMES
73
The theory behind such measurements and a more detailed description of the method, as well as the results of such tests with the Radiotechnique 56AVP multiplier are contained in (S). In fig. 3 is the narrowest time spectrum obtained at the optimum discrimination level using windows set near the upper edge of the Compton electron spectrum from Co 60 7-rays. The scintillators were cylinders, ¼ inch in diameter and 3 inch high. In fig. 4 are the results of the tests described above. The quantity C/R is the fraction of the total integrated pulse amplitude R, at which the fast discriminator is triggered. Qualitatively these curves have the characteristics predicted by the theory of Gatti and Svelto. The results presented are best appreciated by comparison with similarly measured curves for the 56 AVP type multiplier shown in fig. 5 of (S). The new multiplier shows minimum widths approximately 6 0 ~ of those for the 56 AVP. In addition, the optimum values of C/R are appreciably smaller than those obtained for the 5 6 A V P which were --~0.18. This reduction in the optimum value of C/R is expected for the improved single electron response of the phototube. TIME ( x l O * i ° s e c )
Io*
I° 3
WI/2=2.1XIO-I°sec
Ti/2= 2.9 xlO
-IIsec
o
z
ta3 O
io
~c
30
40 50 60 CHANNEL NUMBER
Fig. 3. P r o m p t s p e c t r u m obtained with Co 6° source.
G. PRESENT et al.
74
2.5~
/ 9 3 0 keV
Y 3
,5!
,oo
! 0
i
i
i
T
i
0.1
02
0.3 c/R
0.4
0.5
Fig. 4. Observed values o f w½ ~ / E vs C/R with different energy loss in plastic scintillator.
These measurements were performed at a total high voltage of 2150 V on each photomultiplier. Tests were also performed at increased high voltage (--~500 V higher) at the Co 6° Compton edge (930 keV loss) and showed very little change in either the minimum obtainable resolving time or the optimum value of CIR. The manufacturer's tests 9) of the multiplier show an improvement in rise time with increasing high voltage but this seems to have little effect on the time jitter obtained through the resolving time curves. This lack of improvement of resolving time with increased high voltage has also been consistently observed with the 56 AVP photomultipliers. Tests were also performed using a larger diameter scintillator. The XP1020 shows extremely good characteristics with respect to variations of time with position of irradiation of the photocathode surface. Using scintillators of 1¼ inch in diameter by 1¼ inch high, the resolving curves obtained with 930 keV loss in the phosphor show a broadening of only 20%. This result again indicates a significant improvement over the 56 AVP. The results which we have obtained should not be considered to be the ultimate resolution obtainable
with these phototubes. The complex system of scintillator, multiplier voltage division and fast and slow pulse processing leaves the experimentalist an almost infinite variety of variables which may slightly affect the overall system capabilities. We have attempted to vary only a small number of these parameters and were guided in our choice of variables by past experience with other photomultipliers and some theoretical considerations. Also, we are in possession of only two phototubes of the type to be tested and these are among the earliest production models. Undoubtedly a selection of phototube, as well as improvements in their design and quality will permit slightly better time resolution than we have obtained. Although the curves of fig. 4 agree qualitatively with the theory of Gatti and Svelto, there seems to be some limitation of the resolving time other than those considered in the theory. In order to obtain a prediction of the resolving times from this theory the properties of the phototube single electron response must be known. The critical quantity2), epn, has not been measured for the XP1020. However, one may attempt to correlate the widths of the predicted minimum resolving times with the optimum value of CIR. Such a correlation suggests that in going from the 56 AVP where the measured optimum value of C/R is ,--0.18 to the XP1020 where the measured optimum value of C/R is ~ 0 . 0 7 there should be an improvement of at least a factor of three in resolving time. However, experimentally only a ~ 3 0 % improvement is realized. We believe that the limitation is due to the properties of the scintillators and a quantitative estimate of these effects is contained in the next section.
