B. Rusnak

Neutron production from feedback controlled thermal cycling of a pyroelectric crystal

The LLNL Crystal Driven Neutron Source is operational and has produced record ion currents of approximately 10 nA and neutron output of 1.9(+/-0.3)x10(5) per thermal cycle using a crystal heating rate of 0.2 degrees C/s from 10 to 110 degrees C. A 3 cm diameter by 1 cm thick LiTaO(3) crystal with a socket secured field emitter tip is thermally cycled with feedback control for ionization and acceleration of deuterons onto a deuterated target to produce D-D fusion neutrons. The entire crystal and temperature system is mounted on a bellows which allows movement of the crystal along the beam axis and is completely contained on a single small vacuum flange. The modular crystal assembly permitted experimental flexibility. Operationally, flashover breakdowns along the side of the crystal and poor emitter tip characteristics can limit the neutron source. The experimental neutron results extend earlier published work by increasing the ion current and pulse length significantly to achieve a factor-of-two higher neutron output per thermal cycle. These findings are reviewed along with details of the instrument.

Recent results in the development of fast neutron imaging techniques

We are proceeding with the development of fast (≈12 MeV) neutron imaging techniques for use in NDE applications. Our goal is to develop a neutron imaging system capable of detecting sub-mm-scale cracks, cubic-mm-scale voids and other structural defects in heavily-shielded low-Z materials within thick sealed objects. The final system will be relatively compact (suitable for use in a small laboratory) and capable of acquiring both radiographic and full tomographic image sets. The design of a prototype imaging detector will be briefly reviewed and results from several recent imaging experiments will be presented. The concurrent development of an intense, accelerator-driven neutron source suitable for use with the final production imaging system will also be briefly discussed.

Design and initial results from a kilojoule level dense plasma focus with hollow anode and cylindrically symmetric gas puff

We have designed and built a Dense Plasma Focus (DPF) Z-pinch device using a kJ-level capacitor bank and a hollow anode, and fueled by a cylindrically symmetric gas puff. Using this device, we have measured peak deuteron beam energies of up to 400 keV at 0.8 kJ capacitor bank energy and pinch lengths of ∼6 mm, indicating accelerating fields greater than 50 MV/m. Neutron yields of on the order of 10(7) per shot were measured during deuterium operation. The cylindrical gas puff system permitted simultaneous operation of DPF with a radiofrequency quadrupole accelerator for beam-into-plasma experiments. This paper describes the machine design, the diagnostic systems, and our first results.

Intense pulsed neutron emission from a compact pyroelectric driven accelerator

Intense pulsed D–D neutron emission with rates of >1010 n/s during the pulse, pulse widths of approximately hundreds of nanoseconds and neutron yields of greater than 10 000 per pulse, are demonstrated in a compact pyroelectric accelerator. The accelerator consists of a small pyroelectric LiTaO3 crystal that provides the accelerating voltage and an independent compact spark plasma ion source. The crystal voltage versus temperature is characterized and compares well with theory. Results show neutron output per pulse that scales with voltage as V∼1.7. These neutron yields match a simple model of the system at low voltages but are lower than predicted at higher voltages due to charge losses not accounted for in the model. Interpretation of the data against modeling provides understanding of the accelerator and in general pyroelectric LiTaO3 crystals operated as charge limited negative high voltage targets. The findings overall serve as the proof of principle and basis for pyroelectric neutron generators that...

An accelerator system for neutron radiography

The field of x-ray radiography is well established for performing non-destructive evaluation of a vast array of components, assemblies, and objects. While x-rays excel in many radiography applications, their effectiveness diminishes rapidly if the objects of interest are surrounded by thick, high-density materials that strongly attenuate photons. Due to the differences in interaction mechanisms, neutron radiography is highly effective in imaging details inside such objects. To obtain a high intensity neutron source suitable for neutron imaging, a 9-MeV linear accelerator is being evaluated for delivering a deuteron beam into a high-pressure deuterium gas cell. Since a windowless aperture is needed to transport the beam into the gas cell, a low emittance is needed to minimize losses along the high-energy beam transport (HEBT) and the end station. A description of the HEBT, the transport optics into the gas cell, and the requirements for the linac will be presented.

