  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.
