PHYSICAL REVIEW C 91, 014609 (2015) Angular momentum dependence of the nuclear level density in the A ≈ 170–200 region M. Gohil, Pratap Roy, * K. Banerjee, C. Bhattacharya, S. Kundu, T. K. Rana, T. K. Ghosh, G.Mukherjee, R. Pandey, H. Pai, V. Srivastava, J. K.Meena, S. R. Banerjee, S. Mukhopadhyay, D. Pandit, S. Pal, and S. Bhattacharya Variable Energy Cyclotron Centre, 1/AF, Bidhan Nagar, Kolkata 700064, India (Received 16 September 2014; revised manuscript received 5 December 2014; published 14 January 2015) Neutron evaporation spectra along with γ multiplicity has been measured from 201 Tl ∗ , 185 Re ∗ , and 169 Tm ∗ compound nuclei at the excitation energies of ∼27 and 37 MeV. Statistical model analysis of the experimental data has been carried out to extract the value of the inverse level density parameter k at different angular-momentum (J ) regions corresponding to different γ multiplicities. It is observed that, for the present systems the value of k remains almost constant for different J . The present results for the angular-momentum dependence of the nuclear level density (NLD) parameter ˜ a (=A/k), for nuclei with A ∼ 180 are quite different from those obtained in earlier measurements in the case of light- and medium-mass systems. The present study provides useful information to understand the angular-momentum dependence of the NLD at different nuclear mass regions. DOI: 10.1103/PhysRevC.91.014609 PACS number(s): 25.70.Jj, 25.70.Gh, 24.10.Pa I. INTRODUCTION An accurate determination of nuclear level density (NLD) and information on its dependence on key nuclear parameters such as excitation energy and angular momentum (spin) is essential for precise estimation of nuclear reaction rates using statistical models. Although several theoretical as well as experimental attempts have been made in the past to understand the excitation-energy dependence of NLD; the information on its angular-momentum dependence is quite limited. Information on the angular-momentum dependence of nuclear level density can be obtained experimentally by measuring light-particle evaporation spectra in coincidence with the low-energy γ -ray multiplicity, which is directly related to the angular momentum populated in the nucleus. With the development of advanced γ -ray multiplicity detector arrays, it has been possible to carry out such measurements in recent times [1–6]. On the theoretical side, where the information on the variation of NLD over a wide range of excitation energy (E ∗ ) and angular momentum (J ) comes only from the phenomenology based semi-empirical formulations, the spin dependence in NLD is accounted for by two different approaches. In the first approach, applicable mostly at moderate E ∗ and J , the angular-momentum dependence is incorporated through the spin-dependent rotational energy [7], E rot =  2 2 eff J (J + 1), (1) with  eff = 0 (1 + δ 1 J 2 + δ 2 J 4 ). (2) Here  eff and  0 are the effective and rigid-body moment of inertia of the system and δ 1 and δ 2 , known as the deformability coefficients, are adjustable parameters that provide a range of choices for the spin dependence of the level density [8]. The rotational energy is subtracted from the excitation energy and * pratap_presi@yahoo.co.in the effective energy is used to calculate the NLD by using the standard level density formula [9]. In the second approach, mostly applicable for low E ∗ and J , the spin dependence is introduced in the total level density by a multiplicative Gaus- sian function [exp [−(J + 1 2 ) 2 /(2σ 2 )]] [10], where the width of the Gaussian is determined by the temperature-dependent (T -dependent) spin cutoff factor σ = [( 0 T )/ 2 ] 1/2 . At high excitation energy (i.e., for E ∗  E rot ) these two approaches become equivalent. In both the approaches the spin depen- dence in NLD has been incorporated independently and there is no additional dependence of the level density parameter on angular momentum or deformation. These prescriptions have been tested mostly with the inclusive particle spectra and found to be reasonable to explain the experimental data. However, recent data from exclusive measurements with respect to angular momentum have not been properly explained by the available prescriptions of spin dependence of nuclear level density [2–4]. In these cases additional dependence on angular momentum was required, which was incorporated through the variation of the level density parameter with angular momentum. In one such measurement of angular- momentum-gated neutron evaporation spectra for A ∼ 118, populated at excitation energies E ∗ ∼ 31 and 43 MeV and angular momentum J ∼ 10 to 20, it has been observed that the inverse level density parameter decreases with increasing angular momentum, indicating a relative enhancement of NLD for higher J [3]. In another recent study, we have simultaneously measured all the (significant) light-particle evaporation spectra along with γ -ray multiplicity emitted from the compound nuclei 97 Tc ∗ and 62 Zn ∗ populated at E ∗ ∼ 36 MeV and J in the range of 10 to 20. From the analysis of the all three (n, p, and α particle) light-particle spectra, a strong variation of the level density parameter with angular momentum was observed for both systems [2]. In this case also, the inverse level density parameter was found to decrease with increasing angular momentum. A strong variation of the inverse level density parameter with angular momentum was also reported in the measurement of angular-momentum-gated α-particle spectra for a number of nuclei with A ∼ 120, 0556-2813/2015/91(1)/014609(5) 014609-1 ©2015 American Physical Society