© Published under licence by IOP Publishing Ltd. The identification of nitrogen by neutron activation has been utilized in both explosive detection and in-vivo metabolic analysis. The 10.8 MeV gamma ray line emitted by thermal neutron capture provides a unique signature, however, due to its high energy its registration is non-trivial. Conventional approaches have used large dense inorganic scintillators which inevitably entail considerable expense. We examine the capabilities of arrays of smaller scintillation detectors and the use of glass Cherenkov detectors as an alternative

A Cole

A Kirby

Ahnen M

Cole A

J E McMillan

Knoetig M

Litz M

M L Ahlen

Stamatelatos I

Journal of Physics : Conference Series

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Journal of Physics Conference Series

Scintillators and Cherenkov detectors for the registration of 10.8 MeV gamma rays

McMillan, J.E.

Cole, A.

Kirby, A.

Ahnen, M.L.

White Rose Research Online

Journal of Physics: Conference SeriesPAPER • OPEN ACCESSScintillators and Cherenkov detectors for theregistration of 10.8 MeV gamma raysTo cite this article: J E McMillan et al 2016 J. Phys.: Conf. Ser. 763 012011 View the article online for updates and enhancements.Related contentFast timing and trigger Cherenkov detectorfor collider experimentsV A Grigoryev, V A Kaplin, T LKaravicheva et al.-Cherenkov detectors for high-energyneutrino astrophysicsZh-A M Dzhilkibaev, G V Domogatsky andO V Suvorova-Ground detectors for the study of cosmicray showersH Salazar and L Villasenor-This content was downloaded from IP address 143.167.30.107 on 03/11/2017 at 12:07Scintillators and Cherenkov detectors for theregistration of 10.8MeV gamma raysJ E McMillan1, A Cole1, A Kirby1 and M L Ahnen21 Department of Physics and Astronomy, The University of Sheffield, Sheffield, SouthYorkshire, S3 7RH, UK2 ETH Zürich, Institute for Particle Physics, Otto-Stern-Weg 5, 8093 Zürich, SwitzerlandE-mail: j.e.mcmillan@sheffield.ac.ukAbstract. The identification of nitrogen by neutron activation has been utilized in bothexplosive detection and in-vivo metabolic analysis. The 10.8 MeV gamma ray line emittedby thermal neutron capture provides a unique signature, however, due to its high energy itsregistration is non-trivial. Conventional approaches have used large dense inorganic scintillatorswhich inevitably entail considerable expense. We examine the capabilities of arrays of smallerscintillation detectors and the use of glass Cherenkov detectors as an alternative.1. IntroductionHigh energy gamma rays are produced as a result of neutron activation of materials. There isparticular interest in detecting the 10.8 MeV gamma-ray line produced as a result of thermalcapture on nitrogen. The cross-section for the capture reaction on nitrogen is σγ,all = 79.0mbwhile the component generating 10.8 MeV gamma-rays is σγ,10.8 = 11.3mb. This reaction hasthe potential for use as an indicator of the presence of explosives, since all commercial andmost improvised explosives are rich in nitrogen [1]. There has also been some work on in-vivonitrogen/carbon ratios as a diagnostic of human metabolic disorders [2].Few elements, and certainly no other common element, has significant gamma emission at10.8 MeV or higher [2]. While the detection of these high energy gamma-rays offers a uniquesignature, actual detection is more problematic, due to the size and density of detectors requiredfor total absorption. The task is made more difficult by the fact that the gamma detectors areinevitably operated in a mixed field also containing neutrons of all energies from productiondown to thermal.High-purity germanium (HPGe) detectors are ill-suited to the task, having low efficiency andrequiring cryogenic cooling. Germanium suffers from activation problems [3], while the expenseof HPGe and other semiconductor detectors rules them out for deployment in field deployedexplosive detector systems.Conventional wisdom is to use large dense inorganic scintillators, and the obvious choice issodium iodide (NaI(Tl)). Data from Saint-Gobain [4] indicate that at 10.8 MeV, 50mm NaI(Tl)crystals have an absorption efficiency of 50% and that 120mm crystals have an efficiency ofaround 80%. The studies by Stamatelatos et al.[3], McFee et al.[5] and Gierlik et al.