Gregory Joseph Schmid

Absolute measurements of neutron yields from DD and DT implosions at the OMEGA laser facility using CR-39 track detectors

The response of CR-39 track detectors to neutrons has been characterized and used to measure neutron yields from implosions of DD- and DT-filled targets at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], and the scaling of neutron fluence with R (the target-to-detector distance) has been used to characterize the fluence of backscattered neutrons in the target chamber. A Monte-Carlo code was developed to predict the CR-39 efficiency for detecting DD neutrons, and it agrees well with the measurements. Neutron detection efficiencies of (1.1±0.2)×10−4 and (6.0±0.7)×10−5 for the DD and DT cases, respectively, were determined for standard CR-39 etch conditions. In OMEGA experiments with both DD and DT targets, the neutron fluence was observed to decrease as R−2 up to about 45 cm; at larger distances, a significant backscattered neutron component was seen. The measured backscattered component appears to be spatially uniform, and agrees with predictions of a neutron-transport code. A...

Prototypes of National Ignition Facility neutron time-of-flight detectors tested on OMEGA

Prototypes of several National Ignition Facility (NIF) neutron time-of-flight (nTOF) detectors have been built and tested on OMEGA. One group uses a plastic scintillator coupled with a microchannel plate (MCP) photomultiplier tube (PMT), either a single-stage (gain up to 103) MCP PMT and a two-stage (gain up to 106) MCP PMT. Two ultrafast scintillators—BC-422 and BC-422Q—were used. Another nTOF prototype is based on a synthetic diamond wafer produced by the chemical vapor deposition. The nTOF detectors were tested on DD (2.45 MeV) and DT (14.1 MeV) neutron-producing implosions on OMEGA. Based on the results of these tests, a set of nTOF detectors is proposed for use on the NIF to measure ion temperature and DD and DT neutron yields from 109 to 1019.

CVD diamond as a high bandwidth neutron detector for inertial confinement fusion diagnostics

We characterize the response of chemical vapor deposition (CVD) diamond detectors to inertial confinement fusion (ICF) neutrons generated at the OMEGA laser fusion facility in Rochester, NY. Four detectors are tested: three utilizing “optical grade” CVD diamond, and one utilizing “electronic grade” CVD diamond. Using a 50 Ω measurement system, we find that the optical grade wafers, biased to 1000 V/mm, have an average sensitivity of 0.24 μV ns/n for 2.5 MeV (DD fusion) neutrons and 0.62 μV ns/n for 14.0 MeV (DT fusion) neutrons. At the same E field, the electronic grade wafer has a sensitivity of 0.56 and 1.43 μV ns/n for 2.5 and 14 MeV neutrons, respectively. Linear dynamic range for the optical grade material is shown to be at least 105. Average full width at half maximum response times, as measured with pulsed laser and 3 GHz scope, are 376 and 880 ps for optical and electronic grades, respectively. These characteristics make CVD diamond suitable for ICF applications such as neutron time-of-flight spec...

Development of a gated scintillation fiber neutron detector for areal density measurements of inertial confinement fusion capsules

A detector for fuel areal density measurements in inertial confinement fusion capsules has been designed. Observation of neutrons scattered in an imploded deuterium capsule (0.27–0.6 MeV) is a promising method for areal density measurements in the National Ignition Facility DD surrogate capsules. In order to detect scattered neutrons, we need to (1) suppress interference due to the strong direct neutron burst and (2) suppress the background produced by neutrons scattering on nontarget material (mainly from the target chamber). In our detector system, we suppress direct neutrons by gating the detector. We suppress the nontarget background neutrons by placing the detector outside the target chamber and limiting the view of the detector with collimators. In addition, we are developing a lithium-glass scintillation-fiber detector (LG-SCIFI) to detect the scattered neutrons. The LG-SCIFI will work as a multichannel scintillator array. The scintillation signal will be amplified by a microchannel plate image int...

