Dense stars with exotic configurations

In this paper, we consider dense stars with configurations expected from the SU(3)_C×SU(2)_W× U(1) standard model of strong and electroweak interactions. Following a recent suggestion that strange matter, a form of (uds) quark matter, may be the true ground state of hadronic matter, we investigate the prospect for the existence of dense stars consisting partially, or entirely, of strange matter by comparing the relative stability between neutron matter and strange matter. It is found that the restriction on the maximum star mass holds in all cases, including a pure strange star, a pure neutron star, and a neutron star with a quark core. It is also found that the choice of both the bag constantB and the strong coupling constant _s has a decisive effect on the relative stability between strange matter and neutron matter. For currently accepted values of (B, _s), anA= dense starcannot consist entirely,nor partially, of strange matter. Nevertheless, such conclusion may be subject to change if corrections ofO ( _s ² ) or other effects are taken into account. Finally, we use the framework of Tolman, Oppenheimer, and Volkoff to analyze two cases of boson stars: gluon stars and stars consisting of massive scalar particles (massive bosons). It is found that, in the case of gluon stars, the presence of the bag constant in the QCD vacuum yields results very similar to that found in quark stars. On the other hand, soliton stars consisting of massive bosons exist if there is some background pressure which plays the role similar to the bag constant for lowering the matter pressure. The stability problem for both gluon stars and soliton stars is briefly discussed.     

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.     

Experimental estimates on radiative strength function of low-energy γ-quanta following even-odd heavy nuclei decay

The applicability of the method of summed amplitudes of coinciding pulses to determine radiative strength functions of heavy nuclei by measuring two-step cascade-ray spectra is discussed. Experiments based on thermal neutron capture in¹³⁶Ba and¹⁸⁰Hf were carried out to test this method. The values obtained for the radiative strength functions of a wide range of primary transitions were compared with the predictions of two modifications of the giant dipole resonance model. Some insight into the characteristics of the-decay of compound-states is provided by analyzing the information obtained on the two-step-decay of a number of nuclei in the mass region 137A187.Authors' thanks are due to Mrs. T.F. Drozdova and Dr. M.A. Ali for their help in the preparation of the English version of this paper.     

Coulomb corrections to Delbrück scattering

The emission of high-energy-rays in the deexcitation of hot⁴⁰Ca and³⁹K nuclei formed in heavy-ion fusion reactions at excitation energyE _x 90 MeV has been studied. The high energy-rays were measured in coincidence with evaporation residues or light charged particles. The spectrum from the self-conjugated compound nucleus⁴⁰Ca shows an appreciable yield suppression in the Giant Dipole Resonance (GDR) energy region with respect to the³⁹K, due to isospin selection rules in the dipole-decay. The spectral line shapes of the spectra are well reproduced by using a statistical code which treats explicitly the isospin quantum number in evaluating level densities and transmission coefficients. The GDR parameters determined from the present coincidence measurements are in good agreement with the systematic in theA 40 mass region at lower bombarding energy based on the analysis of inclusive spectra.We thank M. Caldogno for technical support in the development of evaporation residues detectors. We acknowledge the participation of M. Anghinolfi, P. Corvisiero, M. Taiuti and A. Zucchiatti in the early stages of this work. Thanks are due to M. Kicinska-Habior for providing the isospin-dependent code. Discussions with B. Fornal are also gratefully acknowledged.     

Excited states of111I and the observation of a 21 ns isomer

Proton rich nuclei close to ¹⁰⁰ Sn have been investigated inbeam using the NORDBALL detector array. A beam of 270 MeV⁵⁸ Ni was used to bombard a ⁵⁴ Fe target. Reaction channel separation was achieved with a 4 charged particle multi-detector set-up together with a 1 neutron detector wall placed in the forward direction. Gamma-ray transitions belonging to the ¹¹¹ I nucleus were identified for the first time. A level scheme constructed from --particle-coincidence analysis is proposed. The level structure is discussed within the framework of the shell model and the systematics of the heavier odd iodine nuclei.     

Properties of theρ-meson in dense nuclear matter

The properties of-mesons in dense nuclear matter are studied in a model which satisfies unitarity and current conservation. The important coupling of the-meson to two pions as well as the strong mixing of pions and delta-nucleon-hole states in nuclear matter are included. The-meson self energy in nuclear matter is evaluated with in-medium pion propagators and the corresponding vertex corrections required by current conservation. We find that the-meson width grows drastically with increasing density while its mass remains almost unchanged.     

