neutron detector based on lithiated sol–gel glass

Nuclear Instruments and Methods in Physics Research A 483 (2002) 764–773

Neutron detector based on lithiated sol–gel glass

Steven Wallacea,*, Andrew C. Stephana, Laurence F. Millera, Sheng Daib

aUniversity of Tennessee, Nuclear Engineering Department, Knoxville, TN 37996-2300, USAbOak Ridge National Laboratory, USA

Received 18 October 2000; received in revised form 1 August 2001; accepted 2 August 2001

Abstract

A neutron detector technology is demonstrated based on 6Li/10B doped sol–gel glass. The detector is a sol–gel glassfilm coated silicon surface barrier detector (SBD). The ionized charged particles from (n; a) reactions in the sol–gel filmenter the SBD and are counted. Data showing that gamma-ray pulse amplitudes interfere with identifying chargedparticles that exit the film layer with energies below the gamma-ray energy is presented. Experiments were performedshowing the effect of 137Cs and 60Co gamma rays on the SBD detector. The reaction product energies of the triton and

alpha particles from 6Li are significantly greater than the energies of the Compton electrons from high-energy gammarays, allowing the measurement of neutrons in a high gamma background. The sol–gel radiation detection technologymay be applicable to the characterization of transuranic waste, spent nuclear fuel and to the monitoring of stored

plutonium. r 2002 Elsevier Science B.V. All rights reserved.

PACS: 07.77.�n

Keywords: Fast neutron detector; Sol–gel dopants; Lithiated glass; Activated silver; Surface barrier detector; Spent nuclear fuel.

1. Introduction

Neutrons are an unequivocal sign for active/passive detection of transuranic (TRU) elementsassociated with nuclear power generated pluto-nium and declared enriched uranium and pluto-nium derived from the disassembly of nuclearweapons. Commercial neutron detection systemsbased upon national laboratory technologies areavailable from several companies. For severalyears the systems and their applications to fissilematerial management have been described in the

annual Proceedings of the Institute of NuclearMaterial Management [1].Many systems use pressurized 3He gas tubes for

neutron detection [2]. Most systems are designedby Monte Carlo modeling of tube geometry withina moderator blanket to yield maximum captureefficiency for neutrons above thermal energycoming from fissile source material [3]. Lithiatedglass, an excellent neutron detector, is commer-cially available. Glass detectors comparable in sizeto 3He tubes are not used. Large area detectionmay be accomplished by using lithiated glass fibertechnology developed over several years at PacificNorthwest National Laboratory [4]. This technol-ogy is being commercialized, and performance hasbeen reported for several applications [5].

*Corresponding author. Tel.: +1-865-966-6446; fax: +1-

865-974-0668.

E-mail address: [email protected] (S. Wallace).

0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 9 3 2 - 5

Lithiated glass fibers are drawn from a boulein this method. Ionizations in the fiber fromtriton and helium reaction products of 6Liproduce photons via Ce+3 converter atoms. Asmall percentage of these photons are guidedto a photomultiplier tube, producing a signalpulse.The present work is an alternative path to a

lithiated glass detector. Rather than manufactur-ing fibers from a glass boule, the glass is madethrough a condensation reaction from liquidchemical glass precursors. The sol–gel processand chemistry are elucidated in Sol–Gel Science[6]. In recent years, the sol–gel process has beenrecognized as an extremely useful means oftrapping dopants in a glass matrix, many of whichcannot be introduced into a high temperature glassmelt [7–9]. Doped sol–gel glasses have been usedfor chemical identification, pH measurement, andneutron detection [10,11].Neutron capture yields prompt emission of

reaction products from dopants 10B and 6Li. Athin sol–gel layer was applied to the face of asilicon surface barrier detector (SBD). The sol–gelthickness was greater than the ranges of thereaction products [12,13]. SBD charged particledetectors have been used for detecting alphaparticles for monitoring airborne U/Pu particles[14,15]. The direct coat of these alpha particledetectors with doped sol–gel glass converts theminto neutron detectors.A neutron energy spectrometer that uses two

150 mg/cm2 6Li fluoride coated SBDs in a coin-cidence mode to determine the energy above theQ-value is commercialized. The system incorpo-rates ten NIM modules, two preamps, and adetector head [16]. The goal of our sol–gel coateddetector is a low-cost neutron detector based upona simple chemical manufacturing process. Thedesign objectives include maximum loading of 6Liin g/cm3 and a thin film cured to a thickness justbeyond the range of the reaction particles. Max-imum counting efficiency supercedes any consid-eration of resolution. An 6Li fluoride film with athickness exceeding the triton and alpha particlecharged-particle ranges, resulting from a neutroncapture, should yield results comparable to thoseobserved using a lithiated sol–gel glass film.

