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Performance of a liquid scintillator at high count-rates for fusion applications
774
Nuclear Instruments and Methods in Physics Research A278 (1989) 774-778 North-Holland, Amsterdam
PERFORMANCE OF A LIQUID SCINTILLATOR AT HIGH COUNT-RATES FOR FUSION APPLICATIONS T. ELEVANT Royal Institute of Technology, Stockholm, Sweden
Received 30 January 1989 Energy resolution, peak position and n/y discrimination have been measured for a liquid scintillator (Bicron BC 501 3 .75 cm diameter X2 .5 cm) as a function of count-rate for two different neutron energies, 2.4 and 14.1 MeV. Energy resolutions (FWHM) equal to 150 keV (at 2.4 MeV) and 610 keV (at 14 .1 MeV) were achieved at low count-rates, i.e. < 5 X 10 4 c/s. Modified standard electronics were used at count-rates >_ 10 5 c/s and yielded resolutions approximately equal to 250 and 700 keV respectively . At count-rates < 2.0X 10 5 c/s n/y discrimination better than 99% was observed . Detector systems with such properties will be able to resolve center-of-mass neutron energy line broadening from thermonuclear plasmas with ion temperatures T; >_ 4 keV (D-D) and >_ 10 keV (D-T). 1. Introduction Liquid scintillators with n/y pulse-shape discrimination properties have been used extensively for measurements of neutron fluxes and energies for more than twenty years [1-4]. Detection efficiencies of such scintillators are given mainly by the (n, p) elastic scattering cross section and the discrimination level set by the pulse-height electronics . Typical detection efficiencies are 1-20% for neutrons in the MeV range and scintillator thicknesses equal to 1-10 cm . Such efficiencies are high when compared with other detectors with similar energy resolution . Unfolding of the proton recoil spectra has provided energy resolutions from --- 4% (at - 20 MeV) to - 10% (at - 2 MeV) . Pulse shape separation between neutron and gamma events has been demonstrated for large dynamic pulse-amplitude ranges . This type of scintillator has previously been used for flux measurements of D-D neutrons in Ormak [5], PLT [6-8] and Alcator [9]. In the case of large fusion experiments (e .g . TFTR and JET) the D-D neutron emission rate is in the region 10 14 -1017 n/s, and emissions from D-T plasmas are expected to be two orders of magnitude larger . In addition, the neutron production also varies by several orders of magnitude during one discharge and from one discharge to another depending on plasma conditions . The energy confinement time is a fraction of a second and the discharge time is in the 1-20 s range. Also, plasma compression may occur in tens of ms . Thus, it is important to obtain sufficient data in both counting and spectral measurements over small time intervals (10-50 ms). Consequently the detectors must have a © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
0168-9002/89/$03 .50
large (- 10 4) dynamic range while maintaining detection efficiency, stable pulse-height and n/y discriminating properties . Neutron diagnostic instruments for fusion applications include multicollimator systems using liquid scintillators as spectrometers with energy resolutions of a few percent and the ability to operate at high count-rates [10] . We report results from measurements of energy resolution (FWHM), peak position and n/y discrimination stability at count-rates ranging from 10 3 to 10 6 c/s for a detector system consisting of scintillator, photomultiplier and standard electronics . In section 2 the experimental setup is described including a special high voltage divider and use of a light pipe. Section 3 describes the results and in section 4 the results are discussed . 2. Equipment and testing 2.1 . Neutron generator
Two different sealed-tube neutron generators were used with 180 keV accelerating potentials, producing < 5 X 10 1° 14 .1 MeV and < 5 X 10 8 2.4 MeV neutrons per second . The geometry was such that the predicted energy dispersion of the neutrons interacting with the scintillators was < 100-200 keV. An independent measurement with a silicon surface barrier detector gave a width < 150 keV for D-T neutrons [111 . The energy dispersion of the 2.4 MeV neutrons was measured as being 92 keV using a 3 He sandwich detector . The observation angle was 95' with respect to the incident beam in all measurements.
