Energy levels of ${}^{249}\mathrm{Bk}$ populated in the α decay of ${}_{99}^{253}\mathrm{Es}$ and ${\beta }^{-}$ decay of ${}_{96}^{249}\mathrm{Cm}$

The level structure of ${}^{249}\mathrm{Bk}$ has been investigated by measuring the γ-ray spectra of an extremely pure ${}^{253}\mathrm{Es}$ sample obtained by milking this nuclide from ${}^{253}\mathrm{Cf}$ source material produced in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Additional information on the ${}^{249}\mathrm{Bk}$ levels was obtained from the ${\beta }^{-}$-decay study of ${}^{249}\mathrm{Cm}$, produced by neutron irradiation of ${}^{248}\mathrm{Cm}$. Using the results of the present study together with the data from previous ${}^{248}\mathrm{Cm}$($\alpha ,t$) and ${}^{248}\mathrm{Cm}$(${}^3\mathrm{He},d$) reactions, the following single-particle states have been identified in ${}^{249}\mathrm{Bk}$: 7/2${}^{+}$[633], 0.0 keV; 3/2${}^{-}$[521], 8.78 keV; 1/2${}^{+}$[400], 377.55 keV; 5/2${}^{+}$[642], 389.17 keV; 1/2${}^{-}$[530], 569.20 keV; 1/2${}^{-}$[521], 643.0 keV; 5/2${}^{-}$[523], 672.9 keV; and 9/2${}^{+}$[624], 1075.1 keV. Four vibrational bands were identified at 767.9, 932.2, 1150.7, and 1223.0 keV with tentative assignments of {7/2${}^{+}$ [633] $\otimes 1^{-}$}9/2${}^{-}$, {7/2${}^{+}$ [633] $\otimes \hspace{1em}{0}^{-}$}7/2${}^{-}$, {7/2${}^{+}$ [633] $\otimes \hspace{1em}{1}^{-}$}5/2${}^{-}$, and {7/2${}^{+}$ [633] $\otimes \hspace{1em}{0}^{+}$}7/2${}^{+}$, respectively. A band at 899.9 keV was observed in $\gamma \text{−}\gamma$ coincidence measurements and given a tentative spin assignment of 3/2. It is possibly associated with a 2${}^{-}$ phonon coupled to the ground state, with configuration {7/2${}^{+}$ [633] $\otimes \hspace{1em}{2}^{-}$}3/2${}^{-}$. Three levels at 624.3, 703.5, and 769.1 keV were assigned spins of 5/2, 7/2, and 9/2, respectively. These could be the members of the 3/2${}^{+}$ [651] band, expected in this energy region.

Figure 1

A ${}^{253}\mathrm{Es}$ γ-ray spectrum measured with a 5-cm${}^2\times$10-mm LEPS spectrometer one week after chemical separation. The sample strength was ∼16 mCi, and it was sandwiched between two 1-mm quartz plates. The source-to-detector distance was 18 cm and the counting time was 61 h. A set of Cu and Al absorbers was used to attenuate the Bk L x rays. B denotes Pb K x rays.

Figure 2

A ${}^{253}\mathrm{Es}$ γ-ray spectrum measured with a 5-cm${}^2\times$10-mm LEPS detector two weeks after chemical separation. The source was sandwiched between two 0.5-g/cm${}^2$ Pt disks, and it had a strength of ∼4 mCi. A set of Ta, Cd, Cu, and Fe absorbers was used to attenuate Bk K x rays and low-energy γ rays. The source-to-detector distance was ∼1.0 cm and the counting time was 100 h. B denotes the 333.4-keV γ ray of ${}^{249}\mathrm{Cf}$ and the 511.0-keV annihilation radiation.

Figure 3

γ-ray spectrum of a ∼4 mCi ${}^{253}\mathrm{Es}$ source, sandwiched between two 0.5-g/cm${}^2$ Pt disks, measured with a 25% Ge detector through a set of Ta, Cd, Cu, and Fe absorbers, 2 days after chemical purification. The source-to-detector distance was 4.0 cm, and the counting time was 100 h.

Figure 4

γ-ray spectrum measured with GAMMASPHERE in coincidence with the 227.0-keV γ ray. γ rays with energies 672.8, 664.0, 633.0, and 590.1 keV are in coincidence with the 227.0-keV γ ray. The spectrum was obtained by subtracting a spectrum gated on background counts from the spectrum gated on the 227.0-keV peak. The stars denote Compton-scattered γ rays. Source strength was ∼16 mCi.

Figure 5

γ-ray spectrum of a ${}^{249}\mathrm{Cm}$ source measured with an 8-cm${}^3$ Ge(Li) detector. The break on the y axis shows the change in the scale.

Figure 6

γ-ray spectrum of a ${}^{253}\mathrm{Es}$ source measured with a 25% Ge detector showing the fission γ rays. This is the high-energy portion of the spectrum shown in Fig. 3. The 1274.54-keV γ ray belongs to the ${}^{22}\mathrm{Na}$ impurity in the sample and the 1460.8-keV γ ray is due to ${}^{40}\mathrm{K}$ background.

Figure 7

Energy levels of ${}^{249}\mathrm{Bk}$ determined from the study of ${}^{253}\mathrm{Es}$ α decay. Most of the levels were known from previous studies. Bandheads are shown by thicker lines, and all energies are in keV. Nilsson configuration, K${}^{\pi }$[Nn${}_z\Lambda$], is also given. The 643.0-keV level has a mixed configuration of 1/2${}^{-}$[521], 1/2${}^{-}$[530], and 1/2${}^{-}$ collective components.

Figure 8

Energy levels of ${}^{249}\mathrm{Bk}$ showing the decay of high-lying states. Bandheads are shown by thicker lines. All energies are in keV. Nilsson configuration, K${}^{\pi }$[Nn${}_z\Lambda$], is also given. Bands, with no configuration shown, are vibrational bands with configurations {7/2${}^{+}$[633] $\otimes \hspace{1em}{1}^{-}$}9/2${}^{-}$ and {7/2${}^{+}$[633] $\otimes \hspace{1em}{0}^{-}$}7/2${}^{-}$.

Figure 9

Energy levels of ${}^{249}\mathrm{Bk}$ deduced from γ singles and $\gamma \text{−}\gamma$ coincidence data. Bandheads are shown by thicker lines. All energies are in keV. The band at 899.9 keV has a likely assignment of {7/2${}^{+}$[633] $\otimes \hspace{1em}{2}^{-}$}3/2${}^{-}$, and the 624.3, 703.5, and 769.1 keV levels are most likely the members of the 3/2${}^{+}$[651] band. Bands, with no configuration shown, are vibrational bands with configurations {7/2${}^{+}$[633] $\otimes \hspace{1em}{1}^{-}$}5/2${}^{-}$, and {7/2${}^{+}$[633] $\otimes \hspace{1em}{0}^{+}$}7/2${}^{+}$.

Figure 10

Energy levels of ${}^{249}\mathrm{Bk}$ deduced from the ${\beta }^{-}$ decay of ${}^{249}\mathrm{Cm}$. Bandheads are shown by thicker lines. Nilsson configuration, K${}^{\pi }$[Nn${}_z\Lambda$], is also given. All energies are in keV.

Figure 11

Comparison of extracted level energies of ${}^{249}\mathrm{Bk}$ with the energies calculated with a Woods-Saxon potential [26]. Extracted energies represent experimental level energies from which contributions of pairing correlations have been removed. For the Woods-Saxon potential, the deformation parameters used were ${\nu }_2=0.255,{\nu }_4=0.01$, and ${\nu }_6=0.015$.