The structure of28Si above 10 MeV excitation energy II: Assignments of quantum numbers

New assignments of quantum numbers have been obtained for more than 50 levels in²⁸Si between 8953 and 15915 keV excitation energy. They are based on the measurement of -ray angular distributions or anisotropies on 29 resonances of the²⁷Al(p, ) reaction and on the²⁴Mg(a, ) resonance atE =3355 keV. A total of 15 high-spin states withI=5–7 has been obtained and the spectrum ofT=1 states has been identified up to 15915 keV excitation energy and a maximum spinI=6.     

Energy dependence of the isotopic effect in the (n,p) reaction on the germanium isotopes

Cross sections for^70,72,73,74Ge(n, p)^70,72,73,74Ga,⁷⁰Ge(n, 2n)⁶⁹Ge,⁷²Ge (n,)⁶⁹Zn ^m and⁷⁴Ge(n, )⁷¹Zn ^m reactions are measured in the energy range from 13.0 to 16.6 MeV by the activation method using Ge(Li) detector-ray spectroscopy and compared with predictions of the reaction model incorporating preequilibrium and equilibrium emission mechanisms to interpret the energy dependence of the isotopic effect occuring in the (n, p) reaction. The fitted single-particle state-density parametersg, determined here for the germaniums are discussed together with theg-values found previously for the Se, Zr and Pd isotopic chains. A validity of the consistency condition between the precompound and compound models, which relatesg to the experimental level-density parametera viaa= ² g/6 is demonstrated.     

Study of neutron rich neon isotopes

The half-lives andP _n -values of the neutron rich isotopes^26–29Ne have been determined. The results are compared to shell-model calculations and good agreement is found except for²⁹Ne, where the half-life exceeds the predictions by more than an order of magnitude. This unexpectedly long half-life can be explained as due to a fp intruder configuration.     

Estimates of hypernuclear production via the exclusive (p,K+) reaction

We investigate the possibility of producing hypernuclei with proton beams via (p, K ⁺) reaction. We present differential cross sections calculations utilizing the distorted wave impulse approximation in momentum space. We consider the reactions¹²C(p,K ⁺) ¹³ and¹⁶O(p,K ⁺) ¹⁷ within the energy region 0.8 GeV1.2 GeV. We study both the case of formation of in S-state (¹²C) and P-state (¹⁶O). We take into account the contribution of both one-step and two-step processes when K pair is produced directly by incoming proton and intermediate pion, respectively. It is found that practically in all cases the two-step processes give significant contribution.Work supported by KFA Julich     

Microscopic study of the low-energy3He(3He, 2p)4He and3H(3H, 2n)4He fusion cross sections

We have calculated the³He(³He, 2p)⁴He and³H(³H, 2n)⁴He reaction cross sections at low energies within the microscopic multichannel resonating group method. For both reactions, we find good agreement with experiment. For the³H(³H, 2n)⁴He reaction, our calculated energy dependence reproduces that of each individual low-energy experimental data set, except for a normalization constant. Using this fact, we derive at a low-energy³H(³H, 2n)⁴He rate by taking the averaged mean of these fits.This work has been supported in part by the National Science Foundation, Grants PHY86-04197 and PHY88-17296.     

The structure of28Si above 10 MeV excitation energy I: gamma-decay modes and radiative widths of levels

The systematics of-decay modes and radiative widths of highly excited states in²⁸Si has been extended by taking-ray spectra on 60 resonances of the²⁷Al(p,) reaction in the range 1097 keVE _p 4492 keV (12643 keVE _x 15915 keV) and on the²⁴Mg(,) resonances atE =3355, 3431, 4003 keV (E _x =12860, 12925, 13 415 keV). The-decay modes of levels in the sub-resonance region (E _x =10–12.5 MeV) were studied with both the²⁷ A1(p, ) reaction and the²⁷ A1(d, n) reaction at E_d=4,5,6 MeV. Information on radiative widths of levels was obtained by measuring the resonance strengthsS =(2I+1) _p / for 52 resonances of the²⁷Al(p, ) reaction with an emphasis on prospective T=1 states.     

Bogoliubov compensation principle in the electro-weak interaction: value of the gauge constant,muon g-2 anomaly,predictions for Tevatron and LHC

We apply the Bogoliubov compensation principle to the gauge electro-weak interaction to demonstrate the spontaneous generation of an anomalous three-boson gauge invariant effective interaction. The non-trivial solution of the compensation equations uniquely defines the values of the parameters of the theory and the form factor of the anomalous interaction. The contribution of this interaction to the running EW coupling, α _ew(p ²), gives its observable value α _ew(M _W ²)=0.0374, in satisfactory agreement to the experiment. The anomalous three-boson interaction gives a natural explanation of the well-known discrepancy in the muon g−2. The implications for EW studies at Tevatron and LHC are briefly discussed.     

