[en] Highlights: • Percolation thresholds and critical exponents are determined for four Sierpinski carpets. • Critical exponents are dependent on three dimension numbers of the fractal. • Hyperscaling relations are governed by the Hausdorff dimension. • Percolation threshold is controlled by the topological Hausdorff dimension. • Sierpinski carpets and square lattice belong to the same universality class. - Abstract: We study how fractal features of an infinitely ramified network affect its percolation properties. The fractal attributes are characterized by the Hausdorff ($D_H$), topological Hausdorff ($D_{tH}$), and spectral ($d_s$) dimensions. Monte Carlo simulations of site percolation were performed on pre-fractal standard Sierpiński carpets with different fractal attributes. Our findings suggest that within the universality class of random percolation the values of critical percolation exponents are determined by the set of dimension numbers ($D_H$, $D_{tH}$, $d_s$), rather than solely by the spatial dimension (d). We also argue that the relevant dimension number for the percolation threshold is the topological Hausdorff dimension $D_{tH}$, whereas the hyperscaling relations between critical exponents are governed by the Hausdorff dimension $D_H$. The effect of the network connectivity on the site percolation threshold is revealed.

[en] When linear accelerators operate above 8 MV, neutrons are produced in (, n) and (e, e´n) reactions in nuclei via the Giant Dipole Resonance. In linear accelerators for medical applications, the neutron contamination is a radiation protection issue where a non-negligible, and non-desired, dose is delivered to the patient. Moreover, neutrons around linear accelerators for medical applications have been associated with the cancer recurrence and the induction of new tumours. In the past decades several efforts to determine the neutron characteristics around linacs have been carried out; novel procedures and innovative measuring devices have been developed to perform such characterizations. The Bonner sphere spectrometer has been largely used to measure the neutron spectra around linacs; however, the time required to perform the measurements in the treatment hall is large, requiring many linac runs due to the number of spheres in the spectrometer. Aiming to reduce the measuring time, a procedure using a Bonner sphere spectrometer has been developed using a single linac exposure. The procedure was used to measure the photoneutron spectrum in a 15 MV linear accelerator. (author)

[en] Quarry tuffs are volcanic rocks made of an ash matrix with grain sizes ranging from fine clay minerals up to silt-sized material containing lithic, vitric and crystal clasts. They are mostly soft and porous rocks used as building stones. The aim of this work was to estimate the shielding characteristics of quarry tuffs using Monte Carlo methods and the XCOM code for 0.03, 0.07, 0.1, 0.3, 0.662, 1, 2, and 3 MeV mono-energetic photons. The relative transmission of Kerma in air, Ka, and the ambient dose equivalent, H*(10), was also determined. The main results follows. The linear attenuation coefficient of quarry tuff varies from approximately 2.290 to 0.065/cm for 30 keV to 3 MeV photons respectively. For 30 keV photons the half value layer of cantera is 0.3 cm and 10.7 cm for 3 MeV -rays. Photons with energy less than 60 keV are effectively stopped because the main interaction with cantera is through photoelectric interaction. Thus, the cantera is a good shielding material for rooms with X-ray units working below 60 kV. This X-ray equipment includes radio diagnostic X-rays for mammography

[en] The neutron transport equation describes neutrons throughout phase-space; however, exact, analytical solutions can only be obtained for highly idealized problems. In its application to real world problems, approximations to the neutron transport equation are used: some require numerical methods, while others, like the diffusion equation, can be solved obtaining exact solutions used as a first approach to transport theory. In the neutron diffusion equation, boundary conditions are applied to the general solution to obtain a particular solution. One of these conditions is known as the source condition. In this work, a general and formal expression for the source condition is used, where neutron leakage from finite media and neutron absorption in infinite media are included. In this work, the diffusion equation in three coordinate systems for two energy groups was solved in order to determine the radial distribution of neutrons in water. This distribution was compared with the measured neutron flux along the radius of a water-filled cylindrical container with a ²⁴¹AmBe neutron source at its centre and with the results obtained using Monte Carlo methods. The solution obtained compares well with both. (author)

[en] We report on the λ = 5-36 μm Spitzer Infrared Spectrograph (IRS) spectra of 79 young stellar objects in the very young nearby cluster NGC 1333. NGC 1333's youth enables the study of early protoplanetary disk properties, such as the degree of settling and the formation of gaps and clearings. We construct spectral energy distributions (SEDs) using our IRS data as well as published photometry and classify our sample into SED classes. Using 'extinction-free' spectral indices, we determine whether the disk, envelope, or photosphere dominates the spectrum. We analyze the dereddened spectra of objects that show disk-dominated emission using spectral indices and properties of silicate features in order to study the vertical and radial structure of protoplanetary disks in NGC 1333. At least nine objects in our sample of NGC 1333 show signs of large (several AU) radial gaps or clearings in their inner disk. Disks with radial gaps in NGC 1333 show more nearly pristine silicate dust than their radially continuous counterparts. We compare properties of disks in NGC 1333 to those in three other well-studied regions, Taurus-Auriga, Ophiuchus, and Chamaeleon I, and find no difference in their degree of sedimentation and dust processing.

[en] We present 5-36 μm mid-infrared spectra of 82 young stars in the ∼2 Myr old Chamaeleon I star-forming region, obtained with the Spitzer Infrared Spectrograph (IRS). We have classified these objects into various evolutionary classes based on their spectral energy distributions and the spectral features seen in the IRS spectra. We have analyzed the mid-IR spectra of Class II objects in Chamaeleon I in detail, in order to study the vertical and radial structure of the protoplanetary disks surrounding these stars. We find evidence for substantial dust settling in most protoplanetary disks in Chamaeleon I. We have identified several disks with altered radial structures in Chamaeleon I, among them transitional disk candidates which have holes or gaps in their disks. Analysis of the silicate emission features in the IRS spectra of Class II objects in Cha I shows that the dust grains in these disks have undergone significant processing (grain growth and crystallization). However, disks with radial holes/gaps appear to have relatively unprocessed grains. We further find the crystalline dust content in the inner (∼<1-2 AU) and the intermediate (∼<10 AU) regions of the protoplanetary disks to be tightly correlated. We also investigate the effects of accretion and stellar multiplicity on the disk structure and dust properties. Finally, we compare the observed properties of protoplanetary disks in Cha I with those in slightly younger Taurus and Ophiuchus regions and discuss the effects of disk evolution in the first 1-2 Myr.