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Scintillation decay time of anthracene using low energy electrons
NUCLEAR INSTRUMENTS
AND METHODS
156 ( 1 9 7 8 )
609-611
; (~) N O R T H - H O L L A N D
P U B L I S H I N G CO.
SCINTILLATION DECAY TIME OF ANTHRACENE USING LOW ENERGY ELECTRONS* SEAN LAWRENCE PRUNTY t
Department o]" Electrical Engineering, University College, Cork, Ireland Received 12 June 1978 The scintillation decay time (z) of anthracene crystals using low energy incident electrons is determined. No difference in the decay time arising from 100 eV, 10 eV and 5 eV electrons is observed.
This note describes measurements on the decay time of anthracene using incident electrons of less than 100eV. In the usual way for this type of measurement the distribution of the differences in time between excitation of the scintillator mounted on a photomultiplier and the formation of individual photoelectrons at the cathode is recorded. "]?his is achieved by using the gating system of the previously reported time-of-flight (TOF) spectrometer I) to provide the " s t a r t " signal ("zerotime" signal), the " s t o p " signal being provided by the photomultiplier. The time interval between '"start" and " s t o p " signals is converted to a voltage height and the distribution of voltage heights is displayed on a multichannel analyser (MCA). The experimental arrangement used to measure r is shown in fig. 1. Electrons from a conventional electron gun are accelerated into a drift tube 0.5 m long which has gating plates at the input and output ends. An electron can enter the drift tube only if it arrives in the short time interval when the gate is open and it will only leave the drift tube if its transit time corresponds to the pre-set time delay (fixed by the pulse generator) between the opening of the input and output gates. It is therefbre possible to select electrons of specified flight time and hence energy. To ensure that only a single electron is in flight during any one cycle the e,lectron current is sufficiently reduced so that most openings (N99%) of the gates produce no electrons. The remaining openings will then essentially produce single electrons of well defined energy. On leaving the energy analyser the electrons drift a further short distance before striking the *- Work performed at the Department of Physics, Trinity College, University of Dublin,- Ireland. t Present address: Culham Laboratory, Abingdon, Oxfordshire, England.
freshly cleaved anthracene crystal (N 1 mm thick) mounted on a RCA 8850 photomultiplier tube. The incident electron energy is such as to give rise to only single photoelectrons at the cathode. Part of the "zero-time" signal to gate 1 is fed directly to the " s t a r t " input of a time-to-amplitude converter (TAC). (The choice between either gate as the source of the " s t a r t " signal does not affect the decay time being measured since the time interval between the opening of both gates is fixed by the pulse generator). The single photoelectron formed at the cathode produces an amplified pulse at the output of the photomultiplier. After additional amplification this pulse is used to generate the delayed " s t o p " signal to the TAC, the output from this being fed to the MCA. Oscilloscope traces (from the TOF and MCA) showing the general features of the scintillation decay arising from both 100 eV and 10 eV incident electrons appear in fig. 2. The decay time spectra
Electrons
I "[" Drift Tube I !iiiii: ultiplieri I
-, il '--I
F
Pulse
F
I
Generator
start~ Fig. 1. Block diagram showing the experimental method used to measure the scintillation decay time of anthracene.
610
s . L . PRUNTY
show a very rapid rise on the left-hand side with a much slower decreasing tail on the right-hand side. The overall schapes of these curves are of the
a
lOOeV 2Ons
;alibration points
b
loev
form e x p ( - t / r ) slightly modified by the resolution of the TOF system together with the dynamic response of the photomultiplier. An electron multiplier (EM) time spectrum (fig. 2c), obtained in the same way as the crystal-photomultiplier combination (see ref. 1), was run in order to check that the shape of the decay time spectrum was real, this being confirmed by the symmetrical shape of the EM spectrum as one would expect in this case. The width of this spectrum is largely determined by statistical fluctuations occurring in the long transit time ( ~ 7 0 ns) of the electrons in the EM tube, the transit times being distributed symmetrically about this average value. Returning to a consideration of the decay time measurements it is necessary to consider the effect of random "stop" signals arising from the photomultiplier's noise-count. These give rise to a constant background superimposed on the true decay curve. The background due to these random "stop" signals was in fact negligible, being typically smaller than the peak intensity by a factor of approximately 400. The width of the time channels into which
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Fig. 2. (a,b) Anthracene decay curves. (c) EM transit time spectrum.
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Fig. 3. Anthracene decay time spectrum and corresponding log plot.