A mini transmissionγ-ray Compton polarimeter and measurements of the sign of magnetic moments of radioactive nuclei

A mini transmission-ray Compton polarimeter was developed for the measurement of the sign of magnetic moments of radioactive nuclei with low-temperature nuclear orientation. The signs of the magnetic moments of the following isotopes were determined:¹⁹³Os [=+0.7297(16) _N];^191m Ir [=+6.20(9) _N];¹⁹²Ir [=+1.924(10) _N];¹⁹⁴Ir [=+0.39(1) _N];^195m Pt [=–0.605(15) _N].Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Complete spectroscopy of51,52V via the50,51V(n, γ) reactions

The gamma-ray spectra emitted after thermal neutron capture in highly enriched⁵⁰V and^natV targets have been studied using in-pile targets at the ILL high flux reactor and pair and germanium detectors. The neutron binding energies in⁵¹V and⁵²V were determined to beB _n (⁵¹V)=11051.11(17) keV andB _n (⁵²V)=7311.22(26) keV. The thermal neutron capture cross-section in⁵⁰V was measured to be 21 _–2 ⁺⁴ b. From 724 lines attributed to⁵¹V, 330 transitions, comprising 90% of the-ray flux, were placed into the level scheme. Fifty-nine primary dipole transitions from the 11/2⁺ or 13/2⁺ capture states in⁵¹V were established from which the E1 strength distribution was deduced. The energy scaling of these primary transitions was found to follow the E1-giant dipole resonance dominance. Many new levels were established; a number of states proposed in previous (d,p) and (n,) work were confirmed from their primary population and decaying secondary radiations. The density of levels in the high spin (9/2I15/2) region was parametrized with the Fermi gas model. The spin distributions of⁵¹V were analyzed and a spin cut-off parameter =2.8 (3) was deduced. — A nearly complete level scheme of⁵²V up to 3.5 MeV excitation and similar results concerning the level density and the primary -ray spectrum were obtained in the⁵¹V (n, )⁵²V study.Supported by Deutsches BMFT under contract 06GÖ141     

Precision measurement of theg-factor of the 2+ γ-vibrational state of160Dy and test of theF-spin purity

The rotations of the 392-966 keV and the 299–966 keV directional correlations of¹⁶⁰Dy in the static hyperfine field of DyTb have been measured with high precision. A neutron activated Tb single crystal was used which was magnetized to saturation at 4.2 K by an external magnetic field of 4.5 T applied in the direction of theb-axis. The agreement of both rotations proves that the relaxation of the 4f-shell after the-decay takes less than 10 ps. A quantum mechanical calculation within the magnon model gave a relaxation time of the order of 1 ps. Theg-factor of the 2⁺ -vibrational state was derived as

The low-energyD(α, γ)6Li and6Li+208Pb→D+α+208Pb cross sections

We have calculated theE1 andE2 contributions to the low-energyD(,)⁶Li fusion and to the⁶Li+²⁰⁸Pb D++²⁰⁸Pb Coulomb dissociation cross sections within a multichannel Resonating Group calculation based on many-body deuteron+ configurations and pseudo-states. For both reactions experimentally determined cross sections are reasonably well reproduced. We find that dipole capture contributes noticeably to the fusion cross sections atE500 keV, while it is negligible in the Coulomb dissociation data.     

The cross section for (n, γ) production of the 145 keV (11/2−) isomeric level in125Te

The¹²⁴Te(n,)¹²⁵Te reaction has been studied. Absolute intensities of transitions ending at the isomeric state in the¹²⁵Te level scheme were obtained. Cross section for production of 145 keV isomeric level in¹²⁵Te in (n,) reaction was determined as 1.0^m5.3 b.     

The decay of spin-isospin modes in a semi-infinite slab

The surface response for charge exchange (p, n) and (³He, T) reactions is studied in the-region using the semi-infinite slab model. The contribution to the total response from different decay channels, (NN, N, ), is calculated. These decay channels corresponds to the exclusive channels, (pp, p ⁺, ⁺), measured in recent (p, n) and (³He, T) experiments. The in-medium properties of the-resonance is taken into account by using microscopic calculations of the-width in nuclear matter. From the-width in nuclear matter a non-local imaginary-potential, as well as a local potential, is constructed for the semi-infinite slab model. The results in the semi-infinite slab model gives a qualitative understanding of the exclusive experiments. The exclusive (NN, N, ) channels are more sensitive, than the total response, tog-correlation parameters,-width and the absorption function used at the external vertex. Our calculations suggest low values of theg-correlation parametersg _N andg , (0.3). The results with the-width represented as a non-local or a local-potential are very similar, with only minor differences in the exclusive channels.Supported in part by the Swedish Natural Science Research Council and by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Nuclear Physics Division of the U.S. Department of Energy under Contract No. DE-ACO3-76SF00098     

