To be sensitive to an effective Majorana neutrino mass, [m(v)], on the orde
r of 0.1 eV. the NEMO 3 double beta (beta beta (0v)) decay experiment requi
res precise knowledge and control of the backgrounds. The effect of neutron
s and gamma -rays from the Frejus underground laboratory (LSM, Laboratoire
Souterrain de Modane) has been studied during 10 700 h of data collection.
The data were taken with a NEMO 2 prototype detector using different shield
configurations. Monte Carlo calculations with GEANT/MICAP code and a new l
ibrary of gamma -rays from neutron captures are presented. The implied cons
equences for the NEMO 3 detector which is under construction are discussed.
The neutrinoless double beta decay background induced by neutrons and gamm
a -rays within the LSM will be suppressed to the very suitable level of 0.1
event in a live time of five years given the appropriate shield and magnet
ic field, (C) 2001 Elsevier Science B.V. All rights reserved.