On the Present Status of the Problem of Charge Dependence and Charge Asymmetry of the Nucleon-Nucleon Force

The charge dependence of the nucleon-nucleon force can be derived from the low-energy scattering parameters. The p-p and n-p parameters were measured in direct scattering experiments. The n-n parameters a_nn = (-16.70 ± 0.38) fm and r_nn = (2.78 ± 0.13) fm have been determined from few particle reactions. The difference of the scattering lengths (a_nn - a_nn) ≈ 7 fm demonstrates the violation of the charge independence. Information on the charge symmetry is still doubtful due to the inaccuracy of the charge correction for the p-p parameters. Whereas the n-n and the charge corrected p-p parameters known at present yield a weaker n-n force than the p-p force, the opposite result follows from the binding energy difference for ³He and ³H and the meson theory of the nuclear forces.     

On the limit of nuclear stability — The region of double magic100Sn and bound-state β-decay of187Re75+

Experiments which led to the discovery of the heaviest, self-conjugated double magic nucleus¹⁰⁰Sn and the bound-state β-decay of completely ionized¹⁸⁷Re are presented.¹⁰⁰Sn was produced by fragmentation of 1.1 A GeV¹²⁴Xe beams, separated and implanted in a 4π Si-stack detector. From its decay a half-lifeT _1/2=0.94(+0.54, −0.27) s and a β⁺ endpoint energy ofE _β=3.4(+0.7, −0.3) MeV were determined for the 0⁺−1⁺ Gamov-Teller transition. Completely stripped¹⁸⁷Re⁷⁵⁺ was produced and stored in a coolerring with an energy of 351 A MeV for several hours. The products of bound-state β-decay of¹⁸⁷Re⁷⁵⁺ were detected by two independent methods, which allowed to determine a half-life ofT _1/2=33±6 y for this decay, which is of importance for the calibration of the Re/Os nucleosynthesis chronometer.     

Resonance states of superheavy hydrogen nuclei obtained in transfer reactions with exotic beams

A resonance state situated at 1.8±0.1 and, most likely, another state positioned at 2.7±0.1 MeV above the t+n+n decay threshold were observed in the missing mass energy spectrum of the ⁵H nucleus produced in the reaction ³H(t,p)⁵H. The peak located close to $E_{^5 H} = 1.8$ MeV also was seen in the ⁵H spectrum obtained from the energy distributions of ³H nuclei emitted in the reaction ²H(⁶He,⁵H)³He. The width (Γ_obs ≤ 0.5 MeV) obtained for the two ⁵H resonance states is surprisingly small. A state of ⁴H with E _res = 3.3 MeV and γ² = 2.3 MeV was obtained in the reaction ²H(t,p)⁴H from the proton spectrum.     

Dispersion relations and low-energy elastic scattering of charged particles from nuclei: (II). Application to N-4He scattering

A “Coulomb-modified” dispersion relation is applied to low-energy p-⁴He forward elastic scattering. To check the validity of the modification, the results are compared with those deduced from n-⁴He elastic scattering. Completely analogous information is obtained for the two processes. The exchange of three bound nucleons is found to contribute strongly to N-⁴He forward scattering. The corresponding ⁴He-³H-p and ⁴He-³He-n coupling constants are evaluated as R_p = 3.8±0.3 and R_n = 3.0±0.3, respectively. These constants are related to the strength of the asymptotic wave function of nucleons in ⁴He and thus to the tail of the nucleon distribution. A comparison of R_p with the empirical proton distribution in ⁴He as deduced from e-⁴He elastic scattering shows excellent agreement. From R_p and R_n the effective ranges of the singlet ³H-p and ³He-n interactions in the ground state of ⁴He are determined to be equal, in accordance with charge symmetry, and to have a value of r_eff = 1.072±0.006 fm.     

The formation of the superheavy hydrogen isotope <Superscript>6</Superscript>H in the absorption of stopped π<Superscript>−</Superscript>-mesons by nuclei

An experimental search for the superheavy hydrogen isotope ⁶H was conducted through studying the absorption of stopped π^?-mesons by ⁹Be and ¹¹B nuclei. A structure in the missing mass spectrum caused by the resonance states of ⁶H was observed in three reaction channels, namely, ⁹Be(π^?, pd)X, ¹¹B(π^?, d³He)X, and ¹¹B(π^?, p⁴He)X. The parameters of the lowest state E_r=6.6±0.7 MeV and Γ=5.5±2.0 MeV (E_r is the resonance energy with respect to the disintegration into the triton and three neutrons) are evidence that ⁶H is a more weakly bound system than ⁴H and ⁵H. Three excited states of ⁶H were observed. Their resonance levels (E_1r=10.7±0.7 MeV, Γ_1r=4±2 MeV, E_2r=15.3±0.7 MeV, Γ _2r=3±2 MeV, and E_3r=21.3±0.4 MeV, Γ_3r=3.5±1.0 MeV) are energetically capable of disintegrating into six free nucleons.     

