The retention and release of 37Ar from samples of human bone examined in vitro and a review of the implications for argon transfer from bone in vivo.

The release rates and long term retention of 37Ar produced by the reaction 40Ca(n, alpha) 37Ar, following fast neutron irradiation, have been measured in vitro, in three samples of dried human femur. For each sample, the release rates were fitted to three exponential curves, of half-times approximately 2, 20 and 300 min. Some 60-80% of the 37Ar released was contained in the first component. The release rates for samples of trabecular and cortical bone were qualitatively similar, but after some 20 h the retained fraction of 37Ar was 28 +/- 0.6% in a sample of predominantly trabecular bone, and 33 +/- 0.7% in samples of predominantly cortical bone. The processes affecting the trapping and release of 37Ar from bone crystals are discussed. It is suggested that 37Ar escapes from bone mineral by back-diffusion along recoil damage tracks. The factors affecting the release of 37Ar from bone into the venous blood supply are discussed in the context of in vivo measurements of body calcium by the 37Ar method. The physiological processes and compartments likely to give rise to the long 37Ar exhalation half-times following irradiation in vivo: transfer from the mineralised bone matrix to blood capillaries, and uptake and washout from bone marrow, are considered. Uptake in bone marrow may be responsible for both of the observed half-times.