Reference Chamber Research Articles

Assessment of small volume ionization chambers as reference dosimeters in high-energy photon beams

LNE-LNHB is involved in a European project aiming at establishing absorbed dose-to-water standards for photon-radiation fields down to 2 × 2 cm2. This requires the calibration of reference ionization chambers of small volume. Twenty-four ionization chambers of eight different types with volume ranging from 0.007 to 0.057 cm3 were tested in a 60Co beam. For each chamber, two major characteristics were investigated: (1) the stability of the measured current as a function of the irradiation time under continuous irradiation. At LNE-LNHB, the variation of the current should be less than ±0.1% in comparison with its first value (over a 16 h irradiation time); (2) the variation of the ionization current with the applied polarizing voltage and polarity. Leakage currents were also measured. Results show that (1) every tested PTW (31015, 31016 and 31014) and Exradin A1SL chambers demonstrate a satisfying stability under irradiation. Other types of chambers have a stability complying with the stability criterion for some or none of them. (2) IBA CC01, IBA CC04 and Exradin A1SL show a proper response as a function of applied voltage for both polarities. PTW, Exradin A14SL and Exradin A16 do not. Only three types of chambers were deemed suitable as reference chambers according to LNE-LNHB requirements and specifications from McEwen (2010 Med. Phys. 37 2179–93): Exradin A1SL chambers (3/3), IBA CC04 (2/3) and IBA CC01 (1/3). The Exradin A1SL type with an applied polarizing voltage of 150 V was chosen as an LNE-LNHB reference chamber type in 2 × 2 cm2 radiation fields.

Measured and Monte Carlo calculated kQ factors: Accuracy and comparison

The journal Medical Physics recently published two papers that determine beam quality conversion factors, k(Q), for large sets of ion chambers. In the first paper [McEwen Med. Phys. 37, 2179-2193 (2010)], k(Q) was determined experimentally, while the second paper [Muir and Rogers Med. Phys. 37, 5939-5950 (2010)] provides k(Q) factors calculated using Monte Carlo simulations. This work investigates a variety of additional consistency checks to verify the accuracy of the k(Q) factors determined in each publication and a comparison of the two data sets. Uncertainty introduced in calculated k(Q) factors by possible variation of W/e with beam energy is investigated further. The validity of the experimental set of k(Q) factors relies on the accuracy of the NE2571 reference chamber measurements to which k(Q) factors for all other ion chambers are correlated. The stability of NE2571 absorbed dose to water calibration coefficients is determined and comparison to other experimental k(Q) factors is analyzed. Reliability of Monte Carlo calculated k(Q) factors is assessed through comparison to other publications that provide Monte Carlo calculations of k(Q) as well as an analysis of the sleeve effect, the effect of cavity length and self-consistencies between graphite-walled Farmer-chambers. Comparison between the two data sets is given in terms of the percent difference between the k(Q) factors presented in both publications. Monitoring of the absorbed dose calibration coefficients for the NE2571 chambers over a period of more than 15 yrs exhibit consistency at a level better than 0.1%. Agreement of the NE2571 k(Q) factors with a quadratic fit to all other experimental data from standards labs for the same chamber is observed within 0.3%. Monte Carlo calculated k(Q) factors are in good agreement with most other Monte Carlo calculated k(Q) factors. Expected results are observed for the sleeve effect and the effect of cavity length on k(Q). The mean percent differences between experimental and Monte Carlo calculated k(Q) factors are -0.08, -0.07, and -0.23% for the Elekta 6, 10, and 25 MV nominal beam energies, respectively. An upper limit on the variation of W/e in photon beams from cobalt-60 to 25 MV is determined as 0.4% with 95% confidence. The combined uncertainty on Monte Carlo calculated k(Q) factors is reassessed and amounts to between 0.40 and 0.49% depending on the wall material of the chamber. Excellent agreement (mean percent difference of only 0.13% for the entire data set) between experimental and calculated k(Q) factors is observed. For some chambers, k(Q) is measured for only one chamber of each type--the level of agreement observed in this study would suggest that for those chambers the measured k(Q) values are generally representative of the chamber type.

