Ionization Quenching Factor Measurement of Helium 4
  The ionization quenching factor (IQF) is defined as the fraction of energy released by a recoil in a medium through ionization compared with its total kinetic energy. At low energies, in the range of a few keV, the ionization produced in a medium falls rapidly and systematic measurements are needed. We report measurements carried out at such low energies as a function of the pressure in He4 at 350, 700, 1000 and 1300 mbar. In order to produce a nucleus moving with a controlled energy in the detection volume, we have developed an Electron Cyclotron Resonance Ion Source (ECRIS) coupled to an ionization chamber by a differential pumping. The quenching factor of He4 has been measured for the first time down to 1 keV recoil energies. An important deviation with respect to the phenomenological calculations has been found allowing an estimation of the scintillation produced in He4 as a function of pressure. The variation of the IQF as a function of the percentage of isobutane, used as quencher, is also presented. 
D. Santos (LPSC), F. Mayet (LPSC), O. Guillaudin (LPSC), Th. Lamy   (LPSC), S. Ranchon (LPSC), A. Trichet (LPSC), P. Colas (...