Dosimetry of low-energy neutrons using low-pressure proportional counters

Measurements in nearly monoenergetic beams of 144, 24.5, and 2 keV neutrons and of thermal neutrons have been performed with low-pressure proportional counters. The suitability of a tissue-equivalent proportional counter (TEPC) for dosimetry of low-energy neutrons has been investigated. In contrast to higher neutron energies, the modification of the primary radiation field by the detector wall and the contribution of secondaries produced in the gas are significant. These effects have been investigated by additional measurements with a carbon-walled proportional counter. The various physical processes of neutron interaction with wall and gas of the TEPC have been analyzed, and absorbed dose, kerma, and kerma contributions from the various processes are presented. In addition, dose contributions from contaminating neutrons and photons have been obtained for the calibration fields used. The results have been related to neutron fluence. The comparison with tabulated kerma factors shows excellent agreement, indicating the suitability of the TEPC method for dosimetry of low-energy neutrons.     

The response of tissue-equivalent proportional counters to heavy ions

The paper presents a theoretical model for the response of a tissue-equivalent proportional counter (TEPC) irradiated with charged particles. Heavy ions and iron ions in particular constitute a significant part of radiation in space. TEPCs are used for all space shuttle and International Space Station (ISS) missions to estimate the dose and radiation quality (in terms of lineal energy) inside spacecraft. The response of the tissue-equivalent proportional counters shows distortions at the wall/cavity interface. In this paper, we present microdosimetric investigation using Monte Carlo track structure calculations to simulate the response of a TEPC to charged particles of various LET (1 MeV protons, 2.4 MeV alpha particles, 46 MeV/nucleon 20Ne, 55 MeV/nucleon 20Ne, 45 MeV/nucleon 40Ar, and 1.05 GeV/nucleon 56Fe). Data are presented for energy lost and energy absorbed in the counter cavity and wall. The model calculations are in good agreement with the results of Rademacher et al. (Radiat. Res. 149, 387-389, 1998), including the study of the interface between the wall and the sensitive region of the counter. It is shown that the anomalous response observed at large event sizes in the experiment is due to an enhanced entry of secondary electrons from the wall into the gas cavity.     

Effect of the plasma production rate on the implosion dynamics of cylindrical wire/fiber arrays with a profiled linear mass

Results are presented from experimental studies on the implosion of arrays made of wires and metalized fibers under the action of current pulses with an amplitude of up to 3.5 MA at the Angara-5-1 facility. The effect of the parameters of an additional linear mass of bismuth and gold deposited on the wires/fibers is investigated. It is examined how the material of the wires/fibers and the metal coating deposited on them affect the penetration of the plasma with the frozen-in magnetic field into a cylindrical array. Information on the plasma production rate $\dot m_{Bi}$ for different metals is obtained by analyzing optical streak images of imploding arrays. The plasma production rate for cylindrical arrays made of the kapron fibers coated with bismuth is determined. For the initial array radius of R ₀ = 1 cm and discharge current of I = 1 MA, the plasma production rate is found to be $\dot m_{Bi} \approx 0.095 \pm 0.015$ μg/(cm² ns).     

A comparison of instrument performance for six liquid scintillation counters

The counting characteristics of six liquid scintillation counters are compared. Each instrument was tested to determine the figure of merit ($\text{E}{}^2\text{B}$), the change in percentage counting efficiency and external standard ratio over sample volumes from 1 to 20 ml, the reproducibility of chemical quench curves over sample volumes from 5 to 20 ml, and the effects of increasing sample activity on the actual and calculated counting efficiency. Tests were performed using both ³H and ¹⁴C. The results show that differences in counting efficiency are more important than maximum sensitivity ($\text{E}{}^2\text{B}$). All instruments demonstrated the same response to changes in sample volume. The external standard ratio varied with sample volume in all instruments except one. Quench curves were essentially volume independent, with two exceptions. This study demonstrates that periodic performance tests must be conducted to assure that these instruments are operating in an efficient and reproducible manner.     

Comparisons of LET distributions measured in low-earth orbit using tissue-equivalent proportional counters and the position-sensitive silicon-detector telescope (RRMD-III)

Determinations of the LET distribution, phi(L), of charged particles within a spacecraft in low-Earth orbit have been made. One method used a cylindrical tissue-equivalent proportional counter (TEPC), with the assumption that for each measured event, lineal energy, y, is equal to LET and thus phi(L) = phi(y). The other was based on the direct measurement of LETs for individual particles using a charged-particle telescope consisting of position-sensitive silicon detectors called RRMD-III. There were differences of up to a factor of 10 between estimates of phi(L) using the two methods on the same mission. This caused estimates of quality factor to vary by a factor of two between the two methods.     

