Kq Values Research Articles

Extraction, Identification and Photo-Physical Characterization of Persimmon (Diospyros kaki L.) Carotenoids.

Carotenoid pigments were extracted and purified from persimmon fruits using accelerated solvent extraction (ASE). Eleven pigments were isolated and five of them were clearly identified as all-trans-violaxanthine, all-trans-lutein, all-trans-zeaxanthin all-trans-cryptoxanthin and all-trans-β-carotene. Absorption and fluorescence spectra were recorded. To evaluate the potential of 1O2 quenching of the purified carotenoids, we used a monocarboxylic porphyrin (P1COOH) as the photosensitizer to produce 1O2. The rate constants of singlet oxygen quenching (Kq) were determined by monitoring the near-infrared (1270 nm) luminescence of 1O2 produced by photosensitizer excitation. The lifetime of singlet oxygen was measured in the presence of increasing concentrations of carotenoids in hexane. Recorded Kq values show that all-trans-β-cryptoxanthin, all-trans-β-carotene, all-trans-lycopene and all-trans-zeaxanthin quench singlet oxygen in hexane efficiently (associated Kq values of 1.6 × 109, 1.3 × 109, 1.1 × 109 and 1.1 × 109 M−1·s−1, respectively). The efficiency of singlet oxygen quenching of β-cryptoxanthin can thus change the consideration that β-carotene and lycopene are the most efficient singlet oxygen quenchers acting as catalysts for deactivation of the harmful 1O2.

Measurements of charge density on powdery or granular media using the “open” Faraday's cage

Application of the measurements of a charge induced on the flat electrode placed at the bottom of a container, for the determination of total accumulated charge is discussed in the paper. The value of correction factor KQ, important for the numerical linking of both (accumulated and induced) charges, was determined numerically and experimentally. The values of KQ factors determined by various methods differ as 1:1.7, however application of the method of "open" Faraday cage in technological conditions is fully justified.

Nonadiabatic Renner-Teller quantum dynamics of OH(X2Π) + H+ reactive collisions.

Following previous studies on the O(3P) + H2+(X2Σg+) collisions, we present the nonadiabatic quantum dynamics of the reactions OH(X2Π) + H'+ → OH'(X2Π) + H+, exchange (e), → OH+(X3Σ-) + H'(2S), quenching (q), and → OH'+ (X3Σ-) + H(2S), exchange-quenching (eq). The reactants and products correlate via the ground X[combining tilde]2A'' and first excited Ã2A' electronic states of OH2+, which are the degenerate components of linear 2Π species. Therefore, they are strongly perturbed by nonadiabatic Renner-Teller (RT) effects, opening the (q) and (eq) channels that are closed in the Born-Oppenheimer approximation. Using accurate potential energy surfaces (PESs) and RT matrix elements, initial-state-resolved reaction probabilities, real-time dynamics, cross sections, and rate constants of the product channels are obtained through the time-dependent real wavepacket (WP) method and full coupled-channel calculations. Owing to the nonadiabatic couplings, the WP jumps from the excited Ã2A' surface to the X[combining tilde]2A'' ground PES, avoiding any barrier, opening the quenching channels, and giving many collision complexes into the deep minima of both PESs, as it is clearly shown by the oscillations of the reaction probabilities and by the time-dependent WP dynamics. All the results show that the nonadiabatic-RT channels (q) and (eq) are highly reactive, much more than the adiabatic one (e), pointing out large RT effects. The reactivity of the quenching channels is similar, accounting for 97% of the overall reactivity. In fact, the maximum values of the (q) and (eq) cross sections σq and σeq are equal to 31.6 Å2, whereas the maximum σe value equals 1.34 Å2, and the maximum values of the rate constants kq, keq, and ke are 2.07 × 10-10, 2.45 × 10-10, and 0.23 × 10-10 cm3 s-1. Some calculations show that the centrifugal-sudden and the truncated coupled-channel approximations cannot be employed for the (q) channel. After a sharp increase at the threshold, σq and σeq decrease at larger collision energies while σe and the rate constants depend slightly on the collision energy and temperature, respectively. These findings are consistent with the barrierless nature of both PESs and the exoergicity of the quenching products, with the small role played by the centrifugal and RT barriers in the reactant channel, and with the large RT couplings in the OH2+ intermediates. Finally, we contrast the present results with those for the opposite reactions O + H2+ and for the nonadiabatic-induced quenchings NH + H' and OH + H'.

