Low background gamma spectroscopy and neutron activation analysis for Double Chooz

To check the radiopurity of detector components of the reactor neutrino oscillation experiment Double Chooz, low background gamma spectroscopy measurements have been performed at the Garching underground lab using a 150% germanium counter surrounded by active and passive shielding systems. The active shielding consists of an anti-Compton veto and a muon veto. Upper limits on the activities of radioisotopes originating from the uranium and thorium decay chains, as well as potassium-40, can be given in the order of 10-10 g/g. An even higher sensitivity can be obtained by neutron activation analysis performed on the wavelength shifter PPO and the acrylics used for the detector tank. The samples were irradiated for ten minutes at the FRM2 with a thermal neutron flux of (1.63±0.05).1013 cm-2s-1. Thereafter, the spectra of the irradiated samples were recorded using the germanium counting system mentioned above, mainly focussing on the isotope potassium-42. The content of potassium-40 could be determined to be of the order of 10-11 g/g.