[6] showthat there is little to be gained by using recent dense inorganic scintillators such as bismuthgermanate (BGO) or lanthanum bromide (LaBr3). These materials have activation problemsApplications of Novel Scintillators for Research and Industry 2016 (ANSRI 2016) IOP PublishingJournal of Physics: Conference Series 763 (2016) 012011 doi:10.1088/1742-6596/763/1/012011Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd 1Figure 1. Gamma spectrum produced on 10kg urea sample in 300 minutes.and the improvements in detection efficiency are outweighed by the huge increase in cost overNaI(Tl). Overall detection efficiency can be improved more cost-effectively by deploying a largerarea of NaI(Tl).Stamatelatos et al.[3] emphasize that the measurement is a signal-to-noise problem andthat reducing background is more important than increasing signal. They stress that, in theenvironment of a neutron source, it is necessary to shield the detectors adequately from bothgamma-rays coming from directions other than the target and from neutrons. They suggestthe use of bismuth for the gamma shielding as it activates to a much lesser extent than leadand the resulting activation products are not gamma emitters. Further they suggest a layeredshielding approach with both neutron and gamma absorbing materials. Closest to the crystal,they used a layer of resin loaded with 6LiF, to absorb thermal neutrons without the emission ofgamma-rays. 6LiF is now too expensive to be considered for shielding, but we have found thata 10mm layer of a 40/60 mixture of wax and unenriched lithium carbonate intended for ceramicglazes is similarly effective at suppressing thermal neutrons.Stamatelatos et al.[3] also note that significant background is caused by pulse pile-up in whichtwo lower energy gamma-rays interact in the crystal within the decay time of 250ns. They pointout that this can be reduced by using a number of crystals 50mm diameter by 100mm long, eachwith its own readout, rather than a large single crystal.2. Measurements with DT generatorUsing an NSD-Fusion 14 MeV DT neutron generator and pelleted urea as an explosives surrogate,we have demonstrated this technique [7]. The emitted gamma-rays were recorded with a single4′′×4′′ NaI crystal fed to a pulse height analyzer. The crystal was lead shielded in all directionsbut the forward. The pulses were time gated in order to be optimally sensitive to activationcaused by thermal neutrons. Typically three hour exposures were employed on 10kg urea sampleswhich provided a sufficiently detailed spectrum to understand the processes.A typical observed spectrum is shown in figure 1. The background caused by pulse pile-up is particularly severe in the region 4–7 MeV. By subtracting the background polypropylenespectrum from the urea spectrum, the 10.8 MeV emission can clearly be seen as in figure 2.Despite the pulse pile-up and broadening which it causes, a statistically significant peak couldApplications of Novel Scintillators for Research and Industry 2016 (ANSRI 2016) IOP PublishingJournal of Physics: Conference Series 763 (2016) 012011 doi:10.1088/1742-6596/763/1/0120112Figure 2. 10.8MeV peak obtained from 10kg urea sample in 300 minutesbe obtained in around ten minutes. Further details of these measurements can be found inCole [7].3. Cherenkov detectorsTo achieve similar results in shorter time, it is clear that a larger area of detector must be used.As arrays of NaI(Tl) scintillators are likely to remain expensive, it was decided to investigatewhether much cheaper Cherenkov detectors could be used.Cherenkov detectors with dense radiators have found extensive use as charged particledetectors but are also useful for gamma-ray detection where fast timing is required and wheregood energy resolution is not important. Sowerby [8] examined acrylic Cherenkov detectors foruse over the range 1.1–2.6 MeV and Sober et al. [9] consider lead-glass detectors of 153mmdepth for the region 10-20 MeV. They show there is a threshold below which no signal isproduced, while at 10.8 MeV the efficiency can be expected to be around 60%. Grannis etal. [10] give indications regarding low-cost production of these detectors. Ayaz-Maierhafer etal. [11] have recently demonstrated the utility of glass Cherenkov detectors in the detection offission gammas. Ayaz-Maierhafer et al. [12] have also performed experimental measurements onneutron activation of Cherenkov glasses which suggest that their long term operation in mixedneutron-gamma fields is not likely to be problematic. Hayward et al. [13] provide a detailedsimulation of the response of glass Cherenkov detectors to 6 MeV gamma rays.It is evident from these studies, that 10.8 MeV is at the lower end of the useful energy rangeof Cherenkov detectors. However, they offer large size, high density and