~Alamos LA-UR- Development of a Neutron POD Spectrometer. Sy Stange, Douglas Mayo, Rollin Lakis, and M. William Johnson

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1 LA-UR- Apprved fr public release; distributin is unlimited. Title: Develpment f a Neutrn POD Spectrmeter Authr(s): Sy Stange, Duglas May, Rllin Lakis, and M. William Jhnsn Intended fr: Institute f Nuclear Materials Management 2011 Annual Meeting ~Alams NATIONAL LABORATORY --- EST Ls Alams Natinal Labratry, an affirmative actin/equal pprtunity emplyer, is perated by the Ls Alams Natinal Security, LLC fr the Natinal Nuclear Security Administratin f the U.S. Department f Energy under cntract DE-AC52-06NA By acceptance f this article, the publisher recgnizes that the U.S. Gvernment retains a nnexclusive, ryalty-free license t publish r reprduce the published frm f this cntributin, r t allw thers t d s, fr U.S. Gvernment purpses. Ls Alams Natinal Labratry requests that the publisher identify this article as wrk perfrmed under the auspices f the U.S. Department f Energy. Ls Alams Natinal Labratry strngly supprts academic freedm and a researcher's right t publish; as an institutin, hwever, the Labratry des nt endrse the viewpint f a publicatin r guarantee its technical crrectness. Frm 836 (7/06)

2 Develpment f a Neutrn POD Spectrmeter Sy Stange, Duglas R. May, Rllin E. Lakis, and M. William Jhnsn Ls Alams Natinal Labratry, Ls Alams, New Mexic LA-UR-ll- Abstract Ls Alams Natinal Labratry has a lng histry f research, develpment, test and evaluatin, implementatin, and installatin f radiatin detectin equipment. The Nuclear Nnprliferatin Divisin has dne s in supprt f dmestic and internatinal safeguards, emergency respnse, nndestructive analysis, and criticality safety. One f the systems that we have develped and deplyed fr use in the field is the NPOD-III. The system is designed t perfrm crrelated neutrn measurements cupled with ttals neutrn cunting, multiplicity, Feynman variance-t-mean, and Rssi-alpha analyses t determine the mass, multiplicatin, and alpha-n rati, but many mre capabilities have yet t be explred. We are develping an algrithm and determining the applicability f using the current design as a lw-reslutin neutrn spectrmeter, that may differentiate between different impurities present with alpha emitting materials. We are initially fcusing n well-knwn lo-g PU02 standards that cntain specific impurities. In additin we will be cmparing ther surces that have different chemical frms and impurity cntents. A brief descriptin f the detectr, experimental cnstraints, and experimental results will be presented. 1 Intrductin N single tl is universally emplyed fr neutrn spectrmetry. Rather, a variety f techniques have been develped t meet the requirements f specific applicatins. [1 J These include threshld radiactivatin, prtn recil spectrmetry, Bnner spheres, plastic and liquid scintillatrs, and inizatin chambers. Hwever, few f the existing neutrn spectrmetry methds are well-suited t field measurements, which require a detectr that is prtable and prduces prmpt results. Sme techniques, such as threshld radiactivatin, have analysis prcedures that are t time-cnsuming fr field measurements. Others, including liquid scintillatrs, cntain flammable materials, making their use ptentially hazardus in uncntrlled envirnments. In additin, many neutrn spectrmeters pssess undesireable levels f,-ray sensitivity. The bjective f this prject was t develp a lw-reslutin neutrn spectrmetry system that wuld make use f prtable 3He-based detectrs that are already present in the field fr ther 1