4. Optimum resolving times and scintillator decay properties The theoretical treatments of coincidence systems utilizing scintillating phosphors have always assumed that the light output of the phosphor was a purely exponentially decaying function of time measured from the time of entry of the ionizing radiation into the scintillator. In the old simple theory of Post and Schiffl°), the effect of time dispersion in the photomultiplier was completely neglected; they calculated the dispersion in the arrival time of the first photoelectron from a light pulse which eventually yields N photo-electrons. They showed that this dispersion was of the order of ~s/N where zs is the scintillator light output decay constant. For 0.5 MeV energy loss in the scintillator N is of the order of 100 so that the simple Post-Schiff theory predicts resolving times of the order of 3 × 10-11 sec (for two scintillators). The
THE
XP1020
PHOTOMULTIPLIER
Gatti and Svelto theory of fast coincidence resolving times, which includes the effects of finite time dispersion of the photomultipliers, also assumes pure exponential decay for the light output of the phosphor. A comparison of this theory with experiment is given in (S) and a similar comparison is given in the work of Bartl and Weinzierlll). It is our purpose here to point out a serious fault in these theoretical treatments which has become clear in view of recent experimental studies of the scintillation processl~). It is well known that organic scintillators, both of the solid plastic type as well as liquids are composed of several materials. The plastics consist of a substrate material (commonly polystyrene or polyvinyltoluene), a light emitting solute substance (often PBD) and a wavelength shifter (POPOP) to convert the scintillant light to a frequency to which the material itself as well as the photomultiplier glass is transparent. It has been long known that the process of light emission from these scintillators would thus involve transfer of energy from the substrate, where most of the ionization loss occurs, to the scintillator and finally to the wavelength shifter. The light output of the scintillator therefore is not expected to be characterized by a pure exponential function. The theoretical treatments of coincidence systems always assumed that these transfer processes were sufficiently fast that they could be neglected. However, recent measurements of the light output functions 12) indicate that the transfer times are only several times shorter than the apparent light "decay time" (as observed at long times after the initial energy deposition in the scintillator). For several commonly used liquid and plastic scintillators, Koechlin showed that these light output functions can be expressed as the difference between two exponentials. This is exactly what one would expect for a succession of two exponential decays in cascade with lifetimes ~1 and ~2. The time constants found were typically 0.3 to 1.0 nsec. for the shorter one and 1.6 to 6.0 nsec. for the longer one. Since the transfer times are probably even longer than photomultiplier jitter times, we wish to consider their effect upon the resolving time. To analyze the effects of such a light output function, we assume that any time spreads associated with the photomultiplier and electronics are negligible. It is assumed that an individual pulse from the scintillator results in a total of N separate countable electrons emitted from the photocathode surface. We also assume that a time signal may be derived from each of two scintillator-multiplier combinations corresponding to the emission time of the n th electron
A N D L I M I T S OF R E S O L V I N G
75
TIMES
(n < N) from each cathode. The prompt resolution curve of this idealized system is characterized by a full width at half m a x i m u m resulting from the variance in emission times of the nth electron in the two scintillator-multiplier combinations. An exact calculation of this time dispersion (with two mean lives) has been reported by E1-Wahab and Kane la) as a function of n, N and 31/3z. (It should be noted that for this idealized system n/N is the same quantity as C/R used in the previous section). The result of the introduction of this additional lifetime into the scintillation process is a drastic change in the minimum time dispersion obtainable even though the first lifetime (corresponding to energy transfer), is appreciably shorter than the second lifetime. TABLE 1 n=l
0 0.5 1
I i
0.0067 0.12 0.17
i
2
10
0.023 0.13 0.19
0.073 0.16 0.23
Width o f the time dispersion curve, r~}~, for two scintillators timed on the n th photoelectron ( N - 100) as a function of • 1/$2 and n. The width ¢o~_is given in units o f r2. (The values are obtained f r o m the w o r k o f E l - W a h a b and Kane5)).
Table 1 presents the results of the calculation of E1-Wahab and Kane for N ~ 100. The large difference in the behavior of ~ with n as one introduces a second decay time is obvious from the table. For rl/32 = 0 the value of ~% decreases an order of magnitude for n changing from 10 to 1. However as soon as another decay constant is introduced (i.e., 31 @ 0), little improvement is obtained in reducing n. F r o m table 1 we may now calculate an expected resolving time for this idealized system. The parameters rl and 32 for Naton 136 scintillator have been measured by A. Raviart and Y. Koechlin14). Their results are that r2 = 1.6 × 10 -9 sec and rl = ½ 32. Then o~ for N = 100 and n ~ 10 is o~½ z 2.6 × 10 -10 sec. Although the efficiency of the scintillator and p h o t o cathode are not well known one estimates that N = 100 corresponds to about 500 keV energy loss in the scintillator. Our experimentally determined optimum resolving time for 500 keV (2.8 × 10 -1° sec) is very close to this number. The fact that the experimental resolving time deteriorates as C/R is decreased below ,~0.07 is probably due to the effects of the finite time
76
G. PRESENT et al.
spreads in the real multiplier as suggested b y the w o r k of G a t t i a n d Svelto as well as the effects of multiplier noise pulses which b e c o m e significant as triggering levels are reduced.
