Transmutation detectors for fast neutron fluence measurements

Abstract

Transmutation detectors (TMDs) are based on a similar principle as neutron activation detectors (NADs). In both cases, the neutron fluence is estimated from the number of reactions in the detectors. The main differences between these two detectors are the type of product created by the nuclear reaction and the evaluation of the irradiated detectors. In TMD, the transmuted nuclide is stable, and the number of reactions is gained by analytical methods suitable for measuring low concentrations. In NAD, the nuclear reaction leads to the production of radionuclides and the number of reactions is evaluated from the activity measured by radiometric instruments. While TMDs for thermal neutrons can be easily prepared and evaluated due to their high thermal neutron cross sections for isotopes such as ¹⁰B, ¹¹⁵In or ¹⁹⁷Au, the requirements for fast neutron TMDs are higher. Cross sections for nuclear reactions suitable for the TMD method with fast neutrons are in the range of 1 to 100 mb for the fission spectrum. This leads to concentrations from 0.01 ppm to 1 ppm for fast neutron fluence of 1E19 cm^-2. These low concentrations imply requirements for detector material purity and sensitivity of concentration evaluation equipment. Materials for fast neutron TMDs were proposed, where the relevant parameters, such as energy threshold and minimal detectable fluence are known and convenient. An experimental comparison of some TMD and NAD responses was made in the LVR-15 research reactor using secondary ion mass spectrometry to evaluate the concentration of TMDs.