Low Energy Light Yield of Fast Plastic Scintillators
T. A. Laplace, B. L. Goldblum, J. A. Brown, D. L. Bleuel, C. A. Brand, G. Gabella, T. Jordan, C. Moore, N. Munshi, Z. W. Sweger, A. Ureche, E. Brubaker
aa r X i v : . [ phy s i c s . i n s - d e t ] S e p Low Energy Light Yield of Fast Plastic Scintillators
T.A. Laplace a, ∗ , B.L. Goldblum a , J.A. Brown a,b , D.L. Bleuel c , C.A. Brand a,c , G. Gabella a ,T. Jordan a , C. Moore a , N. Munshi a , Z.W. Sweger a , A. Ureche a , E. Brubaker b a Department of Nuclear Engineering, University of California, Berkeley, California 94720 USA b Sandia National Laboratories, Livermore, California 94550, USA c Lawrence Livermore National Laboratory, Livermore, California 94550 USA
Abstract
Compact neutron imagers using double-scatter kinematic reconstruction are being designedfor localization and characterization of special nuclear material. These neutron imaging sys-tems rely on scintillators with a rapid prompt temporal response as the detection medium.As n-p elastic scattering is the primary mechanism for light generation by fast neutron inter-actions in organic scintillators, proton light yield data are needed for accurate assessment ofscintillator performance. The proton light yield of a series of commercial fast plastic organicscintillators—EJ-200, EJ-204, and EJ-208—was measured via a double time-of-flight tech-nique at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. Using a tunabledeuteron breakup neutron source, target scintillators housed in a dual photomultiplier tubeconfiguration, and an array of pulse-shape-discriminating observation scintillators, the fastplastic scintillator light yield was measured over a broad and continuous energy range downto proton recoil energies of approximately 50 keV. This work provides key input to eventreconstruction algorithms required for utilization of these materials in emerging neutronimaging modalities.
Keywords: organic scintillator, plastic scintillator, proton light yield, neutron detection,time-of-flight
1. Motivation
The ability to detect and image special nuclear material is important for nuclear securityapplications. Current neutron imaging systems are bulky and their sensitivity is limited bytheir geometric efficiency [1, 2, 3, 4]. The development of a compact system capable of high-efficiency fast neutron imaging can enable weak threat source detection on a human-portableplatform, where directional information can be used for background discrimination in lowsignal-to-noise environments. Advanced neutron imagers, including single-volume scattercamera (SVSC) concepts where two neutron scattering events are detected within a single ∗ Corresponding author
Email address: [email protected] (T.A. Laplace) olume, are being designed at Sandia National Laboratories to support these goals [5, 6].The work presented here addresses the measurement of the energy-dependent proton lightyield of candidate scintillator materials for use in these novel neutron imagers.The proton light yield, or the relationship between light output and proton energy de-posited in an organic scintillator, varies non-linearly with recoil energy due to quenchingphenomena [7]. For neutron scatter camera imaging modalities, knowledge of the protonlight yield is necessary to convert the measured light into proton recoil energy, which is thenused along with n-p elastic scattering kinematics to determine the energy and direction ofthe incident neutron. As both the magnitude of the scintillation intensity and the quenchingfactor are properties of the scintillating medium, the proton light yield is a characteristicunique to each scintillator material formulation. Existing physics-based models of protonlight yield give an unreliable extrapolation to recoil energies below 1 MeV [8] and it is inthis low energy region where the prompt fission neutron spectrum of special nuclear materialpeaks. Eljen Technology’s EJ-200, EJ-204, and EJ-208 scintillators are potential candidatesfor the SVSC due to their fast rise times and relatively long ( > .
