CdWO 4 scintillator as a compact gamma ray spectrometer for planetary lander missions

Abstract

The objective of this work is to develop a gamma ray spectrometer (GRS) suitable for use on planetary rover missions. The main characteristics of this detector are low weight, small volume low power and resistance to cosmic ray radiation over a long period of time. We describe a 3 cm diameter by 3 cm thick CdWO 4 cylindrical scintillator coupled to a PMT as a GRS for the energy region 0.662–7.64 MeV. Its spectral performance and efficiency are compared to that of a CsI(Tl) scintillator 2.5 cm diameter by 6 cm thick coupled to a 28 mm×28 mm PIN photodiode. The comparison is made experimentally using 137Cs, 60Co, 6.13 MeV gamma rays from a 13C(α,γn)O 16* source, 7.64 MeV thermal neutron capture gamma rays emitted from iron bars using a 252Cf neutron source, and natural radioactivity 1.46 MeV 40K and 2.61 MeV 232Th gamma rays. We use a Monte Carlo method to calculate the total peak efficiency of these detectors and the full energy, first and second escape peak efficiencies. The experimental and calculated results agree well. We investigated the usefulness of these detectors for a GRS on a Mars lander mission. Although both detectors meet desired specifications, it was found that CdWO 4 has advantages over CsI(Tl) being a more compact detector of higher efficiency. Using a shaping amplifier of 24 ms, CdWO 4 spectrometer exhibited a 6.8% FWHM at 662 keV. At 6.13 MeV, CdWO 4 detector possesses an intrinsic total and full energy peak efficiencies of 16.7% and 6.3%, respectively. These efficiencies are nearly a factor of 1.6 and 4 greater than the corresponding efficiencies of the CsI(Tl) detector. A proposed gamma ray spectroscopy system to be placed on a rover, consists of a central detector surrounded by a Compton suppressor shield. The central detector is a cylindrical CdWO 4 detector and the Compton suppressor shield is made of segmented CdWO 4, coupled to PIN photodiodes. The shield also prevents thermal neutron activation of the central detector.