Dennis R. Slaughter

PLEIADES: A picosecond Compton scattering x-ray source for advanced backlighting and time-resolved material studies

The PLEIADES (Picosecond Laser-Electron Inter-Action for the Dynamical Evaluation of Structures) facility has produced first light at 70 keV. This milestone offers a new opportunity to develop laser-driven, compact, tunable x-ray sources for critical applications such as diagnostics for the National Ignition Facility and time-resolved material studies. The electron beam was focused to 50 μm rms, at 57 MeV, with 260 pC of charge, a relative energy spread of 0.2%, and a normalized emittance of 5 mm mrad horizontally and 13 mm mrad vertically. The scattered 820 nm laser pulse had an energy of 180 mJ and a duration of 54 fs. Initial x rays were captured with a cooled charge-coupled device using a cesium iodide scintillator; the peak photon energy was approximately 78 keV, with a total x-ray flux of 1.3×106 photons/shot, and the observed angular distribution found to agree very well with three-dimensional codes. Simple K-edge radiography of a tantalum foil showed good agreement with the theoretical divergence-...

The "nuclear car wash": a scanner to detect illicit special nuclear material in cargo containers

There is an urgent need to improve the reliability of screening cargo containers for illicit nuclear material that may be hidden there for terrorist purposes. A screening system is described for the detection of fissionable material hidden in maritime cargo containers. The system makes use of a low-intensity neutron beam for producing fission and the detection of the abundant high-energy /spl gamma/ rays emitted in the /spl beta/-decay of short-lived fission products and /spl beta/-delayed neutrons. The abundance of the delayed /spl gamma/ rays is almost an order of magnitude larger than that of the delayed neutrons normally used to detect fission, and they are emitted on about the same time scale as the delayed neutrons, i.e., /spl sim/1 min. The energy and temporal distributions of the delayed /spl gamma/ rays provide a unique signature of fission. Because of their high energy, these delayed /spl gamma/ rays penetrate low-Z cargoes much more readily than the delayed neutrons. Coupled with their higher abundance, the signal from the delayed /spl gamma/ rays escaping from the container is predicted to be as much as six decades more intense than the delayed neutron signal, depending upon the type and thickness of the intervening cargo. The /spl gamma/ rays are detected in a large array of scintillators located along the sides of the container as it is moved through them. Measurements have confirmed the signal strength in somewhat idealized experiments and have also identified one interference when 14.5-MeV neutrons from the D, T reaction are used for the interrogation. The interference can be removed easily by the appropriate choice of the neutron source.

Flyspec: A simple method of unfolding neutron energy spectra measured with NE213 and stilbene spectrometers

Abstract We have developed a simple method for unfolding recoil proton pulse height spectra obtained with NE213 and stilbene detectors to obtain neutron energy spectra. The derivative method used is not new, but its present implementation allows fast data reduction in primitive computers such as the LSI-11. It has been used in a portable field instrument which displays environmental neutron energy spectra, and it allows an array of primitive parallel processors to reduce data from many spectrometers simultaneously as is required in plasma diagnostics. The present implementation is in FORTRAN, but a PASCAL version is nearly completed. We compared our results to those obtained with more sophisticated data-reduction codes and found that our method provides nominally the same unfolded results on a variety of input spectra.

Calibration of a depangher long counter from 2 keV to 19 MeV

Abstract We have measured the sensitivity of a dePangher precision long counter (PLC) relative to 7 Li(p, n), T(p, n), and T(d, n) differential cross sections over the neutron energy range 10 keV–19 MeV. We also measured absolute sensitivity at 2 keV using a scandium filtered beam at a reactor. Results obtained with errors in the range 5–30% are consistent with the assumption of a nearly constant sensitivity over the range 2 keV–6 MeV, but exhibit a reduction above 12 MeV.

A highly sensitive silver-activation detector for pulsed neutron sources

Abstract A silver-activation detector has been built and calibrated to measure the neutron yield produced by pulsed D(d,n) and T(d,n) sources. The detector has three principle virtues. Its high sensitivity (registering as little as 20 n/cm 2 ) results from 1.7 kg of silver incorporated into a plastic scintillator. Its modular design allows several efficiency vs energy responses to be obtained, including one configuration in which the efficiency is the same at 2.5 and 14 MeV. Also, the dynamic range spans five to seven decades of useful neutron intensity. Construction details and measured efficiencies are given.