Multifluid interpenetration mixing in directly driven inertial confinement fusion capsule implosions

Mixing between the shell and fuel in directly driven single shell capsule implosions causes changes in yield, burn history, burn temperature, areal density, x-ray image shape, and the presence of atomic mix. Most observations are consistent with a mix model using the same values of its single free parameter as with indirectly driven single shell and double shell capsules. Greater mixing at lower gas pressure fills reduces capsule yield. Time dependent mixing growth causes truncation of the burn history. This emphasizes early yield from the center of the capsule, raising the observed burn temperature. Mixed fuel areal densities are lower because fuel moves through the shell and the observation weights earlier times when areal density is lower. Shell x-ray emission mixing into the fuel fills in the limb brightened image to produce a central peak. Implosions of 3He filled capsules with a layer of deuterated plastic show substantial atomic mix.

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.

Development of high-energy neutron imaging for use in NDE applications

We are currently developing a high-energy (10–15 MeV) neutron imaging system for use in NDE applications. Our goal is to develop an imaging system capable of detecting cubic-mm-scale voids or other structural defects in heavily-shielded low-Z materials within thick sealed objects. The system will be relatively compact (suitable for use in a small laboratory) and capable of acquiring tomographic image data sets. The design of a prototype imaging detector and multi-axis staging system will be discussed and selected results from recent imaging experiments will be presented. The development of an intense, accelerator-driven neutron source suitable for use with the imaging system will also be discussed.

Calibration of National Ignition Facility neutron detectors in the energy range E<14 MeV

We examine various options for calibration of the National Ignition Facility neutron detectors in the energy range E<14 MeV. These options include: downscatter of D–T fusion neutrons using plastic targets; nuclear reactions at a Tandem Van de Graaf accelerator; and “white” neutrons from a pulsed spallation source. As an example of the pulsed spallation option, we present calibration data that was recently acquired with a single crystal chemical vapor deposition diamond detector at the Weapons Neutron Research Facility at Los Alamos National Laboratory.

CVD diamond detectors for current mode neutron time-of-flight spectroscopy at OMEGA/NIF

We have performed pulsed neutron and pulsed laser tests of a CVD diamond detector manufactured from DIAFILM, a commercial grade of CVD diamond. The laser tests were performed at the short pulse UV laser at Bechtel Nevada in Livermore, CA. The pulsed neutrons were provided by DT capsule implosions at the OMEGA laser fusion facility in Rochester, NY. From these tests, we have determined the impulse response to be 250 ps fwhm for an applied E-field of 500 V/mm. Additionally, we have determined the sensitivity to be 2.4 mA/W at 500 V/mm and 4.0 mA/W at 100 V/mm. These values are approximately 2 to 5x times higher than those reported for natural Type IIa diamond at similar E-field and thickness (1mm). These characteristics allow us to conceive of a neutron time-of-flight current mode spectrometer based on CVD diamond. Such an instrument would sit inside the laser fusion target chamber close to target chamber center (TCC), and would record neutron spectra fast enough such that backscattered neutrons and (gamma) rays from the target chamber wall would not be a concern. The acquired neutron spectra could then be used to extract DD fuel areal density from the downscattered secondary to secondary ratio.

Development of lower-energy neutron spectroscopy for areal density measurement in implosion experiment at NIF and OMEGA

Areal density ((sigma) R) is a fundamental parameter that characterizes the performance of an ICF implosion. For high areal densities ((sigma) R>0.1 g/cm2), which will be realized in implosion experiments at NIF and LMJ, the target areal density exceeds the stopping range of charged particles and measurements with charged particle spectroscopy will be difficult. In this region, an areal density measurement method using down shifted neutron counting is a promising alternative. The probability of neutron scattering in the imploded plasma is proportional to the areal density of the plasma. The spectrum of neutrons scattered by the specific target nucleus has a characteristic low energy cut off. This enables separate, simultaneous measurements of fuel and pusher (sigma) Rs. To apply this concept in implosion experiments, the detector should have extremely large dynamic range. Sufficient signal output for low energy neutrons is also required. A lithium-glass scintillation-fiber plate (LG-SCIFI) is a promising candidate for this application. In this paper we propose a novel technique based on down shifted neutron measurements with a lithium-glass sctintillation-fiber plate. The details of instrumentation and background estimation with Monte Carlo calculation are reported.