Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter.

The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions.

Energy loss and charge transfer of argon in a laser-generated carbon plasma.

This Letter reports on the measurement of the energy loss and the projectile charge states of argon ions at an energy of 4 MeV/u penetrating a fully ionized carbon plasma. The plasma of n(e)≈10(20) cm(-3) and T(e)≈180 eV is created by two laser beams at λ(Las)=532 nm incident from opposite sides on a thin carbon foil. The resulting plasma is spatially homogenous and allows us to record precise experimental data. The data show an increase of a factor of 2 in the stopping power which is in very good agreement with a specifically developed Monte Carlo code, that allows the calculation of the heavy ion beam's charge state distribution and its energy loss in the plasma.

Active manipulation of the spatial energy distribution of laser-accelerated proton beams.

The spatial energy distributions of beams of protons accelerated by ultrahigh intensity (>10(19)Wcm2) picosecond laser pulse interactions with thin foil targets are investigated. Using separate, low intensity (<10(13)Wcm2) nanosecond laser pulses, focused onto the front surface of the target foil prior to the arrival of the high intensity pulse, it is demonstrated that the proton beam profile can be actively manipulated. In particular, results obtained with an annular intensity distribution at the focus of the low intensity beam are presented, showing smooth proton beams with a sharp circular boundary at all energies, which represents a significant improvement in the beam quality compared to irradiation with the picosecond beam alone.

Radiological characterisation of photon radiation from ultra-high-intensity laser-plasma and nuclear interactions.

With the increasing number of multi-terawatt (10(12) W) and petawatt (10(15) W) laser interaction facilities being built, the need for a detailed understanding of the potential radiological hazards is required and their impact on personnel is of major concern. Experiments at a number of facilities are being undertaken to achieve this aim. This paper describes the recent work completed on the Vulcan petawatt laser system at the CCLRC Rutherford Appleton Laboratory, where photon doses of up to 43 mSv at 1 m per shot have been measured during commissioning studies. It also overviews the shielding in place on the facility in order to comply with the Ionising Radiation Regulations 1999 (IRR99), maintaining a dose to personnel of less than 1 mSv yr(-1) and as low as reasonably practicable (ALARP).

Mastery motivation in young children with Down's syndrome: relations with cognitive and adaptive competence.

BACKGROUND: Prior research on relations between motivation and competence have focused on cognitive competence, despite theoretical predictions that mastery motivation spurs behaviour that is effective in meeting the demands of one's environment, i.e. adaptive competence. Issues of adaptive competence are especially relevant for children with developmental delay since functional independence is an important long-term goal for these children. METHODS: In the present study, mastery motivation was examined in relation to both cognitive and adaptive competence in 5-year-old children with Down's syndrome (n = 41). RESULTS: Scores on mastery task and parent-report measures of mastery motivation were generally low, but positively related to scores on standardized measures of cognitive competence and adaptive competence. CONCLUSIONS: This is the first study of motivation-competence relations in pre-school-age children. The findings have implications for both developmental theory and early intervention efforts, and the authors hope that they will serve as a stimulus for future research on cross-domain relationships in atypical populations.

The interpersonal context of romantic autonomy in adolescence.

This study explored the interpersonal context of romantic autonomy in mid adolescence by examining within-relationship links with romantic intimacy and affiliation, links across relationships with mothers and friends, and individual differences due to age and gender. Two hundred and thirty adolescents in grades 9, 10 and 11 with a current romantic partner, completed self-report questionnaires assessing their relationships. The results indicated that romantic autonomy was positively linked to romantic intimacy and affiliation, although this link differed according to the duration of the romantic relationship. The results also indicated that romantic autonomy was higher among girls than boys. Finally, romantic autonomy was uniquely linked to intimacy and autonomy in both maternal and friend relationships. Overall, this study highlights the interconnection of intimacy and autonomy in romantic relationships and the interconnections across relationships.

Dating experiences of bullies in early adolescence.

In this study, 196 young adolescents who reported that they bullied their peers were identified out of a sample of 1,758 students in Grades 5 through 8. After selecting from the total sample a group of nonbullying youth who were matched on gender, school, and grade, a comparison was made of the groups' dating experiences, quality of friend and boyfriend or girlfriend relationships, and acts of physical and social aggression. The results indicated that bullies started dating earlier and engaged in more advanced dyadic dating than comparison adolescents. Bullies were highly relationship oriented, yet their views of their friends and boyfriends or girlfriends were less positive and less equitable than the comparison adolescents. Finally, bullies were more likely to report physical and social aggression with their boyfriends or girlfriends. Although the bullies reported more advanced pubertal development, this factor did not fully account for their dating precocity and negative romantic relationships. The results confirmed our hypotheses that adolescents whose peer relationships are characterized by bullying are at risk in their development of healthy romantic relationships.