2. Experimental setup

The neutron source used is a Neutron-PacHowitzer Model 1 manufactured by the formerNUMEC of Apollo, PA. The source consists of a 2Ci Pu–Be pellet encased in stainless steel. Thesource is located in the center of a hydrogenousfilled stainless steel drum lined with cadmium. A2.2MeV gamma-ray field is produced by neutroncapture in the hydrogen. The drum is about 1mhigh and two-third meter in diameter. TheHowitzer model was designed primarily for con-ducting activation experiments. The source sits atthe bottom of a central 3.5 cm diameter by 48.3 cmdeep channel covered by a removable hydrogenousplastic plug. Three side channels with dimensionsequal to those of the central channel are equallyspaced on a 10.2 cm diameter circle around thecentral channel. They hold plastic plugs withhollow chambers at the bottom for activationtargets. Fig. 1 shows a diagram of the drum.The SBD was positioned at different locations inthe center and side channels in order to vary theenergy spectrum of the Pu–Be neutrons inthe experiment described below.Highly energetic reaction products from neutron

absorption in 6Li and 10B produce pulses in thedetector. The 6Li neutron capture reaction has aQ-value of 4.79MeV, while the 10B capturereaction has a Q-value of 2.79MeV with aprobability of 7% and 2.31MeV with a probabilityof 93%. The latter decay branch includes a0.48MeV gamma ray from de-excitation of the10B reaction product. A significant fast fluxproduces reaction products with energies augmen-ted by the neutron kinetic energy as seen in Fig. 2,where pulses are observed with energy greater thanthe Q-value. The measurements that produced thespectrum in Fig. 2 were performed with the SBDpositioned with the detector in near contact withthe source in the center channel of the drum. Theneutron flux is estimated to be about 15 000 n/s/cm2 with a mix of fast neutrons and thermalneutrons. A count time of 2850 s achieved thespectral definition in Fig. 2. The kinetic energy ofthe Pu–Be neutrons can impart up to about4.5MeV above the Q for the capture reaction.The transfer of momentum from an energetic

S. Wallace et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 764–773 765

neutron moving toward the detector can cause oneor both of the charged particles to enter thedetector with at least one carrying more energythan that predicted by the Q-value. Although thecapture cross-section falls as 1=v; the probability ofgreater energy deposition in the detector volumerises linearly with neutron energy for an incoming

neutron approaching the detector surface nor-mally.The SBD sol–gel coating contains enriched

lithium (95% 6Li) and boron (99% 10B). Theresults obtained show that the use of 10B isunnecessary and degrades the efficiency that isbeing sought. One ml of a sol–gel formulation was

Fig. 1. Diagram of NUMEC Howitzer Pu–Be source and drum. The location of the source and channels are shown.

Fig. 2. A typical pulse height spectrum for the 6Li doped sol–gel coated SBD neutron detection system. The low energy peak is

produced by Compton electrons. Pulses above this peak are produced by one or both of the 6Li reaction products entering the detector

volume.

S. Wallace et al. / Nuclear Instruments and Methods in Physics Research A 483 (2002) 764–773766

pipetted onto the face of a 450mm2 Ortec 26-448DSBD and allowed to polymerize. The curedthickness of the film exceeds the range of the 6Lireaction products. A cross-section calculation isnot possible because 10B is present and thisintroduces self-shielding in a thick film thatcompetes with the capture of neutrons by 6Li inan unknown way. The range for the 2.7MeVtriton in silicon is 40 mm and the range for the2.05MeV alpha particle is 7 mm in silicon [17]. Thelow kinetic energy of the 10B reaction productsplaces their contribution in the low-energy peak asseen in Fig. 2 and cannot be distinguished from thegamma ray produced Compton electrons. Sincethe 10B cross-section is about a factor of fourgreater than that of 6Li, neutrons are competitivelylost to the 10B. Few neutron capture events in 10Bare counted because the shorter range of thecharged particles result in most of the energydeposition occurring in the glass. When the 6Liabsorbs a neutron, one or both reaction productscan enter the SBD producing a trail of ionizedelectrons in the depletion region. The bias sweepsthe electrons into the pre-amp/amp, which is theinput to the multichannel analyzer (MCA). De-tector data was fed into a computer equipped withthe data acquisition software Gamma Visions viaan MCA card, and the pulses sorted into an 8192channel spectrum [18]. In the experiments, theSBD was exposed to different neutron spectra,gamma fluxes, and beta particles from an activatedsilver disk.A lead brick pile was constructed to shield

