Experimental investigation of vibrational excitations in230Th

Nuclear levels in²³⁰Th have been investigated in the decay of²³⁰Pa, the²³⁰Th(d, pn) reaction and the²³²Th(p, t) reaction. TheK =0 ₁ ⁺ , 2 ₁ ⁺ , and 2 ₂ ⁺ bands with band heads at 635, 781, and 1010 keV were observed up to the 8⁺, 9⁺, and 7⁺ levels, respectively. A second excited 0⁺ level was identified at 1297 keV which might be interpreted as the usual shape-oscillation. The branching ratios of theE2 transitions from the 0 ₁ ⁺ , 2 ₁ ⁺ , and 2 ₂ ⁺ bands are explained in the framework of the rotational model by taking into account the coupling of these bands with the ground-state band, and the coupling between the 0 ₁ ⁺ and 2 ₁ ⁺ band. A strong enhancement ofE2 transitions from the 2 ₁ ⁺ to the 0 ₁ ⁺ band reported earlier is not confirmed. For the octupole vibrations withK =0^–, 1^–, and 2^– theE1 branching ratios are analyzed in terms of the Coriolis coupling of these bands. An almost complete experimental set ofE1 transition moments from these negative-parity bands to the 0 _g ⁺ , 0 ₁ ⁺ , and 2 ₁ ⁺ bands was obtained. It is suggested that octupole correlations might be important in explaining theseE1 moments.We appreciate the help of Mr. Assmus with the proton irradiations and the financial support of the Kernforschungszentrum Karlsruhe GmbH. This work was supported by the Deutsche Forschungsgemeinschaft (grants Bo 1109/1-1, Gr 894/2-1, Gu 179/3-1 and He 1316/2-3) and by the Volkswagen Stiftung.     

Photofission in238U,232Th and209Bi at intermediate energy

Fragment energy correlation measurements have been made for²³⁸U,²³²Th and²⁰⁹Bi with bremsstrahlung of 600 MeV maximum energy. The study was performed with silicon surface barrier detectors. Distributions of kinetic energy as a function of fragment mass are presented. The results are compared with earlier photofission data and, in the case of bismuth, with calculations based on the liquid drop model.     

Lifetime of the two-phonon vibrational state in232Th

We report on a measurement of the lifetime of the 1414 keV state in²³²Th by the Doppler-shift recoil-distance method. This 4⁺ state shows all characteristics of a nearly-harmonic two-phonon-vibrational excitation. The result for the B(E2) value obtained from the lifetime,=3.2 ± 0.7 ps, is in agreement with an earlier estimate of the collectivity from Coulomb-excitation yields and supports the interpretation of the 1414 keV state as a two-phonon-vibrational excitation.We would like to thank the accelerator crew of the Max-Planck-Institut für Kernphysik for the excellent running conditions they maintain. We are also indebted to Dr. H.J. Maier, University Munich for supplying us with the²³²Th targets. This work was supported by the Bundesminister für Forschung und Technologie under contract numbers 06BN601 and 06HD5251.     

Systematics of the mean energy and the mean multiplicity of prompt neutrons originating from 232Th fission

The cross section for the neutron-induced fission of ²³²Th target nuclei, ?? _f (E _n ), was described within statistical theory. The spectra of the mean multiplicity, $\bar v$ (E _n ), and the mean energy, $\bar E$ (E _n ), of secondary neutrons accompanying ²³²Th fission induced by neutrons of energy extending up to E _n = 20 MeV were analyzed on the basis of the chance structure of the cross section.     

Experimental configuration, F-F spin-spin coupling, and potential energy surface of difluoromethane-Ar

The rotational spectra of normal and ¹³C species of difluoromethane-argon have been investigated by molecular beam Fourier transform microwave spectroscopy. The obtained results allowed for the determination of the configuration of the complex, based on experimental data. The tunnelling splitting originated by van der Waals motions has been determined to be 193.740(1) and 162.16(8) MHz for the normal and ¹³C species, respectively. The two vibrational splittings have been used to evaluate a tunnelling barrier of 94(5) cm⁻¹. The fluorine-fluorine nuclear spin-spin coupling constant has been determined, D_aa = 0.19(1) MHz.     

