Fission of209Bi induced by6He ions

Secondary beams of⁴He and⁶He were produced through the transfer reactions with the 18.5 MeV/u¹¹B and 35.5 MeV/u⁷Li primary beams. The fusion-fission cross sections have been measured for the^4,6He +²⁰⁹Bi reactions at energies E_cm>1.5*B_fus. In the present experiment it was found that the fission excitation function for the neutron rich nuclei⁶He is significantly higher than for the⁴He nuclei.     

Fission induced by heavy-ion collisions

Fission fragment angular distributions from sequential fission induced in heavy-ion reactions will depend on the angular momentum exchange occuring in such reactions in considerable detail. Theoretical expressions for the angular distributions of fission products from totally and partly polarized nuclei are presented. They are compared with recent measurements and discussed in relation to the deep inelastic reaction mechanism.     

Na离子和Cl离子引起He原子直接多重电离过程的研究

利用2～8 MeV的Naq+、Clq+(q=2,3,4,5)轰击氦原子,对碰撞的直接多重电离过程进行研究.实验采用反冲离子-散射离子飞行时间符合技术,通过反冲离子飞行时间谱区分不同价态反冲离子;利用静电偏转和位置灵敏探测技术区分不同电荷态散射离子;结合CAMAC-PC多参数获取系统得到一定价态散射离子所对应的反冲离子电荷态分布谱;经分析该谱得到直接多重电离截面与直接单电离截面之比R21.讨论了R21随入射离子速度和电荷态的变化关系.     

Fission of medium mass elements induced by 126 MeV 14N ions

Ni, Se, Mo, Ag, Ho and Au targets were bombarded by 126 MeV ¹⁴N ions in order to study compound nucleus fission. The large angular momentum brought in by the projectile implies much greater fission probabilities than those obtained when the same compound nuclei are created with light projectiles. We employed two surface-barrier detectors. The results obtained are fission cross sections, angular correlations and kinetic energy and mass distributions. Fissilities are found to decrease exponentially with the ratio Z²/A of the fissioning nucleus until Z²/A = 19 (molybdenum) and then to increase again for lower Z²/A. The calculations of fissilities performed with the fission barrier of Myers and Swiatecki, and including angular momentum effects in evaporation-fission competition, gives the same variation of fissilities as the experimental one, but the results obtained are lower than the experimental values. The total kinetic energies measured are higher than the predictions of the liquid drop model by around 10 MeV (Nix). The shapes of the mass distributions indicate that the value of x_BG (Businaro-Gallone point) is lower than 0.4 (silver).     

Neutralisation of fast helium ions by cesium

The absolute cross-section for charge-exchange in cesium vapor has been measured for 5 to 25 keV He⁺ ions. The proportion of He¹ metastable triplet in the post charge-exchange beam has been estimated.     

Swelling of metallic surfaces irradiated by helium ions

At doses close to threshold for exfoliation or bubble formation the surfaces have been investigated by SEM and, after isolation of surface layers, also by RBS. Swelling as a function of energy and material has been determined. The data are interpreted in the form of volume occupied by a helium atom. We find an indication for a volume decrease with increasing energy. A special example of blister formation is also discussed, where we find evidence for a non-uniform fracture plane.     

Single electron loss by fast ions in helium

The single electron loss cross sections for fast B, N, and Ne ions colliding with He atoms are calculated. In the calculation of the ionization amplitude, the He nucleus screening is taken into account by both the sum rule and the summation over a finite number of excited 1snl (n < 5, l < 2) states and states of the continuous spectrum. The calculated results agree satisfactorily with experimental data.     

Atoms and ions in superfluid helium

Optical spectra of atomic impurities in liquid helium have been investigated. Comparison is made between the wavelength of the free atomic and ionic lines and those in the liquid helium matrix. Simultaneously, the line width and a possible asymmetry is recorded. Presence and absence of radiative transitions depend on the species of the atom implanted in the quantum fluid. The absence of any optical transitions from states lying as low as 1,8 eV below the ionization limit will be explained in this paper.     

Atoms and ions in superfluid helium

Optical spectra, level population and nonradiative deexcitation processes of atomic impurities in liquid helium have been investigated. The recombination of electrons and positive metal ions are described by the tunneling model. This model explains the population gap of 1.8 eV below the ionisation limit which was observed for several neutral defect atoms in liquid helium. In the framework of a pseudopotential theory excited singlet levels of Ba, Ca and Na atoms are recalculated and compared to experimental data. Non-spherical atomic defects for excited p-states are treated also. Quadrupole vibrations of these distorted defect structures are assumed to be responsible for inducing multiphonon transitions between excited atomic states.     

Single ionization of helium atoms by energetic fully stripped carbon ions

A four-body distorted wave approximation is presented for theoretical investigations of the single ionization of groundstate helium atoms by fully stripped carbon ions at impact energies of 2 MeV/amu and 100 MeV/amu. The nine-dimensional integrals for the partial quantum-mechanical transition amplitudes of the specified reaction are reduced to some analytical expressions or one-dimensional integrals over real variables. Fully differential cross sections(FDCSs) are calculated and compared with their experimental values as well as the results obtained from other theories. Despite the simplicity and quickness of the proposed quadrature, the comparison shows that the obtained results are in reasonable agreement with the experiment and are compatible with those of other complicated theories.     