Screening and Binding Analysis of Flavonoids with Alpha-Amylase Inhibitory Activity from Lotus Leaf

Lotus leaf is gaining growing popularity due to its various benefits and widely usage. In this paper, ten flavonoids in lotus leaf extract were analyzed by high performance liquid chromatography (HPLC). Centrifugal ultrafiltration combined liquid chromatography was used to screen alpha-amylase inhibitors from ten flavonoids mixture and the binding degrees ranged from 2.34 to 94.1%. The alpha-amylase inhibition and the 1,1-diphenyl-2-picrylhydrazyl (DPPH) antioxidant activity of ten flavonoids were verified as well. Apigenin, kaempferol and isorhamnetin with relatively higher binding degrees showed higher inhibition on alpha-amylase as well. The interactions between these flavonoids and alpha-amylase were investigated by spectroscopic method. As a result, the fluorescence quenching could be considered as static quenching because the quenching rate constant (Kq) values were higher than 2.0 × 1010 L mol -1 s-1. The binding constants (log10 Ka) of ten flavonoids were in the range of 5.971 to 7.417 L mol-1. The hydrogenation of C2=C3 double bond of apigenin and quercetin decreased the affinity for alpha-amylase (1.92- and 2.82-fold). The hydroxylation on 3 and 3' position decreased the affinity for alpha-amylase. Moreover, the glycosylation with different sugar moiety improved in varying degrees of affinity. The hydrogen bond force might be important in the binding between alpha-amylase and flavonoids.

A Mechanism of Synergistic Effect of Streptomycin and Cefotaxime on CTX-M-15 Type β-lactamase Producing Strain of E. cloacae: A First Report

A blaCTX-M-15 gene is one of the most prevalent resistant marker found in member of enterobacteriaceae. It encodes cefotaxime hydrolysing β-lactamase-15 (CTX-M-15) causing resistance against beta lactam antibiotics. Since single antibiotic therapy fails to control infection caused by multidrug resistance strain, therefore combination therapy was came into practice as an effective treatment. We have first time explained the mechanism where two antibiotics of different classes work against resistant strains. Binding parameters obtained by spectroscopic approach showed significant interaction and complex formation between drugs and CTX-M-15 enzyme with decreased ksv and kq values. CD analysis showed altered conformation and significant changes in alpha helical content of CTX-M-15 enzyme on interaction with streptomycin in combination with cephalosporin. Steady state kinetics revealed decrease in hydrolytic efficiency of enzyme to about 27% by cooperative binding behavior upon sequential treatment of enzyme with streptomycin and cefotaxime. Therefore, the study concludes that combination therapy against CTX-M-15 producing strain with Cefotaxime/Streptomycin in 1:10 molar ratio, decreases CTX-M-15 efficiency significantly because of the fact that streptomycin induced structural changes in CTX-M-15 hence cefotaxime was not properly bound on its active site for hydrolysis rather available for the target to inhibit bacterial cells.

A Spectroscopic Approach with Theoretical Studies to Study the Interaction of 9-aminoacridine with Certain Phenols

Abstract The influence of phenols upon the fluorescence quenching of 9-aminoacridine (9-AA) was examined in acetonitrile solution by employing steady state and time-resolved fluorescence measurements. On increasing the concentration of quencher molecules the absorption spectra of 9-AA change with significant bathochromic shift. The fluorescence intensity of 9-AA change in presence of quencher molecules were measured at various temperatures as a function of the quencher concentrations. The observed bimolecular quenching rate constant (kq) depends on the nature and electronic effect of substituent present in the quencher molecules. The bimolecular quenching rate constant (kq) decreases on increasing the oxidation potential of quencher molecules. To examine the quenching behavior, kq values were correlated with the free energy change (ΔG). To get forthcoming in the quenching process, fluorescence quenching experiments were carried out in different solvents of varying polarities. The observed result suggest the involvement of charge-transfer quenching mechanism. Lifetime measurements support static quenching. Further, the radical scavenging potential is calculated from density functional theory (DFT) calculations to address the quenching behavior of the quencher molecules. DFT result reveals that electronic features are important in tuning the quenching ability of the quencher molecules and found to agree with the obtained experiment result.

