Angular momentum population in the projectile fragmentation of ${}^{238}\mathrm{U}$ at $750\text{MeV}∕\text{nucleon}$

A systematic study of the population probabilities of nanosecond and microsecond isomers produced following the projectile fragmentation of ${}^{238}\mathrm{U}$ at $750\text{MeV}∕\text{nucleon}$ has been undertaken at the SIS∕FRS facility at GSI. Approximately 15 isomeric states in neutron-deficient nuclei around $A\sim 190$ were identified and the corresponding isomeric ratios determined. The results are compared with a model based on the statistical abrasion-ablation description of relativistic fragmentation and simple assumptions concerning $\gamma$ cascades in the final nucleus (sharp cutoff). This model represents an upper limit for the population of isomeric states in relativistic projectile fragmentation. When the decay properties of the states above the isomer are taken into account, as opposed to the sharp cutoff approximation, a good agreement between the experimental and calculated angular momentum population is obtained.

Figure 1

Schematic view of the experimental setup. For details see the text.

Figure 2

Example of the ion identification procedure. A sample of the full statistics is shown. (a) Position of the ions in the middle of the FRS vs $A∕q$. The parallelograms show the selection of events which do not change charge state in the middle of FRS and those which pick up one electron, respectively. (b) Spectrum of $Z$ as measured by the ionization chamber. (c) Position in the middle of FRS vs $A∕q$ for Pb ions selected in (a) and (b). While there is a good separation between different isotopes, each ion with mass $A$ and atomic number $Z$ is contaminated by nuclear species with $A+2$ and $Z+1$.

Figure 3

Delayed $\gamma$-ray spectra associated with ${}^{194}\mathrm{Pb}$, ${}^{195}\mathrm{Bi}$, ${}^{196}\mathrm{Pb}$, ${}^{197}\mathrm{Bi}$, ${}^{198}\mathrm{Po}$, and ${}^{200}\mathrm{Po}$. The time spectra with fitted mean lifetimes are given in the insets. The $\gamma$ rays labeled with an asterisk in ${}^{195}\mathrm{Bi}$ are transitions observed for the first time in the present experiment.

Figure 4

Samples of position spectra at the final focal plane of the separator. They illustrate the applied procedure to separate the nucleus of interest from the contamination, as described in the text.

Figure 5

(a) Experimental isomeric ratios for the ${12}^{+}$ isomers in ${}^{188}\mathrm{Hg}$ (represented by a star), ${}^{192,194,196}\mathrm{Pb}$ (circle), and ${}^{198,200}\mathrm{Po}$ (diamond). (b) The ratio of the isomeric ratios calculated using the ABRABLA code and the analytical formula for the same isomers.

Figure 6

(a) The ratio of the experimental and theoretical (ABRABLA in the sharp cutoff approximation) isomeric ratios for the ${12}^{+}$ isomeric states in ${}^{188}\mathrm{Hg}$ (represented by a star), ${}^{192,194,196}\mathrm{Pb}$ (circle), and ${}^{198,200}\mathrm{Po}$ (diamond). (b) Ratio of the experimental and theoretical angular-momentum population for spin 12 in the same nuclei.