Electron capture decay of 58-min ${}_{92}{}^{229}\mathrm{U}$ and levels in ${}_{91}{}^{229}\mathrm{Pa}$

Electron capture decay of ${}^{229}\mathrm{U}$ is investigated by measuring the $\gamma$-ray and conversion electron spectra of mass-separated and unseparated ${}^{229}\mathrm{U}$ sources with high-resolution germanium and silicon detectors, respectively. Gamma-gamma coincidence measurements are also performed using germanium detectors. These studies provide level energies and level ordering in ${}^{229}\mathrm{Pa}$. Single-particle assignments are given to these levels which are in agreement with the systematics in this region and also with theory. In a previous study, we report the observation of a 5/$2^{\pm }$ parity doublet in the ${}^{229}\mathrm{Pa}$ ground state, which is a signature of octupole deformation. The present analysis of the data still shows a splitting of $60\pm 50$ eV, but with this large uncertainty the existence of the doublet is not certain.

Figure 1

The $\gamma$-ray spectrum of a mass-separated ${}^{229}\mathrm{U}$ source measured with a 5-$\mathrm{cm}{}^2\times 10$-mm LEPS detector. $\gamma$-ray energies (in keV) are indicated by the peaks. An asterisk denotes $\gamma$ rays belonging to ${}^{225}\mathrm{Th}$ or ${}^{221}\mathrm{Ra}$ $\alpha$ decay.

Figure 2

The $\gamma$-ray spectrum of an unseparated ${}^{229}\mathrm{U}$ source measured with a 15% Ge(Li) detector. A set of Cu and Al absorbers was used to reduce the counts in the low-energy $\gamma$-ray and x-ray peaks, thus reducing $\gamma \text{−}\gamma$ summing. $\gamma$-ray energies (in keV) are indicated by the peaks. Counting was started $\sim 2$ h after the end of irradiation. An asterisk denotes $\gamma$ rays belonging to ${}^{225}\mathrm{Th}$ or ${}^{221}\mathrm{Ra}$ $\alpha$ decay.

Figure 3

The $\gamma$-ray spectrum of an unseparated ${}^{229}\mathrm{U}$ source measured with a 5-$\mathrm{cm}{}^2\times 10$-mm LEPS detector showing $\gamma$ rays in the 240-keV region. A set of Cu and Al absorbers was used to reduce the counts in the low-energy $\gamma$-ray and x-ray peaks, thus reducing $\gamma \text{−}\gamma$ summing. $\gamma$-ray energies (in keV) are indicated by the peaks. Counting was started $\sim 2$ h after the end of irradiation. An asterisk denotes $\gamma$ rays belonging to ${}^{225}\mathrm{Th}$ $\alpha$ decay.

Figure 4

The $\gamma$-ray spectrum of an unseparated ${}^{229}\mathrm{U}$ source measured with a 5-$\mathrm{cm}{}^2\times 10$-mm LEPS detector in coincidence with Pa $K_{\alpha }$ x rays. $\gamma$-ray energies (in keV) are indicated by the peaks. The $K_{\alpha }$ x rays were detected with a 20% LEPS spectrometer.

Figure 5

Electron spectrum of an unseparated ${}^{229}\mathrm{U}$ source measured with an 80-$\mathrm{mm}{}^2\times 3$-mm cooled Si(Li) spectrometer with a resolution (FWHM) of 1.0 keV at 100-keV electron energy. The spectrum was generated by subtracting a later spectrum from an early spectrum. Strong electron lines from ${}^{230}\mathrm{Th}$ and ${}^{229}\mathrm{Pa}$ were present in both spectra. Electron and/or transition energies (in keV) are indicated by the peaks. An asterisk denotes electron lines belonging to ${}^{225}\mathrm{Th}$ or ${}^{221}\mathrm{Ra}$ $\alpha$ decay.

Figure 6

Electron spectrum of a mass-separated ${}^{229}\mathrm{U}$ source measured with a cooled Si(Li) spectrometer in coincidence with Pa $K_{\alpha }$ x rays. Electron and/or transition energies (in keV) are indicated by the peaks.

Figure 7

Low-energy portion of the ${}^{229}\mathrm{Pa}$ level scheme. $\gamma$-ray energies are given in kilo–electron volts, and $\gamma$-ray intensities (in % per ${}^{229}\mathrm{U}$ EC decay) are given in parentheses. The half-life in the figure was measured during the course of this work. Gray (red) horizontal lines represent the band heads.

Figure 8

High-energy portion of the partial EC decay scheme of ${}^{229}\mathrm{U}$ deduced in the present work. Gray (red) horizontal lines represent the band heads.

Figure 9

The $\gamma$-ray spectra of a ${}^{229}\mathrm{U}$ source measured in coincidence with 211.0-keV (top) and 198.8-keV (bottom) $\gamma$ rays. $\gamma$-ray energies (in keV) are indicated by the peaks. The upper spectrum shows that most of the 211.0-keV $\gamma$-ray intensity originates at the 241.80-keV level (see Fig. 7).

Figure 10

A comparison of the ${}^{229}\mathrm{Pa}$ energy levels deduced in the present work with the prediction of Chasman [1]. In Ref. [2], the energies of the 5/$2^{+}\left[642\right]$, 5/$2^{-}\left[523\right]$, and 1/$2^{-}\left[530\right]$ single-particle states were calculated to be 0.0, 0.4, and 56 keV, respectively.

Figure 11

A comparison of the experimental ${}^{229}\mathrm{Pa}$ and ${}^{231}\mathrm{Pa}$ energy levels. The figure shows that despite both nuclei having 91 protons, their structures are quite different.