Measurement of isospin mixing at a finite temperature in80Zr via giant dipole resonance decay

Abstract

Isospin mixing in the hot compound nucleus 80 Zr was studied by measuring and comparing the γ -ray emission from the fusion reactions 40 Ca + 40 Ca at E beam = 200 MeV and 37 Cl + 44 Ca at E beam = 153 MeV. The γ yield associated with the giant dipole resonance is found to be different in the two reactions because, in self-conjugate nuclei, the E1 selection rules forbid the decay between states with isospin I = 0. The degree of mixing is deduced from statistical-model analysis of the γ -ray spectrum emitted by the compound nucleus 80 Zr with the standard parameters deduced from the γ decay of the nucleus 81 Rb. The results are used to deduce the zero-temperature value, which is then compared with the latest predictions. The Coulomb spreading width is found to be independent of temperature.The issue of isospin impurity in nuclei has been a longstanding open problem in nuclear physics. In particular, its knowledge is interesting in connection with the properties of the isobaric analog states (IAS) and with the Fermi β decay of the N ≈ Z nuclei around the proton drip line. The evaluation of the isospin impurity provides an important correction to the Fermi-transition rates allowing the extraction, in a nucleusindependent way, of the up-down quark-mixing matrix element of the Cabibbo-Kobayashi-Maskawa matrix [1,2]. Concerning the IAS, they are known to have a narrow spreading width ↓ related to the isospin impurities [3,4] originating from the Coulomb interaction coupling them to states of different isospin.In general, the breaking of isospin symmetry can be observed using, as a magnifying lens, the decays which would be forbidden by the selection rules if isospin mixing was not to occur. This is the case of the neutron decay from the IAS [5] and of the E1 decay from self-conjugate nuclei [6].The giant dipole resonance (GDR), where the maximum strength of the E1 transitions is concentrated, is the ideal excitation mode where this selection rule of E1 decay can be fully exploited. This approach was employed to measure the E1 decay of the GDR in nuclei at a finite temperature produced with fusion-evaporation reactions [7][8][9][10][11]. Fusion-evaporation reactions allow the production of self-conjugate compound nuclei (CN) at high excitation energy which, in many cases, * Present address: CEA Saclay, F-91191 Gif-sur-Yvette, France. are far from the β-stability valley. The use of a self-conjugate projectile and target ensures that the CN produced in fusion reactions has isospin I = 0. Therefore, E1 emission associated with the decay of the GDR is hindered due to the fact that, if the isospin of the initial state is pure, only the less-numerous I = 1 final states can be reached in the decay [12]. Conversely, if the initial state is not pure in isospin but contains an admixture of I = 1 states, it can decay to the more numerous I = 0 final states. Thus, the first-step γ yield depends on the degree of isospin mixing of the CN. In addition, at a finite temperature one expects a partial restoration of the isospin symmetry b...

Isospin Mixing inZr80: From Finite to Zero Temperature

“…For 81 Rb the best-fitting values (see the corresponding FOM in the top right-hand panel of Fig. 2) for the centroid, width, and strength of GDR were found to be E GDR ¼16.4AE0.2MeV, Γ GDR ¼7.0AE0.2MeV, and S GDR ¼ 0.90 AE 0.05, in agreement with the systematics and liquid drop model calculations [15,21].…”

“…[15] the isospin mixing was investigated at N ¼ Z ¼ 40 at T ¼ 3 MeV, while previous works concern CN with smaller N ¼ Z values only. The work on 80 Zr [15] showed that by using the latest prediction of the isospin mixing value (based on EDF models [16]) together with the expression giving the T dependence of the isospin mixing [11], one finds a good agreement with the measured value at T ¼ 3 MeV. This finding indicates that with an additional experimental point at another T one could deduce, from the combined data analysis, the value at T ¼ 0.…”

“…These were obtained by dividing the measured and calculated spectra with a statistical model calculation in which the BðE1Þ has a constant value, instead of the standard Lorentzian function, (see Fig. 3) [12,15]. To provide a more convincing evidence of the effect of the Coulomb spreading width, calculations were also made assuming full mixing [see the dashed blue line in Fig.…”

“…They were analyzed with the statistical model and the corresponding calculations (folded with the detector response function and normalized to the data at around 5 MeV) were obtained with the CASCADE code [19,20] 2015 222502-2 including the isospin formalism (as in Ref. [15]). The analysis of the spectrum for the 81 Rb compound nucleus provided the GDR parameters as derived from the best fit to the data in the region between 8 and 15 MeV.…”

Spectroscopy of Kr70 and isospin symmetry in the T=1 fpg shell nuclei

“…In a further example, shell-model calculations, intended to predict the location of the driplines, have recently had isospin nonconserving (INC) interactions specifically included [3]. To date, such studies have largely focused on nuclei in the sd and lower fp shells and has involved studies of the energies of analog states (see [4,5] and references therein) as well as studies of the hinderance of E1 decays due to selection rules in N = Z nuclei [6][7][8]. Detailed investigations in the f 7 2 shell region have revealed a need to include an isospin nonconserving (INC) force beyond the Coulomb interaction (see Ref.…”