Characterization of a bright, tunable, ultrafast Compton scattering X-ray source

The Compton scattering of a terawatt-class, femtosecond laser pulse by a high-brightness, relativistic electron beam has been demonstrated as a viable approach toward compact, tunable sources of bright, femtosecond, hard X-ray flashes. The main focus of this article is a detailed description of such a novel X-ray source, namely the PLEIADES (Picosecond Laser–Electron Inter-Action for the Dynamical Evaluation of Structures) facility at Lawrence Livermore National Laboratory. PLEIADES has produced first light at 70 keV, thus enabling critical applications, such as advanced backlighting for the National Ignition Facility and in situ time-resolved studies of high- Z materials. To date, the electron beam has been focused down to σ x = σ y = 27 μm rms, at 57 MeV, with 266 pC of charge, a relative energy spread of 0.2%, a normalized horizontal emittance of 3.5 mm·mrad, a normalized vertical emittance of 11 mm·mrad, and a duration of 3 ps rms. The compressed laser pulse energy at focus is 480 mJ, the pulse duration 54 fs Intensity Full Width at Half-Maximum (IFWHM), and the 1/ e 2 radius 36 μm. Initial X rays produced by head-on collisions between the laser and electron beams at a repetition rate of 10 Hz were captured with a cooled CCD using a CsI scintillator; the peak photon energy was approximately 78 keV, and the observed angular distribution was found to agree very well with three-dimensional codes. The current X-ray dose is 3 × 10 6 photons per pulse, and the inferred peak brightness exceeds 10 15 photons/(mm 2 × mrad 2 × s × 0.1% bandwidth). Spectral measurements using calibrated foils of variable thickness are consistent with theory. Measurements of the X-ray dose as a function of the delay between the laser and electron beams show a 24-ps full width at half maximum (FWHM) window, as predicted by theory, in contrast with a measured timing jitter of 1.2 ps, which contributes to the stability of the source. In addition, K -edge radiographs of a Ta foil obtained at different electron beam energies clearly demonstrate the γ 2 -tunability of the source and show very good agreement with the theoretical divergence-angle dependence of the X-ray spectrum. Finally, electron bunch shortening experiments using velocity compression have also been performed and durations as short as 300 fs rms have been observed using coherent transition radiation; the corresponding inferred peak X-ray flux approaches 10 19 photons/s.

Fusion reactivities for several beam and target ion distributions

Fusion reactivities 〈σv〉 in the mean ion energy range 0.1–200 keV have been calculated for the reactions: D(d,n), T(d,n), D(d, p), 3He(d, p) and T(t,2n); and for several different ion energy and angular distributions considered plausible in hot, magnetically confined plasmas. Calculations were carried out to a numerical accuracy of about 5% and the resulting variation of reactivity with mean ion energy is found to be well represented by a simple polynomial function. The polynomial functions are plotted as a function of mean ion energy and their coefficients given. Results of the calculations show that fusion reactivity is very sensitive to the functional form of the speed distribution at mean energies below a few keV, but reactivity is roughly independent of the form of the speed distribution (within a factor of 2) above 10 keV for most of the reactions investigated. Similarly, the importance of the ion angular distribution is large at energies below a few keV but small at high ion energies.

Plasma Neutron Diagnostic Techniques with Good Spatial and Energy Resolution

A neutron-detection system has been assembled to provide both spatial and energy information from the neutrons produced in advanced fusion experiments. Techniques described are applicable to experiments where the neutron pulse is on the order of one-second duration. The system gives spatial resolution of about 1 cm at distances of 1 to 2 m and energy resolution of 0.6 MeV at 14 MeV. In all cases, pulseshape discrimination is used to distinguish neutrons from gamma rays.

Effect of ion speed distribution on the spectral shape of the 2.5‐MeV neutron line produced by DD fusion

Ion mean energy may be determined from Doppler broadening of the 2.5‐MeV neutron line in Maxwellian deuterium plasma. Since many plasmas are non‐Maxwellian a Monte‐Carlo simulation has been used to predict the spectral shape produced by a variety of simple speed distributions with mean energies in the range 1–100 keV. In the case of isotropic ions considered here, the results are Gaussian to high order and their width varies as the square root of the mean ion energy. However, the linewidth varies substantially among different ion speed distributions with the same mean energy. The similarity in shape of these spectra prevent the determination of the speed distribution type from detailed measurements of the neutron spectrum, unlike the situation where co‐counter beams produce anisotropic ion distributions.

Nuclear techniques for determining the spatial and energy distribution of fast‐confined alpha particles in ignited D–T plasma

There are several nuclear reactions between fast‐confined α particles and low‐z ions in ignited D–T plasma. Some of them produce unique and highly penetrating neutron or γ radiation. A study has begun to evaluate the feasibility of using these reactions to determine the spatial profile of fast‐confined reaction product α particles, and to determine a crude envelope of their speed distribution as they thermalize. Particular attention is paid to the means of detecting these radiations, measuring their spectra, and most important of all, reducing and discriminating against the very much larger background of interfering radiation.