Spin inhibition in $\gamma$-decay probabilities for states above $S_n$ in Sm and Dy nuclei

When a compound nucleus is formed at an excitation energy above the neutron-separation threshold, it is assumed that its de-excitation will proceed via neutron emission. However, if the excited nucleus is in a high-spin state, but does not have enough excitation energy to conserve angular momentum by either photon emission after neutron emission or relative angular momentum carried off by the neutron, the nucleus will de-excite via $\gamma$-emission instead. This effect, the spin inhibition, provides an insight into the structure of the compound nuclei and can aid the understanding of the distribution of the populated spin states in a compound nuclear reaction. In this work, the effects of spin inhibition on the $\gamma$-decay probabilities from states in ${}^{146,147}\mathrm{Sm}$ and ${}^{160}\mathrm{Dy}$ are presented. For high-spin states above the $S_n$, spin inhibition is able to suppress neutron emission, and de-excitation via $\gamma$-ray emission is observed for states up to 3 MeV above the $S_n$.

Figure 1

Example experimental data obtained from the ${}^{148}\mathrm{Sm}(p,d){}^{147}\mathrm{Sm}$ reaction measured with Hyperion: (a) Linearized particle identification (PID) plot used to select the (p,d) reaction that populates ${}^{147}\mathrm{Sm}$ nucleus. Red contours denote coincidence gates applied to select protons, deuterons and tritons in the outgoing particle channel. (b) $E_{ex}\text{−}{E}_{\gamma }$ matrix. The red dashed line denotes $E_{ex}=E_{\gamma }$. Discrete transitions appear to dominate below about 3.5 MeV, after which point the $\gamma$-decay seems to follow a probabilistic, continuous behavior. The arrow indicates the neutron-separation energy in ${}^{147}\mathrm{Sm}$. (c) Particle singles spectrum for $(p,d){}^{147}\mathrm{Sm}$ channel. The arrow indicated the neutron-separation energy in ${}^{147}\mathrm{Sm}$. (d) $\gamma$-ray spectrum from the ${}^{148}\mathrm{Sm}(p,d)$ reaction. The blue histogram shows $\gamma$-ray transitions in coincidence with excitation energies between 3.0-5.0 MeV, where only the $(p,d)$ channel is present. The red histogram represents $\gamma$-ray transitions in coincidence with excitation energies between 8.0-10.0 MeV, where both the $(p,d)$ and $(p,dn)$ channels are open.

Figure 2

(Left) A comparison between the geant4 simulation (red dashed line) and measured spectra (black) for ${}^{60}\mathrm{Co}$. The differences in the spectra are primarily due to contaminants that were present in the singles spectra. (Right) A histogram of residual differences between between the measured ${}^{60}\mathrm{Co}$ spectrum and a simulated one. A Gaussian fit (red dashed line) was used to determine the width of the distribution.

Figure 3

A comparison between the photopeak efficiency extracted from the simulation and from source data, both for a single clover. Sources used for this figure are ${}^{152}\mathrm{Eu}$, ${}^{137}\mathrm{Cs}$, and ${}^{60}\mathrm{Co}$. Each data point represents the photopeak efficiency for a $\gamma$-ray energy for a given source. The red band represents the simulated efficiency and its $1\sigma$ uncertainty band.

Figure 4

Extracted $\gamma$-decay probabilities for the first three transitions in ${}^{147}\mathrm{Sm}$, via the $(p,d)$ reaction channel. The solid line denotes the neutron separation energy $S_n$, and the dotted lines correspond to the energies of the first few discrete states in ${}^{146}\mathrm{Sm}$. The spins and parities of the ${}^{146}\mathrm{Sm}$ levels are noted on the figure.

Figure 5

Extracted $\gamma$-decay probabilities for the first three transitions in ${}^{146}\mathrm{Sm}$, via the $(p,dn)$ reaction channel. The solid line denotes the neutron separation energy $S_n$, and the dotted lines correspond to the energies of the first few discrete states in ${}^{146}\mathrm{Sm}$. The spins and parities of the ${}^{146}\mathrm{Sm}$ levels are noted on the figure.

Figure 6

Extracted $\gamma$-decay probabilities for the transitions in ${}^{147}\mathrm{Sm}$, via the $(p,d)$ reaction channel. The solid line denotes the neutron separation energy $S_n$, and the dotted lines correspond to the energies of the first few discrete states in ${}^{146}\mathrm{Sm}$. The spins and parities of the ${}^{146}\mathrm{Sm}$ levels are noted on the figure. Transitions via 430.4, 634.4, 747.2 and 900.8 keV $\gamma$ rays correspond to ${}^{146}\mathrm{Sm}$ and demonstrate the spin inhibition after neutron emission.

Figure 7

Extracted $\gamma$-decay probabilities for the transitions in ${}^{146}\mathrm{Sm}$, via the $(p,t)$ reaction channel. The solid line denotes the neutron separation energy $S_n$, and the dotted lines correspond to the energies of the first few discrete states in ${}^{145}\mathrm{Sm}$. The spins and parities of the ${}^{145}\mathrm{Sm}$ levels are noted on the figure. Transitions via 1105.0 and 1432.2 keV $\gamma$ rays correspond to ${}^{145}\mathrm{Sm}$ and demonstrate the spin inhibition after neutron emission.

Figure 8

Extracted $\gamma$-decay probabilities for the transitions in ${}^{160}\mathrm{Dy}$, via the $(p,t)$ reaction channel. The solid line denotes the neutron separation energy $S_n$, and the dotted lines correspond to the energies of the first few discrete states in ${}^{159}\mathrm{Dy}$. The spins and parities of the ${}^{159}\mathrm{Dy}$ levels are noted on the figure.