Determination of the neutron electric form factor from the reaction 3 He(e,e'n) at medium momentum transfer

The electric form factor of the neutron G_En has been determined in double polarized exclusive ³ He(e,e'n) scattering in quasi–elastic kinematics by measuring asymmetries A _⊥, A _∥ of the cross section with respect to helicity reversal of the electron, with the nuclear spin being oriented perpendicular to the momentum transfer q in case of A_⊥ and parallel in case of A_∥. The experiment was performed at the 855 MeV c. w. microtron MAMI at Mainz. The degree of polarization of the electron beam and of the gaseous ³ He target were each about 50%. Scattered electrons and neutrons were detected in coincidence by detector arrays covering large solid angles. Quasi–elastic scattering events were reconstructed from the measured electron scattering angles ϑ_e, φ_e and the neutron momentum vector p _n ^′ in the plane wave impulse approximation. We obtain the result <G _En>_{(0.27 < Q2c2/GeV2 < 0.5)}= 0.0334 ± 0.0033_stat± 0.0028_syst which is averaged over the indicated range of Q ², the squared momentum transfer. This G _En value is significantly smaller than measured from the D(e,e'n) reaction under similar kinematical conditions. To what extent final state interactions in ³He quench the G _En result is subject of calculations currently in progress elsewhere. Received: 29 April 1999     

Measurement of the tensor analyzing power Ayy in the inelastic scattering of deuterons in the vicinity of excitation of baryonic resonances

The tensor A_yy and vector A_y analyzing powers in the inelastic scattering of deuterons with a momentum of 4.5 GeV/c on beryllium at an angle of 80 mr in the vicinity of baryonic resonances excitation have been measured. The A_yy data being in good agreement with the previous results obtained at a zero angle demonstrate an approximate t scaling up to - 0.9 (GeV/c)². The results of the experiment are compared with the predictions of the multiple-scattering and -meson exchange models. Received: 20 April 2000 / Accepted: 29 May 2000     

Nuclear matter properties and relativistic mean-field theory

Nuclear matter properties are calculated in the relativistic mean-field theory by using a number of different parameter sets. The result shows that the volume energy a₁ and the symmetry energy J are around the acceptable values 16MeV and 30MeV, respectively; the incompressibility K₀ is unacceptably high in the linear model, but assumes reasonable value if nonlinear terms are included; the density symmetry L is around 100MeV for most parameter sets, and the symmetry incompressibility K _s has positive sign which is opposite to expectations based on the nonrelativistic model. In almost all parameter sets there exists a critical point (,), where the minimum and the maximum of the equation of state are coincident and the incompressibility equals zero, falling into ranges 0.014fm^-3 < < 0.039fm^-3 and 0.74 < ≤0.95; for a few parameter sets there is no critical point and the pure neutron matter is predicted to be bound. The maximum mass M _NS of neutron stars is predicted in the range 2.45M ?M _NS? 3.26M , the corresponding neutron star radius R _NS is in the range 12.2km ?R _NS? 15.1km. Received: 5 May 2000 / Accepted: 28 November 2000     

High-spin states in the A= 39 mirror nuclei 39Ca and 39K

High-spin states of the mass A= 39 mirror pair ³⁹K and ³⁹Ca were investigated via the fusion-evaporation reaction ²⁸Si +¹⁶O at 125 MeV beam energy. The Gammasphere array in conjunction with the 4π charged-particle detector array Microball and neutron detectors was used to detect γ rays in coincidence with evaporated light particles. The results of the first high-spin study of the T _z=−1/2 nucleus ³⁹Ca are discussed in terms of mirror symmetry and compared to spherical shell-model calculations in the 1d _3/2–; 1f _7/2 configuration space. Received: 18 August 1999     

Observation of signature inversion in odd-odd 178Ir

High spin states in ¹⁷⁸Ir have been studied via the ¹⁵²Sm(³¹P, 5nγ)¹⁷⁸Ir reaction through excitation functions, X-γ and γ-γ-t coincidence measurements. According to the band structure characteristics and the measured intraband B(M1)/B(E2) ratios, two rotational bands are identified and assigned to be associated with the πh _9/2⊗νi _13/2 and πh _11/2⊗νi _13/2 configurations, respectively. Both bands show the signature inversion feature. Received: 23 March 2000 / Accepted: 31 July 2000     

Gamow-Teller transitions from 58Ni to discrete states of 58Cu

Under the assumption that isospin is a good quantum number, symmetry is expected for the transitions from the ground states of T = 1, T _z = ±1 nuclei to the common excited states of the T _z = 0 nucleus situated between the two nuclei. The symmetry can be studied by comparing the strengths of Gamow-Teller (GT) transitions obtained from a (p, n)-type charge-exchange reaction on a target nucleus with T _z = 1 with those from the β-decay of the T _z = - 1 nucleus. The A = 58 system is the heaviest for which such a comparison is possible. As a part of the symmetry study, we measured the GT transitions from ⁵⁸Ni (T _z = 1) to ⁵⁸Cu (T _z = 0) by using the zero-degree (³ He, t) reaction at 150 MeV/nucleon. With the achieved resolution of 50 keV, many hitherto unresolved GT states have been identified. The GT transition strengths were obtained for states up to 8 MeV excitation, i.e., near to the Q window limitation ( Q _EC = 9.37 MeV) of the β-decay from ⁵⁸Zn (T _z = - 1) to ⁵⁸Cu. The strength distribution is compared with that from shell-model calculations. Received: 24 November 2001 / Accepted: 30 January 2002     

