A magnetically guided slow positron beam with variable energy up to 50 keV has been built in the positron laboratory of CERI (Orleans). The accelerator is coupled with a Doppler broadening spectrometer. A 4 mum polycrystalline W foil is used to moderate the positrons. The speed selection of positrons is performed with a 90 degrees elbow. An activity of about 6 mu Ci was measured on the sample holder with a conventional 60 mCi Na-22 positron source of 6 mm in diameter. We measured the electron momentum distributions in the first micron of heavy ions (Pb, Ar) implanted n-type Ge. Different ion fluences are compared.