The reaction 7Li(π+,π−)7B and its implications for 7B1

The reaction 7Li(π+,π−)7B has been measured at incident pion energies of 30–90 MeV. 7Li constitutes the lightest target nucleus, where the pionic charge exchange may proceed as a binary reaction to a discrete final state. Like in the Δ-resonance region the observed cross sections are much smaller than expected from the systematics found for heavier nuclei. In analogy to the neutron halo case of 11Li this cross section suppression is interpreted as evidence for a proton halo in the particle-unstable nucleus 7B.