A detector material or configuration that can provide an unambiguous
indication of neutron capture can substantially reduce random coincidence
backgrounds in antineutrino detection and capture-gated neutron spectrometry
applications. Here we investigate the performance of such a material, a
composite of plastic scintillator and
$^6$Li$_6^{nat}$Gd$(^{10}$BO$_{3})_{3}$:Ce (LGB) crystal shards of ~1 mm
dimension and comprising 1% of the detector by mass. While it is found that the
optical propagation properties of this material as currently fabricated are
only marginally acceptable for antineutrino detection, its neutron capture
identification ability is encouraging.Comment: 6 pages, 7 figures, submitted to Nuclear Instruments and Methods 

Abbes

Agostinelli

Aoyama

Bowden

Czirr

N.S. Bowden

P. Nelson

English

arXiv

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Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment

Investigation of large LGB detectors for antineutrino detection

A detector material or configuration that can provide an unambiguous indication of neutron capture can substantially reduce random coincidence backgrounds in antineutrino detection and capture-gated neutron spectrometry applications. Here we investigate the performance of such a material, a composite of plastic scintillator and $^6$Li$_6^{nat}$Gd$(^{10}$BO$_{3})_{3}$:Ce (LGB) crystal shards of ~1 mm dimension and comprising 1% of the detector by mass. While it is found that the optical propagation properties of this material as currently fabricated are only marginally acceptable for antineutrino detection, its neutron capture identification ability is encouraging

Nelson, P.

Bowden, N. S.

