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Active Interrogation of Depleted Uranium Using a Single Pulse, High-Intensity Photon and Mixed Photon-Neutron Source Ceri D. Clemett, Member, IEEE, Philip N. Martin, Member, IEEE, Cassie Hill, Member, IEEE, James R. Threadgold, Member, IEEE, Robert C. Maddock, Ben Campbell, John O’Malley, Richard S. Woolf, Member, IEEE, Bernard F. Phlips, Anthony L. Hutcheson, Eric A. Wulf, Member, IEEE, Jacob C. Zier, Member, IEEE, Stuart L. Jackson, Member, IEEE, Robert J. Commisso, Fellow, IEEE, and Joseph W. Schumer, Senior Member, IEEE
Abstract—Active interrogation is a method used to enhance the likelihood of detection of shielded special nuclear material (SNM); an external source of radiation is used to interrogate a target and to stimulate fission within any SNM present. Radiation produced by the fission process can be detected and used to infer the presence of the SNM. The Atomic Weapons Establishment (AWE) and the Naval Research Laboratory (NRL) have carried out a joint experimental study into the use of single pulse, high-intensity sources of bremsstrahlung x-rays and photoneutrons in an active interrogation system. The source was operated in both x-ray-only and mixed x-ray/photoneutron modes, and was used to irradiate a depleted uranium (DU) target which was enclosed by up to of steel shielding. Resulting radiation signatures were measured by a suite of over 80 detectors and the data used to characterise detectable fission signatures as a function of the areal mass of the shielding. This paper describes the work carried out and discusses data collected with proportional counters, NaI(Tl) scintillators and Eljen EJ-309 liquid scintillators. Results with the x-ray-only source demonstrate detection ( ) of the DU target through a minof steel, dropping to when imum of using a mixed x-ray/photoneutron source. The proportional counters demonstrate detection ( ) of the DU target through steel shielding deployed for both the maximum photon and mixed x-ray/photoneutron sources. Index Terms—Active interrogation, bremsstrahlung x-ray, electron accelerators, nuclear security, photoneutron, special nuclear material (SNM).
Manuscript received June 03, 2014; revised October 24, 2014; accepted December 24, 2014. Date of publication March 19, 2015; date of current version April 10, 2015. This work at the Naval Research Laboratory was supported by AWE and funded by the U.K Government through the Defence Threat Reduction Agency. C. D. Clemett, P. N. Martin, C. Hill, J. R. Threadgold, and J. O’Malley are with the Nuclear Security Science Group, Atomic Weapons Establishment, Reading RG7 4PR, U.K. (e-mail:
[email protected]). R. C. Maddock and B. Campbell are with the Radiation Science Group, Atomic Weapons Establishment, Reading, RG7 4PR, U.K. J. C. Zier, S. L. Jackson, R. J. Commisso, and J. W. Schumer are with the Plasma Physics Division, U.S. Naval Research Laboratory, Washington, DC 20375 USA. 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, DC 20375 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2015.2403777
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I. INTRODUCTION
UCLEAR SECURITY, and the threat posed by nuclear terrorism are topics of global concern. The International Atomic Energy Authority’s Incident and Trafficking Database [1] has recorded over 2000 incidents of nuclear or other radioactive material being handled outside of regulatory control in the last two decades. Sixteen of these incidents involved special nuclear materials (SNM), either highly enriched uranium (HEU) or plutonium [1]. Against this background, a significant amount of research has been directed at developing and improving methods for detection of SNM, particularly within shipping containers. Active interrogation is a technique that has been studied extensively in recent years [2], as it offers the possibility of greatly increasing the likelihood of detecting shielded SNM. The basis of active interrogation is the utilisation of an external radiation source to induce fission within SNM and to use the radiation emitted as a result of the fission process as a means of detecting the SNM. The majority of sources used for active interrogation employ neutron or photon radiation. Many methods for generating the required radiation have been proposed: neutrons from deuterium-deuterium (DD) [3] or deuterium-tritium (DT) fusion generators [4] or other ion-based reactions [5], mono-energetic gamma-rays from ion-based reactions [6], and bremsstrahlung x-rays from electron accelerators [7]. These sources are most commonly operated in a pulsed mode, using linear accelerators (LINAC) or pulsed neutron generators