The Radioactive Ion Beam Factory (RIBF) has recently been commissioned and is now capable of delivering the world's most intense secondary beams with energies between 100 and 300 MeV/u. These high energies implicate that de-excitation γ-rays observed in-flight are largely Doppler shifted in the laboratory frame. Thus, the opening angles of γ-ray detectors and the velocity spread of the de-exciting nuclei cause a Doppler broadening, and hence the intrinsic energy resolution of the detectors cannot be attained. In present state-of-the-art γ-ray spectrometers the focus lies therefore either on a high full energy peak (FEP) efficiency with scintillation detectors as in DALI2, while Ge-based spectrometers have a better energy resolution, which is, however, far away from the detectors' intrinsic values.
The new scintillation material LaBr3(Ce) possesses energy resolutions of 2 and 3 % (FWHM) at Eγ = 1333 and 662 keV, respectively, unprecedented for scintillators. Due to its highly reduced price per volume compared to Ge material and the high γ-ray absorption coefficient, LaBr3(Ce) material is presumably an excellent choice to build a next generation γ-ray spectrometer for the RIBF. We have therefore proposed to build the Scintillator based Hig-resOlution γ-ray spectrometer for Unstable Nuclei (SHOGUN). The aims for the complete 4 π array are set to an energy resolution of 3 % (FWHM) and a FEP efficiency of 40 % for 1 MeV γ-rays emitted at 100 MeV/u.
The geometry of the indiviual detector crystals shall be very simple, preferably all having and identical boxed shape. This will ensure a high amount flexibility for any desired changes in the configuration of the spectrometer. To minimize the opening angle caused Doppler broadening, we plan to granulize the 4 π array into about one thousand individual detectors. The arrangegemt will be in such a way that the Doppler broadening is kept constant for all detectors, resulting in different distances to the target depending on the detectors' angular positions. To reduce the Al housing material, part of the crystals will be put into common housings of two and three crystals, respectively.