Selective laser ionization of N≥82 indium isotopes: The new r-process nuclide 135In

Production yields and β-decay half-lives (T _1/2) of very neutron-rich indium isotopes were determined at CERN/ISOLDE using isobaric selectivity of a resonance-ionization laser ion-source. Beta-delayed neutron (βdn) multiscaling measurements have yielded improved T _1/2 for 206(6) ms ¹³²In, 165(3) ms ¹³³In and 141(5) ms ¹³⁴In. With 92(10) ms ¹³⁵In, a new r-process nuclide has been identified which acts as an important “waiting point” in the In isotopic chain for neutron densities in the range n _n≃ 10²⁴-10²⁶ n/cm³, where the r-matter flow has already passed the A≃ 130 abundance peak region. Received: 17 January 2002 / Accepted: 30 January 2002     

Tensor polarization of 12C[2+ 1] in the 16O(13C,12C)17O reaction at 50 MeV

The ¹⁶O(¹³C,¹²C)¹⁷O reaction at 50 MeV has been investigated using the kinematical coincidence method. Polarization tensors t ₂₀ and t ₄₀ of ¹²C[2⁺ ₁] for the quantization axis taken along the direction of propagation have been measured by analyzing the energy spectrum of ¹²C[2⁺ ₁], modulated by the effect of γ ray emission. The deduced t ₄₀ values significantly deviate from zero, contrary to the prediction of the distorted-wave Born approximation theory based on one-step p shell neutron stripping without spin-dependent interactions. The phenomenological spin–orbit interaction necessary to reproduce the magnitude of measured t ₄₀ is found to be much larger than the folding model prediction. It is shown that the experimental polarization tensors as well as the cross sections can be reproduced by introducing multi-step processes involving excitations in ¹²C and ¹³C without introducing spin-dependent interactions. Received: 2 August 1999 / Revised version: 3 February 2000     

Thick skin in neutron/proton-rich sodium isotopes

Nucleon (both neutron and proton) density distributions of the chain of sodium isotopes are calculated using a semi-phenomenological model of nuclear density which incorporates correctly the asymptotic behaviour and the behaviour near the centre. The experimental charge root-mean-square radii and the single neutron and proton separation energies, required as input, are used. The calculated interaction cross-sections using these densities in the Glauber model agree well with the experiment. The calculated neutron rms radii r _n and the nuclear skin thickness ( r _n - r _p) closely agree with the corresponding experimental values and also are consistent with the Relativistic Hartree-Bogoliubov (RHB) calculations. Received: 24 April 2001 / Accepted: 28 June 2001     

Yrast bands in N = 91 isotones

The in-beam spectroscopy on N = 91 isotones has been carried out using the ¹²C + ¹⁵⁰Nd reaction. The νi _13/2 and νh _11/2 bands of ¹⁵³Sm have been extended up to 33/2⁺ and 31/2^-, respectively. Two new γ-rays have been located on the top of the unfavored band with νi _13/2 configuration in ¹⁵⁷Dy. Two identical relationships have been established in the low-spin region of the yrast νi _13/2 configuration between ¹⁵³Sm and ¹⁵⁵Gd, and between unfavored bands of ¹⁵⁵Gd and ¹⁵⁷Dy. Here all these nuclei have the same neutron number N = 91. Received: 28 June 2000 / Accepted: 20 October 2000     

Necessity of extra repulsion in hypernuclear systems: Suggestion from neutron stars

Neutron star models with hyperon-mixed core are studied by a realistic approach to use the YN and the YY interactions consistent with hypernuclear data. From the compatibility of the theoretical maximum mass with the observed neutron star mass 1.44 M _⊙ of PSR1913+16, the necessity of some extra repulsion in hypernuclear systems, e.g., a repulsion from three-body force, is stressed. It is noted that the increase of baryon degrees of freedom to avoid the short-range repulsion effectively is an essential mechanism causing the Y-mixed phase. Received: 1 May 2001 / Accepted: 4 December 2001     

Nucleon properties in the covariant quark-diquark model

In the covariant quark-diquark model the effective Bethe-Salpeter (BS) equations for the nucleon and the Δ are solved including scalar and axial vector diquark correlations. Their quark substructure is effectively taken into account in both, the interaction kernel of the BS equations and the currents employed to calculate nucleon observables. Electromagnetic current conservation is maintained. The electric form factors of proton and neutron match the data. Their magnetic moments improve considerably by including axial vector diquarks and photon induced scalar-axial vector transitions. The isoscalar magnetic moment can be reproduced, the isovector contribution is about 15% too small. The ratio μG _E/G _M and the axial and strong couplings g _A, g _NN, provide an upper bound on the relative importance of axial vector diquarks confirming that scalar diquarks nevertheless describe the dominant 2-quark correlations inside nucleons. Received: 3 July 2000 / Accepted: 15 August 2000     

