Figure 1

6-NM-64 parameterisation. (a) Vertical cross section (to scale) of the radiation transport model developed and used to represent the 6-NM-64 benchmark. Moving from the inside out, comprising six 6\(^{\prime \prime }\) dia. BP28-BF\(_3\) proportional counters (blue), air gap, 2 cm thick HDPE moderator tube and 5.1 cm thick winged Pb rings (one per counter) and outer 7.5 cm thick HDPE reflector with dimensions as indicated. (b) Single counter representation of the parameterised cavity model used as an equivalent to the NM-64 standard with key parameters indicated. NB: this figure represents a single cavity; all simulation performed in this study were for six cavities as per the 6-NM-64 benchmark. The wings of the Pb rings were omitted from the standard NM-64 design for simplicity. (c) Producer mass in kg against total (i.e., summed across all six detectors) count rate in cnts s \(^{-1}\) for randomly generated parameter configurations of the parameterised cavity model shown in (b). The orange marker, located at the intercept of the orthogonal dashed lines, indicates the 6-NM-64 benchmark performance; a count rate of \(\sim\)43 cnts  s\(^{-1}\) for a producer mass of \(\sim\)10 tonne. The region shaded red signifies parameter configurations that exceed the count rate performance of the 6-NM-64 benchmark but for a greater producer mass. The region shaded green signifies parameter configurations that exceed the count rate performance of the 6-NM-64 benchmark but for a lower producer mass. The few parameter configurations in the region shaded green indicates a well optimised NM-64 configuration.