3 measurements. Such a system wuld prvide additinal infrmatin fr field and treaty verificatin measurements, and wuld als be useful fr facility measurements in which a prtable system is needed. A neutrn spectrmetry system wuld assist in the identificatin f impurities present in special nuclear materials (SNM). Nuclear radiistpes, like all ther nuclei with mass number A greater than /"V 150, are unstable against a-decay. When lw-z impurities are present, a-particles may underg (a, n) reactins with light nuclei. The average energies f neutrns prduced thrugh (a, n) reactins are a few Me V and are characteristic f the target nucleus. The cncept n which ur neutrn spectrmeter is based invlves psitining detectrs, cnsisting f 3He tubes embedded in plyethylene slabs, s there are significant differences in the distances between individual tubes and a surce f (a, n) neutrns. By analyzing the respnse prfiles f the detectrs t neutrns with knwn energies, it may be pssible t develp an algrithm capable f analyzing the respnse prfile f an SNM surce f unknwn cmpsitin and identifying the impurities present. 2 Experimental Methdlgy The neutrn spectrmeter utilizes Neutrn POD-III (NPOD) detectrs, which are well-characterized systems that are frequently used in the field. An NPOD cnsists f a plyethylene slab with atm 3He tubes in embedded in the plyethylene in tw rws, as shwn in Figure 1. Data utput is prvided by an ethernet cnnectr. The data stream cntains 17 ptential channels, 15 crrespnding t the individual 3He tubes, ne dedicated t a 100 Hz clck tick, t be used in synchrnizing multiple NPODs, and ne pass-thrugh channel. Standard 10-gram PU02 surces with different impurities, as well as PuBe and AmLi, were used as surces f (a, n) neutrns t evaluate the suitability f the system fr neutrn spectrscpy. The impurities fund in the PU02 surces and the average energies f neutrns prduced thrugh (a, n) reactins with 5.2-MeV a-particles (the average energy f a-particles emitted by plutnium [2]) are given in Table 1. The cmpsitin f surce was the subject f sme uncertainty; institutinal knwledge gave the cmpsitin as PuF 4, but it was unclear whether this was an impurity in a PU02 surce r the cmplete cmpsitin. The average neutrn energies expected frm the PU02 surces ranged frm 1.0 t 2.9 MeV; the inclusin f a PuBe surce, which prduces (a, n) neutrns with an average energy f 4.2 MeV, and an AmLi surce, which emits (a, n) neutrns with an average energy f 0.3 MeV, served t extend the energy range. As shwn in Figure 1, the neutrn spectrmetry cnfiguratin used tw NPODs, ne placed with its narrw end facing the surce and the ther placed with its face directed twards the surce. Bth detectrs were placed at a distance f 25 cm frm the surce with the detectr centers cm abve the flr. A Shielded Neutrn Assay Prbe (SNAP) was als used fr neutrn cunting. Its frnt face (excluding the plyethylene cver) was psitined 100 cm frm the center f the surce. Tw 15-minute measurements were perfrmed fr each surce, ne with the plyethylene mderatr plate attached t the SNAP detectr and ne withut the mderatr. The surce was placed n a stand cm abve the flr. The data stream frm each NPOD was cllected in a binary file, which listed the timestamp and 3He tube f each interactin and the 100 Hz clck ticks. In additin, bth pass-thrugh channels were used. 2

4 Type PU02 I Other I Surce ID PuBe AmLi Impurity PuF4* Oxide Al Mg B Fluride Be Li Avg. Neutrn Energy fr 5.2 Me V Alphas [2] Neutrn Yield per MeV Alphas [2] Table 1: Surces measured using the neutrn spectrmeter. Institutinal knwledge abut the cmpsitin f surce was ambiguus. face-n NPOD 25cm end-n NPOD I, 1/\ ]) L...- _O_O_O_O_O_O_CD_' --1// I ' surce 100cm Figure 1: Experimental cnfiguratin f the tw NPODs and single SNAP detectr used fr spectrmetry. Channel numbering scheme f the NPODs is als shwn. 3

5 -In Q).c ~ -(ij - ~ -r:: ~ ~~~~~.... ~~~~, Face-On NPOD - - _. End-On NPOD Time (s) Figure 2: Ttal cunt rate bserved in face-n and end-n NPODs during ne I5-minute measurement f surce , PU02 with brn impurities. The clck utput f ne f the NPODs was sent int the pass-thrugh channel f the ther, t prvide a single recrd f the timing f the system. The ther pass-thrugh channel was used t cllect the SNAP detectr utput. 3 Results and Analysis Due t the greater slid angle seen by the face-n NPOD, its ttal cunt rate was significantly higher than the cunt rate bserved in the end-n NPOD fr all (a, n) surces. A typical example can be seen in Figure 2. The rati between end-n and face-n cunt rates was typically clse t 0.25, but differed slightly fr the surces measured, as shwn in Figure 3, prviding a qualitative indicatin f the neutrn energy. The rati measured fr the PuF 4 surce was slightly higher than the rati fr the PU02 with fluride impurity, suggesting that the tw surces d nt pssess identical cmpsitins. Since neutrn stpping distance is dependent n energy, analysis f the variatin in respnse rates amng the 3He tubes was expected t differentiate between lw-energy neutrns, such as thse emitted by (a, n) reactins n lithium and aluminum, and high-energy neutrns, such as thse emitted by brn and beryllium. The respnse prfiles f the individual 3He tubes in bth NPODs can be seen in Figures 4 and 5. These figures list the tubes in gemetrical (see Figure 1) rather than numerical rder and are rganized frm lwest-energy neutrns (AmLi, at 0.3 MeV) t highest-energy neutrns (PuBe, at 4.2 MeV). 4

