Our collaboration with scientists from the Department of Neutron Research, Uppsala University, goes back to 1992, and comprises a few different projects, which are concentrated on the experimental work at the OSIRIS mass separator at Studsvik. The OSIRIS facility is an excellent source of neutron-rich exotic nuclides, and has been particularly successful in reaching the highly interesting doubly closed shell regions at 78Ni and 132Sn. The purpose of the present research is to determine the atomic masses of far unstable nuclides in these regions with a high accuracy and also other information on nuclear structure. The mass determinations are performed using a b - spectroscopy method with hyper pure germanium and other solid state detectors. This is a very precise and efficient method, which has opened new possibilities for nuclear mass determinations, in particular for the nuclei near closed shells having very high decay energies.
A considerable part of our first visits to Sweden and Studsvik was spent on improving the method by studies of the spectrometer response to high energy b - particles. These studies have been decisive for reaching the present level of precision. The data obtained are of substantial scientific interest. In the vast area with atomic mass A>56, only one case of doubly closed shells was well known previously, at 208Pb. Our new data for the second region of this kind, at 132Sn (which is as much as about 15 neutrons away from stability), with uncertainties in mass values of less then about 0.4 ppm (corresponding to less then 60 keV) make it possible to perform detailed empirical comparisons of binding and interaction energies at two such regions among heavy nuclides.
The project is now in a rather productive phase. It was shown that the proton-neutron effective interaction energies derived from mass values for the 132Sn region are equal or very close to the ones for the 208Pb region , if (l,j) and (l+1,j+1) - configurations for both protons and neutrons are compared. This result supports the idea of a universal shell model description of different magic regions of heavy nuclides, which may allow to predict properties of unknown magic nuclides throughout the nuclidic chart.
We plan also to continue the mass determinations in some other particularly interesting regions. These include the third double shell closure at 78Ni (Z=28, N=50), where some data already have been obtained in experiments at the OSIRIS mass separator. This region is of particular significance also for astrophysical reasons since the starting point of the r-process path, as seen on the chart of nuclides, is not far from the doubly magic 78Ni and goes through it and nearest nuclei. The properties of these nuclei such as atomic masses, neutron binding energies, half-lives and neutron-emission probabilities, which will be extracted from experimental data, have a crucial importance for a proper understanding of the astrophysical rapid neutron captureprocess, which is responsible for the nucleosynthesis of heavy elements in Nature.
The region above the “magic cross” at A = 78 is definitely
feasible for experiments at OSIRIS - facility in Studsvik. The location
of the r - process path here can be derived from precise determinations
of neutron separation energies. These data can be obtained by Qb
- measurements for nuclides in the isobaric chains A and A+1. The study
of “waiting-point” nuclei along N = 50 will provide the experimental information
for further calculations of the stable isotopes r - abundances in the A
= 80 abundance peak. It is possible for many cases of astrophysical interest
to obtain the beta-delayed neutron branching ratios using the method of
the g - spectroscopic analysis, which can be
more accurate in some cases than other methods. The present plan for upgrading
the production of very exotic nuclei at OSIRIS is a strong argument for
a continuation and extension of the present research program on atomic