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1278 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 62, NO. 3, JUNE 2015

Pulsed Power Active Interrogation of ShieldedFissionable Material

Richard S. Woolf, Member, IEEE, Bernard F. Phlips, Anthony L. Hutcheson, Eric A. Wulf, Member, IEEE,Jacob C. Zier, Stuart L. Jackson, Member, IEEE, Donald P. Murphy, Member, IEEE,Robert J. Commisso, Fellow, IEEE, Joseph W. Schumer, Senior Member, IEEE,

Ceri D. Clemett, Member, IEEE, John O’Malley, Member, IEEE, Cassie Hill, Member, IEEE,Robert C. Maddock, Member, IEEE, Philip N. Martin, Member, IEEE, and James R. Threadgold, Member, IEEE

Abstract—We irradiated a depleted uranium ( ) target withintense, single 50 ns pulses of bremsstrahlung to study the behaviorof , , NaI(Tl), and liquid scintillation detectors in a harshradiological environment. The target was exposed unshielded,and shielded with borated high-density polyethylene, or steel, anddelayed -ray and neutron signatures were measured. We foundthat a high confidence measurement of the delayed emission couldbe obtained in this environment and show the results from eachdetector array, for varying amounts of shielding, in terms of thesignal-to-noise ratio vs. time and the relationship between themean of the signal-to-noise ratio vs. areal mass density.Index Terms—Active interrogation, bremsstrahlung photons,

gamma-ray detection, neutron detection, pulsed power.

I. INTRODUCTION

D ETECTION and interdiction of smuggled nuclear ma-terial at points of entry are important capabilities to

develop for homeland security purposes. A standard detectiontechnique is to seek out the radiation signature naturally emittedby such a material, namely the neutron and - ray emission.Two approaches are typically employed for detection–passivemeasurements or active interrogation. In active interrogation,the strength of the characteristic radiation associated withnaturally occurring fission are significantly enhanced so thatthese signals are more readily measured compared with thepassive approach that measures what can be relatively weakand easily attenuated radiations associated with natural decayof fissionable material [1], [2].Runkle et al. provides an extensive review of the techniques

and reasons for active interrogation [1]. Specifically, there arearguments laid out for using both x-ray photons and neutrons as

Manuscript received December 03, 2014; revised February 27, 2015;accepted April 18, 2015. Date of publication nulldate; date of current versionJune 12, 2015. This work was supported by the Atomic Weapons Establishment(AWE) through the Defense Threat Reduction Agency (DTRA).R. S. Woolf, B. F. Phlips, A. L. Hutcheson, and E. A. Wulf are with the

Space Science Division, U.S. Naval Research Laboratory, Washington, DC20375 USA (e-mail: richard.woolf@nrl.navy.mil).J. C. Zier, S. L. Jackson, D. P. Murphy, R. J. Commisso, and J. W. Schumer

are with the Plasma Physics Division, U.S. Naval Research Laboratory, Wash-ington, DC 20375 USA.C. D. Clemett, J. O’Malley, C. Hill, R. C. Maddock, P. N. Martin, and

J. R. Threadgold are with the Nuclear Security Science Group, AtomicWeaponsEstablishment, Reading RG7 4PR, U.K.Digital Object Identifier 10.1109/TNS.2015.2427152

the interrogating source. Slaughter et al. [3] has shown effec-tive detection of fissile material using the “nuclear car wash”approach with 3 MeV–7 MeV neutrons as the interrogating par-ticle and the resulting detection of MeV -delayed rays.With x-ray photons as the interrogating source, there has been agreat deal of work done using electron linear accelerators (linac)to produce multiple pulses of bremsstrahlung radiation. In thisapproach linac-produced pulses, separated on the order of tensof milliseconds, are used to irradiate an object then measure thedelayed emission between pulses.Several groups in Idaho (Idaho Accelerator Center, Idaho

State University, and Idaho National Laboratory) have em-ployed linac systems for active interrogation applications.Reedy et al. [4] describes the ability to differentiate between fis-sionable and non-fissionable materials by measuring temporaldifferences measured between interrogating pulses. Kinlaw etal. [5] have incorporated a system to measure the time depen-dency of delayed neutron emission to yield fissionable isotopeidentification. Lastly, Jones et al. [6] have developed a cargocontainer scanning system for the detection of nuclear materialshielded by either high- or low- material using an array ofneutron and -ray detectors.Additionally, Wehe et al. [7] showed that after inducing fis-

sion by interrogating a target with a linac, one could thenobserve the delayed emission for several minutes after the in-terrogation.The previously described works using x-ray photons as the

