Nuclear Physics at the ILL: Spectroscopy of microsecond isomers with the Lohengrin fission-product spectrometer

The region around the doubly magic nucleus 132Sn (Z=50, N=82) is one of only two neutron-rich, heavy, doubly magic regions currently experimentally accessible, the other being 78Ni. The nuclei here are particularly suitable for testing shell-model predictions as they have a simple structure, just a few particles or holes outside a closed core. As the nucleus 132Sn has a high ratio of neutrons to protons, N/Z=1.64, then it is of great interest to study the nuclei of this region as theoretical predictions concerning the properties of the nuclei here differ. Open questions remain regarding the behaviour of nucleon-nucleon interactions when a large excess of neutrons is present. These nuclei are situated far from the stability line and are difficult to produce and study. At the Lohengrin spectrometer we have studied many isomeric states in the region around 132Sn. These studies have given access to intermediate spin states in these nuclei. Our measurements can be compared to state-of-the-art shell model calculations and we can see that the predictions of these models work well for nuclei with proton particles outside 132Sn, even for very neutron-rich nuclei, such as 136Sb. The calculations do not work quite as well for In and Cd hole-hole nuclei, with respected to 132Sn, when one goes more than a few holes from this closed core.

A rapid change in shape from a spherical to a deformed one occurs for the ground state of nuclei in the neutron-rich mass 100 region when going from 58 to 60 neutrons. It is worth noting that the shape of the whole nucleus changes due to the addition of these two neutrons. The nuclei of this region have also been studied in detail using the Lohengrin spectrometer including the first observation of excited states in  nuclei such as 95Kr and 96Rb. With the aid of spontaneous fission studies using large arrays of Ge detectors, our collaboration (LPSC, ILL, Warsaw) has proposed a phenomenological explanation for these shape changes in terms of deformation driving (1/2[550], 3/2[541]) and resisting (9/2[404]) neutron Nilsson orbitals (orbitals in a deformed potential). Open questions still remain regarding the role played by the protons in such shape changes and are the subject of current investigations.

The exact nature of octupole correlations in the neutron-rich mass 150 region, and how they evolve with increasing quadrupole deformation, is also unclear. New isomeric states have been observed in 154Nd and 155Sm and the spins of several other known isomers determined. Our isomeric data have also been combined with information from spontaneous-fission measurements using Euroball or Gammasphere to produce a more complete picture of these nuclei. These studies show that octupole correlations disappear rather rapidly with increase neutron and proton numbers.

The fission, spallation and fragmentation reactions are able to produce these very exotic nuclei. The Lohengrin mass spectrometer uses the former reaction to produce nuclei in the above regions and is one of only a handful of instruments worldwide giving access to these nuclei. The short flight time (~2 microseconds) of fission products through the spectrometer means that if a microsecond isomer exists in the nucleus of interest, then its excited states can be studied. Being able to correlate the arrival of a mass separated nucleus with a detected gamma or conversion-electron decay in a short time window (a few microseconds) is a very sensitive technique, allowing the study of very weakly produced nuclei (a few per minute).


Gamma-ray detectors and ionisation chamber at the Lohengrin (ILL) focal point.

Fission reactions are produced at Lohengrin via neutron-capture reactions on different thin actinide targets which are placed close to the core of the reactor, in a flux of 5x1014 n/s/cm2. At the focal point of the spectrometer the ions are identified by ionisation chambers which are made at the LPSC. These chambers are usually optimised for the type of experiment being performed (gamma-ray spectroscopy, conversion-electron spectroscopy, lifetime measurements). Gamma rays emitted by isomeric states are detected by Clover germanium detectors, purchased as part of the ILL's Millennium program upgrade. As the energy spread of the ions at the Lohengrin focal point is small (~1 MeV) then we can stop the fission fragments at a precise distance from the end of a thin (6 micron) Mylar foil. A segmented, liquid-nitrogen cooled Si(Li) detector is placed behind this foil and as any conversion electrons emitted by the isomeric cascades lose little energy passing through the foil then we can perform conversion-electron spectroscopy down to very low energies (~10 keV). These low-energy, isomer-decay experiments of fission fragments are uniquely possible at Lohengrin. With this setup we have studied the structure of some 60 neutron-rich nuclei!