Electronic grade diamond such as single crystal (SC) chemical vapor deposition (CVD) diamond has excellent optoelectronic properties, which enables it to be used as a detector material for high energy detector applications. However, SC CVD diamond suffers from loss of signal contrast and performance over a period of continuous use due to polarization of the detector. When a SC CVD diamond is continuously exposed to radiation, trapped charge carriers build up at defect sites, resulting in the creation of a secondary electric field opposite to the bias field. The emergence of the secondary electric field causes charge collection efficiency loss and reduces the overall performance of the detector. In this work, the effect of polarization on the direct current (DC) neutron response of a 500 μm SC CVD diamond detector was investigated by irradiating the detector with 14.1 MeV neutrons produced from a deuterium-tritium neutron generator. Depolarization techniques were employed to de-trap charge carriers and decrease the strength of the secondary electric field. This was primarily through reverse biasing the applied field yet included scenarios with and without short-lived neutron irradiation. The results indicated that both without and with neutron irradiation techniques improved the stability of the detector response. The latter showed a superior stability at higher bias fields.
An avalanche effect yielding inherent gain can be exploited in thin, single-crystal chemical vapor deposition (scCVD) diamond. It occurs when a high enough bias is applied across the diamond thickness while avoiding breakdown. This charge multiplication effect was studied previously with alpha particles and heavy ions either by using the transient current technique or by measuring the energy spectrum. The measurements we obtained to evaluate the charge multiplication performance of a 10 μm thick scCVD diamond detector used a novel approach—we employed an electrometer to characterize the response of the detector by performing directly coupled current measurements (time-averaged charge, at 1 Hz sampling) when exposed to 14.1 MeV neutrons from deuterium-tritium fusion. We measured both the dark and irradiated currents from the detector over a range of applied displacement field values from 2 to 75 V/μm. A histogram method with central mean and standard deviation width was used to determine the current over each measurement duration typically from 100 to 300 seconds. The dark-subtracted irradiated current (i.e., contrast) was used to evaluate the gain of the detector at each applied displacement field. The contrast at an applied displacement field between 15 and 20 V/μm was higher than the expected linear increase in contrast proportional to the increased applied bias, indicating the possible presence of avalanche events in the diamond. The detector response also indicated possible polarization and charge depletion effects. These results provide an opportunity to further explore the use of thin scCVD diamond as a fast neutron current mode detector with inherent gain.
During the dynamic compression of a subcritical object that is simultaneously receiving a neutron pulse, its fission 𝛾-ray signal can be measured and the die-off in its time-response is directly correlated with peak reactivity and neutron multiplication within. Hence, the signal measured by a time-of-flight (TOF) gamma detector array operating in currentmode is a convolution of the incident neutron pulse, detector time response, and fission signal from the object. Accurate determination of the true fission 𝛾-ray emission rate from the object requires a detector that is both highly sensitive (to preserve statistics) and fast (to minimize distortion of the signal shape from the object). In collaboration with scientists at Mission Support and Test Services (MSTS) and the Nevada National Security Site (NNSS), a TOF 𝛾-ray detection system was designed at Los Alamos National Laboratory (LANL) to meet these experimental objectives. This system consists of a 3-m-diameter array of ∼ 150 hexagonal detector pixels, each operating in current mode. Each pixel consists of a large-volume, fast-plastic scintillator coupled to a 5-in photomultiplier tube via a plastic light guide that uses total internal reflection for optical transport to the photocathode. Development details of this pixel design using statistical and time response metrics, laboratory measurements of full pixels and photomultiplier tubes, and high-fidelity GEANT4 simulations, are given. In closing, fielding considerations and expected performance capabilities for the full detector array are also described.
Diamond photoconductive detectors have been shown to detect fast neutrons with high gamma insensitivity. Depending on the application and the incident neutron energy, there are many possible choices when considering how diamond elements may be sized, arranged, and instrumented. As part of our design effort, we are using Geant4 and MCNP6.2 to simulate the effects of fast neutrons impinging on diamond detectors ranging in thickness from a few microns to a few hundred microns that are 4 mm on a side with intervening materials and other physical parameters. The models may be used to compare diamond detector measurements with incident neutrons ranging from ~1 to 14.1 MeV to better understand the nuclear and atomic physics effects contributing to an electronic signal. We are investigating pulse height, signal-to-noise ratio, and timing characteristics of prototype single-crystal chemical vapor deposition diamond detectors.
A pulsed neutron source is used to interrogate a target, producing secondary gammas and neutrons. In order to make
good use of the relatively small number of gamma rays that emerge from the system after the neutron flash, our detector
system must be both efficient in converting gamma rays to a detectable electronic signal and reasonably large in volume.
Isotropic gamma rays are emitted from the target. These signals are converted to light within a large chamber of a liquid
scintillator. To provide adequate time-of-flight separation between the gamma and neutron signals, the liquid scintillator
is placed meters away from the target under interrogation. An acrylic PMMA (polymethyl methacrylate) light guide
directs the emission light from the chamber into a 5-inch-diameter photomultiplier tube. However, this PMMA light
guide produces a time delay for much of the light.
Illumination design programs count rays traced from the source to a receiver. By including the index of refraction of the
different materials that the rays pass through, the optical power at the receiver is calculated. An illumination design
program can be used to optimize the optical material geometries to maximize the ray count and/or the receiver power. A
macro was written to collect the optical path lengths of the rays and import them into a spreadsheet, where histograms of
the time histories of the rays are plotted. This method allows optimization on the time response of different optical
detector systems. One liquid scintillator chamber has been filled with a grid of reflective plates to improve its time
response. Cylindrical detector geometries are more efficient.
We present the desired performance specifications for an advanced optical imager, which borrows practical concepts in
high-speed microchannel plate (MCP) intensified x-ray stripline imagers and time-dilation techniques. With a four-fold
speed improvement in state-of-the-art high-voltage impulse drivers, and novel atomic-layer deposition MCPs, we tender
a design capable of 5 ps optical gating without the use of magnetic field confinement of the photoelectrons. We analyze
the electron dispersion effects in the MCP and their implications for gating pulses shorter than the MCP transit time. We
present a wideband design printed-circuit version of the Series Transmission Line Transformer (STLT) that makes use of
50-ohm coaxial 1.0 mm (110 GHz) and 1.85 mm (65 GHz) hermetically sealed vacuum feedthroughs and low-dispersion
Teflon/Kapton circuit materials without the use of any vias. The STLT matches impedance at all interfaces with a 16:1
impedance (4:1 voltage) reduction, and delivers a dispersion-limited sharp impulse to the MCP strip. A comparison of
microstrip design calculations is given, showing variances between method of moments, empirical codes, and finite
element methods for broad, low-impedance traces. Prototype performance measurements are forthcoming.
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