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Very feeble radioactivity measurements

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Looking for very feeble radioactive decays has been a soaring thema at CENBG for the last fifteen years, thanks to the neutrino group and its quest for the neutrino-less double-beta decay. Indeed, the long sought-after signal in this latter case is so seldom that even the materials used by the NEMO detector have to be as radioactive-free as possible in order to avoid any fake two-neutrino emission. Therefore, one had to develop some method to monitor and select all building bricks for the detector.

Gamma spectometry with germanium detectors has a great many advantages in this domain, which led the neutrino group together with Canberra-Eurysis to develop low background noise detectors. This "low background" assumption means that these detectors are able to measure very feeble activities, 4 to 5 orders of magnitude lower than that of natural radioactivity. For instance, these detectors are capable of measuring a few mBq/kg, while the human body contains already around 50 Bq/kg of 40K!

Obviously, to obtain such a sentivity, the spectrometer herself has to be made of radioactive-free materials! Moreover, one has to shade her from the natural radioactivity, mostly a mix of several gamma rays. This implies the need for a wraparound shelter of heavy material like lead and copper. A 400 cm3 co-axial germanium detector being installed in Modane is shown below on the right-hand side.

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Germanium detector

For the time being, there are two co-axial Ge spectrometes at LSM (Laboratoire Souterrain de Modane), both 400 cm3 in volume. Both detectors are measuring materials for the BiPo detector and the SuperNEMO prototypes. Some powders of 82Se (a double-beta emitter) are also being investigated in order to ascertain the natural radioactivity levels and to check pysical and chemical purification methods used in the USA and Russia. Another foreseen candidate for the double-beta decay in SuperNEMO, 150Nd is also to be measured by these apparatuses.

In addition to the Ge detectors from LSM and before the PRISNA facility opened, we had several gamma spectrometers installed in the basement of building of the University of Bordeaux 1. They were partly dedicated to radio-purity measurements for the SuperNEMO experiment. In this former hall, 2.5 meters of concrete was enough to insure an almost complete wash-out of the hadron component of cosmic rays (protons and neutrons), but could in no way hinder the muon flux. Therefore, a novel type of shielding was of the order to minimize all the effects from muons and fast neutrons coming from the spallation of residual hadrons with lead. A borated polyethylene layer helped thermalizing and capturing fast neutrons before any interaction with the germanium crystal. On top of the detectors, plastic scintillators acted as a cosmic-ray veto. All teses spectrometers are now at hand at the PRISNA facility on the CENBG grounds.

Loosing an order of magnitude for the background noise compared to the one in LSM is a burden. Albeit, having gamma spectrometers at sea level can be justified in the following way:

  1. before sending samples to Modane for a final analysis, they are selected (time-sparing) and this diminishes all riske of polluting the LSM detectors.
  2. other research themas can be developed locally, like multi-disciplinary ones, for instance for geology, oceanography, archaelogy, for INRA, or even with the DGCCRF (Direction Générale de la Concurrence, de la Consommation et de la Répression des Fraudes).
  3. this is a real educational tool for the understanding of radioactivity measurement techniques.

Apart from the ultra-low background gamma spectrometry, the neutrino group pursues another activity at the LSM, still bearing in mind our specialty for feeble radioactivities. This activity has to do with radon monitoring of the gas used in the NEMO experiment. Indeed, because of mechanical leaks, there is a contamination in the order of 15 mBq/m3 of the gas mixture (He + 4% alcohol) due to the radon content in the LSM air. 214Bi being the great grand-daughter of radon induces a disastrous huge component in the bacground noise of the experiment. To avoid this contamination, the NEMO detector has to be surrounded by an "impermeable" tent inside which blows a 150 m3/h pollution-free air. This air is the laboratory air going through an active carbon column cooled down to -50°C. Monitoring the radon level exhiting from the anti-radon factory and inside the tent is done by two identical, low-background radon detectors which were developed by aour japanese collegues from the University of Saga. Their sensitivity goes down to 1 mBq/m3 which corresponds to the detection of one alph event per day.

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The air in LSM contains some 15 Bq/m3. At the exit of the factory, it drops to around 20 mBq/m3 and nears 150 mBq/m3 inside the tent. Regarding the NEMO gas, 2 mBq/m3 are obtained, a well-accepted value by the collaboration.

In the following figure, the evolution of radon content in the NEMO tent is shown for the period January 11, 2005 to September 13, 2005. Each point corresponds to a 6 hour measurement. What shows up is that every incident (maintenance, current breakdown, overture of the shielding for calibration purpose...) is followed by a very rapid revovery of the radon level, just a matter of a few hours.

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Spectre double beta