gamma rays emitted by the Pu–Be source. Whenusing the pile, the Pu–Be source was removed fromthe drum and placed in a cavity in the pile. This allbut eliminated the 2.2MeV captured gamma rays.The lead housing wall facing the detector positionwas 15.2 cm thick. The cavity size permittedinsertion of graphite blocks for additional neutronmoderation. The pile yielded a neutron flux withlittle gamma radiation.

3. Results

A typical 6Li doped sol–gel coated on an SBDyields a pulse height spectrum as shown in Fig. 2.

The spectrum is comprised of the pulses from thereaction products formed in the thin film coatingthat enter the silicon depletion zone and pulsesfrom Compton scattered electrons. Without at-tenuation of the energy of the charged particleswithin the film layer, two peaks would be seen; oneat 2.05MeV and the other at 2.74MeV. Loss ofenergy can be approximated by a linear loss ofenergy with the distance from the SBD surface intothe film at which the neutron was absorbed. Thisresults in plateaus rather than peaks. Severalimportant features are observed in the sol–gel-coated SBD spectrum. A sharp peak at the low-energy end of the spectrum is one feature.Compton electrons from scattered 137Cs, 60Co,and hydrogen capture gamma rays (2.2MeV)incident on the detector cause this peak. Werethe low-energy gamma/beta peak not present andin the absence of 10B, the neutron counts wouldremain constant at the level of the first plateau tothe zero energy. Two successive plateaus followabove the energy of the Compton peak, afterwhich the counts trail off monotonically up to8.1MeV. This fall-off reflects the decreasing cross-section with neutron energy for capture by 6Li.The SBD neutron measurements consistentlyyielded this profile, the differences observed beingin the relative size of the features.Fig. 3 shows a 257 000 s 230Th standard alpha

spectrum count. An SBD without a sol–gel coatingwas set directly upon an SS disk that had a thinfilm plating of 230Th. The main peak is the4.69MeV 230Th alpha peak followed by the5.49MeV 222Rn, 6.00MeV 218Po, and 7.69MeV214Po alpha particles. A comparison with thespectrum in Fig. 2 indicates energies of 2.5MeVat the first shoulder, 4.7MeV at the secondshoulder, and 8.1MeV at the endpoint (channel160).The response of the coated SBD to a neutron

field changed with the placement of a silveractivation disk over its face. 109Ag (48.2% naturalabundance) activates to 110Ag, which decays with24.6 s half-life by beta particle emission with anendpoint of 2.981MeV. The beta particles, with anaverage energy of around 1MeV, pass through thesol–gel into the SBD. The silver disk effect on thepulse height spectrum is seen in Fig. 4. The low-


energy peak amplitude was sharply increased bysilver beta particles when compared with the peakmeasured without the presence of silver. Theamplitudes of the two plateaus were also affected,the lower-energy plateau (less than 2.5MeV) beingdepressed and the higher-energy (greater than2.5MeV) being slightly reinforced by the presence

of the silver foil. The measurements that pro-duced the spectra in Fig. 4 were performed withthe SBD positioned at a height of 0 cm, i.e. directlyin a side channel next to the source. The high-energy plateau with a shoulder at 4.8MeV thatappears in Fig. 2 is greatly diminished at thisposition.

Fig. 3. Energy calibration of an SBD w/o a sol–gel coating using an SS disk plated with a thin film of 230Th. The SBD is set upon the

disk. The lowest-energy peak is from beta particles. The higher-energy peaks includes a 4.69MeV 230Th alpha peak. Others are

5.49MeV 222Rn, 6.00MeV 218Po, and 7.69MeV 214Po alpha peaks.

Fig. 4. The pulse height spectra for the SBD detector w and w/o the silver activation disk. The beta peak increases with the silver disk,

and the thermal plateau is reduced because of the absorption of thermal neutrons by silver.