Calhoun, Institutional Archive of the Naval Postgraduate School

Calhoun: The NPS Institutional ArchiveFaculty and Researcher Publications Faculty and Researcher Publications2011Investigation of Large LGB Detectors forAntineutrino DetectionNelson, P.http://hdl.handle.net/10945/35449Investigation of Large LGB Detectors for Antineutrino DetectionP. Nelsona,1, N. S. Bowdenb,∗aDepartment of Physics, Naval Postgraduate School, Monterey, CA 93943, USAbLawrence Livermore National Laboratory, Livermore, CA 94550, USAAbstractA detector material or configuration that can provide an unambiguous indication of neutron capture can sub-stantially reduce random coincidence backgrounds in antineutrino detection and capture-gated neutron spectrome-try applications. Here we investigate the performance of such a material, a composite of plastic scintillator and6Linat6 Gd(10BO3)3:Ce (LGB) crystal shards of ≈ 1 mm dimension and comprising 1% of the detector by mass. Whileit is found that the optical propagation properties of this material as currently fabricated are only marginally acceptablefor antineutrino detection, its neutron capture identification ability is encouraging.Keywords: thermal neutron detection, capture-gated neutron spectrometry1. IntroductionNeutron detection systems incorporating a neutroncapture agent or neutron capture indicating detectorhave many applications. These include thermal neutrondetection (e.g. 3He tubes), capture-gated neutron spec-trometry [1], and reactor antineutrino detection [2, 3].Materials or systems that provide an unambiguous in-dication of a neutron capture are preferred in all theseapplications, as this provides a means of rejecting back-ground. In all of the applications listed above, gammaray rejection is of high importance, while in antineutrinodetection rejection of multiple neutrons from a singlecosmic ray could also be advantageous.Here we describe an investigation of a detector ma-terial that can identify neutron captures and there-fore reject a large fraction of these background events.The material, produced by MSI/Photogenics, consistsof ≈ 1 mm shards of an inorganic scintillator dis-tributed in a plastic scintillator matrix, loaded to 1% byweight. The inorganic scintillator 6Linat6 Gd(10BO3)3:Ce(“LGB”), has a very high neutron capture cross sec-tion, high light output, and has an index of refectionwell matched to that of plastic scintillator [4]. Neutroncaptures on 6Li or 10B are relatively easy to identify viaPulse Shape Discrimination (PSD) techniques, since the∗Corresponding Author. Tel.: +1 925 422 4923.Email address: nbowden@llnl.gov (N. S. Bowden)1Present Address: United States Military Academy, West Point,NY 10996 USAresulting heavy ions are fully contained within the crys-tal shards and the inorganic scintillator has a relativelylong (≈ 200 ns) decay time, compared to the organicscintillator (≈ 3 ns).Previous studies of this composite material have usedhigher crystal loadings so as to maximize neutron cap-ture efficiency [5, 6]. These higher loadings led to rela-tively poor optical properties, and therefore limited theuseable volume. Here, we consider the effect of moremodest loadings, and assess the suitability of the inho-mogeneous material for larger detectors like those re-quired for for antineutrino detection and large area fastand thermal neutron detectors.2. Detector DescriptionsTwo detectors were acquired from MSI/Photogenicsfor this investigation. The first was a cylinder of12 cm diameter and 12.3 cm length containing crystalshards ranging in linear dimension between 0.5 mm and1.5 mm. The second was a cylinder of 12 cm diameterand 34.8 cm length containing crystal shards ranging inlinear dimension between 1.5 mm and 3.0 mm.Each detector was loaded with 1% by weight of LGBcrystal shards. A photograph of the smaller detector isshown in Fig. 1. Clearly, the optical transmission prop-erties of the base plastic material in the visible lightspectrum are affected by the inclusion of the crystalshards and a trapped air bubble is apparent. HamamatsuH6527 12.5 cm Photomultipler Tube (PMT) assembliesPreprint submitted to Nuclear Instruments and Methods July 1, 2011arXiv:1106.6083v1  [physics.ins-det]  29 Jun 2011Figure 1: A photograph of the smaller of the two detectors examined.The LGB crystal shards can be clealry, seen, as can a trapped air bub-ble.were coupled to each end of the detector cylinders usingoptical grease.3. Data Acquisition SystemA schematic of the QDC-based system is shown inFig. 2a. Two copies of each PMT signal are integratedby a CAEN V792N QDC. The delay between these twocopies is carefully adjusted with respect to the QDCgate so that one copy is fully integrated, while only thetail portion of the second copy is. As demonstrated inFig. 2b this approach yields a different value for the ra-tio of integrated amplitudes for events with different de-cay constants. The full charge integral can be used foran event energy determination. This method is equiv-alent to implementing two different QDC gates duringthe charge integration of each PMT pulse. The ratioof the amplitudes thus determined (“tail/full”) dependsupon the decay time constant of the PMT pulse. Forevents occurring within the fast decaying plastic scintil-lator this ratio will be small, whereas for events occur-ring within the slow decaying LGB crystals it will belarge.4. Detector CalibrationA relationship between recorded QDC charge andelectron-equivalent energy deposition in the plastic!Figure 0: Schematic diagram of scintillation light attenuation and neutron capture efficiency experimental apparatus. (a) (b) Figure 2: (a) A schematic diagram of the QDC-based DAQ system.Two copies of the PMT signal, with a relative delay, are integratedusing a CAEN V792N QDC. (b) A schematic timing diagram for theQDC DAQ. Two copies of the PMTs pulses are input to the QDC witha relative delay of ≈ 50 ns.scintillator component of the two detectors was estab-lished using the 511 keV and 1275 keV