Intense pulsed active detection (IPAD) is a promising technique for detecting fissile material to prevent the proliferation of special nuclear materials. With IPAD, fissions are induced in a brief, intense radiation burst and the resulting gamma ray or neutron signals are acquired during a short period of elevated signal-to-noise ratio. The 8 MV, 200 kA Mercury pulsed-power generator at the Naval Research Laboratory coupled to a high-power vacuum diode produces an intense 30 ns bremsstrahlung beam to study this approach. The work presented here reports on Mercury experiments designed to maximize the photofission yield in a depleted-uranium (DU) object in the bremsstrahlung far field by varying the anode-cathode (AK) diode gap spacing and by adding an inner-diameter-reducing insert in the outer conductor wall. An extensive suite of diagnostics was fielded to measure the bremsstrahlung beam and DU fission yield as functions of diode geometry. Delayed fission neutrons from the DU proved to be a valuable diagnostic for measuring bremsstrahlung photons above 5 MeV. The measurements are in broad agreement with particle-in-cell and Monte Carlo simulations of electron dynamics and radiation transport. These show that with increasing AK gap, electron losses to the insert and outer conductor wall increase and that the electron angles impacting the bremsstrahlung converter approach normal incidence. The diode conditions for maximum fission yield occur when the gap is large enough to produce electron angles close to normal, yet small enough to limit electron losses.

Summary form only given. Pulsed bremsstrahlung from the Hermes-III generator produces photofission for research in Intense Pulsed Active Detection (IPAD)2. Hermes-III is operated in three modes producing bremsstrahlung with different endpoint energies: 8, 12, and 16 MeV, all with 30 ns x-ray pulse width. The electron-beam current increases with output voltage, from 300 kA at 8 MV to 600 kA at 16 MV. Previous experiments demonstrated detection of delayed neutrons from fissions induced in a depleted uranium (DU) plate. The work reported here focuses on detecting prompt fission neutrons, which are about 100 times more numerous than delayed neutrons. Neutrons are detected using 3He tubes surrounded by two different thicknesses of polyethylene (PE) to moderate the neutrons. A thin layer of cadmium absorbs neutrons with energy less than 1 eV. The “low-energy” detector has 2.5 cm PE and its response peaks at 6 keV. The “high-energy” detector has 12 cm PE and its response peaks at 1.6 MeV. MCNPX calculations for Hermes-III experiments at 8 MV indicate the high-energy detector is capable of detecting prompt fission neutrons (from DU) while effectively excluding photoneutrons produced in the environment or from a lead target. The detector electronics produce pulses that are counted as a function of time after a Hermes-III shot. Scattered x-rays make the detector signals useless for the first 10 microseconds, the time range when prompt neutrons first reach the detectors. But the thick PE disperses the prompt neutrons so they are recorded in the 3He gas at later times. Prompt neutrons are detected between 20 and 500 microseconds, in agreement with MCNPX predictions. When Hermes is operated in the 8-MV mode, the photoneutron signal from the background (no target) or a lead target is zero for times less than 500 microseconds. In the 12-MV mode, photoneutron signals are not negligible, but the presence of fission neutrons can be detected by comparing pulse counts from the two detectors, essentially identifying higher energy neutrons than expected from nonfissionable materials. At 16 MV, photoneutron energies are comparable to the prompt fission neutron energies and detection of fission neutrons by this method is more difficult.

