Reference Chamber Research Articles

11. Study of the feasibility of a daily constancy check for VMAT using the Stealth® transmission chamber

Introduction Volumetric Modulated Arc Therapy (VMAT) is a complex technique that requires additional controls over regulation. The aim of this study was to set up a fast and reliable daily constancy check of the VMAT to detect a problem in the beam delivery parameters. Methods For this work, a Stealth® transmission chamber (IBA Dosimetry) was used, it is directly attached to the head of the linac, connected to a Dose1 (IBA Dosimetry) electrometer. This chamber is normally intended to be used as reference chamber during dosimetry commissioning especially for small fields. The control was performed on a Synergy Beam Modulator Accelerator (Elekta AB, Sweden). Previously, the detector response was studied in terms of repeatability, linearity and influence of the dose rate value with static beams. Then VMAT (clockwise and counter-clockwise) beams with a sliding slit with variation of the dose rate, gantry rotation speed and leaves speed were studied in terms of repeatability and constancy. The measured signal was corrected from the daily output and compared to the routine measurement done with a Farmer-type ionization chamber (FC65-P, IBA Dosimetry) placed in a 15 × 15 × 5 cm3 Plexiglas phantom centered on the couch. Results The first tests demonstrated the need for pre-irradiation of at least 2000 MU. A relative standard deviation of less than 0.1% was obtained for static beam repeatability and 0.4% for VMAT beams. The detector showed a dependence on the dose rate with a deviation of 1.2% for the extreme dose rate values. The study of linearity showed a good correlation (R2 = 1). The monitoring of the constancy check of the VMAT beams with the Stealth® chamber seems to follow the same trend of variation as that obtained with the Farmer chamber. Conclusions This study seems to show the possibility to use this detector for a constancy check of VMAT beams. In order to validate this result, a sensitivity study will be carried out by introducing errors into the VMAT beams.

Extended utilization of a 68Ge/68Ga reference source as a mock 18F source in the ionization chamber calibration

Possible extended utilization of a 68Ge/68Ga source as a surrogate 18F reference source was studied. Some effects and uncertainties might arise from the difference of constructions between chambers that were used in the source calibration and those practically in use. To extend utilization of the surrogate source, response ratio of real 18F to 68Ge/68Ga sources were determined for three models of chambers. Monte-Carlo simulation using EGS5 code was also carried out to evaluate uncertainties expected. The present result shows that such a surrogate source would adapt to calibration even for chambers have with different construction from the reference chamber used as the determination of 18F equivalent activity of the 68Ge/68Ga surrogate source.

A portable precision ionization chamber: The transfer ionization reference chamber

A portable instrument was developed at the Institute of Radiation Physics (IRA) for on-site radionuclides measurements. It will enable the measurement of short-lived radionuclides in nuclear medicine departments or isotope production centres. The system involves an ionization chamber read directly by an electrometer and it was optimized to ensure a good reproducibility through the selection of an appropriate vial, filling volume and source position in the well chamber, as well as monitoring the external background and performing a detailed uncertainty estimation. The calibration factors were determined using solutions whose activity concentration is traceable to the international reference system using the IRA reference chamber. Currently the transfer ionization chamber is calibrated for F-18, C-11, O-15, N-13, I-131 and Tc-99m. The final instrument is able to measure isotope activities with a relative standard uncertainty not larger than 1%.

Electron beam water calorimetry measurements to obtain beam quality conversion factors.