T. Elevant / Performance of a liquid scintillator at high count-rates a -HV
775
tube selected for its high quantum efficiency was used . The effect of a banded (1 cm at the top) light-pipe [12] (3 .75 cm diameter x 3 .75 cm) was investigated . Two different high voltage divider networks equipped with stabilized electronic circuits were used, one having the anode at ground potential and the other having the 10th dynode at ground (see figs . l a and 1b) . Standard modular NIM electronics were used, including an amplifier with shortened (0.4 l s) pulse length (see fig. 2) and a coincidence/anticoincidence circuit to discriminate against pulse pileup within a time interval of = 0 .4 ~Ls. 2 .3 . Unfolding procedure The code FLYSPEC [13] was used to unfold proton recoil energy spectra. The program uses the derivative unfolding method . A digital filter, which performs least-squares fits to a parabola, estimates the derivative of the pulse height . Cubicle spline functions (continuous up to the second derivative) represent scintillation light output as a function of proton energy, and are fit by a least-squares technique to the data of Verbinski et al .
Fig. 1 . (a) Anode grounded high voltage divider used at high count-rates . (b) The dynode grounded divider used at low count-rates . Transistors are 2N3439, diodes IN914 and resistors 0 .5 W . 2 .2 . Scintillators, PM tubes and electrons A Bicron scintillator (BC 501 3 .75 cm diameter x 2 .5 cm) mounted on a 12-stage RCA 8850 photomultiplier
The code assumes isotropic scattering in the centreof-mass system . The anisotropy in the scattering of neutrons of less than 15 MeV energy is at most a few percent and could be taken into account as an empirical efficiency correction but this is not done here . The loss of proton counts due to multiple scattering of a single neutron is treated by calculating the self-attenuation of the detector and is applied as a correction factor to the detection efficiency . A semiempirical formula corrects for proton escape. These corrections are good when multiple scattering and proton escape are not large . Elastic and inelastic scattering from carbon are not taken into account but in principle could be included . All necessary cross section data are contained within the program . 2 .4. n/y discrimination
Fig. 2. Standard (modified) electronics used. TFA - timing filter amplifier, CFD - constant fraction discriminator, PSD pulse shape discriminator and ADC - analog to digital converter .
For the n/y discrimination test the standard detector, phototube, and the anode grounded base together with the electronics were used both with and without pulse pileup rejection circuitry . The detector was exposed to either a 130 l.LCi or 1 MCi 137CS source . The count-rate was varied by changing the source to detector distance . The total y-ray count-rate was measured by a 100 MHz discriminator with its discrimination level set at 1% of the pulse height of the Compton edge from the 1 .3 MeV -y-rays of 22 Na . Adjustment of the PSD was accomplished by use of a TAC (time to amplitude converter) and ADC at low count-rates (103 c/s). The n/y discrimination level was set at the bottom of the valley between the -y-ray and neutron time peaks (from the TAC) observed by viewing a 10 252 Cf source . W Ci
T. Elevant / Performance of a liquid scintillator at
776 3. Results
high
count-rates
2F
To optimize the energy resolution of the scintillator-photomultiplier system, the dynode grounded modified high voltage divider network (see section 2.2 and fig. lb) and standard amplifier with 1 ~ts pulse width were used . The resolution obtained at count-rates - 5 X 10 4 c/s without the light pipe was typically 150 keV. Use of the light pipe gave a resolution equal to - 210 keV . For tests at high count-rates (<_ 10 6 c/s), the modified amplifier and the anode-grounded high voltage divider (see section 2.2 and fig. la) were used . The resolution and peak position for different count-rates are shown in fig. 3. Alterations both in resolution and peak position occur at count-rates exceeding - 2 X 10 5 c/s. The ratio of neutron counts above a threshold corresponding to 511 keV y-energy to total counts is about 10%. An example of a proton recoil spectrum collected at a total rate equal to 6 X 10 5 c/s is shown in fig. 4a, and the corresponding unfolded spectrum in fig. 4b . 3.2. D- T neutrons For D-T neutrons the best resolution, = 570 keV (FWHM), was obtained with the banded light pipe, 1 ws pulse-length electronics, and low count-rates, _ 104 c/s. Without light pipe the resolution was measured to 610 keV. For higher count-rates < 10 6 c/s, the shortpulse electronics provided resolutions and peak positions shown in fig. 5. Some results obtained without the anticoincidence circuit are also shown. Here the resolution and peak position deteriorate at count-rates > 106 400
DD-Neutrons
Y 300 x 200 § w a 100 10 103 104 5 Counts/s
106
a
8 x 10 5 c/s
3.1 . D-D neutrons U
b
0.11
0
50
100 150 Channel Number
200
1.
