Neutron time-of-flight detectors (nTOF) used at Sandia's Z-Machine

Neutron time-of-flight (nTOF) detectors have been used on Sandia National Laboratories' Z-Machine for inertial confinement fusion and magnetized liner fusion experiments to infer physics parameters including the apparent fuel-ion temperature, neutron yield, the magnetic-radius product (BR), and the liner rho-r. Single-paddle, dual-paddle, and co-axial scintillation nTOF detectors are used in axial lines-of-sight (LOS) and LOS that are 12° from the midplane. Detector fabrication, characterization, and calibration are discussed.

Superlinearity, saturation, and the PMT-Tailoring and calibration methodology for prompt radiation detectors

This work illustrates predominant measureable nonlinearities in photomultiplier tubes (PMTs) and introduces a controllable one called "Superlinearity," signifying both a positive nonlinear response and the ability to extend linear operation by counteracting gain saturation mechanisms - charge depletion, space-charge field limitation, and secondary emission surface effects. Recognizing superlinearity and its effect on the temporal step response leads to a true definition of linearity, free of a small-signal linear assumption. Furthermore, given the prevalent use of glass microchannel-plate (MCP) PMTs in favor of a faster impulse response in spite of a small charge limit, we are motivated to examine their nonlinear amplitude response and deploy tailored gain bias string methods to fully harness the maximum linear gain as is usually done for transmissive metal mesh and reflective metal dynode PMTs. Our characterization methodology applies standard NIST-traceable calibrated laboratory equipment with absolute input-referenced techniques, examining step responses over many orders of magnitude in controlled illumination. By doing so, we quantitatively reveal the superlinearity strength independent of charge depletion, yielding true linear responsivity and effectively doubling the small-signal linear limit; this is very relevant to PMT modeling and charge deconvolution efforts. With further development, the tailoring strategies we introduce could be applied to MCP detectors, extracting all useful capillary charge with a significant improvement in large linear signal quality.

Reproducible, rugged, and inexpensive photocathode x-ray diode

The photoemissive cathode type of x-ray diode (XRD) is popular for measuring time and spectrally resolved output of pulsed power experiments. Vitreous carbon XRDs currently used on the Sandia National Laboratories Z-machine were designed in the early 1980s and use materials and processes no longer available. Additionally cathodes used in the high x-ray flux and dirty vacuum environment of a machine such as Z suffer from response changes requiring recalibration. In searching for a suitable replacement cathode, we discovered very high purity vitreous-carbon planchets are commercially available for use as biological substrates in scanning electron microscope (SEM) work. After simplifying the photocathode mounting to use commercially available components, we constructed a set of 20 XRDs using SEM planchets that were then calibrated at the National Synchrotron Light Source at Brookhaven National Laboratory. We present comparisons of the reproducibility and absolute calibrations between the current vitreous-carbon XRDs and our new design.