Unidirectional Focusing Dipole Magnet as a Gamma-Ray Spectrometer in the Intermediate MeV Range

We report on the design of a compact electron and positron spectrometer based on tapered NdFeB magnets. Using RADIA and fourth order Runge-Kutta Particle Tracking code, 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. We address the effect of incident beam divergence on the efficiency and resolution of the detector. We demonstrate a prototype matching the design specifications and discuss the adaptation of the prototype for the detection of gamma-ray in the multi-MeV range. We confirm the energy transfer of gamma-ray to electrons and positrons via Compton Scattering and Pair Production in a low or intermediate Z-converter by using Monte Carlo based FLUKA simulations. We show that for suitable thickness of the converter, the incident gamma-ray energy spectrum directly correlates with the spectrum of electrons and positrons in a constrict acceptance angle. We investigate the effects of divergence on the spectral deconvolution of incident gamma photon. We present the first series of data obtained with this spectrometer at Texas Petawatt Facility.