Extraction of neutron-capture cross sections on ⁹²Zr using the charge-exchange Oslo method

The ⁹³Nb(t,³He) reaction at 115 MeV/u was studied to demonstrate that nuclear level densities and $γ$-ray strength functions can be extracted from charge-exchange reactions at intermediate energies using the Oslo technique. The matrix of excitation energy in ⁹³Zr, reconstructed from the (t,³He) reaction, versus the energy of $γ$ rays emitted by the excited ⁹³Zr nuclei, was obtained in an experiment with the S800 Spectrograph operated in coincidence with the GRETINA $γ$-ray detector. The extracted level density and $γ$-ray strength function obtained by applying the Oslo method to this matrix were used to estimate the ⁹²Zr(n,$γ$)⁹³Zr cross section by combining the new results with other experimental data and theoretical calculations for E1 and M1 strength functions at higher energies. Good agreement with direct measurements of the ⁹²Zr(n,$γ$)⁹³Zr cross section was found. The contribution from the upbend in the extracted $γ$-ray strength function was important to achieve the consistency as the neutron-capture cross section without this contribution is significantly below the direct measurements otherwise. Since charge-exchange reactions at intermediate energies have long been used for extracting Gamow-Teller strengths, the successful demonstration of the charge-exchange Oslo method enables experiments in which (n,$γ$) cross sections and Gamow-Teller strengths can be measured simultaneously, which is of benefit for astrophysical studies.