Selfcalibrated alanine/EPR dosimeters: A new generation of solid state/EPR dosimeters

Alanine/EPR dosimeters are well established as secondary, reference dosimeters for high-energy radiation. However, there are various sources of uncertainty in the evaluation of absorbed dose. This arises primarily from the necessity to calibrate each EPR spectrometer and each batch of dosimeters before their use. In order to overcome this disadvantage, a new generation alanine/EPR dosimeter has been developed, and its possibilities as a radiation detector are reported. Principally, it is a mixture of alanine, some quantity of EPR active substance, and a binding material. The EPR active substance, acting as an internal EPR standard, is chosen to have EPR parameters which are independent of the irradiation dose. The simultaneous recording of the spectra of both the sample and the standard under the same experimental conditions and the estimation of the ratioI _alanine/I _Mn as a function of the absorbed dose strongly reduces the uncertainties. The response of these dosimeters for⁶⁰Co γ-radiation exhibits excellent linearity and reproducibility in the range of absorbed dose, 10²−5·10⁴ Gy.     

An international intercomparison on “self-calibrated” alanine EPR dosimeters

The results, obtained by six independent electron paramagnetic resonance (EPR) laboratories, of the dose response coefficients (K_dr) of “self-calibrated” solid-state EPR dosimeters containing alanine as a radiation-sensitive material and Mn²⁺/MgO as an internal reference material, are reported. The intercomparison trial was divided into three steps. It started with the distribution of dosimeters among the participating EPR laboratories with the purpose of irradiating them with known doses of γ-rays and to estimate the K_dr. The percentage standard deviation (PSD) of the K_dr obtained at individual labs was in the range of 1.4–4.6%. The interlaboratory PSD of the K_dr was 8%, primarily pointing to variations in irradiation procedures and EPR spectrometer settings. Further investigations showed that the main source of the interlaboratory PSD is differences in the calibrations of irradiators and settings of EPR acquisition parameters. In order to provide reproducible estimates of the K_dr, low microwave power and modulation amplitude using a combination of sweep time and time constant that gives a distortion-free EPR spectrum should be utilized. In the third step following such a procedure, measuring the same irradiated alanine dosimeter at the respective laboratories, spectrometers (12 instruments of 6 different models and 3 producers) and 10 operators gave an interlaboratory PSD of 3.1%. In conclusion, EPR dosimetry using “self-calibrated” alanine dosimeters may be used as a secondary standard, although a careful calibration of the EPR spectrometer must be performed in order to further reduce the uncertainty.     

Solid state "self-calibrated" EPR-dosimeters-advantageous and shortcomings

EPR dosimetry method with alpha-alanine as radiation sensitive material (RSM) is widely used in high dose dosimetry laboratories. However, it is not suitable for routine industrial applications mainly because of difficult EPR measurement procedure. In order to simplify quantitative EPR dosimetry measurements Yordanov and Gancheva developed so-called "self-calibrated" (sc) dosimeters consists of RSM (alpha-alanine, sugar, other ones), Mn2+/MgO as internal EPR intensity standard (IES) and a binder. The aim of this work was to check dosimetric properties of two experimental batches of sc EPR dosimeters with alpha-alanine and sugar as RSM, Mn2+/MgO as IES and paraffin as a binder. The percentage content of the components was 60, 5 and 35% (w/w), respectively. It was established that the investigated alanine sc-dosimeters are about two times more sensitive than the sugar ones. The dose-response coefficient, K(dr) of sc-alanine dosimeters was stable in all investigated dose range from 1 to 23 kGy. The K(dr)-value of sugar sc-dosimeters decreased with the dose what was in a contradiction to the results pointed to the high stability of radiation generated sugar radicals. The observed effect arose probably from the special chemical procedure used for the sc-sugar dosimeters production. The results confirmed an expectation that the position of sc dosimeter in the cavity is not important factor for accurate dose evaluation. It allows to read-out dosimetric signals in shorter time, with lower uncertainty and on less sophisticate EPR-spectrometers than that commonly used till now. The main shortcomings of sc dosimeters are: (a) the limitation of RSM suitable for sc dosimeters to these ones having strictly linear signal to dose characteristic; (b) necessity to assure very good homogeneity of dosimeter material; and (c) the cost of IES present in the amount of some percent in each sc dosimeter.     