6 J 0.5~r ' :0: r (1S f: a: 0.45 r: ~ 'E f: 5 0.4f: ~ (1S ~ 0.3- c w ~. ~ I 0.5 I 1 I 1.5 I I I I I I 2 2~ 3 3~ 4 4~ Avg. Neutrn Energy (MeV) Figure 3: Rati between end-n and face-n NPOD cunt rates fr PU02, AmLi, and PuBe surces as a functin f average energy f (a, n) neutrns. The circular pint at 1.2 MeV represents the rati fr the PuF 4 surce. Several phenmena are nticeable in the respnse prfiles. The largest number f neutrn interactins ccur nt in the 3He tubes that are clsest t the surce, but in the tubes behind them, since neutrns are mre likely t be captured fllwing mderatin. Thus, tube 12 detects mre cunts than tubes 4 and 5 fr the face-n NPOD, and tube 15, rather than tube 1, detects the largest number f cunts fr the end-n NPOD. The effect f mderatin may be seen generally fr the face-n NPOD by cmparing the respnses f the frnt tubes (numbers 1-8) with the respnses f the back tubes (numbers 9-15) in Figures 4 and 5. With the exceptin f the AmLi prfile, all surces shw cnsistently higher cunts in the back 3He tubes. The AmLi surce prduces a different prfile because its average neutrn energy is sufficiently lw t allw neutrns t be captured after few scattering interactins. The respnse prfiles fr the end-n NPOD fall ff quickly after the first few 3He tubes, as neutrns escape r are captured in the plyethylene. This effect appears t be energy-dependent - as the neutrn energy increases, a smaller fractin f neutrns are captured in the first tw tubes (numbers 1 and 15) and mre neutrns are detected in mre-distant tubes. 4 Cnclusins and Future Wrk Measurements using tw NPODs have been perfrmed n eight (a, n) surces t assess the ptential f the NPOD system as a neutrn spectrmeter. Initial examinatins f the prfiles f bth detectrs indicate that neutrn energy affects the rati in which neutrns are detected 5

7 AmLi Surce.. 1: ::l Surce : AI Surce : F II 10 7 Surce : PuF 4 11 IS c ,.----~ _. 1: ::l Surce : 0 11 II 10 Surce : Mg Figure 4: Nrmalized prfiles fr bth NPOD detectrs_ Green lines represent head-n detectr; blue lines represent end-n detectr. Prfiles are displayed in rder frm lwest-average-energy neutrns (AmLi) t highest (PuBe, in Figure 5). 6

8 I Surce : B I PuBe Surce C ::J <'3 1~ ~--~ ' Figure 5: Nrmalized prfiles fr bth NPOD detectrs. Green lines represent head-n detectr; blue lines represent end-n detectr. Prfiles are displayed in rder frm lwest-average-energy neutrns (AmLi, in Figure 4) t highest (PuBe). in the tw NPODs, as well as the identity f the individual tubes in which detectins ccur. Analysis methds fr the NPOD data that are under cnsideratin primarily invlve the ratis f 3He tubes. Fr example, Figures 4 and 5 suggest that the difference between the facen detectr's frnt and back tubes increases with neutrn energy, particularly fr tubes lcated near the detectr center. Ratis such as this ne will be analyzed t determine whether their relatinship t neutrn energy can be quantified. Other ptential analysis methds include the use f dubles data frm the NPODs and the incrpratin f data cllected by the SNAP detectr. Additinal measurements will als be perfrmed t imprve the breadth f the data set, amng them, measurements f an additinal surce, PU02 with silicn impurities, which has an average neutrn energy f 1.2 MeV. References [l]f. D. Brks and H. Klein. Neutrn spectrmetry - histrical review and present status. Nucl. Instr. Meth. Phys. Res. A 476:1-11, [2]N. Ensslin, in: D. Reilly, N. Ensslin, H. Smith, Jr. (Eds.), Passive Nndestructive Assay f Nuclear Materials (NUREG/CR-5550), U.S. Gvernment Printing Office, Washingtn, D. C., 1991, pp