Binding studies of lophirone B with bovine serum albumin (BSA): Combination of spectroscopic and molecular docking techniques

The interaction between the transport protein bovine serum albumin (BSA) and the natural product lophirone B, was investigated by spectroscopic techniques combined with a computational method (molecular docking). From the KSV and kq values it was concluded that lophirone B quenches the fluorescence of BSA by dynamic and static mechanisms. The Ka values, of the order of 104 M−1, and the number of binding sites (n ≈ 1), indicate that the binding is moderate and there is just one main binding site in BSA for lophirone B. The negative ΔG° values are in accordance with the spontaneity of the process and the positive ΔH° and ΔS° values indicate that the binding is entropically driven; the main binding forces for the association BSA:lophirone B are probably lipophilic interactions. Circular dichroism (CD) studies show there is not a significant perturbation on the secondary structure of the albumin upon the binding process. In order to better understand the spectroscopic results, a computational method was applied: molecular docking suggests Trp-213 site, as the main binding site for the ligand. Lophirone B seems to be exposed to the aqueous media as well as accommodated inside the protein cavity, resulting in a moderate affinity for the albumin. The Arg-198, His-287, Lys-294 and Lys-439 residues are interacting via hydrogen bonding with lophirone B, whereas the interaction with Trp-213 residue occurs through a lipophilic interaction.

Praseodymium Doped Ceria Oxygen Surface Exchange Coefficient and Film Stress Measurements Via the Curvature Relaxation Technique

Introduction Praseodymium doped ceria (PCO) is a mixed ionic electronic conductor (MIEC) that can be used as a cathode material in solid oxide fuel cells (SOFCs). Under SOFC operating conditions, PCO undergoes the following reaction [1]: ½ O2 + 2Pr’ Ce + VӦ à 2PrCe + Oo x The kinetics of this oxygen exchange reaction are represented by the oxygen surface exchange coefficient (kchem ). In this work, simultaneous kchem and stress measurements on PCO thin films atop yttria-stabilized zirconia (YSZ) substrates were performed from 625-725 oC using the curvature relaxation (κR) technique. Experimental Methods Phase pure Pr0.1Ce0.9O1.95 powder was prepared through glycine nitrate combustion and subsequent powder calcination in air. (001) oriented 9.5% YSZ substrates (Crystec, GmbH) were pre-annealed at 1450oC for 20 hours. PCO thin films were sputtered onto YSZ substrates at room temperature and then sintered for 1 hour at 1100oC. The bilayer structures were tested using a multi beam optical stress sensor (MOSS) from 625oC to 725oC with 25oC increments. The κR technique determines kchem by fitting a solution of Fick’s 2nd Law to the curvature response of mechano-chemically active film | inert substrate bilayers reacting to sudden changes in oxygen partial pressure (synthetic air (21% O2 - 79% Ar) and 10% synthetic air - 90% Ar) [2,3]. Film stress values were calculated using Stoney’s equation [2,3]. In order to compare the kchem values with those in the literature, electrically-measured PCO kq values from the literature [4] were turned into kchem values by multiplying them by the corresponding PCO thermodynamic factor from Ref [5]. Results Figure 1 shows how the kchem value measured via the κR technique compares with literature [4,5] values. Conclusions The PCO kchem and activation energies measured here are significantly different than those reported in the literature. Although additional work is needed before these effects can be explained in full, these discrepancies may be caused by the different synthesis techniques (sputtering vs. pulsed laser deposition) used to produce the two films shown in Figure 1. Acknowledgements This material is based upon work supported by the Department of Energy under Award Number DE-FE0023315.

Effect of quencher and temperature on fluorescence intensity of laser dyes: DETC and C504T

