The matter radius of the doubly magic $$^{56}$$
56
Ni 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 $$E_{kin}=390.2$$
E
kin
=
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 $$<r_{m}^{2}>^{1/2}=3.74^{+0.03}_{-0.06}$$
<
r
m
2
>
1
/
2
=
3
.
74
-
0.06
+
0.03
fm was extracted for the doubly-magic self-conjugate nucleus $$^{56}$$
56
Ni.