250
b 6
6.8 x 105 c/s
Û 12 4 C Û b
FWHM 250keV
2
0 1JI
2 .0
1
2.4
w.tti...~ - E (MeV) 3.0
Fig. 4. Proton recoil spectrum (a) and corresponding unfolded spectrum (b) at 6.8 X 10 5 c/s with 2.4 MeV neutrons. c/s. The recoil energy spectrum is shown in fig. 6a for energies above a threshold corresponding to gamma energies equal to 1.3 MeV and the corresponding unfolded neutron spectrum is shown in fig. 6b . 3.3. n/ y discrimination The results of the n/y discrimination test are shown in fig. 7. The fraction of y events misinterpreted as neutron is plotted as a function of total count-rate . As can be seen, the variation is approximately linear with count-rate up to - 6.5 X 10 5 c/s. Above this value pulse pileup becomes severe and the number of y-rays
Y
900
_ 700 u_ ~ 500
a- 2.4 m a 2 .3 103
10^
Counts/s
10,
106
Fig. 3. Resolution and peak position as a function of count-rate for 2.410 MeV neutrons . Circles indicate results obtained with modified electronics and anode grounded high voltage divider. Squares indicate results with standard electronics and dynode grounded high voltage divider.
0
15
0
14
a m
â
13
11 0'
--
104
Counts/s
5
6
Fig. 5. Same as fig. 3 but for 14.1 MeV neutrons. X indicate tests with the anticoincidence circuit disconnected.
T. Eleoant / Performance of a liquid scintillator at high count-rates a
777
4. Discussion of the results 4.1 . Low count-rate
L
h 0.5 c 0 U
s
b
J 0 .1
0
"
50
100 150 200 Channel Number 00
3 x 10 5
250 b
c/s
48 .1 keV/ch ,a " ~-FWHM
. . 13
14
E~(MeV) 15
Fig. 6. Proton recoil spectrum (a) and corresponding unfolded spectrum (b) at 3.3 X 10 5 c/s with 14.1 MeV neutrons .
The measured resolution has to be compared with the intrinsic resolution of the system . Assuming a scintillation efficiency of 1 photon/120 eV [14], a reflection and transmission efficiency of 0.7, and a photocathode quantum efficiency = 0.35, Poisson photon statistics gives [15] i1 E = 100 keV for 2.4 MeV recoil protons. The corresponding number for 14 .1 MeV recoil protons is = 280 keV including 50% losses in the light guide transmission . Quantum efficiency variation over the cathode surface may vary as much as 10% and will contribute to the width of the pulse height distribution . The electron collection efficiency varies over the cathode and the use of a light-pipe will even out this effect over the photocathode . On the other hand, a light-pipe will cause a reduction of available light due to internal absorption and to surface reflection . The experimental results suggest that the photon statistics is the limiting factor for the energy resolution of 2.4 MeV neutrons while quantum efficiency and electron collection efficiency variations over the cathode are the limiting factors for 14 MeV neutrons . 4.2 . High count-rate
misinterpreted as neutrons increases dramatically . Below 2.0 X 10 5 c/s the fraction of erroneous counts is < 1%, while the fraction stays below 7.5% up to about 6.5 X 10 5 total c/s .
10-1
Fraction of ti-events in neutron channels " Pulse pileup rejector used + No pulse pile up rejector
10-~
Electronics with pulse lengths - 0.4 lts and shaping constants - 0.04 jLs were used for high count-rate tests, together with the stabilized high voltage divider with anode at ground potential. The resolution was - 150 keV for 2.4 MeV neutrons and = 610 keV for 14.1 MeV neutrons at 10 4 c/s. At count-rates from 5 X 10 5 to 10 6 c/s significant deterioration occurs both in energy resolution and peak position stability (see figs . 3 and 5) . In the case of D-D neutrons, about 10% of the events suffer from pileup at a count-rate equal to 10 6 c/s due to the finite pulse-pair resolution of about 100 ns in the CFD unit used to generate anticoincidence pulses . For D-T neutrons, a module with improved pulse-pair resolution - 20 ns was used, giving approximately 2% pileup at 10 6 c/s. 5. Sununary
10 -3
103
10^ 10, Total Count Rate (c/s)
10,
Fig. 7. Fractions of counts in neutron channel is shown as a function of total count-rate when detector is exposed to a -y-source (137cS).