Fluorescence quenching of 7- Diethylamino-3-thenoylcoumarin (DETC) and 2,3,6,7-tetrahydro-1,1,7,7-tetramethyl11-oxo-1H,5H,11H- [1]benzopyrano[6,7,8-ij]quinolizine-10-carboxylic acid, ethyl ester (C504T) by aniline(AN), dimethylaniline (DMA) and diethylaniline (DEA) was investigated in toluene by steady state and transient methods. The quenching parameters like frequency of encounter (kd), probability of quenching per encounter (p), quenching rate parameters (kq) and activation energy of quenching (Ea) were determined experimentally. The kq values determined by steady state and time-resolved methods for the both dyes were found to be same, indicating the dynamic nature of interaction. Magnitudes of p and Ea suggested that the quenching reaction is predominantly controlled by material diffusion. The quenching mechanism is rationalized in terms of electron transfer (ET) from donors (aromatic amines) to the acceptors (coumarin derivatives) confirmed by correlating kq with free energy changes (ΔG°). Further, an effect of temperature on fluorescence intensity was carried out in toluene and methanol solvents. Fluorescence intensity of both the dyes decreases with increase in temperature. Temperature quenching in case of C504T is due to intersystem crossing S1→T2, whereas for DETC, quenching is due to intersystem crossing S1→T2 and ICT→TICT transition.

Improved fracture behavior and microstructural characterization of thin tungsten foils

This study is focused towards the development of the technique for investigating the fracture behaviour of 100µm thick rolled tungsten foils, with a purity of 99.97%. Electron backscatter diffraction (EBSD) scans reveal that the grains are elongated along the rolling direction of the foil, which has a very strong {100}<011> texture. The test specimens were fabricated by electrical discharge machining (EDM) and cracks were initiated by consecutively using a diamond wire saw, a razor blade and a focused ion beam (FIB) workstation. Fracture experiments were performed at temperatures from −196°C to 800°C. The investigation of fracture appearance shows an improved behavior and significantly higher values of conditional fracture toughness Kq compared to bulk W-materials, which can be related to a higher degree of deformation during the production process. A high toughness at room temperature (RT) and 200°C, slowly decreases when approaching the highest testing temperature of 800°C. The most significant result reveals that the ductile to brittle transition temperature (DBTT) is around RT, which is an extraordinary result for any tungsten material. The fracture surfaces, investigated with a scanning electron microscope (SEM), show a transition from cleavage fracture at liquid nitrogen temperature, through pronounced delamination within the foil plane at ambient temperatures to ductile fracture at the highest testing temperatures.

SU‐F‐T‐488: Comparison of the TG‐51 and TG‐51 Addendum Calibration Protocols

Purpose:To quantify differences between the TG51 and TG51 addendum calibration protocols.Methods:Beam energies of 6X, 6XSRS, 10X, 15X, 23X, 6XFFF, and 10XFFF were calibrated following both the TG51 and TG51 addendum protocols using both a Farmer and a scanning ionization chamber with traceable absorbed dose‐to‐water calibrations. For the TG51 addendum procedure, the collimating jaws were positioned to define a 10×10cm2 radiation field, a lead foil was only used for kQ measurements of FFF energies, and a volume‐averaging correction was applied based on crossline and inline dose profiles. For the TG51 procedure, the collimating jaws were set to 10×10cm2 according to the digital readout, and a lead foil was used for kQ measurements of energies greater than 10MV.Results:For beam energies with a flattening filter, absorbed dose‐to‐water determined by the two protocols differed by 0.1%–0.3%. For FFF beam energies, differences between the protocols were up to 0.2% and 0.8% for the scanning and Farmer ionization chambers, respectively. Differences between the protocols were due to kQ determination, volume‐averaging correction, and measurement of raw ionization. Differences in kQ values between the two protocols were up to 0.4% and 0.2% for the scanning and Farmer ionization chambers, respectively. Volume‐averaging corrections were less than 0.1% for the scanning ionization chamber, and up to 0.4% and 0.6% for the Farmer ionization chamber in beams with a flattening filter and FFF beams, respectively. Raw ionization measurements differed up to 0.3%±0.07% due to differences in jaw settings.Conclusion:The TG51 and TG51 addendum calibration protocols differed less than 0.3% for the scanning ionization chamber. For the Farmer chamber in FFF energies, volume‐averaging corrections of up to 0.6% contributed to calibration differences of up to 0.8%. Failure to verify the radiation field size can produce calibration differences of up to 0.3%.

