  We have performed the first direct measurement of the 83Rb(p,g) radiative
capture reaction cross section in inverse kinematics using a radioactive beam
of 83Rb at incident energies of 2.4 and 2.7 A MeV. The measured cross section
at an effective relative kinetic energy of Ecm = 2.393 MeV, which lies within
the relevant energy window for core collapse supernovae, is smaller than the
prediction of statistical model calculations. This leads to the abundance of
84Sr produced in the astrophysical p process being higher than previously
calculated. Moreover, the discrepancy of the present data with theoretical
predictions indicates that further experimental investigation of p-process
reactions involving unstable projectiles is clearly warranted.