2. Methods
Neutrons were produced by impinging a 16-MeV H + beam from the 88-Inch Cyclotron atLawrence Berkeley National Laboratory onto a 3-mm-thick Be breakup target located in thecyclotron vault. This deuteron-breakup neutron source provides an intense, broad spectrumneutron beam for experimental inquiry [9]. The target scintillator to be characterized (i.e.,EJ-200, EJ-204, or EJ-208) was placed in beam, as shown in Figure 1. Out of beam and atdifferent forward angles, eleven EJ-309 pulse-shape-discriminating organic liquid scintillatorsfrom Eljen Technology were positioned to serve as observation detectors in the coincidentmeasurement of scattered neutrons from n-p elastic scattering events in the target cell.Using the method of Brown et al. [10, 11], the neutron TOF between the breakup targetand the target scintillator was used to determine the energy of the incident neutron, E n .Likewise, the neutron TOF between the target scintillator and the observation detectors wasused to determine the energy of the scattered neutron, E ′ n . The proton recoil energy, E p ,was then calculated using the incoming neutron energy and the known neutron scatteringangle, θ : E p = E n sin θ. (1)The proton recoil energy associated with a given target scintillator pulse was determinedon an event-by-event basis to provide a continuous measurement of the proton light yieldrelation. The outgoing TOF was used for disambiguation of the incident beam pulses in caseswhere fast neutrons from a given cyclotron pulse had the same observable TOF as slowerneutrons from previous pulses [12]. This approach also provides strong rejection criteria formultiple scatters in the active target volume.2 igure 1: Experimental setup for proton light yield measurements. The neutron beam traveled along the x -axis (illustrated with a dashed line) through the dual-PMT target scintillator cell. Eleven observationdetectors were positioned at forward angles with respect to the incoming neutron beam. The experimental setup was configured to target proton recoil energies down to tens ofkeV. The observation detectors were placed such that the lowest angles were approximately10 ◦ with respect to the axis defined by incoming beam. Table 1 provides a measurementof the detector locations for the experiment. The origin of the coordinate system is definedat the beamline center along the west wall of the experimental area. The x -coordinate wastaken as the distance from the west wall, the y -coordinate as the distance north from thebeamline center, and the z -coordinate as the distance above and below the beamline center.The flight path between the Be breakup target and the origin of the coordinate system was646 . ± . x -axis, resulting in incident neutron flight paths to the targetscintillators of approximately 7 m. The flight path between the target scintillator and theobservation detectors ranged from 1 . . × × ± . ± . − . ± . ± . ± . − . ± . ± . ± . − . ± . ± . ± − . ± . ± . ± − . ± . ± . ± − . ± . ± . ± − . ± . ± . ± − . ± . ± . ± − . ± . ± . ± − . ± . ± . ± − . ± . ± . ± − . ± . ± . ± − . ± . ± . ± − . ± . Table 1: Summary of detector materials, identification (ID) numbers, locations, and estimated uncertaintiesin the measurements. an event from both target PMTs and an observation cell occurred. The bias voltage of thePMTs in the dual-PMT target configuration was selected to roughly gain match the detec-tors by setting the full height of signals corresponding to the Compton edge of the 662 keV γ ray from a Cs source at 75% of the 2 V full scale range of the digitizer. The targetcell gain settings ranged from 1650 V to 2000 V. To provide a systematic check and cover abroader proton recoil energy range, the EJ-204 proton light yield data were measured usingtwo independent bias voltage settings of the dual-PMT target.Data were acquired over a period of approximately 10 h for each bias voltage settingof the target scintillators with a beam current of approximately 40 nA. The data wererecorded using a CAEN V1730 500 MS/s digitizer. The arrival time of the scintillatorsignals was established using the CAEN digital constant fraction discrimination algorithm,with a 75% fraction and 4 ns delay [13]. The timing pick-off for the cyclotron RF signal wasaccomplished using leading edge discrimination. Full waveforms of the events were writtento disk in list mode with global time stamps for pulse processing and event reconstructionin post-processing.
