EPJ A Highlight - Achieving high resolution in binary nuclear reactions with outgoing fast neutrons – at last!

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Published on 28 October 2014

To date, the two-nucleon pick-up and stripping counterparts of the (p,t) and (t,p) reactions, the (³He,n) and (n, ³He) reactions, have been poorly investigated due to the difficulty in performing high-resolution measurements of fast-neutron energies. The best time-of-flight (³He,n) measurements report resolutions not better than 250 keV. This lack of experimental resolution has hindered a full understanding of the role of proton pairing in nuclei.

In the present work, this experimental constraint is addressed by detecting the γ-ray decay of populated excited states in an array of escape-suppressed HPGe detectors in coincidence with neutron detectors placed near θ_lab = 0^o. High selectivity is obtained and a large rejection factor of unwanted reaction channels, of the order of 1 in 10³, is demonstrated. The population strength of excited states is deduced with an energy resolution better than 3 keV. This allows the proton occupancy of excited states, populated selectively in direct two-proton stripping reactions, to be measured.

We use the ⁵⁹Co(³He,n)⁶¹Cu reaction at E_lab = 22.5 MeV to populate 2p-1h proton states across the Z = 28 closed shell. The observed relative L = 0 two-proton stripping strengths are compared with large-basis shell-model calculations. Discrepancies with theory suggest that proton occupancies of the f7/2 shell are not currently well reproduced. Systematic measurements in various mass regions are underway in order to address the present lack of high-resolution experimental data.