Fast neutron instrumentation for beam diagnostic

This thesis concerns the development of fast neutron instrumentation for beam diagnostic. Two kind of detectors have been developed. The first is a diamond detector for fast neutron measurements at the ChipIr beamline of the ISIS spallation neutron source (Didcot, UK). ISIS is a 50Hz-pulsed source in which neutrons are produced by 800 MeV protons interacting on a heavy metal target. The second is a Gas Electron Multiplier (GEM) detector developed for measurements of the neutron emission map in the deuterium beam prototype facility for the ITER fusion reactor under construction at the RFX site (Padova). Measurements of the so-called Single Event Effects (SEE) are the main application of the ChipIr beamline. SEEs are a potential threat to the robustness of integrated circuits featuring dimensions of tens of nanometers. SEEs occur when a highly energetic particle causes a disruption of the correct operation of an electronic component by striking its sensitive regions. Recent studies have shown that the neutron component above 1 MeV of the cosmic ray radiation is the primary contribution to SEEs for heights < 10 km. In order to evaluate the sensitivity of electronic devices to SEEs, fault-tolerant design techniques must be employed, and extensive analyses are needed to qualify their robustness. Experiments with atmospheric neutrons can be carried out but, due to the low intensity, they require very long periods of data acquisition. Neutron sources represent an opportunity due to the availability of high intensity fluxes which allow for accelerated irradiation experiments. Recent experiments performed at ISIS on the VESUVIO beamline demonstrated the suitability of ISIS for this kind of application. The new ChipIr beamline will provide an atmospheric-like neutron spectrum with a multiplication factor around 10^8. A crucial task for ChipIr design is the development of a neutron beam monitor for measurements of the neutron fluence in the MeV energy range. The detector developed in this thesis as a beam monitor for ChipIr is a Single-crystal Diamond Detector (SDD). Neutron detection using diamonds is based on the collection of the electrons/holes pairs produced by the energy deposited in the crystal following neutron reactions with carbon. First tests were performed in 2009 using a prototype SDD. The device features a p-type/intrinsic/metal Schottky barrier structure where the active (intrinsic) detection layer is obtained by chemical-vapour deposition. Both Time of Flight (ToF) only and biparametric (ToF and pulse height) measurements were successfully performed. Measurements were also performed using a Fission Diamond Detector (FDD). A FDD is a device based on a single crystal diamond coupled to a natural uranium converter foil. The biparametric data collection allowed us to distinguish events from 235U, 238U and from carbon break-up reactions inside the diamond. Limitations to quantitative analysis due to the initial choice of detector thickness and instrumental settings were highlighted by the tests. In a new set of experiments performed in July 2010, April 2011 and October 2011 a new fast neutron detector was tested. The measurements showed three characteristics regions in the biparametric spectra: -background events of low pulse heights induced by gamma-rays; -low pulse height events in the neutron ToF region corresponding to En in the range 2.4-5.7 MeV which are ascribed to elastic scattering on 12C; -large pulse height events in the ToF region corresponding to En>6 MeV which are ascribed to 12C(n,α)9Be and 12C(n,n')3α reactions. Neutron energy information was found to be contained both in the pulse height and in the ToF data, which suggests that SDDs are good candidate detectors for spectroscopy in fast neutron irradiation experiments. The use of diamond detectors as beam monitors requires further characterization of their response to monoenergetic neutrons. The second detector developed in this thesis is a nGEM detector able to map the neutron intensity produced in the SPIDER/MITICA beams at the Consorzio RFX in Padova. The ITER neutral beam test facility under construction in Padova will host two experimental devices: SPIDER, a 100 keV negative hydrogen/deuterium beam, and MITICA, a full scale, 1 MeV deuterium beam. A number of diagnostics will be deployed in the two facilities to qualify the beams. The aim of this thesis was to design a neutron diagnostic for SPIDER, as a first step towards the application of this diagnostic technique to MITICA. The proposed detection system is called CNESM which stands for Close-contact Neutron Emission Surface Mapping. CNESM is placed right behind the beam dump, as close as possible to the neutron emitting surface. It shall provide the map of the neutron emission on the surface of the beam dump. The latter is a rectangular panel made of water cooled pipes used to stop the incoming beam. The CNESM diagnostic system uses nGEM as neutron detectors. These are Gas Electron Multiplier detectors equipped with a cathode that also serves as neutron-proton converter. The diagnostic was designed on the basis of simulations of the different steps, from the deuteron beam interaction with the beam dump to the neutron detection in the nGEM. The deuteron deposition inside the dump was simulated with the TRIM code in order to provide the deposition profile. Neutron emission occurs via fusion reactions between the deuterium beam and the deuterons implanted in the beam dump surface. Neutron scattering in the beam dump was simulated using the MCNPX code. The nGEM cathode is at about 30 mm from the beam dump front surface. It is composed of two layers (polyethylene + aluminum) each ~50μm thick. The aluminum layer stops all protons that are emitted from the polyethylene at an angle higher than 40° relative to the normal to the cathode surface. This means that most of the detected neutrons at a point of the nGEM surface are emitted from the corresponding 40X22 mm^2 beamlet footprint on the dump front surface. The nGEM readout pads (area 20X22 mm^2) will record a useful count rate of ~5 kHz providing a time resolution of better than 1 s. Each nGEM detector maps the neutron emission from a group of 5X16 beamlets: as many as 16 nGEM detectors would be needed to cover the entire beam dump. The effect of the directional detector response due to the Al foil is to decrease the FWHM value to about 30 mm. This level of spatial resolution is adequate for unfolding the neutron source intensity from the 2D event map in the nGEM detector. The first nGEM detector prototype was tested at the FNG neutron source in Frascati, where the directional response of the nGEM cathode to neutrons was verified. The successful design of the CNESM neutron diagnostic for SPIDER provides the basis for its application to MITICA (X100 larger neutron fluxes expected), where it will be particularly useful to resolve the horizontal beam intensity profile.