Other experimental set-ups: IGISOL JANOSIK CUP Si Detectors BARRIERS SYRENA ICARE EAGLE

COULOMB EXCITATION
AT THE WARSAW CYCLOTRON

the Warsaw Coulex Group:

Julian Srebrny, Katarzyna Hadyńska-Klęk, Jędrzej Iwanicki, Paweł Napiorkowski, Daniel Piętak, Kasia Wrzosek-Lipska, Magda Zielińska

The Warsaw Coulex Group was gathered in the early nineties around Tomek Czosnyka, who was its leader until his death in 2006. Under his supervision three PhD and two MSc theses were completed, all pursuing the research using Coulomb excitation method.

Remembrance about Tomek and his legacy and sympathies we received.

Coulomb excitation method

Coulomb excitation (Coulex) is a powerful method to study nuclear structure. Proper selection of the beam energy assures that the interaction between the colliding beam and target particles is purely electromagnetic, thus allowing for the model-independent description in terms of classical electrodynamics, free from the assumptions concerning the nuclear forces. The use of different beams and impact parameter information in principle allows to measure full sets of the parameters of the electromagnetic structure up to high-spin levels - transition matrix elements and static moments, the values which determine spectroscopic observable - lifetimes, gamma intensities, branching and mixing ratios. Finally, the knowledge of the full sets of matrix elements can be used to reproduce the shape of the nucleus independently for each excited level, thus serving as "nuclear microscope". Heavy ion Coulex provides a stringent test of existing models in a purely experimental way.

Recently new interest in Coulex is drawn by commissioning the radioactive beam facilities opening the perspectives to study the structure of unstable nuclei.

Heavy Ion Laboratory is capable of performing the full-scale Coulex projects. The cyclotron offers a wide range of heavy ion beams and the flexibility of adjusting the beam energy according to the requirements.

Coulex setups at HIL

At present two particle detector arrays, dedicated for Coulomb excitation, are used at HIL. The older setup, CUDAC (Coulomb Universal Detector Array Chamber) was built in the Laboratory in the early nineties and installed as one of the permanent experimental stands. Recently a new scattering chamber, previously used with the NORDBALL setup, has been adapted to work with the gamma detector array OSIRIS-II.

Both setups are built on the same principle. Coincident detection of gamma radiation and scattered particles makes possible to uniquely and precisely ascribe the observed gamma rays to the kinematics of collision. This in turn enhances the efficiency of experiments - one physical accelerator run can be off-line divided into several data sets differing by impact parameter. Such an operation is equivalent to performing several experiments at the same real time.

In both scattering chambers small silicon detectors, so-called PIN-diodes, are used for particle detection. As PIN-diodes are not position-sensitive, the information on particle detection angle is obtained by using many diodes in one setup. The relatively small area of a single detector provides sufficiently precise information on scattering angle.

The particle energy measurement is not essential in Coulomb excitation experiments. With thin targets the information on the particle scattering angle allows one to determine the whole kinematics of the scattering process. However, the information on the particle energy is useful to reject the events caused by noise or scattering on admixtures in target. It is also crucial when analysing the experiments with thick targets when the incident energy changes due to the beam stopping in target.

CUDAC

The CUDAC chamber is equipped with 32 PIN-diodes (1 x 1cm), placed at backward angles, which correspond to the strongest excitation of the investigated nucleus.

To detect the deexcitation γ-radiation three HPGe detectors are used, working in coincidence with PIN-diodes.

Fig. 1, left: Schematic view of the CUDAC set-up. right: The CUDAC chamber open to show the silicon detectors inside.

CUDAC has proven to be an excellent tool to study the properties of well deformed nuclei, such as 165Ho. However, for the less deformed ones, such as Mo isotopes, the gamma detection efficiency was too low and therefore we decided to use a more compact Coulex scattering chamber with a multi-detector germanium array OSIRIS-II.

It should be mentioned, that CUDAC was used for the experiments other than Coulomb excitation, e.g.. for ionization or fusion barrier studies.

The data collected using the CUDAC setup were a basis for 3 PhD theses (two on Coulomb excitation: J. Iwanicki (2002), M. Zielińska (2006) and one on fusion barrier distribution: Ł. Świderski (2005)) as well as for several MSc theses.

New scattering chamber

The scattering chamber, which has been recently succesfully integrated into OSIRIS-II, was constructed at LMU Munich as a dedicated Coulomb excitation setup designed for use with the NORDBALL germanium array.

Its small size (10cm in diameter) allows to place HPGe detectors at close distances from the target, thus increasing the gamma detection efficiency. The chamber can accomodate up to 110 PIN-diodes of 0.5 x 0.5cm active area, covering the backward angles (from 110 to 170 degrees with respect to the beam direction). At present 48 PIN-diodes are used. An original electronics was replaced by three 16-fold digitally controlled fast/slow amplifiers (CAEN N568/LC) and constant fraction discriminators (CAEN C808).

The first experiment using this new dedicated Coulex setup was performed in 2006. The future Coulex experiments at HIL can benefit from higher overall efficiency and improved peak-to-Compton ratio of the OSIRIS-II array (comprising 12 BGO-shielded HPGe detectors) and its planned successor EAGLE.

Fig. 2:The backward hemisphere of the new scattering chamber.

Data analysis

Last, but not least, HIL Coulex team is in command of the sophisticated software necessary to analyze the wealth of data from the experiments. This includes both processing of "raw" data and further physical analysis. The analysis computer codes, GOSIA and SIGMA, developed, maintained and updated by Warsaw Coulex group members, are now used by numerous world laboratories dealing with Coulomb excitation. A list of collaborating centers include Rochester and Argonne (USA), Saclay and Orsay (France), Liverpool (UK), Jyvaskyla (Finland), Muenchen (Germany), Chandigarh (India), Tokai, Chiba, Osaka and Kyushu (Japan).