TH-CD-BRA-05: First Water Calorimetric Dw Measurement and Direct Measurement of Magnetic Field Correction Factors, KQ,B, in a 1.5 T B-Field of An MRI Linac

Purpose: Reference dosimetry in MR-guided radiotherapy is performed in the presence of a B-field. As a consequence the response of ionization chambers changes considerably and depends on parameters not considered in traditional reference dosimetry. Therefore future Codes of Practices need ionization chamber correction factors to correct for both the change in beam quality and the presence of a B-field. The objective was to study the feasibility of water calorimetric absorbed-dose measurements in a 1.5 T B-field of an MRLinac and the direct measurement of kQ,B calibration of ionization chambers. Methods: Calorimetric absorbed dose to water Dw was measured with a new water calorimeter in the bore of an MRLinac (TPR20,10 of 0.702). Two waterproof ionization chambers (PTW 30013, IBA FC-65G) were calibrated inside the calorimeter phantom (ND,w,Q,B). Both measurements were normalized to a monitor ionization chamber. Ionization chamber measurements were corrected for conventional influence parameter. Based on the chambers’ Co-60 calibrations (ND,w,Q0), measured directly against the calorimeter. In this study the correction factors kQ,B was determined as the ratio of the calibration coefficients in the MRLinac and in Co-60. Additionally, kB was determined based on kQ values obtained with the IAEA TRS-398 Code of Practice. Results: The kQ,B factors of the ionization chambers mentioned above were respectively 0.9488(8) and 0.9445(8) with resulting kB factors of 0.961(13) and 0.952(13) with standard uncertainties on the least significant digit(s) between brackets. Conclusion: Calorimetric Dw measurements and calibration of waterproof ionization chambers were successfully carried out in the 1.5 T B-field of an MRLinac with a standard uncertainty of 0.7%. Preliminary kQ,B and kB factors were determined with standard uncertainties of respectively 0.8% and 1.3%. The kQ,B agrees with an alternative method within 0.4%. The feasibility of water calorimetry in the presence of B-fields was demonstrated by the direct determination of Dw and kQ,B. This work was supported by EMRP grant HLT06. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

Phosphorescence quenching of neutral and cationic iridium(III) complexes by molecular oxygen and aromatic electron acceptors

Phosphorescence quenching of neutral and cationic Ir(III) complexes by molecular oxygen and aromatic electron acceptors (AEAs) have been systematically investigated in acetonitrile. The phosphorescence quenching rate constant (kq) by AEAs increased with decreasing Gibbs energy change (ΔGel) of electron transfer and gave a diffusion-controlled rate constant (kd) of 2.0 × 1010 M−1s−1. The occurrence of electron transfer was substantiated by transient absorption measurements, although the separated ion yields were as low as 0.10–0.17. The oxygen quenching of the Ir(III) complexes originated mainly from two competing pathways through singlet and triplet encounter complexes: a noncharge-transfer (nCT) channel, which is regarded as internal conversion to produce singlet oxygen (1O2), and a charge-transfer (CT) channel, whose rate is governed by ΔGel. The kd value for the Ir(III) complex/O2 systems was estimated to be 4.1 × 1010 M−1s−1. For the systems with ΔGel ≲ −0.8 eV, the limiting kq value was 2.5 × 1010 M−1s−1 and the efficiencies (fΔ) of 1O2 production from triplet states quenched by oxygen were 0.40. The limiting kq value being greater than (4/9)kd and the fΔ value being larger than 0.25 suggested the involvement of 1O2 formation through intersystem crossing between singlet, triplet, and quintet CT complexes.

Fracture Toughness of Explosively Welded Al/Ti Layered Material in Cryogenic Conditions

In the paper results of comparative analysis of the fracture toughness of explosively welded Al/Ti composite determined for the ambient temperature and for the cryogenic conditions were presented. Testing of mechanical properties of inhomogeneous materials very often presents a lot of difficulties connected with the experimental procedures as well as with the interpretation of study results. It is also in the case of layered Al/Ti material, for which cracking history has different nature for both layers: aluminium and titanium alloy. However, from the viewpoint of practical application, the knowledge of ‘global’ material properties is important. Due to differences of properties of both materials constituting the Al/Ti bimetal, it is hard to talk about determination of material property, which is plane strain fracture toughness K1C. In such case more adequate is fracture toughness KQ value, which can refers to the system of two materials. In investigations compact tension (CT) specimens were used. For specimens used in the tests the ratio of width to thickness W/B was 4. The initial pre-cracks were made by cyclic axial loading of specimens. The experimental tests were made with the use of the axial servohydraulic test system equipped with an original cryogenic chamber. During the tests all parts including specimen, clevis grips, extensometer were immersed in liquid nitrogen. The influence of cryogenic temperature on the fracture toughness was observed as a result of the tests.