A pair of excited superdeformed bands in196Pb

Two excited superdeformed bands have been found in¹⁹⁶Pb using the GASP-ray spectrometer array. They are signature-partner bands with a small signature splitting developing at higher frequencies, similar to bands 2 and 3 in the isotone¹⁹⁴Hg. The bands are probably built on an excited neutron configuration. They show an unusual incremental alignment of 1/2 with respect to the¹⁹⁴Pb core.Communicated by: B. Herskind     

Photofission of235U and238U at intermediate energies: absolute cross sections and fragment mass distributions

The total photofission cross section _,F for²³⁵U and²³⁸U has been measured in the energy range 50E 800 MeV at the 855 MeV Mainz Microtron MAMI using energy and time tagged photons (Glasgow Tagger) and a 4 arrangement of position sensitive fragment detectors. Besides the absolute photofission cross section _F , which almost completely exhausts the total photon absorption cross section for these nuclei, fragment mass distributions in this energy domain were determined via time of flight techniques (TOF). The results for the total photofission cross sections _,F normalized to the atomic numberA for both isotopes coincide, and agree in the-resonance region, within the systematic errors, with the socalledUniversal Curve _,T /A of the total photon absorption cross section _,T . At higher energies the cross sections exhibit a smooth behaviour. In particular, it is shown for the first time that there isno resonance-like shape near the D₁₃ resonance (at 710 MeV) as observed for the free proton. This complete suppression of the D₁₃ resonance in complex nuclei is not yet understood on a microscopic level. The fragment mass distributions show a predominantly mass symmetric fission. However, contributions from mass asymmetric fission at some photon energies may give a hint of an increased mass asymmetric fission after the onset of the pion and two pion channels.Excerpt from D26 Supported by the Deutsche Forschungsgemeinschaft, SFB 201 Mainz     

Investigation of the40Ca(γ,N) reaction at 60 MeV

The values of the angle-integrated cross section for the decay to various low-lying states in³⁹K following the⁴⁰Ca(,) reaction at a mean photon energy of 60 MeV are reported on, together with the values of the reduced Legendre coefficientsa _i . The⁴⁰Ca(,gp) and⁴⁰Ca(,n) reactions yield the same angular cross-section values and these agree with the corresponding RPA-calculations.Senior Research Associate NFWO     

The proton g9/2 isomer in theN=50 nucleus87Rb and M2 transition rates in85,87Rb

Low-lying states of theN=50 nucleus⁸⁷Rb have been investigated in the (, ') reaction measuring- and delayed-rf-coincidences. A half-life ofT _1/2=6(1) ns was obtained for the level at 1578.1 keV. This experimental result confirms the 9/2⁺ assignment and the proton 1g_9/2 single-particle character of this state. Experimental M2 transition strengths in^85,87Rb are compared with predictions of the shell model and the particle-core coupling model.This work was partly supported by the Bulgarian National Research Foundation.     

Level density fluctuations in the generalized interacting boson model

Level density fluctuations are calculated within the generalized interacting boson model proposed for even-even nuclei, in dependence on the truncation parameterk. For the case =2 corresponding to theSU(3) dynamical symmetry of the interacting boson model the fluctuation pattern is close to Poissonian. For cases 2, including the anharmonic vibrator model for which =, a rapid transition to the fluctuation pattern close to GOE is obtained.     

Lifetime measurements and particle-rotor calculations for 42 105 Mo63

The half-lives of seven levels in¹⁰⁵Mo have been determined with high precision at the fission-product separator Josef. The-- triple coincidence method with plastic and BaF₂ scintillators was used for a determination of the time between the _q feeding of a level of interest and its depopulation. A value of _q =0.38 (1) for the deformation parameter has been deduced from the half-lives of the first and second excited states. The results of Particle-Rotor model-calculations with the Nilsson parameters=0.084 and=0.28 support the assignment of the Nilsson orbital [532] 5/2^– to the ground state of¹⁰⁵Mo.The authors acknowledge with pleasure fruitful discussions with Profs. W.R. Phillips, I. Ragnarsson and O.W.B. Schult.     

Nuclear charge distributions from 4π-β measurements on mass separated fragments from235U(nth,f)

A 4- detector was used at the Lohengrin mass separator to measure the-activity of mass chains 91, 93, 94, 95, 137 and 139 produced in the thermal neutron induced fission of²³⁵U. This activity was analysed intoZ-yields. The different possible corrections were considered and applied to the data. TheZ-yields from this work are compared with the diverse existing results obtained with other methods. The need for and significance of this method to determine theZ-distribution of fission fragments of the heavy group is discussed.