Data reduction was accomplished in an object-oriented C ++ framework, which used el-ements of the ROOT data analysis framework [14]. Waveforms from the scintillator cellswere reduced to pulse integrals with 200 ns integration lengths ensuring collection of 95% ofthe scintillation light. The PMT response functions were measured for all target PMTs at4ach of the run-time bias voltages using a finite difference method adapted from Friend etal. [15] and the linearity of the responses was confirmed.To finely balance the signals from the two target PMTs in post-processing, a Cssource was placed > z -dimension and pulse integral spectra were taken. The minimum χ method was used to adjust the pulse integral of one signal relative to the other assuming alinear scaling with a resolution term to allow for variability amongst the response of the twoPMTs [16]. The signals from each PMT were then combined using a geometric average toobtain the total scintillation light independent of the interaction location [17]. The resultingsignal was then calibrated as described in the following section. Given the known non-proportionality of the electron response of EJ-200 [18, 19] and thesimilar formulation of EJ-200, EJ-204, and EJ-208, the electron-equivalent light unit is nota useful quantity for these materials. As such, the light output was determined relative tothat of a 477 keV electron, as evaluated using the Compton edge of a 662 keV γ ray. Using a Cs source placed > γ ray was also performed using Geant4 [20] and convolved with a detector reso-lution function [16]. The simulation geometry included the target scintillator cell, the thinaluminum sleeve, and the PMT borosilicate glass window and magnetic shield. The channelnumber corresponding to the Compton edge of the 662 keV γ ray was obtained by minimiz-ing χ between the measured and simulated source spectra. The parameter minimizationswere performed using the SIMPLEX and MIGRAD algorithms from the ROOT Minuit2package, and error matrices were obtained using the HESSE algorithm [14]. The relativelight unit was then defined as the light corresponding to a given proton recoil energy relativeto the light produced by a 477 keV recoil electron. Timing calibrations were performed for both the incident and outgoing TOF measure-ments as described in Ref. [10]. The incoming TOF, t inc , can be represented as: t inc = t + t − t RF − t c + n × T cyc , (2)where t and t are the clock times of the event in the top and bottom PMTs of the dual-PMTtarget, respectively, t RF is the clock time of the cyclotron RF signal, t c is a time calibrationconstant, n is an integer offset corresponding to the different cyclotron pulses, and T cyc isthe cyclotron pulse period. A coincidence window was applied in post-processing for thetwo target PMT signals, enforcing that events observed were within 10 ns of each other.To obtain the time calibration constant, a histogram of the arrival time of events in thetarget cell was constructed. A binned maximum likelihood estimation between the photonflash arising from deuteron breakup on the Be target and the superposition of a normaldistribution with a linear background term was performed. The centroid of the photon of5 ncoming TOF [ns]50 − − − − − − − − − C oun t s / n s Figure 2: (Color online) The blue curve shows a histogram of time differences between the cyclotron RFsignal and γ -ray events in the EJ-200 target detector. The width of the pulse is primarily reflective of thespatial spreading of the beam pulse, with σ = 5 . the flash was obtained and t c was then set using the speed of light and the known flightpath. This is illustrated in Fig. 2 for the EJ-200 target scintillator. The uncertainty in theincoming TOF determination is dominated by the uncertainty in t c with the spreading ofthe photon flash being σ = 5 . T cyc , was 158.5 ns, resultingin an integer ambiguity, n , in the incoming flight time due to frame overlap, where slowerneutrons from previous pulses have the same arrival time as fast neutrons from a given pulse.The timing calibration of the outgoing TOF was performed using γ − γ coincidencesbetween the dual-PMT target and observation detectors in a procedure similar to thatdescribed above. The outgoing TOF, t out , was taken as: t out = t − t + t − t c , (3)where t is the clock time of the event in the observation cell. The calibration time constantwas determined by creating a histogram of the γ − γ coincidences, with pulse shape dis-crimination applied in the observation detectors, and fitting using the procedure describedabove. To resolve the integer ambiguity, n , in the incoming TOF described in Eq. 2, anevent-by-event comparison was made between the measured incoming TOF and expectedincoming TOF as determined via kinematics using the scattering angle and outgoing TOF.The event reconstruction included a requirement that the absolute time difference betweenthe expected and measured incoming TOF was less than 15.8 ns, or <
10% of the cyclotron6 elative Light Yield L i gh t U n i t s - × C oun t s / . (a) E p = 64 keV, ID 9 − Relative Light Yield L i gh t U n i t s - × C oun t s / . (b) E p = 86 keV, ID 9 − Relative Light Yield C oun t s / . L i gh t U n i t s (c) E p = 202 keV, ID 5 − Relative Light Yield C oun t s / . L i gh t U n i t s (d) E p = 451 keV, ID 5 − Relative Light Yield C oun t s / . L i gh t U n i t s (e) E p = 653 keV, ID 0 − Relative Light Yield C oun t s / . L i gh t U n i t s (f) E p = 1344 keV, ID 0 − Figure 3: (Color online) Relative light yield spectra (blue curves) for a given proton energy bin fit witha piecewise power law and a normal distribution (red dashed curves) for the EJ-200 measurement. Thecentroid of the normal distribution corresponds to the mean light production for n-p elastic scatteringevents within the bin. The ID numbers specify the observation detectors used to observe this feature. period.
