TIARA: A large solid angle silicon array for direct reaction studies with radioactive beams

Abstract

A compact, quasi-4π position sensitive silicon array, TIARA, designed to study direct reactions induced by radioactive beams in inverse kinematics is described here. The Transfer and Inelastic All-angle Reaction Array (TIARA) consists of 8 resistive charge division detectors forming an octagonal barrel around the target and a set of double-sided silicon-strip annular detectors positioned at each end of the barrel. The detector was coupled to the γ-ray array EXOGAM and the spectrometer VAMOS at the GANIL Laboratory to demonstrate the potential of such an apparatus with radioactive beams. The reaction, well known in direct kinematics, has been carried out in inverse kinematics for that purpose. The observation of the ground state and excited states at 7.16 and 7.86 MeV is presented here as well as the comparison of the measured proton angular distributions with DWBA calculations. Transferred l-values are in very good agreement with both theoretical calculations and previous experimental results obtained in direct kinematics

Emergence of theN=16shell gap inO21

“…The large energy difference between the 3/2 + and 1/2 + states is indicative of the emergence of the N=16 shell gap, which is estimated to be 5.1(11) MeV. For the higher-lying resonance, which has a character consistent with a spinparity assignment of 3/2 + or 7/2 − , a 71(22)% branching ratio to the first 2 + state in 20 O has been observed. The magic numbers that explain the structure of nuclei close to stability have their origin in the gaps created by the single-particle eigenstates of the mean-field.…”

“…In addition, a second unbound level was observed. Whilst a definitive spin-parity assignment could not be made, the observation of neutron decay to the first excited 2 + state of 20 O indicates a considerable coreexcited component. The experiment was performed at the SPIRAL facility using a 10.5 AMeV 20 O beam with an integrated intensity of 1.65 x 10 9 particles and a typical rate of 10 4 pps.…”

“…The bound first-excited state of 21 O was determined to lie at 1.21(4) MeV. As in our earlier work [20,21], the target was also surrounded at 90 • by 4 EXOGAM clover detectors in a close packed geometry (photopeak efficiency, including addback [20], of 10(1)% at 800 keV). The coincident gamma-rays indicated an excitation energy of 1.213 (7) MeV.…”

“…The spectroscopy of 21 O has been investigated using a radioactive 20 O beam and the (d,p) reaction in inverse kinematics. The ground and first excited states have been determined to be J π = 5/2 + and 1/2 + respectively.…”

“…The target was a 0.59 mg/cm 2 thick CD 2 foil. The (d,p) reaction channel was selected using the identification of the beam-like residue in the VAMOS spectrometer and the kinematics of the proton, as measured using the TIARA silicon array [20] from 36 • to 169 • in the laboratory frame. The excitation energy in 21 O and the centre-of-mass (cm) scattering angle were obtained from the measured energy and angle of the recoiling protons.…”