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Biomedical subjects

P Gibbon

Publications and source records attributed to P Gibbon.

At least 19 recordsLinked to original sources

Comparison of experimental and simulated Kalpha yield for 400 nm ultrashort pulse laser irradiation.

Ti Kalpha emission yields from foils irradiated with approximately 45 fs, p-polarized pulses of a frequency-doubled Ti:sapphire laser are presented. A simple model invoking vacuum heating to predict absorption and hot electron temperature was coupled with the cross section for K -shell ionization of Ti and the Bethe-Bloch stopping power equation for electrons. The peak predicted Kalpha emission was in generally good agreement with experiment. This contrasts strongly with previous work at the fundamental frequency. Similar predictions using particle-in-cell (PIC) code simulation to estimate the number and temperature of hot electrons also gave good agreement for yield.

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Food cost and availability in a rural setting in Australia.

INTRODUCTION: The burden of chronic diseases is rapidly increasing worldwide. In Australia rural populations have a greater burden of disease. Chronic diseases are largely preventable with diet as a key risk factor. With respect to diet-related chronic disease, dietary risk may be due to poor food access, namely, poor availability and/or the high cost of healthy food. It is likely that poor food access is an issue in rural areas. OBJECTIVE: To assess food access in rural south-west (SW) Victoria, Australia. METHODS: A total of 53 supermarkets and grocery stores in 42 towns participated in a survey of food cost and availability in the rural area of SW Victoria. The survey assessed availability and cost of a Healthy Food Access Basket (HFAB) which was designed to meet the nutritional needs of a family of 6 for 2 weeks. RESULTS: Seventy-two percent of the eligible shops in SW Victoria were surveyed. The study found that the complete HFAB was significantly more likely to be available in a town with a chain-owned store (p <0.00). The complete HFAB was less likely to be available from an independently owned store in a town with only one grocery shop (p <0.004). The average cost of the HFAB across SW Victoria was AU380.30 dollars +/- 25.10 dollars (mean +/- SD). There was a mean range in difference of cost of the HFAB of 36.92 dollars. In particular, high variability was found in the cost of fruits and vegetables. CONCLUSIONS: Cost and availability of healthy food may be compromised in rural areas. IMPLICATIONS: Improvements in food access in rural areas could reduce the high burden of disease suffered by rural communities.

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Spatial characteristics of Kalpha x-ray emission from relativistic femtosecond laser plasmas.

The spatial structure of the Kalpha emission from Ti targets irradiated with a high intensity femtosecond laser has been studied using a two-dimensional monochromatic imaging technique. For laser intensities I<5 x 10(17) W/cm(2), the observed spatial structure of the Kalpha emission can be explained by the scattering of the hot electrons inside the solid with the help of a hybrid particle-in-cell/Monte Carlo model. By contrast, at the maximum laser intensity I=7 x 10(18) W/cm(2) the half-width of the Kalpha emission was 70 microm compared to a laser-focus half-width of 3 microm. Moreover, the main Kalpha peak was surrounded by a halo of weak Kalpha emission with a diameter of 400 microm and the Kalpha intensity at the source center did not increase with increasing laser intensity. These three features point to the existence of strong self-induced fields, which redirect the hot electrons over the target surface.

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Optimization of Kalpha bursts for photon energies between 1.7 and 7 keV produced by femtosecond-laser-produced plasmas of different scale length.

The conversion efficiency of a 90 fs high-power laser pulse focused onto a solid target into x-ray Kalpha line emission was measured. By using three different elements as target material (Si, Ti, and Co), interesting candidates for fast x-ray diffraction applications were selected. The Kalpha output was measured with toroidally bent crystal monochromators combined with a GaAsP Schottky diode. Optimization was performed for different laser intensities as well as for different density scale lengths of a preformed plasma. These different scale lengths were realized by prepulses of different intensities and delay times with respect to the main pulse. Whereas the Kalpha yield varied by a factor of 1.8 for different laser intensities, the variation of the density scale length could provide a gain factor up to 4.6 for the Kalpha output.

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Theory of K(alpha) generation by femtosecond laser-produced hot electrons in thin foils.

An analytical model of femtosecond K(alpha) x-ray generation from laser-irradiated foils is presented. Expressions are found for the photon emission yield in both forward and backward directions in integral form as a function of hot-electron temperature and target thickness. It is found that for any given target material, there is a foil thickness and a hot-electron temperature at which the K(alpha) emission is maximized. Conversion efficiencies are consistent with contemporary measurements of K(alpha) radiation produced with femtosecond lasers.

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Femtosecond silicon K alpha pulses from laser-produced plasmas.

Ultrashort bursts of silicon K alpha x-ray radiation from femtosecond-laser-produced plasmas have been generated. A cross-correlation measurement employing a laser-triggered ultrafast structural change of a CdTe crystal layer (320 nm) shows a K alpha pulse duration between 200 fs and 640 fs. This result is corroborated by particle in cell simulations combined with a Monte-Carlo electron stopping code and calculations on the structural changes of the crystal lattice.

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Mev X rays and photoneutrons from femtosecond laser-produced plasmas.

We demonstrate a novel method to monitor the total angular distribution of the spectrum of hard x-ray emission from a plasma generated with femtosecond laser pulses with an intensity of 5 x 10(18) W/cm2 on a solid target. Measured and calculated angular distributions of x rays show a pronounced anisotropy for MeV photon energies. We complemented the spectral information by demonstrating a (gamma,n) nuclear reaction with a tabletop laser system.

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Phase modulation of intense ultrashort laser pulses reflected from steep, dense plasmas.

The phase modulation of intense ( I = 10(18) W/cm(2)) ultrashort laser pulses ( tau(p) = 70 fs) after reflection from steep, dense plasmas has been temporally resolved for the first time in particle-in-cell simulations. The position of the turning point from where the pulse reflects has been compared to the phase modulation, over a range of angles of incidence. At normal incidence or s polarization the phase modulation almost exactly represents the movement of the turning point due to the light pressure. As the angle of incidence is increased for p polarization, the simple Fresnel relationship between phase modulation and displacement of the reflection position, Delta phi(t) = -2k(0)Delta x(t)cos theta(0), increasingly breaks down.

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Yield optimization and time structure of femtosecond laser plasma kalpha sources

The generation of femtosecond Kalpha x rays from laser-irradiated plasmas is studied with a view to optimizing photon number and pulse duration. Using analytical and numerical models of hot electron generation and subsequent transport in a range of materials, it is shown that an optimum laser intensity I(opt) = 7x10(9)Z4.4 exists for maximum Kalpha yield. Furthermore, it is demonstrated that bulk targets are unsuitable for generating sub-ps x-ray pulses: instead, design criteria are proposed for achieving Kalpha pulse durations </=100 fs using foils of approximately 2 &mgr;m thickness.

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