The two physics objectives of this research project are
i) to investigate the relation between a anisotropies and nuclear deformation, which is at present not well understood and;
ii) to obtain the first precise data on the quantum numbers of fission channels, which is necessary for a better understanding of nuclear fission and for the development of a microscopic theory for this phenomenon.
The understanding of both phenomena is at present very partial due to the lack of a sufficiently large set of experimental data. This is caused by the high technical complexity of both
i) the methods to prepare samples of nuclei that are useful to investigate these two topics and of;
ii) the experiments that are to be carried out to obtain the required physical information.
In order to realize these objectives one has to study the possibilities for the preparation of pure samples of deformed superheavy actinide nuclei (241Am, 243Am, 253Es, 255Es, 255Fm and 257Fm) and to prepare these samples. Once available they will be used to measure
i) the anisotropy of the a particles emitted by these nuclei using the low temperature (e.g. T < 100mK) nuclear orientation method (LTNO) in combination with high resolution a particle detection at 4.2 Kelvin and;
ii) to measure the anisotropy of the spontaneous fission fragments emitted by oriented 255Es and 257Fm nuclei, again using the LTNO method.
The americium activities will be prepared at the Russian Federal Nuclear Centre RFNC in Sarov. The S-2 mass separator of the Radiochemistry Division of this research centre was specially designed to mass separate and implant a-active isotopes of the transuranium elements americium, curium, plutonium, and uranium in order to obtain samples of very high-purity for nuclear physics and applied research. The einsteinium and fermium activities will be prepared at the Research Institute for Atomic Reactors (RIAR) in Dimitrovgrad (Russia), which has a long standing experience in the accumulation of superheavy nuclei using a high flux reactor facility as well as in the radiochemical reprocessing of the irradiated targets. The Es and Fm samples for the LTNO experiments will be prepared at the University of Bonn where one has a large experience in the electromagnetic isotope separation of a.o. several heavy elements above bismuth.
Once suitable samples of the above mentioned nuclei are available these will be loaded in 3He-4He dilution refrigerators after which the anisotropy of the a particles of oriented 241Am, 243Am, 253Es, 255Es, 255Fm and 257Fm nuclei and of the spontaneous fission fragments of oriented 255Es and 257Fm nuclei will be measured as a function of temperature (down to 5milliKelvin).
These anisotropies will be measured with particle detectors, which operate inside the 4.2 Kelvin radiation shield of the refrigerators with properties similar to particle detectors operating at room temperature, a technique that was developed by the group from the University of Leuven. All four groups from the University of Bonn, the University of Leuven, the Russian Academy of Sciences in Moscow and the Moscow State University are experts in low temperature nuclear orientation and are using 3He-4He dilution refrigerators since many years already.
From the measured anisotropies for the favoured a transitions in the decay of the Am, Es and Fm isotopes the mixing ratios d02 between the L = 0 and L = 2 partial a waves can be obtained. For nuclei with a clear static deformation this type of information exists for only three cases up to now, e.g. 221Fr, 227Pa and 229Pa, all with a deformation parameter b2 £ 0.190. Thus, since six new cases will be added, all with a deformation parameter b2 > 0.190 and up to 0.236, the set of isotopes for which mixing ratios d02 are available will be significantly extended. This will allow to further extend the understanding of the origin of anisotropy in the angular distribution of a particles, and in particular its relation to nuclear deformation.
From the measured anisotropies of the fragments emitted in the spontaneous fission of oriented 255Es and 257Fm nuclei the projection K of the spin on a symmetry axis of the nucleus will be obtained. Together with the known spin and parity of the fissioning ground state of these nuclei one will then have the complete set of quantum numbers describing this state at its disposal, which is the necessary input for the development of a microscopic theory of nuclear fission. In addition, one may expect to obtain from these experiments new information on the "transition effects" in nuclear fission that are predicted for this region of nuclei, while it will also be investigated whether it is possible to experimentally test the hypothesis of parity violation in spontaneous fission.