To provide results of water calorimetry and ion chamber measurements in high-energy electron beams carried out at the National Research Council Canada (NRC). There are three main aspects to this work: (a) investigation of the behavior of ionization chambers in electron beams of different energies with focus on long-term stability, (b) water calorimetry measurements to determine absorbed dose to water in high-energy beams for direct calibration of ion chambers, and (c) using measurements of chamber response relative to reference ion chambers, determination of beam quality conversion factors, kQ , for several ion chamber types. Measurements are made in electron beams with energies between 8MeV and 22MeV from the NRC Elekta Precise clinical linear accelerator. Ion chamber measurements are made as a function of depth for cylindrical and plane-parallel ion chambers over a period of five years to investigate the stability of ion chamber response and for indirect calibration. Water calorimetry measurements are made in 18MeV and 22MeV beams. An insulated enclosure with fine temperature control is used to maintain a constant temperature (drifts less than 0.1mK/min) of the calorimeter phantom at 4°C to minimize effects from convection. Two vessels of different designs are used with calibrated thermistor probes to measure radiation induced temperature rise. The vessels are filled with high-purity water and saturated with H2 or N2 gas to minimize the effect of radiochemical reactions on the measured temperature rise. A set of secondary standard ion chambers are calibrated directly against the calorimeter. Finally, several other ion chambers are calibrated in the NRC 60 Co reference field and then cross-calibrated against the secondary standard chambers in electron beams to realize kQ factors. The long-term stability of the cylindrical ion chambers in electron beams is better (always <0.15%) than plane-parallel chambers (0.2% to 0.4%). Calorimetry measurements made at 22MeV with two different vessel geometries are consistent within 0.2% after correction for the vessel perturbation. Measurements of absorbed dose calibration coefficients for the same secondary standard chamber separated in time by 10yr are within 0.2%. Drifts in linac output that would affect the transfer of the standard are mitigated to the 0.1% level by performing daily ion chamber normalization measurements. Calibration coefficients for secondary standard ion chambers can be achieved with uncertainties less than 0.4% (k=1) in high-energy electron beams. The additional uncertainty in deriving calibration coefficients for well-behaved chambers indirectly against the secondary standard reference chambers is negligible. The kQ factors measured here differ by up to 1.3% compared to those in TG-51, an important change for reference dosimetry measurements. The measurements made here of kQ factors for eight plane-parallel and six cylindrical ion chambers will impact future updates of reference dosimetry protocols by providing some of the highest quality measurements of this crucial dosimetric parameter.

The impact of water temperature on absolute dose calibration of photon beams

The Task Group (TG) 51 protocol prescribes that dose calibration of photon beams be done by irradiating an ionization chamber in a water tank at pre-defined depths. Methodologies are provided to account for variations in measurement conditions by applying correction factors. However, the protocol does not completely account for the impact of water temperature. It is well established that water temperature will influence the density of air in the ion chamber collecting volume. The temperature, however, will also influence the size of the collecting volume via thermal expansion of the cavity wall, and the density of water. In this work the overall effect of water temperature on absolute dosimetry has been investigated. Dose measurements were made using a Farmer-type ion chamber for 6 and 23 MV photon beams with water temperatures ranging from 10 °C to 40 °C. A reference chamber was used to account for fluctuations in beam output between successive measurements. For the same beam output, the dose at the depth of calibration, determined using TG 51, was found to be dependent on the temperature of the water in the tank. The dose for both photon energies studied followed a similar trend reaching a maximum around 26 °C. For the 6 MV beam, the dose was found to increase 0.7% from 10 °C to 26 °C and then decrease 0.18% from 26 °C to 40 °C. Similarly, for the 23 MV beam the dose increased 0.95% from 10 °C to 26 °C and then decreased 1.25% from 26 °C to 40 °C. There is a measurable effect of water temperature on absolute dose calibration. To account for this effect, a reference temperature can be defined and a correction factor applied to account for deviations from this reference temperature during beam calibration. Such a factor is expected to be of similar magnitude to most of the existing TG 51 correction factors.

New System for Measuring the Photochemical Ozone Production Rate in the Atmosphere.

We have developed a new system for measuring photochemical ozone production rates in the atmosphere. Specifically, the system measures the net photochemical oxidant (Ox: the sum of ozone (O3) and nitrogen dioxide (NO2)) production rates (P-L(Ox)). Measuring Ox avoids issues from perturbations to the photostationary states between nitrogen oxides (NOx) and O3. This system has "reaction" and "reference" chambers. Ambient air is introduced into both chambers, and Ox is photochemically produced in the reaction chamber and not generated in the reference chamber. Air from the chambers is alternately introduced into an NO-reaction (NO: nitric oxide) tube to convert O3 to NO2, and then the Ox concentration is measured as NO2 using a laser-induced fluorescence technique. P-L(Ox) was obtained by dividing the difference in Ox concentrations between air samples from the two chambers by the mean residence time of the air in the reaction chamber. In this study, the P-L(Ox) measurement system was characterized, and the current detection limit of P-L(Ox) was determined to be 0.54 ppbv h-1 with an integration time of 60 s (S/N = 2), assuming an ambient Ox concentration of 100 ppbv. Field measurements of P-L(Ox) were conducted using the system at a remote forest location.