Summary form only given. An intense bremsstrahlung x-ray pulse is generated by the 8-MeV, 200-kA, 50-ns Mercury inductive voltage adder. 1 A study of the diode configuration was undertaken to optimize the forward-directed radiation. To this end, the diode AK gap was varied between 23 and 43 cm and an ID-reducing insert in the vacuum chamber wall was added to adjust the incidence angle and the electron charge at the tantalum anode converter. 2 . Anode current monitors measure the portion of load-region current reaching the converter. Arrays of CaF 2 TLDs, x-ray pin diodes and an x-ray pinhole camera are used to measure the x-ray dose distributions. Anode current data are presented which show that electron losses to the insert and to the outer-conductor wall increase with AK gap, in agreement with pinhole camera measurements. Pin diode signals are analyzed to determine beam dynamics during the pulse. TLD data are presented which show that, as the AK gap increases, the angular dose distribution narrows and that the on-axis dose increases, until the 43-cm AK gap configuration. Here, axial dose decreases as electron losses to the insert and vacuum chamber wall override any further benefit due to smaller incidence angle for electrons striking the anode. Bremsstrahlung produced by electrons at large radius impacting the walls is removed from the x-ray beam by a thick steel collimator. Though wall losses with a small, 23-cm gap are low, LSP/ITS simulations predict large electron impact angles, which would reduce the on-axis dose downstream of the diode. Research has begun to modify such smaller-gap diodes to magnetically steer electrons to converter impact angles closer to the normal 3 .

Introduction: Passive and active detection of hidden fissile material are subjects of intense interest. In active detection, the naturally emitted characteristic radiations associated with radioactive decay are externally enhanced by inducing fission, thus increasing the strength of these radiations compared with what could be measured passively. One approach to active detection uses the bremsstrahlung produced when an energetic electron beam strikes a high-atomic-number material. Bremsstrahlung photons of energy greater than about 6 mega-electron-volts (MeV) can induce photo-fission in fissionable material, the products of which are detected. Bremsstrahlung-based interrogation using linear accelerators (LINACs) is typically used for active detection, but the LINAC approach requires minutes of exposure to produce a detectable signal above background. The Pulsed Power Physics Branch of NRL’s Plasma Physics Division has pioneered and demonstrated an intense pulsed active detection (or IPAD) approach1–3 using single-pulse, terawatt-level, pulsed-power technology to achieve an inspection time of a few seconds. We have focused on the use of single intense pulses of bremsstrahlung (x-rays) to induce photo-fission, and have also effectively applied the IPAD approach using both neutrons and characteristic gamma rays generated using energetic ion-beam interactions with appropriate targets as the interrogation sources.2 The use of a single, very intense interrogating pulse of either high-energy photons or neutrons has several unique advantages over lower-power systems that repetitively pulse over hundreds of seconds. (1) The short irradiation time leads to a detection time of order seconds (vs minutes for the LINAC-based approach), minimizing the passive background, which is proportional to the detection time.1 Also, the short irradiation time-window (~100 ns) allows for the possibility to measure radiations associated with prompt, short-, and long-lived fission products. (2) Pulsed accelerators driving high-power diodes, when used with a ruggedized commercial off-the-shelf detection system, can rapidly scan moving targets of interest within seconds. (3) Bremsstrahlung can efficiently generate both high-energy photons and neutrons (via photo-neutron processes). This can be useful in scenarios where lowor high-atomic-number shielding is placed around the fissile material. (4) Unique alternative signatures (such as neutronbased activation analyses and delayed-prompt neutron emissions for differentiation of 235U and 238U) may be accessible following a single, intense pulse.

High-current (1-3 MA) reflex triodes are being developed as intense pulsed sources of moderate energy (200-1200 kV endpoint) bremsstrahlung. A triode consists of three coaxial electrodes: two cathodes and a flat anode foil between them. The grounded cathodes emit electrons that produce bremsstrahlung in the positive high voltage anode. The anode is made from thin tantalum to minimize attenuation of the low energy portion of the x-ray spectrum. The electrons must make many passes, or “reflex” through the foil for efficient x-ray production.

