<title>CVD diamond detectors for current mode neutron time-of-flight spectroscopy at OMEGA/NIF</title>

Abstract

We have performed pulsed neutron and pulsed laser tests of a CVD diamond detector manufactured from DIAFILM, a commercial grade of CVD diamond. The laser tests were performed at the short pulse UV laser at Bechtel Nevada in Livermore, CA. The pulsed neutrons were provided by DT capsule implosions at the OMEGA laser fusion facility in Rochester, NY. From these tests, we have determined the impulse response to be 250 ps fwhm for an applied E-field of 500 V/mm. Additionally, we have determined the sensitivity to be 2.4 mA/W at 500 V/mm and 4.0 mA/W at 1000 V/mm. These values are approximately 2 to 5x times higher than those reported for natural Type IIa diamond at similar E-field and thickness (1mm). These characteristics allow us to conceive of a neutron time-of-flight current mode spectrometer based on CVD diamond. Such an instrument would sit inside the laser fusion target chamber close to target chamber center (TCC), and would record neutron spectra fast enough such that backscattered neutrons and γ rays from the target chamber wall would not be a concern. The acquired neutron spectra could then be used to extract DD fuel areal density from the downscattered secondary to secondary ratio.

Response Of A CVD Diamond Detector To ''Typical'' Deuterium-Tritium NIF Implosions For Areal Density Measurement

“…Sensitivity, ξ, of a CVD diamond detector for a 1mm thick wafer, an oscilloscope terminated by 50 Ω, E = 1000 V/mm, and for two wafer grades, optical and electronic. 3,9,10 Neutron energy (MeV) Optical grade (μV.ns/n) Electronic grade (μV.ns/n) 2.5 0.24 0.56 14.1 0.62 1.43 Table 1 gives measured values of sensitivities, ξ, for optical and electronic grades of CVD diamond, measured with DD neutrons (2.5 MeV) and DT neutrons (14.1 MeV). 9 The sensitivity curve will vary (scale up and down) depending on the value of the sensitivity measured (given in table 1), which itself depends on the detector thickness, type (optical or electronic grade for example), and voltage bias.…”

“…Chemical Vapor Deposition (CVD) diamond wafers are being tested to evaluate the feasibility of measuring areal density by determining the ratio of downscattered to primary neutrons (downscattered neutrons are neutrons scattered inside the target). [1][2][3] As part of this evaluation, the response of a CVD diamond detector to "typical" Deuterium-Tritium (DT) NIF implosions has been calculated at several distances from target chamber center. The response of the detector was found by weighting the neutron spectra with the sensitivity of the detector.…”

Charge Carrier Density and signal induced in a CVD diamond detector from NIF DT neutrons, x-rays, and electrons

“…Previous work found a ξ of 0.62 μVns/n and 1.24*10 -17 C/n (14 MeV) for a 1 cm 2 x 1 mm optical grade CVD polycrystalline diamond, d ≈ 50 μm, at E = 1V/μm, with a 50Ω measuring system. 10,28 Both values of ξ have to be scaled to the data of figure 1: ξ = 1 μVns/n and ξ = 1.24 *10 -17 /0.62 = 2*10 -17 C/n. For the 2D fizzle case, we can then calculate the expected S and Q meas that we would obtain using the earlier data: S exp = 1* 4*10 8 = 4*10 8 μVns and Q meas,exp = 2*10 -17 * 4*10 8 = 8 nC (values close to the ones in table 2).…”