The production rate of natural tritium

Abstract

A detailed evaluation is made of the production rate of natural tritium in the pre-thermonuclear epoch. Deuterium and tritium analyses on the same precipitation samples are used to establish the uncontaminated tritium levels in precipitation sampled before the Castle tests, and the tritium balance is calculated for the North American troposphere. The global mean production rate Q, calculated from the geochemical inventory, is found to be 0.5 ± 0.3 atoms T/cm² sec. This value is three to four times smaller than values found previously by such calculations because of the following developments: 1The deuterium and tritium data show that the increases in tritium content observed during early thermonuclear tests before Castle are due to addition of synthetic tritium rather than to random fluctuations. The deuterium-tritium relationships are used to establish the general pattern of tritium variations over the North American continent and to evaluate the uncontaminated tritium levels.2The mean stratospheric residence time for tritium is found to be about 1.6 years from studies on fission product fallout and from the latitudinal variation of stratospheric cosmic ray production.3Stratospheric tritium is preferentially injected into the troposphere at high latitudes, as shown by fallout observations. The tritium influx into the North American troposphere is therefore higher than the mean global value.The predicted production rate is calculated from cosmic ray and nuclear cross section data using the star production rates in the atmosphere. The predicted mean global tritium production rate during an average solar cycle is found to be 0.25 ± 0.08 atoms T/cm² sec. The variation in the production rate over an average solar cycle is found to be ± 4.5%. Within the uncertainties of the data and calculations, the production rates calculated from the geochemical inventory and from the cosmic ray data are in agreement, and there is thus no observational evidence for accretion of tritium from an extra-terrestrial source.