5. Conclusions W i t h the use of XP1020 p h o t o m u l t i p l i e r s we have o b t a i n e d an i m p r o v e m e n t in resolving time of a b o u t 30~o c o m p a r e d to other systems. Results of theoretical considerations of the effect of transfer times in the scintillator a n d the a t t e n d a n t g r o w t h a n d decay of the light o u t p u t indicate t h a t this is p r o b a b l y the limiting factor in o b t a i n a b l e time resolution. R e s e a r c h on the d e v e l o p m e n t of scintillators with very s h o r t transfer times w o u l d be of great value for the fast coincidence m e t h o d . Very crude estimates indicate t h a t time resolutions u n d e r a tenth n a n o - s e c o n d m a y be o b t a i n able with XP1020 p h o t o m u l t i p l i e r s if the transfer times in scintillators could be a p p r e c i a b l y reduced. The a u t h o r s wish to t h a n k Prof. W . B r a n d t for his c o n s t a n t interest in this w o r k a n d Dr. Y. Koechlin for discussions on the light o u t p u t functions of organic scintillators.
Note added in proof: W e have recently received a c o m m u n i c a t i o n f r o m G a t t i a n d Svelto to the effect t h a t they have f o u n d significant m a t h e m a t i c a l errors in their earlier paper2). They n o w find t h a t the m i n i m a exhibited in their calculated curves o f width vs C/R were spurious; their new t h e o r y predicts m o n o t o n i c b e h a v i o u r o f this function with a m i n i m u m value at C/R = 0. The e x p e r i m e n t a l results o b t a i n e d by us (and several o t h e r e x p e r i m e n t a l groups) however, exhibit m i n i m a as suggested in their earlier paper. O n the basis o f calculations presented in G a t t i a n d S v e l t o ' s new w o r k 15) we suspect t h a t the origin o f the e x p e r i m e n t a l l y d e t e r m i n e d m i n i m a m a y be i n t i m a t e l y c o n n e c t e d with the g r o w t h a n d decay o f the light o u t p u t function for the scintillator as discussed above.
W e wish to p o i n t o u t t h a t due to the errors in G a t t i a n d Svelto's original calculation s o m e o f the r e m a r k s at the end o f section 3 o f this p a p e r m a y be unjustified. However, we emphasize t h a t all o u r calculations given in s e c t i o n 4 are still correct. O u r calculations in s e c t i o n 4 in which we d o n o t include any effects o f multiplier time dispersion yield resolving times close to those m e a s u r e d . W e therefore still infer t h a t the multiplier time spreads have only a small effect on the e x p e r i m e n t a l l y d e t e r m i n e d resolutions.
References 1) A. Schwarzschild, Nucl. Instr. and Meth. 21 (1963) 1. 2) E. Gatti and V. Svelto, Nucl. Instr. and Meth. 4 (1959) 189. 3) H. Kallmann and G.J. Brucker, Phys. Rev. 108 (1957) 1122, H. Kallmann, Phys. Rev. 117 (1960) 36. 4) y. Koechlin, Thesis, Paris (1961); C.E.A. Report No. 2194. 5) M.A. El-Wahab and J.V. Kane, Nucl. Instr. and Meth. 15 (1962) 15. 6) The XP1020 photomultipliers are available from Philips, except in the U.S.A. where they are sold by Arnperex, Hicksville, N.Y. Model 101 dual discriminator and Model 105 time to pulse-height converter are manufactured by Chronetics, Inc., Yonkers, N.Y. 7) R.L. Chase, Rev. Sci. Instr. 31 (1960) 945. 8) R. Sugarrnan, F.C. Merritt and W.A. Higinbotham, unpublished report; BNL 711 (T-248) (1962). 9) G. Pietri, private communication. 10) R.F. Post and L.I. Schiff, Phys. Rev. 80 (1950) 1113. u) W. Bartl and P. Weinzierl, Rev. Sci. Instr. 34 (1963) 252. t2) y. Koechlin, A. Raviart and M. Furst, Compt. Rend. 256 (1963) 1096. 1~) The paper of EI-Wahab and Kane presents a calculation, using a simple model of the overall resolution of the scintillator-multiplier system. Their assumption was that the scintillator light output function was purely exponential and that the time dispersions in the photomultiplier could be approximated by another exponential decay. They then treated the statistical processes in an exact manner and computed time dispersions with a fast computer. Although their model of the system is physically very different from ours, the mathematical treatment is identical so that their numerical results are directly applicable. 14) A. Raviart and Y. Koechlin, private communication. 15) E. Gatti and V. Svelto, Nucl. Instr. and Meth. 30 (1964) 213.