Towards a high-precision calculation for the pion-nucleus scattering lengths

We calculate the leading isospin-conserving few-nucleon contributions to pion scattering off ²H, ³He, and ⁴He. We demonstrate that the strong contributions to the pion-nucleus scattering lengths can be controlled theoretically to an accuracy of a few percent for isoscalar nuclei and of 10% for isovector nuclei. In particular, we find the π-³He scattering length to be (62 ± 4 ± 7) × 10⁻³ m _π⁻¹ where the uncertainties are due to ambiguities in the π-N scattering lengths and few-nucleon effects, respectively. To establish this accuracy we need to identify a suitable power counting for pion-nucleus scattering. For this purpose we study the dependence of the two-nucleon contributions to the scattering length on the binding energy of ²H. Furthermore, we investigate the relative size of the leading two-, three-, and four-nucleon contributions. For the numerical evaluation of the pertinent integrals, a Monte Carlo method suitable for the momentum space is devised. We observe that, so far, no power counting is able to provide a quantitative understanding of the relative strength of N- and (N + 1)]]-nucleon operators. Empirically, we find a relative suppression by a factor of 5 compared to a factor of 50 predicted from dimensional analysis. On the other hand, the relative importance of different contributions within each class of N-nucleon operators can be understood within Weinberg counting. The relevance of our findings for the extraction of the isoscalar π-N scattering length from pionic ²H and ⁴He is outlined. We also discuss the applicability of heavy pion effective field to the π-²H system.     

Untersuchung der vier ersten angeregten Zustände des6Li-Kernes durch Elektronenstreuung

Inelastic electron scattering cross sections were measured for energies below 60 MeV and momentum transfersq between 0.2 and 0.6 fm^?1. Ground state radiative widths Γ _γ ⁰ and transition radiiR _tr were deduced. 2.18 MeV: Γ _γ ⁰ (E2)=(4.40±0.34) · 10^?4 eV,R _tr=(4.28±0.39)fm; 3.56MeV: Γ _γ ⁰ (M1)=(8.31±0.36)eV,R _tr=(2.90±0.10)fm; 4.27 MeV: Γ _γ ⁰ (E2)=(5.4±2.8) · 10^?3 eV,R _tr=(3.4±1.2) fm. The excitation of the 5.37 MeV level shows a transverse angular dependence.     

Structure en bande du noyau 22Na: Le premier niveau Jπ = 6+

We investigate the two-body photodisintegration of ³He and its inverse, radiative p-d capture using bound-state functions corresponding to the N-N interaction being given by the Reid soft-core potential. For the two-body photodisintegration of ³He Coulomb effects and the final-state interactions between the proton and deuteron are not included. At low energy the shape of the angular distribution agrees well with experiment, but the 90° cross section exhibits an anomalous peak at 15 MeV due to electric dipole transitions connecting the deuteron and ³He D-states. The low-energy cross section is 25–40 % too small. The intermediate-energy angular distribution peaks too near the forward direction, and, contrary to experiment, has a minimum at 100°. At higher energy the 90° cross section in the center-of-momentum frame is at least an order of magnitude too small, but does display the correct energy dependence. This energy dependence is related to the properties of the bound-state wave functions and it is plausible that it will persist in an improved treatment (e.g. one which includes exchange currents) which properly accounts for the magnitude of the cross section. Contributions from the ³He S-states are negligible for photon energies between 100 and 150 MeV, but are dominant outside this energy region.     

Ground state structure of some double-λ hypernuclei by a three-body model using a simple coordinate space approach

New experimental data on the binding energyB _λλ of_λλ6He, reported very recently, come up with the valuesB _λλ = 725 ±0.14 MeV and ΔB_λλ = 101 ±0.2 MeV which are substantially lower than the old dataB _λλ = 109 ±0.8 MeV and ΔB_λλ = 4.7±10 MeV in use in literature since 1966. In view of the new data we decided to undertake a re-study of the _λλ ⁶ He hypernucleus using the same three-body model (α-λ-λ) with a simple coordinate space variational approach which was employed earlier with the old data on_λλ/6He. After fitting different λ-λ potentials to the new data of _λλ ⁶ He we have applied our method to study some double-λ hypernuclei in light, medium and heavy mass regions and have determined the structural quantities like Bλλ, the r.m.s. values of core-λ (〈r_core-λ〉〉) and λ-λ (〈r_λ-λ〉〉) distances theoretically. The core-λ interaction considered is of Woods-Saxon type. The strength and the range of the core-A potential have been adjusted to reproduce the λ-binding energy(B _λ) . These are in good agreement with the relativistic mean field (RMF) results. Our study shows that the λ-λ bonding energy ΔB_λλ decreases with increasing mass number from _λλ ¹⁰ Be to _λλ ²¹⁰ Pb of a double-A hypernucleus     