The relativistic energy-time relationship was employed to determine the incoming andoutgoing neutron energy from the respective TOF. With the known scattering angle, θ ,the proton recoil energy was then determined on an event-by-event basis using Eq. 1. Thedata were grouped into three sets based on the ID number of the observation detector (seeTable 1): ID 0-4, ID 5-8, and ID 9-10, covering an angle range of 31 − ◦ , 17 − ◦ , and10 − ◦ , respectively, with respect to the axis defined by the incoming beam.The data were reduced into a two-dimensional histogram of light output versus protonrecoil energy, where the proton energy discretization corresponded to the lowest energy reso-lution for the detector group considered. The resolution was calculated using the uncertaintyin the incoming TOF calibration, the uncertainty in the flight path, and an angular varianceof 0.8 ◦ , the latter obtained via a Monte Carlo simulation of the detector array. To helpensure events included in the histogram arose from single n-p elastic scattering events inthe dual-PMT target, pulse shape discrimination was used to isolate neutron events in the7bservation cells and a maximum outgoing neutron energy constraint was applied reflectiveof the beam energy and known detector configuration. To produce a series of data points,projections of individual proton energy bins were made on the light yield axis. The resultinghistograms were fit using a binned maximum likelihood estimation considering the peak to benormally distributed and the background to be represented by a continuous piecewise powerlaw with two subdomains. Representative fits of the relative light yield spectra for protonrecoil energies from 64 keV to 1.34 MeV for EJ-200 are shown in Fig. 3. The coincidenttrigger of the dual-PMT target, the trigger detection threshold, and n- γ rejection criteria inthe observation detectors are potential sources of bias in the relative light distributions forlow proton recoil energies. The sources of systematic uncertainty in the measurement of the light output corre-sponding to a given proton recoil energy include uncertainty in the determination of theincoming and coincident TOF temporal calibration constants, uncertainty in the detectorlocations, and uncertainty in the measured distance from the breakup target to the west wallof the experimental cave. To address the uncertainty from these potential sources of bias, aMonte Carlo method was implemented where the reduction from histogrammed quantitiesto data points was repeated while the input to calculations of the proton energy and lightoutput were varied. The uncertainty in the channel number corresponding to the
Cs edgedetermination was added in quadrature with the uncertainty obtained from the Monte Carlosimulation to provide the total uncertainty on the relative light yield.
3. Results and Discussion
Figure 4 shows a plot of the relative proton light yield of EJ-200. As described in Sec. 2.3,the relative light unit is the light corresponding to a given proton recoil energy relative to thelight produced by a 477 keV recoil electron. The data from this work are presented in threegroups corresponding to three different neutron scattering angular ranges. The x -error barsreflect the proton energy resolution for the lowest scattering angle in a given observationdetector grouping to provide a conservative estimate. The y -error bars include both thestatistical and systematic uncertainty and are generally smaller than the data points. Thereexists a kinematic overlap, where multiple observation detectors view the same proton recoilenergy range in the regions of approximately 150 −
250 keV and 0 . − roton Energy [MeV] − −
10 1 10 R e l a t i v e L i gh t Y i e l d − −
10 1
EJ-200 ID 0-4EJ-200 ID 5-8EJ-200 ID 9-10Zhang et al.
