Fallout and Natural Background in the San Francisco Bay Area

Abstract

Abstract

On or about 20 March 1958 a significant increase in the background radiation levels was noticed at the University of California Radiation Laboratory. It was soon determined that this increase was associated with radioactive material deposited during a series of rainstorms. The Health Physics department began a series of measurements in the San Francisco Bay area in order to contribute to the assessment of the extent and amount of the deposited radioactivity. The instrument used for the field surveys was a NaI(Tl) crystal 3 in. long by 3 in. in diameter and viewed by a DuMont K1197 phototube, the output of which drives a count-rate meter. This same crystal was used for γ-ray pulse-height analysis in conjunction with a 50-channel pulse-height analyzer. Sample counting is done inside of a 4 in. thick lead shield. In making the survey, the points were chosen to be as evenly spaced and as representative of altitude, rainfall and rural and urban areas as possible. One area, an area of dense fern growth, showed the highest radiation levels and it was decided to use this fern for pulse-height analysis. Samples were ashed and then analyzed. The results of this analysis did bear out the belief that this was fallout-produced radiation. The prominent peaks at 0.15, 0.49 and 0.75 MeV are characteristic of fallout spectra of this age. The 1.62 MeV La140 peak was also present. The results of the several surveys enabled us to establish a value for the natural background radiation for these areas as none was available prior to the deposition of the fallout. By the use of γ-ray spectroscopy in conjunction with a γ-ray dose field survey, it is possible to measure changes in the radiation level due to fallout that in magnitude do not exceed 10 per cent of the natural background. A continuing program is being carried out both in the field surveys and the spectroscopy.