A thin alanine-polyethylene film dosimetry system with diffuse reflection spectrophotometric evaluation

Characteristics of a new alanine dosimeter in the shape of a thin film, with the measurement of optical absorption of the CH₃CHCOO^– radical is described. That type of dosimeter, ALA/DRS (for diffuse reflection spectrophotometry) is compared, to an alanine dosimeter with EPR evaluation (ALA/EPR for short). In many respects the simple ALA/DRS version, as the alanine-polyethylene composite is superior. The paper shows the importance of the new experimental approach to free radical research in solid state radiation chemistry.     

Magnetic properties of the mixed-valence compounds CaMn3O6 and CaMn4O8

The preparation of two new mixed-valence compounds, CaMn₃O₆ and CaMn₄O₈, are described and their magntic and EPR behavior investigated. The Mn moments in both compounds have nearly spinonly values. CaMn₃O₆ and CaMn₄O₈ order ferrimagnetically near 3 and 89 K, respectively. The broad, Lorentzian EPR lines indicate a significant exchange interaction between Mn³⁺ and Mn⁴⁺ ions. The magnetic and EPR results suggest a strong ferromagnetic interaction between Mn³⁺ and Mn⁴⁺ ions and a comparable antiferromagnetic Mn³⁺Mn³⁺ and/or Mn⁺Mn⁴⁺ interaction.     

Properties of the ammonium tartrate/EPR dosimeter

The EPR response of γ-irradiated ammonium tartrate on the absorbed dose of γ-rays up to 22 kGy as well as the changes in the shape of the EPR spectrum upon applied modulation amplitude and microwave power are reported. Also the possibility to use ammonium tartrate together with Mn²⁺ magnetically diluted in MgO as an internal reference material is evaluated. The influence of the microwave power and the modulation amplitude on their dose response is investigated. The results show that the radiation-induced EPR spectrum of ammonium tartrate, obtained at a low microwave power is complex consisting several patterns and is more easily saturated than the Mn²⁺ EPR spectrum. In this case the following settings of the EPR parameters are recommended: H_mod⩽0.05 mT and 10⩽P_MW⩽13 mW. Using these parameters the dosimeters can be considered for use in intercomparisons.     

Synthesis and Crystal Structure of [Mn2(H2sal)2(Hsal)2(H2O)4]. First Example of the Reductive Synthesis of a Binuclear Manganese(I) Salicylate Complex

The synthesis and characterization of the first unique binuclear manganese(I) salicylate complex is described. The structure of the complex is unequivocally established with the help of an X-ray crystallographic study. The structure consists of two [MnO₆] units, containing octahedral Mn^I ions, linked together by a bridging salicylato (Hsal⁻) ligand and each Mn^I is chelated with a (H₂sal) ligand. The complex possesses a metal oxidation state of +1 and is a rare example of a noncarbonyl or cyanide complex of a binuclear Mn^I species. The effective magnetic moment per binuclear molecule (μ_eff. at 27 °C is 8.07 μ_b, that also describes the manganese (+1) oxidation state.     