S C I N T I L L A T I O N DECAY TIME OF A N T H R A C E N E TABLE 1 Scintillation decay time (ns) for three separate electron energies. 100 eV
10 eV
5 eV
26.3 25.1 24.8 22.6
29.3 26.8 25.7 25.0 24.7 28.3 26.6
25.7
the counts of the spectra were sorted was obtained in an auxiliary experiment using a standard switch-selected delay module. The decay curve and corresponding log plot due to 10 eV incident electrons is shown in fig. 3 and a complete list of the measurements appears in table 1 (errors in individual measurements here are approximately 8%). A least-squares fit to the data points (fig. 3) corresponds to a decay time of (26__+2) ns which is equal to the average for each of the 5, 10 and 100 eV readings in table 1. An excellent fit to the measured decay curve of fig. 3 was obtained by a convolution of a single exponential having a decay time of 26 ns and a resolution function having a Gaussian shape with a transit time spread of 5 ns. This value of 26 ns compares remarkably well with other results previously reported using different methods of excitation and detection2,3), and with the results obtained under photofluorescence excitation4,5). The electron energies (particularly 5 eV and 10 eV) used in the present experiment are sufficiently low to produce direct excitations as distinct from indirect ones arising from internally
611
generated secondary electrons produced by the passage of a much higher energy particle, and theoretical calculations performed by Stern 6) would tend to support this. On this basis and on the resuits of the time measurements, it appears that excitation by low energy electrons occurs into nsinglet states, the normal fluorescence lifetime being due to optical transitions taking place from the first n-singlet state to the ground state, similar to photofluorescence excitation. The similarity in decay time over the energy range investigated here indicates that the final stage of the scintillation process is the same in each case. It must therefore be the initial stage that determines the relative efficiency for photon production at these energies. Consequently, direct excitations may play a significant role in explaining non-linearities obtained 1,7) in photon output at low electron energies. The author wishes to thank Prof. C. F. G. Delaney for helpful discussions on all aspects of the work. The support of the National Science Council of Ireland and the Department of Education are gratefully acknowledged.
References 1) S. L. Prunty, C. F. G. Delaney and D. S. Walton, IEEE Trans. Nucl. Sci. NS-24 (1977) 248. 2) H. Kallmann and G. J. Brucl
AND METHODS
156 ( 1 9 7 8 )
609-611
; (~) N O R T H - H O L L A N D
P U B L I S H I N G CO.
SCINTILLATION DECAY TIME OF ANTHRACENE USING LOW ENERGY ELECTRONS* SEAN LAWRENCE PRUNTY t
Department o]" Electrical Engineering, University College, Cork, Ireland Received 12 June 1978 The scintillation decay time (z) of anthracene crystals using low energy incident electrons is determined. No difference in the decay time arising from 100 eV, 10 eV and 5 eV electrons is observed.
This note describes measurements on the decay time of anthracene using incident electrons of less than 100eV. In the usual way for this type of measurement the distribution of the differences in time between excitation of the scintillator mounted on a photomultiplier and the formation of individual photoelectrons at the cathode is recorded. "]?his is achieved by using the gating system of the previously reported time-of-flight (TOF) spectrometer I) to provide the " s t a r t " signal ("zerotime" signal), the " s t o p " signal being provided by the photomultiplier. The time interval between '"start" and " s t o p " signals is converted to a voltage height and the distribution of voltage heights is displayed on a multichannel analyser (MCA). The experimental arrangement used to measure r is shown in fig. 1. Electrons from a conventional electron gun are accelerated into a drift tube 0.5 m long which has gating plates at the input and output ends. An electron can enter the drift tube only if it arrives in the short time interval when the gate is open and it will only leave the drift tube if its transit time corresponds to the pre-set time delay (fixed by the pulse generator) between the opening of the input and output gates. It is therefbre possible to select electrons of specified flight time and hence energy. To ensure that only a single electron is in flight during any one cycle the e,lectron current is sufficiently reduced so that most openings (N99%) of the gates produce no electrons. The remaining openings will then essentially produce single electrons of well defined energy. On leaving the energy analyser the electrons drift a further short distance before striking the *- Work performed at the Department of Physics, Trinity College, University of Dublin,- Ireland. t Present address: Culham Laboratory, Abingdon, Oxfordshire, England.
freshly cleaved anthracene crystal (N 1 mm thick) mounted on a RCA 8850 photomultiplier tube. The incident electron energy is such as to give rise to only single photoelectrons at the cathode. Part of the "zero-time" signal to gate 1 is fed directly to the " s t a r t " input of a time-to-amplitude converter (TAC). (The choice between either gate as the source of the " s t a r t " signal does not affect the decay time being measured since the time interval between the opening of both gates is fixed by the pulse generator). The single photoelectron formed at the cathode produces an amplified pulse at the output of the photomultiplier. After additional amplification this pulse is used to generate the delayed " s t o p " signal to the TAC, the output from this being fed to the MCA. Oscilloscope traces (from the TOF and MCA) showing the general features of the scintillation decay arising from both 100 eV and 10 eV incident electrons appear in fig. 2. The decay time spectra
Electrons
I "[" Drift Tube I !iiiii: ultiplieri I
-, il '--I
F
Pulse
F
I
Generator
start~ Fig. 1. Block diagram showing the experimental method used to measure the scintillation decay time of anthracene.
