RESUMEN
The relative response of neutron rem detectors has previously been shown to increase after prolonged exposure to a neutron flux. This increase has been referred to as the "soak factor." The cause of the increased response has been previously unexplained. This note reports on a search for the underlying cause of the increased response. Testing involved gamma-ray spectroscopy of activated neutron rem detector components and testing of instrument response at various rates of neutron flux and accumulated fluence. The proposed primary cause of the increased response is activation of copper in the brass collar at the fill end of the gas-filled detector tube. This component allows for attachment of the extension housing on the tube containing BF3 or He gas. Under a neutron flux, some of this copper activates to Cu. In addition, Mn was detected in activated components but does not seem to be a significant contributor to the detector response. Cadmium isotopes did not appear to be significant. The activated component causes an increase in indicated neutron dose rate due to decay photons from activated components. Photons normally have little impact on neutron rem detector readings because the electrical pulses produced in the probes are below the lower-level discriminator. However, the photon pulses do impact the overall count rate when they occur simultaneously with normally uncounted, lower amplitude, wall-effect neutron pulses.
RESUMEN
Dust loading on air sample filters is known to cause a loss of efficiency for direct counting of alpha activity on the filters, but the amount of dust loading and the correction factor needed to account for attenuated alpha particles is difficult to assess. In this paper, correction factors are developed by statistical analysis of a large database of air sample results for a uranium and plutonium processing facility at the Savannah River Site. As is typically the case, dust-loading data is not directly available, but sample volume is found to be a reasonable proxy measure; the amount of dust loading is inferred by a combination of the derived correction factors and a Monte Carlo model. The technique compares the distribution of activity ratios [beta/(beta + alpha)] by volume and applies a range of correction factors on the raw alpha count rate. The best-fit results with this method are compared with MCNP modeling of activity uniformly deposited in the dust and analytical laboratory results of digested filters. A linear fit is proposed to evenly-deposited alpha activity collected on filters with dust loading over a range of about 2 mg cm to 1,000 mg cm.