A new biochemical radiation dosimeter

Summary A bilirubin-chloroform solution was tested as a gamma-radiation dosimeter (SALPILL dosimeter) in the 0-100 Gy range, and at dose rates between 0.01 to 3.18 Gy ^. min^-1, which displayed certain advantages over the conventional Fricke, TLD and diode dosimeters when examined under identical experimental conditions. The principle of operation involved gamma-irradiation of micro-molar quantities of the unconjugated specimen with a ¹³⁷Cs source (662 keV gamma-rays), and measurement of the (degraded) bilirubin absorption at 453 nm. The relationship of bilirubin depletion and radiation dose was linear, which remained invariant with oxic and anoxic exposure, denoting excellent reproducibility under diverse experimental conditions. Further validation of performance was achieved by repeated in-air trials, which produced a reproducibility within ±2% (n = 5). Stringent comparative tests conducted against currently accepted gamma-radiation dosimeters favoured the SALPILL dosimeter in all the relevant areas. The merits of using chloroform as a solvent in place of water was considered. The SALPILL dosimeter has the following distinctive features: prolonged “shelf-life” (before and after irradiation), insensitive into oxygen, operational at relatively low dose rates, linear functionality at low doses (0-5 Gy), solvent stability, solute integrity, reliability, convenient and cost-effective. The drawbacks of SALPILL are minimal, which makes it a facile dosimeter for certain applications.     

Powder state alanine ESR dosimeter for measuring the absorbed dose of electron beam

This paper describes the application of powder state alanine/ESR dosimeter for measurement of the absorbed dose of electron beam, for transfer the dose standards and for quality controls of the products processed by electron beam irradiation.The dosimeter is a sealed plastic container containing pre-treated alanine powder. Spectra of the internal standard and the alanine sample are measured simultaneously by using dual - cavity of a ESR spectrometer. The internal standard is a CuCl₂. 2H₂O monocrystal which gives stable ESR signals. A diamond sample is regard as working standard. With these two standards, the measurement accuracy can be improved apparently. The standard dose value is determined with a electron beam calorimeter made in our laboratory for dose calibration purpose.The advantages and the dosimetry characteristics of the application of powder state alanine/ESR dosimeters are discussed. This method is proved accurate and easy to use. In the region of 10-10⁴Gy, the dose response show a linear relationship and the precision is better than ± 2%.     

A new label dosimetry system

A new label dosimeter which changes its color by large radiation doses has been developed. The green color of the unirradiated dosimeter gradually turns to brown then to red at high doses. The label dosimeter was prepared with a peal-off paper backing, allowing it to stick by self-adhesion to a product box. Three types of dosimeters having different sensitivities to radiation doses were prepared. Correlations were established between absorbed doses and color scale or the green/red axis of the irradiated dosimeters, using a micro color unit equipped with a data station. The data were analyzed to determine the reproducibility of the reflectance measured from the label dosimeters exposed to different doses of γ radiation. These dosimeters showed great stability on extended storage before and after irradiation.Detailed measurements of absorbed dose extremes (D_min and D_max) in product boxes, processed in the Egypt Mega Gamma I radiation processing facility, were obtained using these dosimeters. These dosimeters are currently available in large quantities and are inexpensive, which makes them suitable for routine high-dose applications in radiation processing of materials.     

EPR Spectra of Some Salts of Bis[η5-1,2-Dicarbollyl]nickeliate(III)(1-) Anion [NiIII (η 5-1,2-B9C2H11)2]?

Some salt-like complexes of the cluster anion [Ni^III (η⁵-1,2-B₉C₂H₁₁ )₂]⁻ ([NiCb₂]⁻), containing paramagnetic Ni³⁺ ion, with cations Cs⁺, (CH₃)₄N⁺, [MnPhen₃]²⁺ (where Phen is 1,10-phenanthroline) are studied by EPR method at 77 K and 300 K. A neutral complex [MnPhen₂(NCS₂] is also studied for comparison. The synthesis procedure and X-ray diffraction analysis of [MnPhen₃][NiCb₂]₂ complex with paramagnetic ions Mn²⁺ (3d ⁵) and Ni³⁺ (3d ⁷) are described. The EPR data of isostructural complexes [MnPhen₃][NiCb₂]₂ and [MnPhen₃][CoCb₂]₂ are reported. No exchange or dipole-dipole interaction was observed between two paramagnetic ions (Mn²⁺ and Ni³⁺) simultaneously present in a complex structure. The temperature changes in EPR spectra of solid compounds are caused by rearrangements in the Mn²⁺ surrounding. In the case of a salt with a compact spherical Cs⁺ ion, the local perturbation in a second coordination sphere of [NiCb₂]⁻ anion leads to redistribution of the electron density and changes in g-factor.__________Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 6, 2005, pp. 403–414.Original Russian Text Copyright © 2005 by Nadolinny, Polyanskaya, Volkov, Drozdova.     