interrogation source are important in the field of active interro-gation research, but have been done using a train of multiplebremsstrahlung pulses with detection time measured in min-utes. Our work concentrates on interrogating a fissionable ma-terial with a single, short (of order 100 nanoseconds) intensepulse of bremsstrahlung radiation produced by a high power(1 terawatt) pulsed power generator. Simulation studies haveshown that interrogation via a single, high-power pulse results indetection with increased signal-to-noise compared to multiple-pulsed linac-based systems [8] (see Section VI. for further de-tails). Concurrent with our work, other groups, including co-au-thors on this manuscript, have investigated active interrogationvia pulsed power techniques using the HERMES-III facility atSandia National Laboratory in Albuquerque, NM [9]–[11].Withthis approach, some of the advantages are that detection canbe achieved after a few seconds and there is little-to-no influ-ence from passive background due to the short observation time.

0018-9499 © 2015 EU

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WOOLF et al.: PULSED POWER ACTIVE INTERROGATION OF SHIELDED FISSIONABLE MATERIAL 1279

The first is an obvious advantage in any detection scenario andthe second allows the detection process to occur with minimumdose. The main disadvantage is contending with the harsh en-vironment produced by the initial pulse and allowing the radi-ation detectors to recover on a short timescale (on the order ofmilliseconds) such that a high-confidence measurement can bemade.The work described herein reports on a collaborative test

campaign carried out at the U. S. Naval Research Laboratory’s(NRL) Mercury pulsed power facility [12][13], operating in theIntense Pulsed Active Detection (IPAD) mode. The goal of thiscampaign was to demonstrate that a high-confidence measure-ment of an unshielded/shielded fissionable target (DU: depleteduranium, ) could be achieved using a wide variety of neu-tron and -ray detection arrays in the harsh active interrogationenvironment in a timeframe of s.This manuscript is intended to complement the results pre-

sented in Clemett et al. [14], which describes the portion ofthis campaign conducted using three types of interrogatingsources while the DU target was shielded with high- materialonly. Measurement of the delayed - ray signature, as well asthe delayed neutron signature, using NaI(Tl) detectors and asmall array of moderated thermal and fast neutron detectors,respectively, are discussed in that publication. Here, and in[15], we concentrate on experiments conducted using oneirradiation source (bremsstrahlung x-ray photons only) and themeasurements made by two large arrays of moderated thermalneutron detectors and a full complement of fast neutron/ -ray detectors, with shielding comparisons of both low- andhigh- materials encapsulating the DU target. We additionallydiscuss the delayed - ray measurements made by an array ofNaI(Tl) detectors observing the DU target shielded with low-material.

II. EXPERIMENTAL METHODS

The main goal of the experiment was to irradiate a DU targetusing the Mercury facility in the IPAD mode to measure the re-sultant radiation with a wide variety of neutron and - ray de-tection systems located 2 m to 12 m from the target (Fig. 1).The Mercury pulsed power generator is capable of producing8-MeV endpoint bremsstrahlung x-ray photons at a peak currentof 200 kA and a pulse width of 50 ns (FWHM) [16]. The intense,short burst of high-energy photons can be used as an active in-terrogator to induce fissions in fissionable material (providing

MeV). Photofissions within the material produce both- ray and neutron emission that can then be detected. The pro-

duction timescale ranges from: prompt (order of nanoseconds)to delayed (order of milliseconds to minutes) after the interro-gating burst. Given the unavoidable active background signaland the length of time needed for detector recovery, this workfocused on measurement of the delayed emissions.A large suite of instrumentation was set up to detect and

measure the delayed -ray and neutron signatures from acm cm cm DU metal plate ( kg).

The test campaign consisted of a series of single IPAD pulses,i.e., “shots,” employing incremental shielding configurationsof high- (steel) and low- (2% borated high-density poly-ethylene, BPE) material encapsulating the DU target. The

Fig. 1. Top view of the test facility showing the position and distance relative tothe target, detector arrays and Mercury. The distance from the radiation source(as indicated by the yellow star) to the target is 2.5 m.

incremental steel thicknesses were 1.27 cm, 3.81 cm, 6.35 cm,8.89 cm, and 11.43 cm, providing areal mass densities ( ) of

g cm , g cm , g cm , g cm , and g cm ,respectively. An additional data point with steel thickness of19.05 cm provided a of g cm . The incremental BPEthicknesses were 10.2 cm, 30.5 cm, and 50.8 cm, providing aof g cm , g cm , and g cm , respectively. To obtaina baseline measurement for comparison with shielded DU, abare DU shot (no shielding) and shots with shielding-only (noDU) were completed. The shielding-only shots were taken atthe same as the high- and low- material used when theDU was present. Additional shots were completed withoutany DU or shielding present (blank target) and with the beamblocked to generally limit the emitted radiation into the room.All of this provided a full measure of the active backgroundsignal observed in the interrogating environment that served asa reference as the target shielding increased and the measuredsignatures decreased.The mean of the output dose for the shots taken, as tabulated

by thermoluminescent dosimetry (TLD) measurementsat 1 m from the source, was found to be 7.759 Gy (775.9 rad).Applying scaling, the dose at the detectors ranged from1.76 Gy (2 m) to Gy (12 m).