Wave-front control of a large-aperture laser system by use of a static phase corrector.

In large-aperture, ultrahigh-intensity laser systems, such as Vulcan at the Rutherford Appleton Laboratory, one of the most important factors that determines the ultimate on-target focused intensity is the wave-front quality of the laser pulse. We report on a wave-front analysis carried out on Vulcan to determine the nature and contribution of the aberrations present in the laser pulse that effectively limited the available on-target intensity. We also report on a significant improvement to the wave-front quality that was achieved by static correction of the main aberration, resulting in an increase of focused intensities by a factor of 4.

Generation of terawatt pulses by use of optical parametric chirped pulse amplification.

Optical parametric chirped pulse amplifiers offer exciting prospects for generating new extremes in power, intensity, and pulse duration. An experiment is described that was used to investigate the operation of this scheme up to energies approaching a joule, as a step toward its implementation at the petawatt level. The results demonstrate an energy gain of 10(10) with an energy extraction efficiency of 20% and close to diffraction-limited performance. Some spectral narrowing during amplification was shown to be compatible with the time-varying profile of the pump beam and consistent with the measured recompressed pulse durations of 260 and 300 fs before and after amplification, respectively.

Peer involvement in bullying: insights and challenges for intervention.

The purpose of this research was to examine the peer processes that occur during bullying episodes on the school playground. These processes were examined from a social learning perspective, allowing us to consider the effects of various types of reinforcement among bullies, victims, and peers. Fifty-three segments of video tape were examined. Each segment contained a peer group (two or more peers) that viewed bullying on the school playground. Peers were coded for actively joining or passively reinforcing the bully, and for actively intervening on behalf of the victim. On average, four peers viewed the schoolyard bullying, with a range from two to 14 peers. Averaged across all episodes, peers spent 54% of their time reinforcing bullies by passively watching, 21% of their time actively modelling bullies, and 25% of their time intervening on behalf of victims. Older boys (grades 4-6) were more likely to actively join with the bully than were younger boys (grades 1-3) and older girls. Both younger and older girls were more likely to intervene on behalf of victims than were older boys. The results were interpreted as confirming peers' central roles in the processes that unfold during playground bullying episodes. We discuss the results in terms of the challenges posed to peer-led interventions. Peers' anti-bullying initiatives must be reinforced by simultaneous whole-school interventions.

Binary phase zone-plate arrays for laser-beam spatial-intensity distribution conversion.

We report on the theory and development of a diffractive element composed of a binary phase zone-plate array. This component conditions the intensity distribution in the focal plane of a conventional refractive lens to generate efficiently (82%) a flattop intensity envelope on target. Analysis of the design indicates that manufacturing tolerances are not critical. Experimental performances on target from x-ray emission and shock-breakout measurements are also presented.

Binary-phase zone plate arrays for the generation of uniform focal profiles.

The generation of uniform focal intensity profiles is important for a number of applications, including laser-plasma interaction experiments. We report on a focusing system that uses a novel binary-phase optic capable of producing efficient two-dimensional uniform top-hat intensity optical and x-ray profiles.

The mental health of Portuguese children.

This study investigated the mental health of Portuguese children in Canada. Preliminary work involved a survey of professionals serving the Portuguese community and the translation and assessment of a standardized child behaviour checklist. Forty-five Portuguese children and 45 non Portuguese children referred to a children's mental health centre were compared on demographic and family indicators and their referral source. There were similar proportions of boys and girls in the two groups, similar types of services were requested, and they had similar treatment histories. The Portuguese children were older at the time of referral and were more likely to be referred by educational agencies than the non Portuguese children. Portuguese families appeared to experience different stresses than non Portuguese families. Implications of these findings for the provision of culturally sensitive interventions for Portuguese children and their families are discussed.

The influence of down's syndrome on sibling interaction.

Home observations were done on sibling interactions in 31 families with a child having Down's syndrome and a non-handicapped sibling. The siblings with Down's syndrome initiated less prosocial and agonistic behaviour, but imitated more frequently than their non-handicapped siblings. These effects were found regardless of birth order. There were no effects of gender. Higher levels of prosocial behaviours among large interval dyads and in dyads with a second-born Down's syndrome child were primarily due to the age of the non-handicapped sibling. Results were similar to those in previous "normative" studies of sibling interactions.