Neutral pion production in subthreshold heavy ion collision with16O (38 MeV/u) and20Ne (200 MeV/u) beams

Inclusive production of neutral pions has been studied in¹⁶O +²⁷Al,¹⁹⁷Au reactions at 38 MeV/u and²⁰Ne +²⁷Al reaction at 200 MeV/u. A setup of 10 Pb glass Cherenkov detector telescopes was used to detect the twoγ rays from theπ ⁰ decay.     

Neutral pion photoproduction from protons at forward angles in the energy region between 350 MeV and 1175 MeV

The differential cross section for the reaction γp → π⁰p at forward angles has been measured in the energy region between 350 MeV and 1175 MeV. A phenomenological multiple analysis was carried out on the present data together with other data.     

On-line γ-ray investigation of the14N+27Al reaction at 30 MeV/u

The cross-sections of target-like residues following the¹⁴N+²⁷Al reaction at 30 MeV/u have been measured from in-beam and radioactiveγ-ray spectra. The recoil velocities of some fragments have been estimated from theγ-ray Doppler broadening.     

Incomplete deep-inelastic scattering in20Ne+197Au collisions at 20 MeV/nucleon

Correlations between emission angles and energies of coincidentα-particle projectile-fragment pairs have been measured for the²⁰Ne+¹⁹⁷Au system at 390 MeV beam energy. Theα-particles observed close to the direction of the projectile-like fragment were found to result essentially from sequential projectile decay. Starting from the close-geometry data, the contribution of sequentially emittedα-particles was calculated by Monte Carlo simulations for the entire angular range. For deep inelastic events, the measured angular correlations exhibit a spectacular excess over the calculated correlation. This excess is centred close to the beam direction but on the opposite side of it with respect to the detected projectile-like fragment. The correspondingα-particles have velocities around 85% of the projectile velocity indicating emission in an early reaction phase.     

π− Production in16O +27Al at 38, 65 and 93 MeV/u

Negative pion production cross-sections are measured at 38, 65 and 93 MeV per nucleon in O + Al reaction. Rough estimations of the total cross sections are given and compared to previous neutral pion data. Double differential cross sections at 93 and 65 MeV/u show enhancement for pions with velocity close to the beam. At variance no effect is observed at 38 MeV, where π^? and π⁰ distributions are identical. Fewπ ⁺ detected at 38 MeV support this result. This indicates that at the lowest incident energy nearly the whole projectile participates in the production reaction.     

Emission of light charged particles in the reactions of 120 MeV 20Ne with 27Al

The emission of H and He isotopes in the reactions of 120 MeV ²⁰Ne with ²⁷A1 has been studied using light particle-heavy ion coincidence measurements. The energy spectra and angular correlations of the light particles emitted in coincidence with products of $\text{Z ≧ 14}$ agree well with the predictions of the statistical model applied to the decay of the compound nucleus ⁴⁷V. Results on α-particle emission associated with deep inelastic reactions leading to carbon and oxygen products are also reported.     

Light fragments produced in40Ar+natAg reactions at 27MeV/u

Angular and energy distributions have been measured for products of 4≦Z≦12 from reaction of 1,095 MeV⁴⁰Ar+^natAg. In addition to sources located near the projectile and fusion system velocities, the data show the presence of a source located aroundV _beam/2 which cannot be explained by any existing model. A possible interpretation is proposed.     

Investigation of the Kr + Au system at 22 MeV/u

With laser-atomic-beam-spectroscopy the hyperfinestructure splitting constants A, B and the lifetimes τ of the Mn I levels 3d⁵4s4p z⁴p_3/2,5/2 were determined from the UV intercombination lines 321,7 nm and 322,5 nm. We obtained: A=?821(3)MHz, B=?40(30)MHz, τ=1120(50)nsec for the J=3/2 level and A=?607,8(1,5)MHz, B=+75(15)MHz, τ=970(50)nsec for the J=5/2 level.     

‘Slow’ residues observed in the reaction20Ne+208Pb at E/A=8.6, 11.4 and 15.0 MeV/u

Velocity distributions of heavy residuesA _Res>A _tar,Z _Res>Z _tar identified by means ofα spectroscopy, have been investigated at the velocity filter SHIP in reactions²⁰Ne+²⁰⁸Pb at projectile energies E/A=8.6, 11.4 and 15.0 MeV/u. Besides products from complete or nearly complete fusion, characterized by velocity distributions peaking atν/ν _CN?0.8–1.0, heavy residues with mean velocities of about half of the compound nucleus velocity were observed. The Z-distribution of this component was found to peak atZ=87. It is interpreted as residues from fusion of target nuclei with projectile fragments produced by nearly symmetric break-up. The experimental results were compared with predictions of theoretical models: cross sections for incomplete fusion were calculated using the sum-rule model of Wilcynski et al., while residue cross sections were calculated using the evaporation code HIVAP. A fair agreement between experimental and calculated mass distributions of heavy residues and transferred projectile fragments is achieved if an energy dissipation of ?23% (at E/A=8.6 MeV/u) and ?8% (at E/A=11.4 MeV/u) of the incident projectile energy is introduced. The observed peak of theZ-distribution atZ=87 is predominantly effected due to a higher fission probability of products withZ>87 during the deexcitation process and experimental limitations in the identification of products withZ≦86 by means of a spectroscopy, which cause a decrease of the observed production rates towards lowerZ.     

