Preface

Full text: This Annual Report summarizes the research activities of the Nuclear Physics Division in the year 2001. The scientific reports are grouped in two sections: - Reaction Mechanisms and Nuclear Structure; - Experimental Methods and Instrumentation. The current research program of our Division includes ''in-house'' activities using the beams from the Warsaw Cyclotron of the Heavy Ion Laboratory as well as involvement in research at large accelerator facilities around the world. Most of the work described throughout this report was carried out as joint efforts of various international collaborations. During the last year we continue our participation in the FOPI, TAPS and WASA-PROMICE international collaborations. We are strongly involved in the FOPI upgrade project, especially in the construction of the modified TOF detector. The upgrade of scintillation sub-detector BARREL has been completed and TOF resolution of 110-140 ps for minimum ionizing particles was achieved. The analysis of ''event-by-event'' fluctuations in nuclear collisions has been the subject of interest for quite some time. Some new results concerning the possible effect of unphysical fluctuations generated by the response function of the FOPI detector as well as by mixing of events of different centrality are presented in this report. A new approach in studying of chemical equilibrium in dense, hot nuclear matter was tested. For this purpose, fluctuations of rare particles (K+, K-) produced in 58Ni + 58Ni and 96Ru + 96Ru nuclear collisions were used. Unfortunately, the number of double-kaon events from these experiments was too small to obtain conclusive results. The question of mass dependence of sub-threshold neutral pion production in heavy-ion collisions was studied with the TAPS spectrometer. The analysis of experimental data obtained with the Ar beam at 95 AMeV on several targets was completed and neutral pions have been identified through invariant mass analysis. A new approach to the analysis of fusion excitation functions and fusion-barrier distributions was proposed. About 50 precisely measured fusion excitation functions were perfectly reproduced within the phenomenological model and the systematics of the parameters were obtained allowing to predict not yet measured fusion- or capture excitation functions. This result is important for better planning of future experiments on synthesis of superheavy elements. During the last year, we continued our study of high-energy γ-ray emission in heavy-ion reactions in an energy range of 5-11 MeV/u. The purpose of this work is to investigate the properties of hot, fast rotating compound nuclei and to extract information on GDR built on excited states - as a function of the nuclear temperature. In our resent experiments with the modified JANOSIK set-up, measurement of high energy γ-rays have been combined with detection of light charged particles. In such a way a more accurate information on the excitation energy and the mass and charge of the decaying nucleus could be obtained. Using the beam of the Warsaw Cyclotron and the OSIRIS experimental set-up, lifetimes of high spin rotational levels in 131La were measured. The mean lifetimes were extracted with DSA method and were obtained for six high-spin rotational levels of the yrast band built on the h11/2 proton orbital. The extracted values range from 0.3 ps for the 43/2- state to 1.3 ps for 23/2- state. Some interesting results have been obtained for nuclei in the atomic mass region around A∼170. For example , 174Yb was produced in the 170Eu(7Li, pxn) reaction and identified with the GASP spectrometer and the charge particle-telescope system ISIS. A set of four identical γ bands originating from orbitals of different shells was found. Among contributions concerning experimental methods and instrumentation, I would like to mention the interesting proposal of using EUROBALL CLUSTER sub-detectors as in-beam polarimeters. Properties of CLUSTER detectors have been studied and compared with the results obtained for CLOVER detectors. As was already mentioned, most of the work presented in this Annual Report results from close collaboration with our colleagues from many foreign as well as Polish institutes an universities. In this place, I would like to express our deep gratitude to all our friends and collaborators around the world. I would also like to acknowledge the financial support of the Polish State Committee for Scientific Research (KBN). (author)