Silicon measurement in a lung phantom by neutron inelastic scattering

A study was made to assess the feasibility of determining the silicon level in human lungs in vivo by measuring the gamma rays arising from the neutron inelastic scattering reaction 28Si (n, n'γ) 28Si. Neutron energies in the range 5--8 MeV represent the best compromise between the conflicting requirements of high energy for good detection statistics and low energy to minimize the dose to the subject. The sensitivity of measurement was enhanced by pulsing the neutron beam and counting only during the period of bombardment. This effectively reduced the background counts emanating from thermal neutron reactions in the phantom and from the fast neutron reaction 31P(n,α)28. In measurements with an anthropomorphic phantom, no interference peaks from other prompt inelastic scattering reactions were observed. With one Ge(Li) detector of 19% relative efficiency, a detection limit of 0.6 g of silicon per 10 mSv was obtained. On this basis, it is estimated that six Ge(Li) detectors of 25% efficiency each would be capable of measuring 0.15 g of silicon in the lungs, the average level found in nonoccupationally exposed adults