A neutron spectrometer system based on proton recoil spectra measurement in a liquid scintillator (type BC 501 3.75 cm diameter X 2.5 cm) which was optically coupled to a photomultiplier tube having a special stabilized high voltage divider was shown to have an energy resolution t1 E .- 150-250 keV (FWHM) for 2.4 MeV neutrons and DE = 610-750 keV for 14 .1 MeV neutrons and a stable peak position (within 2%) up to a count-rate equal to 5 X 10 5 c/s. Furthermore, tests with
778
T. Elevant / Performance of a liquid scintillator at high count-rates
y sources showed 99% n/y discrimination capability at count-rates equal to < 2 x 10 5 c/s. Use of a banded
light-pipe improved the energy resolution for 14 .1 MeV neutrons to 570 keV but reduced it for 2.4 MeV neutrons, mainly due to limitations in photon counting statistics.
Acknowledgements We would like to thank Dr . H. Hendel, Princeton Plasma Physics Laboratory, who suggested the work and Mr . K. Chase for operating of the neutron genera-
tor. The author is also grateful to Dr. N. Jarvis for valuable comments to the manuscript .
References [1] V.V . Verbinski et al ., Nucl. Instr. and Meth. 65 (1968) 8. [2] W.R. Burrus and V.V. Verbinski, Nucl . Instr. and Meth . 67 (1969) 181.
[3] J. Kalyna and I.J. Taylor, Nucl . Instr. and Meth . 88 (1970) 277. [4] A. Bertin and A. Vitale, Nucl . Instr. and Meth . 91 (1971) 649. [5] G.L. Morgan and A.C . England, Nucl. Instr. and Meth. 129 (1975) 1. [6] P. Colestock et al ., Phys . Rev. Lett. 43 (1979) 768. [7] M. Brusati et al ., Nucl. Fus. 18 (1978) 1205 . [8] J.D. Strachan et al., Joint Varenna- Grenoble Int. Symp. on Heating in Toroidal Plasmas, vol. 1 (Grenoble, 1978) p_ 25 . [9] D.S. Pappas et al., PFC/RR-82-14, Plasma Fusion Center, Massachusetts Institute of Technology (August 1982). [10] J.M . Adams, private communications . [11] T. Elevant et al ., Rev. Sci. Instr. 57 (Aug . 1986). [12] H. Klein and H. Schlölermann, IEEE Trans. Nucl. Sci. NS-26, no . 1 (1979) ; see also Nucl . Instr. and Meth. 169 (1980) 25 . [13] D. Slaughter and R. Strout, Nucl . Instr. and Meth . 198 (1982) 349. [14] G. Kettering, Nucl . Instr. and Meth. 131 (1975) 451. [15] G.A. Morton et al ., Appl . Phys . Lett . 13 (1968) 356.
Nuclear Instruments and Methods in Physics Research A278 (1989) 774-778 North-Holland, Amsterdam
PERFORMANCE OF A LIQUID SCINTILLATOR AT HIGH COUNT-RATES FOR FUSION APPLICATIONS T. ELEVANT Royal Institute of Technology, Stockholm, Sweden
Received 30 January 1989 Energy resolution, peak position and n/y discrimination have been measured for a liquid scintillator (Bicron BC 501 3 .75 cm diameter X2 .5 cm) as a function of count-rate for two different neutron energies, 2.4 and 14.1 MeV. Energy resolutions (FWHM) equal to 150 keV (at 2.4 MeV) and 610 keV (at 14 .1 MeV) were achieved at low count-rates, i.e. < 5 X 10 4 c/s. Modified standard electronics were used at count-rates >_ 10 5 c/s and yielded resolutions approximately equal to 250 and 700 keV respectively . At count-rates < 2.0X 10 5 c/s n/y discrimination better than 99% was observed . Detector systems with such properties will be able to resolve center-of-mass neutron energy line broadening from thermonuclear plasmas with ion temperatures T; >_ 4 keV (D-D) and >_ 10 keV (D-T). 1. Introduction Liquid scintillators with n/y pulse-shape discrimination properties have been used extensively for measurements of neutron fluxes and energies for more than twenty years [1-4]. Detection efficiencies of such scintillators are given mainly by the (n, p) elastic scattering cross section and the discrimination level set by the pulse-height electronics . Typical detection efficiencies are 1-20% for neutrons in the MeV range and scintillator thicknesses equal to 1-10 cm . Such efficiencies are high when compared with other detectors with similar energy resolution . Unfolding of the proton recoil spectra has provided energy resolutions from --- 4% (at - 20 MeV) to - 10% (at - 2 MeV) . Pulse shape separation between neutron and gamma events has been demonstrated for large dynamic pulse-amplitude ranges . This type of scintillator has previously been used for flux measurements of D-D neutrons in Ormak [5], PLT [6-8] and Alcator [9]. In the case of large fusion experiments (e .g . TFTR and JET) the D-D neutron emission rate is in the region 10 14 -1017 n/s, and emissions from D-T plasmas are expected to be two orders of magnitude larger . In addition, the neutron production also varies by several orders of magnitude during one discharge and from one discharge to another depending on plasma conditions . The energy confinement time is a fraction of a second and the discharge time is in the 1-20 s range. Also, plasma compression may occur in tens of ms . Thus, it is important to obtain sufficient data in both counting and spectral measurements over small time intervals (10-50 ms). Consequently the detectors must have a © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
