PHYSICAL REVIEW C 79, 024602 (2009) Gamow-Teller unit cross sections of the ( p,n) reaction at 198 and 297 MeV on medium-heavy nuclei M. Sasano, 1,* H. Sakai, 1 K. Yako, 1 T. Wakasa, 2 S. Asaji, 2 K. Fujita, 2 Y. Fujita, 3 M. B. Greenfield, 7 Y. Hagihara, 2 K. Hatanaka, 4 T. Kawabata, 6 H. Kuboki, 5 Y. Maeda, 9 H. Okamura, 4 T. Saito, 1 Y. Sakemi, 8 K. Sekiguchi, 5 Y. Shimizu, 6 Y. Takahashi, 1 Y. Tameshige, 4 and A. Tamii 4 1 Department of Physics, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan 2 Department of Physics, Kyushu University, Higashi, Fukuoka 812-8581, Japan 3 Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan 4 Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan 5 The Institute of Physical and Chemical Research, Wako, Saitama 351-0198, Japan 6 Center for Nuclear Study, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan 7 Department of Physics, International Christian University, Mitaka, Tokyo 181-8585, Japan 8 Cyclotron and Radioisotope Center, Tohoku University, Aoba-ku, Sendai 980-8578, Japan 9 Department of Applied Physics, University of Miyazaki, Kibanadai-nishi, Miyazaki-shi 889-2192, Japan (Received 11 September 2008; published 3 February 2009) Gamow-Teller (GT) unit cross sections, ˆ σ GT , are obtained at 198 and 297 MeV by measuring the double differential cross sections at 0 ◦ for the (p,n) reaction on 58 Ni, 70 Zn, 114 Cd, 118 Sn, and 120 Sn. The mass dependence of ˆ σ GT and the ratio of ˆ σ GT to the Fermi unit cross section, ˆ σ F ,(R 2 ) are also derived in the mass region of 58  A  120. The ˆ σ GT value for 90 Zr at 297 MeV interpolated using the A-dependence obtained herein agrees with that used in a previous analysis where the GT transition strength over a wide energy region up to the continuum was discussed. However, the deduced 64 Ni ˆ σ GT value at 198 MeV is 20% smaller than that obtained from the analysis of a previous (n,p) measurement. The present R 2 values in the mass region heavier than 42 Ca are larger than those in the region up to 42 Ca and increase as a function of A. DOI: 10.1103/PhysRevC.79.024602 PACS number(s): 25.40.Kv I. INTRODUCTION Nuclear spin and isospin processes have attracted much attention because their collectivities are closely related to the properties of nuclear interactions in the spin-isospin channel [1,2]. Among them, the Gamow-Teller (GT) transition is the simplest because both spin and isospin transfers change their respective quantum numbers by one unit, but the other quantum numbers remain the same. Collective aspects of the GT transition have been studied extensively for light to heavy nuclei over a wide excitation-energy region including the GT giant resonance (GTGR) [3–5]. In particular, fp-shell nuclei [6–8] have been intensely studied due to their astrophysical significance. Knowledge of weak processes in stellar cores, especially the GT transition, is essential to the understanding of stellar-core evolution, which leads to supernovae [9]. The β -decay provides the most reliable experimental source for the GT transition strength, B (GT). Unfortunately, GT states of interest are normally located in a high excitation-energy region which is inaccessible by the β -decay. Thus, the (p,n) reaction at intermediate energies (T p  100 MeV) is a powerful probe to explore such relatively high excitation-energy regions. When this reaction excites a GT state, angular momentum is not transferred from the target nucleus to the projectile (L = 0), which implies that the angular distribution of the differential cross section peaks at 0 ◦ . The GT cross section measured at 0 ◦ , σ GT (0 ◦ ), is expected to be related to the * sasano@nucl.phys.s.u-tokyo.ac.jp; http://nucl.phys.s.u-tokyo.ac. jp/sasano corresponding B (GT) value through the proportionality, σ GT (0 ◦ ) = ˆ σ GT F (q,ω)B (GT), (1) where the proportionality factor, ˆ σ GT , which is called the “GT unit cross section,” depends on the incident energy and target mass [10]. Herein the ˆ σ GT values for a variety of masses are determined. The definition of B (GT) is often a source of confusion. Herein B (GT) ≡ |f || ∑ k  σ k t − k ||i | 2 / (2J i + 1), where |i  and |f  are the initial and final states, respectively, ∑ k  σ k t − k is the so-called GT operator, the sub- script, k, runs over all the neutron orbits of the target nucleus,  σ k is the usual Pauli spin operator, t − k is the isospin lowering operator, and J i is the total spin of the initial state. F (q,ω), which gives the dependence of σ GT on the momentum (q ) and energy (ω) transfers, is defined as the ratio of the cross section at finite (q,ω) values to that at (q = ω = 0) and can be reliably calculated by employing a distorted wave impulse approxima- tion (DWIA). On the other hand, theoretical estimates of ˆ σ GT suffer from uncertainties in the relative strength of the isovector part of the NN interactions, the effect of distortions, etc. Hence, to extract reliable B (GT) values, ˆ σ GT must be calibrated empirically for every incident-energy and mass region. Aˆ σ GT value can be determined by measuring the cross section at 0 ◦ for a GT transition whose B (GT) value is deduced from a ft -value for the corresponding β -decay. In most cases, an accurate ft -value exists only for the transition between the ground states. Therefore, the tran- sition to the ground state must be separated from those to the excited states. However, the relatively poor energy resolution in (p,n) experiments where kinetic energies of neutrons are determined by the time-of-flight (TOF) method 0556-2813/2009/79(2)/024602(9) 024602-1 ©2009 The American Physical Society