MONTE-CARLO DESIGN STUDY OF A MODERATED CF-252 SOURCE FOR IN-VIVO NEUTRON-ACTIVATION ANALYSIS OF ALUMINUM

The Monte Carlo computer code MCNP has been used to design a moderated Cf-252 neutron source for in vivo neutron activation analysis of alum inium (Al) in the bones of the hand. The clinical motivation is the ne ed to monitor Al body burden in subjects with renal dysfunction, at ri sk of Al toxicity. The design involves the source positioned on the ce ntral axis at one end of a cylindrical deuterium oxide moderator. The moderator is surrounded by a graphite reflector, with the hand inserte d at the end of the moderator opposing the source. For a 1 mg Cf-252 s ource, 15 cm long x 20 cm radius moderator and 20 cm thick reflector, the estimated minimum detection limit is 0.5 mg Al for a 20 min irradi ation, with an equivalent dose of 16.5 mSv to the hand. Increasing the moderator length and/or introducing a fast neutron filter (for exampl e silicon) further reduces interference from fast-neutron-induced reac tions on phosphorus in bone, at the expense of decreased fluence of th e thermal neutrons which activate Increased source strengths may be ne cessary to compensate for this decreased thermal fluence, to allow mea surements to be made within an acceptable time limit for the comfort o f the patient.