Irradiation of samples for 40Ar/39Ar dating using the Geological Survey TRIGA reactor

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Abstract

The characteristics of the Geological Survey TRIGA Reactor (GSTR) as a source of fast neutrons for the 40Ar/3 Ar technique of K-Ar dating have been determined using data from more then 45 irradiations in the central thimble (core) facility. The GSTR has a flux over the entire energy spectrum of 1.1 x 1017 n/cm2 -MWH and a fast/thermal ratio on the centerline of the central thimble of 117 for fast neutron energies greater than 0.6 MeV. Production of 39Ar is about? x 10~13 mole/gram-percent KgO MWH, and the cross section for the reaction 39K(n, p) 39Ar is 65 4 millibarns for epithermal (> 0.6 MeV) neutrons. Most 4 Ar/ Ar dating applications require about 10-40 hours of irradiation in the GSTR at the maximum continuous power level of 1 MW. The peak neutron flux in the central thimble is 4 cm above the physical centerline, and the verticle flux gradient in the centermost 20 centimeters varies from a small fraction of a percent to a maximum of about 3.5 percent per centimeter. The effect of this gradient can be effectively cancelled by suitable sample encapsulation and the use of a sample holder designed for the purpose. The horizontal flux gradient is less than 0.5 percent over the width of the central thimble. Self-shielding in a solid core of diabase 2.40 cm in diameter and 2.54 cm high is approximately 3 percent from the outside to the center, but self-shielding is probably negligible for the smaller samples usually irradiated for K-Ar dating.

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