Event Identification in 3He Proportional Counters Using RisetimeDiscrimination

T. J. Langforda,b,∗, C. D. Bassc,1, E. J. Beisea, H. Breuera, D. K. Erwina, C. R. Heimbachc, J. S. Nicoc

aDepartment of Physics, University of Maryland, College Park MD, 20742 USAbInstitute for Research in Electronics and Applied Physics, University of Maryland, College Park MD, 20742 USA

cNational Institute of Standards and Technology, Gaithersburg MD, 20899 USA

Abstract

We present a straightforward method for particle identification and background rejection in 3He pro-portional counters for use in neutron detection. By measuring the risetime and pulse height of thepreamplifier signals, one may define a region in the risetime versus pulse height space where the eventsare predominately from neutron interactions. For six proportional counters surveyed in a low-backgroundenvironment, we demonstrate the ability to reject alpha-particle events with an efficiency of 99%. Byapplying the same method, we also show an effective rejection of microdischarge noise events that, whenpassed through a shaping amplifier, are indistinguishable from physical events in the counters. Theprimary application of this method is in measurements where the signal-to-background for countingneutrons is very low, such as in underground laboratories.

Keywords: Helium proportional counters, Neutron detectors, Thermal neutrons, Pulse shapediscrimination, Particle identification, Microdischarges

1. Introduction

Helium-3 proportional counters are used as neu-tron detectors in a variety of fields in physicssuch as neutron scattering and nuclear and par-ticle physics, particularly those experiments thatoperate underground in a low-background environ-ment. They also have a commercial applicationas neutron detectors in health physics, medicalphysics, and nuclear safeguards. 3He has the ad-vantages of a large thermal neutron capture crosssection, easily detected charged reaction products,and low gamma-ray sensitivity. Although the ba-sic properties of 3He proportional counters havebeen known for decades [1], more recent applica-tions in counting low rates of neutrons has spurredwork in better understanding their backgrounds.Methods for improving background rejection inthe counters [2, 3] can be a significant benefitfor those experiments or applications where theneutron signal-to-background ratio is particularlypoor.

Specifically, alpha-particle decays are a perni-cious background for 3He detectors used in lowcounting rate applications. The rate of alpha-particle decays originating from the walls of the

∗Corresponding Author: tlangfor@umd.edu1Current Address: Thomas Jefferson National Acceler-

ator Facility, Newport News VA, 23606 USA

counter can be comparable to, or even exceed, therate of neutron captures. Alpha particles arise pri-marily from daughters in the uranium and thoriumdecay chains. The amount of energy deposited inthe detector can be from essentially zero to severalMeV, spanning the region of the neutron captureproducts. Thus, energy discrimination alone is notsufficient to reject background events from alphaparticles. The method of risetime discriminationprovides a straightforward method to reject a largefraction of these events.

For experiments operating in deep undergroundenvironments, cosmic-ray induced neutron eventscan be a particularly difficult background be-cause one cannot turn off the source of the back-ground [4, 5]. The experiments can only reducethe ambient neutron background through shield-ing and precisely quantifying the remaining neu-tron flux [6]. Solar neutrino experiments haveused 3He proportional counters as triggers for neu-tral current interactions and monitors of muon-induced neutron backgrounds [7]. Dark matter ex-periments are particularly susceptible to neutronbackgrounds, due to neutron recoil events that canmimic WIMP dark matter interactions. To thisend, many experiments use 3He proportional coun-ters to monitor and characterize the neutron fluxat their experimental site [8, 9].

In one notable example, the Sudbury Neutrino

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Observatory (SNO) collaboration took consider-able effort in constructing ultra-low background3He proportional counters containing low concen-tration of alpha emitters. That effort involvedmanufacturing proportional counters from mate-rials with low concentrations of uranium and tho-rium, as well as working in a radon-free environ-ment to prevent surface contamination [10, 11].For experiments that rely upon neutron countersas their triggers, these alpha-particle interactionsin the proportional counter can be a leading sourceof background. Therefore, experiments such asSNO have distinguished alpha-particle backgroundfrom neutron signals either through risetime anal-ysis [12, 13, 14] or chi-squared fitting of the entirepulse waveforms [15].

