DALI2: A NaI(Tl) detector array for measurements of γ rays from fast nuclei

Abstract

A NaI(Tl) detector array called DALI2 (Detector Array for Low Intensity radiation 2) has been constructed for in-beam γ-ray spectroscopy experiments with fast radioactive isotope (RI) beams. It consists typically of 186 NaI(Tl) scintillators covering polar angles from ∼15 • to ∼160 • with an average angular resolution of 6 • in full width at half maximum. Its high granularity (good angular resolution) enables Doppler-shift corrections that result in, for example, 10% energy resolution and 20% full-energy photopeak efficiency for 1-MeV γ rays emitted from fast-moving nuclei (velocities of v/c ≃ 0.6). DALI2 has been employed successfully in numerous experiments using fast RI beams with velocities of v/c = 0.3 − 0.6 provided by the RIKEN RI Beam Factory.

Collectivity of neutron-rich Cr and Fe toward N=50

“…The reach is aided by the novel combination of the DALI2 [8] and MINOS [9] systems, which will be described below, resulting in an excellent experimental sensitivity. Several experiments within the SEASTAR campaigns (first in spring 2014, second in spring 2015) already targeted specific isotopes, or isotopic chains, for which discussions with respect to their underlying shell structure are ongoing.…”

Triaxiality of neutron-rich Ge84,86,88 from low-energy nuclear spectra

“…Reaction vertices were determined by a time projection chamber (TPC) surrounding the LH 2 target. De-excitation γ rays were observed in the NaI(Tl) scintillator array DALI2 [36] covering polar angles from 10 to 128 deg with respect to the central beam axis and the center of MINOS. By a simulation within the GEANT4 framework [37], full-energy peak detection efficiencies of 35% (23%) 85 Ge(p,pn) 84 Ge, (b) 87 As(p,2p) 86 Ge, and (c) 89 As(p,2p) 88 Ge are shown as black data points, superimposed with the fit of the whole spectrum (black solid line).…”

“…A calibration error of 1.5 keV and an energy resolution of 9% (6%) FWHM at 662 keV (1.332 MeV) was obtained, in agreement with the analyses from Refs. [36,38,39]. The Doppler-corrected γ -ray spectra of 84,86,88 Ge are shown in Fig.…”

In-Beam Performances of the MINOS TPC for the Spectroscopy of Very Exotic Nuclei

“…The SEASTAR (Shell Evolution And Search for Two-plus energies At the RIBF) program [12] combines the MINOS device and the DALI2 gamma array [13] for in-beam spectroscopy experiments. In May 2014, the first physics campaign took place at the F8 area of the Radioactive Isotope Beam Factory (RIBF) with the setup shown in Fig.…”