The SBD response was characterized as afunction of neutron energy spectrum shape.Source placement in the lead box without addedmoderator yielded the highest energy spectrumtested. This yielded a second plateau so largerelative to the first that the transition betweenthem could not be seen. Positioning of the SBD atthe bottom of the center channel immediatelyadjacent to the source (still comparatively high-energy but less so than in the lead box) gave afairly large second plateau relative to the first asseen in Fig. 2. As the SBD was moved up thecenter channel away from the source, the firstplateau became steadily larger relative to thesecond as seen in Fig. 4. Faster spectra consistentlyyielded large second plateaus, while thermalizedspectra exhibited small second plateaus. Repeti-tion of the experiment in a side channel producedthis same qualitative behavior.

4. Discussion

Analysis of the silver activation SBD resultsshowed that the silver betas (average energy of1MeV, maximum of 3MeV) deposited were notmore than 1.8MeV in the detector. Gamma rays

of both 60Co and 137Cs deposited less than1.1MeV. One MeV corresponds closely to 20channels in Figs. 2–6. Some statistical effects areevident in the silver beta particle data. Hydrogencapture gammas (2.2MeV) exhibited an energycutoff around 1.4MeV in the SBD. Since beta andgamma ray pulses will no more deposit their fullenergies in the detector, any larger pulses detectedwill be from neutrons. Were the low-energy beta/gamma peak not present in the SBD pulse heightspectrum, the lowest-energy neutron plateauwould be flat to the lowest energy bins in thespectrum. Since about half of the SBD neutroncounts occur above 2.2MeV hydrogen capturegamma cutoff (some extend up to 8MeV), veryreliable neutron–gamma separation may beachieved using pulse height analysis even in a2.2MeV gamma-ray background. However, theintrinsic efficiency for detecting high-energy cap-ture events may limit some applications of the sol–gel coated SBD in which gamma-ray pileup effectsare significant.The SBD pulse spectrum shape at the bottom of

the side channel was recorded with the silver diskpresent and absent and then compared. The netdifference is shown in Fig. 6. Neutrons producingpulses in the first plateau are strongly affected.

Fig. 5. Contributions to the pulse height spectrum from 137Cs and 60Co Compton electrons and 3MeV (maximum) beta particles from

activated silver. A comparison with Fig. 2 shows that pulse height analysis can be an effective method for separating neutron and

gamma pulses.


109Ag absorbs the thermal neutrons approachingthe SBD. The cross-section for 109Ag activationbecomes inconsequential above several hundredeV [19]. Therefore, pulses in the first plateau are inlarge part coming from neutrons in the thermalrange and pulses in the second plateau areproduced by neutrons above thermal energy.Referring to Fig. 6, the large gain in counts below1.3MeV is caused by an increase in beta particlesfrom activated 110Ag decay. A count reduction isobserved in the energy range from 1.3 to 2.5MeVfrom the absorption of thermal neutrons. Essen-tially, no change beyond 2.5MeV is observed.Note that a contribution from betas is not possibleabove 3MeV in the SBD, whereas the thermalneutron count plateau (one reaction productenters the detector) extends to 2.5MeV. Therefore,the silver overcoat on the SBD absorbed thermalneutrons and did not absorb fast neutrons.Experiments involving SBD exposure to differ-

ent neutron energy spectra indicated a similardependence of pulse amplitude on neutron energy.The SBD pulse-height spectrum shape can bequalitatively described by the ratio of counts in thesecond plateau (designated as ‘‘fast’’) and the trail-off region to the number in the first plateau from1.3 to 2.5MeV (designated as ‘‘thermal’’). Since

this qualitative terminology establishes whetherthe pulse from a neutron falls in the first or secondplateau in the detector, it is not possible to assign aspecific energy value to the crossover point from‘‘thermal’’ to ‘‘fast’’ for this detector. Recall thatthe transfer of momentum to the charged particleby the neutron determines whether a pulse is in thesecond plateau or the first and that the probabilityof transfer of a large (forward) momentum riseswith increasing neutron energy and is continuous.Since the presence of the silver overcoat, with asignificant absorption cross-section of severalhundred eV, did not appreciably affect the secondcount plateau, neutrons of several hundred eVenergy are considered ‘‘thermal’’ in this terminol-ogy. The purpose of describing the spectra usingthis terminology is to empirically identify therelative moderation of the neutron flux.Calculation of the fast to thermal ratio for the

SBD in the side channel showed a maximum of0.19 when approximately level with the Pu–Besource, where the fast flux is maximized. The ratiowas minimized (0.07) at the top and bottom of thechannel (where the flux would be most therma-lized). Center channel measurements yielded thesame qualitative behavior as in the side channelbut with higher ratios. Figs. 7 and 8 show the