Figure 4: (Color online) Relative light yield of EJ-200 as a function of proton recoil energy. The ID numbersspecify the observation detectors used to obtain the result in a given proton recoil energy range. excellent agreement was observed between the two measurements. The data for the highgains measurement extend down to 47 keV, providing a new low energy limit for protonlight yield data on this material. A comparison to a measurement by Boccaccio et al. of acommercial equivalent, Nuclear Enterprise’s NE-104, obtained using an edge characterizationtechnique also demonstrates good agreement [23].Figure 6 shows a plot of the relative proton light yield of EJ-208. The data were comparedwith a measurement from Renner et al. on a commercial equivalent, Nuclear Enterprise’sNE-110, obtained using edge characterization without a Monte Carlo normalization [24].The results demonstrate general agreement over the full energy range examined within theestimated uncertainty, though the data from Renner et al. tend to lie systematically abovethe EJ-208 result. A summary table of the light yield data obtained in this work is providedin Appendix A.
4. Summary
The proton light yield of EJ-200, EJ-204, and EJ-208 was measured using a double TOFtechnique over a proton recoil energy range of approximately 50 keV to 5 MeV. These werecompared to proton light yield data from commercial equivalents and a general consistencyin the results was observed. Similar proton light yield relations were also observed for thethree materials, which is reasonable given that they each employ the same polyvinyltoluenepolymer base and have similar carbon and hydrogen atomic densities. The low energy pro-ton light yield data obtained in this work enable the benchmarking of physics-based models9 roton Energy [MeV] − −
10 1 10 R e l a t i v e L i gh t Y i e l d − −
10 110
EJ-204 High GainsEJ-204 Low GainsBoccaccio et al.
Figure 5: (Color online) Relative light yield of EJ-204 as a function of proton recoil energy.
Proton Energy [MeV] − −
10 1 10 R e l a t i v e L i gh t Y i e l d − −
10 1
EJ-208Renner et al.
Figure 6: (Color online) Relative light yield of EJ-208 as a function of proton recoil energy.
10f the specific luminescence of organic scintillators, which can in turn enhance understand-ing of organic scintillator physics and support the design of new detector materials. Thiswork further supports the development of passive neutron imaging systems for nuclear se-curity applications by enabling improved accuracy and neutron energy resolution in imagersemploying double-scatter kinematic reconstruction.
Acknowledgements
The authors thank the 88-Inch Cyclotron operations and facilities staff for their helpin performing these experiments. Thanks also to Walid Younes for expert advice and use-ful insights. This material is based upon work supported by the U.S. Department of En-ergy, National Nuclear Security Administration, Office of Defense Nuclear NonproliferationResearch and Development (DNN R&D) through the Nuclear Science and Security Consor-tium under Award DE-NA0003180, Lawrence Berkeley National Laboratory under ContractDE-AC02-05CH11231, and Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Sandia National Laboratories is a multimission laboratory managed andoperated by National Technology and Engineering Solutions of Sandia LLC, a wholly ownedsubsidiary of Honeywell International Inc., for the U.S. Department of Energy’s NationalNuclear Security Administration under contract DE-NA0003525.
Appendix A. Light Yield Data
The relative proton light yield data for EJ-200 and EJ-208 obtained using a 200 nsintegration length are summarized in Table A.2. The relative proton light yield data for EJ-204 obtained using a 200 ns integration length for different gain settings of the dual-PMTtarget are summarized in Table A.3. The asymmetric proton recoil energy error bars arereflective of the non-uniform distribution of proton energies within a given bin.
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