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Continuum y-ray spectra from decays of 'Cu formed at initial excitation energies of 22.5 to 77.4 MeV and maximum spin up to 40k, using He+' Co, Li+' Fe, ' C+ "V, and "0+ 'Sc entrance channels, have been measured and analyzed. The parameters of the giant dipole resonance strength function have been extracted using a statistical code in a nonlinear least-squares fitting routine. Except for the cases of He and Li at the highest bombarding energies, which show evidence for nonstatistical effects, spectra are well reproduced by statistical calculations. The mean energy and strength of the giant dipole resonance built on excited states of 'Cu are close to the ground-state values, while the width varies smoothly from -5 MeV for the ground-state giant dipole resonance up to 10.6+0.6 MeV in the temperature range up to 1.9 MeV and mean spin in the range 0 -23k. The large range of energies studied permitted different level density formulations to be tested. Measured spectra from Li+ 'Mo and Li+ ' 'Ta at E~,b( Li) = 36 MeV show a strong nonstatistical enhancement at high y-ray energies.

Excited states in 133 Sn were investigated through the β decay of 133 In at the ISOLDE facility. The ISOLDE Resonance Ionization Laser Ion Source (RILIS) provided isomer-selective ionization for 133 In, allowing us to study separately, and in detail, the β-decay branch of 133 In J π = (9/2 +) ground state and its J π = (1/2 −) isomer.

We have deduced compound nuclear isospin mixing in 28 Si* and 26 A1* at excitation energies between 33 and 65 MeV. The 7-ray yield from the decay of giant dipole resonances built on excited states is found to be suppressed, implying that isospin mixing is small. By comparison with data from the literature at lower excitation energy, we conclude that isospin is a better symmetry in these compound nuclei at high excitation energy.PACS numbers: 24.80. Dc, 24.30.Cz, 24.60.Dr, 25.70.Gh Over 30 years ago, Wilkinson predicted that at high excitation energies, the compound nucleus, whose lifetime is determined primarily by the strong interaction, should not live long enough for isospin symmetry to be broken by the relatively weak Coulomb interaction [1], More recently, Kuhlmann [2] and Harney, Richter, and Weidenmuller [3] have proposed that compound nuclear isospin mixing should be characterized by an isospinviolating spreading width F^ which is approximately constant with mass number and excitation energy. This proposal was made on the basis of a large body of data, mostly measurements in different compound nuclei. However, spreading widths deduced from these data scatter over more than 1 order of magnitude [2, 3].The concept of a constant isospin-violating spreading width leads to a definite expectation concerning the excitation energy E* dependence of isospin mixing within a given nucleus. At energies where the compound nuclear decay width F is much greater than D, the level spacing, compound nuclear reaction theory implies that the mixing should depend on T^/F. Hence at high E*, where r ^> r^, the mixing should become small as Wilkinson suggested. At very low E* where F is small, the mixing is weak, on the average, because levels are widely spaced. Thus an additional expectation is that the mixing should be a maximum at energies where F ~ D.Although some previous experiments have suggested a decrease in isospin mixing over a narrow interval in E* (see Ref.[3]), the general form of the energy dependence of isospin mixing has not been demonstrated clearly. An additional motivation for a better understanding of compound nuclear isospin symmetry breaking stems from the recent interest in applying similar concepts to the interpretation of measured and proposed compound-nuclear parity [4] and time-reversal violation experiments [5], These are all examples of a partially broken symmetry in a quantum statistical mechanical system. The underlying symmetry breaking mechanism should be best understood in the case of isospin, making isospin violation a potentially ideal case to study the nature of broken symmetries in complex systems [3].In this Letter we present evidence that isospin mixing is small in 28 Si and 26 A1 compound nuclei at high E*, where F is very large. Our results, when combined with those from other experiments at lower E*, support the predicted energy dependence over a wide range of excitation energies.Our technique involves comparing measured and calculated inclusive 7-ray cross sections for the statistic...

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