Two-Species Reaction-Diffusion System with Equal Diffusion Constants: Anomalous Density Decay at Large Times

We study a two-species reaction-diffusion model where A+A->0, A+B->0 and B+B->0, with annihilation rates lambda0, delta0 > lambda0 and lambda0, respectively. The initial particle configuration is taken to be randomly mixed with mean densities nA(0) > nB(0), and with the two species A and B diffusing with the same diffusion constant. A field-theoretic renormalization group analysis suggests that, contrary to expectation, the large-time density of the minority species decays at the same rate as the majority when d<=2. Monte Carlo data supports the field theory prediction in d=1, while in d=2 the logarithmically slow convergence to the large-time asymptotics makes a numerical test difficult.Comment: revised version (more figures, claim on exactnes of d=2 treatment removed), 5 pages, 3 figures, RevTex, see related paper Phys. Rev. E, R3787, (1999) or cond-mat/9901147, to appear in Phys. Rev.

POUŽITÍ RADIOCHROMNÍHO GELOVÉHO DOZIMETRU PRO VERIFIKACI DÁVKOVÉ DISTRIBUCE VE STEREOTAKTICKÉ RADIOCHIRURGII

DOSE DISTRIBUTION VERIFICATION IN STEREOTACTC RADIOSURGERY USING RADIOCHROMIC GEL DOSIMETER. 3D verification in stereotactic radiosurgery is not a trivial task since there is not available appropriate detector and the process of verification is time consuming as well. The aim of the study was to perform assessment of radiochromic gel dosimeter based on Turnbull blue dye for verification of patient treatment plan at Leksell Gama Knife Perfexion. Gel dosimeter was fixed with stereotactic frame and irradiated with a prescription dose of 40 Gy to 50% isodose. Dosimeter was evaluated two hours after irradiation using home made optical CT scanner. The comparison between the gel measurements and the treatment planning system calculation are presented in the form of 2D isodoses, 1D profile and gamma analysis (1 mm 3%) for the central slices. Measured relative dose distribution in a central slice and measured profiles show excellent correspondence with TPS. There were observed small discrepancies in low dose region according to gamma analyses results. This was probably caused by problems with dose distribution coregistration and set up positioning errors

Irradiation of luminescence dosimeters in pulsed mixed radiation fields

UHDpulse - Metrology for Advanced Radiotherapy using beams with Ultra-High Pulse Dose Rates is a European project aimed at developing novel dosimetry standards, as well as improving existing ones, for FLASH radiotherapy, very high energy electrons radiotherapy, and laser-driven medical accelerators. Within the scope of this project, Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) detectors are used to measure stray radiation fields. Experiments performed with conventional pulsed particle-beams allow to characterize the dosimeters in known and controllable radiation fields. In turn, this allows to develop models and predict their behavior in complex radiation fields, such as those at laser-driven and FLASH facilities. TL and OSL detectors were irradiated at the Microtron MT25 electron accelerator in Prague, Czech Republic. GAFChromicTM films and plastic nuclear track detectors were used to study the beam profile and the neutron background respectively. The responses of the different detector to the pulsed mixed radiation fields of the Microtron MT25 are compared among each other and presented in this paper

Isotropic-nematic phase equilibria in the Onsager theory of hard rods with length polydispersity

We analyse the effect of a continuous spread of particle lengths on the phase behavior of rodlike particles, using the Onsager theory of hard rods. Our aim is to establish whether ``unusual'' effects such as isotropic-nematic-nematic (I-N-N) phase separation can occur even for length distributions with a single peak. We focus on the onset of I-N coexistence. For a log-normal distribution we find that a finite upper cutoff on rod lengths is required to make this problem well-posed. The cloud curve, which tracks the density at the onset of I-N coexistence as a function of the width of the length distribution, exhibits a kink; this demonstrates that the phase diagram must contain a three-phase I-N-N region. Theoretical analysis shows that in the limit of large cutoff the cloud point density actually converges to zero, so that phase separation results at any nonzero density; this conclusion applies to all length distributions with fatter-than-exponentail tails. Finally we consider the case of a Schulz distribution, with its exponential tail. Surprisingly, even here the long rods (and hence the cutoff) can dominate the phase behaviour, and a kink in the cloud curve and I-N-N coexistence again result. Theory establishes that there is a nonzero threshold for the width of the length distribution above which these long rod effects occur, and shows that the cloud and shadow curves approach nonzero limits for large cutoff, both in good agreement with the numerical results.Comment: 20 pages, 13 figure