Gradient Magnet Design for Simultaneous Detection of Electrons and Positrons in the Intermediate MeV Range

We report the design and development of a compact electron and positron spectrometer based on tapered Neodymium Iron Boron magnets. We show that the tapered design forms a gradient magnetic field component allowing energy dependent focusing of the dispersed charged particles along a chosen detector plane using RADIA, a code developed by European Synchrotron Radiation Facility for solving three-dimensional magnetostatics configuration, and a fourth order Runge-Kutta Particle Tracking code. The mirror symmetric design allows for simultaneous detection of pairs i.e. electrons and positrons with energies from 2 MeV to 500 MeV. We have developed a prototype matching the design specifications. We investigate the effects of beam divergence on the energy resolution and signal conversion efficiency for a photo-stimulated luminescencebased Imaging Plates (IPs). The optimal entrance aperture of the magnet is found to be elliptical and bigger than that of conventional pinhole aperture-based spectrometer designs even for a divergent beam originating from a point source at 20 cm away (i.e. solid angle of ~8 milli steradians). The signal efficiency in BAS-IP of SR type ranges from 1% to 5% for a parallel beam incident on a circular aperture of 20 mm diameter type at a chosen detection plane whereas it drops by up to a factor of 3 in the presence of divergence of ~ 8 milli steradians. The proposed gradient magnet is suitable for the detection of low flux and/or monoenergetic type electron/positron signals with finite transverse sizes. It offers unparalleled advantages for gammaray spectroscopy in the intermediate MeV range.