According to International Atomic Energy Agency (IAEA) recommendations, the calibration of ionization chambers used for radiotherapy dosimetry follows the substitution method which demands the use of a reference ionization chamber. This work introduces the Design and fabrication of Cylindrical ionization chamber for dosimetry to be used by standard dosimetry laboratories. The result of the quality control tests in Secondary Standard Dosimetry Laboratory (SSDL) of the Atomic Energy Organization Which was carried out in accordance with the standards and limits set by the International Atomic Energy Agency, are evidence of the accuracy of the claim that this chamber can be used as a reference dosimeter in standard dosimetric laboratories. The low polarity effect and leakage current, high ion collection efficiency and stability, linear response of the detector to the dose and the rate of dosing are some of this instrument characteristic through optimum design of guard ring and collecting electrode and also the proper choice of materials in fabrication.

Background and Objective: Cylindrical ionization chamber is used for measurements of patient dose in radiation therapy. By determination the effective point of measurement we can determine the dose distribution and the parameters of treatment. Therefore in this research we introduce an analytical approach for determination this point in CC13 Cylindrical ionization chamber that used for dosimetry of linear accelerator.Subjects and Methods: In this study a CC13 ionization chamber was used for dose measurement of 6 and 18 photon beams of Variant accelerator in different field sizes, 5*5 cm2 up to 35*35cm2. Measurements were performed in blue phantom, up to 5cm depth and data fit software was used for evaluation of experimental data.Results: with our formalism the Pdis quantity in 6MV for 5x5 thought 35x35cm2 were measured 3.088 to 2.88 mm and in 18 MV are measured 4.62 to 4.20 mm.Conclusion: The first critical point of each curve is caused by changing environment from air to water during measuring of ionization. In fact it can be considered critical point as effective point of measurement of ionization chamber. Investigation shows that any increasing in field size, decrease and increasing of energy increase the depth of this critical point.

Introduction: Recently, modern radiotherapy techniques have been extensively developed to deliver doses only to therapeutic volume. The objective of the present study was to empirically evaluate the performance capability of the ionization chamber in comparison to the edge detector. Material and Methods: Firstly, the performance of the ionization chamber, compared to that of the edge diode detector, was validated at a 10×10 cm2 field size. Then, the percentage depth dose (PDD), percentage surface dose, and transverse profile doses at the 2×2, 3×3, and 4×4 cm2 field sizes were evaluated for both dosimeters. The empirical and statistical results in a water phantom were compared to those reported for the edge diode detector as the reference field dosimeter using the 6 MV Elekta linear accelerator. Results: The empirical and statistical results of the transverse profiles of the ionization chamber and edge detector are in agreement for the reference field of 10×10 cm2. However, a small difference between the two dosimeters could be observed in small fields. A discrepancy of less than 1% was observed between the results of PDDs for two dosimeters in small fields. Conclusion: The dosimetric characteristics of the ionization chamber and edge detector illustrated some differences, especially in terms of transverse profiles at the small-field size. This discrepancy could be related to the volume effect of the chamber which affects the penumbra. Due to the importance of sensitive organs, it is recommended to utilize the ionization chamber for small radiotherapy fields.

IntroductionFor dose measurement in Megavoltage (MV) photon beams with ion chambers, the effect of volume occupied by the air cavity is not negligible. Therefore, the result of measurement should be corrected with a displacement perturbation correction factor (Pdis) or using an effective point of measurement (EPOM). The aim of this study is to calculate the EPOM for Cylindrical ion chamber and to evaluate the fixed EPOM that was recommended by standard dosimetry protocols.Materials and MethodsPercent depth doses (PDDs) for 6 MV and 18 MV were measured with two types of chambers for different depths and field sizes. The EPOM was calculated using results obtained from measurement data for two types of chambers, comparison of the readings, and using dosimetry, mathematical, and statistical consideration. For displacement correction factor Dr=0, Dr=0.6r and different Dr, the minimum standard deviations ratio (SDRs) were calculated at several depths and field sizes.ResultsMaximum level of SDRs was about 0.38% and 0.49% (when assuming variable Dr) for 6 MV and 18 MV, respectively (which was less than 0.5% and acceptable). This quantity was greater than one (for assuming = 0.6r) and greater than 2 when there was no shift (Dr =0)ConclusionThe results show that the recommended shift for Cylindrical ion chamber in dosimetry protocols (upstream of 0.6r) is not correct and using a fixed value for the EPOM at all photon beam energies, depths, and field sizes is not suitable for accurate dosimetry.

In this paper, the effect of cold air on the fluid flow inside the Cylindrical combustion chamber and its wall temperature distribution have been studied computationally, taking into account the effect of radiative heat transfer from hot gases. The results have been compared with the case that radiative heat transfer was neglected. It is observed that the reattachment length increases when increasing the expansion ratio and mixing of two fluid flows are not affected by temperature. As mixing is directly related to turbulence, higher turbulence will cause uniform mixture in a shorter length and sudden expansion is helpful for faster mixing. The results show that the combustion chamber wall temperature due to gas radiation will increase very quickly at the entrance and will then decrease rapidly, while neglecting the radiation effect; the wall temperature will increase monotonically.

