Results of the material screening program of the NEXT experiment

The 'Neutrino Experiment with a Xenon TPC (NEXT)', intended to investigate neutrinoless double beta decay, requires extremely low background levels. An extensive material screening and selection process to assess the radioactivity of components is underway combining several techniques, including germanium gamma-ray spectrometry performed at the Canfranc Underground Laboratory; recent results of this material screening program are presented here.


Introduction
The NEXT experiment [1] will operate at the Laboratorio Subterrneo de Canfranc (LSC), Spain, a high- * Attending speaker pressure xenon time projection chamber (TPC) to search for neutrinoless double beta decay events of 136 Xe using 100 kg of enriched xenon at 90%. As in any experiment investigating rare event phenomena, ultra-low background conditions are a must and materials used in Available online at www.sciencedirect.com www.elsevier.com/locate/nppp the set-up have to be carefully selected. A thorough material screening program was undertaken to evaluate the radioactivity of all the relevant components of NEXT [2,3]; new results are presented here.
This screening program is mainly based on germanium γ-ray spectrometry using ultra-low background detectors from the Radiopurity Service of LSC (in particular, those named GeOroel, GeAnayet, GeTobazo, Ge-Latuca) operated at a depth of 2450 m.w.e.. Detectors are p-type close-end coaxial 2.2-kg High Purity germanium detectors, from Canberra France. For the measurements presented here, shield consisted of 5 cm of copper in the inner part surrounded by 20 cm of low activity lead, with nitrogen flush to avoid airborne radon intrusion. Detection efficiency is estimated for each sample by GEANT4 simulation. Complementing germanium spectrometry results, measurements based on Glow Discharge Mass Spectrometry (GDMS) and Inductively Coupled Plasma Mass Spectrometry (ICPMS) have been also carried out. GDMS is performed by Evans Analytical Group in France, providing concentrations of U, Th and K. An ICPMS measurement was made at CIEMAT (Unidad de Espectrometria de Masas) in Spain.

Results
Materials analyzed deal with the shielding, pressure vessel, field cage and electroluminescence (EL) components and the energy and tracking readout planes. Results obtained after those presented in [2,3] are summarized in table 1 and described in the following; for germanium measurements, reported errors correspond to 1σ uncertainties and upper limits are given at 95% C.L.. Uncertainties for GDMS results are typically of 20%.
Lead and copper from different suppliers to be used as shielding were studied [2,3]. Finally, refurbished lead from the OPERA experiment with 80 Bq/kg of 210 Pb will be used for external shielding (#1-2) and CuA1 (or ETP) copper will be used for inner shield (#3-4). For the pressure vessel, several samples of 316Ti Stainless Steel were initially screened with germanium detectors: 10-mm-thick for body, 15-mm-thick for end-caps, 50mm-thick for flanges. Now, complementary results have been obtained from GDMS analysis (#5-7).
In summary, complementary activity measurements based on ICPMS, GDMS and germanium spectrometry performed at LSC have been carried out to help both in the design of the set-up and in the construction of the background model of the NEXT experiment. Radiopure enough samples of copper for inner shielding, stainless steel for pressure vessel and polyethylene for field cage have been found: expected contributions from 214 Bi+ 208 Tl at the region of interest are 9.7, 2.9 and 9.4 10 −5 keV −1 kg −1 y −1 respectively. An extensive work has been carried out, but the screening program is still going on and SiPMs and shielding structure components are now under analysis.