Measurement of photoneutron doses in and out of high-energy X-ray beam of a SATURNE-20 medical linear accelerator by ECE polycarbonate detectors

Photoneutron contaminations in and out of high energy X-ray beams of the medical linear accelerator SATURNE 20 (CGR) of the Radiotherapy Department of Omeed Hospital in Isfahan, Iran, have been determined using 250 μm polycarbonate (PC) dosimeters, in strips or in sheets, processed by electrochemical etching (ECE) using specially designed ECE chambers to etch larger sheets. A two dimensional or topographical distribution of neutron contamination was also determined in a full size beam. The neutron dose equivalents (Hn) in the beam of 18 MV X-rays at 80 cm FSD were determined to be linear functions of X-ray dose equivalents (Hx) up to 1400 cSv. The distribution of the Hn at different X-ray doses showed bell-shape profiles with maxima at the isocenter. The ratios of dose equivalents of neutrons to those of X-rays increased as the field size increased having values of 0.22%, 0.28%, 0.31% and 0.37% for field sizes of 10×10, 20×20, 30×30, and 40×40 cm² respectively. Although such neutron dose equivalents can be corrected for patient treatment, it can cause radiation protection problems for workers where the design of the facility is not well planned.     

Development of a new approach to simulate a particle track under electrochemical etching in polymeric detectors

A numerical approach based on image processing was developed to simulate a particle track in a typical polymeric detector, e.g., polycarbonate, under electrochemical etching. The physical parameters such as applied voltage, detector thickness, track length, the radii of curvature at the tip of track, and the incidence angle of the particle were considered, and then the boundary condition of the problem was defined. A numerical method was developed to solve Laplace equation, and then the distribution of the applied voltage was obtained through the polymer volume. Subsequently, the electric field strengths in the detector elements were computed. In each step of the computation, an image processing technique was applied to convert the computed values to grayscale images. The results showed that a numerical solution to Laplace equation is dedicatedly an attractive approach to provide us the accurate values of electric field strength through the polymeric detector volume as well as the track area. According to the results, for a particular condition of the detector thickness equal to 445 μm, track length of 21 μm, the radii of 2.5 μm at track tip, the incidence angle of 90°, and the applied voltage of 2080 V, after computing Laplace equation for an extremely high population of 4000 × 4000 elements of detector, the average field strength at the tip of track was computed equal to 0.31 MV cm⁻¹ which is in the range of dielectric strength for polymers. The results by our computation confirm Smythe’s model for estimating the ECE-tracks.     

Strömung und Wärmeübergang bei der Oberflächenverdampfung und Filmkondensation

Zusammenfassung Mit Hilfe der Mischungswegtheorie wurden Gleichungen zur Berechnung der Geschwindigkeitsprofile und des Druckabfalles bei der turbulenten, abwärtsterichteten Gas/Film-Strömung aufgestellt. Zur Berechnung des Wärmeübergangs wurde die turbulente Temperaturleitfähigkeit aus einem halbempirischen Ansatz bestimmt. Es konnte eine befriedigende Übereinstimmung zwischen den berechneten und gemessenen Nußelt-Zahlen bei der Oberflächenverdampfung erzielt werden. Zur Auslegung von Fallstromverdampfern wurde ein Computerprogramm erstellt. Damit lassen sich Einflußgrößen wie Wandtemperatur, Filmdicke, Verdampfungsrate usw. in Abhängigkeit von der Lauflänge bestimmen.

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Flow and heat transfer in surface evaporation and film condensation

Using the mixing length model, equations were established to calculate the velocity profiles and pressure drop in turbulent downward directed gas/film flow. The thermal diffusivity needed for the calculation of heat transfer was determined from a semiempirical model. The calculated Nußelt-numbers agreed very well with experiments. For the design of falling-film evaporators, a computer program was developed, which enables to evaluate wall temperature, film thickness, evaporation rate etc. as a function of flow-path length.

