30 in positions such that any of the axes common to the targets and filters may be aligned with the main axis 14 of the accelerator the embodiment shown in fig5 is only distinguished from the preceding one in that it comprises two independent supports 40 41 support 40 contains several targets 32a 33a each having a main axis of symmetry whereas support 41 contains several filters 34a 35a each having a main axis of symmetry the positioning means stops 20c with which supports 40 and 41 cooperate allow the axis of symmetry of any filter and the axis of symmetry of any target to be aligned with the main axis 14 of the accelerator with respect to the embodiment shown in fig4 the number of target filter combinations is doubled with the same number of targets and filters depending on the beam characteristics desired the filter may be made from different materials more especially tungsten lead copper titanium stainless steel or graphite in the examples which have just been described the supports are drawers with rectilinear movement as mentioned above they are made essentially from lead but they will advantageously comprise steel slides not shown in the simple case shown the mechanical handling system may be manual if it is desired to have more than two targets and or filters motor driven solutions may be adopted with remote control and servo controlled positioning all these handling systems being within the scope of a man skilled in the art positioning control may also be provided by means of microcontactors and a microprocessor logic system monitoring the state of these contactors fig5.