Publications

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This report documents work done at the Sandia Ion Beam Laboratory to develop a capability to produce 14 Me neutrons at levels sufficient for testing radiation effects on electronic materials and components. The work was primarily enabled by a laboratory directed research and development (LDRD) project. The main elements of the work were to optimize target lifetime, test a new thin- film target design concept to reduce tritium usage, design and construct a new target chamber and beamline optimized for high-flux tests, and conduct tests of effects on electronic devices and components. These tasks were all successfully completed. The improvements in target performance and target chamber design have increased the flux and fluence of 14 MV neutrons available at the test location by several orders of magnitude. The outcome of the project is that a new capability for testing radiation-effects on electronic components from 14 MeV neutrons is now available at Sandia National Laboratories. This capability has already been extensively used for many qualification and component evaluation and development tests.

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Neutron bang times for a series of MagLIF (Magnetic Liner Inertial Fusion) experiments with D₂-filled targets have been measured at the Z facility. The emitted neutrons were detected as current-mode pulses in a multichannel, neutron time-of-flight (nTOF) diagnostic with conventional, scintillator-photomultiplier-tube (PMT) detectors. In these experiments, the detectors were fielded at known, fixed distances L (690-2510 cm) from the target, and on three, non-coplanar (but convergent) lines-of-sight (LOS). The primary goal of this diagnostic was to estimate a fiducial time (bang time) relative to an externally generated time-base for synchronizing all the diagnostics in an experiment. Recorded arrival times (A7) of the pulses were characterized experimentally by three numerical methods: a first-moment estimate (centroid) and two nodal measures — Savitzky-Golay (SG) smoothing and a single point peak estimate of the raw data. These times were corrected for internal detector time delays (transit and impulse-response function) — an adjustment that linked the recorded ATs to the corresponding arrival of uncollided neutrons at each detector. The bang time was then estimated by linearly regressing the arrival times against the associated distances to the source; t_bang (on the system timescale) was taken as the temporal intercept of the regression equation at distance L = 0. This article reports the analysis for a representative shot #2584 for which (a) the recorded ATs — even without detector corrections — agreed by method in each channel to within 1-2 ns; (b) internal corrections were each ~3 — 5 ns; and (c) a 95% uncertainty (confidence) interval for t_bang in this shot was estimated at ±3 ns with 4 degrees of freedom. A secondary goal for this diagnostic was to check that the bang time measurements corresponded to neutrons emitted by the D(d,n)³He reaction in a thermalized DD plasma. According to the theoretical studies by Brysk, such neutrons should be emitted with an isotropic Gaussian distribution of mean kinetic energy $ \overline{E}$ of 2.449 MeV; this energy translates to a mean neutron speed $ \overline{u}$ of 2.160 cm/ns [D. H. Munro, Nuclear Fusion, 56(3) 036001 (2016)]. In the MagLIF series of shots there was no evidence of spatial asymmetry in the time-distance regressions, and it was possible to extract the mean neutron speed from the slope of these fits. In shot 2584 $ \overline{u}$ was estimated at 2.152 cm/ns ± 0.010 cm/ns [95 % confidence, 4 dof] and the mean kinetic energy $ \overline{E}$ (with relativistic corrections) was 2.431 MeV ± 0.022 MeV [95 % confidence, 4 dof] — results supporting the assumption that D-D neutrons were, in fact, measured.

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The apparent ion temperature and neutron-reaction history are important characteristics of a fusion plasma. Extracting these quantities from a measured neutron-time-of-flight signal requires accurate knowledge of the instrument response function (IRF). This work describes a novel method for obtaining the IRF directly for single DT neutron interactions by utilizing n-alpha coincidence. The t(d,α)n nuclear reaction was produced at Sandia National Laboratories' Ion Beam Laboratory using a 300 keV Cockcroft-Walton generator to accelerate a 2.5 μA beam of 175 keV D+ ions into a stationary ErT2 target. The average neutron IRF was calculated by taking a time-corrected average of individual neutron events within an EJ-228 plastic scintillator. The scintillator was coupled to two independent photo-multiplier tubes operated in the current mode: a Hamamatsu 5946 mod-5 and a Photek PMT240. The experimental setup and results will be discussed.

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This work will focus on the characterization of NTOF detectors fielded on ICF experiments conducted at the Z-experimental facility with emphasis on the MagLif and gas puff campaigns. Three experiments have been proposed. The first experiment will characterize the response of the PMT with respect to the amplitude and width of signals produced by single neutron events. A second experiment will characterize the neutron transit time through the scintillator and the third is to characterize the pulse amplitude for a very specific range of neutron induced charged particle interactions within the scintillator. These experiments will cover incident neutron energies relevant to D-D and D-T fusion reactions. These measurements will be taken as a function of detector bias to cover the entire dynamic range of the detector. Throughout the characterization process, the development of a predictive capability is desired. A new post processing code has been proposed that will calculate a neutron time-of-flight spectrum in units of MeVee. This code will couple the experimentally obtained values and the results obtained with the Monte Carlo code MCNP6. The motivation of this code is to correct for geometry issues when transferring the calibration results from a light lab setting to the Zenvironment. This capability will be used to develop a hypothetical design of LOS270 such that more favorable neutron measurements, requiring less correction, can be made in the future.

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