Observation of multiple passes of electrons through thin internal targets of a betatron

We present observations of the radiation produced by electrons making multi ple passes through thin targets in a betatron. We show the influence of mul tiple passes on the distribution of electrons and on the angular distributi on of bremsstrahlung. Measurements of electron-beam spots and bremsstrahlun g distributions have been performed for 15-33 MeV electrons and target thic knesses of 10(-5)-3.5 x 10(-3) radiation lengths. Estimates of the number o f passes were obtained for thin targets by comparing them with 1.3 mm thick W and 1.5 mm thick Cu targets, through which electrons pass only once. The numbers of 33 MeV electron passes observed were 460 for 3 mum Mylar, 200 f or 20 mum Be, 171 for 5 mum Cu and 30 for 15 mum Cu foils. The multiple pas s effect can be useful in the future for increasing the efficiency of novel X-ray sources, such as transition and parametric radiators. (C) 2001 Elsev ier Science B.V. All rights reserved.