Evidence for multiphonon giant resonances in electromagnetic fission of 238U

Differential cross sections for electromagnetic fission of 238U projectiles (500 MeV/u) in C, Sn, and Pb targets are measured and analyzed in terms of single- and multiphonon giant resonance excitations as doorway states to fission. A novel experimental method exploits the linear relationship between neutron multiplicity and the primary 238U excitation energy. Multiphonon states contribute up to 20% of the cross section; a component at high excitation energies is indicated that may arise from three-phonon dipole and two-phonon GDR x GQRiv giant resonance excitations.     

Hard gamma as probe of nuclear dynamics

Experimental results concerning the ⁵⁸Ni + ⁵⁸Ni at 30 MeV/u and ⁵⁸Ni + ¹⁹⁷Au at 30 and 45 MeV/u reactions are presented. The emission mechanism of hard photons has been exploited to get information on the time evolution of the reactions. From the measured hard gamma multiplicity associated to different classes of heavy residues a quantitative estimate of the reaction centrality has been obtained. Moreover, evidence of a prompt Intermediate Mass Fragment (IMF) emission is shown for the Ni + Au reaction at 45 MeV/u for central collisions exploring the correlation function between thermal photons and IMF’s.     

Multiplicity fluctuation analysis of target residues in nucleusemulsion collisions at a few hundred MeV/nucleon

Multiplicity fluctuation of the target evaporated fragments emitted in 290 MeV/u12C-AgBr, 400 MeV/u12C-AgBr, 400 MeV/u20Ne-AgBr and 500 MeV/u56Fe-AgBr interactions is investigated using the scaled factorial moment method in two-dimensional normal phase space and cumulative variable space, respectively. It is found that in normal phase space the scaled factorial moment(ln Fq) increases linearly with the increase of the divided number of phase space(lnM) for lower q-value and increases linearly with the increase of lnM, and then becomes saturated or decreased for a higher q-value. In cumulative variable space ln Fq decreases linearly with increase of lnM. This indicates that no evidence of non-statistical multiplicity fluctuation is observed in our data sets. So, any fluctuation indicated in the results of normal variable space analysis is totally caused by the non-uniformity of the single-particle density distribution.     

Angular correlations of electron-positron pairs produced in heavy-ion collisions

We investigate the strength of angular correlations between coincident electron-positron pairs produced in Pb + Pb collisions at 5.7 MeV/u. The angular correlations are predicted to be observable at electron energies above 200 keV, almost independent of positron energy.     

Discovery of deeply bound π− states in the 208Pb(d,3He) reaction

Zeitschrift für Physik A Hadrons and nuclei - Narrow lines were observed around 133 MeV excitation energy in the 208Pb(d,3He) reaction at T d = 300 MeV/u using the Fragment Separator System at...     

Space-time evolution of the hadronic source in peripheral to central Pb+Pb collisions

Two-particle correlations of negative pions as a function of charged particle multiplicity are studied in Pb+Pb collisions at GeV per nucleon using the NA44 experiment at the CERN Super Proton Synchrotron(SPS). We find that the source size parameters increase with the charged particle multiplicity. However the slope of the source size parameters plotted as a function of charged multiplicity is slightly larger at high multiplicity than at low multiplicity. The value of is independent of charged multiplicity. For Pb+Pb collisions, is larger than and for all multiplicity intervals, whereas these three radius parameters were approximately equal in S+Nucleus collisions. The ratios () and () for Pb+Pb data show almost no dependence on charged multiplicity. The duration of pion emission is constant at 3.81.1 fm/c as a function of the charged multiplicity in Pb+Pb collisions. Effective volume (V) is also calculated as V=, assuming a cylindrically shaped source. We found, within the limited statistics, the effective volume rapidly increases at high multiplicity. Received: 17 July 2000 / Revised version: 3 November 2000 / Published online: 8 December 2000     

Photodisintegration of Pb isotopes

Yields of photonucleon reactions of different multiplicity of natural isotopic mixture of Pb were measured on a bremsstrahlung photon beam with a maximum energy of 67.7 MeV using gamma-activation methods. Cross-sections of photonucleon reactions of different multiplicity were calculated on stable Pb isotopes within the bounds of the combined model. The calculated cross sections are compared to those measured in experiments on quasimonochromatic photon beams. They are also used to split the measured yields into yields of particular photonucleon reactions.     