INSTRUMENTS
AND
METHODS
31
(1964)
71-76;
0
NORTH-HOLLAND
PUBLISHING
FAST DELAYED COINCIDENCE TECHNIQUE : THE XP1020 PHOTOMULTIPLIER OF RESOLVING TIMES DUE TO SCINTILLATOR CHARACTERISTICS? G. PRESENT,
A. SCHWARZSCHILD*,
Department
AND
CO.
LIMITS
I. SPIRN and N. WOTHERSPOON
of Physics, New York University,
New York
Received 18 June 1964
Tests of the new XPlO20 photomultipliers in a delayed coincidence apparatus using plastic scintillators are described. Prompt resolution curves were measured as a function of energy loss in the scintillators and triggering levels of the fast discriminators used to process the multiplier outputs. The best resolution obtained with ~1 MeV loss in each scintillator was 2.1 x lo-10 sec. (fwhm). The effect of finite growth and decay
times of the light output from commonly used the ultimate resolving time is considered. It is growth time, due to finite transfer times in the a drastic effect upon the available time resolution the limiting factor in obtainable resolution with multipliers.
1. Introduction
For these reasons it seemed valuable to perform measurements with these photomultipliers in a working coincidence system. In determining the optimum working conditions for the multipliers, we have performed measurements suggested by the theoretical work of Gatti and Sveltoa) and identical to those described in detail in (S). The results of these tests suggest that the theory of Gatti and Svelto does not properly describe the coincidence system. The source of the discrepancy is that the theory assumes a simple exponential decay for the scintillator light output. In recent years the mechanism of energy transfer and light emission from the scintillators has been studied in detail. The theoretical work of Kallmann and Brucker3) and the recent measurements of Koechlin4) have shown that there is an appreciable time required for energy transfer from the ionizing radiation to the light emitting centers of the scintillator. Thus the light emitted from organic scintillators does not follow a pure exponential decay. The finite transfer time, which produces an effective rise time in the scintillator light output function, has a profound effect upon the ultimate time resolutions obtainable in coincidence systems. We have combined the experimentally determined light output time functions of Koechlin and the theoretical considerations of El-Wahab and Kanes) regarding the statistical processes involved in the coincidence arrangement, to calculate theoretical resolving times neglecting any effects of the photomultiplier. The results of this
The ultimate resolving time of a fast coincidence system is determined mainly by the time constants of the light output from the scintillator and the transit time variation of pulses in the photomultiplier. The technology of circuitry design is sufficiently advanced so that at present the electronics external to the photomultiplier contributes negligible width to the ultimate resolving times obtainable. A survey of the pertinent theoretical treatments of fast concidence systems and the experimental results obtained under various conditions were given by Schwarzschildl) (hereafter referred to as (S)). Since the original writing of that paper (1961) there has been very little improvement of the techniques. Recently, however, a new photomultiplier has become available which is appreciably faster than the one used in tests described in (S). This paper will report on tests made of the time resolution of a fast delayed coincidence system, utilizing the XP1020 Photomultiplier manufactured by La Radiotechnique of Paris. It is common to attempt to study those characteristics of the photomultiplier operation which are contained in the “single electron response”, such as the full width at half maximum of such pulses, the jitter time and the variation of transit time for light incident upon different parts of the photocathode surface. In principle, knowledge of these properties should enable one to predict the possible resolving time of a coincidence system using these multipliers. However, in practice the measurements are extremely difficult to perform, especially as photomultiplier design improves. In addition, the existing theories of the effects of multiplier time dispersion and phosphor properties upon available time resolution give only qualitative agreement with experiments.
scintillators on shown that the scintillator, has and is probably the new photo-
This work partially supported by the U.S. Atomic Energy Commission and the U.S. Army Signal Corps Engineering Laboratories, Fort Monmouth, New Jersey. Permanent address : Brookhaven National Laboratory, Upton, Long Island, New York. 71
72
G. PRESENT et al. slow aynode pulse XPIO2O p ate photomultiplier _
variable 1~7 delay /
_
"\ / scintillators
X'PIO20 slow dynode pulse photomultipfier ~plate
L~
i //clip
j
ping ~
~
~-me topulse height converter ~.