with beam currents in the A range, pulse widths of order s, and frequencies up to several thousand Hz. In contrast, the Atomic Weapons Establishment (AWE) and the Naval Research Laboratory (NRL) have studied active interrogation techniques using a pulsed-power accelerator, which produces intense radiation in a single short pulse, or flash [8]. This flash-based approach to active interrogation offers several possible advantages over the conventional pulsed approach: For a fixed accelerated charge, the delivery of the interrogating radiation in a single, high current pulse generates the maximum fission signature in the shortest time, therefore maximising the obtainable fission signal-to-background ratio. Secondly, the length of the pulse, typically less than 100 ns, provides the possibility
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of gaining access to prompt fission signatures. These potential advantages may however be offset by decreases in detector performance caused by the more intense radiation environment of a flash-based active interrogation. AWE and NRL have previously carried out investigations into active interrogation using flash-based sources of kinematically-collimated neutrons from the reaction [9], mono-energetic gamma-rays from the reaction [10], and 8 MeV-endpoint bremsstrahlung x-rays [11]. Data from these studies suggests that for a flash-based active interrogation system, a bremsstrahlung x-ray source is the most efficient and technically mature [12]. The use of a bremsstrahlung x-ray source opens up the prospect of carrying out mixed photon and neutron interrogation by introducing a photoneutron convertor to the accelerator. The resulting dual-particle source further increases the possibility of interrogating SNM through both hydrogenous and high atomic number (high-Z) shielding. The experiment reported here is an extension of previous work carried out on flash-based bremsstrahlung x-ray active interrogation [11]. Data were collected from a depleted uranium (DU) target irradiated through steel shielding using the source in x-ray-only mode, and through the addition of a convertor target, in mixed x-ray and photoneutron mode. Borated polyethylene (BPE) and lead shielding were also used, although these data are not reported here. Radiation signals generated during interrogations with no DU present were also investigated. Experimental procedures are described, and data from a subset of the radiation detectors fielded are discussed.
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Fig. 1. Experimental layout at the source end of the Mercury cell. Additional radiation detectors, not reported here, were present on the grayed-out stands.
Peak diode voltages were calculated [17] for each interrogation, giving mean electron endpoint energies of MeV using only the large-area x-ray diode and MeV with the or container in place; allowing a maximum photoneutron energy from the reaction of approximately 3 MeV. A pair of thermoluminescent dosimeters (TLDs, Harshaw TLD-400) positioned centrally 2 m downstream of the diode measured the x-ray dose generated by each interrogating pulse. During x-ray-only interrogations, the mean dose normalized to 1 m from the diode was rad , falling to rad with the or container in place.
II. ACTIVE INTERROGATION SOURCE
III. EXPERIMENTAL DETAILS
The NRL Mercury Inductive Voltage Adder (IVA) pulsedpower driver was used as the active interrogation source used for these experiments. This pulsed-power driver is typically used for flash-radiography research [13], [14] and is capable of producing an 8 MeV-endpoint bremsstrahlung x-ray spectrum at a peak electron current of 200 kA and with a full-width half maximum (FWHM) pulse width of 50 ns. A large area x-ray diode was used to generate the x-ray source [15]. Electrons were accelerated into a m thick, 35.56 cm-diameter tantalum convertor layer covered by a m-thick aluminium foil and backed by a 1.91 cm-thick aluminium beam stop. An additional 1.91 cm-thick aluminium plate downstream of the diode served as a vacuum seal. Cylindrical steel collimation extending forwards from the vacuum seal was used to reduce direct source contributions to the detectors. The collimation had a 33.02 cm inner diameter and extended forwards from the vacuum seal for 1.5 m. The initial 62 cm of collimation was 26 cm thick, and the remaining 88 cm length was 12.7 cm thick. The accelerator was adapted to produce a mixed x-ray and photoneutron source by placing a 10 cm-thick plastic container filled with within the collimation against the outer edge of the vacuum seal [16]. This container was interchangeable with an identical container filled with .