Anisotropy of energy losses in high-current Z-pinches produced by the implosion of cylindrical tungsten wire arrays

Results are presented from measurements of the anisotropy of energy losses in high-current Z-pinches produced by the implosion of wire arrays at the ANGARA-5-1 facility at load currents of up to 4MA. The energy losses were measured in the radial direction and along the pinch axis from the anode side. The main diagnostics were time-integrated thermocouple calorimeters, nanosecond X-ray diodes (XRDs) with different filters, and a foil radiation calorimeter with a time resolution of 2 μs. The azimuthal anisotropy of energy losses was measured for different wire array configurations and different shapes of the high-voltage electrode. The presence of strong initial azimuthal inhomogeneity of the wire mass distribution (sectioned arrays), as well as the use of conical electrodes instead of plane ones, does not increase the azimuthal inhomogeneity of the total energy losses. For cylindrical wire arrays, energy losses in the radial direction are compared with those along the pinch axis. According to XRD and calorimetric measurements, the radiation yield per unit solid angle along the pinch axis is two to three times lower than that in the radial direction. In the axial direction, the energy flux density of the expanding plasma is two to three times lower than the radiation intensity. The measured radiation yield across the pinch is 2.5–5 kJ/sr, while that along the pinch axis is 1–2 kJ/sr. The results obtained by means of XRDs agree to within measurement errors with those obtained using the radiation calorimeter. It is found that the energy per unit solid angle carried by the expanding plasma in the radial direction does not exceed 10% of the soft X-ray yield. Analysis of the structure of time-integrated pinhole images and signals from the radial and axial XRDs shows that radiation emitted in the radial direction from the hot central region of the pinch is partially screened by the less dense surrounding plasma halo, whereas radiation emitted in the axial direction is a superposition of the emissions from the hot dense central region of the pinch and the colder less dense peripheral plasma.     

The effect of viewing eccentricity on enumeration

Visual acuity and contrast sensitivity progressively diminish with increasing viewing eccentricity. Here we evaluated how visual enumeration is affected by visual eccentricity, and whether subitizing capacity, the accurate enumeration of a small number (～3) of items, decreases with more eccentric viewing. Participants enumerated gratings whose (1) stimulus size was constant across eccentricity, and (2) whose stimulus size scaled by a cortical magnification factor across eccentricity. While we found that enumeration accuracy and precision decreased with increasing eccentricity, cortical magnification scaling of size neutralized the deleterious effects of increasing eccentricity. We found that size scaling did not affect subitizing capacities, which were nearly constant across all eccentricities. We also found that size scaling modulated the variation coefficients, a normalized metric of enumeration precision, defined as the standard deviation divided by the mean response. Our results show that the inaccuracy and imprecision associated with increasing viewing eccentricity is due to limitations in spatial resolution. Moreover, our results also support the notion that the precise number system is restricted to small numerosities (represented by the subitizing limit), while the approximate number system extends across both small and large numerosities (indexed by variation coefficients) at large eccentricities.     

Experimental and numerical studies of plasma production in the initial stage of implosion of a cylindrical wire array

The features are studied of plasma production in the initial stage of implosion of hollow cylindrical wire arrays at electric-field growth rates of 10¹² V/(cm s). The results are presented from the analysis of both UV emission from the wire plasma and the discharge parameters in the initial stage of the formation of a Z-pinch discharge. It is found that, a few nanoseconds after applying voltage to a tungsten wire array, a plasma shell arises on the wire surface and the array becomes a heterogeneous system consisting of metal wire cores and a plasma surrounding each wire (a plasma corona). As a result, the current switches from the wires to the plasma. A further heating and ionization of the wire material are due primarily to heat transfer from the plasma corona. A model describing the primary breakdown along the wires is created with allowance for the presence of low-Z impurities on the wire surface.     

Effect of SAR on human head modeling inside cylindrical enclosures

This study intends to discuss enclosed a realistic approach to determine and analyze the effects of radio frequency on human exposure inside a cylindrical enclosure. A scenario in which a mobile phone with inverted-F antenna (IFA) operating in the Global System for Mobile Communication (GSM) band (900 MHz) is used inside a cylindrical enclosure. Metallic enclosures are known to have resonance and reflection effects, thereby increasing electric field strength and hence resulting in a change of the human exposure to electromagnetic absorptions. So, this study examines and compares the levels of absorption in terms of specific absorption rate (SAR) values under various conditions. In this study, a human phantom with dielectric properties is designed and its interaction is studied with IFA inside fully enclosed cylindrical enclosures. The results show that SAR values are increased inside cylindrical enclosures compared with those in free space. The method of computation uses method of moments. Simulations are done in FEKO software.     