0168-9002/89/$03 .50
large (- 10 4) dynamic range while maintaining detection efficiency, stable pulse-height and n/y discriminating properties . Neutron diagnostic instruments for fusion applications include multicollimator systems using liquid scintillators as spectrometers with energy resolutions of a few percent and the ability to operate at high count-rates [10] . We report results from measurements of energy resolution (FWHM), peak position and n/y discrimination stability at count-rates ranging from 10 3 to 10 6 c/s for a detector system consisting of scintillator, photomultiplier and standard electronics . In section 2 the experimental setup is described including a special high voltage divider and use of a light pipe. Section 3 describes the results and in section 4 the results are discussed . 2. Equipment and testing 2.1 . Neutron generator
Two different sealed-tube neutron generators were used with 180 keV accelerating potentials, producing < 5 X 10 1° 14 .1 MeV and < 5 X 10 8 2.4 MeV neutrons per second . The geometry was such that the predicted energy dispersion of the neutrons interacting with the scintillators was < 100-200 keV. An independent measurement with a silicon surface barrier detector gave a width < 150 keV for D-T neutrons [111 . The energy dispersion of the 2.4 MeV neutrons was measured as being 92 keV using a 3 He sandwich detector . The observation angle was 95' with respect to the incident beam in all measurements.
T. Elevant / Performance of a liquid scintillator at high count-rates a -HV
775
tube selected for its high quantum efficiency was used . The effect of a banded (1 cm at the top) light-pipe [12] (3 .75 cm diameter x 3 .75 cm) was investigated . Two different high voltage divider networks equipped with stabilized electronic circuits were used, one having the anode at ground potential and the other having the 10th dynode at ground (see figs . l a and 1b) . Standard modular NIM electronics were used, including an amplifier with shortened (0.4 l s) pulse length (see fig. 2) and a coincidence/anticoincidence circuit to discriminate against pulse pileup within a time interval of = 0 .4 ~Ls. 2 .3 . Unfolding procedure The code FLYSPEC [13] was used to unfold proton recoil energy spectra. The program uses the derivative unfolding method . A digital filter, which performs least-squares fits to a parabola, estimates the derivative of the pulse height . Cubicle spline functions (continuous up to the second derivative) represent scintillation light output as a function of proton energy, and are fit by a least-squares technique to the data of Verbinski et al .
Fig. 1 . (a) Anode grounded high voltage divider used at high count-rates . (b) The dynode grounded divider used at low count-rates . Transistors are 2N3439, diodes IN914 and resistors 0 .5 W . 2 .2 . Scintillators, PM tubes and electrons A Bicron scintillator (BC 501 3 .75 cm diameter x 2 .5 cm) mounted on a 12-stage RCA 8850 photomultiplier
The code assumes isotropic scattering in the centreof-mass system . The anisotropy in the scattering of neutrons of less than 15 MeV energy is at most a few percent and could be taken into account as an empirical efficiency correction but this is not done here . The loss of proton counts due to multiple scattering of a single neutron is treated by calculating the self-attenuation of the detector and is applied as a correction factor to the detection efficiency . A semiempirical formula corrects for proton escape. These corrections are good when multiple scattering and proton escape are not large . Elastic and inelastic scattering from carbon are not taken into account but in principle could be included . All necessary cross section data are contained within the program . 2 .4. n/y discrimination
Fig. 2. Standard (modified) electronics used. TFA - timing filter amplifier, CFD - constant fraction discriminator, PSD pulse shape discriminator and ADC - analog to digital converter .
