Imaging Germanium Spectrometer With Rotational Modulation Grid Collimators

Abstract

The WINKLER spectrometer is a matrix of nine high -purity, n -type germanium detectors developed for astrophysical observations and terrestrial radiation monitoring.The spectrometer has been fitted with a set of modulation collimator grids designed for imaging hard x -ray and gamma -ray sources by the method of Mertz, Nakano, and Kilner. This technique employs a pair of gridded collimators in front of each detector with the number of grid bars varying from one to N, where N is the number of detectors. When the collimator pairs are rotated through a full 360 degree angular range, the detector signals provide the information for a two -dimensional bandlimited Fourier reconstruction of order N.Tests of the spectrometer with single and multiple point sources as well as continuous source distributions are reported. The spectrometer field -ofview is 20 deg, and the observed FWHM of the point spread function is 1.6 deg, in good agreement with simulation results.Images have been obtained for gamma -ray energies from 60 keV to 1.3 MeV, although transmission through the grids reduces contrast at the higher energies. Potential capabilities of the spectrometer for locating the position of single point sources or resolving structure in closely-spaced source distributions are discussed, as well as proposed upgrades to improve angular resolution.