THE MEASUREMENT OF SILICON IN A LUNG PHANTOM - A COMPARISON OF 2 NUCLEAR-REACTIONS FOR IN-VIVO ACTIVATION-ANALYSIS

The amount of silica in the human lung may be estimated by measurement of silicon using in vivo neutron activation analysis. A pulsed, fast neutron beam, produced with a 2 MV Van de Graaff generator using the H -2+H-2 reaction, was used to irradiate a Si-doped chest phantom in ord er to determine minimum detection limits (MDL). Two 'in-beam' nuclear reactions on Si were studied; prompt fast neutron inelastic scatter Si -28 (n,n'gamma)Si-28 reaction was measured during the beam burst and t he slow neutron prompt capture reaction was measured between the fast neutron bursts. Although the latter reaction appeared less favourable due to neutron cross section and measurement efficiency considerations , it yielded an MDL Of 1.8 g compared with 2.3 g for the Si-28(n,n'gam ma)Si-28 reaction. A comparison was made with a Cf-252 neutron irradia tion system where a Si MDL of 6.3 g was obtained using the slow neutro n capture reaction. The Van de Graaff system permits 'exposed' Si lung burdens to be measured but not normal levels. Improved measurement se nsitivity may be achieved by reduction of high counting-rate losses an d high background radiation.