More generally, fast neutron spectroscopy canbe done directly using a 3He proportionalcounter [16, 17], and it is also routinely done us-ing Bonner spheres, where fast neutrons are mod-erated in a hydrogenous material and a fractionof those thermalized neutrons may be captured bya central 3He proportional counter [18, 19]. An-other technique for fast neutron detection using3He proportional counters is capture-gated spec-troscopy [20]. The fast neutrons recoil from pro-tons in an active hydrogenous moderator; themoderated neutron diffuses in the medium andmay be subsequently captured on 3He. The time-delayed capture signifies that the preceding in-teraction was a fully thermalized neutron. Thisallows for an accurate measurement of the inci-dent particles energy, as well as a highly efficientmethod for background reduction.

Work in nuclear safeguards has made helium-based neutron detectors the focus of detecting spe-cial nuclear material. Most neutron-detecting por-tal monitors use a 3He proportional counter sur-rounded by a passive neutron moderator [21]. Be-cause the monitors are searching for very low levelsof neutron activity, the technical requirements forthese detectors and those operating in an under-ground environment are very similar. In this case,background events are false positives, and hencealpha particles and noise in the detectors are adetriment to the effectiveness of the portal moni-tors [22].

Improving the particle identification and rejec-tion of events in 3He proportional counters thatare not neutrons has clear relevance to these ap-plications. More specifically, this collaboration isconstructing a fast neutron spectrometer [23] thatemploys a large number of 3He counters, and henceit is critical to understand all the background con-tributions in each counter that may be part of thespectrometer. Approximately 75 3He proportionalcounters of the same model were surveyed as part

of this study.In this paper, we discuss how a combination of

risetime and energy analysis can significantly im-prove the identification of neutron capture eventsin a 3He proportional counter. Section 2 brieflyreviews the operation of 3He proportional coun-ters. In Section 3 we describe the experimentalsetup and analysis method for testing 3He propor-tional counters. Section 4 gives the approach foridentifying the origin of various waveforms thatmay occur in a 3He proportional counters. We ap-ply that method to quantify the rejection of non-neutron events for a specific counter in Section 5.Section 6 describes an auxiliary measurement toquantify alpha-particle rates from the proportionalcounter body, and Section 7 summarizes the re-sults.

2. 3He proportional counters

2.1. Principles of operation

3He provides an excellent medium for detectingthermal neutrons due to the high capture crosssection (5330× 10−24 cm2 ) and a final state thatconsists of charged particles, as opposed to gammarays. A neutron is captured by a 3He nucleus,resulting in a proton and a triton, which share764 keV of kinetic energy

n + 3He→ p + t + 764 keV. (1)

These heavy charged particles ionize the gas in theproportional counter, creating electron-ion pairsthat are proportional to the energy deposited.For cylindrical proportional counters, the electronsdrift towards a central anode wire, and when theelectric field gradient reaches a critical level, beginto avalanche. Thus, the detected current is createdfrom the drifted charge.

Because the neutron capture on 3He has a twobody final state, the charged particles will be emit-ted in opposite directions. If the ionization tracksare parallel to the central anode wire, all of thecharge will be collected at approximately the sametime, leading to a short risetime of the detectedsignal. However, if the particles are emitted per-pendicular to the anode wire, one particle will bemoving towards the wire while the other is mov-ing away. The collected charge will be spread outin time due to the radial variation caused by thetrack geometry [24].

Typically, a 3He proportional counter is biasedthrough a preamplifier, and the resulting signalsare sent through a shaping amplifier to be analyzedwith a multi-channel analyzer or peak-sensing ana-log to digital converter. However, shaping the

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PreAmp'

HV'

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Figure 1: A schematic showing the setup for a sin-gle channel of our measurements. Particles in the3He proportional counters generate current signalsthat go to the preamplifier. The waveform digi-tizer captures the resulting long-tailed signals andrecords them for off-line analysis.

preamplifier signal smoothes out the slight devia-tions in signal shape caused by the track geometry.With high-speed waveform digitizers, it is possibleto study the shape of preamplifier signals directlyand apply digital signal processing to identify thetype of incident radiation.

2.2. Specific energy loss and particle range

The specific energy loss of charged particles withcharge Z, mass m, and kinetic energy Ekin, is ap-proximately proportional to

− dE

dx∝ Z2m

Ekin. (2)

Thus, the track length for a given Ekin decreaseswith increasing mass and charge of the projectile.For neutron capture on 3He, the proton is emittedwith 573 keV and the triton with 191 keV. At theseenergies and using the method of Anderson etal. [25], the proton has a range of 1.88 mg/cm2 andthe triton has a range of 0.71 mg/cm2 in our pro-portional counters. Because the proton and tritonare back-to-back, this yields a total track length of2.6 mg/cm2, compared to a 764 keV alpha particle,for example, with a range of 1.0 mg/cm2.