Introduction: Well type ionization chamber is a measuring device which is used to determine the activity of brachytherapy sources. The chamber has a Cylindrical volume in which a Cylindrical tube is mounted in the middle of the chamber. For the measurements, the brachytherapy sources are transferred to the middle of the tube.Materials and Methods: For designing the wen type chamber, the measurement principals of wen type chambers were considered and MCNP-4C code as a calculation tool was used. The designed chamber was simulated and the response of the chamber was evaluated. In this investigation, the chamber operational parameters such as operating voltage, leakage current, reproducibility, reference measuring point, recombination and polarization factors as well as response stability for 137Cs, 57CO and 241Am sources were studied.Results: The chamber leakage currents at the operating voltage in comparison to the chamber response for the measurement of the above mentioned sources were negligible. The responses of the fabricated chamber for these sources are reproducible and its reference measurement position for these sources was obtained at 6 cm from the bottom of the chamber. The recombination factor for the well type chamber was negligible and the polarization factor is close to 1. Therefore, these two factors were not considered in the measurements. The reproducibility of the measurements in different intervals shows the stability of the chamber response for each source. Also the results of the chamber current measurement in term of source strength were compared to the response of the simulated chamber for different source positions and energy ranges of the used sources.Discussion and Conclusion: The results show that the measurement of the reference positions for each source in the simulated and fabricated chamber is quite in a good agreement. Regarding the reliable operational properties of the fabricated chamber, this chamber can be used to determine the activities of brachytherapy sources in which the energy ranges are greater than 50 KeV.

Free-air well-type ionization chambers are used in brachytherapy medical centers for measurement of brachytherapy source strengths. In this investigation, the HDR-33004 free-air well-type ionization chamber has been simulated using MCNP-4C computer code for several aluminum alloys as chamber wall material. The relative response (electric current) behavior of the simulated chamber for various source positions along the chamber central axis has been calculated and shows good agreement with measured values. We have also observed that the calculated and measured reference points are situated at the same positions. The results showed that the calculated electric current for unit source strength of 137Cs does not depend on the chamber wall aluminum alloy type, but for 57CO and 241 Am sources, a strong dependency on the type of aluminum alloy as chamber wall material is observed. The presence of high atomic number elements such as zinc in some aluminum alloys resulted in higher response of the chamber. It is also found that for similar Al-7091 alloys the calculated and measured responses for 137Cs, 57CO and 241A msources are in good agreement, showing that the alloy type of the chamber wall material would likely be an alloy similar to Al-7091. The results of this investigation can be applied for design of free-air well-type ionization chambers for low energy gamma sources.

Introduction: Computed Tomography scans are a very important tool for diagnosis and assessment of response to treatment in the practice of medicine. Ionizing radiation in medical imaging is undoubtedly one of the most powerful diagnostic tools in medicine. Yet, as with all medical interventions, there are potential risks in addition to the clear potential benefits.Materials and Methods: Two reference dose quantities have been defined in order to promote the use of good technique in CT. These are weighted CT dose index (CTDIw) in (mGy) for a single slice in serial scanning or per rotation in helical scanning, and dose–length product (DLP) per complete examination (mGy.cm), All measurements were performed using a pencil shaped ionization chamber introduced into polymethyl methacrylate Cylindrical brain and body phantoms. This survey was performed on 7 CT scanners in Khorasan Province-Iran.Results: DLP for brain, chest, abdomen and pelvic examinations had a range of 255 - 1026, 76-1277, 48-737, 69-854 mGy.cm, respectively.Conclusion: The results obtained in this study show that the DLP values obtained in this province are lower than European Commission reference dose levels (EC RDL), in other words performance of all the scanners were satisfactory.

Background and Objective: In order to measure the absorbed dose due to electron beam irradiation at the reference point of a medium, part of this medium is replaced by a Cylindrical ionization chambers. Thus changes will appear in attenuation and scattering of the radiation. In order to assess the displacement effect, an effective point of measurement uses as the reference point. This effective point is taken upstream of the geometrical center of the chamber.This effective point of measurement depends on every detail of the chamber design, energy and field size. So, in the different researches on this point, various values have been reported for Cylindrical ionization chamber. The purpose of this study was to analyse the dosimetric information of electron beams and determine the analytical effective point of measurement in CC13 ionization chamber.Subjects and Methods: In this study a CC13 ionization chamber was used for dose determination of 9, 12 and 15MeV electron beams of Varian accelerator of Ahwaz Golestan hospital in different field sizes of 6×6cm2up to 25×25cm2 applicators. Measurement was made in blue phantom, up to 30mm depth. The PDD curve was plotted separately for all treatment electron fields. The table curve 2D software was used for analyzing experimental data. Then critical point for each curve was calculated.Results: The torsion area is caused by changing environment from air to phantom (water) during measuring of ionization.Also, with increased energy and field size, mean effective point of measurement is altered.Conclusion: For CC13 ionization chamber, the mean value is obtained as 0.9r.