Measurement of the total reaction cross section for the mirror nuclei ~(12)N and ~(12)B

The mirror nuclei 12N and 12B are separated by the Radioactive Ion Beam Line in Lanzhou(RIBLL) at HIRFL from the breakup of 78.6 MeV/u 14N on a Be target.The total reaction cross-sections of 12N at 34.9 MeV/u and 12B at 54.4 MeV/u on a Si target have been measured by using the transmission method.Assuming 12N consists of a 11C core plus one halo proton,the excitation function of 12N and 12B on a Si target and a C target were calculated with the Glauber model.It can fit the experimental data very well.The ch...     

The impact parameter dependence of the K MO X-ray emission in208Pb +208Pb collisions

The emission probabilities for quasimolecular K-x-radiation (K-MOR) have been measured in²⁰⁸Pb +²⁰⁸Pb collisions at 4.3 MeV/u and 4.8 MeV/u as a function of the scattering angle using the particle-photon-coincidence technique. The probabilities exhibit a superposition of two exponential slopes with different fall-off constants. We identify the sharp fall off at very small impact parameters (<60 fm) with the K-MOR contribution from the decay of 1sσ-vacancies. The flatter slope for impact parameters larger than 60 fm is attributed to the radiative decay of holes in 2p _1/2 σ molecular states. In this way, experimental “1sσ-MOR-emission probabilities” could be extracted and compared with theory. For 4.3 MeV/u the1sσ- and 2p _1/2 σ-excitation probabilities could also be determined and have been compared to coupled-channel calculations and to a scaling law for 1sσ excitation.     

中能重离子碰撞中带电粒子多重性的同位旋效应

利用兰州4π带电粒子探测器阵列测量带电粒子多重性,研究了55MeV/u ⁴⁰Ar+^58,64Ni核反应中He和中等质量碎片的产额与反应系统同位旋的关系,以及这种同位旋效应与反应系统的碰撞参数(即碰撞的激烈程度)、系统的激发能的变化关系,对两个反应系统,观察到带电粒子多重性中He的比分随带电粒子多重性的增加而增大,带电粒子多重性中IMF的比分随带电粒子多重性的增加而先增大,后减小的规律.两个反应系统虽然具有相同的核电荷数,但轻粒子He和中等质量碎片在多重性中的比分有明显的同位旋相关性。     

The PbPb collision

The reaction ²⁰⁸Pb on ²⁰⁸Pb was studied at bombarding energies of 7.0 and 7.57 MeV/u. One-particle inclusive measurements using a large-area position-sensitive ionisation chamber delivered the kinetic energy, charge and scattering angle of the reaction products. A precise calibration of the stopping power for very heavy ions in the detector gas was performed. The measured Wilczynski diagrams show, for increasing loss of kinetic energy, an increase of the mean scattering angle. It is attributed to the dominance of the repulsive Coulomb forces with respect to the attractive nuclear forces. The element distribution for the ²⁰⁸Pb on ²³⁸U reaction at 7.5 MeV/u was also measured and compared to the PbPb and UU reactions. Fission probabilities are derived as a function of charge and total kinetic energy loss. The most striking result is seen in the σ_z² versus TKEL correlation: the average rate of energy loss per nucleon exchange is abnormally large. It is shown that this behaviour is associated with the double magic closed shell character of the colliding nuclei. Nuclear structure information is extracted through a simple parametrisation.     

Emission of intermediate mass fragments in the heavy ion interaction of (14.0 MeV/u) Pb+Au

We have studied the heavy ion interactions of (14.0 MeV/u) Pb + Au using two threshold detectors, mica and CN-85. A thin layer of Au was deposited on each of the three mica and two CN-85 detector pieces. These target-detector assemblies were exposed to a beam of 14.0 MeV/u Pb ions having the fluence of 1.5×10⁶ cm² at GSI, Darmstadt, Germany. After removing the target material and etching the samples in appropriate etchants, we scanned 32.29 cm² and 24.97 cm² area of mica and CN-85, respectively. Based on the observed number of events of various multiplicities, we have determined the total as well as partial experimental reaction cross-sections. It is shown that a significant number of intermediate mass fragments are emitted alongwith the heavy fragments in the present reaction.     