Clinical commissioning of the INCISE™ multileaf collimator for cyberknife M6™ system

Introduction The National Center for Cancer Care and Research (NCCCR) at Hamad Medical Corporation in Qatar is amongst the first institutions in Asia and Europe to have the CyberKnife® M6 FIM system (Accuray, Inc., Sunnyvale, CA, USA) including Fixed Collimators, Iris Variable Aperture Collimator and InCise™ Multileaf Collimator. Purpose To report our commissioning experiences of the CyberKnife M6 InCise™ Multileaf Collimator system. Materials and methods Accuray guidelines were used to conduct measurements. A detailed description of the equipment used, the mechanical alignment, the verification procedures, and the alignment of the detector motion with the radiation beam is described. Incise MLC testing was conducted as per Accuray procedure. Further MLC tests were developed to verify its feasibility under certain clinical settings. Results Our results were within the manufacturer specifications. Results for both the Sun Nuclear Edge detector and PTW SRS diode 60018 were found to be comparable. Measurement conditions specified by Accuray were sufficient; however the use of a reference chamber introduced additional noise for the continuous profile acquisitions. As compared to fixed and IRIS collimators, the incise MLC produced acceptable results with an average of 20% decrease in treatment delivery time. Conclusions Our results are amongst the first set of data reported on the InCise™ MLC and were within the manufacturer specifications. These results will be used to develop a benchmark reference for periodic Quality Control (QC). Disclosure Authors have nothing to disclose.

Recombination in liquid-filled ionization chambers beyond the Boag limit.

The high mass density and low mobilities of charge carriers can cause important recombination in liquid-filled ionization chambers (LICs). Saturation correction methods have been proposed for LICs. Correction methods for pulsed irradiation are based on Boag equation. However, Boag equation assumes that the charge ionized by one pulse is fully collected before the arrival of the next pulse. This condition does not hold in many clinical beams where the pulse repetition period may be shorter than the charge collection time, causing overlapping between charge carriers ionized by different pulses, and Boag equation is not applicable there. In this work, the authors present an experimental and numerical characterization of collection efficiencies in LICs beyond the Boag limit, with overlapping between charge carriers ionized by different pulses. The authors have studied recombination in a LIC array for different dose-per-pulse, pulse repetition frequency, and polarization voltage values. Measurements were performed in a Truebeam Linac using FF and FFF modalities. Dose-per-pulse and pulse repetition frequency have been obtained by monitoring the target current with an oscilloscope. Experimental collection efficiencies have been obtained by using a combination of the two-dose-rate method and ratios to the readout of a reference chamber (CC13, IBA). The authors have also used numerical simulation to complement the experimental data. The authors have found that overlap significantly increases recombination in LICs, as expected. However, the functional dependence of collection efficiencies on the dose-per-pulse does not change (a linear dependence has been observed in the near-saturation region for different degrees of overlapping, the same dependence observed in the nonoverlapping scenario). On the other hand, the dependence of collection efficiencies on the polarization voltage changes in the overlapping scenario and does not follow that of Boag equation, the reason being that changing the polarization voltage also affects the charge collection time, thus changing the amount of overlapping. These results have important consequences for saturation correction methods for LICs. On one hand, the two-dose-rate method, which relies on the functional dependence of the collection efficiencies on dose-per-pulse, can also be used in the overlapping situation, provided that the two measurements needed to feed the method are performed at the same pulse repetition frequency (monitor unit rate). This result opens the door to computing collection efficiencies in LICs in many clinical setups where charge overlap in the LIC exists. On the other hand, correction methods based on the voltage-dependence of Boag equation like the three-voltage method or the modified two-voltage method will not work in the overlapping scenario due to the different functional dependence of collection efficiencies on the polarization voltage.