Over 80 radiation detectors were positioned within the Mercury radiation cell to measure photon and neutron fission signatures as functions of time and energy, and to gather source performance data. The detector suite included: NaI(Tl), BGO, EJ-200, and EJ-299-33 scintillators; EJ-301 and EJ-309 liquid scintillators; and proportional counters; Rh activation counters and TLDs. Data collected with 20 NaI(Tl), 7 EJ-309, and 6 detectors are discussed in this paper. Fig. 1 details the layout of the Mercury cell, showing the relative positions of the source, target stand, and detector stands holding NaI(Tl), EJ-309 and detectors. Fig. 2 shows an image of the Mercury cell. A. Target and Target Shielding Depleted uranium was used as a surrogate for SNM. A DU slab was placed 2.5 m from the x-ray source. The slab, measuring cm cm cm, was sealed within an aluminium container with 0.32 cm-thick front and back plates and 1.27 cm-thick side walls. It was positioned facing the source with its centre coaxial with the source axis. The composition of the DU included approximately 0.22% of by weight. Shielding of the DU target was provided by 1.27 cm-thick steel plates ( g cm ), which were stacked and arranged into uniform thickness box structures completely
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Fig. 2. Photograph showing the layout of the Mercury radiation cell used for testing x-ray and x-ray/photoneutron active interrogation sources.
enclosing the DU. The experiment used shielding configurations with areal masses of: g cm , g cm , g cm , g cm , g cm , and g cm . These values are single-pass, line-of-sight areal masses between the source and the DU target. B. Detectors NaI(Tl): Twenty NaI(Tl) scintillators were fielded, the cm cm cm in size, NaI(Tl) crystals were with the frontal face of each detector ( cm cm) in direct line-of-sight of the DU target. The detectors were divided into two groups of 10, with each group located on a separate stand in two rows of 5 detectors positioned at the midpoint of the stand. The two stands (A and B in Figs. 1 and 2) were positioned on either side of the accelerator in a reflective geometry (upstream of the target). Stand A detectors were 3.3 m from the DU target at an angle of 55 from the source axis, stand B was at 4 m and 67 . Frontal shielding of the detectors consisted of a 0.64 cm-thick lead sheet sandwiched between two 2.54 cm-thick 2% borated polyethylene (BPE) slabs. Rear shielding consisted of 5.08 cm of lead within the same BPE sandwich. The sides of the detectors were covered by 5.08 cm of lead, and the top and bottom by 5.08 cm of both lead and BPE. On the bottom the outer layer was BPE, reversed on the top. During and interrogations, a m layer of Cd was wrapped around the detectors. EJ-309 Liquid Scintillators: Sixteen Eljen EJ-309 liquid scintillators were positioned in an array facing the edge of the DU target at a distance of 2.5 m (stand C in Figs. 1 and 2). The active volume of the detectors was cm cm cm. They were shielded to the front and sides with 10.16 cm of lead, and on the top and bottom by 5.08 cm of lead. A m layer of Cd was wrapped around 12 of the detectors during and interrogations. Data from 7 of the 16 detectors are used.
proportional counters : Six proportional counters (Canberra 64NH30) were arranged in 3 pairs positioned 1 m from the DU target and in a reflective geometry (stand D in Figs. 1 and 2). Each of the detectors was cylindrical with a 30 cm active length, 2.5 cm diameter, and a 10 bar pressure. Neutron moderation was provided by a cylindrical arrangement of 2 cm-thick polyethylene, 0.06 cm-thick cadmium foil and 1 cm-thick Flexiboron from the tube outwards [18]. This combination is intended to minimize the detection of low-energy scattered neutrons and provides increased sensitivity to delayed fission neutrons.
C. Data Acquisition Detector control and data acquisition systems were positioned within the Mercury control room, with power and data cables feeding through the 2.6 m concrete wall separating the control room from the Mercury cell. Data from each of the NaI(Tl) detectors were routed through a Mesytec MSCF-16 spectroscopic amplifier and the resulting signals digitized in a 12-bit Mesytec MADC-32 peak-sensing ADC [19]. This setup provided individual streams of digitized list mode data for each of the twenty detectors. The detectors were operated at a positive bias with photomultiplier tube (PMT) voltages ranging from 1000 to 1100 V. Output signals from the EJ-309 detectors fed into Struck SIS3320 FADC VME cards [19], providing digital list mode data including pulse shape analysis; allowing for differentiation between detected particle species as either a photon or neutron detection. EJ-309 PMT voltages ranged from V to V. Energy calibration of the NaI(Tl) and EJ-309 detectors was carried out using , , and sources and periodic checks of gain stability were carried out over the course of the experiment.
