Absolute calibration of a compact gamma-ray spectrometer for high intensity laser plasma experiments

Copious amounts of gamma rays are generated in high intensity laser matter experiments, either by bremsstrahlung or by inverse Compton scattering. Measurements of multi-MeV gamma rays in such experiments provide direct indication of hot electrons generated during the interaction. A spectrometer based on forward Compton scattering was recently tested by Espy et al.. We report on an improved design of a similar spectrometer which is significantly more compact (70 cm x 25 cm x 25 cm) and thus extremely convenient to use in laser plasma experiments. In this presentation, we describe the design considerations and recent results of an absolute calibration of the gamma-ray spectrometer. The calibration was performed using a bremsstrahlung source at different electron energies, i.e. 11, 13, 15 and 18 MeV. Experimental spectra show a systematic increase of the maximum cut-off energy, temperature and flux. The results indicate that the spectrometer is effective for an energy range of 4 -20 MeV with 5-10% energy resolution.