Neutron decay of ${}^{15}\mathrm{C}$ resonances by measurements of neutron time-of-flight

The neutron decay of the resonant states of light neutron-rich nuclei is an important and poorly explored property, useful to extract valuable nuclear structure information. In the present paper the neutron decay of the ${}^{15}\mathrm{C}$ resonances populated via the two-neutron transfer reaction ${}^{13}\mathrm{C}({}^{18}\mathrm{O},{}^{16}\mathrm{O}n)$ at 84-MeV incident energy is reported for the first time using an innovative technique which couples the MAGNEX magnetic spectrometer and the EDEN neutron detector array. Experimental data show that the resonances below the one-neutron emission threshold decay to the ${}^{14}\mathrm{C}$ ground state via one-neutron emission with an almost $100\%$ total branching ratio, whereas the recently observed ${}^{15}\mathrm{C}$ giant pairing vibration at 13.7 MeV mainly decays via two-neutron emission.

Figure 1

Schematic of the time diagram used for the determination of the time-of-flight of the neutrons (or $\gamma$ rays) $\mathrm{TO}{\mathrm{F}}_{\mathrm{EDEN}}$ from the target point to each EDEN detector [see Eq. (1)]. $t_0$ is the time at which the reaction takes place. $T_{\mathrm{TDC}}$ is the time difference between the FPD and the EDEN time signal delayed by a fixed quantity $\mathrm{\Delta }{T}_{\mathrm{delay}}$.

Figure 2

(a) Example of fast versus slow distribution relative to a single EDEN detector and a single silicon detector for the ${}^{13}\mathrm{C}({}^{18}\mathrm{O},{}^{17}\mathrm{O})$ data. (b) Plot of $\mathrm{TO}{\mathrm{F}}_{\mathrm{EDEN}}$ for the events gated on γ rays as drawn in panel (a). In the inset a zoomed view of the $\mathrm{TO}{\mathrm{F}}_{\mathrm{EDEN}}$ plot is shown.

Figure 3

Neutron time-of-flight $\mathrm{TO}{\mathrm{F}}_{\mathrm{EDEN}}$ gated on the ${}^{16}\mathrm{O}$ ejectiles detected by MAGNEX (black spectrum) superimposed on the randomly distributed background ${\mathrm{TOF}}_{\mathrm{EDEN}}^{\mathrm{random}}$ (red dashed spectrum).

Figure 4

${}^{15}\mathrm{C}$ excitation energy spectrum for the ${}^{13}\mathrm{C}({}^{18}\mathrm{O},{}^{16}\mathrm{O})$ reaction at 84-MeV incident energy and $3^{\circ }<{\theta }_{\mathrm{lab}}<{14}^{\circ }$. The color filled areas are the different regions selected for the study of the neutron decay spectra in Figs. 6 and 7. $S_n$ and $S_{2n}$ indicate the one- and two-neutron separation energies, respectively, of ${}^{15}\mathrm{C}$.

Figure 5

${}^{15}\mathrm{C}$ level diagram and proposed neutron decay scheme. The observed neutron decays are indicated by dotted arrows, and the deduced γ decays are indicated by the dashed arrows.

Figure 6

(a), (c), (e), (g), and (i) Neutron energy spectra gated on different ${}^{15}\mathrm{C}$ excitation energy regions for the experimental data (solid line) and for the randomly distributed events (dashed line). (b), (d), (f), (h), and (j) Energy spectra with background subtraction. The dot-dashed lines correspond to the decay energy of each ${}^{15}\mathrm{C}$ state $E_n=E_x-S_n$.

Figure 7

(a) and (c) Neutron energy spectra gated on different ${}^{15}\mathrm{C}$ excitation energy regions for the experimental data (solid line) and for the randomly distributed events (dashed line). (b) and (d) Energy spectra with background subtraction.