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Formelzeichen a Temperaturleitfähigkeit - c spez. Wärmekapazität - d Durchmesser - f_m bezogene mittlere turbulente Temperaturleitfähigkeit - Fi /(3²/g)^1/3) Filmkennzahl - Fr Froude-Zahl - g Fallbeschleunigung - Ka ³/g⁴ Kapitza-Zahl - L Rohrlänge - l Mischungsweg - m Massenstrom - Nu (²/g)^1/3/ Nußelt-Zahl - Nu / Nußelt-Zahl des Filmes - p Druck - Pr /a Prandtl-Zahl - q Wärmestromdichte - R Radius - Re Reynolds-Zahl - Re_ü Übergangs-Reynolds-Zahl - Re_w Schubspannungs-Reynolds-Zahl der Flüssigkeit - r radiale Koordinate - T Temperatur - u Geschwindigkeit - u_w Schubspannungsgeschwindigkeit der Flüssigkeit - u Grenzflächengeschwindigkeit - u_T Schubspannungsgeschwindigkeit des Gases - y Wandabstand - y^* y/ dimensionsloser Wandabstand - z axiale Koordinate Griechische Zeichen Wärmeübergangskoeffizient - Filmdicke - dyn. Viskosität - dimensionslose Temperatur - Wärmeleitfähigkeit - kin. Viskosität - Dichte - Oberflächenspannung - Schubspannung Zusatzzeichen und Indizes G Gas - K Kondensation - s Sättigung - t turbulent - w Wand - wi Welleninstabilität - Phasengrenze - - mittlere Größe     

A New Numerical Approach to Evaluate Variation of Electric Field Strength at the End of Particle Trajectory in Nuclear Track Detectors

A geometrical model for an electrochemical etching （ECE） track in a dielectric detector is defined and a primaxy programme is written to generate the track. The generated track is transformed to an M× N matrix of primary voltages. Using a numerical method, the matrix of final voltages is computed, and using another numerical approach, the electric field strengths in the elements of detector volume are computed. The final field strength at the end of particle trajectory is obtained. The results of our numerical computation show that there are exact correlations between the field strength at the end of particle trajectory and the parameters of track under ECE. It is found that although two traditional models of Mason and Smythe in dielectrics can be partly applied for short and long tracks, none of them are able to explain the behaviour of field strength in a ‘general case＇. Furthermore, we find that there is an expressive relationship between the field strength and the incidence angle of impacted particle, while the mentioned traditional models are not able to explain this effect.     

Experimental and numerical study of wing boundary layer behavior in propeller flowfield

Journal of Visualization - In this research, the effects of propeller slipstream on wing boundary layer and transition front were studied through wind tunnel tests and numerical analysis. In this...     

Verification of Mason and Smythe equations for electrical treeing breakdown at the tip of charged particle tracks in ECE polycarbonate detectors

There are two different equations to describe enhancement of field strength at the track tip in ECE polycarbonate detectors: Mason and Smythe equations. In order to evaluate the equations, PC detectors with different thicknesses were exposed to 1 MeV alpha and/or fast neutron. The response of the detectors was studied as a function of applied voltage and average field strength. The ratio of critical voltages for different detectors was estimated experimentally and compared with calculated values. The experimental results are in good agreement with Smythe's equation. The results show that the track density and mean track diameter in the different detectors have the same behaviour, when the macroscopic field strength increases, and also that the field strength at the tip of tracks is proportional to macroscopic field strength.     

Sorption of strontium ions from aqueous solutions by oxidized multiwall carbon nanotubes

Multiwall carbon nanotubes (MWCNTs) oxidized by nitric acid solution were used to investigate the adsorption behavior of strontium from aqueous solutions similar to the nuclear waste media. The physical properties of both as produced and oxidized MWCNTs were studied by Boehm’s titration method and nitrogen adsorption/desorption. The results showed that the surface properties of MWCNTs such as specific surface area, functional groups and the total number of acid sites were improved after oxidation. Furthermore, the effect of solution conditions such as initial concentration of strontium(II), pH, ionic strength, MWCNT concentration and contact time were studied at room temperature. The results of this study showed that the adsorption of strontium(II) was significantly influenced by the pH value and the solution ionic strength. According to the Langmuir model, the maximum adsorption capacities of strontium(II) onto the as produced and oxidized MWCNTs were obtained as 1.62 and 6.62 mg g^?1, respectively. The contact time to reach equilibrium was 100 min. The good adsorption of strontium(II) on oxidized MWCNTs at the lower ionic strength, the relatively high pH and the short equilibrium time indicate that the oxidized MWCNTs have great potential applications in the field of the environmental protection.