Surface-Passive Pressure Sensor by Femtosecond Laser Glass Structuring for Flip-Chip-in-Foil Integration

To allow for smaller sizes, smoothness and robustness of exposed surface, and for integration in flexible sensor arrays, an innovative piezoresistive pressure sensor design has been developed. In contrast to known concepts, the sensing elements and the conducting tracks are positioned within the pressure reference chamber and, thus, protected against environmental influences such as water or particles. Sensing elements are electrically accessible from the backside by vias, thus enabling a fully flat surface totally free of electrical elements as desired for flow experiments. The sensor comprises a thin silicon sensing membrane and a body made from glass holding the reference chamber and the vias. The structuring of the sensor body is performed by femtosecond laser ablation. Steep ablation edges are realized, leading to small sensor dimensions. The sensing membrane is fabricated using potassium hydroxide (KOH) wet etching. The glass body and the silicon membrane can be connected with different techniques; hitherto, adhesive bonding by an epoxy resin layer was successfully tested. A sensitivity of 10 mV/V/bar and stable operation up to 7 bar absolute pressure could already be demonstrated. The new concept simplifies micromanufacture and allows for flip-chip-assembly in foil-based flexible systems that can be used in liquids and harsh environments. [2016-0003]

Plant irradiation device in microwave field with controlled environment

Plant irradiation during the germination period, by using environmental low level microwave radiation (known as electrosmog) alters the plant germination and the growing process. In order to analyze if such a subtle effect is generated only by the microwave irradiation, the environmental parameters (light, temperature, and humidity) must be kept identical for the reference and the irradiated lot. This is a difficult task because the humidity and temperature are interrelated. In order to study the plant behavior under microwaves low power irradiation, the paper describes the design, manufacturing and operating process and device performances on plant growth. To this end, a low power microwave field (average microwaves power density of 3.8mW/m2) under controlled environmental parameters is used. The device consists of a reference chamber (R) and a microwave irradiation chamber (I), of 0.5m3 volume each one, equipped with access doors. The irradiation chamber ensures microwave field distribution with programmable power, frequency and bandwidth in the most commonly used standards for network communication such as: Global System for Mobile Communications (GSM900/1800), Code Division Multiple Access (CDMA), the third generation of mobile telecommunications technology (3G) or 2.4/5GHz Wireless Local Area Network (WLAN). Both chambers provide a radiofrequency (RF) shielding (at least −60dB) against and toward outside, so the electrosmog is shielded and does not interfere with the inner environment. Both chambers are equipped with performing temperature/humidity sensors and controlled LED lighting system (maximum 400μmolcm−2s−1) with a uniformity of ±5μmolcm−2s−1 measured at the bottom level of the chamber. An embedded system (microcontroller) measures the temperature and humidity and proceeds continuously to match the humidity into the chambers pair with less than ±1.5% relative humidity (RH) difference, by using a low flux exhaust ventilation process through a simple innovative method. The accuracy of the temperature measurement is better than ±0.2°C. Humidity and temperature data set are logged (with programmable acquisition rate) during the whole experiment and can be read later by a personal computer. To identify the influence of microwave treatment on bean seeds and plants development, three growing experiments were settled, based on 122 bean seeds each. The number of germinated seeds (determined each day during 8days of experiment), the germination energy (GE [%]) of the seeds, the length of stems (SL) and roots (RL), the germination (G) [%], the seedling vigor index (SVI) and dry matter content (DM%) have been measured and computed. The obtained data showed significant increase for all parameters on microwave irradiation condition.

Determination of the characteristic limits and responses of nuclear track detectors in mixed radon and thoron atmospheres

Closed nuclear track detectors are widely used for the determination of Rn-222 exposures. There are also partial open systems available, which are specially designed for the determination of the exposure to Rn-220, which is a relevant exposure in special workplaces or in specific regions of the world. This paper presents data and a detail analysis of how to determine the cross-correlation by calibration in pure Rn-222 and pure Rn-220 atm. By these means calibration coefficients for the analysis of real mixed atmospheres can be obtained. The respective decision threshold, detection limit and limits of the confidence interval were determined according to ISO 11929 (ISO 11929:2010, 2010).The exposure of detectors was performed at the radon reference chamber and the thoron progeny chamber of the Physikalisch-Technische Bundesanstalt (PTB). The analysis of track response was done at Parc RGM, while the analytical routines were developed in the Leibniz University Hanover, Institute Radioökologie und Strahlenschutz IRS at the working Group AK SIGMA (Arbeitskreis Nachweisgrenzen).

Helium gas permeability measurements of polymeric membranes using the difference method

This paper presents a new permeation measurement facility based on the difference method, which is shown to be capable of measuring gas fluxes through different polymeric membranes. In order to correct for the background pressure increase in the measurement chamber, a reference chamber has been adapted and connected to it. The permeation flux was examined by measuring the pressure difference between the reference and measurement chambers. Pure helium permeation was performed through polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET) membranes using this method. The uncertainty of the permeability was evaluated. The results show that the introduction of reference chamber can mitigate the influence of the residual pressure in the measurement chamber. This new method allows simplified and visualized measurement of the permeation fluxes.