The neutron/proton ratio of fast nucleon emission in antiproton annihilation on nuclei

The measurement of fast protons and neutrons emitted after antiproton annihilation at rest on²³⁸U and⁶³Cu reveals a large neutron/proton ratioR. Its value for⁶³Cu is larger than expected from the conventional model of intranuclear pion rescattering. A value ofR essentially constant over the whole range of mass number, from¹²C up to²³⁸U, is also announced by the experimentalists. It is shown that, on the contrary, the conventional scheme predicts a regular increase ofR with mass number. Alternative explanations of the effect are looked for. Within the usual scheme, an excess of negative pions, leading to more emitted neutrons, is not compatible with final pion multiplicities. The influence of meson resonances on the neutron/proton ratio is considered, as well as the possible occurrence of annihilations on two nucleons. They cannot warrant an increase ofR at low mass targets sufficient to explain a constant value over the whole range of target masses.     

High density behaviour of nuclear symmetry energy

There exists profound discrepancy in the high density behaviour of the nuclear symmetry energy obtained in realistic variational many-body (VMB) calculations and in relativistic mean-field (RMF) calculations. While the symmetry energy decreases to negative values in the former approach it increases monotonically in the latter one. The origin of this discrepancy is discussed and it is argued that VMB prediction is more reliable. It is shown that vanishing of the symmetry energy implies proton-neutron separation instability in dense matter.This work was partially supported by KBN grants 2 0204 91 01 and 2 0054 91 01.     

Preequilibrium emission in (p,x n) reactions on palladium isotopes

Energy spectra of neutrons from the^{104, 105, 106, 108, 110}pd(p, xn) reactions atE _p=26.1 MeV have been measured using the time-of-flight technique with good overall energy resolution (< 250=" kev).=" the=" hard=" part=" of=" angle-integrated=" spectra=" corresponding=" to=" low=" residual=" excitations=">U6MeV) exhibits pronounced structure varying from isotope to isotope. The observed structure can be reasonably well described by preequilibrium-model calculations with two-quasiparticle state densities for proton-particle neutron-hole configurations based on Nilsson-model single-particle levels instead of equidistant ones. The influence of nuclear deformation and pairing on preequilibrium particle emission is explored.Supported in part by the exchange program of the Universities of Hamburg and Zagreb, Internationales Büro — KFA Jülich, and by the BMFT (Contract 06 HH 142)     

Proton decay of spin isospin modes excited by the12C(6Li,6He)12N reaction

First investigations of the reaction¹²C(⁶Li,⁶He)-¹²N(p)¹¹C were used to study spin-isospin strength in the nucleus¹²N at E_Li=156 MeV. While the⁶He ejectiles were detected at _He=0° using a magnetic spectrograph, the decay protons were analysed in a wide range of backward angles (100°_p<170°) with=" an=" arrangement=" of=" semiconductor=" strip=" detectors.=" singles=" spectra=" were=" extracted=" as=" well=" as=" coincident=" excitation=" strength=" for=" the=" decay=" to=" low=" lying=" levels=">¹¹C. For the decay to the¹¹C ground state angular correlations were evaluated for six different energy regions in¹²N^*.The authors wish to thank the staff of the Karlsruhe Cyclotron Laboratory for their cooperation. This work has been funded by the German Federal Minister for Research and Technology (BMFT) under contract no. 06ER262I and by the Kernforschungs-zentrum Karlsruhe.     

Transient field measurements on32S(21 +) ions in Gd at the 1s electron Bohr velocity

With the knowng-factor of the Coulomb excited first 2⁺-state in³²S the transient magnetic field was determined for sulphur ions traversing Gd at a mean velocity of 16 ₀ ( ₀=c/137). The degree of polarization deduced for the dominating H-like ions, ¯p_1s =0.10(3), agrees very well with that obtained at lower velocities. In addition, an upper limit of a transient electric field gradient was deduced from the particle--angular correlation which is expected on theoretical grounds.The authors thank the operating staff and in particular Dr. R. Repnow of the accelerator facility at the Max-Planck-Institut für Kernphysik in Heidelberg for providing excellent beam conditions. They are indebted to Profs. D. Habs and U. Schmidt-Rohr for their continuous interest and generous support during the experiment. They are grateful to Dr. P. Maier-Komor and Mrs. A. Meens for preparing the Gd-targets and He-implantation. Support by the BMFT and the Deutsche Forschungsgemeinschaft is acknowledged.     

Absolute pionic x-ray intensities in Mg and Fe

Absolute pionic x-ray intensities have been measured for Mg and Fe. The accuracy achieved was 2% of the line intensity for the most prominent transitions. In the Balmer series of Mg and the Paschen series of Fe the crossover transitions were found to be even more attenuated than the transitions between circular states, as deduced, for example, by comparison with muonic atoms; no crossover transitions were observed in the Fe Balmer series. According to cascade calculations based on the experimental intensities more than half of the pions are absorbed in Fe already from levels with principal quantum numbern>17 by strong interaction.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Coherent electromagnetic radiation from the fragments of nuclear fission

The electromagnetic radiation from two vibrating fission fragments is classically treated. The rotational and translational motion of the fragments during emission and the interference of the radiation from the two individual fragments are taken into account. It is shown that the angular distribution of the radiation contains valuable information on the fission process and that the spectral distribution depends sensitively on the damping of the shape vibration.Dedicated to Professor P. Armbruster and P. Kienle on the occasion of their 60th birthday     

Some asymptotic formulae for one-electron two-center problem

Asymptotic formulae of some expectation values related to the relativistic corrections in inverse powers of the internuclear distance R for the lsσ _g electron state of hydrogen molecular ion H₂⁺ and the lsσ molecule-like state of antiprotonic helium atom He⁺ are obtained with the use of the first-order perturbation function. Using these asymptotic formulae, the relativistic correction of order mα⁶ for these states in reciprocal powers of the internuclear distance R is derived to accuracy of (R ⁻⁴). The article is published in the original.     