For the n/y discrimination test the standard detector, phototube, and the anode grounded base together with the electronics were used both with and without pulse pileup rejection circuitry . The detector was exposed to either a 130 l.LCi or 1 MCi 137CS source . The count-rate was varied by changing the source to detector distance . The total y-ray count-rate was measured by a 100 MHz discriminator with its discrimination level set at 1% of the pulse height of the Compton edge from the 1 .3 MeV -y-rays of 22 Na . Adjustment of the PSD was accomplished by use of a TAC (time to amplitude converter) and ADC at low count-rates (103 c/s). The n/y discrimination level was set at the bottom of the valley between the -y-ray and neutron time peaks (from the TAC) observed by viewing a 10 252 Cf source . W Ci
T. Elevant / Performance of a liquid scintillator at
776 3. Results
high
count-rates
2F
To optimize the energy resolution of the scintillator-photomultiplier system, the dynode grounded modified high voltage divider network (see section 2.2 and fig. lb) and standard amplifier with 1 ~ts pulse width were used . The resolution obtained at count-rates - 5 X 10 4 c/s without the light pipe was typically 150 keV. Use of the light pipe gave a resolution equal to - 210 keV . For tests at high count-rates (<_ 10 6 c/s), the modified amplifier and the anode-grounded high voltage divider (see section 2.2 and fig. la) were used . The resolution and peak position for different count-rates are shown in fig. 3. Alterations both in resolution and peak position occur at count-rates exceeding - 2 X 10 5 c/s. The ratio of neutron counts above a threshold corresponding to 511 keV y-energy to total counts is about 10%. An example of a proton recoil spectrum collected at a total rate equal to 6 X 10 5 c/s is shown in fig. 4a, and the corresponding unfolded spectrum in fig. 4b . 3.2. D- T neutrons For D-T neutrons the best resolution, = 570 keV (FWHM), was obtained with the banded light pipe, 1 ws pulse-length electronics, and low count-rates, _ 104 c/s. Without light pipe the resolution was measured to 610 keV. For higher count-rates < 10 6 c/s, the shortpulse electronics provided resolutions and peak positions shown in fig. 5. Some results obtained without the anticoincidence circuit are also shown. Here the resolution and peak position deteriorate at count-rates > 106 400
DD-Neutrons
Y 300 x 200 § w a 100 10 103 104 5 Counts/s
106
a
8 x 10 5 c/s
3.1 . D-D neutrons U
b
0.11
0
50
100 150 Channel Number
200
1.
250
b 6
6.8 x 105 c/s
Û 12 4 C Û b
FWHM 250keV
2
0 1JI
2 .0
1
2.4
w.tti...~ - E (MeV) 3.0
Fig. 4. Proton recoil spectrum (a) and corresponding unfolded spectrum (b) at 6.8 X 10 5 c/s with 2.4 MeV neutrons. c/s. The recoil energy spectrum is shown in fig. 6a for energies above a threshold corresponding to gamma energies equal to 1.3 MeV and the corresponding unfolded neutron spectrum is shown in fig. 6b . 3.3. n/ y discrimination The results of the n/y discrimination test are shown in fig. 7. The fraction of y events misinterpreted as neutron is plotted as a function of total count-rate . As can be seen, the variation is approximately linear with count-rate up to - 6.5 X 10 5 c/s. Above this value pulse pileup becomes severe and the number of y-rays
Y
900
_ 700 u_ ~ 500
a- 2.4 m a 2 .3 103
10^
Counts/s
10,
106
Fig. 3. Resolution and peak position as a function of count-rate for 2.410 MeV neutrons . Circles indicate results obtained with modified electronics and anode grounded high voltage divider. Squares indicate results with standard electronics and dynode grounded high voltage divider.
0
15
0
14
a m
â
13
11 0'
--
104
Counts/s
5
6
Fig. 5. Same as fig. 3 but for 14.1 MeV neutrons. X indicate tests with the anticoincidence circuit disconnected.
