Fig. 3: Pulse-height spectra recorded for various radiation sources.

a Pulse-height spectrum of the natural background (no sources present) recorded at Los Alamos National Laboratory (elevation 2198 m above sea level) for a measurement duration of 49089 s; this background was subtracted from all raw pulse-height distributions measured for sources present (b and c). b Pulse height spectrum (blue dots) measured using irradiation with a 10.5 µCi ²⁵²Cf fission neutron source (measurement duration 600 s). The solid red line is a least-squares fit of a Gaussian function to the neutron capture peak using the data in the 1310–2000 channel range. The peak is at channel 1560 with a full width at half maximum (FWHM) of 239 channels, yielding an energy resolution of 15.3%. c Pulse height spectra due to gamma were measured using irradiation with ¹³⁷Cs (orange dots, measurement duration 4006 s), ⁶⁰Co (red dots, measurement duration 7981 s), and ²³²Th (green dots, measurement duration 6000 s) gamma sources. The count rates are shown on a log-scale. The pulse height spectrum from Fig. 3b is shown for reference (blue dots). The channel 1310–2000 region of interest (ROI) is indicated by a dashed blue vertical line. The dashed black lines are double exponential fits to the Compton tails of the respective pulse-height spectra.