TH-E-211-08: The Energy Dependence of Small Volume Ionization Chambers and Solid State Detectors at Diagnostic Energy Ranges for CT Dosimetry - Assessment in Air and In Phantom

Purpose: While several ionization chambers and solid state detectors are commercially available for CT, their energy dependency has yet to be determined. This work evaluates the energy response of a 0.6-cc ionization chamber and solid state dosimeter against an ionization chamber specifically calibrated at several diagnostic energy levels (reference chamber) as a function of HVL for in-air and in-CTDI phantom measurements. Methods: For two CT scanner models of known difference in beam spectra, HVLˈs for all available kVpˈs were obtained with measurements (in-air) and from Monte Carlo simulations (in head and body CTDI phantom). The reference ionization chamber was calibrated by the UWADCL using reference beams with HVLˈs in the CT range. Calibration factors for the reference chamber at each measured/simulated HVL were calculated based on polynomial fit of the UWADCL-beam factors. For each scanner, exposure conditions were established at each kVp by varying mAs to obtain similar results in the reference chamber. Both test dosimeters were then scanned at these conditions and the exposure (or dose) relative to the reference chamber was obtained; no specific energy corrections were applied for either test device. The same procedure was performed with the head and body CTDI phantoms for both CT scanners. Results: HVLˈs from the scanners ranged from 3.49 to 9.7mm Al in-air and from 3.1 to 8.8mm Al in-phantom. Doses measured by the test chamber matched those of the reference chamber across scanners and kVpˈs to within 3.5% in-air, 4.9% in head phantom and 11.8% in body phantom. Differences in the solid state device were larger: as high as 16%, 21%, and 37% for in-air, head, and body phantom, respectively. Conclusions: While ionization chambers have some energy dependence in the diagnostic range, solid state detectors exhibit a more pronounced energy dependence and correction factors should be applied based on HVL.

On the wall perturbation correction for a parallel‐plate NACP‐02 chamber in clinical electron beams

In recent years, several Monte Carlo studies have been published concerning the perturbation corrections of a parallel-plate chamber in clinical electron beams. In these studies, a strong depth dependence of the relevant correction factors (p(wall) and P(cav)) for depth beyond the reference depth is recognized and it has been shown that the variation with depth is sensitive to the choice of the chamber's effective point of measurement. Recommendations concerning the positioning of parallel-plate ionization chambers in clinical electron beams are not the same for all current dosimetry protocols. The IAEA TRS-398 as well as the IPEM protocol and the German protocol DIN 6800-2 interpret the depth of measurement within the phantom as the water equivalent depth, i.e., the nonwater equivalence of the entrance window has to be accounted for by shifting the chamber by an amount deltaz. This positioning should ensure that the primary electrons traveling from the surface of the water phantom through the entrance window to the chamber's reference point sustain the same energy loss as the primary electrons in the undisturbed phantom. The objective of the present study is the determination of the shift deltaz for a NACP-02 chamber and the calculation of the resulting wall perturbation correction as a function of depth. Moreover, the contributions of the different chamber walls to the wall perturbation correction are identified. The dose and fluence within the NACP-02 chamber and a wall-less air cavity is calculated using the Monte Carlo code EGSnrc in a water phantom at different depths for different clinical electron beams. In order to determine the necessary shift to account for the nonwater equivalence of the entrance window, the chamber is shifted in steps deltaz around the depth of measurement. The optimal shift deltaz is determined from a comparison of the spectral fluence within the chamber and the bare cavity. The wall perturbation correction is calculated as the ratio between doses for the complete chamber and a wall-less air cavity. The high energy part of the fluence spectra within the chamber strongly varies even with small chamber shifts, allowing the determination of deltaz within micrometers. For the NACP-02 chamber a shift deltaz = -0.058 cm results. This value is independent of the energy of the primary electrons as well as of the depth within the phantom and it is in good agreement with the value recommended in the German dosimetry protocol. Applying this shift, the calculated wall perturbation correction as a function of depth is varying less than 1% from zero up to the half value depth R50 for electron energies in the range of 6-21 MeV. The remaining depth dependence can mainly be attributed to the scatter properties of the entrance window. When neglecting the nonwater equivalence of the entrance window, the variation of p(wall) with depth is up to 10% and more, especially for low electron energies. The variation of the wall perturbation correction for the NACP-02 chamber in clinical electron beams strongly depends on the positioning of the chamber. Applying a shift deltaz = -0.058 cm toward the focus ensures that the primary electron spectrum within the chamber bears the largest resemblance to the fluence of a wall-less cavity. Hence, the influence of the chamber walls on the perturbation correction can be separated out and the residual variation of p(wall) with depth is minimized.

