Detection of 236U: a possible 100-million year neutron flux integrator

Abstract

It is anticipated that accelerator mass spectrometry (AMS) assay of actinides, in particular 236U and 233U in uranium and 229Th and 230Th in thorium, will become valuable for identifying the previous human use of such materials. A discussion is included concerning the possibility of using 236U as a thermal neutron monitor having an integration period of up to 100 million years using AMS to fragment all molecular interferences. Using the detection of 236U 238U at levels below 10 −13 as a design goal, a description is provided of the geometry of a proposed broad-range precision AMS spectrometer that includes a 3 MV terminal tandem accelerator and a terminal gas stripper. The elimination of interfering molecular ambiguities and backgrounds is discussed. A post-acceleration stripping procedure for M Q ambiguity elimination is presented. A novel acceleration and fragmentation protocol is also described where molecular fragmentation is carried out at ground, outside of the tandem, rather than by stripper fragmentation at high voltage. It is shown that this new procedure simplifies AMS especially for heavy elements and leads to the possibility of using smaller tandems and analysis components. Multiple scattering during terminal charge changing is considered together with its effects upon accuracy and efficiency.