gamma raysemitted by a 22Na source. A “fan” collimator of leadbricks separated by a 0.4 cm gap was used to prefer-entially illuminate the center portion of each detector.The “Full” response was recorded and is plotted for eachPMT and detector in Fig. 3. The energy scale was estab-lished using a GEANT4 [7] simulation of the calibra-tion configuration - two features corresponding to theCompton edges of the two gamma rays can be clearlyobserved in Fig. 3 . Detector resolutions of 15% and25% were found, qualitatively, to be a good representa-tion of the properties of the 12.3 cm and 34.8 cm detec-tors, respectively.Using the reasonable approximation of exponentialattenuation of light as it propagates through the detec-2Energy (MeV)0.2 0.4 0.6 0.8 1 1.2 1.4 1.6Normalized Counts00.0020.0040.0060.0080.01 source at 0 cmγNa 22Left PMT with  source at 0 cmγNa 22Right PMT with with 15% Energy ResolutionSimulation of Detector Response(a)Energy (MeV)0.2 0.4 0.6 0.8 1 1.2 1.4 1.6Normalized Counts00.0020.0040.0060.0080.010.012 source at 0 cmγNa 22Left PMT with  source at 0 cmγNa 22Right PMT with  with 25% Energy ResolutionSimulation of Detector Response(b)Figure 3: Calibration spectra taken with a collimated 22Na sourceplaced at the center of the (a) 12.3 cm and (b) 34.8 cm detectors iscompared to the simulated response.tors, we can combine the recorded signals of the “left”and “right” PMTs (ELand ER) to correct for interactionposition dependencies in the analysis that follows. As-suming an effective attenuation length (αe f f ), that in-corporates the effect of the both non-perfect reflectionat the detector boundaries and optical absorption withinthe detector, we can write the event energy (E) as:√ELER =√(Ee−x/αe f f )(Eex/αe f f ) = E. (1)That this is a reasonable approach here can be seenin Fig. 4, where 22Na spectra taken at several collima-tor positions are compared. Each is in good qualitativeagreement with that taken at the center (the energy cali-bration position).5. Optical Attenuation MeasurementsThe collimated gamma ray source used for energycalibration was also used to measure the effective op-tical attenuation length for each detector. The variationin the absolute position (measured in QDC channels) ofthe spectral features due to the 511 keV and 1.275 MeVADC Channel0 200 400 600 800 1000Counts020004000600080001000012000140001600018000 CenterRight 2 cmRight 4 cmRight 5 cm(a)ADC Channel0 100 200 300 400 500 600Counts02000400060008000100001200014000 CenterRight 4 cmRight 10 cmRight 14 cm(b)Figure 4: Spectra taken with a collimated 22Na source placed at vari-ous positions relative to the center of the (a) 12.3 cm and (b) 34.8 cmdetectors. The good agreement between these spectra indicates thatposition dependencies are largely accounted for by the appropriatecombination of the two PMT amplitudes.γ-ray energy Effective Attenuation Length (cm)12.3 cm detector 34.8 cm detector511 keV 17.7 ± 2.0 cm 18.4 ± 1.2 cm1275 keV 16.8 ± 2.0 cm 18.0 ± 1.2 cmTable 1: Effective attenuation lengths measured for both detectors attwo γ-ray energies.γ-rays was recorded for each PMT from various sourcepositions. Representative results for the “left” PMT ofboth detectors are shown in Fig 5 for the 511 keV fea-ture. The inferred optical attenuation lengths thus mea-sured are given in Table. 1. The two independent mea-surements made for each detector are in good agree-ment.The modest attenuation lengths measured suggestthat this material, as currently manufactured, would notsupport detector lengths at the 1 m scale ideal for an-tineutrino detection applications. In a 1 m length detec-tor, light from an interaction at one end of the detectorwould be attenuated by least 2 orders of magnitude be-fore reaching the PMT at the other end. This wouldmake position reconstruction and correction unreliable.However, lengths of ≈ 50 cm appear feasible, and mightbe sufficient, given the attractive neutron capture identi-fication properties of the material.3Distance from PMT face (cm)0 2 4 6 8 10 12QDC channel050100150200250(a)Distance from PMT face (cm)10 12 14 16 18 20 22 24 26QDC channel020406080100120140160180200(a)(b)Figure 5: Relative optical intensity of the Compton edge due 511 keVγ-rays as a fucntion of γ-ray source position. Data are shown for the12.3 cm (a) and 34.8 cm (b) detectors.6. Neutron Capture Identification ResultsThe neutron capture response of both detectors wasinvestigated using a bare 2.5 µCi 252Cf source placed30 cm from the detector center. Plots of electron-equivalent energy aginst the PSD parameter (“tail” en-ergy/“full” energy) from 1 hour acquisitions are dis-played in Fig. 6 for the 12.3 cm detector and in Fig. 6for the 34.8 cm detector. A background acquisition isalso shown for comparison.Two distinct features with PSD parameters close tounity due to neutron capture can be observed in theseplots: one near an electron equivalent energy of ≈2.2 MeVee due to captures on 6Li, and the other near0.75 MeVee due to captures on 10B. Good separationfrom events in the plastic scintillator is especially evi-dent for the 6Li captures. The 10B capture feature spansa greater range of values in the PSD parameter, since93% of these events are accompanied by a 478 keVγ ray. Compton scatters of those γ rays add to the en-ergy of the capture event and reduce the PSD parametervalue, since they increase the “Full” integral value, butnot that of the “tail” integral.The 10B capture feature is also more difficult to dis-tinguish from background γ ray interactions due to itslower average energy, which is due both to the lowerQ-value of the neutron capture reaction and the higherquenching of the heavier daughter products. Low en-ergy Compton electrons can deposit a significant frac- (MeVee)fullE0 0.5 1 1.5 2 2.5 3 3.5 4 full/EtailE00.20.40.60.811.21.4110210310410(a) (MeVee)fullE0 0.5 1 1.5 2 2.5 3 3.5 4 full/EtailE00.20.40.60.811.21.4110210310410(b)Figure 6: (a) Data demonstrating the ability of the 12.3cm detector toidentify neutron captures via PSD when irradiated by a 252Cf source.