Influence of the Protonatable Site in the Photo-Induced Proton-Coupled Electron Transfer between Rhenium(I) Polypyridyl Complexes and Hydroquinone

In the present work the influence of the distance of the protonatable site of different ancillary ligands to the metal center on the luminescence quenching of ReI polypyridyl complexes by hydroquinone are evaluated by means of experimental and theoretical studies. In these systems, it is expected the occurrence of proton-coupled electron transfer (PCET) reactions upon excitation, which is a key process in solar-to-fuels energy conversion. The series fac-[Re(CO)3(2,2-bpy)(L)]PF6, L = pyridine, 1,4-pyrazine, 4,4'-bipyridyl, 1,2-bis-(4-pyridyl)ethane were synthesized and the luminescence quenching rate constant (kq) by hydroquinone in CH3CN and 1:1 CH3CN/H2O were determined by steady-state and lifetime measurements. In bare acetonitrile, the 1,4-pyrazine exhibits the higher kq (3.49 ± 0.02) × 109 L mol-1 s-1 among the species investigated, followed by 4,4'-bipyridyl (kq = 2.50 ± 0.02) × 109 L mol-1 s-1. In 1:1 CH3CN/H2O, the kq values for all complexes are very similar evidencing the role of water molecules as proton acceptor following the reductive quenching of the complexes by hydroquinone. In CH3CN, the proton release for the solvent is not spontaneous and the higher basicity of the coordinated 1,4-pyrazine and 4,4'-bipyridyl in relation to 1,2-bis-(4-pyridyl)ethane after metal-to-ligand charge transfer (MLCT) excitation contributes to the proton transfer step. These results are corroborated by time-dependent density functional theory (TD-DFT) calculations. Moreover, the low H/D kinetic isotope effect (KIE) in 3:1 CH3CN/X2O (X = H or D) confirms that the major PCET pathway is the electron transfer followed by proton transfer, but for 1,4-pyrazine and 4,4'-bipyridyl the concerted proton-electron transfer seems to play a role at high hydroquinone concentrations.

Determination of kQ using MLC-collimated rectangular fields for absolute dosimetry of the CyberKnife.

Traditional CyberKnife (CK) calibration uses TG‐51, which requires kQ to be defined using the standard reference condition of 100 cm SSD in a 10 cm×10 cm field. Since the CK is calibrated using a 6 cm fixed‐aperture collimating cone at 80 cm SAD, the BJR‐25 method is commonly used to relate circular‐field PDDs to square‐field PDDs for kQ determination. Using the InCise MLC system, the CK is able to deliver rectangular fields, allowing a more direct measurement of %dd(10 cm) using conventional reference conditions. We define the PDD correction factor (CPDD) as the ratio of %dd(10 cm) measured using CK reference conditions to that measured using standard TG‐51 reference conditions. Using four ionization chambers (A1SL, CC08, CC13, and A19), %dd(10 cm) is measured using a 6 cm fixed cone at 80 cm SSD and at 100 cm SSD using an effective 10 cm×10 cm MLC‐collimated field. These values are used to calculate CPDD, while the latter is used to directly calculate a kQ value. This direct kQ value is then compared to values determined using the BJR‐25 method. Using the MLC system, this study demonstrates conversion between the %dd(10 cm) measured using CyberKnife reference conditions and TG‐51 reference conditions. These values provide the means for derivation of a kQ curve as a function of direct measurements of %dd(10 cm) using a 6 cm fixed‐aperture collimating cone at 80 cm SSD.PACS number: 87.55.Qr

Point/Counterpoint. Calibration of radiotherapy ionization chambers using Co-60 is outdated and should be replaced by direct calibration in linear accelerator beams.

Point/Counterpoint. Calibration of radiotherapy ionization chambers using Co-60 is outdated and should be replaced by direct calibration in linear accelerator beams.

