Gamma-ray spectrometry is a reference technique for measurements carried out during the decommissioning process. Compton continuum is a major contribution to the signal-to-noise ratio when considering low-energy peaks measurement. Lead shielding is oftenly used to reduce background contribution but its weight is a major drawback.
This work investigates a compact Compton Suppression System (CSS) based on a High-Purity Germanium (HPGe) diode as a primary spectrometer. Among a wide range of guard detectors, including BGO, NaI(Tl), and LaBr3, a plastic scintillator is selected on account of its lightweightness.
For this purpose, CEA investigates a compact Compton Suppressor System “CSS” consisting of an HPGe “High Purity Germanium” spectrometer coupled with a plastic “EJ 200” scintillator. Hence, when an event temporally coincides in both detectors, corresponding to at least on Compton scattering event, it is rejected by an anti-coincidence filter. The signal-to-noise ratio improvement of total-absorption peaks over Compton continuum hence betters the Minimum Detectable Activity “MDA”.
The present article is focused on the experimental study of said Compton Suppression System. It consists in using HPGe detectors of various relative efficiencies (10 %, 20 % and 40 % at 1.3 MeV), set inside a plastic annulus scintillator from Scionix Holland. The acquisitions were carried out using “Hexagon” and “DSPEC 502A” digital MCAs, from CAEN SpA and AMETEK Ortec respectively. The sources and peaks of interests shall be 59 keV (for 241Am, 238U’s 63 keV and 133Xe’s 81 keV), 122 keV (for the 100 – 200 keV region of Plutonium), 137Cs’s 662 keV, 60Co’s 1.17+1.33 MeV.
The figures of merit of interest quantized with and without anti-Compton filtering are the overall Compton continuum “C(E)” reduction at different energies “E”, and the P(E)/C(E) ratio between total absorption peak “P” and the Compton continuum; the latter corresponding to the overall improvement of the measurement.