The elusive β − p + decay was observed in 11 Be by directly measuring the emitted protons and their energy distribution for the first time with the prototype Active Target Time Projection Chamber (pAT-TPC) in an experiment performed at ISAC-TRIUMF. The measured β − p + branching ratio is orders of magnitude larger than any previous theoretical model predicted. This can be explained by the presence of a narrow resonance in 11 B above the proton separation energy.

In the Letter, we presented the first direct observation of the elusive β −-delayed proton emission (β − p þ) in 11 Be using a time projection chamber. There is an error in the extracted logðftÞ, which should read 2.8(4). The corrected logðftÞ value does not affect the conclusions presented in the Letter, since it was only used to assert the allowed character of the transition, a conclusion that is not changed. We would like to point out that, assuming a pure Gamow-Teller (GT) transition, it yields BðGTÞ ¼ 5.5 þ8.3 −3.3 , thus, including the BðGTÞ < 3 limit for a single neutron decay within one sigma. It should be noted that, due to the small energy window, the uncertainty in the resonance energy amounts to nearly 75% of the error budget for the logðftÞ value. Thus, a more precise measurement of the resonance energy could bring the B(GT) to within theoretical limits without significantly affecting the measured branching ratio.

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Background: Neutron-rich nuclei around neutron number N = 60 show a dramatic shape transition from spherical ground states to prolate deformation in 98 Sr and heavier nuclei. Purpose: The purpose of this study is to investigate the single-particle structure approaching the shape transitional region. Method: The level structures of neutron-rich 93,94,95 Sr were studied via the 2 H( 94,95,96 Sr, t ) one-neutron stripping reactions at TRIUMF using a beam energy of 5.5 AMeV. γ -rays emitted from excited states and recoiling charged particles were detected by using the TIGRESS and SHARC arrays, respectively. States were identified by gating on the excitation energy and, if possible, the coincident γ radiation. Results: Triton angular distributions for the reactions populating states in ejectile nuclei 93,94,95 Sr were compared with distorted wave Born approximation calculations to assign and revise spin and parity quantum numbers and extract spectroscopic factors. The results were compared with shell-model calculations and the reverse (d, p) reactions and good agreement was obtained. Conclusions: The results for the 2 H( 94 Sr, t ) 93 Sr and 2 H( 95 Sr, t ) 94 Sr reactions are in good agreement with shellmodel calculations. A two-level mixing analysis for the 0 + states in 94 Sr suggest strong mixing of two shapes. For the 2 H( 96 Sr, t ) 95 Sr reaction the agreement with the shell-model is less good. The configuration of the ground state of 96 Sr is already more complex than predicted, and therefore indications for the shape transition can already be observed before N = 60.

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