Beta emitters, electrons from γ-interactions,and cosmic rays leave long tracks and deposit littleenergy resulting in small signals with a wider rangeof risetimes. Alpha particles with much higherspecific energy loss leave shorter tracks and de-posit more of their energy, which yield large sig-nals with a relatively fast risetime. These fea-tures can be exploited in analysis to help identifythe original radiation. Details about the energyloss of charged particles and the resulting pulseshapes in proportional counters can be found else-where [25, 26, 24, 27, 28].

3. Experimental setup

The experimental setup is shown in Fig. 1.We used 2.54 cm outer diameter, 46.3 cm active

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Figure 2: Typical energy spectrum of neutron cap-ture events in one of the 3He proportional countersused in this study. The dominant feature is thefull-deposition peak at 764 keV. The two edges inthe spectrum near 200 keV and 600 keV are relatedto partial energy deposition in the gas when eitherthe proton or the triton interacts in the wall.

length, aluminum-bodied, cylindrical 3He pro-portional counters manufactured by GE-ReuterStokes2. The 3He partial pressure in the counterswas 404 kPa (4 atm) with a buffer gas consistingof 111 kPa (1.1 atm) of krypton. Helium does nothave a high stopping power for heavy charged par-ticles. Therefore, manufacturers tend to add in abuffer gas with higher stopping power to ensurefull energy deposition from the capture products.The krypton increases the stopping power of thegas for charged particles but has little effect onneutron capture.

The proportional counters were biasedthrough a charge sensitive preamplifier, andthe traces were recorded for offline analysis by a125 MSamples/s 12-bit GaGe waveform digitizeror a 250 MSamples/s 12-bit CAEN waveformdigitizer. Data were taken at different voltagelevels to test the variability of the counters’response. For these specific counters, 2000 V wasa reasonable operating voltage. An explorationof the effect of different counter bias voltageindicated that there was little variation of therisetime distribution.

Figure 2 shows a typical energy spectrum mea-sured by one of the proportional counters usedin this work. Note that although the neutron

2Certain trade names and company products are men-tioned in the text or identified in an illustration in order toadequately specify the experimental procedure and equip-ment used. In no case does such identification imply rec-ommendation or endorsement by the National Institute ofStandards and Technology, nor does it imply that the prod-ucts are necessarily the best available for the purpose.

3

capture reaction is monoenergetic, there are stillfeatures in the energy spectrum related to whenone of the particles interacts in the counter wall,leading to reduced energy deposition. This well-known wall effect is clearly seen in the two edgesaround 200 keV and 600 keV in Fig. 2. Events be-low 200 keV are largely betas liberated from thecounter body by gammas from the 252Cf source.

3.1. Analysis method

The digitized waveforms were analyzed, andboth the amplitude and risetime of the signalswere extracted. Figure 3 shows sample traces cor-responding to three different sources: a neutronevent, an alpha-particle event, and a microdis-charge from the proportional counter. Microdis-charges are electronic artifacts originating fromcurrent leakage in the bias voltage breakdown ap-plied to the counter [29, 30, 31]. To determine thepulse height of the waveform, one first removes thebaseline offset by subtracting off the average of thefirst few hundred samples of the trace, well beforethe pulse onset. To remove high-frequency noise,a Gaussian smoothing routine was applied to thewaveform, and the amplitude was determined fromthe resulting wave. Because the signals were sentthough a charge-sensitive preamplifier, the energyof the particle is proportional to the amplitude ofthe recorded signal. The fall time of the signalswere a property of the AC coupling and decay timeof the preamplifier. The detectors were calibratedusing the thermal neutron capture peak to set theenergy scale.

For this work, the risetime was defined as thetime difference between the signal reaching 10%and 50% of its maximum amplitude. This is thesame interval used in Ref. [12]. This choice wasbased upon a reasonable separation of the threesignal types. If the interval was too small (lessthan 20%), there was not adequate separation be-tween the microdischarges and the real signals.However, the interval could not be made too large,otherwise the slow component of the risetime, dueto the drift of positive ions [27], decreases the sep-aration of alpha-particle and neutron events. Forboth the alpha particles and neutron traces shownin Fig. 3, this begins at about 75% of the maxi-mum signal amplitude.

