Measurement of the Thermal Neutron Fluence Rate in the Instrument Thimble 11 Position in the Advanced Test Reactor Critical

The Advanced Test Reactor Critical (ATRC) is a full-scale replica of the Advanced Test Reactor (ATR) core, utilized primarily for evaluating experiment impacts on power distribution. This study focuses on neutron fluence measurements within Instrument Thimble 11 (IT-11) by means of radiometric activation of passive dosimetry packages. These packages contained indium and gold thin foils as well as gold, cobalt, and nickel wire utilized during the 23-3 and 23-4 ATRC irradiations within IT-11 to evaluate the neutron field present within the test position. Measurement variances were observed between the various dosimetry methods, likely due to self-shielding effects in dosimetry foils and potential epithermal neutron contributions. Future work includes refining self-shielding corrections, exploring neutron field directionality, and developing epithermal corrections to resolve discrepancies. The findings aim to establish a more accessible and affordable test platform for nuclear instrumentation development and testing at the ATR facility.