T. Eleoant / Performance of a liquid scintillator at high count-rates a
777
4. Discussion of the results 4.1 . Low count-rate
L
h 0.5 c 0 U
s
b
J 0 .1
0
"
50
100 150 200 Channel Number 00
3 x 10 5
250 b
c/s
48 .1 keV/ch ,a " ~-FWHM
. . 13
14
E~(MeV) 15
Fig. 6. Proton recoil spectrum (a) and corresponding unfolded spectrum (b) at 3.3 X 10 5 c/s with 14.1 MeV neutrons .
The measured resolution has to be compared with the intrinsic resolution of the system . Assuming a scintillation efficiency of 1 photon/120 eV [14], a reflection and transmission efficiency of 0.7, and a photocathode quantum efficiency = 0.35, Poisson photon statistics gives [15] i1 E = 100 keV for 2.4 MeV recoil protons. The corresponding number for 14 .1 MeV recoil protons is = 280 keV including 50% losses in the light guide transmission . Quantum efficiency variation over the cathode surface may vary as much as 10% and will contribute to the width of the pulse height distribution . The electron collection efficiency varies over the cathode and the use of a light-pipe will even out this effect over the photocathode . On the other hand, a light-pipe will cause a reduction of available light due to internal absorption and to surface reflection . The experimental results suggest that the photon statistics is the limiting factor for the energy resolution of 2.4 MeV neutrons while quantum efficiency and electron collection efficiency variations over the cathode are the limiting factors for 14 MeV neutrons . 4.2 . High count-rate
misinterpreted as neutrons increases dramatically . Below 2.0 X 10 5 c/s the fraction of erroneous counts is < 1%, while the fraction stays below 7.5% up to about 6.5 X 10 5 total c/s .
10-1
Fraction of ti-events in neutron channels " Pulse pileup rejector used + No pulse pile up rejector
10-~
Electronics with pulse lengths - 0.4 lts and shaping constants - 0.04 jLs were used for high count-rate tests, together with the stabilized high voltage divider with anode at ground potential. The resolution was - 150 keV for 2.4 MeV neutrons and = 610 keV for 14.1 MeV neutrons at 10 4 c/s. At count-rates from 5 X 10 5 to 10 6 c/s significant deterioration occurs both in energy resolution and peak position stability (see figs . 3 and 5) . In the case of D-D neutrons, about 10% of the events suffer from pileup at a count-rate equal to 10 6 c/s due to the finite pulse-pair resolution of about 100 ns in the CFD unit used to generate anticoincidence pulses . For D-T neutrons, a module with improved pulse-pair resolution - 20 ns was used, giving approximately 2% pileup at 10 6 c/s. 5. Sununary
10 -3
103
10^ 10, Total Count Rate (c/s)
10,
Fig. 7. Fractions of counts in neutron channel is shown as a function of total count-rate when detector is exposed to a -y-source (137cS).
A neutron spectrometer system based on proton recoil spectra measurement in a liquid scintillator (type BC 501 3.75 cm diameter X 2.5 cm) which was optically coupled to a photomultiplier tube having a special stabilized high voltage divider was shown to have an energy resolution t1 E .- 150-250 keV (FWHM) for 2.4 MeV neutrons and DE = 610-750 keV for 14 .1 MeV neutrons and a stable peak position (within 2%) up to a count-rate equal to 5 X 10 5 c/s. Furthermore, tests with
778
T. Elevant / Performance of a liquid scintillator at high count-rates
y sources showed 99% n/y discrimination capability at count-rates equal to < 2 x 10 5 c/s. Use of a banded
light-pipe improved the energy resolution for 14 .1 MeV neutrons to 570 keV but reduced it for 2.4 MeV neutrons, mainly due to limitations in photon counting statistics.
Acknowledgements We would like to thank Dr . H. Hendel, Princeton Plasma Physics Laboratory, who suggested the work and Mr . K. Chase for operating of the neutron genera-
tor. The author is also grateful to Dr. N. Jarvis for valuable comments to the manuscript .
References [1] V.V . Verbinski et al ., Nucl. Instr. and Meth. 65 (1968) 8. [2] W.R. Burrus and V.V. Verbinski, Nucl . Instr. and Meth . 67 (1969) 181.
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