The HERMES-III generator is used for research in photofission and induced background. This work follows experiments using the 8-MV, 200 kA, 40 ns bremsstrahlung pulse from the Mercury generator to induce photofission in a variety of targets.2 HERMES is operated in two modes: half-machine at 8 MV and 300 kA, similar to Mercury, and full-machine at 16 MV and 600 kA. The objective of the research is to understand radiation background in the environment as the bremsstrahlung endpoint is increased. Standard electrical and X-ray diagnostics are used to characterize the bremsstrahlung source.The x-ray beam is apertured by a 46 cm square hole in a 25 cm thick steel panel, and by two 10 cm thick lead doors, located 12 m from the x- ray source. Targets are located 6 m beyond the aperture. Targets investigated include depleted uranium (DU), lead, borated polyethylene, steel and water.Neutrons are diagnosed using a rhodium counter (time integrated), two 3He tubes and two plastic scintillators coupled to photomultiplier tubes (time resolved). The detectors are located varying distances from the target. More neutron pulses are recorded from the DU target than from the lead target, possibly evidence of photofission. Far more neutrons are detected in the 16-MV mode than the 8- MV mode, as expected. Many pulses are recorded for about 20 ms from all target types. The neutron signals persist for several seconds from the DU target, evidence of delayed neutrons from photofission.

High-current (1-3 MA) reflex triodes are being developed as intense pulsed sources of moderate energy (200-1200 kV endpoint) bremsstrahlung. The triode consists of three coaxial electrodes: two hollow cylindrical cathodes and a flat anode foil between them. The grounded cathodes emit electrons that produce bremsstrahlung in the positive high voltage anode. The anode is made from thin tantalum (small fraction of the electron range) to minimize attenuation of the low energy portion of the x-ray spectrum. The electrons must make many passes through the tantalum to efficiently lose their energy and produce x-rays. This process is called reflexing. The triode configuration is thought to be the best way to accomplish this.

A pinch-reflex ion diode is fielded on the pulsed-power machine Mercury (R. J. Allen, et al., 15th IEEE Intl. Pulsed Power Conf., Monterey, CA, 2005, p. 339), which has an inductive voltage adder (IVA) architecture and a magnetically insulated transmission line (MITL). Mercury is operated in positive polarity resulting in layered MITL flow as emitted electrons are born at a different potential in each of the adder cavities. The usual method for estimating the voltage by measuring the bound current in the cathode and anode of the MITL is not accurate with layered flow, and the interaction of the MITL flow with a pinched-beam ion diode load has not been studied previously. Other methods for determining the diode voltage are applied, ion diode performance is experimentally characterized and evaluated, and circuit and particle-in-cell (PIC) simulations are performed. Results indicate that the ion diode couples efficiently to the machine operating at a diode voltage of about 3.5 MV and a total current of about 325 kA, with an ion current of about 70 kA of which about 60 kA is proton current. It is also found that the layered flow impedance of the MITL is about half the vacuum impedance.

In intense, pulsed active detection, a single, intense pulse of radiation is used to induce photofission in fissionable material, increasing its detectability. The Mercury pulsed-power generator was converted to positive polarity (+3.7 MV, 325 kA, 50-ns FWHM) to drive an intense, pulsed radiation source based on the FIGARO active detection concept. The probing radiation source consisted of an ion-beam diode and a thick PTFE (Teflon) converter where 6-7 MeV γ-rays were produced via the 19F(p,αγ)16O reaction. A suite of radiation detectors was used both to detect the presence of irradiated fissionable material and to characterize the probing radiation source. Four types of detectors were used for the source characterization. Thermoluminescent dosimeters were used to measure the angular distribution of the dose associated with x-rays and γ-rays from the ion-beam diode. Plastic scintillator-photodiode detectors were used to characterize the time dependence of this dose. A plastic scintillator-photomultiplier detector was used to monitor the γ-ray intensity of the probing radiation source and to monitor changes in the production of background neutrons by the diode and PTFE converter. A set of rhodium foil activation counters was used to measure the absolute yield of these background neutrons. Two types of detectors with comparable sensitivities were used to measure delayed neutrons resulting from photofission: 3He proportional counters and a 6Li-loaded-glass-scintillator detector. The neutron detection rate from each detector following the probing radiation pulse was over 100 times higher with depleted uranium present than with lead.

