Figure 6. Demonstration of background rejection by imaging spectroscopy using a ¹⁵²Eu radioisotope.

(a) Energy spectra of gammas of ¹⁵²Eu (0.72 MBq) placed off-axis at a polar angle of 30° with a distance of 2.4 m. Black, purple, and red crosses are the energy spectra for all the events, reconstructed gamma-ray events (see text for the filtering criteria), and the events after the imaging selection (δ < 10°), respectively. (b) Energy spectra with the remaining background subtracted, which is accumulated from a symmetrical region indicated in Fig. 5d. Vertical blue dashed lines are major gamma-line energies of ¹⁵²Eu between 100 and 1200 keV.