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TABLE I INTERROGATION SUMMARY
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was a simple time-varying ratio, R(t), of the net detected signal to the standard deviation of the measured background distribution: (1) Where is the cumulative total signal in counts, detected in the time t, is the mean background signal expected in the same time t, and the standard deviation of the expected background. For the remainder of the text, the parentheses showing time dependence have been dropped. A. Defining the Background
Distribution of interrogations carried out with each source type for the various target and shielding configurations used during the experiments These interrogations were carried out with 30.48 cm of steel ( cm plates) inserted into the machine collimation and an empty target.
The proportional counters were directly attached to Canberra ACHNA98 charge amplifiers whose output fed into a Canberra Multiport II multichannel analyzer. Data were collected in multichannel scalar (MCS) mode with 16384 bins and a dwell time of 30 ms, giving approximately 8 minutes of acquisition time. Additionally, four of the detectors were connected to a digital oscilloscope which recorded the first second of data at a sampling rate of 10 MS/s. Initialization of the NaI(Tl) and EJ-309 data acquisition systems was carried out manually prior to each interrogation. The electrical trigger pulse used to fire Mercury was recorded by both data acquisition systems for use as a timing fiducial. The data acquisition system was triggered automatically by the Mercury trigger pulse, so that the interrogating pulse was contained within the first MCS time bin. D. Experimental Sequence Seventy four interrogations were carried out over the course of the experiment, divided into two main types: DU interrogations and null interrogations. Both types were further sub-divided according to the amount of steel shielding present and source-type. Three different interrogation sources were used: x-ray-only, x-ray plus a photoneutron convertor (referred to as mixed source), and x-ray plus an attenuator (referred to as attenuated x-ray). Multiple interrogations were carried out for most combinations of interrogation type, shielding level and source; as described in Table I. IV. DATA ANALYSIS METHOD The primary objective of the experiment was to measure the change in the detectable fission signatures as a function of active interrogation source type and areal mass of steel shielding surrounding the DU target. The method used to quantify detection
The measured background in an active interrogation has two components: the standard passive radiation background, which can be measured prior to an interrogation, and any active signal not due to the presence of SNM. We define the active background as the total signal received by a detector system during an interrogation in which no SNM is present. Direct detection of the interrogating pulse, detection of scattered radiation from the interrogating pulse, and radiation from reactions induced in inert materials (non-SNM) all contribute to the active background. The characteristics of the active background are therefore sensitive to changes in target configuration and accelerator output. Null interrogations, in which complete shielding configurations with the DU removed were irradiated, were used to investigate the active background. This allowed measurements of the effects on the active background of varying interrogation pulse intensity and of increasing levels of shielding. B. Analysis Methodology Energy thresholds were applied to the raw data to remove as much of the passive background signature as possible. In the case of null interrogations, the cumulative data were used to define an empirical distribution of background data for each individual detector, from which the time-varying cumulative mean and standard deviation of the background were derived, giving inputs for and in (1). Active background distributions were defined for each of the three source types. Cumulative data from DU interrogations were used as inputs for in (1), generating the time varying ratio, , for each DU interrogation. A mean value of , denoted , was calculated over the first 10 s of data. These values were used as the detection value for the detector at the relevant areal mass of shielding. The behaviour of as the areal mass of shielding increases gives an indication of the relative detection of the DU with each detector and source-type. C. List Mode Data Digital list mode data were acquired for the NaI(Tl) and EJ-309 detectors. No hardware cuts were applied to the data during the acquisition process, allowing post-experiment analysis to define any combination of energy cut, time cut, time binning, or pulse shape discrimination (PSD) cut. NaI(Tl): Data from NaI(Tl) detectors were extracted using a 3 MeV lower energy cut. Rate histograms with a time bin width of 10 ms were generated with the energy–cut data. A time
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Fig. 3. and (dotted lines: ) from null interrogation data collected with a single NaI(Tl) detector. Also shown is the mean passive background mea). surement from the same detector (shaded area:
