High Energy Gamma-ray spectroscopy

One of the challenging topics of research in nuclear physics is the evolution of nuclear properties as a function of temperature and angular momentum. Hot (excited) and rapidly rotating (high angular momentum) nuclei can be produced using heavy ion fusion reactions. The decay modes of the excited nuclei carry the signals from which the nuclear properties like the nuclear shape and the parameters of the fundamental vibrational modes can be deciphered. The prominent mode of decay at high excitation energy is via particle (proton, neutron or alpha) emission with a small branch for the gamma decay. However, the study of gamma decay has the advantage that the electromagnetic interaction is well understood and so the nuclear properties can be extracted in a cleaner way. One of the fundamental vibrational modes was manifested about 50 years ago by the observation of the giant dipole resonance (GDR) in the absorption of high energy photons by various nuclei. In a macroscopic picture, this corresponds to the oscillation of protons and neutrons against each other. In a spherical nucleus, vibrations along all three axes are identical and correspond to a single resonance frequency. However, in deformed nuclei the vibration along each axis will have a different energy corresponding to different axial dimensions leading to more than one component in GDR spectra. The separation between theses various components and their relative strengths provide information on the shape of nucleus. These properties can be deduced by studying the (GDR) in excited nuclei. The width of GDR arises due to the dissipation of the collective vibrational energy to the uncorrelated single particle excitations as well as due to fluctuations in the shape of the nucleus. The dependence of the width on temperature and angular momentum can provide valuable information on the damping mechanism. The GDR properties are studied from the gamma decay of these resonances populated in heavy ion fusion reactions.

The study of these GDR gamma rays (~5 to 35 MeV) needs a large efficiency multi-detector (high granularity) array. In our laboratory, large volume NaI(Tl) and BaF₂ detector arrays (consisting of 7 hexagonal elements) have been setup for high energy gamma ray spectroscopy. By measuring the number of low-energy gamma rays emitted in the reaction (multiplicity, which is roughly proportional to the angular momentum) in coincidence with the high energy gamma rays, it is possible to disentangle the effect due to the temperature and angular momentum. The multiplicity filter of BGO/NaI(Tl) detectors is used for this purpose

Systematic studies have been made in different mass regions of nuclear mass number A~80 to 200. A general feature that has been extracted is that the nuclei become deformed, straddling different shapes and orientations at high angular momentum or temperature, irrespective of the ground state shapes. In the A~200 region the deformation as well as the triaxiality (departure from the cylindrical symmetry) is seen to increase with angular momentum, whereas in the A~80 region the effect of temperature is relatively more important than that of the angular momentum.

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