III. INSTRUMENTATION

A. Detectors

Four separate detection arrays were fielded in the experiment:the two dedicated to neutron detection consisted of 11 borontrifluoride ( ) detectors and six helium-3 ( ) detectors; a- ray dedicated array consisting of 20 NaI(Tl) detectors; and

16 liquid scintillation detectors (Eljen 309, para-xylene) for the

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1280 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 62, NO. 3, JUNE 2015

dual detection of neutron and - rays. (Note: 15 liquid scintil-lation detectors were used in the analysis due to complicationswith one unit).The detectors are cylindrical tubes with a fill gas at

a pressure of Pa (0.9 atm), an active length of 1.83 mand a diameter of 11.4 cm. Each detector was surrounded by1.27-cm-thick foam (for padding), then moderated with 2.5-cm-thick polyethylene (PE) inside 2.5-cm-thick BPE (5 cm total) onall sides. An additional 2.5 cm of BPE was used on the target-facing side of the detectors. Structural support for the array wasprovided by a lumber frame composed of cm cmpine wood boards. The tubes were positioned 11.6 m fromthe target in the rear of the Mercury cell at an angle ofwith respect to the target center, subtending a solid angle of

.The detectors are cylindrical tubes with a fill gas at

a pressure of Pa (2.7 atm), an active length of 64 cmand a diameter of 15 cm. Each detector (surrounded by 1.27-cm-thick foam) was moderated by 2.5-cm-thick PE inside 2.5-cm-thick BPE on all sides. The tubes were arranged side-by-side as a horizontal array, located at a distance of 2.1 m fromthe target and at an angle of 77 with respect to the target center,subtending a solid angle of sr.Each of the NaI(Tl) detectors are cm cm cm

in total active area. The shielding around these detectors con-sisted of a 0.6-cm-thick sheet of lead on the target-facing sidewith 5-cm-thick lead on the top, sides and the side opposite thetarget (rear). The lead was sandwiched between two sheets ofBPE with a total thickness of 5 cm. This work concentrated onthe group of ten located at a distance of 3.1 m from the target andat an angle of 53 with respect to the target center, subtending asolid angle of sr.The liquid scintillation detectors are cm cmcm in volume and arranged in a array located at a

distance of 2.5 m from the target and at an angle of 99 withrespect to the (front of the) target center (positioned directlyabove the array), subtending a solid angle of sr.The array was surrounded by 10-cm-thick lead shielding on thefront and sides of the array and 5-cm thick on the top and bottomof the array.

B. Data AcquisitionThe signals from each detector array were processed through

either standard NIM electronics and VME peak-sensing ADCsor read directly into a flash analog-to-digital converter (FADC).The NIM/VME data acquisition system received inputs from

the , and NaI(Tl) arrays. The signal output from eachwas run into a preamplifier, which was then routed into a

16-channel Mesytec shaping amplifier (MSCF-16) [17]. Sim-ilarly, the PMT output signal from each NaI(Tl) detector wasread directly into two MSCF-16 modules. The detectorprovided a discriminated TTL signal to a level translator NIMmodule, which then provided TTL and NIM signals for gating.A TTL beam trigger signal was sent from the pulsed powergenerator to the data acquisition system to provide a fiducialshowing when the shot occurred relative to the data acquisitionstart time. The beam trigger into the system occurred s priorto the shot. A pulse generator providing NIM pulses at 2 kHz

Fig. 2. Simulated response of a (detector only) as a function of incidentneutron energy. The addition of moderating material (HDPE and BPE) sur-rounding the detector results in increased detection efficiency in the fastneutron energy range.

provided a reference for estimating the recovery time of the dataacquisition system.Two 32-channel peak-sensing Mesytec ADCs (MADCs),

running in 12-bit mode, collected and digitized the inputsfrom the arrays. Each MADC can accept system trigger gatesof differing widths. The first ADC read in the signals fromthe twenty NaI(Tl) detectors, produced by the two MSCF-16NIM modules. The common trigger (1- s width) producedby the MSCF-16 module, the pulser and the beam NIM-outtrigger were processed in a multi-input logic unit, producingan OR logic gate output signal that was then gate stretchedwith a gate-and-delay generator and used to trigger the ADCwith a 2.2- s gate. The second ADC read in the signals fromthe MSCF-16 containing the detectors and the TTL sig-nals from the detectors. The common trigger from theMSCF-16, the logic fan-in/fan-out from the NIM-out signalof the detectors, the pulser and the beam NIM-out triggerwere processed in a multi-input logic unit, producing an ORlogic gate output signal that was then gate stretched to triggerthe second ADC with a 20- s gate.The fast signal output by a liquid scintillator was read directly

