A new GEM based 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 hydrogen/deuterium RF source, and MITICA, a full scale, 1 MeV deuterium beam injector. A number of diagnostics will be deployed in the two facilities to qualify the beams. This paper reports the development of a neutron diagnostic called CNESM (Close-contact Neutron Emission Surface Mapping) which aims to provide the map of the neutron emission from the beam dump surface. The CNESM uses nGEM as neutron detectors. These are Triple GEM (Gas Electron Multipliers) detectors, equipped with a cathode that also serves as neutron-proton converter foil. The cathode is at about 30 mm from the beam dump front surface and it is composed by two layers (polyethylene + aluminum, overall thickness = 100 mu m). The aluminum layer stops all the protons that are emitted from the polyethylene in an angle higher than 45 degrees with respect to the incoming neutron direction, thus ensuring the directionality property of this detector, needed in order to correctly reconstruct the neutron beamlets composition. The nGEM readout pads (area 20 x 22 mm(2)) will record a useful count rate of 5 kHz, providing a time resolution better than 1 s and a space resolution better than 5 mm. The readout electronics system that will be employed is based on carioca chips. Each nGEM detector has an active area of 35.2 x 19 cm(2) and maps the neutron emission from a group of 5x16 beamlets: 16 nGEM would be needed to cover the entire beam dump. Preliminary measurements have been performed using a nGEM detector prototype with an active area of 10 x 10 cm(2) equipped with the converter cathode: the neutron detection efficiency, directionality and gamma background level have been measured using neutrons coming from D-D reaction. The final dimension prototype for SPIDER (active area 35.2 x 19 cm(2)) is currently being built and will be tested in 2012.