| 1. |
- Danilczuk, M., et al.
(författare)
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Ammonium Dithionate – a New Material for Highly Sensitive EPR Dosimetry
- 2008
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Ingår i: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy. - : Institutionen för fysik, kemi och biologi. - 1386-1425 .- 1873-3557. ; 69:1, s. 18-21
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Tidskriftsartikel (refereegranskat)abstract
- Polycrystalline ammonium dithionate has been examined for its radiation response in the low dose range (< 5 Gy) using EPR technique. The •SO3- radical ion was detected as a single EPR line with a peak-to-peak derivative width of ca. 0.44 mT in irradiated samples and its intensity was found to vary linearly with dose. At equal and moderate settings of microwave power and modulation amplitude ammonium dithionate was at least 7 times more sensitive than L-alanine which is the most common EPR dosimeter standard. Pulse experiments were performed on the powder samples to obtain the longitudinal relaxation time. These and microwave saturation experiments served to indicate the optimal microwave power to be applied during measurements as an EPR dosimeter for best sensitivity of this material. It is thus claimed that ammonium dithionate has excellent potential to become an EPR dosimeter with a low limit of the measurable dose for cases where tissue equivalence is not required or can be corrected for.
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| 2. |
- Danilczuk, M., et al.
(författare)
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Development of nickel-doped lithium formate as potential EPR dosimeter for low dose determination
- 2007
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Ingår i: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. - : Elsevier BV. - 1386-1425. ; 67:5, s. 1370-1373
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Tidskriftsartikel (refereegranskat)abstract
- EPR dosimetry employing l-α-alanine has been in vogue during the past few years, due to its tissue equivalence and linear dose response. However, l-α-alanine dosimetry has been improved during the past years, the sensitivity of this material is still too low for clinical applications. Polycrystalline lithium formate doped with NiCl2 was therefore examined for radiation response in the dose range of clinical interest (<5 Gy) using CW EPR and pulse EPR techniques. At equal and moderate settings of microwave power and modulation amplitude lithium formate doped with 1.6 wt% of NiCl2 was almost four times more sensitive compared to l-α-alanine, which is the most common EPR dosimeter standard. It was shown that the nickel-doped lithium formate has an excellent radiation response with a low limit of the measurable dose, and a linear dose response in the range 1–5 Gy. The relaxation and power saturation studies showed that high microwave power can be applied during measurements to improve the sensitivity of this material as an EPR dosimeter. These results show that lithium formate doped with Ni(II) exhibits promising properties required for further development of an EPR dosimeter in the dose range typical for clinical dosimetry.
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| 3. |
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| 4. |
- Gustafsson, Håkan, et al.
(författare)
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Enhanced sensitivity of lithium dithionates doped with rhodium and nickel for EPR dosimetry
- 2005
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Ingår i: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. - : Elsevier BV. - 1386-1425. ; 62:1-3, s. 614-620
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Tidskriftsartikel (refereegranskat)abstract
- Electron paramagnetic resonance (EPR) studies of X-irradiated lithium dithionate, Li2S2O6·2H2O, doped with Ni and Rh have shown that these impurities enhance the yield of radicals formed by X-irradiation at room temperature. The signal in the doped samples, measured peak-to-peak of the single EPR derivative line attributed to the SO3− anion was about 3–4 times that of the pure lithium dithionate and more than 10 times stronger than the alanine signal. These impurities also shortened the spin-lattice relaxation time, T1, which gives the possibility to measure the doped samples at a higher microwave power. This implies that sensitivity could be further enhanced in the already sensitive EPR dosimeter material lithium dithionate.
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| 5. |
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| 6. |
- Sastry, M.D., et al.