4. Event identification through risetimeanalysis

4.1. Physical signals from source radiation

Events in the counters arise from other radiationsources or bias voltage breakdown in the counter.

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Figure 3: Example of the raw preamplifier tracesshowing the risetimes from three different sources.Shown here, in sequence of increasing risetime,are a microdischarge (red), an alpha-particle trace(gray) and a neutron trace (black).

Data were collected under a few different condi-tions to characterize the 3He proportional coun-ters’ response to different sources of radiation. Bydisplaying the risetime versus the pulse height ofeach signal, it is possible to identify regions of thistwo-dimensional space where events from differentsource radiation will occur. Figure 4 shows theproportional counter response to thermal neutron,gamma, beta, alpha, and microdischarge events.

Thermal neutrons were generated from neutronsfrom 252Cf that were moderated by a 5 cm thickpiece of polyethylene placed between the sourceand proportional counter. Figure 4a illustrates theregion in both risetime and pulse height where thecounter detects the neutron-capture reaction prod-ucts. The large concentration of events around764 keV correspond to the full energy deposition.The risetime varies depending upon the orienta-tion of the decay products in the counter with re-spect to the central anode wire.

To test the counter’s response to beta andgamma radiation, 90Sr and 60Co sources wereplaced directly above the counter in separate mea-surements. Figures 4b and 4c show that these in-teractions are comparatively low energy that canbe easily discriminated from neutron events. Notethat there are still neutron events from the ambi-ent background in the laboratory where the mea-surements were performed.

Figure 4d illustrates why alpha particles are themost difficult background to address. Many eventsoverlap with neutrons in both energy and risetime.For this measurement, the source of the alpha par-ticles was the trace amount of uranium and tho-rium contamination in the wall of the proportionalcounter body. To emphasize the alpha-particleevents, the counter was wrapped in boron-loaded

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Figure 4: Scatter plots showing the risetime versus pulse height from different sources of radiation andmicrodischarges in 3He proportional counters: a) neutron source data; b) gamma source data; c) betasource data; d) alpha-particle background data; and e) microdischarges. The solid black lines illustrateregions where the indicated events occur. Further description is found in the text of Section 4.

5

silicone rubber to absorb most of the external am-bient background of thermal neutrons. As the typ-ical alpha-particle detection rates for the countersin this work are quite low, data were taken over thecourse of a few days to acquire sufficient statistics.Note that with such a long run, there is still someleakage of thermal neutron events into the spec-trum.

We characterized the events from about 75 3Heproportional counters for this risetime study. Wenote that they had a wide range of alpha activ-ity as well as variations in spectral shape, eventhough they all came from the same manufac-turer. Some counters had rates consistent with orhigher than the raw aluminum expectation, dis-cussed in Section 6, while some were considerablylower. The counter with the lowest alpha-particlerate had approximately 6×10−4 s−1, and the high-est alpha-particle rate counter was approximately3 × 10−2 s−1. Certain counters exhibited a broadspectrum of alphas with no peaks while others dis-played peaks at energies consistent with isotopesin the decay chains of uranium and thorium. Dur-ing construction of the counters, a thin layer ofnickel was deposited onto the inner surface of thecylinders. This was done by the manufacturer tominimize the contribution of alpha particles to thebackground [32]. We speculate that deviations inthe thickness of this coating could cause the ob-served variation.

In counters with high alpha-particle rates,events were observed with pileup of a large alpha-like signal on top of a small gamma-like signal.These events are possibly from correlated decays,where a nucleus alpha-decays to an excited state,which subsequently emits a gamma. An exampletrace of this process is shown in Fig. 5. The low-amplitude gamma signal will appear first becauseits track is extended and closer the anode. Thealpha particles predominately originate from thecounter walls, the furthest distance from the an-ode, and therefore arrive after the gamma event.As a result, these signals have longer risetimes,which could lead to misidentification as neutron-like signals with this analysis. Other techniques,such as waveform fitting, should be able to identifysuch events.

