Structure of ${}^{155}\mathrm{Nd}$ and ${}^{163}\mathrm{Gd}$ from ${}^{252}\mathrm{Cf}$ spontaneous fission

Background: A puzzle has arisen recently caused by the apparent shift in maximum deformation from the expected ${}^{66}\mathrm{Dy}$ isotopic chain to the ${}_{60}\mathrm{Nd}$ isotopic chain in the $82<N<126$ and $50<Z<82$ midshell region.

Purpose: This work provides data for two specific nuclei with odd neutron numbers, ${}^{155}\mathrm{Nd}$ and ${}^{163}\mathrm{Gd}$, useful for constraining parameters in models that seek to answer the six proton shift in maximum deformation.

Method: Data from the spontaneous fission of ${}^{252}\mathrm{Cf}$ were taken by the Gammasphere detector array at Lawrence Berkeley National Laboratory to observe the excited states of ${}^{155}\mathrm{Nd}$ and ${}^{163}\mathrm{Gd}$.

Results: The structure of ${}^{163}\mathrm{Gd}$ has been expanded with the addition of two new levels and three new $\gamma$ rays, which are found to be consistent with previously published calculations and the structure of ${}^{165}\mathrm{Dy}$. In ${}^{155}\mathrm{Nd}$, nine new levels and 12 new $\gamma$ rays are observed. The spins and parities of the previously known levels in ${}^{155}\mathrm{Nd}$ have been reassigned from a $\nu 3/2^{-}\left[521\right]$ ground state configuration to a $\nu 5/2^{+}\left[642\right]$ isomeric configuration by comparison of these newly observed levels with levels in ${}^{153}\mathrm{Nd}$ and ${}^{155}\mathrm{Sm}$.

Conclusion: Further experimentation is required to determine the energy of the newly reassigned $\nu 5/2^{+}\left[642\right]$ level in ${}^{155}\mathrm{Nd}$ with respect to the suspected $\nu 3/2^{-}\left[521\right]$ ground state. Additionally, more experiments should be conducted to further determine the structure of neutron rich nuclei, rarely produced in the spontaneous fission of ${}^{252}\mathrm{Cf}$, such as ${}^{163}\mathrm{Gd}$.

Figure 1

Lowest first $2^{+}$ energies in the $82<N<126$ and $50<Z<82$ midshell region. The data show in this figure are compiled from Refs. [6, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35].

Figure 2

The level scheme of ${}^{155}\mathrm{Nd}$ as found in this work. All level and $\gamma$ energies are in keV. Red transitions and levels are newly observed in this work. Table 1 has the precise energies for these levels and $\gamma$ rays.

Figure 3

A double gate on $836.7\text{–}1308.7$ keV, the first two yrast transitions from ${}^{94}\mathrm{Sr}$, the three neutron fission partner of ${}^{155}\mathrm{Nd}$. Transitions from ${}^{155}\mathrm{Nd}$ are labeled with their rounded energy. For peaks from other sources the source is given as the label. Labels ending in a question mark indicate that the transition is previously unidentified. Transitions with contributions from multiple sources are combined with plus ($+$) signs.

Figure 4

Two triple gates showing transitions in the structure of ${}^{155}\mathrm{Nd}$. Transitions from ${}^{155}\mathrm{Nd}$ are labeled with their rounded energy. For peaks from other sources the source is given as the label. (a) A triple gate on the $273.1\text{–}341.3\text{–}407.5$ keV within the structure of ${}^{155}\mathrm{Nd}$. (b) A triple gate on 836.7 keV from ${}^{94}\mathrm{Sr}$ and $109.4\text{–}314.8$ keV from ${}^{155}\mathrm{Nd}$.

Figure 5

Systematics of bands across ${}^{153,155}\mathrm{Nd}$, and ${}^{155}\mathrm{Sm}$. Data for ${}^{153}\mathrm{Nd}$ are from [11] and ${}^{155}\mathrm{Sm}$ come from [37, 38]. Part (a) shows the $\nu 3/2^{-}\left[521\right]$ and $\nu 5/2^{+}\left[642\right]$ bands for each isotope, with the lowest energy level in ${}^{155}\mathrm{Nd}$ set to zero to ensure that its levels fit in the plot better. Part (b) contains a plot of the spacing between any two levels whose spins differ by 1. The dashed lines indicate the $\nu 3/2^{-}\left[521\right]$ bands while the solid lines indicate the $\nu 5/2^{+}\left[642\right]$ bands. The line for the $\nu 3/2^{-}\left[521\right]$ band of ${}^{155}\mathrm{Nd}$ is what the spins would be if the only observed band were that configuration, rather than the $\nu 5/2^{+}\left[642\right]$ configuration assumed by this work.

Figure 6

Plots of the ${}^{252}\mathrm{Cf}$ SF yields of Sr isotopes as function of Nd mass number ($A$) from Musangu et al. [42].

Figure 7

The level scheme of ${}^{163}\mathrm{Gd}$ as observed in this work. The width of the arrows indicates intensity (white is theoretical internal conversion). In red are the new transitions and levels newly observed in this work. The half-life of the $\approx 138\mathrm{keV}$ isomer is taken from [13]. The precise level and $\gamma \text{−}\mathrm{ray}$ energies are shown in Table 2.

Figure 8

A double gate on $704.1\text{–}863.6$ keV, the first two yrast transitions in ${}^{86}\mathrm{Se}$, in which all the transitions observed in this work for ${}^{163}\mathrm{Gd}$, its three neutron fission partner, are visible. Transitions from ${}^{163}\mathrm{Gd}$ are labeled with their rounded energy. For peaks from other sources the source is given as the label. Transitions with contributions from multiple sources are combined with plus ($+$) signs.