(model ~ IO5} Ioutput
j
1
channel
[.,~t..... triple co,ne. ]
~
__l
J slowC3~.... )triplecoine[ f l
GATE
_DISPLAY
MULTI- CHANNEL
analyzer
j
Fig. 1. Block diagram of delayed coincidence system.
calculation show that the resolving time obtained with the XPI020 photomultiplier is close to the limit determined by the scintillator properties alone.
frequency compensated attenuators (50 g~) at the discriminator input. Time calibrations of the converter system are obtained with the use of General Radio air-dielectric fixed lines.
2. The coincidence system 2.2. PHOTOMULTIPLIER AND SCINTILLATOR
2.1. THE ELECTRONICS Our electronic system is shown in fig. 1. The components are all commercially available 6) and are completely transistorized. The slow circuitry such as linear amplifiers, single channel analyzers and slow coincidence system are all of the Chase 7) design. The fast discriminators and the time to pulse height converter are similar in design to those described by Sugarman et al.S). The tunnel diode discriminator triggers on current pulses of 2 m A and its output is a constant height pulse which is delay line clipped to about 10 nsec. before entering the time to pulse height converter. Tests were made of the overall electronic resolution. If both discriminators are triggered (at equal levels) by the output of a single multiplier, the resulting time spectrum exhibits a width of less than 65 picosec. Since the level of triggering of the fast discriminators is fixed, variations in the effective signal triggering level are effected by insertion of fixed,
The photomultiplier dynode voltage division was performed according to the manufacturers specifications. A negative high voltage supply was used, thus enabling relatively high bypass capacity between the last few dynodes and ground. The multiplier and phosphor were wrapped in aluminum foil which was connected to the photocathode potential. The voltage divider carried a static current of 4 mA at 2000 V. The focussing electrodes were adjusted for all measurements to yield maximum signal output but extensive measurements were not performed to determine their effect upon the time or pulse height resolution. The slow signal used for pulse height analysis was derived from the eighth dynode. (There are a total of twelve dynodes in the XP1020.) The derivation of the pulse from the multiplier plate was performed in an unconventional manner for this phototube. The anode signal leaves this multiplier in a coaxial connector. The shield of this connector is
THE
XPI020 P H O T O M U L T I P L I E R
t DYNODE DIVIDER f _ TOGRID
Ii
-
_~ ~50K~ [I ~>
o.i/~r
T ~~ ~ /'L~ MULTIPLIER_ /
I
. I PLATE 0 05~u.F ~
DYNODE ~12 f\
'
/-
PHOTOMULT, PLIE~ COAXIAL CONNECTER
T27.0~ O
50,0, COAX.
DISCRIMINATOR
<
~509. ._.~ov~
Fig. 2. O u t p u t circuit for XP1020 photomultiplier.
connected to the last dynode and must be at a negative potential with respect to the anode. The manufacturer suggests that the anode voltage supply therefore be an independent positive voltage source and that the coaxial shield be grounded. The coaxial output is to be fed directly into a 100 ~2 transmission line. It is claimed that this arrangement produces less ringing than conventional tube design. Since we are using the anode signal to trigger a discriminator which is a regenerative device and is unaffected by the pulse shape beyond the initial triggering time, it was felt that this "coaxial h o o k u p " was overly cumbersome for our application. A diagram of our anode circuitry is shown in fig. 2. The phosphor used in all our measurements was Naton 136. The scintillator was machined, polished and optically coupled to the photomultiplier with silicone grease. 3. Measurements The measurements consist basically of a determination of the full width at half maximum of time spectra obtained as functions of the fraction of the integrated p.m. output pulse required to trigger the fast discriminators. The measurements were performed for several different total energy losses in the scintillator. In all cases narrow pulse height windows (--~20~) in both counters were used to minimize the effects of pulse amplitude variation and to define the energy loss in the scintillator. In all cases the mean energy loss in both scintillators was the same, and the fraction of the pulse output used for discriminator triggering was equal for both multipliers.
