Measurement of the quasifree $p\overrightarrow{n}\mathrm{pn}\eta$ reaction

The first measurement of the cross section of the quasifree $p\overrightarrow{n}\mathrm{pn}\eta$ reaction has been carried out at the CELSIUS storage ring using a deuterium internal cluster jet target. The energy dependence of the cross section is extracted using a fixed incident proton energy of $T_p=1350\mathrm{}\hspace{0.5em}\mathrm{MeV}$ and exploiting the Fermi momentum of the struck neutron. The data cover a range of center-of-mass excess energies from 16 to 109 MeV. The shape of the excitation function is broadly similar to that of the $p\overrightarrow{p}\mathrm{pp}\eta$ reaction, though with a cross section about a factor of 6.5 larger.