Spectroscopy of $^{35}$P using the one-proton knockout reaction
Résumé
The structure of $^{35}$P was studied with a one-proton knockout reaction at
88~MeV/u from a $^{36}$S projectile beam at NSCL. The $\gamma$ rays from the
depopulation of excited states in $^{35}$P were detected with GRETINA, while
the $^{35}$P nuclei were identified event-by-event in the focal plane of the
S800 spectrograph. The level scheme of $^{35}$P was deduced up to 7.5 MeV using
$\gamma-\gamma$ coincidences. The observed levels were attributed to proton
removals from the $sd$-shell and also from the deeply-bound $p_{1/2}$ orbital.
The orbital angular momentum of each state was derived from the comparison
between experimental and calculated shapes of individual ($\gamma$-gated)
parallel momentum distributions. Despite the use of different reactions and
their associate models, spectroscopic factors, $C^2S$, derived from the
$^{36}$S $(-1p)$ knockout reaction agree with those obtained earlier from
$^{36}$S($d$,\nuc{3}{He}) transfer, if a reduction factor $R_s$, as deduced
from inclusive one-nucleon removal cross sections, is applied to the knockout transitions.
In addition to the expected proton-hole configurations, other states were observed
with individual cross sections of the order of 0.5~mb. Based on their shifted
parallel momentum distributions, their decay modes to negative parity states,
their high excitation energy (around 4.7~MeV) and the fact that they were not
observed in the ($d$,\nuc{3}{He}) reaction, we propose that they may result
from a two-step mechanism or a nucleon-exchange reaction with subsequent neutron
evaporation. Regardless of the mechanism, that could not yet be clarified, these
states likely correspond to neutron core excitations in \nuc{35}{P}. This
newly-identified pathway, although weak, offers the possibility to selectively
populate certain intruder configurations that are otherwise hard to produce
and identify.
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