into one channel of two separate eight-channel FADCVME slotcards (Struck Systeme Innovative, model SIS3320) [18]. Eachchannel of the SIS3320 was internally triggered and sampledeach input waveform at a rate of 200 MHz (100 MHz band-width). The beam trigger signal, which was used as a fiducial inthe previously described data acquisition system, was not usedin the FADCs. Instead the initial burst (EM, photons) from theaccelerator (corresponding to ) provided a fiducial in eachchannel of the FADC system.Digital pulse shape discrimination (PSD) analysis was per-

formed via the charge integration method [19] by integratingover multiple components (gates) of the waveform. The shortand long integration gates widths are defined by the user and de-pend on the detector type and application. The short gate was onthe order of 50 ns; integration starts just prior to the fast-risingleading edge of the pulse. The long gate was on the order of500 ns; integration starts just prior to the fast-rising leading edge

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WOOLF et al.: PULSED POWER ACTIVE INTERROGATION OF SHIELDED FISSIONABLE MATERIAL 1281

Fig. 3. (a) Counts vs. time in the NaI(Tl) array for a bare DU shot compared to a blank-target shot and the passive background (with the DU present). (b) Countsvs. time in the array for a bare DU shot compared to a blank-target shot and the passive background (with the DU present).

of the pulse and extends the entire pulse width. By taking thesimple ratio of the difference between the short and long gatesto the long gate, expressed as where ( ) isthe short (long) integration gate, and plotting a 2- scatter ofthis quantity versus the long gate (i.e., the pulse height, or en-ergy), reveals inherent differences in the tail of the de-excitationscintillation light pulse. These differences are produced by par-ticles with differing dE/dx (i.e., - ray induced recoil electronsand neutron-induced recoil protons). This method is used to dif-ferentiate between incoming species and was employed to makeprimary neutron and secondary - ray measurements.Each detector’s bias voltage was applied by either a low-

or high-current ISEG [20] high voltage supply (low-current:A, and high-current: 1.3 mA, ) operated via

a Wiener MPOD VME/HV hybrid crate [21].

C. Performance

The operating voltage of the PMT-based liquid scintillatorand NaI(Tl) detectors were optimized such that a maximum inthe gain (to optimize the detector performance) was achievedwhile minimizing the effects of PMT recovery after the inter-rogating pulse. Additional efforts to mitigate these effects weretaken by adding a high-capacitance bleeder string, via in-housemodifications, to the voltage divider on the Scionix-manufac-tured 14-pin PMT bases. These detectors were biased with thehigh-current ISEG high voltage supply. The and oper-ating voltages were determined by the manufacturers to be 2600and 1900 V, respectively, to ensure that good separation of theneutron-induced reaction products and the low amplitude eventswas achieved.We observed recovery times–the time after which the elec-

tronics begins to count individual events–on the order of 10 msfor the liquid scintillation detectors operating with the FADCs;250 ms–500 ms for the , and NaI(Tl) detectors oper-ating in the MADCs. These times varied based on the target andshielding configuration, therefore to account for the recoveryof each detector group, we imposed conservative time cuts of

greater than 100 ms for the liquids, and greater than 500 ms forthe , and NaI(Tl). Time cuts additionally ensure thatpiled-up pulses, which will cause PSD selected events to be in-correctly categorized as either a neutron or γ ray, were negli-gible.The dynamic range for the detection systems was driven by

the energy range of delayed emission from the DU target. Thedelayed neutron spectrum peaks at approximately 750 keV, thendecays exponentially out to MeV (where the endpoint of thespectrum is dependent on the endpoint of the bremsstrahlungbeam) [22]. To accommodate this energy range, the neutronpulse shape selected events in the liquid scintillation detectors(given in terms of proton equivalent energy, p.e.) was MeVto 15 MeV. The moderating material surrounding the gas detec-tors increased the detection efficiency in the fast neutron energyrange ( keV and above) while reducing the efficiency forthermal neutrons. Fig. 2 shows the intrinsic efficiency vs. energyobtained via Monte Carlo simulations, demonstrating the effectof the moderating material on the response of a detector;a combination of the HDPE and BPE effectively increases thefast neutron efficiency, compared to that observed with a baredetector. In these simulations, input events were usedto generate these data.For the build-up of PE and BPE around the detectors,

the Monte Carlo simulations show that the intrinsic efficiencyfrom thermal to fast neutrons varied from 0.1% - 10% with 3%efficiency at 1 MeV. The intrinsic efficiency for the detec-tors over the same energy range is higher by a factor of 2. Theliquid scintillation detectors, with neutron pulse shape selectedevents, have an intrinsic efficiency in the 2 MeV–8 MeV energyrange of .The delayed - ray spectrum is a decaying exponential above