Measuring Bipolar Charge and Mass Distributions of Powder Aerosols by a Novel Tool (BOLAR).

The Bipolar Charge Analyzer (BOLAR) was evaluated for measuring bipolar electrostatic charge and mass distributions of powder aerosols generated from a dry powder inhaler. Mannitol powder (5, 10, and 20 mg) was dispersed using an Osmohaler inhaler into the BOLAR at air flow rates of 30 or 60 L/min. As the aerosol sample was drawn through the BOLAR, the air flow was divided into six equal fractions. Five of them entered individual detection tubes with a defined cutoff diameter in the range of 0.95 to 16.36 μm (depending on the flow rate) and the remaining (i.e., the sixth) fraction passed through a reference chamber. The aerosols that entered the detection tubes were separated according to the particle charge polarity (positive, negative, or neutral) and charge was measured by separate electrometers. The deposited powder of a single actuation from the inhaler was chemically assayed using high performance liquid chromatography. Additionally, the aerosol measurements were conducted on a modified Classic Electrical Low Pressure Impactor (ELPI) for comparison of the net specific charge per size fraction. Spray-dried mannitol carried significantly different positively and negatively charged particles in each of the five defined particle size fractions. The charge-to-mass ratio (q/m) of positively charged particles ranged from +1.11 to +32.57 pC/μg and negatively charged particles ranged from -1.39 to -9.25 pC/μg, resulting in a net q/m of -3.08 to +13.34 pC/μg. The net q/m values obtained on the modified ELPI ranged from -5.18 to +4.81 pC/μg, which were comparable to the BOLAR measurements. This is the first full report to utilize the BOLAR to measure bipolar charge and mass distributions of a powder aerosol. Positively and negatively charged particles were observed within each size fraction, and their corresponding q/m profiles were successfully characterized. Despite some potential drawbacks, the BOLAR has provided a new platform for investigating bipolar charge in powder aerosols for inhalation.

Evaluation of a novel reference chamber “stealth chamber” through Monte Carlo simulations and experimental data

Purpose: To evaluate a novel reference chamber (Stealth Chamber by IBA) through experimental data and Monte Carlo simulations for 6 and 15 MV photon energies. Methods: Monte Carlo simulations in a water phantom for field sizes ranging from 3×3 and 25×25 cm 2 were performed for both energies with and without the Monte Carlo model of the Stealth Chamber in the beam path, and compared to commissioning beam data. Percent depth doses (PDDs), profiles, and gamma analysis of the simulations were performed along with an energy spectrum analysis of the phase-space files generated during the simulation. Experimental data were acquired in water with IBA three-dimensional (3D) blue phantom in a set-up identical to the one used in the Monte Carlo simulations. PDD comparisons for fields ranging from 1×1 to 25×25 cm 2 were performed for photon energies. Profile comparison for fields ranging from 1×1 to 25×25 cm 2 were executed for the depths of dmax, 5, 10 and 20 cm. Criteria of 1%, 1 mm to compare PDDs and profiles were used. Transmission measurements with the Stealth Chamber and a Matrixx detector from IBA were investigated. Measurements for 6 and 15 MV with fields ranging from 3×3 to 25×25 cm 2 dimensions were acquired in an open field with and without the Stealth Chamber in the path of the beam. Profiles and gamma analysis with a 1%, 1 mm gamma analysis criterion were performed. Results: Monte Carlo simulations of the PDDs and profiles demonstrate the agreement between both simulations. Furthermore, the gamma analysis (1%, 1 mm) result of the comparison of both planes has 100% of the points passing the criteria. The spectral distribution analysis of the phase spaces for an open field with and without the chamber reveals the agreement between both simulations. Experimental measurements of PDDs and profiles have been conducted and reveal the comparability of relative dosimetric data acquired with the Stealth Chamber and our gold standard the CC13 chamber. Transmission data measured with an ion chamber array (Matrixx) showed the small attenuation caused by the use of the Stealth Chamber. Conclusion: Simulations and experimental results from this investigation indicate the benefits associated with chamber positioning and time expended during the acquisition of the relative measurements of PDDs and profiles for the beam commissioning of photon beams when the Stealth Chamber is used as a reference chamber to perform these tasks. The results demonstrate that relative profiles and PDDs scanned with the Stealth Chamber in place are consistent with those made using a CC13 chamber within a 1% and 1 mm criterion.