Advancements in accuracy of the alanine dosimetry system. Part 1. The effects of environmental humidity

A one-year study of the EPR signal of γ-irradiated (⁶⁰Co) l-α-alanine with simultaneous monitoring of the cavity Q-factor was undertaken. The widespread opinion that the EPR signal remains absolutely stable under normal laboratory storage conditions is inaccurate. At 0% humidity, the signal can be regarded as stable within ±1% of its initial value for 6 months for 1 and 10 kGy doses, but for only 3 months for 100 kGy. When stored at the same relative humidity values up to 60%, the fading rates for dosimeters irradiated to 1 and 10 kGy are similar, whereas signals of dosimeters irradiated to 100 kGy fade considerably faster for all humidities. The rates of fading increase with the relative humidity, especially above 60% R. H. Environmental humidity also deteriorates the accuracy of alanine dosimetry by changing the resonant cavity Q-factor. This is particularly important when irradiated alanine dosimeters are used as instrument calibration standards. Short-term changes in alanine EPR signal amplitudes were recorded upon removal of the irradiated dosimeters from their storage environments. The importance of an in situ standard to correct for measurement errors due to environmental effects is demonstrated.     

Effect of the high pressure on the structure and intercalation properties of lithium-nickel-manganese oxides

Lithium-nickel-manganese oxides (Li_1+x(Ni_1/2Mn_1/2)_1−xO₂, x=0 and 0.2), having different cationic distributions and an oxidation state of Ni varying from 2+ to 3+, were formed under a high-pressure (3 GPa). The structure and cationic distribution in these oxides were examined by powder X-ray diffraction, infrared (IR) and electron paramagnetic resonance (EPR) in X-band (9.23 GHz) and at higher frequencies (95 and 285 GHz). Under a high pressure, a solid-state reaction between NiMnO₃ and Li₂O yields LiNi_0.5Mn_0.5O₂ with a disordered rock-salt type structure. The paramagnetic ions stabilized in this oxide are mainly Ni²⁺ and Mn⁴⁺ together with Mn³⁺ (about 10%). The replacement of Li₂O by Li₂O₂ permits increasing the oxidation state of Ni ions in lithium-nickel-manganese oxides. The higher oxidation state of Ni ions favours the stabilization of the layered modification, where the Ni-to-Mn ratio is preserved: Li(Li_0.2Ni_0.4Mn_0.4)O₂. The paramagnetic ions stabilized in the layered oxide are mainly Ni³⁺ and Mn⁴⁺ ions. The disordered and ordered phases display different intercalation properties in respect of lithium. The changes in local Ni,Mn-environment during the electrochemical reaction are discussed on the basis of EPR and IR spectroscopy.     

Chiral Versus Non-Chiral [MnIII6MnIINaI], [MnIII6MnII2NaI2] and [MnIII3MnIINaI] Clusters Derived from Schiff Bases or the Fight for Symmetry

The reaction in basic media of manganese chloride with Schiff bases derived from the condensation of o-vanillin with different chiral/racemic aminoalcohols yielded in a family of complexes in which the nuclearity, symmetry and magnetic behavior is controlled by changing the position of the chiral carbon. Chiral and racemic clusters with [Mn^III₆Mn^IINa^I], [Mn^III₆Mn^II₂Na^I₂] and [Mn^III₃Mn^IINa^I] metallic core have been structurally and magnetically characterized. The racemic clusters with an odd number of chiral ligands exhibit the anomalous mixing of ligands with different conformation. Related racemic compounds have been reviewed.     