Characterization of a gated fiber‐optic‐coupled detector for application in clinical electron beam dosimetry

Assessment of the fundamental dosimetric characteristics of a novel gated fiber-optic-coupled dosimetry system for clinical electron beam irradiation. The response of fiber-optic-coupled dosimetry system to clinical electron beam, with nominal energy range of 6-20 MeV, was evaluated for reproducibility, linearity, and output dependence on dose rate, dose per pulse, energy, and field size. The validity of the detector system's response was assessed in correspondence with a reference ionization chamber. The fiber-optic-coupled dosimetry system showed little dependence to dose rate variations (coefficient of variation +/- 0.37%) and dose per pulse changes (with 0.54% of reference chamber measurements). The reproducibility of the system was +/- 0.55% for dose fractions of approximately 100 cGy. Energy dependence was within +/- 1.67% relative to the reference ionization chamber for the 6-20 MeV nominal electron beam energy range. The system exhibited excellent linear response (R2 = 1.000) compared to reference ionization chamber in the dose range of 1-1000 cGy. The output factors were within +/- 0.54% of the corresponding reference ionization chamber measurements. The dosimetric properties of the gated fiber-optic-coupled dosimetry system compare favorably to the corresponding reference ionization chamber measurements and show considerable potential for applications in clinical electron beam radiotherapy.

Development of a Low-Flow Meter for Measuring Gas Production in Bioreactors

Accurate measurement of gas production from biological processes is important in many laboratory experiments. A gas flow rate measurement system, consisting of an embedded controller operating three gas meters, was developed to measure volumetric flows between 0 and 9 mL min-1 (1 atm, 273 K). The gas meter system builds on earlier pressure-based designs by adding a reference chamber to avoid errors associated with atmospheric pressure fluctuations. The controller has a keypad and LCD that guides users through operation, logs data, and can be connected to a computer for data retrieval using a custom graphical user interface. Each of the three gas meters was calibrated using a bubble meter as the standard. The error in gas flow rates measured by the calibrated gas meter is expected to be within 0.17 mL min-1 95% of the time. The gas metering system was used to measure biogas production from batch anaerobic reactors that were operated for biohydrogen production using non-fat milk as the substrate. The results show that the gas meter can be used to collect accurate and high-resolution gas flow data from laboratory bioreactors.

Investigation and Performance Tests of a Designed and Constructed Cylindrical Ionization Chamber(0.6cc)in Iran

There are a number of codes, reports and protocols by national and international organizations, including IAEA, which provide physicists with a systematic approach to dosimetry of photon beams. Most of these dosimetry recommendations have explicitly recognized the advantages of using cylindrical ionization chambers for dosimetry of therapeutic beams, especially for photon beams with energies from kilo voltage (kV) to megavoltage (MV) x-ray. A commercial cylindrical ionization chamber (W-30001) was used as the reference chamber for compare measurements. The homemade 0.6cc ionization chamber (CC1) have been designed, fabricated, tested and calibrated. Measurements were made using a Farmer type 2670 electrometer together with these chambers. Leakage current, short-term stability, cable effect, repeatability and angular dependence of the CC1 and W-30001 were all tested and found to be in consistence with the international standard IEC 60731. Ion collection efficiency and polarity effect were determined during calibration of the chambers in Co-60. According to the preliminary test results the CC1 homemade chamber is found to be in consistence with the international standard IEC 60731. An advantage of CC1 chamber is a very low leakage current i.e. its specific insulation design and material.

Calibration of radiation protection area monitoring instruments in Sudan

This article presents results of measurements for the calibration of radiation protection area monitoring instruments carried out during the period 2006–2008 at Secondary Standard Dosimetry Laboratory of Sudan. The work performed included quality assurance measurements, measurements for the dosimeter calibrations and uncertainty analysis. Calibrations were performed using 137Cs gamma ray sources produced by OB 85 and OB 34/1 gamma calibrators producing air kerma rate that ranged from 10 μGy/h to 50 mGy/h. Area monitoring instruments were calibrated in terms of ambient dose equivalent, H*(10) derived using air kerma to ambient dose equivalent conversion coefficients. Results are presented for 78 area monitoring instruments representing most commonly used types in Sudan. Radioactive check source measurements for the reference chamber showed deviation within 1% limit. The accuracy in the beam output measurements was within 5% internationally considered as acceptable. The results highlighted the importance of radiation protection calibrations. Regulations are further need to ensure safety aspect really meet the required international standards.