Fission barrier of 209Bi measured by electron induced fission

For ²⁰⁹Bi an electrofission cross section below 10^?36 cm² was measured by the observation of correlated binary fission tracks in mica detectors. From the cross-section data between 28 and 41 MeV electron energy, a fission barrier of 25.5 ± 1.5 MeV was deduced. The method offers the possibility to measure fission barriers ar low angular momentum and for nuclei not accessible otherwise.     

Neutralization of helium ions backscattered from tungsten

Measurements and computer simulations of the energy spectra of helium ions backscattered from clean, polycrystalline tungsten at 138° have been combined to obtain estimates of the probability of survival against neutralization. For incident ion energies of 300, 1180 and 2600 eV, the estimated survival probabilities for ions backscattered near the surface are 0.05%, 0.2% and 1%, respectively. The estimated survival probability for subsurface backscattered ions has been found to range from a value of 1% at 2600 eV, comparable to that of near-surface backscattered ions, down to near zero at 300 eV incident ion energy. Evidence is presented that the final charge state of ions which backscatter from beneath the surface is not controlled by their emergence speed but by their previous history in the solid.     

Energy-loss straggling for protons and helium ions

Energy-loss straggling for protons and helium ions at 50–500 keV per nucleon has been measured in molecular and atomic gases. It is found that composition of the target and charge-state fluctuations of the projectile play an important role in straggling.     

Fission modes in charged-particle induced fission

The population of the three fission modes predicted by Brosa's multi-channel fission model for the uranium region was studied in different fissioning systems. They were produced bombarding²³²Th and²³⁸U targets by light charged particles with energies slightly above the Coulomb barrier. Though the maximum excitation energy of the compound nucleus amounted to about 22 MeV, the influences of various spherical and deformed nuclear shells on the mass and total kinetic energy distributions of fission fragments are still pronounced. The larger variances of the total kinetic energy distributions compared to those of thermal neutron induced fission were explained by temperature dependent fluctuations of the amount and velocity of alteration of the scission point elongation of the fissioning system. From the ratio of these variances the portion of the potential energy dissipated among intrinsic degrees of freedom before scission was deduced for the different fission channels. It was found that the excitation remaining after pre-scission neutron emission is mainly transferred into intrinsic heat and less into pre-scission kinetic energy.     

Ionization of atomic vapors and helium by collision with relativistic heavy ions

We have studied collisions of atomic vapors and helium with relativistic, highly charged ions having Z v c. Formulas for the cross-sections of ionization and excitation of discrete levels of the atomic vapor and the cross-section of single and double ionization of helium atoms have been obtained. Comparisons with experimental data of the single and double ionization of helium are presented.Tashkent University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 23–29, February, 1995.     

Fission of232Th induced by 4–11 MeV protons

The excitation function for the²³²Th(p,f) reaction has been determined in the energy range from 4 to 11 MeV. A discrepancy existing in the literature is resolved. The mass yield distribution has been studied at 8 MeV proton energy and the peak-to-valley ratio has been determined to be 9.2. The fission data, together with evaporation residue data from an earlier experiment, are understood rather well within the framework of a statistical decay model which derives its parameters from independent data. The absorption cross sections for protons in the vicinity of and below the Coulomb barrier are also discussed and confronted with optical model predictions using published global proton potentials.     

Structure of negative impurity ions in liquid helium

The experimental mobilities of negative halogen (Cl^?, F^?, and I^?) and metal (Ba^? and Ga^?) impurity ions in superfluid ⁴He are close to each other and much lower than the mobilities of not only He⁺ ions but also electron bubbles. It has been shown that the formation of multiatomic complexes (clusters or bubbles) around ions is responsible for this low mobility. Although the mobilities are similar, the structures of the resulting complexes are qualitatively different in the cases of halogens and metals: solid clusters, which are similar to a well-studied cluster at the He⁺ ion, are formed near halogen ions, which exhibit high electron affinities, whereas metal ions are localized in bubbles, which are similar to electron bubbles. The temperature and pressure dependence of the mobility of these complexes is qualitatively different. Experiments in this area, most likely, performed with a wider variety of negative ions, would enhance the understanding of the structure of charged complexes in liquid helium.     

Observation of cold, long-lived antiprotonic helium ions

Cold, two-body antiprotonic helium ions p 4He2+ and p 3He2+ with 100-ns-scale lifetimes, occupying circular states with the quantum numbers ni=28-32 and li=ni-1 have been observed. They were produced by cooling three-body antiprotonic helium atoms in an ultra-low-density helium target at temperature T approximately 10 K by atomic collisions, and then removing their electrons by inducing a laser transition to an autoionizing state. The lifetimes of p 3He2+ against annihilation induced by collisions were shorter than those of p 4He2+, and decreased for larger-ni states.