Excited states in the neutron-deficient nuclei ${}^{197,199,201}\mathrm{Rn}$

Excited states of the extremely neutron-deficient radon isotopes with $N=111,113,115$ have been studied for the first time in a series of in-beam experiments performed at the Accelerator Laboratory of the University of Jyväskylä. The reactions used were: ${}^{118}\mathrm{Sn}$(${}^{82}\mathrm{Kr}$, $3n$)${}^{197}\mathrm{Rn}$, ${}^{120}\mathrm{Sn}$(${}^{82}\mathrm{Kr}$, $3n$)${}^{199}\mathrm{Rn}$, ${}^{150}\mathrm{Sm}$(${}^{52}\mathrm{Cr}$, $3n$)${}^{199}\mathrm{Rn}$, and ${}^{122}\mathrm{Sn}$(${}^{82}\mathrm{Kr}$, $3n$)${}^{201}\mathrm{Rn}$. The γ rays emitted from excited states in the different isotopes were identified using the recoil-α-decay tagging technique. The estimated cross section for the production of ${}^{197}{\mathrm{Rn}}^m$ was 7(3) nb, which is the lowest cross section reported so far for an in-beam study. The energies of the $(17/2^{+})$ levels built on the isomeric $(13/2^{+})$ states in ${}^{197,199,201}\mathrm{Rn}$ indicate a transition from an anharmonic vibrational structure toward a rotational structure at low spins for these nuclei. However, the transition is not as sharp as predicted by theory.

Figure 1

Energies of the α particles detected in correlation with a DSSD-implanted recoil. The maximum correlation times which were used to obtain the above spectra are noted in each spectrum. The upper panel shows the α-decay energies correlated with the DSSD-implanted recoils in the ${}^{200}\mathrm{Rn}$ compound nucleus reactions. The middle panel shows a similar spectrum for the ${}^{202}\mathrm{Rn}$ compound nucleus reactions and the lower panel shows the α spectrum for the ${}^{204}\mathrm{Rn}$ compound nucleus reactions.

Figure 2

RDT γ-ray spectra observed in the JUROGAM Ge detector array at the target position. Spectrum (a) shows the prompt γ rays tagged with the α decay of the $(13/2^{+})$ isomer in ${}^{197}\mathrm{Rn}$${}^m$ with the maximum recoil-α correlation time set to 125 ms. Spectrum (b) shows the total recoil-tagged JUROGAM spectrum from this experiment. Panel (c) shows the prompt γ rays tagged by the α decay of the $(13/2^{+})$ isomer in ${}^{199}\mathrm{Rn}$${}^m$ with an upper limit for the correlation time of 1.5 s. Spectrum (d) shows the total recoil-tagged JUROGAM spectrum from the ${}^{199}\mathrm{Rn}$ experiment and the lowest spectrum (e) displays the ${}^{199}\mathrm{Rn}$ $(3/2^{-})$ ground-state α-tagged JUROGAM spectrum with a maximum correlation time of 2 s. The 376-keV transition in the lowest spectrum, marked with an asterisk, is also observed in the ${}^{199}\mathrm{Rn}$${}^m$ α-tagged JUROGAM spectrum.

Figure 3

Plot showing the mother nucleus α energy on the x-axis and the subsequently decaying daughter α energy on the y-axis. The distribution of events corresponding to the decay of ${}^{197}\mathrm{Rn}$${}^m$ followed by the decay of ${}^{193}\mathrm{Po}$${}^m$ is visible in the figure. The recoil-${\alpha }_m$ correlation upper time limit for the mother decay was set to 250 ms and the ${\alpha }_m\text{−}{\alpha }_d$ maximum correlation time for the daughter decay was set to 2 s. The α-particles emitted from the ${}^{199}\mathrm{Rn}$${}^m$ decay are visible in the figure, indicating contamination in the ${}^{118}\mathrm{Sn}$ target from heavier Sn isotopes.

Figure 4

Spectrum showing the energies of the prompt γ rays tagged by the α particles following the decay of the $(13/2^{+})$ state in ${}^{201}\mathrm{Rn}$, within the recoil-α search time of 15 s. The level scheme as obtained from the $\gamma \text{−}\gamma$ coincidence analysis and their energy relationships is also presented in the figure.

Figure 5

The upper spectrum shows the ${}^{201}\mathrm{Rn}$ $(3/2^{-})$ ground-state α-tagged γ-rays with a maximum correlation time of 20 s. Transitions marked with an asterisk are also observed in coincidence with the α decays of the $(13/2^{+})$ isomers. The lower spectrum shows the total recoil-tagged JUROGAM spectrum from this experiment.

Figure 6

Systematics of low-spin states in the neutron-deficient radon isotopes. The data on ${}^{198,200,202,203}\mathrm{Rn}$ are taken from Refs. [4, 5, 8, 26].

Figure 7

The exitation energy vs. the neutron excess for the Rn and Po isotopes with $N⩽126$. The levels in the odd-mass Rn isotopes are marked with filled triangles and the levels in the even-mass Rn isotopes are marked with open triangles. The odd- and even-mass Po isotopes are marked with filled and open circles, respectively.

Figure 8

Total Routhian Surface calculations performed at a frequency of $\hslash \omega =0.04$ MeV for the neutron configuration: ($\pi ,\alpha$) = ($+,+1/2$). The energy minimum is at an oblate shape with $\left|{\beta }_2\right|\approx 0.2$ for ${}^{197}\mathrm{Rn}$${}^m$ (a), and at a prolate (near-spherical) shape with ${\beta }_2\approx 0.1$ for ${}^{199,201}\mathrm{Rn}$${}^m$, (b) and (c), respectively. The energy maxima in the surfaces are at prolate deformations with ${\beta }_2\approx 0.3$. The energy spacing between the contours is about 100 keV.