Measurement of Ozone Production Sensor

Abstract. A new ambient air monitor, the Measurement of Ozone Production Sensor (MOPS), measures directly the rate of ozone production in the atmosphere. The sensor consists of two 11.3 L environmental chambers made of UV-transmitting Teflon film, a unit to convert NO2 to O3, and a modified ozone monitor. In the sample chamber, flowing ambient air is exposed to the sunlight so that ozone is produced just as it is in the atmosphere. In the second chamber, called the reference chamber, a UV-blocking film over the Teflon film prevents ozone formation but allows other processes to occur as they do in the sample chamber. The air flows that exit the two chambers are sampled by an ozone monitor operating in differential mode so that the difference between the two ozone signals, divided by the exposure time in the chambers, gives the ozone production rate. High-efficiency conversion of NO2 to O3 prior to detection in the ozone monitor accounts for differences in the NOx photostationary state that can occur in the two chambers. The MOPS measures the ozone production rate, but with the addition of NO to the sampled air flow, the MOPS can be used to study the sensitivity of ozone production to NO. Preliminary studies with the MOPS on the campus of the Pennsylvania State University show the potential of this new technique.

Measurement of ionization chamber absorbed dose factors in megavoltage photon beams

Absorbed dose beam quality conversion factors (k(Q) factors) were obtained for 27 different types of ionization chamber. The aim was to obtain objective evidence on the performance of a wide range of chambers currently available, and potentially used for reference dosimetry, and to investigate the accuracy of the k(Q) calculation algorithm used in the TG-51 protocol. Measurements were made using the 60Co irradiator and Elekta Precise linac facilities at the National Research Council of Canada. The objective was to characterize the chambers over the range of energies applicable to TG-51 and determine whether each chamber met the requirements of a reference-class instrument. Chamber settling, leakage current, ion recombination and polarity, and waterproofing sleeve effects were investigated, and absorbed dose calibration coefficients were obtained for 60Co and 6, 10, and 25 MV photon beams. Only thimble-type chambers were considered in this investigation and were classified into three groups: (i) Reference chambers ("standard" 0.6 cm3 Farmer-type chambers and their derivatives traditionally used for beam output calibration); (ii) scanning chambers (typically 0.1 cm3 volume chambers used for beam commissioning with 3-D scanning phantoms); and (iii) microchambers (very small volume ion chambers (< or = 0.01 cm3) used for small field dosimetry). As might be expected, 0.6 cm3 thimble chambers showed the most predictable performance and experimental k(Q) factors were obtained with a relative uncertainty of 0.1%. The performance of scanning and microchambers was somewhat variable. Some chambers showed very good behavior but others showed anomalous polarity and recombination corrections that are not fully explained at present. For the well-behaved chambers, agreement between measured and calculated k(Q) factors was within 0.4%; for some chambers, differences of more than 1% were seen that may be related to the recombination/polarity results. Use of such chambers could result in significant errors in the determination of reference dose in the clinic. Based on the experimental evidence obtained here, specification for a reference-class ionization chamber could be developed that would minimize the error in using a dosimetry protocol with calculated beam quality conversion factors. The experimental k(Q) data obtained here for a wide range of thimble chambers can be used when choosing suitable detectors for reference dosimetry and are intended to be used in the upcoming update/addendum to the AAPM TG-51 dosimetry protocol.