(författare)
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Dynamical effects and ergodicity in the dipolar glass phase : Evidence from time-domain EPR and phase memory time studies of AsO44- in Rb1-x(NH4)xH2PO4 (x = 0,0.5,1)
- 2006
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 18:17, s. 4265-4284
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Tidskriftsartikel (refereegranskat)abstract
- Three-pulse electron spin echo envelope modulation (ESEEM), hyperfine sublevel correlation spectroscopy (HYSCORE) investigations and two-pulse electron spin echo (ESE) measurements of phase memory time TM, were carried out, in the 20-200 K temperature range, on an AsO44- paramagnetic probe stabilized in RbH2PO4 (RDP), NH4H2PO4 (ADP), and dipolar glass Rb0.5(NH4)0.5H2PO4 (RADP). The results obtained on ADP revealed hyperfine interaction of the probe ion with the 14N of the ammonium ion, the coupling constant satisfying the condition of 'cancellation' at a field of 480 mT. The ammonium ion was found to be in two different sites in ADP, which became indistinguishable on the formation of dipolar glass RADP. These results were confirmed by HYSCORE spectral measurements. The fast Fourier transform (FFT) spectra of three-pulse ESEEM decays have clearly revealed the interaction with protons in the bond both in ADP and RDP, and in RADP with an averaged coupling constant. The phase memory times in RADP exhibited strong temperature dependence and were found to be dependent on the nuclear spin quantum number mI of 75As. The temperature dependence of TM exhibited a well-defined maximum around 90 K, coinciding with the temperature of onset of 'freezing' in Rb0.5(NH4)0.5H2PO 4. This is symptomatic of dynamic fluctuations in the dipolar glass phase, with onset around 150 K, going through a maximum around 90 K and slowing down on further cooling. These results suggest that in RADP, a dynamical mechanism with progressive slowing down below 90 K is operative in the glass formation. This implies that the RADP system, with x = 0.5, exists in an ergodic relaxor (R)-state in the 20-200 K temperature range wherein every fluctuating monodomain can be viewed as statistically representative of the whole sample. © IOP Publishing Ltd.
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| 7. |
- Danilczuk, M., et al.
(författare)
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Conduction electron spin resonance of small silver particles
- 2006
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Ingår i: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy. - : Elsevier BV. - 1386-1425 .- 1873-3557. ; 63:1, s. 189-191
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Tidskriftsartikel (refereegranskat)abstract
- Electron spin resonance (ESR) spectroscopy has been used to study the conduction electron spin resonance (CESR) of small silver particles stabilized in dehydrated Ag-rho zeolite. Silver particles were produced by hydrogen reduction at elevated temperatures and diameter of the stabilized particles was calculated based on the linewidth of CESR signal using Kawabata theory. © 2005 Elsevier B.V. All rights reserved.
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| 8. |
- Danilczuk, M., et al.
(författare)
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EPR and DFT study on the stabilization of radiation-generated methyl radicals in dehydrated Na-A zeolite
- 2006
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 110:48, s. 24492-24497
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Tidskriftsartikel (refereegranskat)abstract
- Electron paramagnetic resonance (EPR) spectroscopy was applied to study paramagnetic species stabilized in Na-A zeolite exposed to gaseous methane and ?-irradiated at 77 K. Two types of EPR spectra were recorded during thermal annealing of zeolite up to room temperature. Owing to the results for the zeolite exposed to 13CH4 the multiplet observed at 110 K was assigned to a ?CH3?Na+ complex. After decay of the multiplet, the isotropic quartet of methyl radical was recorded in the temperature range of 170-280 K. On the basis of the EPR parameters it is postulated that ?CH3 radicals in this temperature region are able to freely rotate inside the zeolite cage. The structures of the ?CH3?Na+ adsorption complex and respective hyperfine coupling constants were calculated by applying DFT quantum chemical methods. Two different models were applied to represent the zeolite framework: the 6T structure of one six-membered ring and the 3T cluster. The hyperfine coupling constants calculated for the ?CH3?Na+ adsorption complex for both applied models show very good agreement with those obtained experimentally. © 2006 American Chemical Society.
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| 9. |
- Danilczuk, M, et al.
(författare)
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Interaction of silver atoms with ethylene in Ag-SAPO-11 molecular sieve : an EPR and DFT study
- 2004
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 6:6, s. 1165-1168
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Tidskriftsartikel (refereegranskat)abstract
- The formation of silver-ethylene complexes in dehydrated Ag-SAPO-11 molecular sieve have been observed by electron paramagnetic resonance spectroscopy (EPR) after gamma-irradiation at 77 K. Such reactive intermediate can play an important role in catalytic conversion of ethylene on silver loaded molecular sieves. The Ag(C2H4)(2) stabilized inside relatively small channels of SAPO-11 molecular sieve it is observed even at 290 K. The 'gas-phase' geometries of Ag(C2H4) and Ag(C2H4)(2) complexes, and respective hyperfine coupling constants were calculated applying DFT quantum chemical methods. The hyperfine coupling constants calculated for Ag(C2H4)(2) complex of D-2h symmetry are in very good agreement with those obtained experimentally.
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| 10. |
- Lund, Anders, et al.
