New isotopes and isomers in the neutron-rich Pb region
by M. Hellström and M. Pfützner

Projectile fragmentation in combination with magnetic separation of the reaction products has proven to be a very successful tool in nuclear science, in particular for the production of secondary radioactive beams to study exotic nuclei at the limits of stability. Previous fragmentation studies have shown that isomeric states are populated with large probabilities, typically of the order of 50% [1]. This opens up attractive possibilities to perform experiments with beams of nuclei in metastable states [2]. In addition, information crucial for testing and developing nuclear-structure models for nuclei far from stability may be obtained by studying the decays of isomeric states in exotic nuclei inaccessible by other techniques. Isomers with half-lives in the 10 ns to 1 ms range can be studied by measuring coincidences between identified, stopped fragments and the delayed gamma-rays de-exciting the isomer.

We have performed an experiment using projectile fragmentation of ²³⁸U at relativistic energies to populate and study neutron-rich nuclei in the region around doubly-magic ²⁰⁸Pb. Until recently, this region of the nuclear chart was extremely difficult to reach experimentally. The heaviest polonium isotope previously identified was ²¹⁸Po (then known as RaA) was studied in 1904 by E. Rutherford [3], and ²¹⁰Tl (RaC'') was investigated by O. Hahn and L. Meitner in 1909 [4]! The current work demonstrates that fragmentation of relativistic uranium beams provides a novel and efficient method for accessing heavy neutron-rich nuclei. These nuclei are of interest because they offer favorable conditions for the formation of isomeric states, due to the presence of high-j orbitals such as neutron i13/2, neutron g9/2, neutron j15/2, and proton h11/2 which reside close to both the proton and neutron Fermi surfaces. The location of isomeric states provides a useful empirical calibration of shell model residual interactions between specific high-j orbitals [5]. In addition, the neutron separation energy is predicted [6] to be reduced to a few MeV for heavy nuclei with N>126, potentially creating conditions for weakly bound isomeric states. These may exhibit features analogous to the ground-state properties of nuclei near the neutron drip line. Since it is extremely difficult, if not impossible, to reach the neutron drip line for heavy nuclei experimentally, the study of such metastable states may represent the best means to study phenomena that have been predicted to occur at the drip line [7], such as the change of pairing interaction or the coupling between bound and continuum states.

	Figure 1: Gamma-ray spectra measured in coincidence with 203Tl and 212Pb fragments, corresponding to 12400 and 370 implanted ions, respectively. The inserts show the observed gamma-line decay time patterns. On the right, mass-to-charge spectra for elements transmitted at the setting for 214Pb are shown. The framed numbers indicate new isotopes. The figure is taken from Ref. [8].

The isotopes of interest were produced by fragmentation of a 1000 MeV/nucleon, ²³⁸U primary beam from the SIS synchrotron at GSI-Darmstadt in a 1g/cm² Be target located at the entrance of the projectile fragment separator FRS. Data were collected at three different separator settings, optimized to transmit nuclei around ²⁰⁸Pb, ²¹²Pb, and ²¹⁴Pb, respectively. The atomic number Z and mass-to-charge ratio A/Q of reaction product ions reaching the final focus were determined by combining measurements of time of flight (TOF) with information from position-sensitive multi-wire counters and the magnetic field values (see Fig. 1). The identified ions were slowed down in a variable-thickness degrader and subsequently stopped in a catcher placed between two Ge detectors. Gamma-rays from the decay of any isomeric states were recorded within a 30µs time gate, which was started with the detection of a heavy ion event. In order to determine the isomer half-lives, the time interval between implantation and the delayed gamma-ray emission was measured using both time-to-amplitude converters (30µs range) and time-to-digital converters (1ms range).

The isomeric ratio F is the ratio between the production yield for the isomeric state and the total production of that isotope. The yield of the isomeric state is obtained by correcting the number of observed isomeric gamma-transitions according to detection efficiency and in-flight decay losses. The production rates were determined by correcting the measured counting rates for ion-optical transmission losses in the spectrometer, losses due to secondary reactions in the target and materials at the intermediate focal plane, losses due to non-fully stripped charge states, and the dead time of the data acquisition system. In the present work, isomeric ratios ranging between 10% (I=20^{+ 204}Tl) and 50% (I=7^{- 206}Pb, I=(8⁺) ²¹²Pb) were observed. These values can be compared with estimates derived from calculated [9] distributions of angular momentum in the final products of relativistic fragmentation reactions under the extreme simplifying assumption that F is equal to the probability that the final fragment is produced with an angular momentum larger than that of the isomer. The agreement with the measured values is surprisingly good.

Previously unobserved isomeric states were found in ²⁰³Tl, ²⁰⁴Tl, ²¹¹Bi, and ²¹²Pb. The decay time analysis of the strongest lines, using a maximum likelihood method, yielded half-lives of 7.7(5)µs, 2.6(2)µs, 1.4(3)µs, and 5(1)µs, respectively. Additionally, there is strong evidence for a second isomer in ²⁰⁴Tl with a half-life of 1.6(2)µs. A detailed shell-model analysis of these isomers will be published separately [10]. As an example, we propose that the isomeric state in ²⁰³Tl can be interpreted as a I=(25/2,29/2)⁺ level with proton h11/2 neutron i13/2p1/2 or proton h11/2 neutron i13/2f5/2 quasiparticle structure. A similar I=25/2⁺ isomer has been observed in ²⁰⁵Tl [11].

To the top!