New Exposure System To Evaluate the Toxicity of (Scooter) Exhaust Emissions in Lung Cells in Vitro

A constantly growing number of scooters produce an increasing amount of potentially harmful emissions. Due to their engine technology, two-stroke scooters emit huge amounts of adverse substances, which can induce adverse pulmonary and cardiovascular health effects. The aim of this study was to develop a system to expose a characterized triple cell coculture model of the human epithelial airway barrier, to freshly produced and characterized total scooter exhaust emissions. In exposure chambers, cell cultures were exposed for 1 and 2 h to 1:100 diluted exhaust emissions and in the reference chamber to filtered ambient air, both controlled at 5% CO(2), 85% relative humidity, and 37 degrees C. The postexposure time was 0-24 h. Cytotoxicity, used to validate the exposure system, was significantly increased in exposed cell cultures after 8 h postexposure time. (Pro-) inflammatory chemo- and cytokine concentrations in the medium of exposed cells were significantly higher at the 12 h postexposure time point. It was shown that the described exposure system (with 2 h exposure duration, 8 and 24 h postexposure time, dilution of 1:100, flow of 2 L/min as optimal exposure conditions) can be used to evaluate the toxic potential of total exhaust emissions.

Development of a Constant Pressure Type Flowmeter for the Vacuum Standard

A constant pressure type flowmeter was developed to generate precise gas flow for calibrations of vacuum gauges by an orifice flow method and standard leaks by a comparison method. The flow rate is derived from the constant pressure and the volume reduction rate. For accurate measurement of volume reduction, a relation of the bellows length and its volume was measured as mounted in the flowmeter. Volume change at 25 mm was 32.19 ml. Measurement uncertainty is 0.24%. In order to keep the pressure in the main chamber constant, a reference chamber was installed to detect pressure change in the main chamber by the differential pressure between two chambers. Pressure was kept constant within 0.01% during the operation of flowmeter. By changing the initial pressure in the flowmeter from 0.16 Pa to 5000 Pa for nitrogen, flow rate of 4×10−9∼8×10−4 Pa m3/s was generated with a relative standard uncertainty of 5.7%∼0.36%. A gas is effused through a metal capillary attached to the main chamber. Repeated conductance measurements of the capillary for 540 days showed that the gas flow rate measurements was stable within∼0.1%.

Heterodimerization of the Sialidase NEU1 with the Chaperone Protective Protein/Cathepsin A Prevents Its Premature Oligomerization

Lysosomal neuraminidase-1 (NEU1) forms a multienzyme complex with beta-galactosidase and protective protein/cathepsin A (PPCA). Because of its association with PPCA, which acts as a molecular chaperone, NEU1 is transported to the lysosomal compartment, catalytically activated, and stabilized. However, the mode(s) of association between these two proteins both en route to the lysosome and in the multienzyme complex has remained elusive. Here, we have analyzed the hydrodynamic properties of PPCA, NEU1, and a complex of the two proteins and identified multiple binding sites on both proteins. One of these sites on NEU1 that is involved in binding to PPCA can also bind to other NEU1 molecules, albeit with lower affinity. Therefore, in the absence of PPCA, as in the lysosomal storage disease galactosialidosis, NEU1 self-associates into chain-like oligomers. Binding of PPCA can reverse self-association of NEU1 by causing the disassembly of NEU1-oligomers and the formation of a PPCA-NEU1 heterodimeric complex. The identification of binding sites between the two proteins allowed us to create innovative structural models of the NEU1 oligomer and the PPCA-NEU1 heterodimeric complex. The proposed mechanism of interaction between NEU1 and its accessory protein PPCA provides a rationale for the secondary deficiency of NEU1 in galactosialidosis.

The effect of coloured light on Ipomoea purpurea growth

Ipomoea purpurea is a climbing ornamental plant native to Mexico. The paper is describing the experimental setup and results for indoor growing plants exposed to white LED light (inside a reference chamber) and four different wavelength LED lights (inside a measure chamber). Four growing experiments of 12–15 days, took place in identical environmental conditions (identical temperature and relative humidity inside the reference and measure chambers, similar lighting conditions and soil moisture). At the end of the experiments, the plant chlorophyll and xanthophylls content have been measured and the plant aspect (vegetal mass, leaves colour and robustness) has been observed. The smallest content in chlorophyll (a and b) was developed by the plants growth in blue light (480 nm), however those plants where 10% taller than plants growth in white light, but less robust. The higher content in carotenoids and xanthophylls was observed in plants which growth in white and red light.

