Near-barrier fission of the system12C+232Th

The fission fragment angular distributions for the system¹²C +²³²Th have been measured in the energy range 0.97<>_c.m./V_B<1.22. the=" measured=" anisotropies=" have=" been=" compared=" with=" the=" predictions=" of=" the=" standard=" saddle=" point=" statistical=" model,=" using=" the=" second=" moment=" of=" the=" compound=" nucleus=" spin=" distribution=">l² deduced from a Wong model fit to the fission excitation function. While the measured anisotropies agree with the standard saddle point statistical model for E_c.m./V_B>1.05, they are abnormally large at lower energies. This is also true for the existing measurements of¹⁶O and¹⁹F +²³²Th systems. Since for this system pre-equilibrium fission contributions are not expected, this anomaly indicates strong channel coupling effects leading to an increase in l² at sub-barrier energies.The authors thank Drs. R.K. Choudhury and A. Saxena for useful discussions, Dr. R.J. Singh for making the thorium target and the Pellectron operation staff for excellent operation of the machine.     

Sub-barrier fission fragment angular distributions for the system19F+232Th

The measurements of fission fragment angular distributions for the system¹⁹F+²³²Th have been extended to the sub-barrier energies of 89.3, 91.5 and 93.6 MeV. The measured anisotropies, within errors are nearly the same over this energy region. However, the deviation of the experimental values of anisotropies from that of standard statistical model predictions increases as the bombarding energy is lowered.     

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.     

The sub-barrier photo-fission and the232Th fission barrier profile

The²³²Th sub-barrier photo-fission data have been analysed in terms of the predicted triple-humped barrier. It is shown that these results and particularly the isomeric shelf seen in these data cannot be explained as being due to the 2nd minimum. However, one can explain these results in terms of the 3rd minimum on the assumptions thatE _III (the energy of the 3rd minimum relative to the ground state energy) ≈ 3 MeV instead ofE _III≈4.5 MeV as generally thought and the isomeric state in this well is fed and decays like the heavier actinides with double-humped barriers.     

Molecular reorientation in collisions of OH + Ar

Orientational effects in rotationally elastic collisions of OH with Ar were studied in a crossed molecular beam setup. A static electric field was applied to orient the molecules before the collision. After the collision the orientation was measured by using a narrow-band laser system to probe the Stark-split states corresponding to different orientations. Differential and integral cross sections for reorientation of the molecular axis have been determined as a function of the initial orientation. The cross section for reorientation by one quantum is 25 A2 with a preference for O-end collisions.     

低能Ar3+ + Ar碰撞反应过程实验研究

测量了入射离子能量为27 keV、45 keV和66 keV时Ar3 Ar碰撞反应各子过程的分截面,通过将其归一化到Bliman,Dousson等人所测量的截面值,获得了绝对截面.结果表明,该能区电子俘获过程仍占主导地位,但转移电离过程已不能忽略.结果还表明,各子过程反应截面基本不随能量变化.与考虑自电离的MCBM(Molecular Coulombic overBarrier Model)模型比较发现,模型所给截面普遍高于实验测量值,这在一定程度上是由于归一化时带来的系统误差.在误差允许的范围内,单电子和双电子俘获截面符合的比较好,但三电子俘获截面却相差一个数量级.对于实验观测到的转移电离过程,模型却预言并没有此过程发生.     

Measurement of the light-nuclide yield from the photon-induced fission of 232Th

The yield of ²⁴Na and ²⁸Mg radionuclides was determined by their gamma activity with the aid of radiochemistry. The corresponding measurements were performed upon the activation of purified Th targets with bremsstrahlung photons having endpoint energies of 12, 16.5, and 24 MeV. Possible background sources are carefully analyzed, and it is concluded that the yield of the above light nuclei is about 10^?6 per event of ²³²Th fission.     

Fragment spin distribution in 400 MeV40Ar+93Nb collisions investigated by light-particle anisotropies

In and out-of-plane angular correlations between light particles and projectile-like fragments from deeply inelastic collisions of 400 MeV⁴⁰Ar+⁹³Nb were measured. At backward angles, the equilibrium emission of a particles and protons from the target-like fragment was identified. By analysing the out-of-plane angular correlations in the framework of a semi-classical statistical model, an average spin of 30±2? and an average alignment ofP _zz =0.80±0.15 for the target-like fragment were deduced.     

On the anomalon interpretation of40Ar + Cu collisions at 0.9 and 1.8 A GeV

The paper of Dersch et al. [1], concerning the results of inelastic⁴⁰Ar + Cu collisions at 1.8 and 0.9 GeV per nucleon, is discussed. The authors have explained their experimental results on the basis of the concept of anomalons of a lifetime of ～2·10^?10 s and/or as “inconsistency with known nuclear physics”. It is shown that such conclusions are untenable, and the data presented in the cited paper can be explained within the frame of known physics.     

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.     

40Ar+238U 实验介绍

主要介绍了利用兰州重离子加速器提供的270 MeV的40Ar离子束轰击238U靶，通过熔合蒸发反应进行试合成Z=110附近的新同位素的实验情况。分析了目前关于超重核研究的现状并描述了这次实验的目的、 可行性分析、 实验装置以及实验过程等。本次实验仍然用氦喷嘴技术对产物进行传输， 并用一套具有数对探测器组的转轮收集探测系统对产物进行收集和测量。 The state of the experiment to produce the new isotopes around Z=110 are presented in this paper. The emphasis is laid upon introducing the experiment purpose, the set up and the feasibility for producing this objective nuclide. In the experiment the new isotopes were produced by the complete fusion evaporation reaction of 238U with 270 MeV 40Ar at the Sector Focus Cyclotron (SFC) of Heavy Ion Research Facility in Lanzhou(HIRFL). The reaction products were also transported and collected by using the helium jet technique and rotating wheel apparatus.     

Comment on fusion cross sections in the40Ar+40Ca and40Ca+40Ca systems

Using a strongly mismatched transfer reaction,¹³⁸Ba (¹⁴C,¹⁵N), the spin orbit potential for¹⁵N projectiles has been derived in terms of the reaction asymmetry. The strength and sign of the L·S-potential are consistent with a previous determination using an (¹⁶O,¹⁵n) reaction; the signis opposite to that for nucleons and agrees with a model where the last P_1/2 nucleon is responsible for the L·S potential.     

Approaching peripheral collisions in the reaction Ar+Au at 27.2 MeV/u by the associated neutron multiplicities

A 4π detector has been used to measure the neutron multiplicity associated with peripheral heavy ion reactions induced at 27.2 MeV/u. A strong correlation is observed between the number of emerging neutrons and the mass of the projectile-like-fragment. However, the large energy dissipation which is observed is still too small to support the idea of a pure massive transfer process.     

Multielectron processes in collisions of Xe23+ ions with Ar atoms

We report the measurements of relative cross sections for multielectron processes in collisions of Xe²³⁺ ions with argon atoms in the velocity range of 0.65–1.32 a.u. By means of the coincidence time-of-flight (TOF) technique, the final charge states of both the projectile and target ions for each collision event are determined. The present experimental data are compared with the scaling law by Selberg et al. [Phys. Rev. A 54, 4127 (1996)] and the extended classical over-barrier (ECB) model.