|Accelerator & Separator (Vassilissa and Shels)|
by- 10 October 2008
In the spring and summer of 2004, GABRIELA was commissionned. The characteristics of the setup and the results of the comissioning runs are described in the article by K. Hauschild, A. Yeremin et al., published in Nucl. Instr. and Meth. Phys. Res. A 560 (2006) 38.
In the fall of 2004, the nuclei 253,254,255No, 255Lr and 227Np were studied with GABRIELA. A new excited state was observed in 249Fm and internal conversion coefficients extracted for the observed transitions. The N=151 systematics of the low-lying isomeric 5/2+ state was extended to 251Fm and 253No. High K isomeric states were observed in 253No and 255Lr. The analysis of the excited states populated in the alpha decay of 255No is still ongoing.
In 2005, a radioactive 210Pb target was used. The aim was to produce 257No via the 5-neutron evaporation channel and study its alpha-decay to 253Fm. Unfortunately, no events were observed due to lack of target material.
The experimental setup was modified in the spring-summer of 2006: the 37 degree magnet was removed and the original 8 degree magnet put back in its place. The ToF detector is now more compact and the secondary-electron-emitting foils thinner in order to reduce the energy losses of the recoils produced in light ion induced reactions. The efficiency of the Germanium array was increased by nearly a factor of 2 by reducing the distance between the Germanium crystal and the front face of the Aluminium end cap of the detector closest to the implantation detector.
The characteristics of the new setup were measured during a test run in mid-September 2006. 22Ne induced reactions products (from Au, Pb, W and Pt targets) were transported to the focal plane of Vassilissa. The background conditions were considerably improved and the transmission/detection efficiency was measured to be 6% (3 times higher than with the old setup!).
The actual 2006 campaign started with a test-run with 238U targets. Initially, we ran with the standard ToF set-up (2 emissive foils) and no 255No alpha-decays were seen at the focal plane of VASSILISSA. Extensive tests have shown that even with a vacuum separator, one can only work with 1 ToF foil due to the important angular straggling of 255No recoils produced in asymmetric fusion-evaporation reactions induced by light projectiles. Finally, 1500 255No alpha-decays could be observed at the focal plane of VASSILISSA and the transmission and detection efficiency of No recoils was estimated to be of the order of 1%. Unfortunately, problems with the fabrication of the 242Pu targets did not allow us to perform the scheduled 3-week experiment to study the decay properties of 259Rf.
During February/March 2008 we performed our 4th one-month long campaign. Our initial plan was to study 259Rf following the bombardment of a 242Pu target with a 22Ne beam. In order to achieve this goal the emissive foils used in the time-of-flight measurement were reduced to 15 μg/cm2. Tests were performed using 22Ne+197Au and a transmission of over 6% was achieved. The next incremental step was to tune the separator parameters using the 22Ne+238U reaction since it has approximately 40 times the cross-section of 22Ne+242Pu. Surprisingly we were unable to detect 255No alpha decay at the focal plane, even though we had managed to do this in the 2006 campaign. We have therefore abandoned the program of light beams on actinide targets until the upgrade of VASSILISSA has been completed. Changing to a 40Ar beam we then studied the decay properties of 245,246Fm, obtained complementary statistics for the decay of 217Pa and, in preparation for isomeric studies in 218U, searched for isomers in 214Th.
We had envisaged to study 256,257Rf but problems with the beam preparation forced us to revert to the detailed study of the isomeric- and alpha-decay properties of 253No using a 48Ca beam. The focal plane detector and its electronics have been modiﬁed and successfully tested for the ﬁrst time during this campaign (Feb-March 2009). The detector is now a 48x48-strip DSSD equipped with 3 pre-ampliﬁcation ranges, which allows it to be sensitive to low-energy electrons (100 keV) as well as high-energy ﬁssion events (∼ a few hundreds of MeV). The transmission of VASSILISSA was also much improved but we suffered from many quadrupole power supply brake downs during the 5 week irradiation time. During the campaign, it was attempted to investigate beta-delayed fission of 196At produced in the reaction 40Ca + 159Tb. This was abandonned because of the power supply problems. Finally, the last days of the campaign were dedicated to collect more statistics on light Np isotopes produced with a Ta target. This run was ended abruptly when the high voltage of one of the electrostatic dipoles failed.