Negative pion production by 800 MeV/A C,Ne, and Ar beams

Energy and angular distributions of π's produced in relativistic heavy-ion collisions have been measured at 800 MeV/A incident energy. Cross sections for low-energy pions (E_cm ≈ 100–200 MeV) are forward and backward peaked in the nucleon-nucleon c.m. frame. Such anisotropy becomes weaker for higher energy pions. The shape of the energy spectra at θ_cm = 90° is approximately exponential. Their slopes are steeper than those observed for protons. The data are discussed in terms of several models.     

TotalK-vacancy production in Ne (10 MeV) traversing Al

Deexcitation of projectile inner shell vacancies created while traversing a solid foil may take place via competing processes: a) vacancy sharing with foil atoms in close impacts, b) radiative and non-radiative electron capture, and c) such X-ray and Auger electron transitions as are possible in the heavy ion projectile. The change inK-vacancy creation with foil thickness can be investigated by measuring either projectile or target X-rays where the vacancies are created by Coulomb excitation and processa. In the system Ne (10 MeV) on Al, detecting AlK X-rays, the NeK-vacancy production probability has been determined.     

Formation and decay of the compound nucleus studied in the reaction20Ne+27Al

Energy and velocity spectra, angular and mass distributions have been measured for evaporation residue products of the^{20, 22}Ne+²⁷Al system in the energy range ofE _L (²⁰Ne)=51 to 395 MeV and in the angular rangeθ=2° and 30°. Calculations with simple assumptions of the velocity and angular distributions of the evaporation residues are presented and compared to the data. The structure seen in the mass distributions, a competition between α-particle and nucleon evaporation in the deexcitation of the compound nucleus, is described well by calculations with the computer code CASCADE. The evaporation residues exhibit mass distributions varying systematically as a function of the excitation energy. The excitation function of the evaporation residue cross section is compared with theoretical models. At higher incident energies contributions of incomplete momentum transfer (incomplete fusion) are observed. A limitation for complete compound nucleus formation with following light particle evaporation is found.     

形变对10.6MeV/u 84Kr+27Al反应裂变前粒子发射的影响

研究了形变效应对10.6MeV/u ⁸⁴Kr+²⁷Al反应中断前粒子发射的影响.发现通过测量的裂变前粒子多重性提取的裂变延迟时间将因此从20×10^－21s减少到5×10^－21s,表明增强的断前粒子发射有一部分来自形变效应的贡献.     

Search for high excitation energy structures in the 20Ne + 208Pb system at EA = 30 MeV

The inelastic scattering of 600 MeV ²⁰Ne from ²⁰⁸Pb has been reinvestigated. Cross sections for giant resonance excitation have been extracted and analysed in terms of DWBA. No evidence has been found for higher excitation energy structures.     

Directed collective flow and azimuthal distributions in 36Ar + 27Al collisions from 55 to 95 MeV/u

A 4π charged particle detector array with a low velocity threshold has been used to detect the products from reactions induced by ³⁶Ar on ²⁷Al at energies ranging from 55 to 95 MeV/u. Well characterized events were selected and sorted as a function of the impact parameter. Two methods were used for sorting these events with respect to their impact parameters and three methods were compared to determine the reaction plane. The transverse momentum analysis has been found to be the best method to extract the direction of the reaction plane for this system and for the experimental set-up used here. The energy of vanishing flow for central collisions has been found to be around 90–95 MeV/u. The azimuthal distributions of mid-rapidity particles exhibit a preferential in-plane emission and no squeeze-out effect.     

A fast-fission component with small mass drift in the reaction84Kr+27Al atE lab=5.9 MeV/u

All reaction products in the range from target- and projectile-like to fission-like fragments were measured for the system⁸⁴Kr+²⁷Al at 5.9 MeV/u beam energy. They are assigned to the various reaction mechanisms on the basis of experimental signatures (energy dissipation, mass and angular distribution). The sum of the measured partial cross sections, including the evaporation residue yield obtained previously, agrees with the total reaction cross section derived from elastic scattering. A small fast-fission component was found, discernible from deep-inelastic reactions by its 1/sin θ angular distribution, and distinguished from compound-nucleus fission by an incomplete mass asymmetry relaxation.     

Nuclear fragmentation processes in the 20Ne + 27Al system at 30 MeV/A

Inclusive energy spectra and angular distributions of Z ≦ 9 fragments from the Ne + AI reaction have been measured at 30 MeV/A. The spectra present three components. The high-energy component has a reduced momentum width σ₀ of approximately 65 MeV/c, which is lower than the values observed in reactions induced by relativistic heavy ions. Conversely, for the low-energy component, the results are rather similar to high-energy heavy-ion data.     

Correlations between fission fragments and light charged-particles in the reaction20Ne+197Au at 13 MeV/nucleon

rgy of light charged-particles has been measured in coincidence with one or two fission fragments in the reaction²⁰Ne+¹⁹⁷Au at a bombarding energy of 13 MeV/u. The fission cross section was found equal to 1,340±260 mb. Assuming that it represents the totality of the fusion cross section, the critical valuel _c is deduced equal to 93±9?, higher thanl _Bf=0=74 in the rotating liquid drop model. The main emission source for protons and alpha particles seems to be a thermally equilibrated composite system. The competition between fission and charged particle emission is unexpected in the frame of the statistical treatment. A high energy component is observed in the forward direction. These results correspond to the first step of a study programme on the evolution of nucleus-nucleus collisions between 10 and 100 MeV per nucleon.