High-speed microchannel plate (MCP)-based imagers are critical detectors for x-ray diagnostics employed on
Z-experiments at Sandia National Laboratories (SNL) to measure time-resolved x-ray spectra and to image dynamic
hohlraums. A multiframe design using eight half strips in one imager permits recordings of radiation events in discrete
temporal snapshots to yield a time-evolved movie. We present data using various facilities to characterize the
performance of this design. These characterization studies include DC and pulsed-voltage biased measurements in both
saturated and linear operational regimes using an intense, short-pulsed UV laser. Electrical probe measurements taken to
characterize the shape of the HV pulse propagating across the strips help to corroborate the spatial gain dependence
Wired array studies are being conducted at the SNL Z accelerator to maximize the x-ray generation for inertial
confinement fusion targets and high energy density physics experiments. An integral component of these studies is the
characterization of the time-resolved spectral content of the x-rays. Due to potential spatial anisotropy in the emitted
radiation, it is also critical to diagnose the time-evolved spectral content in a space-resolved manner. To accomplish
these two measurement goals, we developed an x-ray spectrometer using a set of high-speed detectors (silicon PIN
diodes) with a collimated field-of-view that converged on a 1-cm-diameter spot at the pinch axis. Spectral
discrimination is achieved by placing high Z absorbers in front of these detectors. We built two spectrometers to permit
simultaneous different angular views of the emitted radiation. Spectral data have been acquired from recent Z shots for
the radial and axial (polar) views. UNSPEC1 has been adapted to analyze and unfold the measured data to reconstruct
the x-ray spectrum. The unfold operator code, UFO2, is being adapted for a more comprehensive spectral unfolding
treatment.
Cobalt 60 is a particularly attractive candidate for marking explosives because not only does it have a useful half-life of 5.3 years, but it produces tow highly penetrating gamma rays, at 1.173 and 1.332 MeV, in coincidence. The fact that two strong, monoenergetic gamma rays are produced simultaneously makes it possible to detect a much smaller amount of this element in the presence of terrestrial background radiation than is possible for other radioactive elements that produce only one gamma ray. We have built a test-bed system comprising six large plastic scintillators and their associated hardware to investigate the performance of this technique. Experiments have shown that reliable detection can be done with a sufficiently small amount of cobalt 60 so as not to raise safety concerns.
Silicon positive-intrinsic-negative (p-i-n) diodes have been used in plasma diagnostics by the Los Alamos and Lawrence Livermore National Laboratories (LANL and LLNL) since the early seventies. Since the response bandwidth of these detectors is relatively poor (typically, approximately 5 ns FWHM for 1 cm2 sensitive area and 250 micrometers depletion depth), they are too slow for high-speed applications. GaAs photoconductive detectors (PCD) have been developed since the early eighties at LANL and later at LLNL, and can be tailored by judicious neutron damage to provide the required high-speed bandwidth. Unfortunately, for surface absorbed or non-penetrating radiation, we have discovered that the PCD sensitivity is not flat across its gap, where the incident radiation is perpendicular to the bias electric field. This response non-uniformity can lead to erroneous measurements in cases where the radiation is spatially varying. To overcome this problem, we reoriented the GaAs chip to allow the radiation to be incident through the electrode and parallel to the bias electric field. Then to increase bandwidth, we doped the GaAs crystal with chromium to create trapping sites and provide large resistivity (approximately 109 (Omega) cm), thus creating a semi-insulator detector (SID). We present and discuss the merits of the SID versus PCD and p-i-n diode by showing pulse response data of each detector characterized with 16 MeV endpoint gamma and electron radiation created by the EG&G/EM linear accelerator (Linac) and 5 to 16.5 MeV proton radiation produced by the LLNL Tandem Van de Graaff (TVDG). Application of the SID in Compton electron spectrometry also is discussed.
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