(författare)
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Determining the geometry and magnetic parameters of fluorinated radicals by simulation of powder ESR spectra and DFT calculations : The case of the radical RCF2CF2. in nafion perfluorinated ionomers
- 2007
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 111:32, s. 9484-9491
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Tidskriftsartikel (refereegranskat)abstract
- The ESR spectrum of the chain-end radical RCF2CF 2. detected in Nafion perfluorinated membranes exposed to the photo-Fenton reagent was accurately simulated by an automatic fitting procedure, using as input the hyperfine coupling tensors of the two F a and two Fß nuclei as well as the corresponding directions of the principal values from density functional theory (DFT) calculations. An accurate fit was obtained only for different orientations of the hyperfine coupling tensors for the two Fa nuclei, indicating a nonplanar structure about the Ca radical center. The fitted isotropic hyperfine splittings for the two Fß nuclei in the Nafion radical, 24.9 and 27.5 G, are significantly larger than those for the chain-end radical in Teflon (15 G), implying different radical conformations in the two systems. The excellent fit indicated that the geometry and electronic structure of free radicals can be obtained not only from single-crystal ESR spectroscopy, but also, in certain cases, from powder spectra, by combination with data from DFT calculations. The optimized structures obtained by DFT calculations for the CF3CF 2CF2CF2. or CF3OCF 2CF2. radicals as models provided additional support for the pyramidal structure determined from the spectral fit. Comparison and analysis of calculated and fitted values for the hyperfine splittings of the two Fß nuclei suggested that the radical detected by ESR in Nafion is ROCF2CF2., which originates from attack of oxygen radicals on the Nafion side chain. The combination of spectrum fitting and DFT is considered important in terms of understanding the hyperfine splittings from 19F nuclei and the different conformations of fluorinated chain-end-type radicals RCF2CF2. in different systems, and also for elucidating the mechanism of Nafion fragmentation when exposed to oxygen radicals in fuel cell conditions. © 2007 American Chemical Society.
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| 11. |
- Lund, Eva, et al.
(författare)
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Compounds of Li-6 and natural Li for EPR dosimetry in photon/neutron mixed radiation fields
- 2004
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Ingår i: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy. - : Elsevier BV. - 1386-1425 .- 1873-3557. ; 60:6, s. 1319-1326
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Tidskriftsartikel (refereegranskat)abstract
- Formates and dithionates of Li-6, enriched and Li-7 in natural composition of Li offer a possibility to measure the absorbed dose from photons and thermal neutrons in a mixed radiation field for instance at a boron neutron capture therapy (BNCT) facility. Tests with formates and dithionates of enriched 6Li and lithium compounds with natural composition have been performed at the BNCT facility at Studsvik, Sweden. Irradiations have been performed at 3 cm depth in a Perspex phantom in a fluence rate of thermal neutrons 1.8 x 10(9) n cm(-2) s(-1). The compounds were also irradiated in a pure X-ray field from a 4 MV linear accelerator at 5 cm depth in a phantom with accurately determined absorbed doses. The signal intensity and shape was investigated within 3 h after the irradiation. A single line spectrum attributed to the CO2- radical was observed after irradiation of lithium formate. An increase in line width occurring after neutron irradiation in comparison with photon irradiation of the Li-6 sample was attributed to dipolar broadening between CO2- radicals trapped in the tracks of the alpha particles. A spectrum due to the SO3- radical anion was observed after irradiation of lithium dithionate. The signal amplitude increased using the Li-6 in place of the Li with natural composition of isotopes, in studies with low energy X-ray irradiation. Due to the decreased line width, caused by the difference in g(N) and I between the isotopes, the sensitivity with Li-6 dithionate may be enhanced by an order of magnitude compared to alanine dosimetry. After comprehensive examination of the different combinations of compounds with different amounts of Li-6 and Li-7 regarding dosimetry, radiation chemistry and EPR properties these dosimeter material might be used for dose determinations at BNCT treatments and for biomedical experiments. Interesting properties of the radical formation might be visible due to the large difference in ionization density of neutrons compared to photons. (C) 2003 Elsevier B.V. All rights reserved.
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| 12. |
- Lund, Eva, et al.
(författare)
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Formates and dithionates : sensitive EPR-dosimeter materials for radiation therapy
- 2005
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Ingår i: Applied Radiation and Isotopes. - : Elsevier BV. - 0969-8043 .- 1872-9800. ; 62:2, s. 317-324
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Tidskriftsartikel (refereegranskat)abstract
- Polycrystalline, formates and dithionates are promising materials for EPR dosimetry, as large yields of radiation induced stable radicals are formed with a linear dose response. Rapid spin relaxation rates were detected in many of the substances, indicating that a high microwave power can be applied during EPR acquisition in order to improve sensitivity. Different techniques used to further improve the sensitivity, such as the replacement of Li-7 with Li-6 or exchange of protons with deuterons in the corresponding crystalline matrices and metal ion doping are discussed. It is concluded that formates and dithionates may be up to 10 times as sensitive as L-alpha-alanine. (C) 2004 Elsevier Ltd. All rights reserved.
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