Resonance states of hydrogen nuclei 4H and 5H obtained in transfer reactions with exotic beams

To investigate ⁵H resonance states with a better instrumental resolution, we utilized the two-neutron transfer reaction ³H(t, p)⁵H accomplished with the use of a cryogenic liquid-tritium target and 57.5-MeV triton beam. As a result of this study, a valuable fraction of protons detected at ? _lab=18°–32° in ptn coincidence events was attributed to the states of the ⁵H nucleus. Two resonance states situated at 1.8±0.1 and 2.7±0.1 MeV above the t + n + n decay threshold were obtained in the missing mass energy spectrum of the ⁵H nucleus. The peak located close to E5_H was clearly seen in the ⁵H spectrum obtained from the energy distributions of ³H nuclei emitted in the reaction ²H(⁶He, ⁵H)³He at ? _lab=17°–32°. The width (Γ_obs≤0.5 MeV) obtained for the two ⁵H resonance states is surprisingly small. A state of ⁴H with E _res=3.3 MeV and γ ²=2.3 MeV was obtained in the reaction ²H(t, p)⁴H from the spectra of protons leaving the target at ? _lab=18°–32° and detected in coincidence with neutrons emitted in the decay of ⁴H nuclei.     

Spectroscopy of the <Superscript>5</Superscript>H superheavy hydrogen isotope

The formation of the ⁵H superheavy hydrogen isotope was experimentally sought in the reactions induced by stopped π^? mesons absorbed by ⁹Be nuclei. Peaks in missing-mass spectra were observed in two reaction channels, ⁹Be(π^?, pt)X and ⁹Be(π^?, dd)X, and were attributed to the ⁵H resonance states. The lowest state has parameters Er=5.5±0.2 MeV and Г=5.4±0.5 MeV [E_r is the resonance energy measured from the (triton + two neutrons) threshold]. Therefore, ⁵H is bound more weakly than ⁴H. Excited states of ⁵H were also observed. All three resonance levels (E_1r=10.6±0.3 MeV, Г_1r=6.8±0.5 MeV; E_2r=18.5±0.4 MeV, Г_2r=4.8±1.3 MeV; E_3r=26.7±0.4 MeV, Г_3r=3.6±1.3 MeV) can decay into five free nucleons.     

Determination of the 1S0 neutron-neutron scattering length in the nd breakup reaction at energies in the range En = 40?60 MeV

The first results are obtained in a kinematically complete experiment devoted to measuring the n + d → p + n + n reaction yield at energies in the range E _n = 40−60 MeV and various angles of divergence of two neutrons (Δθ = 4°, 6°, and 8°) in the geometry of neutron-neutron final-state interaction. The ¹ S ₀ neutron-neutron scattering length a _nn is determined by comparing the experimental energy dependence of the reaction yield with the results of a simulation in the Watson-Migdal approximation, which depend on a _nn . For E _n = 40 MeV and Δθ = 6° (the best statistics in the experiment), the value a _nn = −17.9 ± 1.0 fm was obtained. A further improvement of the experimental accuracy will make it possible to remove the existing disagreement of the results from different experiments.     

Determination of low-energy parameters of neutron-proton scattering in the the shape-parameter approximation from present-day experimental data

On the basis of the total cross sections for neutron-proton scattering in the region of laboratory energies below 150 keV, the value of σ₀ = 20.4288(146) b was obtained for the total cross sections for neutron-proton scattering at zero energy. This value is in very good agreement with the experimental cross sections obtained by Houke and Hurst, but it is at odds with Dilg’s experimental cross section. By using the value that we found for σ₀ and the experimental values of the neutron-proton coherent scattering length f, the deuteron binding energy ɛ _t , the deuteron effective radius ρ _t (−ɛ _t , −ɛ _t ), and the total cross section in the region of energies below 5 MeV, the following values were found in the shape-parameter approximation for the low-energy parameters of neutron-proton scattering in the spin-triplet and spin-singlet states: a _t = 5.4114(27) fm, r _0t = 1.7606(35) fm, v _2t = 0.157 fm³, a _s = −23.7154(80) fm, r _0s = 2.706(67) fm, and v _2s = 0.491 fm³.     

