The measurement of silicon in a lung phantom--a comparison of two 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 2H + 2H reaction, was used to irradiate a Si-doped chest phantom in order to determine minimum detection limits (MDL). Two 'in-beam' nuclear reactions on Si were studied; prompt fast neutron inelastic scatter 28Si (n,n' gamma)28Si reaction was measured during the beam burst and the 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 28Si(n,n' gamma)28Si reaction. A comparison was made with a 252Cf neutron irradiation system where a Si MDL of 6.3 g was obtained using the slow neutron capture reaction. The Van de Graaff system permits 'exposed' Si lung burdens to be measured but not normal levels. Improved measurement sensitivity may be achieved by reduction of high counting-rate losses and high background radiation.