Neutron-based analysis of fission rates and ultra-trace concentrations of [formula omitted] using gamma spectrometry and CR-39 (plastic track detector)

Abstract

Fission rates of 235U in SRM2710 Montana Soil, zirconia, coal and coal ash samples were determined using nuclear track technique. Each sample (in the form of a disk) was sandwiched between two pieces, several square millimeter of CR-39. The sample packages were then placed in a cold neutron beam at the NIST Center for Neutron Research reactor. Almost all fission events in these samples resulting from the irradiations were due to 235U(n,f) reactions. The induced fissions were measured by track counting. The uranium contents of the zirconia, determined relative to the SRM2710 by both the fission track density and gamma activity of 239Np, were found to be 354±18 and 339±10.7 ppm , respectively. For comparison, instrumental neutron activation analysis followed by gamma spectrometry was also used to determine the uranium content of all the test samples. The fission track density rates of SRM2710 and zirconia were found to be 3±0.15 and 51.7±2.6 fission cm −2 s −1 , respectively, while the fission track registration sensitivities of CR-39 for both samples are (6.33±0.32)×10 −9 and (7.68±0.38)×10 −9 fission track cm −2 s −1 ppm −1 unit neutron flux −1 , respectively. This technique has been applied for the detection of trace and ultra trace 235U bearing inclusions in two environmental samples, coal ash and coal, in which the 235U contents have been found to be 77±4 and 4.18±0.2 ppb , respectively, while the related experimental fission track registration sensitivities of CR-39 were determined to be (3.26±0.16)×10 −9 and (4.7±0.24)×10 −9 fission track cm −2 s −1 neutron −1 per ppm 235 U , respectively.