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The damping of the nuclear shell effect with excitation energy has been measured through an analysis of the neutron spectra following the triton transfer in the 7 Li induced reaction on 205 Tl.The measured neutron spectra demonstrate the expected large shell correction energy for the nuclei in the vicinity of doubly magic 208 Pb and a small value around 184 W. A quantitative extraction of the allowed values of the damping parameter γ, along with those for the asymptotic nuclear level density parameterã, has been made for the first time. The shell effect is a cornerstone of the mean field theory describing finite fermionic systems. The shell structure in atoms decides the chemical properties of the corresponding elements. In nuclear physics the spin orbit coupling, in addition, plays a dominant role in deciding the shell closures and the associated magic numbers of protons and neutrons. The nuclei having such numbers of neutrons and protons have an extra stability with respect to that expected from the average behaviour described by the liquid drop model (LDM). Many important nuclear phenomena such as the occurrence of super heavy elements [1,2], fission isomers [3,4], super-deformed nuclei [5] and new magic numbers in exotic nuclei [6,7] are the consequences of the shell effect. The shell effect also affects another fundamental property of the nucleus viz. the nuclear level density (NLD). The NLD is an indispensable input to the statistical calculation of compound nuclear decay and thus an important physical quantity for many practical applications, such as the calculations of reaction rates relevant to nuclear astrophysics, nuclear reactors and spallation neutron sources.The NLD was first calculated by Bethe using a noninteracting Fermi gas model, without shell effects, arriving at its leading dependence on excitation energy (E X ) and angular momentum (J) [8,9]. The generic behaviour with respect to E X is described by e 2 √ aE X . Here 'a' is the NLD parameter which is related to the single particle density at the Fermi energy. Direct measurements of the NLD are based on the study of slow neutron resonances, which are mainly s-and p-wave, and are extrapolated to higher J values to estimate the angular momentum summed or total NLD. The total NLD inferred from such a measurement shows that on the average the level density parameter a increases linearly with the mass number (A) of the nucleus as a ≈A/8 MeV −1 . However, there is a significant departure from this liquid drop value at shell closures. This departure is the largest for the doubly magic nucleus 208 Pb, where a (at E X ∼7 MeV) is as low as A/26 MeV −1 . This shell effect on the NLD parameter is expected to damp with excitation energy so that a approaches its liquid drop value at E X ∼ 40 MeV [10]. It is important to make measurements on the damping of the shell effect over a wide E X range. To our knowledge, no such measurement has been reported.Experimental information on the damping of the shell effect can be obtained by measuring the E X dependence...

High-energy gamma rays in the range of ∼4–28 MeV were measured in the reaction 28Si+124Sn at E(28Si) ∼ 185 MeV in coincidence with low-energy gamma ray multiplicities and evaporation residues. The centroid energy and width of the giant dipole resonance were extracted for various multiplicity windows from the statistical model analysis. These extracted widths, along with those from an earlier measurement at E(28Si) ∼ 149 MeV, show a discrepancy with the results of a calculation under the thermal shape fluctuation model which describes the inhomogeneous damping of the resonance. An empirical form of the temperature and angular momentum dependence of the width, describing the data at both the beam energies, has been derived. The present results suggest that the contributions from both the inhomogeneous damping and the intrinsic collisional damping processes should be included.