Production of the 186mRe Isomer in Nuclear Reactor

Abstract

   To provide experiments on stimulating the de-excitation of the isomeric state of 186mRe nuclei (half-life of the isomer T1/2 = 2 × 105 years), the possibility of producing weighed amounts of isomeric 186mRe nuclei by irradiating natural rhenium in the neutron flow of a pressurized water reactor was considered. From a comparison of the activity of the 186mRe isomer in rhenium samples irradiated inside the reactor core and by thermalized neutrons, it was shown that the isomers are formed mainly in the radiative capture of thermal neutrons by 185Re nuclei. The integral cross section for isomer formation is 300 ± 60 mb. Taking into account the burn up of the starting isotopes and the produced isomer, the maximum concentration of the 186mRe isomer of about 0.13 % relative to other rhenium isotopes is achieved at a neutron fluence of 2.2 × 1022 cm–2. To obtain a material with a higher concentration of the 186mRe isomer, one can first purify irradiated rhenium from chemical impurities on an ion-exchange column and then isolate the isotope with a mass number of 186. This will be an almost pure isomer of 186mRe, since the ground state of 186Re nuclei decays with a period of T1/2 = 90 h. For the separation of rhenium isotopes, the gas-centrifugal method using gaseous rhenium hexafluoride is promising; its boiling point is only 33.7 °C. A metal from the pure 186mRe isomer will essentially be a new state of matter.