Microsecond isomers in neutron-rich fission products

J. Genevey, J.A. Pinston (ISN Grenoble), H. Faust, S. Oberstedt (ILL).

The properties of neutron-rich nuclei have received special attention in the last few years. Experiments far from the stability line in very light nuclei revealed unexpected behaviour of the neutrons in these isotopes, which, in some cases, form a neutron halo extending far away out of the nuclear-charge radius. A second phenomenon observed was the disappearance of magic shells, if the isospin goes to extreme values. These findings triggered a programme on the Lohengrin spectrometer to search for isospin effects in the very neutron-rich regions of the fission products. In particular high sensitivity is reached for the spectroscopy of microsecond isomers populated in fission, where ions are observed after passing the separating magnetic and electric fields of the instrument. These isomers are apparently produced abundantly in various fission-product regions, and seem to be of strong single-particle character. Therefore, they are expected to probe directly the nuclear potential, and its dependencies on isospin.

Figure 1: Comparison between total g-spectrum of mass A = 94 without time condition (upper part) and the spectrum obtained by subtraction of the two gates as indicated in the insert (lower part).

Nuclear fission is a particularly favorable method for the investigation of very neutron-rich isotopes throughout a wide region of the nuclear chart, because the fission process is a cold reaction. Only a small fraction of the available Q-value is converted into temperature in the fragments, and a considerable number of isotopes have excitation energies below the neutron evaporation threshold. Isotopes very far from the stability line are therefore produced, and, following mass separation, can be investigated for effects of neutron excess on the nuclear potential parameters.

When going far away from the nuclear stability-line the production rate of the ions decreases, and the observation of microsecond isomers requires a coincidence condition between the incoming signal, which is analysed for energy, mass number and nuclear charge, and the gamma decay appearing according to the microsecond lifetime, which is observed in a germanium detector. Due to the rarity of the events we had to construct a compact ionisation chamber allowing for particle identification, and to place the gamma counter nearby. The ionisation chamber is mounted at the focal position of the instrument, and the ionisation signal, together with the separator field settings determines energy, mass and charge of the incoming fragment. In order to get a fast start signal for the coincidence, we implant the fragment at the end of the ionisation chamber in a thin silicon diode. The gamma decay of the fragment is observed in germanium diodes, and the time between ion impact and decay is recorded. The coincidence requirement considerably enhances the sensitivity of the detection, which is demonstrated in Fig. 1.

In order to optimise count rates a ²⁴¹Pu target of thickness 300 g/cm² was introduced in the in-pile position of Lohengrin, in a thermal neutron flux of 5 . 10¹⁴ n/cm²s. In the first measurements the spectrometer fields were adjusted to the mass region from A = 88 to A = 109. Here, many isomers in the microsecond range were observed. The half-lives of the nuclear states were obtained using one or several gamma-rays where exponential fits to the time distributions were applied. The decay curves of new isomers observed are shown in Fig. 2.

Figure 2: Decay curves of the isomers observed in this experiment. Purple line = fitted curve, dashed orange line = background, green line = background subtracted result.

The level scheme obtained for the decay of the isomer in ⁹⁶Rb is shown in Fig 3. From the data and expectations that in this mass region the spherical shell model may apply, we can deduce a microscopic structure of the isomer to be composed of a neutron in the h 11/2 shell coupled to a proton hole in the f 5/2 shell.

Figure 3: Level scheme of the isomer in 96Rb.

Only a small part of the available fission-product region has been investigated up till now for longer lived nuclear-states. The experiments have shown that high sensitivity for isomer decay can be reached. In the heavy fission-products many isomers are expected to decay via conversion electron-decay because of the high spin characteristic of the isomeric state, leading to high multipole order for the decay, and also in view of the high nuclear charge of the isotopes leading to high conversion coefficients. Following the gamma-spectroscopic studies, we performed a first short test on Lohengrin, which showed that conversion-electron spectra are indeed extremely useful in this type of isomer investigation. In consequence, a new ionisation chamber has been built enabling us to take conversion-electron data. It is foreseen to install high-efficiency gamma-detector arrays at the focal plane of Lohengrin (MINIBALL detectors). This will increase the sensitivity by more than hundred times and make possible spectroscopy even farther away from stability. These investigations will enable us to get a picture of the systematic behavior of the isomeric states over large regions of different isospin.

PN1's focal plane; from left: René Guglielmini and Jean-Alain Pinston, both ISN Grenoble, and Herbert Faust.