610
s . L . PRUNTY
show a very rapid rise on the left-hand side with a much slower decreasing tail on the right-hand side. The overall schapes of these curves are of the
a
lOOeV 2Ons
;alibration points
b
loev
form e x p ( - t / r ) slightly modified by the resolution of the TOF system together with the dynamic response of the photomultiplier. An electron multiplier (EM) time spectrum (fig. 2c), obtained in the same way as the crystal-photomultiplier combination (see ref. 1), was run in order to check that the shape of the decay time spectrum was real, this being confirmed by the symmetrical shape of the EM spectrum as one would expect in this case. The width of this spectrum is largely determined by statistical fluctuations occurring in the long transit time ( ~ 7 0 ns) of the electrons in the EM tube, the transit times being distributed symmetrically about this average value. Returning to a consideration of the decay time measurements it is necessary to consider the effect of random "stop" signals arising from the photomultiplier's noise-count. These give rise to a constant background superimposed on the true decay curve. The background due to these random "stop" signals was in fact negligible, being typically smaller than the peak intensity by a factor of approximately 400. The width of the time channels into which
20as 2.0-
"alibration points
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/ / i o ,/
E x~
"'0 e\'
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J
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C
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~4oi .
130
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"~
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i
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I
I
140
15Q
160
170
180
190
Channel
Fig. 2. (a,b) Anthracene decay curves. (c) EM transit time spectrum.
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L. . . . . . . . . . . 160
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MULTIPUER
:alibrati0n polnls
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TRANSIT TIME SPECTRUM
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E',ect r o s s
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Time
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Decqy
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Number
Fig. 3. Anthracene decay time spectrum and corresponding log plot.
S C I N T I L L A T I O N DECAY TIME OF A N T H R A C E N E TABLE 1 Scintillation decay time (ns) for three separate electron energies. 100 eV
10 eV
5 eV
26.3 25.1 24.8 22.6
29.3 26.8 25.7 25.0 24.7 28.3 26.6
25.7
the counts of the spectra were sorted was obtained in an auxiliary experiment using a standard switch-selected delay module. The decay curve and corresponding log plot due to 10 eV incident electrons is shown in fig. 3 and a complete list of the measurements appears in table 1 (errors in individual measurements here are approximately 8%). A least-squares fit to the data points (fig. 3) corresponds to a decay time of (26__+2) ns which is equal to the average for each of the 5, 10 and 100 eV readings in table 1. An excellent fit to the measured decay curve of fig. 3 was obtained by a convolution of a single exponential having a decay time of 26 ns and a resolution function having a Gaussian shape with a transit time spread of 5 ns. This value of 26 ns compares remarkably well with other results previously reported using different methods of excitation and detection2,3), and with the results obtained under photofluorescence excitation4,5). The electron energies (particularly 5 eV and 10 eV) used in the present experiment are sufficiently low to produce direct excitations as distinct from indirect ones arising from internally
611
generated secondary electrons produced by the passage of a much higher energy particle, and theoretical calculations performed by Stern 6) would tend to support this. On this basis and on the resuits of the time measurements, it appears that excitation by low energy electrons occurs into nsinglet states, the normal fluorescence lifetime being due to optical transitions taking place from the first n-singlet state to the ground state, similar to photofluorescence excitation. The similarity in decay time over the energy range investigated here indicates that the final stage of the scintillation process is the same in each case. It must therefore be the initial stage that determines the relative efficiency for photon production at these energies. Consequently, direct excitations may play a significant role in explaining non-linearities obtained 1,7) in photon output at low electron energies. The author wishes to thank Prof. C. F. G. Delaney for helpful discussions on all aspects of the work. The support of the National Science Council of Ireland and the Department of Education are gratefully acknowledged.
References 1) S. L. Prunty, C. F. G. Delaney and D. S. Walton, IEEE Trans. Nucl. Sci. NS-24 (1977) 248. 2) H. Kallmann and G. J. Brucl