The charge (Z) distributions from intermediate energy heavy-ion reactions depend upon the multiplicity n of intermediate mass fragments through a factor of the form e-cnZ. Experimentally c starts from zero at low values of the transverse energy Et and reaches a saturation value at high Et. In a liquid-gas phase diagram, c = 0 for the saturated vapor, while c > 0 for the unsaturated vapor. It is suggested that in the c ~ 0 regime the source evaporates down to a sizable remnant, while for c > 0 the source vaporizes completely. Percolation of a finite system portrays a. behavior similar to that observed experimentally. Typeset using REVTEX

The behaviour of prestressed concrete structures with unbonded tendons differs significantly from that of those with bonded tendons after the working stage. To assess the safety of such structures, it is necessary to carry out full-range analysis with particular attention to the post-peak behaviour. This paper reports the initial findings of experimental investigations. A number of simply supported externally prestressed concrete beams with either steel or aramid fibre-reinforced polymer tendons are tested to failure, so as to study the effect of external prestressing on the ductility. The specimens tested all have T-sections prestressed by two external tendons with one on each side. In the experiments, different factors are taken into consideration. They include the level of prestressing force, area of non-prestressed steel reinforcement and partial prestressing ratio and so on. It is found that friction may vary significantly when a beam is subjected to large deformations. Comprehensive analysis of internal forces in selected specimens has been carried out. While the tendons play a key role in supporting loads during the initial stage of loading, their contribution decreases with further loading. At the same time, the role of bottom non-prestressed reinforcement increases and remains predominant after the apparent yielding of the specimen.

Fission in 3He and 4He induced reactions at excitation energies between the fission barrier and 140 MeV has been investigated. Twentythree fission excitation functions of various compound nuclei in different mass regions are shown to scale exactly according to the transition state prediction once the shell effects are accounted for. New precise measurements of excitation functions in a mass region where shell effects are very strong, allow one to test the predictions with an even higher accuracy. The fact that no deviations from the transition state method have been observed within the experimentally investigated excitation energy regime allows one to assign limits for the fission transient time. The precise measurement of fission excitation functions of neighboring isotopes enables us to experimentally estimate the first chance fission probability. Even if only first chance fission is investigated, no evidence for fission transient times larger than 30 zs can be found. Talk presented at the XXXV. International Winter Meeting on Nuclear Physics, Bormio, Italy, 1997 and to be published in the proceedings. Electronic address: TRubehn@lbl.gov

OBJECTIVES:To examine parent perception of child behaviour problems and parenting stress in Hong Kong, and to assess the extent to which they are related to socio-demographic factors and the availability of social support.DESIGN:Cross-sectional survey by using a questionnaire.SETTING:Maternal and Child Health Centres, Hong Kong.PARTICIPANTS:Parents of children aged 4 years who were registered with Maternal and Child Health Centres and were living in Hong Kong between September 2002 and February 2003.MAIN OUTCOME MEASURES:Child behaviour problems and parenting stress.RESULTS:A total of 1009 questionnaires were returned giving a participation rate of 67.0%. About one tenth of parents were experiencing difficulties with their children's behaviour. Parenting stress and children's behaviour problems were associated with presence or absence of social support. Parenting stress was also associated with household income.CONCLUSIONS:The prevalence of child behaviour problems in Hong Kong is comparable with international figures. Intervention programmes should be targeted at parents who experience difficulties with their children's behaviour and parenting.

Excitation functions were measured for complex fragments with atomic number Z=5−20 emitted from the compound nuclei 70,76Se produced in the reactions 58,64Ni+12C. Mass asymmetric fission barriers were extracted by fitting the excitation functions with a transition state formalism. The extracted barriers were compared with those calculated from macroscopic nuclear models. The measured barriers for symmetric fission seem to support the hypothesis of a shape-dependent congruence energy, which doubles for fission of strongly indented saddle-point shapes. All of the measured excitation functions can be scaled onto a single straight line according to the transition state prediction.

Multifragment emission following Xe+Au collisions at 30, 40, 50 and 60 AMeV has been studied with multidetector systems covering nearly 4-pi in solid angle. The correlations of both the intermediate mass fragment and light charged particle multiplicities with the transverse energy are explored. A comparison is made with results from a similar system, Xe+Bi at 28 AMeV. The experimental trends are compared to statistical model predictions.

