To produce a detector for gamma ( gamma ) spectroscopy operating under ultra-high vacuum (UHV) inside an enclosure, the enclosure cover is electron-beam welded in vacuo to the enclosure body (1) after the detector (4) has been inserted therein. The detector is protected against thermal stress during the welding process. Vapours produced during welding are condensed outside the detector chamber (3a, 3b), for which purpose there is a chamber (18) in the enclosure which is separated from the detector chamber.