1 MeV and yields an order of magnitude more emission, com-pared to neutrons, above 3 MeV. The 3 MeV–7 MeV energyrange is of particular interest since cutting out events with en-ergy less than 3 MeV eliminates the natural background (i.e.,above the 2.614 MeV line) and thermal neutron activa-tion (if present) of the internal iodine in the NaI(Tl) detectors

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1282 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 62, NO. 3, JUNE 2015

Fig. 4. Integrated counts vs. time for the (a) , (b) , (c) liquid scin-tillation detection ( -ray pulse shape selection) arrays observing DU (greencurve), high- shielding without the DU target present and the passive back-ground. (Shots on low- shielding without the DU target show comparable re-sults). Multiple shots were taken for a given configuration, shown as one colorwith differing (solid or dashed) line patterns. The yellow shading represents thefive-sigma background region.

(producing a continuum up to 2.562 MeV). Analyzing this en-ergy range then allows one to seek out the signature unique

to fissionable material under active interrogation, not presentin other materials [23]. The energy range for the - ray pulseshape selected events in the liquid scintillation detectors (givenin terms of electron equivalent energy, e.e.) was MeVto 8 MeV; for the NaI(Tl) detectors, the energy range wasMeV to 8 MeV. This range was chosen such that there would

be sensitivity to the spectrum above 3 MeV.The NaI(Tl) detectors have an intrinsic efficiency of

for in the 3 MeV–7 MeV energy range. The liquid scintillationdetectors, with -ray pulse shape selected events, have an in-trinsic efficiency of in this same energy range.

IV. ANALYSIS AND RESULTS

A. Time Profile

In Fig. 3 we show the integrated counts vs. time in the ( )NaI(Tl) array and ( ) array to show the baseline delayed-ray and neutron signal observed from a bare DU target com-

pared to a blank-target shot and passive background. The sharpincrease in counts corresponds to the time of the shot, followedby a characteristic die away to nominal background countinglevels on the order of minutes. The observed signal in thearray decays to background levels after approximately four min-utes. The NaI(Tl) array measures counts above the backgroundfor greater than five minutes. When analyzing the -ray signa-ture, we imposed the aforementioned time and energy cut. Onecan see that for a shot without the target present, each detectorarray measures the initial burst, shown as the spike at . Theresulting die away, though, is less than 1 s, clearly indicating thatthe curve corresponding to a shot on target is the signature of thedelayed products from induced fission in the DU target.

B. Active Background

Fig. 4(a)–(c) shows the multitude of background shots (as de-scribed in Section II) observed in neutrons ( and ) andrays (liquid scintillation detectors with PSD selections), dis-

played as integrated counts vs. time in 10-ms bin steps. For com-parison, the delayed signal from the bare DU target is shown.We observe that the signal from the interrogated DU is approx-imately three to four orders of magnitude larger than the back-ground and that there is little shot-to-shot variation in the activebackground based on the shielding only and blank-target shots(the standard deviation of the measured dose, derived from TLDmeasurements, was 9.6%). Note that the detector’s response toshots on shielding only and the blank target are the same, towithin error, as the passive background. The average of the en-semble of shots taken to measure the active background, alongwith the passive background, provides the basis for the back-ground term in themetric of the signal-to-noise ratio (SNR). TheSNR is tabulated as the net integrated counts in terms of the dif-ference in the measured signal and the background over a givenintegration time, divided by the standard deviation of the back-ground ( ), where -valid in time binswhere counting statistics apply, i.e., after the first few secondsof data acquired with our detection systems. In Fig. 4(a)–(c) weshow the background region (shaded) to demonstrate thefluctuation in the active background compared to the DU target.

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WOOLF et al.: PULSED POWER ACTIVE INTERROGATION OF SHIELDED FISSIONABLE MATERIAL 1283

Fig. 5. Signal-to-noise vs. time measured by the: (a) , (b) , (c) liquid scintillation detection (with neutron PSD selections), (d) NaI(Tl) and (e) liquidscintillation detector (with -ray PSD selections) arrays. Curves represent the net integrated counts above background, divided by the of the background for shotson DU, the blank target and DU plus low- shielding configurations of: g cm , g cm and g cm . Multiple shots were taken for a given configuration,shown as one color with differing (solid or dashed) line patterns.

