A recent project has started to provide JET (and in the future ITER tokamak) of a second real time diagnostic for fusion power, measuring the absolute intensity of the 16.7 MeV gamma line from D-T reaction. This diagnostic requires ABSOLUTE EFFICIENCY calibration of the used detector, and thus accurate measurement of the detector sensitive volume. LaBr3 detectors are the best option for high-energy gamma-ray monitoring in neutron-rich environments, but they are provided in sealed mounts that cannot be open without damage. We thus propose to perform a Neutron Tomography measurements of two such sealed detectors. One of them is a "one-of-its-kind" very large (3"x6") detector that will be made available by JET tokamak for a limited amount of time. NT will provide the exact crystal dimensions to be inserted in GEANT4-MCNP simulations for the efficiency estimation.