Summary form only given. The SATURN pulsed-power machine is normally operated in negative polarity using up to 3 cathodes and 4 anodes to drive three electron-beam diodes in parallel. A large-area, reflex triode for <;1MV x-ray production is being tested on SATURN. The first shots with a reflex triode load used convolutes to connect the lines to a tri-axial feed with two cathodes and one anode. A new configuration for SATURN does not use convolutes and drives the reflex triode directly, allowing simplified diagnostic access to both cathodes and the anode and significantly reduces current lost between the MITLs and the triode. Current probes have been mounted in the upper and lower cathodes and in the anode to measure the triode currents. Two vacuum voltmeters were mounted in field-free regions above and below the triode to separately measure the upper and lower cathode voltages. A circuit model of SATURN, from the water transmission lines, to the reflex triode load has been developed. It is able to reproduce the measured triode voltage and current waveforms with good fidelity. The generator current is adjusted by charging only a subset of SATURN'S 36 marx banks. Using 17 banks, the measured upper/lower currents and voltages were 1.2MA/1.2MA and 290kV/340kV at the time of peak x-ray emission. The triode electron beams strike a thin tantalum anode to produce Bremsstrahlung x-rays. Radiation dose, dose-rate and imaging diagnostics monitor the triode performance. These data will be presented. Particle-In-Cell code simulations of electron transport from the MITL plates through the tri-axial feed lines to the triode load region predict efficient transport of the available current. Efforts are underway to increase the current while maintaining fixed voltages.

A high-power, high-impedance ion beam diode has been fielded on the Mercury inductive voltage adder at typical parameters of 4 MV, 360 kA, and 70–90-kA ion current. These results are consistent with theory and with LSP calculations. This beam is focused onto a CF2 target to produce characteristic gammas via the 19F(p,αγ)16O reaction. Diagnostics characterize the ion emission from the anode, the beam distribution on the target, and the gamma yield. Shaping the anode surface improves beam focusing on the target. Our experimental arrangement allows us to resolve the gamma signal in the presence of the diode bremsstrahlung, and to minimize spurious neutron production. The calculated gamma yield exceeds 1011 gammas/sr.

Summary form only given. Calculations to determine the dose scaling from a low- impedance (~1 Omega) reflex triode with voltage and current have been performed. These calculations use the particle-in-cell code LSP to follow the charged particle dynamics in a reflex triode. As electrons interact with the thin anode foil, the LSP code writes out the positions, energies, and direction cosines of the bremsstrahlung photons that are created. These photons are then read into the Monte-Carlo electron-photon transport code, ITS, to predict the radiation field emerging from the foil. In previous work, it was shown that an optimum foil thickness exists which maximizes the dose. When the foil is thicker than the optimum, the 10-100 keV photons are increasingly self absorbed in the converter. For foils thinner than the optimum, the electrons leak out radially and return to ground without making useful radiation. The calculations presented here show that, for foil thicknesses equal to or greater than the optimum and voltages less than 2 MV, the dose per electron from the more complex LSP calculations which accurately predict the electron angles of incidence on the foil can be predicted by simple ITS calculations where normally-incident electrons reflex back and forth through the foil until they are stopped. Therefore, the dose-rate from experiments can be predicted from relatively simple ITS calculations provided the electron and ion current fractions and the voltage can be reliably measured. The LSP simulations show that the electron current increases slowly with voltage and asymptotes at about 1 MA while the ion current increases rapidly as V3/2. Therefore, as the voltage increases the electron current becomes an increasingly smaller fraction of the total current and the ion current dominates. At 1 MV, the LSP simulations show that the ion current fraction is approximately 40% and increases to more than 60% at 2 MV. Furthermore, the absolute X-ray spectrum can be d- duced from the ITS calculations to provide information about the spectral content of the X-rays.