The determination of beam quality correction factors: Monte Carlo simulations and measurements

Modern dosimetry protocols are based on the use of ionization chambers provided with a calibration factor in terms of absorbed dose to water. The basic formula to determine the absorbed dose at a user's beam contains the well-known beam quality correction factor that is required whenever the quality of radiation used at calibration differs from that of the user's radiation. The dosimetry protocols describe the whole ionization chamber calibration procedure and include tabulated beam quality correction factors which refer to 60Co gamma radiation used as calibration quality. They have been calculated for a series of ionization chambers and radiation qualities based on formulae, which are also described in the protocols. In the case of high-energy photon beams, the relative standard uncertainty of the beam quality correction factor is estimated to amount to 1%. In the present work, two alternative methods to determine beam quality correction factors are prescribed—Monte Carlo simulation using the EGSnrc system and an experimental method based on a comparison with a reference chamber. Both Monte Carlo calculations and ratio measurements were carried out for nine chambers at several radiation beams. Four chamber types are not included in the current dosimetry protocols. Beam quality corrections for the reference chamber at two beam qualities were also measured using a calorimeter at a PTB Primary Standards Dosimetry Laboratory. Good agreement between the Monte Carlo calculated (1% uncertainty) and measured (0.5% uncertainty) beam quality correction factors was obtained. Based on these results we propose that beam quality correction factors can be generated both by measurements and by the Monte Carlo simulations with an uncertainty at least comparable to that given in current dosimetry protocols.

SU‐FF‐T‐397: Positioning Recommendations for Parallel‐Plate Ionization Chambers in Clinical Electron Beams According to Different Dosimetry Protocols ‐ a Monte Carlo Study for Three Different Chamber Types

Purpose: Current dosimetry protocols recommend the use of plane‐parallel ionization chambers for the dosimetry of clinical electron beams. Regarding the allowance for the non‐water equivalence of the entrance window and therefore the exact positioning of the parallel‐plate chambers, the recommendations given in these protocols are not unique, resulting in positioning deviations in the range of tenth of millimeter. Method and Materials: The dose and the spectral fluences inside the active volume of three parallel‐plate chamber types (Roos‐, Advanced‐ Markus‐ and NACP‐chamber) were calculated in a water phantom in the reference depth zRef and the half‐value depth R50 for three electron energies (6, 11 and 21 MeV) using the EGSnrc Monte Carlo code. From these results the perturbation correction pwall was calculated as pwall = Dcav/Ddet where Ddet is the dose inside the active volume of the chamber and Dcav the dose inside the air cavity with walls entirely made of water. To inspect the consequences of the different positioning recommendations given in the dosimetry protocols (AAPM TG‐51, IAEA TRS‐398, DIN 6800‐2), the chamber's reference point was shifted by an amount Δz to fulfill the recommendations given in the different protocols. Results: Although the recommended shifts Δz are in the range of tenth of millimeters only, the spectral fluence inside the active volume of the chambers changed strongly with Δz. At the depth zref these spectral fluence changes had no impact on pwall. For all chambers its numerical value is larger than 1, the recommended value given in all protocols. Contrary, at the depth R50 the different positioning recommendations resulted in deviations of pwall up to 15%. Conclusion: The different positioning recommendations have no impact on the results of dose measurements at the clinical relevant depth zref. The resulting deviations in pwall at the depth R50may be of academic interest.

Multiplex pressure measurement in microsystems using volume displacement of particle suspensions

We demonstrate a simple image-based method to measure pressure in microsystems using volume displacement of fluorescent particle suspensions. These micro pressure-sensors are composed of two layers with a poly(dimethylsiloxane) (PDMS) membrane in between: the flow layer includes a flow channel and the sensor layer contains a detection channel filled with suspensions of fluorescent particles. The pressure increase in the flow channel deflects the membrane, and this membrane deformation can be quantified by measuring the cross-sectional areas at specific focal planes. These simple sensors have the advantage that a broad sensing-range can be achieved by tuning the mechanical property and the geometry of the membrane during design and fabrication, and even simpler by tuning the focal plane or the pressure of a reference chamber while in operation. We also demonstrate here a pressure transduction scheme coupled with the image-based sensing method as a multiplex pressure measurement tool for simultaneously detecting pressures in multiple locations in a microsystem. Overall, the image-based pressure sensing method has high precision when operated in both direct and remote detection modes. Compared to conventional mechanical methods of pressure detection, this technique is inexpensive because it does not require complex off-chip equipment to quantify the pressure-dependent membrane deformation. In addition, the image analysis using the software code developed here is fast, and it generates data that are simple to interpret.

