PHYSICAL REVIEW C 102, 054605 (2020) Temperature and isospin dependence of the level-density parameter in the A ≈ 110 mass region G. K. Prajapati, 1 , * Y. K. Gupta, 1, 2 B. V. John, 1, 2 B. N. Joshi, 1 Harjeet Kaur, 3 Nishant Kumar, 1, 2 L. S. Danu, 1 S. Mukhopadhyay, 1 S. Dubey, 1, 4 , † S. R. Jain, 1 D. C. Biswas, 1 and B. K. Nayak 1 1 Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India 2 Homi Bhabha National Institute, Mumbai 400094, India 3 Guru Nanak Dev University, Amritsar 143005, India 4 Physics Department, Faculty of Science, M.S. University of Baroda, Vadodara 390002, India (Received 20 March 2020; revised 1 September 2020; accepted 6 October 2020; published 9 November 2020) α-Particle evaporation spectra were measured at backward angles in 16 O + 94,100 Mo reactions at multiple beam energies. Inverse level-density parameter, K , was determined for Cd nuclei of different isospins by simulating high-energy tail of the measured α-particle evaporation spectra with statistical model code PACE2. An overall increasing behavior of the K value is observed with increasing temperature in the range of 1 to 2.5 MeV. It is observed that in the temperature region below 1.8 MeV, the parameter K is higher for the neutron-rich nuclei by around 1 MeV. Semiclassical calculations were performed which treat single-particle level density of neutron and proton on different footing and this can account for the isospin effects. These calculations reproduce the K values as determined from the statistical model analysis of α-particle evaporation spectra. Present results clearly demonstrate the isospin dependence of the level-density parameter as conjectured by theoretical works. DOI: 10.1103/PhysRevC.102.054605 I. INTRODUCTION Nuclear level-density (NLD) is a fundamental property of atomic nucleus and plays a crucial role to understand several physical phenomena in nuclear physics and astrophysics. It is a key ingredient in the prediction of nuclear reaction cross sections using statistical models. Therefore, it is also a very important input parameter in designing new nuclear technolo- gies. It is often necessary to have an accurate estimate of the NLD of highly excited nuclei as a function of the excita- tion energy, angular momentum, isospin, and other constants of motion. Primarily, it is described in a phenomenological framework, where its excitation energy dependence is given by the Fermi-gas (FG) approximation [1] as ρ (E X ) = √ π 12 exp(2 √ aE X ) a 1/4 E 5/4 X , (1) where E X is the excitation energy of the nucleus and a is the nuclear level-density parameter, which is related to the single- particle level-density g(ε F ) at the Fermi energy (ε F ) through the relation a = (π 2 /6)g(ε F ). Influence of other important factors on the level-density parameter, such as, shell effects, pairing, collectivity, etc., are taken into account through a number of adjustable parameters [2–4]. It is more convenient to use the inverse level-density parameter, K = A/a, where A is the mass number of the nucleus. In the FG approx- imation, the parameter K is constant around 15 MeV. At * shyam@barc.gov.in † Present address: Tata Institute of Fundamental Research, Mumbai 400005, India. excitation energies around the particle emission threshold, the inverse level-density parameter, K , exhibits a dramatic varia- tion around the shell closures due to prominent shell effects. However, an average trend of K = 8–9 MeV is observed while spanning almost the whole nuclear chart. With increasing excitation energies, the shell effects are depleted, and the parameter K approaches to its asymptotic value at nuclear temperatures, T > 1 MeV, and this aspect is understood well [5]. Initially, it was a puzzle that the average magnitude of the NLD parameter, a, is around A/8 MeV −1 , which is sig- nificantly higher than the FG value of A/15 MeV −1 . With increasing excitation energies, however, it is observed that the level-density parameter approaches to the FG value of A/15 MeV −1 [6–13]. Febris et al .[10] has shown from evaporated α-particle spectra in 19 F + 181 Ta reaction that K values in- crease from 8 to 14 MeV while scanning the beam energy from 90 to 140 MeV. Similarly, Roy et al .[14] has reported from evaporated neutron spectra in mass region around A = 210 that K values increase from 7.8 to 10 MeV with increase in temperature from 0.7 to 1.4 MeV. The intriguing variation of K ≃ 8 MeV from low temperature to the FG value of K ≃ 15 MeV at T ≈ 5 MeV has been attributed to the effects arising from finite nuclear size and effective nucleonic mass [8,15,16]. Calculations by Shlomo and Natowitz [8,16] which include finite nuclear size, effective nucleonic mass, and shell effects support the experimental trend of increasing K value with temperature. However, these predict quite different rates of increase in K with temperature in different regions of the nuclear chart. As the nuclear level-density parameter is directly related to microscopic aspects of the atomic nucleus, its temperature dependence is of fundamental importance. 2469-9985/2020/102(5)/054605(11) 054605-1 ©2020 American Physical Society