gas for example a coating based on aluminium or nickel may be used to cover the walls of the acceleration chamber 2 and or the walls of the ionisation chamber 3 the acceleration chamber 2 and the ionisation chamber 3 are connected via a passage 7 in the form of a through hole formed in the inner wall 6 the passage 7 may be provided with a grid 8 generally made of metal an exit 9 is provided in an outer wall of the ionisation chamber 3 opposite the inner wall 6 the exit 9 may be open or fitted with a grid especially if a gas of a similar nature and at a similar pressure is present in the enclosure 4 and around the enclosure 4 if the conditions of pressure and or the nature of the gas are different the exit 9 is generally provided with a seal not shown for example in the form of a part made of synthetic material which is impermeable to gas and at least partly permeable to electrons so as to allow the electron flux generated in the source 1 to escape the seal may also be covered with a layer of metal notably based on metal with a high atomic mass of for example more than 50 with a view to generating x rays under the effect of the electron.