SU‐E‐T‐377: Evaluation of a Novel Transmission Detector as a Reference Chamber for Use in Beam Measurements

Purpose:To evaluate a new transmission chamber for use as a reference chamber in measurements of beam dataMethods:We assessed the performance of a new transmission detector, the Stealth Chamber, manufactured by IBA (IBA Dosimetry). The chamber has an active volume of 249 cm3, with an attenuation equivalent of &lt;0.5mm Al. We mounted the chamber to a TrueBeam linac (Varian Medical Systems, Palo Alto, CA) such that the active area is perpendicular to the beam direction. We performed PDD and profile measurements on field sizes from 1×1 cm2 to 10×10 cm2, as well as for a 4 mm cone. The field detector was either a CC‐13 chamber (IBA Dosimetry) with an active volume of 0.13 cm3 or an Edge Detector (Sun Nuclear, Melbourne, FL) with an active volume of 0.019 cm3. For comparison we repeated all measurements using a CC‐13 or CC‐01 (active volume 0.01 cm3 ) chamber as reference detectors (IBA Dosimetry).All scans were acquired using a Blue Phantom2 (IBA Dosimetry) and IBA OmniPro‐Accept v7.4.24 software.Results:All measurements with the Stealth Chamber were identical to those with ion reference chambers. For both regular and filter free 6MV beams there was agreement in PDDs and profiles, including the penumbra region, for all field sizes. This was true for the 4mm cone measurement, as well. The deviation between the Stealth and ion chamber measurements was on average 0.3%.Conclusion:The Stealth Chamber gives identical beam data as a conventional ion chamber for all field sizes. The advantage of the Stealth chamber over ion chambers is its efficiency. Once mounted, there is no need to reposition the chamber with varying field sizes. This translates into a huge savings in measurement time, as well as a reduction in potential errors due to reference chamber mispositioning.

SU‐E‐T‐406: Evaluation of the Stealth Chamber by IBA Under Large Field Conditions

Purpose:To evaluate the performance of the stealth chamber by IBA under large field conditions.Methods:Measurement of PDDs and profiles for fields of 10×10, 15×15, 20×20 and 25×25 cm2 were acquired in a blue phantom2 scanning system for 6 and 15 MV beams in a Varian TrueBeam linear accelerator. The stealth chamber has a field size limit of 25×25 cm2. Measurements with two different detector set were performed. First a cc13 chamber as field and reference chamber was acquired. And with a cc13 chamber as field detector and the stealth chamber as the reference chamber. PDDs for the fields mentioned above were conducted and profiles with the same fields at depth of dmax, 5, 10 and 20 cm were acquired for comparison.Results:Examination of the PDDs and profile comparison with a criterion of 1% and 1 mm agreement was performed. Very good agreement between the cc13 and stealth chamber set was observed within the chosen criterion.Conclusion:This work indicates the comparable results obtained with the use of the stealth chamber and cc13 as the reference chamber in relative dosimetry measurements.

SU‐E‐T‐156: Can Sr‐90 Check Sources Replace Co‐60 Measurements for Monitoring of Reference Chamber Stability?

Purpose:To determine the ultimate precision of a system for monitoring reference‐class ion chamber stability using a commercial Sr‐90 check source.Methods:A detailed investigation of a commercial Sr‐90 check source (PTW48002) was carried out using a series of Farmer‐type ionization chambers. Investigations included: positioning repeatability (angular variation as chamber is rotated in source, variation in ionization current with vertical alignment); chamber settling; short and long term repeatabilityResults:i) Measurement precision – the ionization current was typically 10 pA, and therefore a high‐precision electrometer is required to prevent electrometer noise/resolution/leakage biaising the results. ii) Chamber settling ‐ the chamber response stabilizes after approximately 10 minutes, which is longer than reported for linac beams and is likely due to the low doserate of the source.iii) The measured response depended at the 1 % level on the orientation of the chamber with respect to the source. However, consistent positioning resulted in repeatability at the 0.05 % level. Care was also required to ensure that the chamber was consistently positioned vertically with respect to the source. The sensitivity to vertical position was found to be &gt; 1 % per mm.iv)With a uniform procedure the long‐term (&gt; 6 month) repeatability was found to be better than 0.1 % for multiple chamber types and potentially a precision of 0.05 % is achievable.Conclusion:A Sr‐90 check source is easy to use and is a viable alternative to Co‐60 for monitoring reference chamber stability.