4.2. Microdischarges

In the course of the measurements, a few coun-ters were found to have spurious signals with veryrapid risetimes. After studying the counters inquestion, the source was determined to be mi-crodischarges from the high voltage feedthroughto the grounded case of the counter. An exam-ple of such a signal is shown in Fig. 3 along withneutron-capture and alpha-particle signals. This is

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Figure 5: A sample trace attributed to an alpha-particle and gamma-ray event in the proportionalcounter. These are possibly from a correlated de-cay, where a nucleus alpha decays to an excitedstate, which then de-excites by emitting a gamma.

a known effect in 3He proportional counters, anda thorough treatment of the origin of these sig-nals can be found in Ref. [33]. The microdischargeis seen by the preamp as a current pulse and istreated as a normal signal. When sent through ashaping amplifier, there is no way to distinguish adischarge from a signal generated by an incidentparticle. By using the preamplifier signal, it ispossible to measure the fast risetime of these spu-rious signals and discriminate against them. Fig-ure 4e shows an example of a detector with a clearband of fast (< 100 ns) risetime events that spana wide range of pulse height. The rate of suchevents may vary significantly among proportionalcounters, even for counters made by the same man-ufacturer.

5. Non-neutron event rejection using en-ergy and risetime discrimination

Utilizing the energy and risetime of these sig-nals provides a straightforward and fast methodto characterize the shape and origin of events in acounter, as well as a rationale to define cuts thatminimize backgrounds specific to a given applica-tion. Figure 6 is a risetime versus pulse height plotfrom one of the 3He proportional counters placedunderground. The black lines indicate where onemay apply cuts in an analysis to isolate the differ-ent events. Two methods of performing cuts on thedata proved useful in this work and are describedin this section. First, one may place restrictivecuts that define an essentially neutron-only region,This is particularly important for the applicationswhere the rate from the neutron signal is expectedto be very low. Second, one may place less re-

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Figure 6: Risetime versus pulse height data from a 3He proportional counter placed underground atKURF. The black lines and labels serve to indicate the predominate type of event in the region. Thelow neutron flux facilitates the study of the internal alpha-particle contamination of the counters. Thiscounter has a broad alpha spectrum and a low microdischarge rate, as shown in the labeled regions.

strictive cuts that eliminate microdischarges andgamma and beta backgrounds but accept the con-tribution from alpha-particle contamination.

5.1. Neutron-only cut region

In the risetime versus pulse height space, onecan define a region essentially free of alpha andbeta/gamma backgrounds, as well as microdis-charge noise. This region is clearly shown in Fig. 6.Three cuts are required to isolate neutron events:two diagonal cuts and one in pulse height only.The first diagonal cut is defined by events abovethe upper edge of the alpha region. The seconddiagonal cut is defined as events below the upperedge of the distribution from neutron source data.Finally, an upper energy threshold is made to re-ject events with energy above 0.8 MeV, which isthe approximate end of the neutron capture peak.These cuts define a triangular region which is es-sentially free from alpha-particle and beta/gammabackgrounds and microdischarge events.

To test leakage of alpha-particle events into theneutron-only region, we operated several of theproportional counters in a low-neutron environ-ment at the Kimballton Underground ResearchFacility (KURF) in Blacksburg, VA. The facility islocated in an active limestone mine at a depth of

1450 meters water equivalent and provides a goodlow-radioactivity counting environment [34]. Sixhelium detectors were counted for approximately16 hours. These detectors showed a variety of al-pha and microdischarge rates; the risetime versuspulse height spectra from one of the counters isshown in Fig. 6.

It is possible to estimate the leakage of alphaevents into the signal region by looking at eventsabove the neutron capture peak in Fig. 6. Thereshould be no neutrons in this region, but the alphaevents should have the same probability to spillinto the cut-region. For the six detectors tested atKURF, we see an average of 1 % of alpha eventswhich are not cut by this method. When these cutsare applied to neutron source data, we find thatapproximately 55 % of the neutrons are rejected.The location of the cuts can be refined to matchdistributions present in specific counters, as wellas the desired signal-to-background ratio.

5.2. Elimination of microdischarge noise only

After surveying the 75 proportional counters us-ing the risetime method, we selected counters thatcontained the lowest alpha-particle backgrounds tominimize the contamination of the neutron signal

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Figure 7: Energy spectra recorded with a siliconsurface barrier detector. The red trace shows thespectrum from the outside surface of a 3He pro-portional counter that had been scrubbed cleanto remove surface contamination. The blue traceshows a background spectrum obtained when the3He counter was removed. The broad spectrumof alpha-particle energies is consistent with bulkcontamination of the counter body material.

region. This permits one to focus on the microdis-charge backgrounds. Because the microdischargeregion is well-separated from the neutron region,we can use this cut to eliminate the microdischargeevents while preserving the entire neutron region.

