FIG. 4: Measured parallel momentum distributions for the ground state and the states at 4382, 4766, 4960 and 5096 keV (displayed in red in the level scheme) of \( ^ { 3 5 } \mathbf { P } \). The bottom panel shows that for \( ^ { 3 6 } \mathbf { P } \) nucleus, as produced from \( ^ { 3 6 } \mathrm { S } \). The verti-cal dashed line, crossing all panels, represents the centroid of the momentum distribution of \( ^ { 3 6 } \mathbf { P } \) nucleus, \( p _ { 0 } \), shifted by the evaporation of a neutron, i.e. \( p _ { 1 } \)=(A-1)/A \( p _ { 0 } \). It is seen that the distributions for all states labeled red in the level scheme are shifted relative to that of the \( ^ { 3 5 } \mathrm { P } \) ground state, that is produced via direct one-proton knockout.

section of about 2.5 mb was measured at similar beam energy for the sd-shell nucleus \( { } ^ { 2 2 } \mathrm { M g } \) [32]. It follows that the TS mechanism would have a rather low cross section, of the order of 0.10 mb, that is a factor of 20 lower than we observe for the sum over all states with shifted mo-mentum distributions. It is therefore likely that a more direct process is needed to account for the measured cross section.

Charge-exchange reactions occur more favorably with an allowed GT operator (\( \Delta L \)=0,\( \Delta J = \)=0 \( , \pm \) 1, \( \Delta \pi \)=no) that would lead to the production of \( 1 + \) states in \( ^ { 3 6 } \mathbf { P } \) starting from the \( 0 ^ { + } \) ground state of \( ^ { 3 6 } \mathrm { S } \). Higher momen-tum transfers are also observed when very strict kine-matic condition for the reaction as well as suitable target excitations cannot be selected, as in the present exper-iment. Moreover, having a closed neutron sd shell, the exchange of an sd proton to an sd neutron orbital that would lead to \( 1 ^ { + } \) states in \( ^ { 3 6 } \mathrm { S } \) is strongly hindered by Pauli blocking. Rather, transfer to the f p or to the high-est \( g - s d \) orbits must become largely competitive. The non-feeding of any of the known \( 1 + \) states in \( { } ^ { 3 6 } \mathrm { P } \) but the population of the negative parity states \( 4 ^ { - } \) (g.s.), \( 3 ^ { - } \) (250 keV) and of the tentatively assigned \( 2 ^ { - } \) (425 keV) through low-energy charge-exchange reactions of Ref. [33] are in line with the above assumptions. In the present experiment, several \( \gamma \) rays are observed in coin-cidence with \( ^ { 3 6 } \mathbf { P } \) (see Fig. 5). However, none of them arise from the deexcitation of the \( ^ { 1 + } \) states at 1303 keV and 2281 keV that were populated in the \( \beta \) decay of \( ^ { 3 6 } \mathrm { S i } \) to \( { } ^ { 3 6 } \mathrm { \mathbf { C } } \) [34] and whose decay occurs through \( \gamma \) rays of 878 keV and (934, 977, 1858 keV), respectively, as in Ref. [33]. Even if the present statistics does not allow ex-tracting \( \gamma \)-\( \gamma \) coincidences, the observation of transitions at 250 keV and about 170 keV suggest the feeding of the \( 3 ^ { - } \) and \( 2 ^ { - } \) states, respectively. Other transitions are ob-served, but none (except perhaps the 2020(20) keV from[35]) were reported in previous works. Interestingly, one transition is seen at 4050 keV, i.e. about 600 keV above the neutron emission threshold of 3.465 MeV. This state seems to decay preferentially through a \( \gamma \)-ray transition rather than neutron decay to the \( 1 / 2 ^ { + } \) ground state of \( ^ { 3 5 } \mathrm { P } \).

FIG. 5: Doppler-corrected \( \gamma \)-decay spectrum for the \( ^ { 3 6 } \mathrm { { P } } \) nu-cleus populated from \( ^ { 3 6 } \mathrm { S } \) in a nucleon-exchange reaction.

The last hypothesis is that these momentum-shifted distributions result from the proton knockout from a state excited in inelastic scattering. Such a TS reaction was discussed as one possibility to account for the obser-vation of a \( \gamma \)-decaying neutron-unbound state in \( ^ { 3 5 } \mathrm { S i } \) at 3.611 MeV (\( S _ { n } \)=2.470(40) MeV) populated in the \( ^ { 3 6 } \mathrm { S i } \)(−1n) reaction [22]. As shown in Fig. 4 of Ref. [22], the momentum distribution associated with the production of this unbound state appears to lie dominantly in the low-momentum tail of the \( ^ { 3 5 } \mathrm { S i } \) distribution, possibly in-dicating that its centroid is shifted to lower momentum. The production of such a state was evaluated in Ref. [22]to be of 0.8(2) mb, a value that is comparable to the par-tial cross sections for the states which are observed with a