Investigation of parity mixing in Ta and Lu by measurement of the circular polarization of γ-rays

The circular polarization of the 482 keV γ-radiation of ¹⁸¹Ta and the 343 keV γ-radiation of ¹⁷⁵Lu was measured with a Compton polarimeter. Spurious effects arising from higher order contributions of quantum electrodynamics to the Compton scattering cross section were discovered. The results for the polarization are P = − (4.1±1.3)10⁻⁶ for ¹⁸¹Ta and P = − (1.5±6.0)10⁻⁶ for ¹⁷⁵Lu. These values are corrected for instrument asymmetries, bremsstrahlung and intensity of other lines.     

Triaxial superdeformation in Lu

High-spin states in ¹⁶³Lu have been investigated using the Euroball spectrometer array. The previously known superdeformed band has been extended at low and high energies, and its connection to the normal-deformed states has been established. From its decay the mixing amplitude and interaction strength between superdeformed and normal states are derived. In addition, a new band with a similar dynamic moment of inertia has been found. The experimental results are compared to cranking calculations which suggest that the superdeformed bands in this mass region correspond to shapes with a pronounced triaxiality (γ≈±20°).     

Observation of two-photon emission in nuclear pion capture

The emission of two photons above 25 MeV energy was observed in the capture of stopped pions by beryllium and carbon with a rate of (1.0 ± 0.1) × 10⁻⁵, respectively (1.4 ± 0.2) × 10⁻⁵ per capture. These rates are about two to four times greater than various free-nucleon estimates based on the π⁻+π⁺→γγ annihilation mechanism alone.     

Isomeric E1 transitions with ΔK = 0 INLu

An experimental arrangement for ns lifetime measurements in nuclear reactions is briefly described. The half-life of the level at 208.1 keV in ¹⁷¹Lu has been determined in the reaction ¹⁶⁹Tm(, 2n) ¹⁷¹Lu as . Delayed and populating γ-ray spectra have been measured, applying the method of delayed γ-γ coincidences. In the radioactive decay of ¹⁷³Hf, the lifetime of the level at 425.1 keV in ¹⁷³Lu has been measured as . Comparisons with Weisskopf and Nilsson estimates have been performed taking into account pairing correlations. A modified Nilsson Hamiltonian has been used. The dependence of El transition probabilities between the states and on the difference of the initial- and final-state deformations is studied. Qualitative agreement with semi-microscopic calculations of equilibrium deformations is found. Lifetimes of the states are estimated.     

The contribution of 2N photon absorption in C(γ,2N) reactions for Eγ = 150–400 MeV

The ¹²C(γ,pn) and ¹²C(γ,pp) reactions have been studied using tagged photons of energy E_γ = 150–400 MeV. Recoil momentum distributions are compared to the results of Monte Carlo calculations based on a two-nucleon photon absorption model and two different phase space models. The ¹²C(γ,pn) data at low missing energy are consistent with absorption on 1p² and 1s1p nucleon pairs.     

Electro-pion production on O to bou nd states of the residual nuclei

Differential cross sections for the reactions 16O(γ, π−π+−)16F16F to the sum of the four lowest lying states in 16F and 16N have been measured as a function of angle for pions with a kinetic energy of 30 MeV. The extracted ratios R = σ(γ, π−)/σ(γ, π+), the first ones to discrete final states as a function of angle, are in fair agreement with results obtained for the nucleon. For positive pions the energy dependence of the cross section has been measured at the angles of 45° and 90°. Distorted wave impulse approximation calculations fail to describe the energy dependence.     

Σ hypernuclear spectra from (K, π) inclusive reactions

We report on DWIA calculations of the pion inclusive spectra related to Σ-formation in (K⁻, π⁺) reactions on nuclei. Realistic distorted waves are used to describe the incoming kaon and outgoing pion. The Σ wave function is calculated in a real Woods-Saxon potential, the depth of which provides information about the underlying Σ N effective interaction. The absorptive effect due to the Σ-Λ conversion process in the nuclear medium is taken into account by effective two-channel coupled equations. Comparisons are made with the available data on ¹²C and ¹⁶O. Using a weak Σ-nucleus potential the overall agreement is satisfactory for the spectrum derived from kaon-in-flight experiments. Concerning the three peaks reported in a stopped kaon experiment on ¹²C, the lowest peak structure can be generated by increasing the depth of the Σ potential in ¹²C. However, the remaining two narrow structures cannot be reproduced as Σ-particle-proton-hole states in our continuum treatment of the Σ spectrum. The difficulties in extracting the strength of the Σ-nucleus spin-orbit potential are also discussed.     

Calculation of the beta decay of Pd. Quasiparticle random phase approximation results

Single beta decay transitions in ^114–120Pd are calculated. Theoreticalβ⁺ and β⁻ strength distributions, for transitions to 1⁺ states in the nuclei ^114–120Ag and ^114–120Rh, are obtained in the framework of the quasiparticle random phase approximation (QRPA). The effective two-body interaction which is used in the calculations is constructed from the Bonn one-boson-exchange potential (OBEP). Particle-hole and particle-particle like channels of the two-body force are included in the definition of the QRPA matrix equations. Effects associated with the particle number violation of the quasiparticle mean field are accounted for by using a particle-number-projected version of the QRPA formalism. Theoretical strength distributions for the β⁻ and β⁺ braches are shown and compared with data.     

Elastic photoproduction of charged pions on He and H in the Δ(1232) resonance region

Photoproduction of charged pions on ³He and ³H has been measured in the Δ(1232)-resonance region. Differential cross sections are presented as functions of the photon energy at fixed momentum transfer and compared to experimental data and theoretical calculations.     

Elastic and charge exchange scattering of pions from He

Elastic and charge exchange scattering of pions from ³He is studied using an optical potential in the momentum representation, which includes the second-order effect as well as the nucleon binding correction. These corrections are found to be appreciable but still not enough to resolve the discrepancy between theoretical calculations and recent experimental data on the reaction ³He(π⁻, π⁰)³H.