721 research outputs found
The longitudinal cascade development of cosmic-ray showers from observations of atmospheric Cerenkov radiation
This thesis is concerned with the measurement of the longitudinal cascade development of large cosmic-ray showers from observations of atmospheric Cerenkov radiation. The purpose of the measurements is to obtain information on the mass composition of the primary cosmic rays and on the gross features of the high-energy hadronic interactions. The characteristics of the Cerenkov light signal which are strongly coupled to the longitudinal shower development are described with reference to computer simulation results. An experiment designed to measure these characteristics was deployed in Dugway, Utah, U.S.A. between October 1977 and March 1980. Measurements were made in showers of primary energy 10(^15) – 10(^18) eV. Existing data analysis techniques have been refined and new procedures developed in order to optimise the reduction of the digital data. A detailed analysis is given of two aspects of the time structure of the Cerenkov light signal in showers of mean primary energy ~ 2 x 10(^17) eV. The measurements are interpreted in terms of the mean depth of electron cascade maximum and the fluctuations occurring between showers. The many results on the cascade development currently available from the Dugway data are summarised. Interpretation of the results is found to be possible in the framework of scaling-based models of the high energy hadronic interactions which incorporate an enhancement of the central-region multiplicity. This allows certain inferences to be made concerning the primary mass composition. Specifically, primaries of energy ~ 10(^16) eV appear to be predominantly heavy nuclei, and the mass composition becomes lighter with increasing primary energy until at least ~ 2 x 10(^17) eV. While no other single experiment has yet been in a position to corroborate these results, broad consistency is found between the Dugway results and the combined results from a number of other experiments
Interactions between Silica Particles in the Presence of Multivalent Coions
Forces between charged silica particles in solutions of multivalent coions
are measured with colloidal probe technique based on atomic force microscopy.
The concentration of 1:z electrolytes is systematically varied to understand
the behavior of electrostatic interactions and double-layer properties in these
systems. Although the coions are multivalent the Derjaguin, Landau, Verwey, and
Overbeek (DLVO) theory perfectly describes the measured force profiles. The
diffuse-layer potentials and regulation properties are extracted from the
forces profiles by using the DLVO theory. The dependencies of the diffuse-layer
potential and regulation parameter shift to lower concentration with increasing
coion valence when plotted as a function of concentration of 1:z salt.
Interestingly, these profiles collapse to a master curve if plotted as a
function of monovalent counterion concentration
Polycyclic aromatic hydrocarbon processing in a hot gas
Context: PAHs are thought to be a ubiquitous and important dust component of
the interstellar medium. However, the effects of their immersion in a hot
(post-shock) gas have never before been fully investigated. Aims: We study the
effects of energetic ion and electron collisions on PAHs in the hot post-shock
gas behind interstellar shock waves. Methods: We calculate the ion-PAH and
electron-PAH nuclear and electronic interactions, above the carbon atom loss
threshold, in H II regions and in the hot post-shock gas, for temperatures
ranging from 10^3 to 10^8 K. Results: PAH destruction is dominated by He
collisions at low temperatures (T < 3x10^4 K), and by electron collisions at
higher temperatures. Smaller PAHs are destroyed faster for T < 10^6 K, but the
destruction rates are roughly the same for all PAHs at higher temperatures. The
PAH lifetime in a tenuous hot gas (n_H ~ 0.01 cm^-3, T ~ 10^7 K), typical of
the coronal gas in galactic outflows, is found to be about thousand years,
orders of magnitude shorter than the typical lifetime of such objects.
Conclusions: In a hot gas, PAHs are principally destroyed by electron
collisions and not by the absorption of X-ray photons from the hot gas. The
resulting erosion of PAHs occurs via C_2 loss from the periphery of the
molecule, thus preserving the aromatic structure. The observation of PAH
emission from a million degree, or more, gas is only possible if the emitting
PAHs are ablated from dense, entrained clumps that have not yet been exposed to
the full effect of the hot gas.Comment: 16 pages, 11 figures, 3 tables, typos corrected and PAH acronym in
the title substituted with full name to match version published in Astronomy
and Astrophysic
Influence of gas pressure on high-order harmonic generation of Ar and Ne
We study the effect of gas pressure on the generation of high-order harmonics
where harmonics due to individual atoms are calculated using the recently
developed quantitative rescattering theory, and the propagation of the laser
and harmonics in the medium is calculated by solving the Maxwell's wave
equation. We illustrate that the simulated spectra are very sensitive to the
laser focusing conditions at high laser intensity and high pressure since the
fundamental laser field is severely reshaped during the propagation. By
comparing the simulated results with several experiments we show that the
pressure dependence can be qualitatively explained. The lack of quantitative
agreement is tentatively attributed to the failure of the complete knowledge of
the experimental conditions.Comment: 7 figure
Relationships between Peak Oxygen Uptake and Arterial Function: a Preliminary Study
Please view abstract in the attached PDF file
Localising vaccination services:Qualitative insights on public health and minority group collaborations to co-deliver coronavirus vaccines
Ethnic and religious minorities have been disproportionately affected by the SARSCoV-2 pandemic and are less likely to accept coronavirus vaccinations. Orthodox (Haredi) Jewish neighbourhoods in England experienced high incidences of SARSCoV-2 in 2020-21 and measles outbreaks (2018-19) due to suboptimal childhood vaccination coverage. The objective of our study was to explore how the COVID-19 vaccination programme (CVP) was co-delivered between public health services and an Orthodox Jewish health organisation. Methods included 28 semi-structured interviews conducted virtually with public health professionals, community welfare and religious representatives, and household members. We examined CVP delivery from the perspectives of those involved in organising services and vaccine beneficiaries. Interview data was contextualised within debates of the CVP in Orthodox (Haredi) Jewish print and social media. Thematic analysis generated five considerations: i) Prior immunisation-related collaboration with public health services carved a role for Jewish health organisations to host and promote coronavirus vaccination sessions, distribute appointments, and administer vaccines ii) Public health services maintained responsibility for training, logistics, and maintaining vaccination records; iii) The localised approach to service delivery promoted vaccination in a minority with historically suboptimal levels of coverage; iv) Co-delivery promoted trust in the CVP, though a minority of participants maintained concerns around safety; v) Provision of CVP information and stakeholders' response to situated (context-specific) challenges and concerns. Drawing on this example of CVP co-delivery, we propose that a localised approach to delivering immunisation programmes could address service provision gaps in ways that involve trusted community organisations. Localisation of vaccination services can include communication or implementation strategies, but both approaches involve consideration of investment, engagement and coordination, which are not cost-neutral. Localising vaccination services in collaboration with welfare groups raises opportunities for the on-going CVP and other immunisation programmes, and constitutes an opportunity for ethnic and religious minorities to collaborate in safeguarding community health.<br/
Medium propagation effects in high harmonic generation of Ar and N
We report theoretical calculations of high harmonic generation (HHG) by
intense infrared lasers in atomic and molecular targets taking into account the
macroscopic propagation of both fundamental and harmonic fields. On the
examples of Ar and N, we demonstrate that these {\it ab initio}
calculations are capable of accurately reproducing available experimental
results with isotropic and aligned target media. We further present detailed
analysis of HHG intensity and phase, under various experimental conditions, in
particular, as the wavelength of the driving laser changes. Most importantly,
our results strongly support the factorization of HHG at the macroscopic level
into a product of a returning electron wave packet and the photorecombination
transition dipole, under typical experimental conditions. This implies that the
single-atom/molecule structure information can be retrieved from experimentally
measured HHG spectra
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