To more clearly elucidate the effects of DU target attenuation,we display the results in Figs. 5–6 in terms of the SNR vs. time(post shot) for each configuration of the target plus shieldingwith either low- or high- material. Lastly, Fig. 7 shows themean SNR for each shot versus the areal mass density for eachshielding case, with first-order exponential fits.

C. (BPE) Shielding

Figs. 5(a)–(e) show the results for BPE shielding withthicknesses of 10.2 cm, 30.5 cm and 50.8 cm. The blanktarget data are from a shot without the target or shielding inplace; it serves as a baseline reference to the shots with the

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Fig. 6. Signal-to-noise vs. time measured by the: (a) , (b) , and(c) liquid scintillation detection (with -ray PSD selections) array. Curvesrepresent the net integrated counts above background, divided by the of thebackground for shots on DU, the blank target and DU plus high- shieldingconfigurations of: g cm , g cm , g cm , g cm , g cm and

g cm ( and only). Multiple shots were taken for a given configuration,shown as one color with differing (solid or dashed) line patterns.

DU target and varying amounts of shielding present. The BPEstrongly affects the signal observed in the neutron detectors.The gas detectors observe a reduction in counts by two orders

of magnitude between the bare DU plate and the addition of10.2 cm of BPE (Figs. 5(a)–(b)); the liquid scintillation detec-tors measure a reduction by an order of magnitude (Fig. 5(c)).The liquid scintillation detectors have a collective threshold of

MeV , while the gas detectors, which have increasedefficiency MeV, are still relatively sensitive to neutrons atand well below 1 MeV. For the gas detectors, although they aremore sensitive to lower energies, and hence one would expectthat the delayed neutrons down scattered in energy would yieldan increase in signal, results show that the loss of low-energyneutrons due to the moderator outweighs the gain due to thedown scattered contribution. The liquid scintillators, whichwere unable to measure neutrons below 1 MeV, were lessaffected. An additional effect is due to the nature of the DUfission spectrum. Because the majority of the delayed neutronsignal from the DU is below the liquid scintillation detectorthreshold, the addition of BPE has less of an effect on theoverall signal in the liquid scintillation detectors compared tothe gas detectors.A similar reduction is observed when the BPE shielding is in-

creased from 10.2 cm to a total of 30.5 cm with all three neutrondetector arrays counting at or marginally above the blank-targetreference shot SNR. At 50.8 cm all three neutron detector arraysobserve a signal commensurate with the background.Figs. 5(d)–(e) show the results for BPE as measured by

the -detection arrays. The resulting counts in the NaI(Tl)array drop by roughly a factor of three for a BPE thickness of10.2 cm, compared to the bare DU target. As the BPE thicknessis increased in 20.3-cm increments, the resulting counts inthe NaI(Tl) array drop by roughly a factor of four to five ineach step (Fig. 5(d)). At 50.8 cm the NaI(Tl) array measuresapproximately 10 times above the blank-target backgroundshot SNR. The liquid scintillation detectors observe a reductionby more than an order of magnitude with 10.2 cm surroundingthe target, compared to bare DU. At 30.5 cm and 50.8 cm theresultant signal is buried in the background with any countsobserved above background consistent with statistical fluctu-ations (Fig. 5(e)). Compared to the NaI(Tl) array, for the bareDU plate the liquid scintillation detector counts are lower bymore than an order of magnitude. This difference is due to thehigher intrinsic efficiency of the NaI(Tl) compared to that ofthe organic liquid for photons in the 3 MeV to 7 MeV energyrange, as well as differences in the total detector geometricarea, their orientation relative to the target ( edge on vs. 53degrees) and in the frontal detector lead shielding (10 cm forthe liquid scintillation detectors compared to 0.6 cm for theNaI(Tl)).

D. (Steel) ShieldingFigs. 6(a)–(b)) display the results for the incremental addi-

tions of steel around the DU target as measured by the neu-tron detection arrays. The detectors observe a signal that isroughly four orders of magnitude greater than the blank-targetshot (Fig. 6(a)). While increasing the steel thickness reduced thecounts, the net count is 100 times greater than the blank-targetshot for the maximum steel thickness of 19.1 cm. Similar re-sults are shown for the array (Fig. 6(b)), where a lower,yet clearly distinguishable, signal is observed above the blank-

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WOOLF et al.: PULSED POWER ACTIVE INTERROGATION OF SHIELDED FISSIONABLE MATERIAL 1285

Fig. 7. The average signal-to-noise vs. areal mass density measured by the: (a) , (b) , (c) liquid scintillation detection (with neutron PSD selections),(d) NaI(Tl) and (e) liquid scintillation detection (with -ray PSD selections) arrays. The error bars represent a 1 standard deviation. First-order exponential fitsare given by the solid lines. Average signal-to-noise values commensurate with the blank-target shot were excluded from the fit.

target shot for a steel thickness of 19.1 cm. The results for theneutron pulse shape selected events in the liquid scintillation de-tection array, as discussed in [14][15], show that the array mea-sured counts that are comparable in magnitude to that observedby the , albeit with a steeper falloff as the shielding was in-creased. At a of g cm the array measured a signal thatis indistinguishable from the blank-target.