consequently highefficiency at low cost. Lead-glass is inherently cheaper than NaI(Tl), as an indication, SchottF2 glass was quoted as £56/kg [14]. Being non-hygroscopic, no special precautions are neededin construction and the resulting detectors are robust. Their time response is limited by therisetime of the photomultiplier used and is typically 10ns, so problems with pulse pile-up aremuch reduced.The obvious disadvantage of Cherenkov detectors derives from the low light output andthe subsequent poor energy resolution at 10.8MeV. In the absence of distinguishable peaksin the output spectra, the calibration of Cherenkov detectors is also problematic. This iscompounded by the unavailability of sources with suitable high-energy emission. Calibrationhas been considered in detail by Rose and Erickson [15].Applications of Novel Scintillators for Research and Industry 2016 (ANSRI 2016) IOP PublishingJournal of Physics: Conference Series 763 (2016) 012011 doi:10.1088/1742-6596/763/1/01201131.00.80.60.40.20.0120100806040200channel number background cobalt-60 thorium-232 neutron captures on nitrogenFigure 3. Normalized pulse height spectra obtained from lead-glass detector3.1. Experimental detectorTo test these ideas, an experimental lead-glass Cherenkov detector was constructed. A block ofSchott F2 lead-glass 80mm×80mm×80mm was obtained. The surfaces of this were shot-blasted,with the exception of a circular output window on one face. The block was then painted withEdmund 53-697 BaSO4 high reflectance paint, which provides good diffuse reflectance, even inthe ultraviolet. The block was then coupled to an ET9265K 70mm diameter photomultiplierusing a transparent silicone pad. The whole assembly was made light-tight with aluminium foiland black tape.The resulting detector worked largely as expected. The 9265K has a rise-time of 7.5ns, so theoutput pulses are around 15ns FWHM. Figure 3 shows normalized pulse height spectra obtainedin response to different sources. There is clearly an offset of some 20 channels on the x-axis,which should be ignored.The spectrum taken with 60Co, having peaks at 1.1 and 1.3 MeV shows little difference frombackground. The 232Th spectrum was taken with an old sample of ThO, so had emissions fromall the progeny, of which 208Tl is the most important as it decays with a major branch emittinga gamma of 2.62 MeV. The neutron captures on nitrogen spectrum was obtained by placing afew kilograms of urea in front of the detector and exposing it to thermal neutrons obtained froman AmBe source in a mass of polypropylene moderator. It can be seen that by channel 60,background and low-energy gammas are negligible. Any events registered with a height greaterthan channel 75 are interesting and indicative of high-energy gamma emission.Further to the spectra shown, there was a population of much larger pulses, with a peak ataround channel 1200 and a high energy tail. The peak was attributed to single cosmic muonstraversing the block, while the higher energy events are probably showers and residual gas eventsin the photomultiplier. As the rate of these events was many orders of magnitude less than thatexpected in a screening scenario, they can be ignored.These results are broadly similar to those obtained by Sober et al. [9] and Rose andErickson [15]. While detailed measurements have not yet been made, it appears that ourexperimental detector has an electron detection threshold at ∼138keV and at 10.8 MeV, adetection efficiency of ∼50% and an energy resolution of ∼50%.Applications of Novel Scintillators for Research and Industry 2016 (ANSRI 2016) IOP PublishingJournal of Physics: Conference Series 763 (2016) 012011 doi:10.1088/1742-6596/763/1/01201144. Detector arraysThe object of the proposed measurements is not to produce a detailed emission spectrum ofthe sample being irradiated, but simply to establish whether the emission is consistent with thepresence of emission at 10.8 MeV. Consequently the analysis of peaks in regions-of-interest canbe replaced with the simple registration of Cherenkov signals exceeding a threshold.A more convincing result may be obtained by combining the signals from a small area ofNaI(Tl) detectors with a larger area of Cherenkov detectors. An array containing a mixture ofthe two types of detectors is envisaged. In the presence of 10.8 MeV emission the NaI(Tl) wouldrecord a small number in the relevant peak region-of-interest, while the Cherenkov detectorswould record a large number of high-energy events. The most obvious conclusion would bethat the Cherenkov