Vitaly Nagy

Accuracy considerations in EPR dosimetry

A brief overview of some of the potential sources of inaccuracies that are common to all kinds of electron paramagnetic resonance (EPR) dosimetry is given. The instrumental sources discussed are inaccuracies of receiver gain calibration, instability of the spectrometer sensitivity, and neglect of inhomogeneity of the fields in the resonators. The discussed sources of inaccuracies related to data processing are violations of basic assumptions of the used version of the least-squares technique, incorrect model selection for fitting the data, and nonoptimal design of the calibration experiment. Methods for detecting and correcting such inaccuracies are briefly discussed and references to the original literature are given.

Advancements in accuracy of the alanine dosimetry system. Part 2. The influence of the irradiation temperature

Abstract Systematic measurements of the temperature coefficient for alanine electron paramagnetic resonance (EPR) response have been performed for irradiation in the temperature range (10–50)°C and in the absorbed dose range (1–100) kGy at the dose rate 9.5 kGy/h. During the 60 Co -ray irradiation, - l -alanine dosimeters were kept in a sealed aluminum holder that provided an effective heat exchange with the temperature-controlled environment. The time between the irradiation and signal measurements was standardized, and a reference sample fixed in the resonant cavity was used to correct the signals for small variations in the spectrometer sensitivity. The temperature coefficient for each dose was determined from approximately 30 experimental points processed by the weighted least-squares technique after the necessary statistical tests were done. The temperature coefficients thus determined were considerably lower than previously reported. The dose dependence of the temperature coefficient features a minimum at (20–30) kGy (about 0.135%/K) with higher values at 1 kGy (0.17%/K) and at 100 kGy ((0.175–0.19) %/K). With the exception of very high doses, no significant distinction was found between the temperature coefficients of Bruker and NIST dosimeters, which differ in shape and binder content.

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

A one-year study of the EPR signal of g-irradiated ( 60 Co) L-a-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 21% 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 eAects is demonstrated. # 2000 Elsevier Science Ltd. All rights reserved.

Complex time dependence of the EPR signal of irradiated L-α-alanine☆

Abstract Measurements of the EPR signal amplitude of γ-irradiated L-α-alanine with use of an adjacent reference sample have revealed variations in the signal intensity within hours and days after irradiation. The character of the time dependence of the amplitude varies with dose and the amplitude changes reach 1–1.5%. This observation favors the hypothesis that irradiated alanine contains several paramagnetic centers. Usefulness of adjacent reference samples in alanine dosimetry is also demonstrated.

Uncertainties in alanine dosimetry in the therapeutic dose range

A method for evaluating the overall uncertainty of alanine EPR transfer dosimetry in the therapeutic dose range is described. The method uses experimental data on EPR signal reproducibility from replicate dosimeters irradiated to low doses (1-5 Gy), estimates of Type B uncertainties, and Monte Carlo simulations of heteroscedastic orthogonal linear regression. A Bruker ECS106 spectrometer and Bruker alanine dosimeters have been used for this evaluation. The results demonstrate that alanine dosimetry can be used for transfer dosimetry in that range with the overall uncertainty 1.5-4% (1sigma) depending on the dose, the number of replicate dosimeters. and the duration of the calibration session (the session should not exceed one working day).

Advancements in accuracy of the alanine EPR dosimetry system: Part III: Usefulness of an adjacent reference sample

Abstract High stability of the radiation-induced radicals in alanine and the high reproducibility of alanine response to radiation make it possible to determine radiation doses accurately with Electron Paramagnetic Resonance (EPR) spectral analysis. Small uncontrollable variations of the EPR spectrometer sensitivity, however, can significantly deteriorate the accuracy of the method. The errors due to these variations can be eliminated or markedly decreased if an adjacent reference sample (such as a synthetic ruby crystal) is permanently present in the cavity, and if ratios of the alanine and ruby signal amplitudes are used throughout the dosimetric session instead of the absolute alanine amplitudes. This paper addresses methodological aspects of using such adjacent reference samples in alanine dosimetry and provides illustrations of the usefulness of this technique.

Characterization of flame-retarded polymer combustion chars by solid-state 13C and 29Si NMR and EPR

We report here on the results of our continuing effort to study the flame-retardant mechanism of silica gel and potassium carbonate. These additives reduce the flammability of a wide variety of common polymers such as polypropylene, nylon, polymethylmethacrylate, poly(vinyl alcohol), and cellulose. In an effort to determine how these additives reduce polymer flammability, we have used electron paramagnetic resonance spectroscopy and solid-state 13C and 29Si nuclear magnetic resonance spectroscopy to characterize the combustion chars or residues. These data indicate that, in the case of poly(vinyl alcohol), the additives do not change the type of char formed, but they do change the rate of char formation relative to the rate of fuel generation. We also found that, using only CP/MAS 13C NMR, there can be significant intensity distortions which complicate interpretation, if the char is hydrogen depleted and contains paramagnetic centres.

e-Calibrations: Using the Internet to Deliver Calibration Services in Real Time at Lower Cost

Abstract The National Institute of Standards and Technology (NIST) is expanding into a new frontier in the delivery of measurement services. The Internet will be employed to provide industry with electronic traceability to national standards. This is a radical departure from the traditional modes of traceability and presents many new challenges. The traditional mail-based calibration service relies on sending artifacts to the user, who then mails them back to NIST for evaluation. The new service will deliver calibration results to the industry customer on-demand, in real-time, at a lower cost. The calibration results can be incorporated rapidly into the production process to ensure the highest quality manufacturing. The service would provide the US radiation processing industry with a direct link to the NIST calibration facilities and its expertise, and provide an interactive feedback process between industrial processing and the national measurement standard. Moreover, an Internet calibration system should contribute to the removal of measurement-related trade barriers.

A fluidized bed process for electron sterilization of powders

Abstract A small capacity (100 g.s −1 ) pilot system is described for presentation of powders and fine aggregates at high velocity, to an electron beam. Electron beam dose rate is continuously monitored in real time, while the thickness of the fluidized bed used to pneumatically transport the product can be monitored and controlled using beta-gauge techniques. Using electron paramagnetic resonance (EPR) techniques, alanine power mixed with the product is used for precise determination of dose delivered to the powder stream. Thin film dosimeters transported in the bed are also used for dose determination. Results with a variety of products are presented using both dose rate and velocity as the independent variables. Lethality data for the bioburdens present in several powdered foodstuffs are discussed.

Temperature stabilization of alanine dosimeters used for food processing and sterilization

The International Atomic Energy Agency has established a dose quality audit service for radiation processing facilities. The objective of the service is to provide an independent check on the routine dosimetry system in use at the facility. The audit service is based on the use of alanine EPR dosimetry. Generally, alanine dosimeters are irradiated at the facility together with a product, and the response is then analyzed at the IAEA laboratory. Practice of the audit service has shown that the main uncertainty in alanine dosimetry is due to absence of temperature control at the irradiation facilities. Here, a method for stabilizing the temperature of the dosimeter during irradiation is proposed.