Fig. 6(c) shows the results for the incremental additions ofsteel around the DU target for the -ray PSD selected eventsin the liquid scintillation detectors. As expected, data for likeshielding configurations are clustered and an overall reductionin counts with increasing steel thickness is measured. How-ever, in the regime of low overall counts above background(for greater than g cm ), the differences for increased steel

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1286 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 62, NO. 3, JUNE 2015

thickness become less pronounced with statistical fluctuationsdominating. The results from the NaI(Tl) array, as discussedin [14][15], show that the unshielded DU target shot resultedin detection three orders of magnitude above the blank-targetshot. For steel shielding with of g cm the array mea-sured roughly three times more signal–above the blank-targetshot–than is required to trigger the data acquisition system.

V. DISCUSSION

The observed attenuation in the neutron detectors for in-creasing BPE shielding around the target is consistent withthe mass attenuation coefficient of hydrogen with an incomingbremsstrahlung photon beam with 7-MeV endpoint energyand subsequent outgoing neutrons losing energy via elasticneutron-proton (n-p) scattering in a hydrogen-rich material.(Second-order effects from neutron-carbon and neutron-boronscattering were not considered). For a neutron energy of

MeV—accounting for the average neutron energy of500 keV for delayed neutrons from the DU [24] and thehigher energy contributions in the tail that extend out toseveral MeV—in the BPE the neutron has a mean-free-path

cm; for an elastic n-p scatter, the neutron deposits,on average, 50% of its incident energy [25]. In 10.2 cm ofBPE, a MeV neutron will scatter five to six times beforeexiting, reducing its energy in the process to a few hundredkeV. For thicker BPE, the neutron will scatter an increasednumber of times while passing through the material, furtherreducing its energy and the probability of neutron capture inthe material. The for neutrons below 100 keV is 0.6 cm,on average, resulting in a MeV neutron undergoing 20additional scatters before thermalizing ( eV), i.e.,approximately 28 cm of BPE if the neutron traveled in a straightline during the exit. (A neutron depositing half of its incidentenergy, on average, would scatter at 45 with each interaction,thus requiring less than 28 cm of BPE for thermalization tooccur). The few counts observed with g cm (30.5 cm)of BPE shielding the DU target correspond to neutrons in thehigh-energy tail that survive; with g cm (50.8 cm) ofBPE shielding the DU target all neutron detector arrays registerbackground levels. (Comparatively, for g cm of steel,all detector arrays clearly detect the DU signature).The observed attenuation in the NaI(Tl) array for in-

creasing BPE shielding around the target is consistent withthe mass attenuation coefficient of hydrogen with an incomingbremsstrahlung photon beam with 7-MeV endpoint energyand 5-MeV outgoing fission delayed rays. The data fromthe NaI(Tl) array unambiguously demonstrates detection for

g cm (50.8 cm) of BPE surrounding the DU target, whilethe liquid scintillation detectors with -ray PSD selectionsclearly show detection with g cm (10.2 cm) of BPEsurrounding the DU target.The observed attenuation in the neutron detectors for in-

creasing steel shielding around the target is primarily due toattenuation of the incoming bremsstrahlung photon beam, whilethe outgoing induced fission neutrons are largely unaffected.For MeV neutrons escaping the target, their in steel is

cm, which is greater than all shielding configurationsexcept for the 19.1 cm case. An interaction occurring in thesteel would result in a maximum energy deposit of 6.8% of the

incident neutron energy [25]. The associated reduction in neu-tron ( and arrays) counts correlate well with the massattenuation coefficient of steel for an incoming bremsstrahlungphoton beam with 7-MeV endpoint energy.The observed attenuation in the liquid scintillation detector

array for increasing steel shielding is consistent with that of anincoming bremsstrahlung photon beam with 7-MeV endpointenergy and 5-MeV outgoing fission rays. Unlike the neutrondetectors, in the case of the -ray detectors, the attenuation fromsteel affects both the incoming bremsstrahlung photon fluencereaching the target and the outgoing induced fission rays ar-riving at the detectors. Thus, the -ray detectors were not ableto unambiguously detect a signal from the DU at the greatestthicknesses of steel shielding.To better represent the relationship obtained from the data in

Figs. 5–6, themean values of the SNRwere tabulated and plottedversus the areal mass density for each system (Fig. 7(a)–(e)).Where applicable, both the low- and high- shielding configu-rations are shown together. The error bars represent 1 standarddeviation. These data were fit with simple first-order exponen-tials using a non-linear least squares fit method. Displaying theresults in this manner allows the reader to observe the shape ofthe exponential fall off with increasingly thick shielding anddetermine the detection significance for a given thickness ofshielding encapsulating a fissionable material.