correction was applied using the timing fiducial provided by the Mercury trigger signal to set the interrogation time to zero. A further time correction was then applied in order to account for detector dead-time caused by the extremely high count rates experienced by the detectors during the interrogating pulse. Individual detectors exhibited varying lengths of zero response following each interrogation, and it was necessary to adjust each dataset individually so that each detector started to operate at the same time. Without this correction, each of the variables in (1) would become non-zero at a different time and lead to skewed measurements of . Time-corrected count rate data were finally converted into cumulative count data. Data acquired from each of the three source types were treated in an identical manner. EJ-309 Liquid Scintillators: There were only minor differences in the treatment of the data from the EJ-309 detectors compared to those from the NaI(Tl) detectors. Cuts were applied to the EJ-309 detector data to remove the majority of the signal seen in the background datasets. Energy calibration showed the lower energy cut to be equivalent to removing events of approximately 2 MeV and below. Using the PSD capability of the EJ-309 detectors, an additional cut was applied based on the pulse shape parameter, which can be used to discriminate between photon and neutron interactions. Initial examination of the datasets suggested that the number of photon counts was far higher than the number of neutron counts. A piecewise pulse shape cut was applied which focused detection of photons only. Time corrections applied to EJ-309 data were similar to those applied to NaI(Tl) detectors. V. RESULTS AND DISCUSSION A. Active Background Measurements Fig. 3, Fig. 4, and Fig. 5 show for each of the three source types, calculated from data collected with NaI(Tl), EJ-309, and detectors during the null interrogations shown in Table I. Also plotted in each case are: , demonstrating the spread
Fig. 4. and (dotted lines: ) from null interrogation data collected with a single EJ-309 detector. Also shown is the mean passive background mea). surement from the same detector (shaded area:
Fig. 5. and (dotted lines: ) from null interrogation data collected proportional counters. Also shown is the mean passive background with the ). (shaded area:
in null interrogation data; and the mean passive background measured over the course of the experiment. NaI(Tl) and EJ-309: X-ray-only and attenuated x-ray active background data are comparable in both the NaI(Tl) and the EJ-309 plots (Fig. 3 and Fig. 4), with the cumulative signals effectively tracking each other and reaching of order 10 counts after a 12 s integration time. Mixed source active backgrounds for these detectors are higher by an order of magnitude. Comparisons with passive radiation measurements for these detectors show that x-ray-only and attenuated x-ray active backgrounds are very similar to the passive measurements, suggesting that these source types do not have a large effect on the measured radiation background. It is thought that this is due to detector dead-time caused by the interrogating pulse: during the experiments, both the NaI(Tl) and EJ-309 detectors exhibited a period of poor response following each interrogation. The length of this period varied greatly: NaI(Tl) detectors
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remained unresponsive for 200 ms or longer (in some cases extending to s), EJ-309 detectors recovered faster, with typical recovery times between 1 ms and 10 ms. The length and nature of the interrogating pulse were such that by the time the detectors had recovered, there were no direct traces of the pulse remaining. In the absence of longer-lived effects, such as photo-activation of materials within the cell by the irradiating pulse, then all that was left for the detectors to measure was the passive radiation background. This is not the case with mixed-source active backgrounds, which show a greatly inflated cumulative signal compared to passive measurements and x-ray-only active backgrounds. While a period of detector dead-time still occurred during these interrogations, the neutron component of the source caused activation of materials within the cell and created an additional, longer lived source of radiation; the biggest contributor to the increase in signal is thought to be activation of the detectors or detector casings. Energy spectra acquired by NaI(Tl) detectors following mixed source interrogations show the characteristics of activation of iodine through the reaction, which exhibits decay with an electron endpoint energy of 2.119 MeV and a half-life of 24.99 minutes. In the case of EJ-309, energy spectra show activation of the aluminium housing surrounding the detectors through the reaction, resulting in emission of a 1.778 MeV -ray with a half life of 2.24 minutes. The 24.99 minutes half-life of the iodine activation is significant. Typically, interrogations were carried out approximately every 60 minutes, meaning that the level of activation within the NaI(Tl) detectors continued to increase over the course of each day, before returning to background levels overnight. The derivation of does not take this variation into account and thus likely overestimates both and . As a consequence, DU interrogations carried out early in the day will be compared to an active background which incorporates data from detectors with a much higher level of activation. Accumulation of activation effects is less of a problem for the EJ-309 detectors owing to the much shorter half-life of the major contributing reaction. : Null interrogation measurements with proportional counters did not show a dependence on source type. Fig. 5 shows that in all cases the cumulative signal was of order 1 count following a 10 s integration time, very similar to the passive background measurements. These detectors also exhibited a period of poor response immediately following each interrogation. Oscilloscope traces showed this period extending for approximately 20 ms after each interrogation, meaning these effects were contained within the first MCS bin (30 ms width) and could be filtered easily from the remaining data. B. Detection of DU Data from a selection of DU interrogations have been plotted as a series of cumulative curves, illustrating the total detected signal ( ) as a function of detection time for each of the shielding configurations tested and source types used. Also shown are shaded regions denoting the region bounded by and , as calculated in Section V-A., which allow an
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Fig. 6. Cumulative signals measured with a single NaI(Tl) detector during one x-ray-only and attenuated x-ray DU interrogation at each shielding configuraas measured for x-ray-only null tion tested. The shaded area denotes interrogations shown in Fig. 3.
