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

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.Corrections for interfering Ar isotopes produced by neutron reactions with Ca are relatively reproducible with values of 2.64 ± 0.017 x 10~4 for ( 36Ar/ 37Ar) Ca and 6.73 ± 0.037 x 1Q~4 for ( 39Ar/37Ar) Ca.The measured values for ( 40Ar/ 39Ar)ic, however, vary by an order of magnitude.This variability, whose cause is unknown, has been reported from other reactors.The corrections for interfering Ar isotopes can be minimized by using optimization curves for the GSTR to choose the best sample size and irradiation time for a given material.Of more than 100 possible neutron reactions in common rocks and minerals, only 26 need be considered for purposes of radiological safety.The activity produced by these reactions upon irradiation of samples can be conveniently and accurately predicted either by a computer program or from graphs specifically devised for the GSTR. SAMPLE ENCAPSULATIONControl rod Transient rod drive _/-1 LazX.SUSa " cr-^> loading tube .'»»•;"°°;Aluniinuin tanker ';.°.^V.^'C!^°«°V.'o»°'»-/?>V05V%:BBi'''wced str"uc'tflral\≮re°tVVC"v»»V^ a ^o o °o A» O- 0 ?0 ?0 ?0 ?0 ?0 ?0 F Q ?0 ,.0 ?0 ?0 AR 37 -Ar DECAY CORRECTION PROGRAM