Upgrade of the JET tangential gamma-ray spectrometer. Conceptual design

A conceptual design for the upgrade of the JET tangential gamma-ray spectrometer (TGRS) has been carried out. The main design target for the TGRS upgrade is to maximize the signal-to background ratio at the spectrometer detector, the ratio being defined in terms of the plasma emitted gamma radiation and the gamma-ray background. A complex system of collimators and shields for both the neutron and gamma radiations define the spectrometer field of view. Two tandem collimators determine the field of view through the tokamak plasma. The entrance aperture to the penetration in the JET Torus Hall south wall is defined by a neutron shield. Two gamma-ray shields together with the existing concrete collimator determine the field of view at the bismuth germanate (BGO) gamma-ray detector. One of the gamma-ray shields (with an embedded neutron attenuator) can be remotely moved in and out of the detector line of sight thus providing flexibility in definition of the neutron and gamma-ray fields at the BGO detector. Neutron attenuators using lithium hydride (LiH) with natural isotopic composition (that meets the requirements for gamma-ray transparency) could provide the necessary attenuation factors (approximately 10
⁴
for the 2.45 MeV neutrons and 10
²
for the 14.1 MeV neutrons). A hot pressing technology has been proposed for the construction of the lithium hydride attenuators. The performance of a simplified geometry TGRS has been evaluated by preliminary neutron and photon transport calculations and the results show that the design parameters could be attained.