VI. CONCLUSIONSWe have presented empirical results from a unique method

of bremsstrahlung active interrogation that has not been done,to this level of detail, in the past. The work was carried outusing a wide variety of neutron and -ray detectors to mea-sure the delayed signatures. The choice of detection system (thetype and number of detectors used) and location with respectto the target were driven by the instrumentation available at thetime of the experiment and the space constraints within theMer-cury facility, respectively. The optimal detection system, dis-tance from the target, or determining the optimal dose from thepulsed power machine to achieve a measurement with a givenlevel of confidence, was beyond the scope of this work.The utility of this novel approach to active interrogation

is that it demonstrates a method in which one could obtain ameasurement with higher confidence and in a shorter amount oftime than could be achieved with traditional systems (linacs).Swanekamp et al. [8] has analytically demonstrated this byusing the receiver operator characteristic (ROC) curve tocompare the performance of a pulsed power system vs. a linacsystem for active interrogation. The ROC curve analysis is astatistical tool for distinguishing a signal from the noise [26].In this approach the ROC curve examines a Gaussian distri-bution of cosmic-ray induced background neutrons measuredby some detection system over a given time interval and thedistribution of this background plus the distribution of thedelayed neutron signal measured by the same detection systemover the same time interval, produced by active interrogationof a fissile material. The mean of the latter distribution isseparated (in terms of total number of counts) from the meanof the former based of the strength of the measured signal.For two overlapping distributions the ROC curve evaluates theprobability of a successful detection vs. the false alarm rate.Summarizing the work in [8], an example is laid out discussing

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WOOLF et al.: PULSED POWER ACTIVE INTERROGATION OF SHIELDED FISSIONABLE MATERIAL 1287

a fissile object undergoing active interrogation by a linac andpulsed power system, each delivering the same integratedbremsstrahlung spectrum, producing the same dose and totalnumber of neutrons in the fissile object. In this scenario, thetotal counting interval for the linac is 120 s, compared to 3 sfor the pulsed power system. The factor of 40 reduction inthe counting interval for the pulsed power system reduces theaverage number of background counts ( ) and hence the widthof the distribution (where ), while the signal strengthproduced in each system is comparable. Hence, there is a largerseparation between the distribution of background neutronsand background plus delayed neutrons from the fissile materialwhen using the pulsed power system. This allows for positivedetection with an extremely small false alarm probability.Comparatively, for the linac system, the ROC curve analysisshows a 73% probability of successful detection at a 10% falsealarm rate. Additionally, it was found that for the pulsed powersystem to have the same detection success rate as the linac,the pulsed power dose (and machine current) would have to bereduced by a factor of five.The potential impact of this method for active interrogation

could be fully realized during a search scenario of heavilyshielded cargo containers at points of entry (e.g., border cross-ings, sea ports, etc.). The short observation time needed toachieve a high-confidence measurement would reduce the scantime needed for assessing whether a threat source is present,minimizing the impact on traffic. While the source used in thisexperiment is not a threat source ( ), one could consider thescenario for a threat source ( ) and the expected signal. Inthis case, by interrogating , one would expect an increase insignal, compared to , based on the well-established fact thatthe photofissioncrosssection is larger than thatof .This approach to active interrogation, with the strengths out-

lined above, does not come without weaknesses. By performingthe experiment we learned about the difficulties involved indetecting fissionable material in the harsh radiological environ-ment. To overcome these difficulties, we worked to optimizethe detector performance after irradiation by a copious fluxof bremsstrahlung x-ray photons, by modifying the voltagedividers on PMT-based detectors with high-capacitance bleederstrings, using sufficient shielding to reduce the low-energy-ray/neutron flux and using fast readout electronics. Another

hurdle to overcome is to reduce the expanse of the pulsed powermachine itself (Mercury is immobile and resides in a large,high-bay facility). Work is currently underway to develop aless cumbersome system [27]. Additionally, the ability to havea repetitive, quick-turnaround system (i.e., smaller betweenshots as current systems are on the order of 10s of minutes)would help facilitate a path towards a real-world search system.Developing a repetitive and reliable mobile system, workingin tandem with -ray detectors of some given geo-metric form, would yield a high-confidence method for activeinterrogation when attempting to interdict threat materials.

ACKNOWLEDGMENT

The authors wish to thank A. Culver, D. Featherstone,E. Featherstone, and D. Phipps for their assistance operatingthe Mercury facility and constructing the various shieldingconfigurations.

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