Fig. 7. Cumulative signals measured with a single NaI(Tl) detector during one mixed source DU interrogation at each shielding configuration tested. The as measured for mixed source null interrogations shaded area denotes shown in Fig. 3.
immediate comparison between the magnitude of the detected signals at each shielding level and measured active backgrounds. Fig. 6 and Fig. 7 show data from NaI(Tl) detectors, Fig. 8 and Fig. 9 show data from EJ-309 detectors, and Fig. 10 and Fig. 11 show data from proportional counters. Qualitatively, these plots show that for all detectors, for x-rayonly and attenuated x-ray sources (Figs. 6, 8, and 10), detected signals from DU interrogations at all shielding levels, with the exception of the thickest shielding for NaI(Tl), were more than above active backgrounds for detection times longer than 1 second. As expected, detected signals from attenuated x-ray interrogations are lower than x-ray-only interrogations for identical shielding configurations. The same is not true for NaI(Tl) and EJ-309 data obtained during mixed source interrogations (Figs. 7 and 9). In the case
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Fig. 8. Cumulative signals measured with a single EJ-309 detector during one x-ray-only and attenuated x-ray DU interrogation at each shielding configuraas measured for x-ray-only null tion tested. The shaded area denotes interrogations shown in Fig. 4.
Fig. 10. Cumulative signals measured with a single proportional counter during one x-ray-only and attenuated x-ray DU interrogation at each shielding as measured for x-rayconfiguration tested. The shaded area denotes only null interrogations shown in Fig. 5.
Fig. 9. Cumulative signals measured with a single EJ-309 detector during one mixed source DU interrogation at each shielding configuration tested. The as measured for mixed source null interrogations shaded area denotes shown in Fig. 4.
Fig. 11. Cumulative signals measured with single proportional counter during one mixed source DU interrogation at each shielding configuration as measured for mixed source null tested. The shaded area denotes interrogations shown in Fig. 5.
of NaI(Tl), signals obtained with g cm of shielding are equivalent to , while data from the g cm configurations are below . This is due to not taking into account increasing detector activation, as noted above. In this case, the DU interrogation shown for the g cm dataset was carried out early in the day, before significant activation had taken place. EJ-309 data from the g cm configuration mixed source interrogations are still above , but are now within the shaded region whose upper bound is . Data from proportional counters (Figs. 10 and 11) are unaffected by the type of interrogation source, with all datasets an order of magnitude or more above . The change in vs. shielding areal mass is shown for NaI(Tl) in Fig. 12, for EJ-309 in Fig. 13 and for in Fig. 14. First order exponential fits to the data were carried out using a least squares method.
Fig. 12 shows NaI(Tl) detection values for x-ray-only and mixed source data diminishing at a similar rate. The greater initial value of for the x-ray-only source leads to this data crossing any given at a level of shielding approximately g cm higher than the mixed source. At , the fit to x-ray-only data lies at a shielding level of g cm , with mixed-source data lying at g cm . The fit to attenuated x-ray data appears to have gradient similar to both other sources and a shielding level at greater than the x-ray-only data. This should be treated with caution given the relative magnitudes of the x-ray-only and attenuated x-ray datasets shown in Fig. 6; it is likely that the apparently better performance of this source type is an artefact of the fitting process being applied to very few data points. Data from the highest g cm configurations have not been used in the fitting process, as they exhibit a noticeable
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Fig. 12. vs. areal mass of steel shielding for NaI(Tl) detectors. Exponential g cm data points. fits (dashed lines) exclude
deviation from the simple exponential behaviour of the rest of the data points. The reason for this deviation can be explained by considering the changing nature of the detected signal as the level of shielding increases. Initially, when little or no shielding is present, the majority of the detected signal comes from fission processes induced in the DU target, and the amount of fission signal detected, and hence , are exponentially attenuated by the shielding. At some point, shielding attenuation diminishes the fission signal enough that the active background becomes the dominant signal. The active background, by its nature, is not greatly affected by the amount of shielding present. Thus the detected signal, and , become constant (within the bounds of the stochastic processes by which the active background is generated). At and above the level of shielding at which this change occurs, measured values of are just samples from the distribution which defined the active background. For the NaI(Tl) detectors, this occurs somewhere between the g cm and g cm data points. Fig. 13 shows a similar situation occurring for the EJ-309 detectors. The mixed source crosses at around the same shielding level as for NaI(Tl) at g cm , while the x-rayonly level at is increased to g cm . Again, the attenuated x-ray curve is a fit to relatively sparse data, and is more useful in suggesting that the performance of this source type is not too far removed from the x-ray only source. Fig. 14 shows that the proportional tubes exhibit improved performance over both the other detectors, with fits to the experimental data suggesting that the shielding level equivalent to is well above the tested shielding thicknesses. This is largely due to the reduced background levels seen by the . In contrast to the other detector types, the addition of a neutron component to the source has not had a detrimental effect on the amount of shielding through which the DU can be detected with , while the attenuated x-ray source has. This difference in the behaviour of for the NaI(Tl) and EJ-309 detectors compared to the proportional counters for
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Fig. 13. vs. areal mass of steel shielding for EJ-309 detectors. Exponential g cm data points. fits (dashed lines) exclude
Fig. 14. vs. areal mass of steel shielding for show exponential fits to data.