Saturn is a pulsed power accelerator consisting of 36 parallel modules producing 10 MA at 1.7 MV in a 40 ns power pulse[1]. Saturn is built to operate in negative polarity. Two methods of inverting the polarity in vacuum and driving up to 2.5 MA into a triplate MITL with a low impedance load have been built and demonstrated. Both rely on the use of a ballast inductance to invert the polarity[2]. The first method uses a dual post-hole convolute while the second method uses no convolutes.

This paper reviews the motivation for, results from, and analyses of 12-cm-diameter argon gas-puff experiments carried out over the last four years on three generators at 3.2- to 6.5-MA peak currents, all with implosion times >= 200 ns. Using the argon K-shell yield as a metric of implosion quality, high-quality implosions are obtained for an appropriate initial radial mass distribution, i.e., a distribution that is peaked on axis. Higher compressed densities and smaller final radii are observed compared to shell-like initial mass distributions. Theory and data suggest that these distributions mitigate the magnetic Rayleigh-Taylor instability. An energy analysis shows that (1) significant electrical energy is directly coupled to the pinch during the K-shell radiation pulse and (2) conversion of radially-directed kinetic energy into thermal energy is not the dominant mechanism responsible for the pinch K-shell radiation.

A vacuum-voltmeter (VVM) was fielded on the Saturn pulsed power generator during a series of argon gas-puff Z-pinch shots. Time-resolved voltage and separately measured load current are used to determine several dynamic properties as the load implodes, namely, the inductance, L(t), net energy coupled to the load, E(coupled)(t), and the load radius, r(t). The VVM is a two-stage voltage divider, designed to operate at voltages up to 2 MV. The VVM is presently being modified to operate at voltages up to 6 MV for eventual use on the Z generator.

Summary form only given. The self-magnetic-pinch [SMP] diode is a high impedance (~40-Ohm), low R/D (typically 4 mm/8 mm) pinched-beam diode that shows promise for high-power X-radiography. The scaling of this diode to higher voltage (and thus power) is critical to the development of next generation radiographic sources. We report here on SMP experiments on the NRL Mercury generator at voltages from 3.5-6 MV and impedances from 35-50 Ohms. Measurements include diode electrical behavior, time-integrated and time-resolved X-ray dose, and time integrated radiographic spot size. We have studied the effects of several variations in electrode geometry, surface coating, gap; and the level of machine prepulse. Extensive modeling using the Sandia ITS codes is used to help interpret the X-ray dose and spot measurements. As the operating voltage increases, we find that a given diode tends to produce a smaller spot but also suffer reduced impedance lifetime, and optimization involves increasing the cathode diameter and diode gap as the voltage increases. We find good quantitative agreement with ITS predictions over the entire data set, assuming an electron incidence angle of 20 degrees. This gives a dose rate that scales (over the range examined) as IV2.2. Over this range, we observe favorable scaling of optimized diode performance with voltage, with good dose scaling and a slight spot size decrease with voltage. Our best results comprise roughly 200 rads at 1 meter with a ~2 mm diameter spot

A vacuum-voltmeter[ 1 ] (VVM) was fielded on the Saturn pulsed-power generator during a series of 12-MA short-circuit, 6-MA aluminum wire-array z-pinch, and 6 MA argon Plasma Radiation Source (PRS) shots. The VVM was connected to the convolute structure in the vacuum chamber above the load. This arrangement permitted the VVM to directly measure the time-resolved voltage between the ground side of the magnetically insulated transmission line (MITL) anode and the negative high-voltage cathode feed to the load. The time-resolved voltage and the separately-measured load current are used to determine several dynamic properties during the wire or gas-puff load implosion, namely, the inductance, L(t), coupled energy delivered to the load, Ecoupled(t), and the load radius, r(t). We report here the results of these tests for a fixed inductance short-circuit load and a 12-cm diameter, argon gas-puff load. We correlate the time dependent electrical parameters with the radiation output from the imploding loads. In particular, we observe electrical energy being delivered to the pinch during and after the radiation pulse.[2][3]