Design study for a 500 MeV positron beam at the Mainz Microtron MAMI

TL;DR

This paper presents a detailed design study for generating a 500 MeV positron beam at Mainz Microtron MAMI, including target geometry, magnetic optics, and expected beam parameters and yield.

Contribution

It provides a comprehensive design and expected performance analysis for a high-energy positron beam at MAMI, focusing on pair production and beam transport optimization.

Findings

Positron beam energy of 500 MeV achieved with specific target and magnetic setup.

Expected positron yield of 13.1 per second at 1 nA electron current.

Beam emittances and spot sizes are precisely estimated for the proposed setup.

Abstract

A design study has been performed for a positron beam with an energy of 500 MeV to be realized at the applied physics area of the Mainz Microtron MAMI. Positrons will be created after pair conversion of bremsstrahlung, produced by the 855 MeV electron beam af MAMI in a tungsten converter target. From the two conceivable geometries (i) pair conversion in the bremsstrahlung converter target itself, and (ii) bremsstrahlung pair conversion in a separated lead foil, the former was considered in detail. Positrons will be energy selected within an outside open electron beam-line bending magnet, and bent back by an additional sector magnet. Magnetic focusing elements in between are designed to prepare in a well shielded positron target chamber about 6 m away from the target a beam with horizontal and vertical emittances of epsilon_v = 0.055 pi mm mrad (1 sigma), and epsilon_h = 0.12 pi mm mrad (1 sigma), respectively, for a 10 micro m thick amorphous tungsten target and negligible momentum spread. At an accepted positron band width of 1 MeV, spots are expected vertically with an angular spread of 0.064 mrad and a size of 5.0 mm (FWHM), and horizontally with an angular spread of 0.64 mrad and a size of 7.7 mm (FWHM). The positron yield amounts to 13.1 per second, 1 MeV positron energy band width, and 1 nA electron beam current.