Shadowing of α-emitting fragments observed in 313 MeV32S+93Nb collisions

Energy- and angular correlations ofα particles in coincidence with deeply inelastic fragments from³²S+⁹³Nb reactions at 313 MeV were measured and the assumption of sequentialα emission from the fragments tested. In the direction of the detected light fragment too fewα particles from the heavy fragment are observed. This pronounced anomaly is ascribed to nuclear shadowing of theα particles by the proximate light fragment.     

Nuclear fragmentation cross-sections of 400 A MeV 36Ar and 40Ar in collisions with light and heavy target nuclei

We have measured fragmentation cross-sections of Ar projectile nuclei at beam energy of 400 A MeV using experimental set-ups with plastic nuclear track detectors and different targets. In this paper total charge changing cross-sections and elemental fragmentation cross-sections for the production of fragments with charges ZF > or = 7 in interactions with H, C, Al, Cu, Ag and Pb target nuclei are presented. The dependence of the cross-sections on the fragment charge number and target charge number are discussed. The experimental results are compared to predictions of semi empirical cross-section models.     

25MeV/u~(40)Ar ~(93)Nb反应中热核的激发能和核温度

利用半导体望远镜探测器和ＰＰＡＣ对２５ＭｅＶ／ｕ４０Ａｒ＋９３Ｎｂ反应中的带电粒子和余核进行了关联测量，对所得α粒子能谱用三源模型进行了拟合，并由余核飞行时间和粒子多重性得到热核激发能．通过对温度的修正，发现在本实验中有激发能Ｅ＊／Ａ为４．３ＭｅＶ，温度Ｔｉｎｉｔ为６．９ＭｅＶ的热核形成。通过与其它实验结果的对比可以看出核物质在轻系统和重系统中行为的差异。     

30 MeV/u 40Ar+natAg反应中类弹碎片发射的时–空演化

报道了30MeV/u ⁴⁰Ar+^natAg反应中中等质量碎片(IMF)发射时间(τ)随发射源空间大小的演化规律,并对类弹碎片的发射时空进行了讨论.结果表明,IMF的发射时标与中等质量碎片关联函数以及发射源的核物质密度(ρ)有关,而与发射源的质量数的关系不大.对于能量较高的类弹碎片来说,在较小的核物质密度下提取的发射时间也较小,因此,在正常核物质密度参数下提取出的发射时间值可作为碎片实际发射时间的上限值.中速碎片的发射时间随密度大小的变化非常缓慢,提取出的发射时间值即可作为实际的发射时间.     

Binary aspects and particle multiplicities of the fragments from natAg+340 MeV 40Ar deep inelastic collisions

Deep inelastic fragments from the reaction ^natAg+340 MeV ⁴⁰Ar have been studied in coincidence. Charged particles (10 ≦ Z ≦ 32) were detected and identified in Z by means of a ΔE?E telescope, while the complementary fragments were detected in a one-dimensional, solid-state position-sensitive detector. Both in-plane and out-of-plane correlations were measured. The results confirm the binary nature of the deep inelastic process for this reaction. From the measured energies and angles of the fragments and the atomic number of one of the fragments, it was possible to determine the total mass loss due to the de-excitation of the fragments as well as the total evaporated charge at symmetry. An iterative procedure is discussed which enables one to determine the masses and kinetic energies of the fragments before evaporation, as well as the total number of particles evaporated by each fragment. The widths of the in-plane and out-of-plane correlations agree with the results of the iterative calculations, as do evaporation calculations which are based on the charge equilibrium model. The experimental results support the charge equilibrium model and indicate that thermal equilibrium is achieved between the fragments at fixed mass asymmetry.     

Peculiarities in fragment mass distribution in the 238U + 40Ar (243 MeV) reaction

A pronounced fine structure (FS) in the form of distinct peaks was observed in neutron gated mass spectra from the decay of the ²⁷⁸110 composite system produced in the reaction ²³⁸U + ⁴⁰Ar (243 MeV) at an initial excitation energy E ^* > 70 MeV. The FS peaks are located in the vicinity of mass numbers 70-80, 100, and 130, which correspond to those of magic nuclei (clusters). In the data there is also evidence for a new type of decay -- collinear cluster tripartition of an excited nucleus. Received: 8 August 2000 / Accepted: 2 February 2001     

Neutron multiplicity in deep inelastic collisions: 400 MeV Cu+Au system

We have detected in nine different positions of space the neutrons associated with the collision of ⁶³Cu on ¹⁹⁷Au at 400 MeV bombarding energy. The deep inelastic products were detected at two different angles: close to the grazing angle and 30° forwards of it. Their masses were measured using a time-of-flight technique. The neutrons were detected in coincidence with the fragments - the efficiency of the neutron detectors was measured relatively to a ²⁵²Cf source during beam time. The neutron threshold was set at 300 keV. Within an accuracy of 10% all the emitted neutrons are evaporated by the fully accellerated deep inelastic fragments. It is shown that the excitation energy is shared between the fragments in proportion to their masses and that the relaxation time for internal equilibration of the composite system is very short (～10^?22 s).     

