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.
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.
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