Three-nucleon calculations without the explicit use of two-body potentials

Using as two-nucleon interaction input the ³S₁-³D₁ and ¹S₀ partial waves, the Faddeev equations are solved for the three-nucleon bound state. The ³S₁³D₁T-matrix is calculated from the Reid potential. Avoiding the usual potential fit, the ¹S₀T-matrix is directly continued off-shell and is constructed consistent with the ¹S₀ phase shift of elastic two-nucleon scattering. The off-shell part of the ¹S₀T-matrix is parametrized and with this parametrization the dependence of the three-nucleon bound-state properties is studied. As a result it is found that the binding energy varies only between 6.2 MeV and 6.8 MeV, while the minimum in the charge form factor for electron scattering from ³He lies between 12.9 fm^?2 and 18.7 fm^?2. The larger (smaller) ³He binding energy is accompanied by a ³He charge form factor whose minimum is at larger (smaller) momentum transfers.     

Distribution of the isotopes produced in the YBa2Cu3O7−x superconductor and PbZr0.54Ti0.46O3 ferroelectric by energetic charged particles

A study has been made of the concentration profiles of radioactive isotopes produced by transmutation in the YBa₂Cu₃O_7−x superconductor and PbZr_0.54Ti_0.46O₃ ferroelectric by energetic protons (10 and 15 MeV), deuterons (4 MeV), and ³He and ⁴He nuclei (20 MeV). Profiles of two types have been observed: monotonic and with a maximum. It is shown that the type of isotope concentration profile is determined by the nature of the cross-section energy dependence of the nuclear reaction producing a given isotope. Fiz. Tverd. Tela (St. Petersburg) 40, 2189–2191 (December 1998)     

Measurement of the coherent neutron scattering length of 3He by neutron interferometry

A precise determination of the coherent scattering length of ³He with a neutron interferometer yields a value a_c = 4.29 ± 0.04 fm. A comparison with varoous theoretical perdictions is made and its relation to the few-body problem is discussed. A combination with other experimental results yield as most probable values for the free singlet- and triplet scattering length a_s = 8.0 ± fm and a_t = 3.05 ± 0.07 fm, respectively.     

Differential cross section for elastic π± helium isotopes scattering from 25 to 65 MeV

Measurements of absolute elastic-scattering differential cross sections for π^±He at T^lab_π = 30.2 MeV, 45.1 MeV and 65.0 MeV and for π^±-⁴?⁴He at T^lab_π = 25.1 MeV and 51.1 MeV have been made in the angular range 40° to 145°. An overall accuracy of about ±5% was achieved. The results are compared with available theoretical calculations.     

A study of excited states of5He by the7Li (d, α)5He-reaction

The interactions of deuterons with⁷Li, in which two α-particles and a neutron are produced in the final state are studied atE _d =1.2 MeV. The dual parameter α-α coincidence spectrum was analyzed with a search program. The results indicate two excited states of⁵He: One at excitation energy 2.9±0.1 MeV and with a reduced width of γ²=1.1±0.4 MeV, and the other at 4.8±0.4 MeV, with γ²=0.9±0.4 MeV. No contributions of sequential decay through the 4⁺ state in⁸Be or direct three body decay was observed.     

Fragmentation of <Superscript>7</Superscript>Li relativistic nuclei on a proton into the <Superscript>3</Superscript>H + <Superscript>4</Superscript>He channel

In a track nuclear photoemulsion exposed to a beam of ⁷Li nuclei accelerated to a momentum of 3 GeV/c per nucleon at the synchrophasotron of the Joint Institute for Nuclear Research (JINR, Dubna), 13 events in which ⁷Li nuclei interacting with protons break up into ³H and ⁴He fragments were detected among 3730 inelastic-interaction events. For this fragmentation channel, the cross section was found to be 8 ± 2 mb. The average value of the fragment total transverse momentum was 214 ± 5 MeV/c. This value exceeds markedly the average value of the transverse-momentum transfer in the coherent dissociation of ⁷Li nuclei on track-emulsion nuclei (166±5MeV/c). The recoil-proton transverse momentum was on average 98% of the total proton momentum. The longitudinal-momentum distribution of protons was characterized by a variance of 16 MeV/c and a mean value of 37 ± 2MeV/c.     

Observation of the 7 MeV excited spin-flip and non-spin-flip partners in 16 ΛO by γ -ray spectroscopy

We have observed three γ -ray transitions in _Λ ¹⁶ O from both 7MeV excited spin-flip and non-spin-flip partners ( 2^-, 1^- ₂ to the ground-state doublet ( 1^- ₁, 0^- via the ¹⁶O(K ^-,π^-) reaction. The 7MeV excitation energies of the spin-doublet members ( 2^-, 1^- ₂ were determined to be 6784±4±4 keV and 6562±1±2 keV, respectively, and thus the spacing was obtained to be 222±4±5 keV. This is the first observation of the spin-flip state directly populated by the ( K ^-,π^- reaction. Moreover, such directly populated spin-flip and non-spin-flip partners were resolved for the first time.