Proton recoil detector of fusion neutrons using natural diamond

Abstract

Diamond, with its high radiation damage resistance, is an attractive alternative to silicon for neutron measurements in next step fusion experiments. A 200-μm-thick type IIa natural diamond with Ti/Au contacts was tested at the LAMPF-WNR facility by time-of-flight neutron energy identification. The crystal, having a carrier lifetime of up to 1 ns, was arranged in a low-energy-resolution, high-sensitivity proton recoil telescope consisting of a polyethylene radiator and a low-energy-proton Teflon filter. This arrangement is similar to the triton burnup monitor of Croft et al. [Rev. Sci. Instrum. 64, 1418 (1993)], where a silicon photodiode was used as a recoil proton detector. The observed sensitivity for 14 MeV neutrons (DT) is (1.25±0.15)×10−3 counts/neutron. However, a high contribution of neutron-induced events in the diamond, mainly carbon (A=12) recoils, was observed. A one-dimensional calculation for the detector response to carbon recoil and proton deposition is compared to the measurements. Poor energy resolution of the diamond detector precludes pulse height discrimination between direct 2.5 MeV neutrons events and proton events corresponding to 14 MeV neutrons. Therefore, an overall DT/DD neutron sensitivity ratio of only ∼6.5 is achieved. This value is much lower than the ratio of 540 reported by Croft et al. in their silicon (A=28) monitor.

Proton recoil telescope based on diamond detectors for measurement of fusion neutrons

“…where ρ=1 gr/cm 3 is the density of the polyethylene, whose chemical formula is C 2 H 4 , A=28 is its the molar mass and n H =4 is the number of H atoms in the molecule, N A is the Avogadro number, ∆ x=70 µm is the converter thickness, The presence of high noise during the charge sensitive chain measurements deteriorated the energy resolution. In the deposited energy spectrum, three regions can be identified: below 4 MeV there is the elastic scattering shoulder, from 4 MeV to 6 MeV there is the (n,3α) shoulder and around 8 MeV there is the 12 C(n,α) 9 Be peak. The discrepancy between measured and simulated spectra in the <4 MeV range could be due to the lack of some inelastic reactions with threshold >10 MeV in Geant4.…”

“…Other most important contributions to coincidences come from 12 C(n,n'), i.e. from the elastic scattering of neutrons on the carbon nuclei in both crystals, and from the 12 C(n,xα), which are essentially 12 C(n,α) 9 Be or 12 C(n,3α)n. In the last two cases, one of the αs can travel from one crystal to the other, to give the coincidence.…”

“…A detailed MCNP [14] input file, describing the geometry and the neutron source was provided by ENEA and allowed to calculate the neutron flux and spectrum in the region of interest. Unfortunately, MCNP is not able to generate the products of nuclear reactions such as 12 C(n,α) 9 Be or (n,p). So the simulation was made in two steps: the neutron flux and spectrum at the converter surface were evaluated using the code MCNP, while the detector response to the neutrons was simulated with Geant4.…”

“…LiF coating on CVD diamonds has been used in JET tokamak [8]. In that configuration, not only the 14-MeV neutrons had a good energy resolution through the 12 C(n,α) 9 Be reaction, but also the thermal neutrons could be measured. Also a plastic converter placed in front of a natural diamond detector was used to measure the burn up fraction in the proton recoil telescope configuration [9].…”

“…In that configuration, not only the 14-MeV neutrons had a good energy resolution through the 12 C(n,α) 9 Be reaction, but also the thermal neutrons could be measured. Also a plastic converter placed in front of a natural diamond detector was used to measure the burn up fraction in the proton recoil telescope configuration [9]. This configuration, though, gave not satisfactory results: in fact, the background due to the elastic scattering of the neutrons on the Carbon atoms compromised the measurements of the 2.5 MeV neutrons.…”