proportional. Dashed lines
the various source types can be explained by the activation effects noted in the NaI(Tl) and EJ-309 detectors, where the apparent reduction in performance for the mixed source derives from the increased variation and intensity of the active background caused by neutron activation, rather than through any diminishment of the efficacy of the source. Conversely, the active backgrounds measured by the proportional counters are not noticeably affected by the mixed source, and here the performance is equivalent to that measured with the x-ray-only source. The similarity in source-dependent active background measurements for the detectors shown in Fig. 5, along with the relative performance of the three source types shown in Fig. 14 is interesting. It suggests that attenuation of the x-ray source caused by the container, the effects of which are demonstrated by the attenuated x-ray results, is compensated for by the additional photoneutron source.
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VI. CONCLUSIONS Detection of a DU target through steel shielding has been successfully demonstrated through detection of fission signatures induced by both flash-based x-ray and flash-based mixed x-ray and photoneutron sources. Data from NaI(Tl), EJ-309 and detectors have been analyzed using a simple ratio of the net detected signal to an empirically-derived active background. Results from x-ray-only interrogation show detected signal levels from a DU target to be in excess of for steel shielding of up to g cm . The addition of a photoneutron component to the source appears to lower the level of steel shielding at which signal levels detected with NaI(Tl) and EJ-309 detectors drop below to g cm . This decrease is thought to be caused by increasing active background levels due to neutron activation of the detectors. Data from proportional counters show detected signal levels to be well in excess of for all levels of shielding fielded. These results show good agreement with work carried out by others on the detection of shielded DU through iron-based shielding [20]. The improved performance of the proportional counters when compared against the NaI(Tl) and EJ-309 detectors is expected, given the relative photon and neutron attenuation properties of the steel shielding material. Shielding materials with a greater hydrogen density, such as water or polyethylene, would likely reverse this pattern. Measurements show that with an x-ray-only source, the interrogating pulse has no major effect on the nature of the detected active background; the intense nature of the initial pulse causes the detectors to exhibit a period of poor response, during which no useful active interrogation data can be acquired. By the time the detectors have recovered, no trace of the initial pulse remains. This suggests that the derivation of a suitable active background term may not be necessary, and that passive background measurements could instead be used. While possibly allowing for an easier determination of the background, this period of detector dead time also means a significant amount of data from fission of the DU target is being missed. With a mixed source, the active background is not unaffected by the pulse, although direct detection of the pulse is still avoided due to detector dead time, neutron activation of material within the interrogation volume, as well as of the detector itself, changes active background measurements from interrogation to interrogation. The accumulation of detector activation, especially in the case of the NaI(Tl) detectors, demonstrates a weakness of the empirically-derived active background as defined in this work. The technique applied is a valid one only if the active background is well understood and known not to vary greatly over the course of multiple interrogations. The proportional counters fielded remained unaffected by either source type, as neutron activation from common materials did not result in an increase in the neutron background. ACKNOWLEDGMENT The authors would like to thank Mercury facility technicians Aaron Miller, Anthony Culver, and Derric Featherstone for their efforts in operating and maintaining the Mercury IVA, as well as
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