Reactions between 12C and heavy target nuclei at 84 MeV/u

Using an experimental setup which allows the coincident investigation of the various projectile and target fragments the reactions between 84 MeV/u ¹²C projectiles and different targets were studied. Impact parameters have been deduced from the size and multiplicity of projectile fragments. Using a “minimum bias” triqqer relative cross sections have been determined for the different reaction channels. New results on the production of medium-heavy fragments are presented.     

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.     

Capture cross-section and rate of the 1 4 C(n, γ) 1 5 C reaction from the Coulomb dissociation of 1 5 C

We calculate the Coulomb dissociation of ¹⁵C on a Pb target at 68 MeV/u incident beam energy within the fully quantum mechanical distorted wave Born approximation formalism of breakup reactions. The capture cross-section and the subsequently rate of the ¹⁴C(n, γ) ¹⁵C reaction are calculated from the photodisintegration of ¹⁵C, using the principle of detailed balance. Our theoretical model is free from the uncertainties associated with the multipole strength distributions of the projectile.     

Inelastic scattering and nucleon transfer in the system232Th+206Pb near the Coulomb barrier

Nucleon transfer accompanied by Coulomb excitation was studied in the system²⁰⁶Pb+²³²Th atE _Lab=6.4 MeV/u. Particle-particle-gamma coincidence techniques were used to identify excited states of reaction products populated through inelastic scattering and nucleon transfer reactions. The mean excitation energy was measured by means of aγ-ray energy and multiplicity filter consisting of 6 NaI detectors. Large cross sections for one-neutron and two-neutron pick-up from²³²Th are observed. The impact-parameter dependence of the neutron transfer is analyzed in terms of semiclassical barrier penetration models. Using realistic neutron potentials with a diffuse surface, the experimental data are in accordance with the assumption of a “cold” transfer to states near the yrast line.     

Comment on isospin mixing in nuclei

The central collisions of 70 GeV/c protons with Ag, Br and Pb nuclei have been studied, with the help of BR-2 photoemulsion and emulsion of the same constitution loaded with Pb nuclei. It is shown that the average multiplicity of produced particles (s-particles) increases slowly with increasing the atomic weight of the target nucleus. The ratio of the average multiplicity for these particles to the average charged particle multiplicity for protonproton interactions equals 3.2 forp-Ag, Br and 3.5 forp-Pb. The average multiplicities of particles from nuclear disintegration with energies for protons from 26 up to 400 MeV (g-particles) are 14.2±0.8 forp-Ag, Br and 23.0±1.0 forp-Pb corresponding to the emission of about half of all nucleons from the nucleus at the first rapid stage of interaction. The difference of thes-particle pseudorapidity distributions forp-Ag, Br, Pb andp-p interactions at pseudorapidities larger than 4.2 can be explained by successive interactions of the incident proton with nucleus nucleons.     

Reaction filters. Charged-particle multiplicity and linear momentum transfer

The relation between charged-particle multiplicity and linear momentum transfer to heavy reaction residues has been investigated with a 4π charged-particle detector for the reactions ³⁶Ar+²³⁸U at E/A=35 MeV and ¹⁴N+²³⁸U at E/A=50 MeV. The multiplicity of charged particles at backward angles (θ > 35°) incrreases linear momentum transfer while the multiplicity of charged particles in the forward direction is almost independent of the linear momentum transfer.     

展宽重离子束的Bragg峰

介绍了自行设计加工的用于对重离子束Bragg峰进行展宽的旋转式射程调制器. 实验上运用双电离室法测定了75MeV/u ¹²C离子束的Bragg曲线以及经该射程调制器调制的75MeV/u ¹⁶O及¹²C离子束的Bragg曲线,调制的离子束展示出明显展宽的Bragg峰,并将测量结果同计算结果进行了比较.     

能量修正的Glauber模型对中能反应总截面的影响

叙述了对Glauber模型中的透射系数进行半经验的能量修正,并利用该修正模型计算了^12—14C,⁶Li,⁷Be,⁸B+¹²C和⁶Li,⁷Be,⁸B+⁹Be以及²⁰Ne+¹²C,¹²C+²⁷Al等系统的激发函数(能区范围10—1000MeV/u),经与实验值比较,能量修正的Glauber理论计算值能够很好地描述中能条件下的反应总截面实验测量值.