One- and two-nucleon emission following the nuclear radiative pion capture

Decay channels in the radiative pion capture reactions are discussed and their respective yields calculated. Specific consequences of a recently suggested quadrupole excitation mode in the ( ^–,) reaction are found and their experimental investigation is urged. A yield of about 19% of the total decay was estimated for the two-nucleon decay channel even for the light nuclei like¹⁶O.On leave of absence fromthe Institute of Nuclear Physics, Czechosl. Acad. Sci., 250 68 e, Czechoslovakia.We a re grateful to the members of Lausanne-Münich-Zürich collaboration, particularly to C.Joseph, G.Strassner and P.Trüol for supplying us with the experimental results prior to publication. M. G. also thanks them and R.Wünsch for interesting discussions. We are grateful to R. A.Sakaev for his help at an early stage of this work.     

Positive parity giant multipole resonances in16O

Distributions of the (J =0⁺, 1⁺, 2⁺, 3⁺, 4⁺) isoscalar and isovector strength in¹⁶O have been calculated in then particle —n hole (n=0, 1, 2) shell model. The isoscalar quadrupole giant resonance comes out fragmentated over eight peaks which exhaust 33% of the EWSR betweenE ^*=17 and 25 MeV. This result agrees nicely with the recent³He and alpha inelastic scattering experiments. Giant monopole isoscalar (isovector) resonance appears to exhaust more than 50% of the EWSR nearE ^*=30 MeV (E ^*=40 MeV). Several collective states of other multipolarities are predicted either near to 30 MeV or between 50 and 60 MeV. The ground state correlations of the 2p2h type give rise to a considerable strength redistributions as compared with the case of the closed shell ground state.On leave fromInstitute of Nuclear Physics, Czechosl. Acad. Sci., 250 68 e, Czechoslovakia.     

Where do we stand with calculations of the two-neutrino double beta decay rates?

Alternative approaches to the calculation of two-neutrino double beta decay amplitudes are suggested which include neither the evaluation of the intermediate nucleus spectrum nor the closure approximation.Dedicated by the co-author to the memory of M. Gmitro.We wish to thank V. Belyaev, S. Bilenky, R. Eramzhyan, A. Ovchinnikova and E. Truhlik for interest and discussions.     

Towards a better understanding of O nuclear structure: Muon capture and radiative pion capture reactions

Muon capture and radiative pion capture reactions on ¹⁶O have been analysed with the aim to extract more information on the importance of higher components of the A = 16 nuclear ground and excited states. Good agreement with the experimental data for both reactions may be achieved if at least 2p2h components of the nuclear wave functions are taken into account. The positive parity excited states in ¹⁶N are shown to contribute 43 % to the radiative π⁻ capture but only ≈ 10 % to the muon capture rate.     

РЕАКЦИЯ ОБРАЗОВАНИЯ ПОЗИТРОНИЯ В ГАЗАХ

The investigation of positronium angular distributions in the positron-positronium pick-up reactions in gases is considered as a new method for studying electron wave functions in molecules. The methods of direct nuclear reactions were used. The basic approximation of the present paper is the Born-approximation.Angular distributions are distinguished by reactions with electrons from atomic and molecular orbits. The most advantageous region of positron energy is the 20–100 eV region.

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Subpicosecond electro-optic measurement of relativistic electron pulses

Time-resolved measurements of the transverse electric field associated with relativistic electron bunches are presented. Using an ultrafast electro-optic sensor close to the electron beam, the longitudinal profile of the electric field was measured with subpicosecond time resolution and without time-reversal ambiguity. Results are shown for two cases: inside the vacuum beam line in the presence of wake fields, and in air behind a beryllium window, effectively probing the near-field transition radiation. Especially in the latter case, reconstruction of the longitudinal electron bunch shape is straightforward.     

Pion photoproduction and inelastic scattering in A=12 nuclei: Transitions to the lowest isovector 1+ and 2+ levels

The continuity equation for the nuclear electric charge and convection current $\text{? ·}\text{j}\text{?}{}_{\mathrm{N}}\text{=}\text{?α}\text{ρ}\text{?}{}_{\mathrm{N}}\text{ρt}$ has been used in an analysis of nuclear transition densities in ¹²C. The results differ considerably from the former derivations. The standard M1 and newly-calculated E2 nuclear structure input is applied to estimate the pionic cross sections. A consistent improvement in several instances over the earlier results has been achieved; the calculations agree nicely with data both in the (3, 3) resonance region and at lower pion energies.     

On the sensitivity of nucleon-nucleon correlations to the form of short-range potential

Nucleon-nucleon correlation characteristics are calculated for several phenomenological and realistic strong potentials. The results show that a square-well potential reasonably well approximates the nucleon-nucleon interaction if one calculates the correlations between nudeons generated in a region with an r.m.s. radius larger than 1·5–2 fm. And vice versa, the correlations of nucleons emitted from a smaller generation region are sensitive to the form of assumed nucleon-nucleon potential.Dedicated to the 30th anniversary of the Joint Institute for Nuclear Research.On leave of absence from theInstitute of Nuclear Physics, Czechosl. Acad. Sci., 250 68 e, Czechoslovakia.On leave of absence from theInstitute of Physics, Czechosl. Acad. Sci., 180 00 Praha, Czechoslovakia.