Some steps towards establishing a tertiary standard dosimetry laboratory at a radiotherapy department

Background: In order to deliver the precise dose to the target in radiotherapy, absorbed dose to water at the reference point should be assessed. When the calibraon procedure is performed for a reference dosimeter in the 60 Co beam of a Secondary Standard Dosimetry Laboratory (SSDL), the total uncertainty in absorbed dose to water (D w) is esmated to be approximately 1.5%. This study a)empts to re-measure the N D,W factors for all available field chambers at Reza Radiotherapy & Oncology Centre (RROC). Materials and Methods: Consistency and linearity checks were performed for a range of available chambers using a check source. The N D,W factors were also measured for the ionizaon chambers. All cylindrical chambers have been cross calibrated at 6 MV photon beam using a Siemens Primus Plus Linac. The Plane Parallel Chamber has been cross calibrated at the highest available electron beam and the ND,W factor has been measured. Results: The tolerance of consistency and linearity checks has been reported to be within 0.3%. The ND,W value for field Farmer chamber was found in agreement with cerf icate within 1%. In contrast for small acve volume chambers, the deviaon from the SSDL reports was 2.3%. For the plane parallel chamber, the difference between SSDL and Home measurements are found to be 12%. Conclusion: Although the calibraon of reference chambers used for absolute dosimetry through a Primary Standard Dosimetry Laboratory (PSDL) or SSDL is recommended, for field chambers this can be done at home department as a Terary Standard Dosimetry Laboratory (TSDL).

Easy-to-make portable chamber for in situ CO&lt;sub&gt;2&lt;/sub&gt; exchange measurements on biological soil crusts

Commercial chambers for in vivo gas exchange are usually designed to measure on vascular plants, but not on cryptogams and other organisms forming biological soil crusts (BSCs). We have therefore designed two versions of a chamber with different volumes for determining CO2 exchange with a portable photosynthesis system, for three main purposes: (1) to measure in situ CO2 exchange on soils covered by BSCs with minimal physical and microenvironmental disturbance; (2) to acquire CO2-exchange measurements comparable with the most widely employed systems and methodologies; and (3) to monitor CO2 exchange over time. Different configurations were tested in the two versions of the chamber and fluxes were compared to those measured by four reference commercial chambers: three attached to two respirometers, and a conifer chamber attached to a portable photosynthesis system. Most comparisons were done on biologically crusted soil samples. When using devices in a closed system, fluxes were higher and the relationships to the reference chambers were weaker. Nevertheless, high correlations between our chamber operating in open system and measurements of commercial respiration and photosynthetic chambers were found in all cases (R 2 > 0.9), indicating the suitability of the chamber designed for in situ measurements of CO2 gas exchange on BSCs.

An experimental set-up to analyse the oxygen consumption of elastomers during ageing by using a differential oxygen analyser

Ageing of polymers becomes more and more important. This can be seen by the increasing number of research projects dealing with this topic. However, the influence of oxygen on changes in the mechanical performance is undisputable and important with respect to the lifetime of polymer products. Therefore, a respirometer offers the potential to detect the smallest amounts of oxygen changes in the polymers’ ambient air. It will be used to analyse the oxygen consumption of rubber which is exposed for different times to elevated temperatures. In this contribution, virgin rubber samples are aged for various times in a sealed chamber at temperatures of 60, 80 and 100°C. The decline of the oxygen concentration in the ambient air is measured by flushing the chamber with dried and cleaned air which is conducted into the respirometer. The oxygen concentration is compared with that in a reference chamber, which is exposed to the same ageing conditions as the sample under investigation. The absorbed oxygen is relevant for ageing and a considerable factor for further investigations. For this reason, an experimental set-up using a differential oxygen analyser is developed, which allows for ageing several samples simultaneously in external climate chambers. The comparison of the change in the mechanical material behaviour after ageing can provide an important contribution for improving constitutive models or ongoing researches on the fatigue strength of polymers. This work shows the development of an improved method for combining mechanical testing and the measurement of oxygen consumption.