Optimization of the sensitivity and stability of the PRESAGE™ dosimeter using trihalomethane radical initiators

The aim of this study is to investigate the effect of trihalomethane radical initiators on the radiological properties, radiation dose sensitivity and post response photo-stability of the PRESAGE dosimeter. Different PRESAGE dosimeters containing 50 and 100 mM of iodoform (CHI₃), bromoform (CHBr₃) or chloroform (CHCl₃) radical initiators where fabricated and irradiated with 6 MV photons for a range of radiation doses from 0 to 30 Gy. A comparison between sensitivity and radiological properties of the PRESAGE dosimeters with the different radical initiators was carried out. Optical density changes of the dosimeters before and after irradiation were measured using a spectrophotometer. The incorporation of different radical initiators in the composition of the PRESAGE dosimeter resulted in variation of the radiation dose sensitivity and radiological properties of the dosimeters depending on the type and concentration of the radical initiator used, with iodoform showing the highest dose-response slope followed by bromoform and chloroform. However, at 100 mM iodoform, the effective atomic number was significantly higher than water (Z_eff=16). This enhancement in dose-response was found to be directly related to the carbon–halogen bond dissociation energy and to the radiological properties of each individual radical initiator used in this study. Furthermore, the post-response stability of the PRESAGE dosimeters over two weeks remained stable regardless of the trihalomethane radical initiator employed, with negligible change in the post-response stability and linearity of the PRESAGE dosimeters.     

Hydrogen peroxide disproportionation by the [TPA2Mn2(μ-Cl)2] complex

The dichloride-bridged [TPA₂Mn₂(μ-Cl)₂]²⁺ complex (I) was synthesized as a structural and functional model complex of the chloride-inhibited manganese catalase, and its catalytic properties in MeCN have been studied. Complex I shows sigmoidal kinetics and the activity is significantly inhibited in the presence of water. The kinetic parameters of the hydrogen peroxide disproportionation by complex I have been successfully fitted with non-Michaelis–Menten kinetics of Hill’s equation, which implies a multiple-step substrate activation of complex I. After termination of the catalysis, the mononuclear [TPA₂Mn](ClO₄)₂ was isolated from the solution. During the catalysis, a new penta-coordinate [TPAMnCl](ClO₄) complex (IV), tending to accumulate at a lower ratio of H₂O₂, was isolated and its X-ray crystallographic structure, as well as physical properties, was determined. Transformation of complex I in the presence of different molar ratios of H₂O₂ was studied by UV–Vis, EPR and ESI-MS spectroscopy. Upon addition of H₂O₂, the catalytic solution turned dark green, with instant evolution of oxygen gas, and the electronic spectra obtained were identical to that of the dark green dioxo bridged [TPA₂Mn₂(μ-O)₂](ClO₄)₃ complex (III). When the catalytic solution was subjected to EPR measurement, the transient peaks corresponding to the electronically localized Mn(II) species developed in a short time at lower concentrations of H₂O₂. The signal was more distinctive in the presence of water, and the complex I·H₂O₂ adduct was suggested as the intermediate species based on ESI-MS measurements. The EPR signal corresponding to complex III was detected at higher concentrations (>800 equiv.) of H₂O₂. As possible catalytic intermediates, the [TPA₂Mn^III₂(μ-O)₂]²⁺ and [TPA₂Mn^III₂(μ-O)(μ-OH)]⁺ species were suggested. A possible catalytic mechanism of H₂O₂ disproportionation by complex I, including the formation of active species and termination of the catalysis, has also been suggested.     

Unusual stabilization of manganese(III) during the cooxidation of dimethylformamide and manganese(II) by chromium(VI)

The kinetics of the formation and decomposition of Mn^III have been investigated spectrophotometrically in acidic media at 25 °C. The complete rate law for Mn^III formation isCr^VI + DMF + Mn^II {H⁺} Mn^III + CO₂ + Me₂NH + Cr^III ... (1)Mn^III + DMF {H⁺} Mn^II + CO₂ + Me₂NH ... (2)expressed by k _obs1 = k ₁ k₁ K _a1[H⁺][DMFH⁺][Mn^II]/{1 + K _a1[H⁺]}. Mn^III reduction by DMF follows the rate law k _obs2 = k ₂ K _h[DMF][H⁺]²/{[H⁺] + K _h}. The above results are accounted for by a mechanism involving the intermediacy of Cr^IV.     

