LENDA, a Low Energy Neutron Detector Array for experiments with radioactive beams in inverse kinematics

The Low Energy Neutron Detector Array (LENDA) is a neutron time-of-flight (TOF) spectrometer developed at the National Superconducting Cyclotron Lab- oratory (NSCL) for use in inverse kinematics experiments with rare isotope beams. Its design has been motivated by the need to study the spin-isospin response of unstable nuclei using (p, n) charge-exchange reactions at intermediate energies (> 100 MeV/u). It can be used, however, for any reaction study that involves emission of low energy neutrons (150 keV - 10 MeV). The array consists of 24 plastic scintillator bars and is capable of registering the recoiling neutron energy and angle with high detection efficiency. The neutron energy is determined by the time-of-flight technique, while the position of interaction is deduced using the timing and energy information from the two photomultipliers of each bar. A simple test setup utilizing radioactive sources has been used to characterize the array. Results of test measurements are compared with simulations. A neutron energy threshold of < 150 keV, an intrinsic time (position) resolution of \sim 400 ps (\sim 6 cm) and an efficiency > 20 % for neutrons below 4 MeV have been obtained.Comment: Version accepted for publication in Nucl. Instr. Methods A. Revised text, 2 new figures added (one in section 4 and one in section 7

Shapes, softness, and nonyrast collectivity in 186W

Nonyrast, excited states in neutron-rich 186W were populated via inelastic-scattering reactions using beams of 136Xe nuclei accelerated to 725 and 800 MeV. Levels populated in the reactions were investigated via particleγ coincidence techniques using the Gammasphere array of high-purity germanium detectors and the compact heavy-ion counter, CHICO2. The Kπ = 2+ (γ), Kπ = 0+ and Kπ = 2− (octupole) rotational side bands were extended to spins 14¯h,12¯ h, and 13¯h, respectively. A staggering pattern observed in the energies of levels in the Kπ = 2+ band was found to be consistent with a potential that gets softer to vibration in the γ degree of freedom with increasing spin. The odd-even staggering of states in the Kπ = 2− band was found to exhibit a phase opposite to that seen in the γ band; an effect most probably associated with Coriolis coupling to other, unobserved octupole vibrational bands in 186W

N=151Pu, Cm and Cf nuclei under rotational stress: Role of higher-order deformations

Fast-rotating N=151 isotones 245Pu, 247Cm and 249Cf have been studied through inelastic excitation and transfer reactions with radioactive targets. While all have a ground-state band built on a νj15/2[734]9/2- Nilsson configuration, new excited bands have also been observed in each isotone. These odd-N excited bands allow a comparison of the alignment behavior for two different configurations, where the νj15/2 alignment is either blocked or allowed. The effect of higher order deformations is explored through cranking calculations, which help clarify the elusive nature of νj15/2 alignments

Shapes, softness, and nonyrast collectivity in 186W^{186}\mathrm{W}

International audienceNonyrast, excited states in neutron-rich W186 were populated via inelastic-scattering reactions using beams of Xe136 nuclei accelerated to 725 and 800 MeV. Levels populated in the reactions were investigated via particle-γ coincidence techniques using the Gammasphere array of high-purity germanium detectors and the compact heavy-ion counter, CHICO2. The Kπ=2+ (γ), Kπ=0+ and Kπ=2− (octupole) rotational side bands were extended to spins 14ℏ, 12ℏ, and 13ℏ, respectively. A staggering pattern observed in the energies of levels in the Kπ=2+ band was found to be consistent with a potential that gets softer to vibration in the γ degree of freedom with increasing spin. The odd-even staggering of states in the Kπ=2− band was found to exhibit a phase opposite to that seen in the γ band; an effect most probably associated with Coriolis coupling to other, unobserved octupole vibrational bands in W186

N=151Pu,Cm and Cf nuclei under rotational stress: Role of higher-order deformations