Excitation functions were measured for complex fragments with atomic number Z = 5–25 emitted from the compound nuclei 90,94,98Mo produced in the reactions 78,82,86Kr + 12C. Mass-asymmetric fission barriers were extracted by fitting the excitation functions with a transition state formalism. The extracted barriers are several MeV higher on average than the calculations of the Rotating Finite-Range Model and substantially lower than predicted by the Rotating Liquid Drop Model. The symmetric fission barriers measured support the hypothesis of a congruence term that doubles for the fission of strongly indented saddle-point shapes. The excitation functions were analyzed to search for atomic number Z- and energy E-dependent deviations from transition-state-method predictions. All of the measured excitation functions can be scaled onto a single universal straight line according to the transition-state predictions. No Z- and/or E-dependent effects that could be attributed to transient effects are visible.

Two-fragment reduced-velocity correlation functions were measured for small-impact-parameter collisions of Kr-86+Nb-93 at E/A=50 MeV and compared to results of many-body Coulomb-trajectory calculations performed for instantaneous and sequential multifragment breakup scenarios. The correlation functions indicate emission on a very shea time scale and appear consistent with an instantaneous breakup scenario, even though they exhibit a pronounced dependence on fragment kinetic energy when fragments are emitted at large transverse momenta. For the case of instantaneous breakup, sensitivities to initial-state momentum correlations due to total momentum conservation and to different emission patterns are investigated. For fragments emitted with large transverse momenta, momentum conservation constraints can cause a dependence of reduced-velocity correlation functions on fragment energy and fragment charge similar to those observed experimentally.

In recent years, multifragmentation of nuclear systems has been extensively studied, and many efforts have been made to clarify the underlying physics[1]. However, no clear consensus exists on the mechanism for multifragmentation. Is the emission of intermediate mass fragments (IMF: 3 < Z > 20) a dynamical process (brought on by the occurrence of instabilities of one form or another) or a statistical process (i.e. the decay probabilities are proportional to a suitably defined exit channel phase space)?

Fission in {sup 3}H and {sup 4}He induced reactions at excitation energies between the fission barrier and 140 MeV has been investigated. Twenty-three fission excitation functions of various compound nuclei in different mass regions are shown to scale exactly according to the transition state prediction once the shell effects are accounted for. New precise measurements of excitation functions in a mass region where shell effects are very strong, allow one to test the predictions with an even higher accuracy. The fact that no deviations from the transition state method have been observed within the experimentally investigated excitation energy regime allows one to assign limits for the fission transient time. The precise measurement of fission excitation functions of neighboring isotopes enables one to experimentally estimate the first chance fission probability. Even if only first chance fission is investigated, no evidence for fission transient times larger than 30 zs can be found.

Coulomb bubbles, though stable against monopole displacement, are unstable at least with respect to quadrupole and octupole distortions. We show that there exists a temperature at which the pressure of the vapor filling the bubble stabilizes all the radial modes. In extremely thin bubbles, the crispation modes become unstable due to the surface-surface interaction.

Fission in 3He and 4He induced reactions at excitation energies between the fission barrier and 140 MeV has been investigated. Twentythree fission excitation functions of various compound nuclei in different mass regions are shown to scale exactly according to the transition state prediction once the shell effects are accounted for. New precise measurements of excitation functions in a mass region where shell effects are very strong, allow one to test the predictions with an even higher accuracy. The fact that no deviations from the transition state method have been observed within the experimentally investigated excitation energy regime allows one to assign limits for the fission transient time. The precise measurement of fission excitation functions of neighboring isotopes enables us to experimentally estimate the first chance fission probability. Even if only first chance fission is investigated, no evidence for fission transient times larger than 30 zs can be found.

Fission excitation functions of He-3 and He-4 induced compound nuclei are shown to scale exactly according to the Bohr-Wheeler transition state prediction once the shell effects are accounted for. The presented method furthermore allows one to model-independently extract values for the shell effects which are in good agreement to those obtained from liquid-drop model calculations. The fact that no deviations from the transition state method have been observed within the experimentally investigated excitation energy regime allows one to assign an upper limit for the transient time of 10 zs.

The experimental emission probabilities of complex fragments by low energy compound nuclei and their dependence upon energy and atomic number are compared to the transition state rates. Intermediate-mass-fragment multiplicity distributions for a variety of reactions at intermediate energies are shown to be binomial and thus reducible at all measured transverse energies. From these distributions a single binary event probability can be extracted which has a thermal dependence. A strong thermal signature is also found in the charge distributions. The n-fold charge distributions are reducible to the 1-fold charge distributions through a simple scaling dictated by fold number and charge conservation.

Fission excitation functions of compound nuclei in a mass region where shell effects are expected to be very strong are shown to scale exactly according to the transition state prediction once these shell effects are accounted for. Furthermore, the method applied in this paper allows for the model-independent determination of the nuclear shell effects. {copyright} {ital 1996 The American Physical Society.}