Fig. 6. The difference in the pulse height spectrum of the SBD neutron detector w and w/o the silver activation disk (see Fig. 4). The

range where the difference is below zero is what we call ‘‘thermal’’ and beyond that is what we call ‘‘fast’’.


number of fast and thermal counts and fast tothermal ratio down the side channel. Placement ofthe detector inside the lead box with the Pu–Besource yielded a fast to thermal ratio of 0.942.Nine cm of lead and 5 cm of graphite betweenthe detector and source lowered the ratio to0.496. Other experiments not detailed here showed

a consistent dependence of fast to thermalcount ratio on the neutron energy spectrum.Thus, a consistent and reliable predictor of therelative thermalization of the neutron flux isachieved.That fast neutrons produce higher amplitude

pulses follows directly from the collision reaction

Fig. 7. The ‘‘fast’’ and ‘‘thermal’’ neutron count rates by the SBD down the side channel in the NUMEC drum.

Fig. 8. The ‘‘fast’’ to ‘‘thermal’’ ratio for neutron counts by the SBD down the side channel in the NUMEC drum. The drop in the

ratio represents the thermalization of the neutrons moving away from the source.


kinematics. Low-energy (below several hundredeV) neutrons have a very low momentum com-pared to the reaction products from neutroncapture by 6Li and 10B. Thus, momentum con-servation results in the reaction products travelingat 1801 to each other. Only one reaction product islikely to enter the SBD and produce a pulse(backscatter of the other reaction product into theSBD is possible but uncommon). The alphaparticle from 6Li neutron capture (Q ¼ 4:79MeV)MeV) has an energy of 2.05MeV and the tritonhas an energy of 2.74MeV, as required for theconservation of momentum. The average of thetwo energies is about 2.4MeV, which correspondsvery closely to the high-energy end of the firstplateau. When neutrons traveling toward thedetector have substantial momentum (i.e. signifi-cant energy), the reaction products will typicallytravel at a smaller angle relative to each other andshow significant enhancement in the forwarddirection. Thus both reaction products can enterthe SBD, resulting in a much larger deposition ofenergy in the SBD and hence, a much larger (sum)pulse. The end of the second plateau was at4.7MeV, which corresponds very well to the4.79MeV Q-value of the reaction. Enhancementof the energies of the reaction products by theadditional kinetic energy of the neutron occurs forreactions involving very high-energy neutrons, andthis corresponds to pulses with energies above theQ-value of the reaction. The greatest effect ofenergy enhancement occurs when the reactionparticle and the neutron both travel toward theSBD. The highest energy pulses observed corre-sponded to energies over 8MeV. Such energies areproduced by 4.5MeV neutrons from the Pu–Besource splitting 6Li atoms. The effect is observablein Figs. 2 and 4, even though the cross-section forthe (n; triton) reaction is about 700 b for thermalneutrons and the fast cross-section at 4MeV isonly about 200mb [19].The first plateau, in summary, corresponds to

counts from single reaction products, primarilyfrom thermal neutrons. The second plateau isproduced either by reaction particles, or anenergy-enhanced single reaction particle, enteringthe SBD. The trail-off above the Q value occurswhen the energies of both reaction particles are

augmented. All cases with energy augmentationinvolve fast neutrons.

5. Conclusions

The sol–gel coated SBD system demonstratedgood discrimination between neutrons and beta/gamma rays. A previous result obtained withscintillation detectors and a thermalized 252Cfneutron spectrum noted that 6Li gives superiorresults compared to doping with B-10 for separa-tion of neutrons and 137Cs gamma-ray pulses [20].Neutron flux spectral information can be easilyacquired from the SBD output. An array of SBDneutron detectors might be applied in some casesof spent nuclear fuel and TRU waste characteriza-tion with little gamma interference depending onthe total radiation field and the gamma ray toneutron ratio. Scalability may be easily accom-plished by adding more detectors to the array. Anarray of detectors can also be located at individualstorage containers of plutonium for surveillancepurposes [21]. A sol–gel lithiated glass film hasbeen tested using PIN photodiodes as the chargedparticle detector. This results in a much lowerdetector cost for monitoring compared to the useof surface barrier detectors.

Acknowledgements

The Oak Ridge National Laboratory is mana-ged for the Department of Energy under contractNo. DE-AC05-00OR22725 by UT-Battelle, LLC.Funding for this work is through the support ofthe Department of Energy NN-20.

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neutron detector based on lithiated sol–gel glass

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