COULOMB EXCITATION AT THE WARSAW CYCLOTRON

the Warsaw Coulex Group:
Paweł Napiorkowski, Katarzyna Hadyńska-Klęk, Jędrzej Iwanicki, Michalina Komorowska, Julian Srebrny, Kasia Wrzosek-Lipska

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 applying the Coulomb excitation method to nuclear structure studies.

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 assumptions concerning the nuclear forces. The use of various beams and covering broad ranges of scattering angles makes it possible to measure full sets of the parameters of the electromagnetic structure up to high-spin levels – transitional matrix elements and static moments, which are related to spectroscopic observables such as lifetimes, gamma-ray intensities, branching and mixing ratios. Finally, the knowledge of a full set of matrix elements can be used to determine the shape of the nucleus for each excited level independently, thus serving as “nuclear microscope”. Heavy ion Coulex provides a stringent test of existing models in a purely experimental way.

Recently new interest in Coulexhas been drawn by commissioning of the radioactive beam facilities that open perspectives to study the structure of unstable nuclei.

Heavy Ion Laboratory is capable of performing full-scale Coulex projects. The U-200P 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

Two particle detector arrays designed for Coulomb excitation studies are used at HIL. The older setup, CUDAC (Coulomb Universal Detector Array Chamber) was constructed in the Laboratory in the early nineties and installed as one of the permanent experimental stands. After some years of succesful CUDAC measurements, a new scattering chamber, previously used with the NORDBALL setup, was adapted to work with the gamma detector array EAGLE

Both setups were 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 is sufficient 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 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 in Coulex studies three HPGe detectors were used, working in coincidence with PIN-diodes.

 

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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.

Nowadays CUDAC is used for experiments other than Coulomb excitation, mostly 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 distributions: Ł. Świderski (2005)) as well as for several MSc theses.

New scattering chamber

The scattering chamber, which was succesfully integrated into OSIRIS-II and then EAGLE, had been 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 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. The 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. All Coulex experiments at HIL performed after this date could benefit from higher overall efficiency and improved peak-to-Compton ratio of the OSIRIS-II array (comprising 12 BGO-shielded HPGe detectors) and its successor EAGLE.

 

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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 extraction of electromagnetic matrix elements from measured gamma-ray intensities. 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), Leuven (Belgium), Liverpool (UK), Jyvaskyla (Finland), Muenchen (Germany), Chandigarh (India), Tokai, Chiba, Osaka and Kyushu (Japan).

It should be stressed that available data from heavy ion Coulex are still scarce, so our Coulex project has obviously wide perspectives for the future. It already is and should remain for the years to come one of the home specialties of the nuclear physics in Warsaw.