TH-D-352-02: New KQ Factors for Reference Dosimetry in High-Energy Photon Beams

Purpose: The AAPM is currently working to provide an update/addendum to the TG‐51 dosimetry protocol. As part of this work measured and calculated kQ factors for a range of thimble‐type ionization chambers not listed in TG‐51 were obtained. The aim was to investigate the accuracy of the calculations and obtain objective evidence on the performance of these chambers for reference dosimetry.Method and Materials: The same formalism and computer programs as employed in TG‐51 were used to calculate kQ factors. As well as 0.6 cm3 reference chambers, factors were calculated for scanning and micro chambers from the major manufacturers. Measurements were made using the 60 Co and Elekta Preciselinac facilities at the National Research Council of Canada. The aim was to characterize the chambers over the range of energies applicable to TG‐51 and determine whether the chamber met the requirements of a reference class instrument. Chamber settling, ion recombination and polarity were investigated and absorbed dose calibration coefficients were obtained for 60 Co and 6, 10 and 25 MV photon beams. Results: As might be expected, 0.6 cm3 thimble chambers showed the most predictable performance and experimental kQ factors were obtained with a relative uncertainty of 0.1%. The performance of scanning and micro chambers was somewhat variable. Some chambers showed very good behaviour and gave reasonable agreement between measured and calculated kQ factors but others showed anomalous polarity and recombination corrections that require further investigation. For the well‐behaved chambers, agreement between measured and calculated kQ factors was within 0.4%. However, for some chambers differences of nearly 1% were seen that may be related to the recombination/polarity issues. Conclusion: Experimental and calculated kQ data have been obtained for a wide range of thimble chambers that can be used by the AAPM and clinical users in choosing suitable detectors for reference dosimetry.

A micromachined microcalorimeter with split-flow microchannel for biochemical sensing applications

This paper reports a micromachined microcalorimeter based on a split-flow microchannel structure for biochemical applications. It is characterized by measuring the reaction heat between biotin and streptavidin. For enhancing its sensitivity, the thermal components of the proposed microcalorimeter are fabricated on a high-thermal resistivity layer, SU-8 in this case, which leads to reduce the parasitic heat transfer to the silicon substrate. For removing additional heating elements, the split-flow microchannel is used. The split-flow microchannel scheme constantly keeps the output of the microcalorimeter near zero level when there is no biochemical reaction; in this state, a heater becomes unnecessary. The microchannel is based on the principle that target biomolecules applied to both the reaction and the reference chambers maintain the chambers at the same temperature, and then capture biochemicals are applied to the reaction chamber on hot junctions, only. Finally, the voltage generated by the biochemical reaction heat in the reaction chamber is measured. In order to verify characteristics of the fabricated sensor, reaction heat due to the biotin–streptavidin reaction was measured. For the experiment, 3.4 mg/ml biotin was used with various streptavidin concentrations of 1.0, 0.5, 0.25, 0.1, and 0.01 mg/ml. Three microliters of titrated biotin and streptavidin of various concentrations were injected into the sensor through a split-flow microchannel. The measurement results show that its sensitivity was 33.7 V/molarity (=0.21 V/cal) as the concentrations of streptavidin.

Compact electrochemical bifunctional NOx/O2 sensor with metal/metal oxide internal reference electrode for high temperature applications

Abstract Simultaneous measurement of total NOx and O2 using two electrochemical methods are demonstrated using metal/metal oxide internal oxygen reference electrode-based sensors at high temperatures. The Pd/PdO-containing reference chamber was sealed within a stabilized zirconia superstructure by a high pressure/temperature plastic deformation bonding method exploiting grain boundary sliding between the ceramic components. Amperometric and potentiometric NOx sensing devices were assembled on the outside of the sensor. Pt-loaded zeolite Y was used to obtain total NOx capability. Both the amperometric and potentiometric type sensors showed total NOx response, with the potentiometric device showing better NOx/O2 signal stability and lower NOx–O2 cross-interference. Since these sensors do not require plumbing for reference air, there is more flexibility in the placement of such sensors in a combustion stream.