Positron yields from a 45-MeV L-band electron linear accelerator

Abstract

The characteristics of a positron beam produced with a converter system located after the first section of a three-section, L-band accelerator have been investigated. The number of positrons emerging from the accelerator per electron incident on the converter was determined as a function of electron bombarding energy over the range from 8 to 15 MeV. Other properties of the positron beam which were studied include the energy spectrum, the spot size, and electron contamination. The influence of the magnetic focussing fields on the positron yields was investigated, and the energy of the positrons leaving the converter which were preferentially accelerated was measured. Results of the yield measurements indicate that the present system, operating with an electron beam power of 4.5 kW on the converter, should yield an average positron beam current at the end of the accelerator of about 1.4 × 10 −9 A within a 2 1 2 % energy interval at 25 MeV. The energy of the positron beam can be varied continuously from several MeV to 33 MeV, and the current within a 2 1 2 % energy bin is approximately proportional to positron energy. Calculations of positron yields and beam characteristics performed for electron bombarding energies of 9 MeV and 17 MeV gave results in good agreement with experimental observations. The calculations also give a conceptual picture of the positron production in the converter and subsequent trapping and acceleration by the accelerator.