Notable Effects of the Metal Salts on the Quenching Reaction of Singlet Oxygen by α-Tocopherol in Ethanol Solution

Abstract Measurement of the quenching rate constant (kQ) of singlet oxygen (1O2) by α-tocopherol was performed in ethanol solution including four kinds of alkali and alkaline earth metal salts (NaClO4, Mg(ClO4)2, MgCl2, and CaCl2) by varying concentrations of metal salts. A remarkable effect of metal salts on the kQ value of α-tocopherol was observed. The 1O2-quenching rate constant (kQ) decreased notably with increasing concentrations of metal salts, and reached a constant value for each salt at high concentration of the salt. For example, the kQ value (9.48 × 107 M−1 s−1) obtained in the presence of 20 mM of Mg(ClO4)2 was 46% of that (2.06 × 108 M−1 s−1) in the absence of metal salt. The kQ value decreased in the order of no metal salt &amp;gt; NaClO4 &amp;gt; CaCl2 &amp;gt; MgCl2 ≈ Mg(ClO4)2 at the same concentration of the metal salts. The metal salts having a smaller ionic radius and a larger charge of the cation gave a smaller kQ value. Effects of anions were negligible. These metal cations coexist with α-tocopherol in foods and biological systems, suggesting that the metal cations may give a notable effect to the 1O2-quenching rate constant (kQ).

A radiation quality correction factor k for well-type ionization chambers for the measurement of the reference air kerma rate of (60)Co HDR brachytherapy sources.

The aim of this study was to investigate whether a chamber-type-specific radiation quality correction factor kQ can be determined in order to measure the reference air kerma rate of (60)Co high-dose-rate (HDR) brachytherapy sources with acceptable uncertainty by means of a well-type ionization chamber calibrated for (192)Ir HDR sources. The calibration coefficients of 35 well-type ionization chambers of two different chamber types for radiation fields of (60)Co and (192)Ir HDR brachytherapy sources were determined experimentally. A radiation quality correction factor kQ was determined as the ratio of the calibration coefficients for (60)Co and (192)Ir. The dependence on chamber-to-chamber variations, source-to-source variations, and source strength was investigated. For the PTW Tx33004 (Nucletron source dosimetry system (SDS)) well-type chamber, the type-specific radiation quality correction factor kQ is 1.19. Note that this value is valid for chambers with the serial number, SN ≥ 315 (Nucletron SDS SN ≥ 548) onward only. For the Standard Imaging HDR 1000 Plus well-type chambers, the type-specific correction factor kQ is 1.05. Both kQ values are independent of the source strengths in the complete clinically relevant range. The relative expanded uncertainty (k = 2) of kQ is UkQ = 2.1% for both chamber types. The calibration coefficient of a well-type chamber for radiation fields of (60)Co HDR brachytherapy sources can be calculated from a given calibration coefficient for (192)Ir radiation by using a chamber-type-specific radiation quality correction factor kQ. However, the uncertainty of a (60)Co calibration coefficient calculated via kQ is at least twice as large as that for a direct calibration with a (60)Co source.

TU-F-CAMPUS-T-01: Calculation of KQ for a Variety of Commercially Available Ionization Chambers in the Presence of An External Magnetic Field for MR-Linac Dosimetry

Purpose: The strong magnetic fields associated with MRI have a significant impact on the dosimetry of radiotherapy beams as well as on the response of the detectors used to measure it. In order to calibrate the dose delivered by these beams, it is necessary to correct for these effects when performing measurements with ionization chambers. Methods: Detailed models of 16 commercially-available ionization chambers were implemented in Geant4. To validate these models, calculations of the beam quality correction factor kQ (defined by TG-51) were performed using the spectrum from a 6 MV Elekta SL25. kQ values were then recalculated for each detector with a 1.5 T external magnetic field applied uniformly across the geometry. Chamber, beam and B-field were all orthogonal. Results: The comparison between the kQ calculations and TG-51 showed agreement within 1.4%. The B-field calculations indicate that the change in kQ values due to the B-field is strongly affected by the volume and material of the ionization chamber. All large volume (>0.5 cm 3) ionization chambers had lower kQ values when used in a B-field. The kQ values for small volume chambers were very sensitive to the chambers’ collecting volume and could be higher or lower when used in a B-field. Conclusions: Of the chambers tested, the PTW 30011 demonstrated the smallest correction of just 1.2%. The next step is to calculate kQ using the energy spectrum from an actual MR-linac. Further validation of these results against measurements will be paramount before they can be used. Funding provided by Elekta Limited. The MR-linac spectrum will be provided by Elekta Limited based on a machine they have designed.