Energy levels of ${}^{247}\mathrm{Cm}$ populated in the $\alpha$ decay of ${}_{98}{}^{251}\mathrm{Cf}$

$\alpha$-Particle, conversion-electron, and $\gamma$-ray singles spectra of ${}^{251}\mathrm{Cf}$ sources containing few Bq activities were measured with high-resolution semiconductor detectors. $\alpha \text{−}\gamma$ coincidence and level half-life measurements were also performed. On the basis of these measurements the following single-particle states have been identified in ${}^{247}\mathrm{Cm}$: $9∕2^{-}\left[734\right]$, 0 keV; $5∕2^{+}\left[622\right]$, 227.4 keV; $7∕2^{+}\left[624\right]$, 285.4 keV; $1∕2^{+}\left[620\right]$, 404.9 keV; $1∕2^{+}\left[631\right]$, $518.6\text{keV}$. The measured single-particle energies are in good agreement with energies calculated with a Woods-Saxon single-particle potential. From the measured half-life of the $227.4\text{−}\text{keV}$ level and the $E3$ branching ratio, a $B\left(E3\right)$ value of $5\text{W.u.}$ has been deduced between the $5∕2^{+}\left[622\right]$ and $9∕2^{-}\left[734\right]$ bands. The large $B\left(E3\right)$ value is an indication of octupole mixing in the $227.4\text{−}\text{keV}$ level.

Figure 1

A ${}^{251}\mathrm{Cf}$ $\alpha$-particle spectrum measured with a $25-{\text{mm}}^2$ PIPS detector. The detector solid angle was $\sim 0.26\text{sr}$ and the source strength was $\sim 10\text{Bq}$. Energy scale is $1.48\text{keV}$ per channel.

Figure 2

A ${}^{251}\mathrm{Cf}$ conversion-electron spectrum measured with a $0.3\text{−}\text{mm}$-thick $6\text{−}\text{mm}\times 6\text{−}\text{mm}$ PIN diode at room temperature. Energy scale is $0.31\text{keV}$ per channel.

Figure 3

$\gamma$-Ray spectrum of a $\sim 400\text{Bq}$ ${}^{251}\mathrm{Cf}$ source measured with a $5{\text{−}\text{cm}}^2\times 10\text{−}\text{mm}$ LEPS spectrometer through a $0.5\text{−}\text{mm}$ Cu and $0.8\text{−}\text{mm}$ Al absorber. The source-to-detector distance was $0.5\text{cm}$. $B$ denotes background peaks.

Figure 4

$\alpha$-Decay scheme of ${}^{251}\mathrm{Cf}$ deduced from the present work.

Figure 5

Decay spectrum of the $227.38\text{−}\text{keV}$ level measured by delayed coincidence technique. $\alpha$-Particles were detected by a Si detector and $\gamma$ rays with a NaI (Tl) crystal. For the start signal the single-channel analyzer was set above ${\alpha }_{405}$ peak and for the stop signal the single-channel analyzer window was set at $80–160\text{keV}$.

Figure 6

Decay spectrum of the $404.9\text{−}\text{keV}$ level in ${}^{247}\mathrm{Cm}$ measured by delayed coincidence technique. The TAC was started by $\alpha$-particles detected by a Si detector and was stopped by $\gamma$ rays detected in a ${\text{BaF}}_2$ crystal. The TAC was gated by the coincidence output of $\alpha$ and $\gamma$ signals. The window on $\gamma$-ray signal was set at $130–220\text{keV}$ and the window on $\alpha$-particles was set below ${\alpha }_{228}$ peak.

Figure 7

(a) A ${}^{251}\mathrm{Cf}$ $\gamma$-ray spectrum measured with a $5{\text{−}\text{cm}}^2\times 10\text{−}\text{mm}$ LEPS spectrometer and gated by $150–370\text{ns}$ delay after the $\alpha$ decay. (b) ${}^{251}\mathrm{Cf}$ $\gamma$-ray spectrum measured with the same detector as (a) but gated by $3.5–102\mu \mathrm{s}$ after $\alpha$ decay.

Figure 8

A comparison of measured and calculated energy level spacings in ${}^{247}\mathrm{Cm}$. The left side shows the extracted energy levels which represent measured level energies from which the pairing contributions have been removed. The right side shows energies calculated with a Woods-Saxon potential whose parameters were taken from Ref. 15. Dashed lines show levels identified in $(d,p)$ and $(d,t)$ reactions [5] only.