The Doppler shift and anisotropy of quasimolecular X-rays emitted in Nb+Nb collisions at 67 MeV

The quasimolecularKX-ray spectrum obtained by bombarding Nb with 67 MeV Nb ions has been found to undergo a Doppler shift with an effective Doppler velocity equal to the mean center-of-mass velocity of the intermediate Nb+Nb molecule, corresponding to the projectile velocity. By applying Doppler shift corrections the intrinsic anisotropy of the quasimolecularKX-radiation is found.     

High transverse momentum proton emission in Ar + Ta collisions at 94 MeV/u

Energy spectra of fast protons arising from Ar + Ta collisions at 94 MeV/u have been measured at large angles. These data are analysed in the framework of a one-body transport theory simulated by the BNV code. The low cross section measured for very energetic protons rules out a possible mechanism of kaon production via incoherent nucleon collisions at such low beam energies.     

Light particle emission in deeply inelastic collisions of 280 MeV40Ar+58Ni: Thermal equilibrium and angular momentum transfer

Protons andα-particles emitted in coincidence with a deep inelastic fragment produced in the reaction⁴⁰Ar (280 MeV)+⁵⁸Ni have been measured. From a detailed study of their in-plane angular distribution, it is shown that the bulk of the light charged particle emission can be attributed to a secondary evaporation process by the two primary deep inelastic fragments. A lowest estimate of the fragment spin (assuming total alignment perpendicular to the reaction plane) is obtained from the anisotropy of the out-of-plane angular distribution of theα-particles. For very asymmetric splitting, the heavy fragment spin estimate as obtained from theα-anisotropy measurement is consistent with the observed proton toα-particle multiplicity ratio. The results are in agreement with the hypothesis of a sticking configuration between two deformed nuclei.     

Evidence for quasi-fission in40Ar+208Pb collisions near the coulomb-barrier

Fission-fragment angular distributions were measured in the reaction of⁴⁰Ar with²⁰⁸Pb near the fusion barrier. For nearly symmetric mass-/charge splits we find angular distributions symmetric around θ=90 degrees, however, with unusually large anisotropies. These develop gradually into forward-backward asymmetric distributions as one moves away from mass-/charge symmetry. This indicates that non-compound fission (‘quasi-fission’) competes with true fusion-fission. The relative contribution of quasi-fission to the total fission cross section is somewhere between 51 and 85%. In the framework of the extra-push model this is equivalent to an extra-extra push energy for compound-nucleus formation inside the true fission saddle point of 4<E _xx <9 MeV in agreement with a recent empirical parametrization of fusion-barrier shifts based on fusion-fission cross sections. On the basis of cross sections for fusion-evaporation residues it had previously been concluded that fusion of⁴⁰Ar with Pb isotopes occurs unhindered. Possible reasons for this apparent discrepancy are discussed.     

Gamma ray multiplicities in deep inelastic collisions for the Cu + Au 400 MeV system

For the Cu + Au 400 MeV system γ-ray multiplicities have been measured. The mean value of the transferred angular momentum, the variances, and the repartition of angular momentum between the fragments have been deduced. It is shown that, early in the reaction the building up of angular momentum is a very fast process. For longer times, additional angular momentum is induced in the fragments. The excitation of collective modes can account for this effect. The properties of such modes have been analysed.     

Excitation energy partition in deeply inelastic collisions between40Ar and Ag at 27 MeV per nucleon

The dynamics of the two partners produced in dissipative collisions has been experimentally studied for the system⁴⁰Ar + Ag at 27 MeV per nucleon. Primary masses of the fragments can then be calculated; the excitation energy partition between the two fragments is derived from the number of particles evaporated by each fragment. We found that this division evolves from equipartition to a repartition close to thermal equilibrium in the excitation energy range 300–350 MeV or interaction times 5-10×l0^?22 s.     

Fragment Angular Distributions of the 600 MeV 40)Ar+197 Au Reaction

The angular distributions of the fragments in the range of 4π solid angle for the 600 MeV ⁴⁰Ar+¹⁹⁷Au reaction have been measured using the beam-off γ-ray spectroscopy technique. The features of the fragment angular distributions and the reaction mechanisms of the production of fragments have been discussed.     

Light-particle emission versus evaporation residue formation in collisions of40Ar onnatCa at 30 MeV/u

The closely associated phenomena of preequilibrium emission and evaporation residue formation in fusion-like reactions were studied in central collisions between⁴⁰Ar and^natCa at 30 MeV/u. Heavy reaction products were taken in coincidence with neutrons and light charged particles. The preequilibrium neutron data agree very well with predictions of a quantal phase-space model which, in addition to the mean field, takes two-body collisions properly into account. Preequilibrium emission ends in thermally equilibrated hot nuclei with an average excitation energy of about 6 MeV/u. The combined results show a striking interrelation between the missing mass and light-particle multiplicities: the mass difference between the full compound mass and the observed residues can be explained quantitatively by the emission of only neutrons and light charged particles withZ2 during the entire course of energy dissipation.Supported by the German Bundesministerium für Forschung und Technologie (BMFT) under contract 06 HD 983I