Fast-rotating N=151 isotones 245Pu, 247Cm and 249Cf have been studied through inelastic excitation and transfer reactions with radioactive targets. While all have a ground-state band built on a νj15/2[734]9/2− Nilsson configuration, new excited bands have also been observed in each isotone. These odd-N excited bands allow a comparison of the alignment behavior for two different configurations, where the νj15/2 alignment is either blocked or allowed. The effect of higher order deformations is explored through cranking calculations, which help clarify the elusive nature of νj15/2 alignments. Keywords: Superheavy, Neutron-rich, Inelastic and transfer reactions, Rotational alignments, Higher-order deformation

Experimental study of Gamow-Teller transitions via the high-energy-resolution O 18 (He 3,t) F 18 reaction: Identification of the low-energy >super>-Gamow-Teller state

Using the high-resolution O18(He3,t)F18 reaction at 0 and at 140 MeV/nucleon, Gamow-Teller (GT) transitions were studied. A high energy resolution of 31 keV was achieved by applying dispersion matching techniques. The main part of the observed GT transition strength is concentrated in the transition to the F18 ground state (g.s.). The absolute values of the reduced GT transition strengths, B(GT), were derived up to Ex=12 MeV assuming proportionality between the B(GT) values and the reaction cross sections at 0. The B(GT) value obtained from the β decay of F18 (g.s.) →O18 (g.s.) was used to determine the proportionality constant. A total B(GT) of 4.06(5) was found and 76(1)% of the strength is concentrated to the ground state of F18. The obtained B(GT) values were compared with those from the O18(p,n)F18 reaction and the mirror symmetric β+ decay of Ne18→F18. The candidates for 1+ states with isospin T=1 were identified by comparison with the O18(p,p′) data. The results of shell-model and quasiparticle-random-phase approximation calculations suggest constructive contributions of various configurations to the F18 ground state, suggesting that this state is the low-energy super GT state

High-resolution study of Tz=+2→+1 Gamow-Teller transitions in the 44Ca(3He,t)44Sc reaction

In order to study the Gamow-Teller (GT) transitions from the Tz=+2 nucleus 44Ca to the Tz=+1 nucleus 44Sc, where Tz is the z component of isospin T, we performed the (p,n)-type (3He,t) charge-exchange (CE) reaction at 140 MeV/nucleon and the scattering angles 0∘ and 2.5∘. An energy resolution of 28 keV, that was realized by applying matching techniques to the magnetic spectrometer system, allowed the study of fragmented states. The GT transition strengths, B(GT), were derived up to the excitation energy (Ex) of 13.7 MeV assuming the proportionality between cross sections and B(GT) values. The total sum of B(GT) values in discrete states was 3.7, which was 31% of the sum-rule-limit value of 12. Shell model calculations using the GXPF1J interaction could reproduce the gross features of the experimental B(GT) distribution, but not the fragmentation of the strength. By introducing the concepts of isospin, properties of isospin analogous transitions and states were investigated. (i) Assuming isospin symmetry, the Tz=+2→+1 and Tz=-2→-1 mirror GT transitions should have the same properties, where the latter can be studied in the β decay of 44Cr to 44V. First, we confirmed that the β-decay half-life T1/2 of 44Cr can be reproduced using the B(GT) distribution from the 44Ca(3He,t) measurement. Then, the 0∘, (3He,t) spectrum was modified to deduce the "β-decay spectrum" and it was compared with the delayed-proton spectrum from the 44Cr β decay. The two spectra were mostly in agreement for the GT excitations, but suppression of the proton decay was found for the T=2 isobaric analog state (IAS). (ii) Starting from the T=2 ground state of 44Ca, the (3He,t) can excite GT states (state populated by GT transitions) with T=1, 2, and 3. On the other hand, the 44Ca(p,p') reaction can excite spin-M1 states (states populated by spin-M1 transitions) with T=2 and 3 that are analogous to the T=2 and 3 GT states, respectively. By comparing the spectra from these two reactions, a T value of 2 is suggested for several GT states in the Ex=11.5-13.7 MeV region. (iii) It has been suggested that the T=2, Jπ=0+ double isobaric analog state (DIAS) at 9.338 MeV in the Tz=0 nucleus 44Ti forms an isospin-mixed doublet with a subsidiary 0+ state at 9.298 MeV. Since no corresponding state was found in the Tz=+1 nucleus 44Sc, we suggest T=0 for the subsidiary state