Crystal Driven Neutron Source: A New Paradigm for Miniature Neutron Sources

Neutron interrogation techniques have specific advantages for detection of hidden, shielded, or buried threats over other detection modalities in that neutrons readily penetrate most materials providing backscattered gammas indicative of the elemental composition of the potential threat. Such techniques have broad application to military and homeland security needs. Present neutron sources and interrogation systems are expensive and relatively bulky, thereby making widespread use of this technique impractical. Development of a compact, high intensity crystal driven neutron source is described. The crystal driven neutron source approach has been previously demonstrated using pyroelectric crystals that generate extremely high voltages when thermal cycled [1–4]. Placement of a sharpened needle on the positively polarized surface of the pyroelectric crystal results in sufficient field intensification to field ionize background deuterium molecules in a test chamber, and subsequently accelerate the ions to ener...

Livermore Accelerator Source for Radionuclide Science (LASRS)

The Livermore Accelerator Source for Radionuclide Science (LASRS) will generate intense photon and neutron beams to address important gaps in the study of radionuclide science that directly impact Stockpile Stewardship, Nuclear Forensics, and Nuclear Material Detection. The co-location of MeV-scale neutral and photon sources with radiochemical analytics provides a unique facility to meet current and future challenges in nuclear security and nuclear science.

Fully kinetic modeling and ion probe beam experiemnts in a dense plasma focus Z-pinch

The Z-pinch phase of a dense plasma focus (DPF) emits multiple-MeV ions from a ~cm length interaction. The mechanisms through which these physically simple devices generate such high-energy beams in a relatively short distance are not fully understood. We are exploring the mechanisms behind these large accelerating gradients using fully kinetic simulations of a DPF Z-pinch and ion probe beam measurements. Our particle-in-cell simulations have successfully predicted ion beams and neutron yield from kJ-scale DPFs1, which past fluid simulations have not reproduced. To access the regime of MJ-scale devices within computational resources, we have developed a handoff simulation starting from a fluid calculation near the end of rundown and continuing fully kinetic through the pinch. To probe the accelerating fields in our tabletop experiment, we inject a 4 MeV deuteron beam along the z-axis. For the first time, we have directly measured the gradients in the DPF and the acceleration of an injected ion beam. We observe > 50 MV/m acceleration gradients during 800 J operation using a fast capacitive driver2. In addition, we have now experimentally measured and observed in simulations for the first time, electric field oscillations near the lower hybrid frequency. This is suggestive that the lower hybrid drift instability, long speculated to be the cause of the anomalous plasma resistivity that produces large DPF gradients, is playing an important role. Direct comparisons between the experiment and simulations enhance our understanding of these plasmas and provide predictive design capability for accelerator and neutron source applications.

Characterization and applications of a bright, tunable, MeV class Compton scattering γ-ray source

We report detailed spectral and spatial characterization of a 0.1-MeV-0.8 MeV tunable ultra-bright laser-based Compton scattering source. Nuclear Resonance Fluorescence experiments with the source are also presented.

Realizing the Opportunities of Neutron Cross Section Measurements at RIA

The Rare Isotope Accelerator will produce many isotopes at never before seen rates. This will allow for the first‐time measurements on isotopes very far from stability and new measurement opportunities for unstable nuclei near stability. In fact, the production rates are such that it should be possible to collect 10 micrograms of many isotopes with a half‐life of 1 day or more. This ability to make targets of short‐lived nuclei enables the possibility of making neutron cross‐section measurements important to the astrophysics and the stockpile stewardship communities. But to fully realize this opportunity, the appropriate infrastructure must be included at the RIA facility. This includes isotope harvesting capabilities, radiochemical areas for processing collected material, and an intense, “mono‐energetic,” tunable neutron source. As such, we have been developing a design for neutron source facility to be included at the RIA site. This facility would produce neutrons via intense beams of deuterons and prot...