AND
LIMITS
OF R E S O L V I N G
TIMES
73
The theory behind such measurements and a more detailed description of the method, as well as the results of such tests with the Radiotechnique 56AVP multiplier are contained in (S). In fig. 3 is the narrowest time spectrum obtained at the optimum discrimination level using windows set near the upper edge of the Compton electron spectrum from Co 60 7-rays. The scintillators were cylinders, ¼ inch in diameter and 3 inch high. In fig. 4 are the results of the tests described above. The quantity C/R is the fraction of the total integrated pulse amplitude R, at which the fast discriminator is triggered. Qualitatively these curves have the characteristics predicted by the theory of Gatti and Svelto. The results presented are best appreciated by comparison with similarly measured curves for the 56 AVP type multiplier shown in fig. 5 of (S). The new multiplier shows minimum widths approximately 6 0 ~ of those for the 56 AVP. In addition, the optimum values of C/R are appreciably smaller than those obtained for the 5 6 A V P which were --~0.18. This reduction in the optimum value of C/R is expected for the improved single electron response of the phototube. TIME ( x l O * i ° s e c )
Io*
I° 3
WI/2=2.1XIO-I°sec
Ti/2= 2.9 xlO
-IIsec
o
z
ta3 O
io
~c
30
40 50 60 CHANNEL NUMBER
Fig. 3. P r o m p t s p e c t r u m obtained with Co 6° source.
G. PRESENT et al.
74
2.5~
/ 9 3 0 keV
Y 3
,5!
,oo
! 0
i
i
i
T
i
0.1
02
0.3 c/R
0.4
0.5
Fig. 4. Observed values o f w½ ~ / E vs C/R with different energy loss in plastic scintillator.
These measurements were performed at a total high voltage of 2150 V on each photomultiplier. Tests were also performed at increased high voltage (--~500 V higher) at the Co 6° Compton edge (930 keV loss) and showed very little change in either the minimum obtainable resolving time or the optimum value of CIR. The manufacturer's tests 9) of the multiplier show an improvement in rise time with increasing high voltage but this seems to have little effect on the time jitter obtained through the resolving time curves. This lack of improvement of resolving time with increased high voltage has also been consistently observed with the 56 AVP photomultipliers. Tests were also performed using a larger diameter scintillator. The XP1020 shows extremely good characteristics with respect to variations of time with position of irradiation of the photocathode surface. Using scintillators of 1¼ inch in diameter by 1¼ inch high, the resolving curves obtained with 930 keV loss in the phosphor show a broadening of only 20%. This result again indicates a significant improvement over the 56 AVP. The results which we have obtained should not be considered to be the ultimate resolution obtainable
with these phototubes. The complex system of scintillator, multiplier voltage division and fast and slow pulse processing leaves the experimentalist an almost infinite variety of variables which may slightly affect the overall system capabilities. We have attempted to vary only a small number of these parameters and were guided in our choice of variables by past experience with other photomultipliers and some theoretical considerations. Also, we are in possession of only two phototubes of the type to be tested and these are among the earliest production models. Undoubtedly a selection of phototube, as well as improvements in their design and quality will permit slightly better time resolution than we have obtained. Although the curves of fig. 4 agree qualitatively with the theory of Gatti and Svelto, there seems to be some limitation of the resolving time other than those considered in the theory. In order to obtain a prediction of the resolving times from this theory the properties of the phototube single electron response must be known. The critical quantity2), epn, has not been measured for the XP1020. However, one may attempt to correlate the widths of the predicted minimum resolving times with the optimum value of CIR. Such a correlation suggests that in going from the 56 AVP where the measured optimum value of C/R is ,--0.18 to the XP1020 where the measured optimum value of C/R is ~ 0 . 0 7 there should be an improvement of at least a factor of three in resolving time. However, experimentally only a ~ 3 0 % improvement is realized. We believe that the limitation is due to the properties of the scintillators and a quantitative estimate of these effects is contained in the next section.
