High-resolution $\alpha$ and electron spectroscopy of ${}_{98}{}^{249}\mathrm{Cf}$

$\alpha$-particle spectra of ${}^{249}\mathrm{Cf}$ have been measured with a double-focusing magnetic spectrometer and with passivated implanted planar silicon (PIPS) detectors. The conversion-electron spectra of ${}^{249}\mathrm{Cf}$ have been measured with a cooled Si(Li) detector and with a room-temperature PIPS detector. Precise energies of $\alpha$ groups in the decay of ${}^{249}\mathrm{Cf}$ have been measured with respect to the known energy of ${}^{250}\mathrm{Cf}$. In addition, $\alpha$-electron, $\alpha \text{−}\gamma$, and $\gamma \text{−}\gamma$ coincidence measurements were also performed to determine the spin-parity of the previously known 643.64-keV level. From electron intensities, conversion coefficients of transitions in the daughter ${}^{245}\mathrm{Cm}$ have been determined. The measured $L_3$ conversion coefficients of the 333.4- and 388.2-keV transitions are found to be in agreement with the theoretical conversion coefficients for pure $E1$ multipolarity. On the other hand, the $K,L_1+L_2,M$, and $N$ conversion coefficients are approximately twice the theoretical values for pure $E1$ transitions. These measurements indicate anomalous $E1$ conversion coefficients for the 333.4- and 388.2-keV transitions, as has been pointed out in earlier measurements. The measured conversion coefficient of the 255.5-keV transition gives an $M1$ multipolarity for this transition which establishes a spin-parity of 7/$2^{-}$ and the 7/$2^{-}\left[743\right]$ single-particle assignment to the 643.64-keV level.

Figure 1

${}^{249}\mathrm{Cf}$ $\alpha$-particle spectra measured with the Argonne double-focusing magnetic spectrometer. The three spectra show different energy ranges. The spectrometer had a resolution (FWHM) of 5 keV. Energy scales for the three spectra are 2.804 keV per channel (top), 2.610 keV per channel (middle), and 2.511 keV per channel (bottom). The ${\alpha }_{853}$ peak in (b) is wide because it contains the 5360.5-keV peak of ${}^{245}\mathrm{Cm}$.

Figure 2

$\alpha$-particle spectrum of a 30-year old mass-separated ${}^{249}\mathrm{Cf}$ source measured with a 25-$\mathrm{mm}{}^2\times 0.3$-mm passivated implanted planar silicon (PIPS) detector at a solid angle of 0.07% of $4\pi$. The energy scale is 1.449 keV per channel and the energy resolution (FWHM) is 13.0 keV. The subscript after $\alpha$ denotes the energy of the level populated by that $\alpha$ group.

Figure 3

Electron spectrum of a mass-separated ${}^{249}\mathrm{Cf}$ source measured with a cooled Si(Li) spectrometer. The spectrometer had a resolution (FWHM) of 1.2 keV for a 100-keV electron line. The spectrum was measured soon after the source preparation and hence it does not contain the ${}^{245}\mathrm{Cm}$ $L_1$ line, present in Fig. 4.

Figure 4

Electron spectrum of a 30-year old mass-separated ${}^{249}\mathrm{Cf}$ source measured with a room-temperature 25-$\mathrm{mm}{}^2\times 0.5$-mm PIPS detector. The detector has a resolution (FWHM) of 2.3 keV for a 100-keV electron line. The energy scale is 0.213 keV per channel.

Figure 5

Electron spectrum of a mass-separated ${}^{249}\mathrm{Cf}$ source measured with a cooled Si(Li) detector in coincidence with $\alpha$ particles with energies less than the ${}^{249}\mathrm{Cf}$ ${\alpha }_{388}$ group. The electron detector had a resolution (FWHM) of 1.2 keV for a 100-keV electron line. The energy scale is 0.306 keV per channel.

Figure 6

$\gamma$-ray spectra of a mass-separated ${}^{249}\mathrm{Cf}$ source measured with a 2-$\mathrm{cm}{}^2\times 10$-mm LEPS detector in coincidence with $\alpha$ particles with energies less than the ${}^{249}\mathrm{Cf}$ ${\alpha }_{388}$ group (top) and in coincidence with the 388.2-keV $\gamma$ ray measured with a NaI(Tl) detector (bottom). The energy scale is 0.085 keV per channel. The resolving time was 0.$5\mu \text{s}$.

Figure 7

A partial $\alpha$-decay scheme of ${}^{249}\mathrm{Cf}$ deduced in the present work. On the left side of the levels, the single-particle state quantum numbers $\left[N{n}_z\Lambda \right]K,I^{\pi }$ are given. On the right side, level energies in keV, $\alpha$ intensities in %, and hindrance factors are given.

Figure 8

Level scheme of ${}^{245}\mathrm{Cm}$ from Ref. [1]. Level energies and $\gamma$-ray energies are given in keV.

Figure 9

A plot of the $E1$ conversion coefficient anomaly against the $\gamma$-ray retardation using data from Ref. [13]. The anomaly is defined as $[{\left({\alpha }_{L1+L2}\right)}_{\exp }-{\left({\alpha }_{L1+L2}\right)}_{\mathrm{theo}}]/{\left({\alpha }_{L1+L2}\right)}_{\mathrm{theo}}$ and the retardation is defined as the ratio of the experimental partial $\gamma$-ray half-life and the theoretical single-particle half-life. Our data in ${}^{245}\mathrm{Cm}$ are shown as filled diamonds.