Advances in chemical vapor deposition techniques have made possible in the last decade to grow synthetic diamonds which can be used for detection of fast neutrons. In particular, detectors based on single crystal diamonds (SCD) of so called electronic grade offer the best available detector performance. This contribution will present fast neutron measurements with SCD for application to fusion plasmas and spallation neutron sources.
Fusion neutrons are produced by the D+D->He3 +n and D+T-> 4He +n reactions and feature a spectral distribution which is characteristic of the plasma neutron source and is centered at the mean energies of 2.5 and 14 MeV, respectively. Neutron emission spectroscopy (NES) measure the emitted neutron spectra from which it is possible to infer information on the plasma state, such as for instance the fuel ion temperature and non-thermal emissions due to the presence of fast ions heated by external mechanism. Dedicated neutron spectrometers have been developed at JET since the end of 1990’s for matching the stringent requirements on energy resolution (FWHM better than 5% ) and high counting rate capability (up to 1 MHz) needed to measure the plasma time evolution. The state of the art of NES measurement is represented by the Magnetic Proton Recoil spectrometer for 14 MeV neutrons and the TOFOR time of flight spectrometer for 2.5 MeV neutrons. CVD single crystal diamonds, besides being used as neutron counters, represent a very interesting recent alternative to these neutron spectrometers since they feature the advantage of being extremely compact and insensitive to magnetic field. A SCD prototype of dimensions 4.5x4.5x0.5 mm3 area and a matrix based on 12 single pixels have been installed at JET and have been collecting data in D plasmas. The neutron detection principle in diamond is based on elastic scattering for neutrons up to 6 MeV and on the reaction 12C(n,a)9Be (Q=-5.7 MeV) for 14 MeV neutrons. In order be able to perform NES measurements at MHz counting rate a dedicated fast electronic chain based on fast charge preamplifiers and 500 MS/14 bit digitizers have been developed. The results have showed that it is possible to achieve energy resolution of 1% at 14 MeV neutrons with counting rate capabilities in excess of 1 MHz. In this contribution we focus especially on the superior energy resolution offered by SCD for NES spectral studies of DT fusion neutrons.
A second application of SCD that will be presented is fast neutron measurements at short pulse spallation sources. Due to their wide response to neutrons, small size and fast signals SCD are used to measure the neutron map and flux of the fast neutron beam line ChipIR of the ISIS neutron spallation sources. ChipIR features a wide fast neutron spectrum up to energies of 800 MeV and is dedicated to the study of single events errors in electronics.