Determination of the Gamow–Teller quenching factor from charge exchange reactions on 90Zr

Abstract

Double differential cross sections between 0 • -12 • were measured for the 90 Zr(n, p) reaction at 293 MeV over a wide excitation energy range of 0-70 MeV. A multipole decomposition technique was applied to the present data as well as the previously obtained 90 Zr(p, n) data to extract the Gamow-Teller (GT) component from the continuum. The GT quenching factor Q was derived by using the obtained total GT strengths. The result is Q = 0.88 ± 0.06, not including an overall normalization uncertainty in the GT unit cross section of 16%.The (p, n) reaction at intermediate energies (T p > 100 MeV) provides a highly selective probe of spin-isospin excitations in nuclei due to the energy dependence of the isovector part of nucleon-nucleon (NN ) t-matrices [1]. The

Effects of Three-Nucleon Forces and Two-Body Currents on Gamow-Teller Strengths

“…The quenching factor depends on the nucleus, with q 2 ≈ 0.84 − 0.92 due to 2BCs for the studied nuclei. We recall that q 2 ≈ 0.88 − 0.92, extracted from experiments on 90 Zr [18][19][20], are within our error band. We also computed the low-lying strengths for β − decay, and found that only 70% -80% of the total strength S ± is exhausted below 10 MeV of excitation energy.…”

“…GamowTeller strength functions [14,15] are of particular interest also because of their astrophysical relevance [16]. Chargeexchange measurements on 90 Zr and other medium mass nuclei have suggested that the total strength for β decay is quenched by a factor of q 2 ≈ 0.88 − 0.92 [17][18][19][20] when compared to the Ikeda sum rule [21]. Similarly, shellmodel calculations [22,23] suggest that g A needs to be quenched by a factor q ≈ 0.75 to match data.…”

Role of momentum transfer in the quenching of the Gamow-Teller strength

“…These experiments, however, were only able to measure the strength up to the excitation energy of the giant resonance. A consistent analysis of (p, n) and (n, p) reaction data from 90 Zr over a much wider range of excitation energies, concluded that the GT strength is actually quenched by approximately 10% [5,6]. Theoretical models systematically overestimate the transition strength compared to the measured values [7].…”

“…Nevertheless, recent experiments reported data on the Gamow-Teller response in 90 Zr, in both β − [5] and β + channels [6], up to 50 MeV excitation energies. Therefore, in the total strength, contributions from higher-order terms in the expansion in q appear, the first of which is the isovector spin monopole (IVSM) mode.…”

“…To obtain precise quantitative data on the total GT strength, and indirectly on the quenching, the contribution of the isovector spin monopole mode and of higher-order terms in momentum transfer, must be subtracted from the measured strength. Although interference occurs between the GT and IVSM modes, these contributions are usually subtracted incoherently from the spectrum because the distribution of GT strength in the IVSM resonance region is unknown [6]. Related to new measurements of nuclear response in unstable nuclei [17], the correct treatment of effects that influence the extraction of the Gamow-Teller strength is all the more important.…”