T. Clancy, J. Caggiano, J. McNaney, M. Eckart, M. Moran, V. Glebov, J. Knauer, R. Hatarik, S. Friedrich, R. Zacharias, A. Carpenter, M. Shoup, T. Buczek, M. Yeoman, Z. Zeid, N. Zaitseva, B. Talison, J. Worden, B. Rice, T. Duffy, A. Pruyne, K. Marshall
This paper describes the engineering architecture and function of the neutron Time-of-Flight (nToF) diagnostic suite
installed on the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL). These
instruments provide key measures of neutron yield, ion temperature, drift velocity, neutron bang-time, and neutron
downscatter ratio.
Currently, there are five nToFs on three collimated lines-of-site (LOS) from 18m to 27m from Target Chamber Center,
and three positioned 4.5m from TCC, within the NIF Target Chamber but outside the vacuum and confinement boundary
by use of re-entrant wells on three other LOS.
NIF nToFs measure the time history and equivalent energy spectrum of reaction generated neutrons from a NIF
experiment. Neutrons are transduced to electrical signals, which are then carried either by coaxial or Mach-Zehnder
transmission systems that feed divider assemblies and fiducially timed digitizing oscilloscopes outside the NIF Target
Bay (TB) radiation shield wall.
One method of transduction employs a two-stage process wherein a neutron is converted to scintillation photons in
hydrogen doped plastic (20x40mm) or bibenzyl crystals (280x1050mm), which are subsequently converted to an
electrical signal via a photomultiplier tube or a photo-diode.
An alternative approach uses a single-stage conversion of neutrons-to-electrons by use of a thin (0.25 to 2 mm) Chemical
Vapor Deposition Diamond (CVDD) disc (2 to 24mm radius) under high voltage bias. In comparison to the scintillator
method, CVDDs have fast rise and decay times (<ns), have very low residual tails, are insensitive to shot gammas, and
are less sensitive to the neutron signal of interest.
Stephan Friedrich, Stephane Terracol, Toshiyuki Miyazaki, Owen Drury, Zaheer Ali, Mark Cunningham, Thomas Niedermayr, Troy Barbee, Jan Batteux, Simon Labov
Superconducting Gamma-ray microcalorimeters operated at temperatures around ~0.1 K offer an order of magnitude improvement in energy resolution over conventional high-purity Germanium spectrometers. The calorimeters consist of a ~1 mm3 superconducting or insulating absorber and a sensitive thermistor, which are weakly coupled to a cold bath. Gamma-ray capture increases the absorber temperature in proportion to the Gamma-ray energy, this is measured by the thermistor, and both subsequently cool back down to the base temperature through the weak link. We are developing ultra-high-resolution Gamma-ray spectrometers based on Sn absorbers and superconducting Mo/Cu multilayer thermistors for nuclear non-proliferation applications. They have achieved an energy resolution between 60 and 90 eV for Gamma-rays up to 100 keV. We also build two-stage adiabatic demagnetization refrigerators for user-friendly detector operation at 0.1 K. We present recent results on the performance of single pixel Gamma-ray spectrometers, and discuss the design of a large detector array for increased sensitivity.
Cryogenic high-resolution X-ray spectrometers are typically operated with thin IR blocking windows to reduce radiative heating of the detector while allowing good x-ray transmission. We have estimated the temperature profile of these IR blocking windows under typical operating conditions. We show that the temperature in the center of the window is raised due to radiation from the higher temperature stages. This can increase the infrared photon flux onto the detector, thereby increasing the IR noise and decreasing the cryostat hold time. The increased window temperature constrains the maximum window size and the number of windows required. We discuss the consequences for IR blocking window design.
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