A DFT investigation of potential energy surface and vibrational properties of hydrogen adsorbed on the Rh(1 1 1) surface

Vibrational properties of hydrogen on the Rh(1 1 1) surface have been investigated theoretically. The potential energy surface for this system has been calculated within the density functional theory. The potential is found to be very anharmonic. The wave functions and their energies for the hydrogen motion on the potential energy surface (PES) have been calculated and assigned by using discrete variable representation. It was found that the vibrational wave function is localized at hollow site in the ground state for hydrogen on Rh(1 1 1). Higher excited states are of delocalized nature and mixed parallel and perpendicular character. Our results are in good agreement with the observed vibrational spectra of hydrogen on the Rh(1 1 1) surface.     

Electrofission of238U and232Th

Absolute electrofission cross sections for²³⁸U and²³²Th in the energy regionE _e =7 ?65 MeV and fission fragment angular distributions forE _e =7–30 MeV have been measured. The angular distributions show strong anisotropies for low energies. The relative dipole and quadrupole contributions as a function of excitation energy are discussed in terms of the low lying fission transition states above the fission barriers. The cross sections show significant deviations from the results of some earlier measurements, in particular in the energy region above the giant dipole resonance. From the difficulties of absolute electrofission cross section measurements and the ambiguities in their interpretation it is concluded that by this time the quantitative analysis of electrofission cross sections with respect to the contributions of the giant quadrupole resonances to the fission decay channel should be regarded as rather tentative.     

Effect of surface potential on field-emission energy distributions and on the surface density of states

We present numerical results for the total energy distribution of electrons field emitted from the (100) plane of tungsten for two different surface barriers and two different values of the applied field. Numerical results are also presented for the surface density of states.     

A secondary minimum in the potential energy surface of Ca

Strutinsky type calculations are extended to lighter nuclei. Secondary minima in the potential energy surface generally considered to account for the existence of fission isomers occur also in the lighter mass region. Results for ⁴⁰Ca are compared with experimental evidences.     

The effect of spurious motion on the collective potential energy surface

Energy corrections to the potential energy surface which are caused by spurious collective motions are investigated. We concentrate on spurious center-of-mass motion, rotational oscillations and spurious vibrations. It is shown that the effects are deformation dependent and thus, that the heights and curvatures of the potential energy surface are changed. The changes are of the order of 1–2 MeV in the range of deformation δ=0 to δ=±0.4.     

Reduced transition probabilities for high-spin states in 232Th

The lifetimes and reduced transition probabilities for spins 6⁺ ? 18⁺ in the ground-state rotational band of ²³²Th have been measured by recoil-distance, Doppler-broadened lineshape, and multiple Coulomb excitation methods using ⁸⁴Kr and ¹³⁶Xe projectiles. Within the limits of experimental uncertainty, transition probabilities from all of the methods are consistent with those of a good rotor for spins 6⁺ ? 18⁺. The descrepancy with the prediction of simple β-band mixing and centrifugal stretching models is discussed.     

Radiative capture of protons by the deformed nuclide232Th

The excitation function for the radiative capture²³²Th(p, γ)²³³Pa has been determined in the proton energy range 7 to 20 MeV by an activation method. The results are compared with a compound nucleus model prediction and earlier experimental data for another deformed nuclide¹⁷⁶Yb. As in previous cases an enhancement over the CN-model prediction is observed and the excitation of the giant dipole resonance via the direct-semidirect reaction process is a likely explanation. Supplementary measurements of the²³²Th (p, f) excitation function in the proton energy range 11–20 MeV have been performed.     

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.     

聚四氟乙烯表面滴状冷凝的实验研究

根据递推法测定滴状冷凝传热系数原理,搭建了滴状冷凝传热系数测定试验台,并对聚四氟乙烯材料表面的传热特性进行了试验测定,分析了影响滴状冷凝传热系数的因素。     

Experimental investigation of magneto-rheological droplet impact on a smooth surface

We present an experimental investigation of the impact of magneto-rheological droplets on a smooth surface. The experimental setup consists of a syringe pump with capillary tube of 1.6 mm diameter located perpendicularly above a dry smooth quartz surface assembled above an electromagnetic module, which enables magnetic flux density control up to 7.8 G. Free surface flow patterns generated during the impact of droplets of 2.2 mm diameter, Reynolds number in the range 15-125 were recorded using a digital high-speed camera. The materials used in this study were commercial ferro-fluids, (hydrocarbon-based fluid containing micron-sized magnetizable particles). These fluids were characterized using a rotational rheometer modified by an electromagnetic module. The results show an up to 40% reduction in maximum spread diameters as well as reductions in spread velocities for droplets subjected to a magnetic field.