First sum spectra obtained with the SuN detector

Example of summing technique principle for 60Co spectrum obtained with a standard source placed in the center of the detector. Spectrum registered by a sungle PMT is shown below. The two cobalt lines can be observed and a small sum peak at 2505 keV is also visible.

When all signals from the three PMTs reading one of the segments are summed, the resolution of the spectrum is improved as can bee seen in the next figure.

Finally, signals from all eight segments are summed.Now, the most dominant is the sum peak. If the summing efficiency was 100% that would be the onle peak in the spectrum. In case of our setup, the summing efficiency is about 85% for cobalt, which means that for some decays only one of the two gammas is registered and as a consequence small peaks at 1.17 and 1.33 MeV are still present.

Due to an additional triggering condition, that requires the total energy deposited in the crystal during one event to be above 1 MeV (to reduce low energy background), there is almost no background in the final sum spectrum. A very small background peak at 1.46 MeV (40K background line) is present.