4. Optimum resolving times and scintillator decay properties The theoretical treatments of coincidence systems utilizing scintillating phosphors have always assumed that the light output of the phosphor was a purely exponentially decaying function of time measured from the time of entry of the ionizing radiation into the scintillator. In the old simple theory of Post and Schiffl°), the effect of time dispersion in the photomultiplier was completely neglected; they calculated the dispersion in the arrival time of the first photoelectron from a light pulse which eventually yields N photo-electrons. They showed that this dispersion was of the order of ~s/N where zs is the scintillator light output decay constant. For 0.5 MeV energy loss in the scintillator N is of the order of 100 so that the simple Post-Schiff theory predicts resolving times of the order of 3 × 10-11 sec (for two scintillators). The
THE
XP1020
PHOTOMULTIPLIER
Gatti and Svelto theory of fast coincidence resolving times, which includes the effects of finite time dispersion of the photomultipliers, also assumes pure exponential decay for the light output of the phosphor. A comparison of this theory with experiment is given in (S) and a similar comparison is given in the work of Bartl and Weinzierlll). It is our purpose here to point out a serious fault in these theoretical treatments which has become clear in view of recent experimental studies of the scintillation processl~). It is well known that organic scintillators, both of the solid plastic type as well as liquids are composed of several materials. The plastics consist of a substrate material (commonly polystyrene or polyvinyltoluene), a light emitting solute substance (often PBD) and a wavelength shifter (POPOP) to convert the scintillant light to a frequency to which the material itself as well as the photomultiplier glass is transparent. It has been long known that the process of light emission from these scintillators would thus involve transfer of energy from the substrate, where most of the ionization loss occurs, to the scintillator and finally to the wavelength shifter. The light output of the scintillator therefore is not expected to be characterized by a pure exponential function. The theoretical treatments of coincidence systems always assumed that these transfer processes were sufficiently fast that they could be neglected. However, recent measurements of the light output functions 12) indicate that the transfer times are only several times shorter than the apparent light "decay time" (as observed at long times after the initial energy deposition in the scintillator). For several commonly used liquid and plastic scintillators, Koechlin showed that these light output functions can be expressed as the difference between two exponentials. This is exactly what one would expect for a succession of two exponential decays in cascade with lifetimes ~1 and ~2. The time constants found were typically 0.3 to 1.0 nsec. for the shorter one and 1.6 to 6.0 nsec. for the longer one. Since the transfer times are probably even longer than photomultiplier jitter times, we wish to consider their effect upon the resolving time. To analyze the effects of such a light output function, we assume that any time spreads associated with the photomultiplier and electronics are negligible. It is assumed that an individual pulse from the scintillator results in a total of N separate countable electrons emitted from the photocathode surface. We also assume that a time signal may be derived from each of two scintillator-multiplier combinations corresponding to the emission time of the n th electron
A N D L I M I T S OF R E S O L V I N G
75
TIMES
(n < N) from each cathode. The prompt resolution curve of this idealized system is characterized by a full width at half m a x i m u m resulting from the variance in emission times of the nth electron in the two scintillator-multiplier combinations. An exact calculation of this time dispersion (with two mean lives) has been reported by E1-Wahab and Kane la) as a function of n, N and 31/3z. (It should be noted that for this idealized system n/N is the same quantity as C/R used in the previous section). The result of the introduction of this additional lifetime into the scintillation process is a drastic change in the minimum time dispersion obtainable even though the first lifetime (corresponding to energy transfer), is appreciably shorter than the second lifetime. TABLE 1 n=l
0 0.5 1
I i
0.0067 0.12 0.17
i
2
10
0.023 0.13 0.19
0.073 0.16 0.23
Width o f the time dispersion curve, r~}~, for two scintillators timed on the n th photoelectron ( N - 100) as a function of • 1/$2 and n. The width ¢o~_is given in units o f r2. (The values are obtained f r o m the w o r k o f E l - W a h a b and Kane5)).
Table 1 presents the results of the calculation of E1-Wahab and Kane for N ~ 100. The large difference in the behavior of ~ with n as one introduces a second decay time is obvious from the table. For rl/32 = 0 the value of ~% decreases an order of magnitude for n changing from 10 to 1. However as soon as another decay constant is introduced (i.e., 31 @ 0), little improvement is obtained in reducing n. F r o m table 1 we may now calculate an expected resolving time for this idealized system. The parameters rl and 32 for Naton 136 scintillator have been measured by A. Raviart and Y. Koechlin14). Their results are that r2 = 1.6 × 10 -9 sec and rl = ½ 32. Then o~ for N = 100 and n ~ 10 is o~½ z 2.6 × 10 -10 sec. Although the efficiency of the scintillator and p h o t o cathode are not well known one estimates that N = 100 corresponds to about 500 keV energy loss in the scintillator. Our experimentally determined optimum resolving time for 500 keV (2.8 × 10 -1° sec) is very close to this number. The fact that the experimental resolving time deteriorates as C/R is decreased below ,~0.07 is probably due to the effects of the finite time
76
G. PRESENT et al.
spreads in the real multiplier as suggested b y the w o r k of G a t t i a n d Svelto as well as the effects of multiplier noise pulses which b e c o m e significant as triggering levels are reduced.