Nuclear matter properties in relativistic mean-field model with σ- ω coupling

The σ-ω coupling is introduced phenomenologically in the linear σ-ω model to study the nuclear matter properties. It is shown that not only the effective nucleon mass M^* but also the effective σ meson mass m _σ ^* and the effective ω meson mass m _ω ^* are nucleon-density-dependent. When the model parameters are fitted to the nuclear saturation point, with the nuclear radius constant r ₀ = 1.14 fm and volume energy a ₁ = 16.0 MeV, as well as to the effective nucleon mass M ^* = 0.85M, the model yields m _σ ^* = 1.09m _σ and m _ω ^* = 0.90m _ω at the saturation point, and the nuclear incompressibility K ₀ = 501 MeV. The lowest value of K₀ given by this model by adjusting the model parameters is around 227 MeV. Received: 23 March 2001 / Accepted: 8 June 2001     

Determination of the neutron electric form factor in the D(e,e′n)p reaction and the influence of nuclear binding

The electric form factor of the neutron G _E,n has been determined at the Mainz Microtron MAMI at the low momentum transfer Q ²= 0.15 (GeV/c)² in a measurement of the recoil polarisation ratio P _x/P_z in the quasifree reaction D(e,e′n)p. At this Q ² the influence of the nuclear binding is strong. A purely kinematical model is used to get some insight into the effect of the initial Fermi momentum distribution of the neutron. The influence of the final state interaction is determined quantitatively by a model of Arenh?vel et al.. After the corresponding corrections a value of G _E,n(0.15 (GeV/c)²) = 0.0481±0.0065_stat±0.0053_syst is obtained. Received: 12 April 1999     

Microscopic three-body force for asymmetric nuclear matter

Brueckner calculations including a microscopic three-body force have been extended to isospin-asymmetric nuclear matter. The effects of the three-body force on the equation of state and on the single-particle properties of nuclear matter are discussed with a view to possible applications in nuclear physics and astrophysics. It is shown that, even in the presence of the three-body force, the empirical parabolic law of the energy per nucleon vs. isospin asymmetry β = (N - Z)/A is fulfilled in the whole asymmetry range 0≤β≤1 up to high densities. The three-body force provides a strong enhancement of the symmetry energy which increases with density in good agreement with the predictions of relativistic approaches. The Lane's assumption that proton and neutron mean fields linearly vary vs. the isospin parameter is violated at high density due to the three-body force, while the momentum dependence of the mean fields turns out to be only weakly affected. Consequently, a linear isospin split of the neutron and proton effective masses is found for both cases with and without the three-body force. The isospin effects on multifragmentation events and collective flows in heavy-ion collisions are briefly discussed along with the conditions for direct URCA processes to occur in the neutron star cooling. Received: 18 February 2002 / Accepted: 16 May 2002     

New data on the p + N → [Σ0K+] + N reaction at E p= 70 GeV and the search for exotic baryons

New data for the diffractive reaction p+N→ [Σ⁰ K ⁺] +N at E _p= 70 GeV were obtained with partially upgraded SPHINX setup. The data are in a good agreement with the results of our previous study of this reaction. In the mass spectrum M(Σ⁰ K ⁺) a structure at the threshold region with a mass ∼1810 MeV and a distinct X(2000) peak with M= 1989 ± 6 MeV and γ= 91 ± 20 MeV are observed. Unusual features of the massive X(2000) state (narrow decay width, anomalously large branching ratio for the decay channel with strange particle emission) make it a serious candidate for cryptoexotic pentaquark baryon with hidden strangeness |qqqsˉs>. We also present new results on the narrow threshold structure X(1810) with M= 1807 ± 7 MeV and γ= 62 ± 19 MeV which is produced in the region of very small P ² _T < 0.01 GeV². The possibility of the Coulomb production mechanism for X(1810) is discussed. Received: 28 April 1999     

Ground-state spin of 59Mn

Beta-decay of ⁵⁹Mn has been studied at PSB-ISOLDE, CERN. The intense and pure Mn beam was produced using the Resonance Ionization Laser Ion Source (RILIS). Based on the measured β-decay rates the ground-state spin and parity are proposed to be J ^π = 5/2^-. This result is consistent with the systematic trend of the odd-A Mn nuclei and extends the systematics one step further towards the neutron drip line. Received: 17 November 2000 / Accepted: 16 February 2001     

New nuclide 139Tb and (EC +β+) decay of 138,139Gd

Excited levels in ¹³³Sn and ¹³⁴Sb, populated in spontaneous fission of ²⁴⁸Cm, were studied by means of prompt-γ spectroscopy, using the EUROGAM 2 multidetector array. The neutron i _13/2 single-particle energy has been determined for the first time in the ¹³²Sn region. The energy of the h _9/2 neutron level, proposed previously, has been confirmed. Received: 13 January 1999 / Revised version: 22 March 1999