Without utilizing the information in the pulserisetime, there is nothing to distinguish microdis-charge events from alpha-particle events. Thus,more proportional counters would have been cate-gorized as high-background that are low in alphacontamination but high in microdischarges. Coun-ters that would otherwise have been unsuitable forlow background experiments can still be used, aslong as the microdischarge events are rejected.

6. Direct measurement of alpha-particlerates from 3He counter walls

To verify that the background attributed to al-pha particles was being correctly identified, weperformed a separate measurement of the alpha ac-tivity at the surface of a 3He proportional counterusing a silicon detector. These measured rateswere compared with existing measurements [35,36] and the internal alpha rates from the coun-ters themselves. The comparison will not exact be-cause some parameters among the counters cannotbe controlled, such as the manufacturing processand knowledge of an absorbing layer deposited onthe inner surface.

To measure the alpha-particle rate from the alu-minum surface, a 3He proportional counter wasplaced inside a vacuum chamber, and a silicon

surface barrier detector was placed on the sur-face. The chamber was lined with thin plasticand evacuated to minimize events originating fromthe chamber and radon in the atmosphere, respec-tively. The silicon detector signal was processedthrough a preamp and shaping amplifier, and thepeak heights were registered on a multichannel an-alyzer. Data were collected under a number ofdifferent conditions in order to identify sources ofbackground. It is important to differentiate be-tween alphas that originate from the metallic sur-face of the 3He counter and those that may comefrom the chamber, detector mounting hardware, orthe silicon detector itself. Contamination on thesurface the 3He counter body is a particular con-cern, and care was taken to clean the surface with amild abrasive and detergent before measurementswere performed.

Figure 7 shows the energy spectrum of parti-cles from a 3He counter along with a backgroundspectrum when the counter was removed. To com-pare with values in the literature, we summed thecounts between 2 MeV and 10 MeV after back-ground subtraction when the counter was re-moved. Taking into account the detector solid an-gle and scaling up to the full size of the propor-tional counter, a typical alpha rate was 3.9× 10−2

s−1. Using the internal surface area of 370 cm2

for a counter, one obtains a count rate per areaof 1.1 × 10−4 cm−2s−1. In Ref. [36], researcherstabulated alpha count rates per area ranging from1.4×10−5 to 1.2×10−4 cm−2s−1 from commercialaluminum. Rates will vary widely due to severalfactors, such as the supplier and surface contami-nation [37], but the values obtained with this pro-portional counter are consistent with both the lit-erature and our own internal measurements. Thisgives additional confidence that the alpha particlein the proportional counters are being correctlyidentified.

7. Summary

We have detailed a straightforward method toidentify neutron, alpha, gamma and beta, and mi-crodischarge events from 3He proportional counterpreamplifier signals collected with waveform digi-tizers. By using the risetime and energy informa-tion of each signal, one can isolate neutron captureevents and eliminate contamination of the neutronsignal from the gamma and beta backgrounds andmicrodischarges. We have verified that the domi-nant background in the neutron risetime and en-ergy window arises from alpha decays from thecounter body itself. We have identified a set of se-lection criteria to improve significantly the neutron

8

signal-to-background ratio in that region. If count-ing statistics are sufficient for given experiment, orif one desires the highest signal-to-background ra-tio, one may choose to sacrifice approximately halfof the neutron events in order to eliminate nearlyall of the alpha-particle events. In many low eventrate applications, that may not be the preferredoption; in which case, this method will provide animportant tool to identify and mitigate the contri-bution of alpha backgrounds in 3He proportionalcounters.

A fast neutron spectrometer utilizing six 3Heproportional counters [23] was placed undergroundat KURF. Using the risetime method to identifyand reduce backgrounds has led to a significantimprovement of the experiment. In future work,we will investigate the improvement in the neutronsignal-to-background from data acquired in a low-background environment.

8. Acknowledgments

We thank Vladimir Gavrin and Johnrid Ab-durashitov of the Institute for Nuclear Research- Russian Academy of Sciences for useful discus-sions. We acknowledge the NIST Center for Neu-tron Research for the loan of the 3He propor-tional counters used in this study. We also ac-knowledge KURF and Lhoist North America, es-pecially Mark Luxbacher, for providing us accessto the underground site and logistical support.The research has been partially supported by NSFgrant 0809696. T. Langford acknowledges supportunder the National Institute for Standards andTechnology American Recovery and ReinvestmentAct Measurement Science and Engineering Fellow-ship Program Award 70NANB10H026 through theUniversity of Maryland.

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