5. Conclusions W i t h the use of XP1020 p h o t o m u l t i p l i e r s we have o b t a i n e d an i m p r o v e m e n t in resolving time of a b o u t 30~o c o m p a r e d to other systems. Results of theoretical considerations of the effect of transfer times in the scintillator a n d the a t t e n d a n t g r o w t h a n d decay of the light o u t p u t indicate t h a t this is p r o b a b l y the limiting factor in o b t a i n a b l e time resolution. R e s e a r c h on the d e v e l o p m e n t of scintillators with very s h o r t transfer times w o u l d be of great value for the fast coincidence m e t h o d . Very crude estimates indicate t h a t time resolutions u n d e r a tenth n a n o - s e c o n d m a y be o b t a i n able with XP1020 p h o t o m u l t i p l i e r s if the transfer times in scintillators could be a p p r e c i a b l y reduced. The a u t h o r s wish to t h a n k Prof. W . B r a n d t for his c o n s t a n t interest in this w o r k a n d Dr. Y. Koechlin for discussions on the light o u t p u t functions of organic scintillators.
Note added in proof: W e have recently received a c o m m u n i c a t i o n f r o m G a t t i a n d Svelto to the effect t h a t they have f o u n d significant m a t h e m a t i c a l errors in their earlier paper2). They n o w find t h a t the m i n i m a exhibited in their calculated curves o f width vs C/R were spurious; their new t h e o r y predicts m o n o t o n i c b e h a v i o u r o f this function with a m i n i m u m value at C/R = 0. The e x p e r i m e n t a l results o b t a i n e d by us (and several o t h e r e x p e r i m e n t a l groups) however, exhibit m i n i m a as suggested in their earlier paper. O n the basis o f calculations presented in G a t t i a n d S v e l t o ' s new w o r k 15) we suspect t h a t the origin o f the e x p e r i m e n t a l l y d e t e r m i n e d m i n i m a m a y be i n t i m a t e l y c o n n e c t e d with the g r o w t h a n d decay o f the light o u t p u t function for the scintillator as discussed above.
W e wish to p o i n t o u t t h a t due to the errors in G a t t i a n d Svelto's original calculation s o m e o f the r e m a r k s at the end o f section 3 o f this p a p e r m a y be unjustified. However, we emphasize t h a t all o u r calculations given in s e c t i o n 4 are still correct. O u r calculations in s e c t i o n 4 in which we d o n o t include any effects o f multiplier time dispersion yield resolving times close to those m e a s u r e d . W e therefore still infer t h a t the multiplier time spreads have only a small effect on the e x p e r i m e n t a l l y d e t e r m i n e d resolutions.
References 1) A. Schwarzschild, Nucl. Instr. and Meth. 21 (1963) 1. 2) E. Gatti and V. Svelto, Nucl. Instr. and Meth. 4 (1959) 189. 3) H. Kallmann and G.J. Brucker, Phys. Rev. 108 (1957) 1122, H. Kallmann, Phys. Rev. 117 (1960) 36. 4) y. Koechlin, Thesis, Paris (1961); C.E.A. Report No. 2194. 5) M.A. El-Wahab and J.V. Kane, Nucl. Instr. and Meth. 15 (1962) 15. 6) The XP1020 photomultipliers are available from Philips, except in the U.S.A. where they are sold by Arnperex, Hicksville, N.Y. Model 101 dual discriminator and Model 105 time to pulse-height converter are manufactured by Chronetics, Inc., Yonkers, N.Y. 7) R.L. Chase, Rev. Sci. Instr. 31 (1960) 945. 8) R. Sugarrnan, F.C. Merritt and W.A. Higinbotham, unpublished report; BNL 711 (T-248) (1962). 9) G. Pietri, private communication. 10) R.F. Post and L.I. Schiff, Phys. Rev. 80 (1950) 1113. u) W. Bartl and P. Weinzierl, Rev. Sci. Instr. 34 (1963) 252. t2) y. Koechlin, A. Raviart and M. Furst, Compt. Rend. 256 (1963) 1096. 1~) The paper of EI-Wahab and Kane presents a calculation, using a simple model of the overall resolution of the scintillator-multiplier system. Their assumption was that the scintillator light output function was purely exponential and that the time dispersions in the photomultiplier could be approximated by another exponential decay. They then treated the statistical processes in an exact manner and computed time dispersions with a fast computer. Although their model of the system is physically very different from ours, the mathematical treatment is identical so that their numerical results are directly applicable. 14) A. Raviart and Y. Koechlin, private communication. 15) E. Gatti and V. Svelto, Nucl. Instr. and Meth. 30 (1964) 213.