Metal–Organic Frameworks (MOFs) of a Cubic Metal Cluster with Multicentered MnIMnI Bonds

MOFs with both multicentered metal–metal bonds and low‐oxidation‐state (LOS) metal ions have been underexplored hitherto. Here we report the first cubic [Mn^I₈] cluster‐based MOF ( 1 ) with multicentered Mn^I? Mn^I bonds and +1 oxidation state of manganese (Mn^I or Mn(I)), as is supported by single‐crystal structure determination, XPS analyses, and quantum chemical studies. Compound 1 possesses the shortest Mn^I? Mn^I bond of 2.372 Å. Theoretical studies with density functional theory (DFT) reveal extensive electron delocalization over the [Mn^I₈] cube. The 48 electrons in the [Mn^I₈] cube fully occupy half of the 3d‐based and the lowest 4s‐based bonding orbitals, with six electrons lying at the nonbonding 3d‐orbitals. This bonding feature renders so‐called cubic aromaticity. Magnetic properties measurements show that 1 is an antiferromagnet. This work is expected to inspire further investigation of cubic metal–metal bonding, MOF materials with LOS metals, and metalloaromatic theory.     

Density Functional Calculations of 55Mn, 14N and 13C Electron Paramagnetic Resonance Parameters Support an Energetically Feasible Model System for the S2 State of the Oxygen‐Evolving Complex of Photosystem II

Metal and ligand hyperfine couplings of a previously suggested, energetically feasible Mn₄Ca model cluster ( SG2009^?1 ) for the S₂ state of the oxygen‐evolving complex (OEC) of photosystem II (PSII) have been studied by broken‐symmetry density functional methods and compared with other suggested structural and spectroscopic models. This was carried out explicitly for different spin‐coupling patterns of the S=1/2 ground state of the Mn^III(Mn^IV)₃ cluster. By applying spin‐projection techniques and a scaling of the manganese hyperfine couplings, computation of the hyperfine and nuclear quadrupole coupling parameters allows a direct evaluation of the proposed models in comparison with data obtained from the simulation of EPR, ENDOR, and ESEEM spectra. The computation of ⁵⁵Mn hyperfine couplings (HFCs) for SG2009^?1 gives excellent agreement with experiment. However, at the current level of spin projection, the ⁵⁵Mn HFCs do not appear sufficiently accurate to distinguish between different structural models. Yet, of all the models studied, SG2009^?1 is the only one with the Mn^III site at the Mn_C center, which is coordinated by histidine (D1‐His332). The computed histidine ¹⁴N HFC anisotropy for SG2009^?1 gives much better agreement with ESEEM data than the other models, in which Mn_C is an Mn^IV site, thus supporting the validity of the model. The ¹³C HFCs of various carboxylates have been compared with ¹³C ENDOR data for PSII preparations with ¹³C‐labelled alanine.     

Electron paramagnetic resonance study of doped synthetic crystals of struvite and its zinc analogue

The electron paramagnetic resonance (EPR) technique has been used to study the Mn²⁺ paramagnetic impurity complexes in synthetic struvite (MgNH₄PO₄β6H₂O) and the zinc isomorph (ZnNH₄PO₄β6H₂O). EPR of VO²⁺ ion complexes in vanadyl doped crystals of the zinc isomorph of struvite has also been studied. Two differently oriented, but otherwise identical complexes of both Mn²⁺ ion and VO²⁺ ion are found in these crystals. The spin Hamiltonian parameters indicate a large orthorhombic distortion of the [Mn²⁺(H₂O)₆] octahedra and an axial symmetry of the vanadyl complexes. The results indicate that in both manganese and vanadyl complexes, the metal ions have covalent bonding with the ligands.