Matter radius of the doubly-magic ⁵⁶ Ni measured in a storage ring

The matter radius of the doubly magic 56Ni was extracted from a measurement of the differential cross section by employing, for the first time, elastic proton scattering in inverse kinematics with a radioactive beam at Ekin= 390.2 MeV/nucleon circulating in a storage ring and passing an internal hydrogen gas-jet target with a revolution frequency of around 2 MHz. The novel experimental scheme is based on UHV-compatible Si detectors operated as active vacuum windows, which were implemented in the ESR storage ring at GSI. A matter radius <rm2>1/2=3.74-0.06+0.03 fm was extracted for the doubly-magic self-conjugate nucleus 56Ni.