Relation between the electric parameters of a Z-pinch discharge and plasma production in the load during the implosion of a cylindrical wire array

A study of the process of implosion of a cylindrical tungsten wire array by electrical and optical methods shows that it involves two phases. In the first phase, the plasma is produced from the dense wire cores under the action of the heat flux from the current-carrying plasma. This plasma then fills the internal space of the liner array. The measured inductance of the liner and its visible diameter vary only slightly in this phase. During the second phase, the total material of the liner is compressed toward the axis and the inductance of the discharge gap increases. The process of the implosion of wire arrays is studied by analyzing the electric parameters (current and voltage) of the load in the Angara-5-1 facility. The time behavior of the load inductance, the average current radius, and the start time of the liner compression are determined. The compression start time determined from the visible size of the liner is found to coincide with that determined from electric measurements. The compression ratio of the liner in terms of the average current radius turns out to be lower than that measured by optical and X-ray diagnostics. The reason is that, by the instant of maximum compression, only a portion of the current flows at the periphery of the initial wire array.     

An abbreviated method for the quality control of pollen counters

We present an abbreviated method for conducting large scale quality control (QC) exercises over limited time periods, which was used for examining the proficiency of technicians involved in the electronic Pollen Information Network (ePIN). The goal was for technicians to have their analysis skills evaluated at least twice: (1) by having at least one of their slides successfully checked by other counters in the ePIN network and (2) by successfully examining at least one additional slide from other sites. Success was judged as a relative difference (RDif %) ≤ 30% between the two daily average pollen concentrations. A total of 21 sites participated in the ePIN QC exercise. All of the results for total pollen had RDif % < 30%. Only five results had RDif > 30%, three for Betula and two for Poaceae pollen. Of these, three were slides containing < 40 pollen/m³ daily average and two were for sites that had microscopes with small fields of view and examined < 10% of the slide surface. More than 80% of the participants had at least two slides successfully checked by someone else in the network, and all of the participants had one slide successfully examined. The latter is comparable to a traditional ring test where only one slide is sent to participating sites. The method described here enabled a large number of technicians to be examined in a short period of time and represents a viable alternative to other approaches that can take many months to complete.     

Effect of the feed frequency on the performance of anaerobic filters

Anaerobic filter technology is suitable for the treatment of biodegradable organic wastes: for example, wastewaters from food industries. These wastewaters are not produced in a continuous mode since they are of seasonal nature and hence their production varies considerably during the year. In this work, a study of the performance of anaerobic filters with changing feeding systems was undertaken. A comparison was made between continuous feeding and different semicontinuous modes of feeding (in which the overall volume to be added into the reactors was divided into several doses and each of them were added at a constant interval of time). Filter performance was characterized by determining the different operational variables: depurative efficiency, methane production, biogas composition and volatile acids. From the obtained results, we conclude that the optimum feed frequency range is 24 doses/day or more. The continuously fed system has both greater stability and degradation efficiency.     

Effect of drop interactions on the performance of hydrocarbon fermentors

A drop-interaction model devised by the authors, which successfully predicts experimentally observed drop-size distributions in stirred batch vessels, has been used to assess the importance of droplet coalescence and redispersion in hydrocarbon fermentation yields. Calculations, which have been made with both n-alkane and gas–oil fermentations, have been compared with previous calculations based on simpler and, hence, less realistic droplet interaction mechanisms. The comparison shows that the differences in yield are not spectacularly different.     

Effect of sludge flocs on the aeration performance of suction-impellers

The authors have investigated a special aspect of the transfer of oxygen: the aeration performance of small-scale suction-impellers in a suspension of activated sludge floes and a performance comparison with clear water. The principle of the impellers is, that in order to disperse air bubbles into the aforementioned liquids, all that we need do is a simple stirring. Due to the impellers the authors could not show a reduction of the oxygen transfer coefficient in the presence of sludge flocs. Former comparative studies of oxygen transfer, using a fixed orifice, did show a reduction in the presence of sludge flocs. Therefore, it seems probable that the effect of activated sludge flocs on the transfer coefficient is linked with the manner in which air is dispersed, i.e., depends on whether an orifice is fixed or moved. The experimental results allow presumption that by developing a high intensity of shear near the location where air bubbles are produced, we are able to minimize the effect of sludge flocs on the process of bubble formation.     

Effect of group organization on the performance of cooperative processes

Problem-solving competence at group level is influenced by the structure of the social networks and so it may shed light on the organization patterns of gregarious animals. Here we use an agent-based model to investigate whether the ubiquity of hierarchical networks in nature could be explained as the result of a selection pressure favoring problem-solving efficiency. The task of the agents is to find the global maxima of NK fitness landscapes and the agents cooperate by broadcasting messages informing on their fitness to the group. This information is then used to imitate, with a certain probability, the fittest agent in their influence networks. The performance of the group is measured by the time required to find the global maximum. For rugged landscapes, we find that the modular organization of the hierarchical network with its high degree of clustering eases the escape from the local maxima, resulting in a superior performance as compared with the scale-free and the random networks. The optimal performance in a rugged landscape is achieved by letting the main hub to be only slightly more propense to imitate the other agents than vice versa. The performance is greatly harmed when the main hub carries out the search independently of the rest of the group as well as when it compulsively imitates the other agents.