The effective half-life of a broad beam 238PuBe total body neutron irradiator

A broad-beam 238PuBe neutron irradiator has been previously developed for exclusive use in in vivo neutron activation analysis in humans. The initial calibrations of the facility provided only a fixed value for the thermal neutron flux. Adjustment of this flux value for decay of the neutron source was later introduced and was based on the physical half-life of 238Pu. The current findings would suggest, however, that a more appropriate value for the effective half life for the total body irradiator is 141.7±2.5 y. In addition, variations in the induced counts for a Mn standard on a yearly basis indicate that seasonal differences of approximately 0.5% can be present between the winter and summer months. (author)