(b) A background spectra is shown for comparison.tion of their total energy in a crystal grain, raisingthe measured value of the PSD parameter for thoseevents. This effect appears to contribute a significantnon-neutron background to the 10B capture region.Measurements of the neutron capture efficiency werecompared to a GEANT4 simulation (Table 2). TheGEANT4 model incorporated randomly distributedLGB inclusions matching the average size quoted by themanufacturer and tallied the number of neutrons cap-tured upon 6Li and 10B for an incident fission neutronspectrum. The error on the simulated efficiency was es-timated by both increasing and decreasing the LGB par-ticle size by 50%, since the particle size distribution isunknown. The 10B efficiency is not considered, sincethere are several reasons why the measured efficiencyis unreliable. First, the relatively poor γ ray discrimi-nation of the 10B capture feature and the multitude ofhigh energy γ rays emitted by 252Cf made reliable neu-tron counting difficult with this nucleus. Second, it wasnoticed via inspection of oscilloscope traces that manyapparent 10B neutron captures did not trigger the DAQsystem. This is because the triggering is based upon avoltage level discriminator - the maximum voltage ofthe slow 10B PMT pulse is considerably lower than thatof a γ ray with an equivalent integrated PMT amplitude.The reasonable agreement between experiment andsimulation allows us to use that same simulation to es-timate the neutron capture efficiency of this material for4 (MeVee)fullE0 0.5 1 1.5 2 2.5 3 3.5 4fullEtailE00.20.40.60.811.21.4110210310410(a) (MeVee)fullE0 0.5 1 1.5 2 2.5 3 3.5 4fullEtailE00.20.40.60.811.21.4110210310410(b)Figure 7: (a) Data demonstrating the ability of the 34.8cm detector toidentify neutron captures via PSD when irradiated by a 252Cf source.(b) A background spectra is shown for comparison.6Li Capture Efficiency (%)12.3 cm detector 34.8 cm detectorMeasured 1.71 ± 0.24 1.38 ± 0.14Simulated 1.47 ± 0.25 1.76 ± 0.25Table 2: Measured and simulated neutron capture efficiencies upon6Li.lower neutron energies (≈ 10 keV) relevant to the an-tineutrino detection application, and to investigate theeffect of varying the LGB crystal isotopic composition.The GEANT4 simulation was repeated for a 1 m3 detec-tor with 10 keV neutrons distributed uniformly withinthe detector volume. The fraction of neutrons that cap-ture on 6Li, 10B, Gd (155Gd or 157Gd), H, or that escapethe detector are given in Table 3 for several differentLGB isotopic compositions.Assuming that a means could be found to reliablyregister all 10B captures, the isotopic composition usedin this investigation would yield a total neutron captureefficiency for inverse beta-decay detection of ≈ 30%,similar to that of previously demonstrated antineutrinodetectors for reactor monitoring [3], but far from ideal.If it were economically feasible to produce gadoliniumdepleted in 155Gd and 157Gd, very attractive capture ef-ficiencies could be achieved. In particular, use of depGdand 11B would be ideal, providing high efficiency forthe easily identified 6Li capture.Isotopics Fraction of neutrons (%)6Li 10B Gd H Escape6LinatGd10B 10.8 21.4 43.0 12.0 12.86LidepGd10B 23.4 49.4 - 14.6 12.66LidepGd11B 63.3 - - 21.5 15.26LinatGd11B 15.0 - 59.9 12.5 12.6Table 3: The predicted fraction of neutrons that capture on particularnuclei or that escape a 1 m3 detector. depGd refers to gadolinium thathas been completely depleted of 155Gd and 157Gd.7. ConclusionThe composite LGB/Plastic scintillator material hasmany attractive features for antineutrino detection. Thevery good PSD separation of 6Li neutron captureswould provide a powerful means to reject random γ raycoincidences. This in turn might allow for a reduction inpassive shielding and a reduction in total device size—an important parameter for the reactor monitoring ap-plication. That that material is based upon solid plasticcould have practical advantages over the more typicallyused liquid scintillator.However, this material, as currently realized, also hassignificant drawbacks. The optical properties do notsupport detector elements of the desired length. Also,despite its very high neutron capture cross section, theincorporation of gadolinium in the inorganic crystal re-sults in a reduced identifiable neutron capture efficiency.Further work to improve the manufacturing process thatincorporates the crystals shards in the plastic would bebeneficial.Other avenues for future investigation include study-ing the effect of crystal shard size upon neutron captureefficiency and attempting to identify other suitable inor-ganic crystals that contain only 6Li as a neutron cap-ture agent. Finally, we note that this material wouldbe ideal for use in a segmented capture-gated neutronspectrometer: appropriate segmentation would allowfor enhanced event-by-event resolution [8], while theunambiguous neutron capture indication would providestrong background rejection.AcknowledgementsLLNL-JRNL-482515.This work was performed under the auspices of theU.S. Department of Energy by Lawrence Livermore Na-tional Laboratory in part under Contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344.5References[1] J. B. Czirr and G. L. Jensen, Nucl. Inst. and Meth. A 284 (1989)365[2] M. Abbes, et al., Nucl. Inst. and Meth. A 284 (1996) 164[3] N. S. Bowden, et. al., Nucl. Instr. and Meth. A. 572 (2007) 985[4] J. B. Czirr et al., Nucl. Inst. and Meth. A 476 (2002) 309[5] M. Flaska, et al., IEEE NSS Conf. Rec. (2008) 3376[6] N. Menaa, et al., IEEE Trans. Nucl. Sci. 56 (2009) 911[7] S. Agostinelli et al., Nucl. Inst. and Meth. A 506 (2003) 250[8] N. S. Bowden, et. al., Nucl. Instr. and Meth. A. 609 (2009) 326

Investigation of Large LGB Detectors for Antineutrino Detection

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Investigation of Large LGB Detectors for Antineutrino Detection

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