events have the same energy distribution as those recorded by the NaI(Tl)detector. The presence of high-energy Cherenkov events with no signal in the NaI(Tl) region ofinterest would be indicative of neutron activation of some other element than nitrogen.5. Combining datasetsThe combination of data from detectors with different resolutions and efficiencies and thedetermination of the presence of 10.8 MeV gamma emission can be examined using objectiveBayesian analysis [16, 17]. An algorithm using Bayes Factors calculated from count rates overspecified periods for the two types of detector is proposed. In the case of the NaI(Tl) detectorthere is a region of interest (ROI) which includes the 10.8 MeV peak, while in the case of theCherenkov detector, the high-energy region of interest is everything above a discriminationthreshold. The Bayes factors B01 are calculated as per equation 3.1 of [17]. Similarly thethreshold is set as a Bayesian z-value as per equation 3.3 of [17].Record background for a certain time toff :Record background with NaI detector,number of events in the 10.8MeV RoI is Noff,NaIRecord background with Cherenkov detector,number of events in the high-energy RoI is Noff,CheRecord sample for a certain time ton:Record sample with NaI detector,number of events in the 10.8MeV RoI is Non,NaIRecord sample with Cherenkov detector,number of events in the high-energy RoI is Non,CheCalculate individual Bayes Factors that events are due to background only:Calculate Bayes Factor for NaI detector with:B01,NaI(Non,NaI, Noff,NaI, ton/toff )Calculate Bayes Factor for Cherenkov detector with:B01,Che(Non,Che, Noff,Che, ton/toff )As Bayes Factors are commutative, the experiments can be performed in any order.The new odds result from previous odds by multiplication:Resulting odds of the background (H0) over signal (H1) hypothesis:P (H0)/P (H1) = B01,NaI ∗B01,CheSet a threshold. If P (H0)/P (H1) < 5.7 × 10−7 then there is 5σ detectionApplications of Novel Scintillators for Research and Industry 2016 (ANSRI 2016) IOP PublishingJournal of Physics: Conference Series 763 (2016) 012011 doi:10.1088/1742-6596/763/1/01201156. Current statusThe shielding of both the NaI(Tl) and Cherenkov detector needs improvement and more detailedcalibration runs of the Cherenkov detector are planned. The Bayesian algorithm is being testedwith simulated data. If the initial experiments look fruitful, a larger detector array will be built.AcknowledgementsThis work was supported by EPSRC EP/G005141/1 “Neutron Screening for Airborne Cargo”.We would like to thank Prof. Dick Lacey of the Home Office Centre for Applied Science andTechnology and Dr. Marieke Navin for the fabrication of the lead-glass block.References[1] Litz M et al. 2012 Neutron-activated gamma-emission: Technology review Tech. Rep. ARL-TR-5871 ArmyResearch Laboratory, Adelphi, MD, USA[2] Molnar G 2004 Handbook of prompt gamma activation analysis (Kluwer Academic Publishers, Dordrecht,NL)[3] Stamatelatos I, Chettle D and Scott M 1993 Phys. Med. Biol 38 411[4] 2014 Efficiency calculations for selected scintillators, Saint-Gobain Ceramics & Plastics, Inc.[5] McFee J E et al. 2009 IEEE Trans. Nucl. Sci. 56 1584–1592[6] Gierlik M et al. 2012 IEEE Trans. Nucl. Sci. 59 230–235[7] Cole A 2015 Installation and Characterization of a pulsed-neutron generator at the University of SheffieldNeutron Facility for use in a Feasibility Study into Pulsed Fast-Thermal Neutron Activation Analysis forCargo Screening Ph.D. thesis The University of Sheffield[8] Sowerby B 1971 Nucl. Instr. Meth. 97 145–149[9] Sober D et al. 1973 Nucl. Instr. Meth. 108 573–579[10] Grannis P, Jaffe D and Marx M 1981 Nucl. Instr. Meth. Phys. Res. 188 239–242[11] Ayaz-Maierhafer B, Laubach M A and Hayward J P 2016 J. Radioanal. Nucl. Chem. 308 919–926[12] Ayaz-Maierhafer B et al. 2013 IEEE Trans. Nucl. Sci. 60 701–707[13] Hayward J P et al. 2013 J. Radioanal. Nucl. Chem. 295 1321–1329[14] 2016 David Bentley, private communication, H.V.Skan Ltd.[15] Rose P and Erickson A 2015 Nucl. Instr. Meth. Phys. Res. A799 99–104[16] Knoetig M 2014 Astrophys. J. 790 106 (8pp.)[17] Ahnen M 2015 34th International Cosmic Ray Conference, POS(ICRC2015)701Applications of Novel Scintillators for Research and Industry 2016 (ANSRI 2016) IOP PublishingJournal of Physics: Conference Series 763 (2016) 012011 doi:10.1088/1742-6596/763/1/0120116

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Scintillators and Cherenkov detectors for the registration of 10.8 MeV gamma rays

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