nGEM neutron diagnostic concept for high power deuterium beams

The ITER neutral beam test facility under construction in Padova will host two experimental devices: SPIDER, a 100 kV negative H/D RF source, and MITICA, a full scale, 1 MeV deuterium beam injector. Detection of fusion neutrons will be used as a means to resolve the horizontal beam intensity profile. The neutron detection system will be placed right behind the SPIDER beam dump, as close to the neutron emitting surface as possible thus providing the map of the neutron emission on the beam dump surface. The system uses nGEM neutron detectors. These are Gas Electron Multiplier detectors equipped with a cathode that also serves as neutron-proton converter foil. The cathode is designed to ensure that most of the detected neutrons at a point of the nGEM surface are emitted from the corresponding 40 x 22 mm super(2) beamlet footprint on the dump front surface. The nGEM readout pads (area 20 x 22 mm super(2)) will record a useful count rate of 5 kHz providing a time resolution of temporal structures in the neutron fluency rate to be measured of better than 1 s. The directional response of the nGEM to neutrons is a key to reducing the scattering contribution to the measured neutron flux. First results achieved using small size nGEM prototypes shows that these detectors own the directionality property and that experimental results are in agreement with MCNP simulation. In addition they have a neutron efficiency detection of about 10 super(-5) and their response scales linearly following the intensity of the neutron flux.