Beryllium neutron activation counter for pulsed D-D fusion sources

Abstract

The fusion reaction occurring in DD plasma is followed by emission of 2.45 MeV neutrons, which carry out information about fusion reaction rate and plasma parameters and properties as well. Neutron activation of beryllium has been chosen for detection of DD fusion neutrons. The cross-section for reaction 9Be (n, α) 6He has a useful threshold near 1 MeV, which means that undesirable multiply-scattered neutrons do not undergo that reaction and therefore are not recorded. The product of the reaction, 6He, decays with half-life T1/2 = 0.807 s emitting β– particles which are easy to measure. Large area gas sealed proportional detector has been chosen as a counter of β– particles which leave activated beryllium plate. The plate with optimized dimensions adjoins the proportional counter entrance window. Such set-up is also equipped with appropriate electronic components and forms beryllium neutron activation counter. The density of neutron flux on beryllium plate can be determined from the number of counts. Therefore, a proper calibration procedure needs to be performed to establish such a relation. The measurements with the use of known β– source have been done. In order to determine the detector response function such experiment has been modelled by means of MCNP5 – the Monte Carlo transport code. It has allowed a proper application of results of the transport calculations of β– particles emitted from the radioactive 6He and reaching the proportional detector active volume. In order to test the counter system and measuring procedure a number of experiments have been performed on PF-6, PF-1000 and PF-4 devices. The experimental conditions have been simulated by means of MCNP5. The correctness of simulation outcome have been proved by measurements with known radioactive neutron source. The results of the DD fusion neutrons measurements have been compared with other neutron diagnostics.