Cross sections of 102 element isotopes formation in the reactions of22Ne+236U and26Mg+232Th

The production cross sections of the isotopes²⁵²102,²⁵³102, and²⁵⁴102 were measured for the heavy ion fusion reactions of²²Ne+²³⁶U and²⁶Mg+²³²Th by using the kinematic separator VASSILISSA. The obtained excitation functions and the maximum production cross sections are compared with the ones for more asymmetric reactions leading to the same compound nucleus²⁵⁸102. The experimental cross sections and the results of statistical model calculations are compared and discussed.The authors express their gratitude to Prof. Yu. Ts. Oganessian for his great interest and support of this work, to Drs. E.A. Cherepanov, Yu.A. Muzychka and B.I. Pustylnik for the calculations and for the useful discussions.     

Limiting angular velocity of realistic relativistic neutron star models

The Keplerian velocity as well as those frequencies at which instability against gravitational radiation-reaction sets in are calculated for rotating neutron star models of gravitational mass 1.5M . The investigation is based on four different, realistic neutron star matter equations of state. Our results indicate that the gravitational radiation instability sets in wellbelow (i.e., 63–71% of) the Keplerian frequency, and thatyoung neutron stars are limited to rotational periods greater than about 1 ms. In young and therefore hot (T10¹⁰ K) neutron stars them=5(±1) modes and in old stars after being spun up and reheated by mass accretion, them=4 and/orm=3 modes may set the limit on stable rotation.Dedicated to Prof. Dr. H.J. Mang on the occasion of his 60th birthday.     

Preequilibrium versus thermalized light-charged particle emissions in the40Ar (1100MeV)+24Mg reaction. Dynamical calculations

In an attempt to separate preequilibrium and thermalised emissions of light particles in low impact parameter heavy-ion collisions, the⁴⁰Ar+²⁴Mg reaction has been studied at 27.5 MeV/nucleon. Exclusive measurements have permitted us to examine, in some detail, heavy fragments and charged particles (p, d, t,-particle). The fragments recognized as evaporation residues have been selected and, due to inverse kinematic conditions, the related preequilibrium and statistical emissions of light particles resulting from incomplete fusion reaction appear to be distinguishable to a fair extent. This separation is fully supported by Monte Carlo calculations. Some experimental characteristics of the light particles have been examined and compared to the predictions of dynamical calculations. These calculations, associating a preequilibrium (interpreted as prompt emitted particles) model with a statistical-decay model, follow the evolution of the collision from the point of contact between the projectile and the target to the final evaporation-residue formation on an event by event basis. The predictions of these calculations have been compared to experimental data and satisfactory agreement is achieved for fragment-mass distribution, proton-energy spectra, and proton-angular distribution.     

Search for scalar and pseudoscalar bosons emitted in nuclear decays via their interactions

A search for light penetrating scalar and pseudoscalar bosons has been performed at the Jülich nuclear-reactor FRJ-1 (Merlin). In nuclear deexcitation processes pseudoscalar particles should be emitted in magnetic transitions and scalar particles in electric transitions. The experiment relies on the detection of the interaction products from the conversion of bosons in the field of an electron or a nucleus with a rotatable multi-scintillator system. The geometrical arrangement of the scintillators allows to distinguish between scalar and pseudoscalar particles via the different angular distributions of the final state photons. We obtain as upper limits for the product of production ratio and interaction cross sectionW for pseudoscalar particles W}<6.9>^–40 cm², and for scalar particles W}<1.3>^–39 cm² with 90% confidence.We gratefully acknowledge the support of this experiment by the KFA Jülich and we are very grateful for most valuable discussions with H. Bechteler, H.R. Koch, O.W.B. Schult, H. Seyfarth and A. Zehnder. We would like to express our thanks to the personnel of the reactor Merlin. We also wish to acknowledge the excellent work of the technical staff of our institute in particular to E. Hermens and R. Pahlke. Thanks to H. Geller and C. Latsch who assisted with the runs.     

Search for spontaneous π−- emission from a252Cf source

High energynp-pairs from a²⁵²Cf source were searched for. They are expected to be emitted with the absorption of negative pions which can be created through spontaneous fissions. The branching ratio is deduced as /_SF 1.3×10^–8 at the confidence level of 90%.We thank Professor M. Ishihara for encouragement. We wish to acknowledge an assistance of Dr. S. Shimoura during the experiment. This work was supported in part by a Grant-in-Aids for Scientific Research No. 6342007, Ministry of Education, Science and Culture of Japan.