Considerations on a revision of the quality factor

A modified analytical expression is proposed for the revised quality factor that has been suggested by a liaison group of ICRP and ICRU. With this modification one obtains, for sparsely ionizing radiation, a quality factor which is proportional to the dose average of lineal energy, y. It is shown that the proposed relation between the quality factor and lineal energy can be translated into a largely equivalent dependence on LET. The choice between the reference parameters LET or y is therefore a secondary problem in an impending revision of the quality factor

Renormalization of the Optical Response of Semiconductors by Electron-Phonon Interaction

In the past five years enormous progress has been made in the ab initio calculations of the optical response of electrons in semiconductors. The calculations include the Coulomb interaction between the excited electron and the hole left behind, as well as local field effects. However, they are performed under the assumption that the atoms occupy fixed equilibrium positions and do not include effects of the interaction of the lattice vibrations with the electronic states (electron-phonon interaction). This interaction shifts and broadens the energies at which structure in the optical spectra is observed, the corresponding shifts being of the order of the accuracy claimed for the ab initio calculations. These shifts and broadenings can be calculated with various degrees of reliability using a number of semiempirical and ab initio techniques, but no full calculations of the optical spectra including electron-phonon interaction are available to date. This article discusses experimental and theoretical aspects of the renormalization of optical response functions by electron-phonon interaction, including both temperature and isotopic mass effects. Some of the theoretical techniques used can also be applied to analyze the renormalization of other response functions, such as the phonon spectral functions, the lattice parameters, and the elastic constants.Comment: Latex 2.09, 28 pages, 13 Figs., 2 Tables, submitted to Phys. Stat. Sol.

The PadR-like transcriptional regulator LftR ensures efficient invasion of Listeria monocytogenes into human host cells

Invasion of the bacterial pathogen Listeria monocytogenes into human host cells requires specialized surface molecules for attachment and induction of phagocytosis. However, efficient invasion is also dependent on factors with house-keeping functions, such as SecA2-dependent secretion of autolysins for post-divisional segregation of daughter cells. Mutations in this pathway prevent degradation of peptidoglycan cross-walls, so that long cell chains are formed that cannot be phagocytosed. The extreme chaining of such mutants manifests as rough colony phenotype. One rough clone was isolated from a transposon library with a transposon insertion in the uncharacterized lmo0720 gene (lftS) together with a spontaneous point mutation in the secA2 gene. We separated both mutations and demonstrated that this point mutation in the intramolecular regulator 2 domain of SecA2 was sufficient to inactivate the protein. In contrast, lftS deletion did not cause a ΔsecA2-like phenotype. lftS is located in an operon with lftR (lmo0719), encoding a PadR-like transcriptional regulator, and lftR deletion affected growth, invasion and day-light dependent coordination of swarming. Inactivation of lftS partially suppressed these phenotypes, suggesting a functional relationship between LftR and LftS. However, the invasion defect of the ΔlftR mutant was only marginally suppressed by lftS removal. LftR regulates expression of the lmo0979–0980 (lieAB) operon, encoding a putative multidrug resistance transporter and lieAB transcription was strongly upregulated in the absence of LftR. Deletion of lieAB in the ΔlftR background restores wild type-like invasion levels. Hence, we conclude that tight transcriptional repression of the lieAB operon is essential for efficient listerial host cell invasion

The Quality Factor for Neutrons in Radiation Protection: Physical Parameters

Recommendations by NCRP and ICRP for a selective increase of the quality factor for neutrons have not been implemented in the practice of radiation protection, but have added urgency to the search for a more consistent new convention on quality factors. In response to this need, a liaison group of ICRP and ICRU has proposed numerical changes, but also a replacement of the reference parameter LET by the microdosimetric variable lineal energy. This would make quality factors and dose equivalents measurable, but would make computations more complicated. To resolve the dilemma, it is proposed to use an equivalence relation between lineal energy and LET which leads to nearly identical definitions whenever one deals with charged particles of sufficiently long range, For photons below 200 keV and for neutrons below 0.5 MeV, the equivalence is imperfect, but it may, even in these cases, be acceptable for the practical purposes of radiation protection

On the Use of LET for a Revised Quality Factor for Photons

Linear energy transfer, L, is currently used as reference parameter for the quality factor in radiation protection, and this practice is likely to be continued in an impending revision of the quality factor. But the numerical convention is not, at present, actually applied to photon or electron fields; their quality factor is, instead, summarily equated to unity. The current trend of tightened dose limits in radiation protection may create the need for precise computations of quality factors even for photon and electron radiations. Such computations can, however, not be performed in terms of unrestricted linear energy transfer. The reasons for this difficulty are explained, and the formulae for the quality factor as an integral over electron fluence and restricted linear energy transfer are given. A correct energy balance in this and in a variety of linked dosimetric relations requires a modification of the definition of restricted linear energy transfer. For large energy cut-off values the correction is of minor importance, but for small cut-off values - such as those invoked in biophysical considerations - the modification is essential

Microdosimetry of therapy electron beams - Measurements and Monte-Carlo simulations

Microdosimetry can be an important tool in fundamental radiobiology towards an improved understanding of the primary mechanisms of radiation action. In addition, its applications in radiation protection and in clinical radiology have been of increasing importance. The variance-covariance method relates to such applications; it permits the determination of the dose averaged microdosimetric parameters in radiation fields of varying intensity. Measurements with the variance-covariance method are here reported for therapy electron beams with acceleration voltages from 5 to 20 MV. The experimental results are compared with Monte-Carlo simulations