Measurements of neutron flux and high energy γ radiation at LSM

A low background sodium iodide crystal detector has been used to measure the high energy photon flux in the Laboratoire Souterrain de Modane. The main difficulty is related to the fact that the detector is sensitive not only to external photons but also to the photons emitted in the neutron radiative capture in the crystal iodine and/or in shielding copper as well. Three different configurations of the shielding around the NaI detector were successively used: without shielding (the detector is sensitive to the thermal neutrons captured by the iodine and to the external photons); 8 cm borated polyethylene shielding (the thermal neutrons are absorbed by the shielding, and the detector is sensitive to only external photons); 10 cm Pb + 5 cm Cu shielding (the thermal neutrons are essentially captured in copper while the external photon flux is strongly attenuated). The photon flux for the energy range 6 - 9 MeV is 1.5 x 10-6 γ/s/cm2 and for the 9 - 32 MeV range is 10 -8 γ/s/cm2. The thermal neutron flux has been estimated to be of the order of 3 x 10-6 n/s/cm2. The measurements show that the photon flux is dominated by the photons emitted from the neutron capture in the rock and different materials of LSM. This hypothesis was verified by using a neutron source at LSM: the counting rate in the region 6 - 9 MeV was multiplied by a factor of 50 while no event occurred at energies above 9 MeV