New Measurements of the 81Kr Atmospheric Abundance

Abstract

We have determined the 81Kr activity to be (0.067 +/- 0.003) decay/min. 1 krypton. Using this activity in conjunction with our new measurement of the 80 KR(n,y)81Kr reaction cross-section of (12 +/- 4)b, we infer that the 81Kr activity is 1.5-2.0 times that which would be calculated calculated using the current cosmic ray flux. This implies that the average cosmic ray intensity in the atmosphere during the 81Kr life-time was greater than the current intensity, a fact that has implications for the 14C time scale.