Chopped cold neutron beam activation analysis (March 2021)

A linear fast neutron beam chopper has been deployed at a prompt gamma activation analysis instrument, enabling in-beam activation analysis of short-lived ( $T_{\mathrm {1/2}} > \sim 10$ ms) neutron capture products without the need for sample transfer. This article describes the design and operation of the chopper system and measurement results from test samples containing Se-77 ( $T_{\mathrm {1/2}} =17.36$ s) and Yb-175m ( $T_{\mathrm {1/2}} =0.0682$ s). The chopper timing was optimized for a specific isotope in each measurement, demonstrating an advantage of the programmable system. The linear response of the total gated counts in the decay phase to the incremented Se and Yb masses demonstrates the suitability of the system for quantitative analysis. Theoretical calculations based on the experimental parameters were in good agreement with the observed results. An example that illustrates the separation of overlapping energy peaks by the use of time-stamped list mode data acquisition is shown in a sample containing Yb-175m and Dy-165 ( $T_{\mathrm {1/2}} =75.4$ s).