75 research outputs found
Craniospinal radiotherapy in children: Electron- or photon-based technique of spinal irradiation
AbstractBackgroundThe prone position and electron-based technique for craniospinal irradiation (CSI) have been standard in our department for many years. But this immobilization is difficult for the anaesthesiologist to gain airway access. The increasing number of children treated under anaesthesia led us to reconsider our technique.AimThe purpose of this study is to report our new photon-based technique for CSI which could be applied in both the supine and the prone position and to compare this technique with our electron-based technique.Materials and methodsBetween November 2007 and May 2008, 11 children with brain tumours were treated in the prone position with CSI. For 9 patients two treatment plans were created: the first one using photons and the second one using electron beams for spinal irradiation. We prepared seven 3D-conformal photon plans and four forward planned segmented field plans. We compared 20 treatment plans in terms of target dose homogeneity and sparing of organs at risk.ResultsIn segmented field plans better dose homogeneity in the thecal sac volume was achieved than in electron-based plans. Regarding doses in organs at risk, in photon-based plans we obtained a lower dose in the thyroid but a higher one in the heart and liver.ConclusionsOur technique can be applied in both the supine and prone position and it seems to be more feasible and precise than the electron technique. However, more homogeneous target coverage and higher precision of dose delivery for photons are obtained at the cost of slightly higher doses to the heart and liver
Molecular absorption lines toward star-forming regions : a comparative study of HCO+, HNC, HCN, and CN
Aims. The comparative study of several molecular species at the origin of the
gas phase chemistry in the diffuse interstellar medium (ISM) is a key input in
unraveling the coupled chemical and dynamical evolution of the ISM. Methods.
The lowest rotational lines of HCO+, HCN, HNC, and CN were observed at the
IRAM-30m telescope in absorption against the \lambda 3 mm and \lambda 1.3 mm
continuum emission of massive star-forming regions in the Galactic plane. The
absorption lines probe the gas over kiloparsecs along these lines of sight. The
excitation temperatures of HCO+ are inferred from the comparison of the
absorptions in the two lowest transitions. The spectra of all molecular species
on the same line of sight are decomposed into Gaussian velocity components.
Most appear in all the spectra of a given line of sight. For each component, we
derived the central opacity, the velocity dispersion, and computed the
molecular column density. We compared our results to the predictions of
UV-dominated chemical models of photodissociation regions (PDR models) and to
those of non-equilibrium models in which the chemistry is driven by the
dissipation of turbulent energy (TDR models). Results. The molecular column
densities of all the velocity components span up to two orders of magnitude.
Those of CN, HCN, and HNC are linearly correlated with each other with mean
ratios N(HCN)/N(HNC) = 4.8 1.3 and N(CN)/N(HNC) = 34 12, and more
loosely correlated with those of HCO+, N(HNC)/N(HCO+) = 0.5 0.3,
N(HCN)/N(HCO+) = 1.9 0.9, and N(CN)/N(HCO+) = 18 9. These ratios
are similar to those inferred from observations of high Galactic latitude lines
of sight, suggesting that the gas sampled by absorption lines in the Galactic
plane has the same chemical properties as that in the Solar neighbourhood. The
FWHM of the Gaussian velocity components span the range 0.3 to 3 km s-1 and
those of the HCO+ lines are found to be 30% broader than those of CN-bearing
molecules. The PDR models fail to reproduce simultaneously the observed
abundances of the CN-bearing species and HCO+, even for high-density material
(100 cm-3 < nH < 104 cm-3). The TDR models, in turn, are able to reproduce the
observed abundances and abundance ratios of all the analysed molecules for the
moderate gas densities (30 cm-3 < nH < 200 cm-3) and the turbulent energy
observed in the diffuse interstellar medium. Conclusions. Intermittent
turbulent dissipation appears to be a promising driver of the gas phase
chemistry of the diffuse and translucent gas throughout the Galaxy. The details
of the dissipation mechanisms still need to be investigated
Dissociative recombination and electron-impact de-excitation in CH photon emission under ITER divertor-relevant plasma conditions
For understanding carbon erosion and redeposition in nuclear fusion devices,
it is important to understand the transport and chemical break-up of
hydrocarbon molecules in edge plasmas, often diagnosed by emission of the CH
A^2\Delta - X^2\Pi Ger\"o band around 430 nm. The CH A-level can be excited
either by electron-impact or by dissociative recombination (D.R.) of
hydrocarbon ions. These processes were included in the 3D Monte Carlo impurity
transport code ERO. A series of methane injection experiments was performed in
the high-density, low-temperature linear plasma generator Pilot-PSI, and
simulated emission intensity profiles were benchmarked against these
experiments. It was confirmed that excitation by D.R. dominates at T_e < 1.5
eV. The results indicate that the fraction of D.R. events that lead to a CH
radical in the A-level and consequent photon emission is at least 10%.
Additionally, quenching of the excited CH radicals by electron impact
de-excitation was included in the modeling. This quenching is shown to be
significant: depending on the electron density, it reduces the effective CH
emission by a factor of 1.4 at n_e=1.3*10^20 m^-3, to 2.8 at n_e=9.3*10^20
m^-3. Its inclusion significantly improved agreement between experiment and
modeling
Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate
The H3+ molecular ion plays a fundamental role in interstellar chemistry, as
it initiates a network of chemical reactions that produce many interstellar
molecules. In dense clouds, the H3+ abundance is understood using a simple
chemical model, from which observations of H3+ yield valuable estimates of
cloud path length, density, and temperature. On the other hand, observations of
diffuse clouds have suggested that H3+ is considerably more abundant than
expected from the chemical models. However, diffuse cloud models have been
hampered by the uncertain values of three key parameters: the rate of H3+
destruction by electrons, the electron fraction, and the cosmic-ray ionisation
rate. Here we report a direct experimental measurement of the H3+ destruction
rate under nearly interstellar conditions. We also report the observation of
H3+ in a diffuse cloud (towards zeta Persei) where the electron fraction is
already known. Taken together, these results allow us to derive the value of
the third uncertain model parameter: we find that the cosmic-ray ionisation
rate in this sightline is forty times faster than previously assumed. If such a
high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy
between chemical models and the previous observations of H3+ can be resolved.Comment: 6 pages, Nature, in pres
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