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Our experimental set-up is installed on one of the beam lines of the U400 cyclotron of the FLNR . This cyclotron should soon be modernised. This modification will improve the quality and intensity of the beams delivered by the cyclotron. It will also allow for a smooth beam energy variations and reduce the consumption of rare isotopes.
The heavy nuclei of interest are produced in fusion-evaporation reactions in which the dominating process is fission. The evaporation residues (ERs) are kinematically and electrically selected from the beam particles and the overwhelming background of fission products by a recoil separator called VASSILISSA.
VASSILISSA has an ion optical scheme Q-Q-Q-E-E-E-Q-Q-Q-D (where
Q stands for Quadrupole lens, E for Electrostatic Dipole, D for Dipole magnet). It is an energy filters since the separation of the reaction products is based on the difference of Eion/Qion ratio (were Eion stands for energy of recoil nucleus or reaction product, Qion – for ionic charge respectively). The present set up is limited to an electric rigidity of recoil nuclei of 2 MV (this means that the separator can transmit nuclei with energy Eion - 40 MeV and charge state Qion - 20+).
The modernization of VASSILISSA has two purposes :
the transmission/detecetion efficiency for very asymmetric combinations like 22Ne + 238U or 16O + 244Pu by a factor of 5 – 6 and
to extend the region of accessible reactions to symmetric
combinations like 136Xe + 136Xe, for which ERs have an electric rigidity about 10 MV.
It is planned to replace the central part of the separator, consisting of 3 electrostatic deflectors, by a
combination of two electrostatic deflectors and two dipole magnets creating a velocity filter instead of an energy one. The new velocity filter will be called SHELS (Separator for Heavy Element Spectroscopy).
The modernized separator will be of Q-Q-Q-E-D-D-E-Q-Q-Q-D type and will operate in
vacuum mode. It will be 12 m long.
A grant was obtained from the Agence Nationale de la Recherche in 2006. This grant will cover part of the necessary funds to modernize VASSILISSA and its detection system.
The dipole magnets were ordered at the end of 2008 and the yokes arrived for Christmas 2009 !
The electric dipole tanks were ordered as well as the vacuum chambers for the dipoles.
In the summer of 2010, the vacuum chambers were ready :
By the beginning of 2011, the new chambers had passed the vacuum tests
and the dipole magnets were ready to be tested.
In the summer of 2011, VASSILISSA was dismounted, the beam line was moved slightly and the base of the new separator fixed to the floor.
The field mapping of the quadruple triplets was performed in 2012 thanks to a specially designed magnetic probe.
In November 2012, VASSILISSA was mounted in the cave and the pumping system was installed.
Vacuum tests were carried shortly after the celebration of the 40th anniversary of the IN2P3-JINR collaboration in Januray 2013.
The plates of the electrostatic dipoles were then carefully lowered into the vacuum tanks and connected to the HV power supplies.
The first HV was then applied in april 2013.
At the beginning of May 2013, the first alpha particles were transmitted at the focal plane of VASSILISSA.
With alpha particles, the transmission was measured to be 50% larger than before.
The transmission measured with heavy ions was found to be a factor of 2 larger for 48Ca and 50Ti.