The gamma-ray response of clear polymethylmethacrylate dosimeter radix RN15®

Basic characteristics of different batches of Radix RN15® dosimeter, a commercially available undyed polymethylmethacrylate dosimeter, was studied for its application to process control of radiation sterilization. Radix has relatively small deviation of thickness, optical absorption spectrum, and optical density values before irradiation. The response curves, in terms of net optical density per unit thickness (ΔOD/mm) as a function of absorbed dose, D_PMMA, are nearly linear up to 15 kGy and become sublinear at higher doses. Scattering of dose response in 5 dosimeter replicates is ± 1% (1 σ). The dose-response slope at 25 kGy while held at irradiation temperatures in the range 0–60°C, relative to those under 25°C, increases with temperature up to 40°C, the maximum point, and decreases at higher temperatures. Over the temperature range of 10–50°C, the variation with temperature of evaluated dose derived from the calibration curve at 25°C is less than 5% when using a dose rate of 6 kGy/h. At lower dose rates (e.g. 0.7 kGy/h) the irradiation temperature dependence is negligible over this temperature range. The temperature dependence at 40 kGy is less severe than that at 25 kGy. The post-irradiation stability of dose response is less than 5% for more than 100 h after 25 kGy irradiation and storage at 0–35°C. The change in optical density of unirradiated dosimeters 3 years after manufacturing is negligible when the dosimeters are stored at controlled laboratory conditions of temperature (25°C) and relative humidity (40%). The 3-year-old dosimeters by irradiation to a dose of 25 kGy show about 2% lower response than that at 25 kGy at the beginning of the 3-year period.     

Dose control in the semi-industrial irradiation plant at Ezeiza Atomic Center

High-dose dosimetry is carried out at Ezeiza Atomic Center (CAE) on a routine basis, within the absorbed dose range 10²–10⁵ Gy using potassium nitrate dosimeters (Dorda and Muñoz, 1985). The reference dosimeter is Fricke and Super Fricke. The next inauguration of a private irradiation plant and the increasing interest for a new technology in the industry led us to use a dosimeter easy to be read, without need for wet chemical analysis: the alanine dosimeter (Regulla and Deffner, 1982). Under the influence of ionizing radiation, the alanine in the crystalline state forms stable paramagnetic free radicals. The radical concentration of irradiated alanine, which is proportional to the absorbed dose, can be measured using electron spin resonance (ESR) spectroscopy. The pellets are made of 90% alanine and 10% paraffin. The procedure for mixing is not simple: temperature control is used to get an homogenous mass. The dose evaluation is performed for the ESR signal by measuring the maximum peak-to-peak amplitude. Intercomparison studies were made between the alanine and potassium nitrate dosimeter with good results. The relative standard deviation of our alanine dosimeter at doses of 25 and 32 kGy and 25°C temperature is 4%.     

Temperature effects on the ethanol-chlorobenzene dosimeter (Dvornik dosimeter)

The effects of elevated temperatures (50 and 80°C) on the dose dependence of the response of the ethanol-chlorobenzene (ECB) dosimeter, namely G(Cl⁻) values as a function of dose, were studied in the dose range 1–25 kGy. Systems containing 4, 10 and 20 vol% CB showed no dose effects of the response at 50°C and system containing 4 vol% CB showed no dose effect of G(Cl⁻) at 80°C, when compared with the radiation chemical yield measured for irradiations at room temperature (20°C). Dose dependence of G(Cl⁻) values in other systems at elevated temperatures were qualitatively similar to those at room temperature; the main difference, however, was that the values of G(Cl⁻) at elevated temperatures were higher. Larger temperature effects were observed at higher temperatures, higher CB concentrations and lower doses. Correct dosimetry is nevertheless possible with any formulation of the system at any dose or temperature within the studied interval by taking appropriate values of the response, either directly or by interpolation.     

Acidic aqueous dichromate solutions as reference dosimeters in the 10–40 kGy range

The radiolytic reduction of dichromate ion in acidic aqueous solution has been investigated as a potential dosimetry system for the 10–40 kGy range of absorbed dose. Dilute sulphuric acid has been shown to be unsuitable as a solvent, and the use of 0.1 mol·dm⁻³ perchloric acid is recommended. A previously reported effect of silver ion in supressing reaction with hydrogen has been confirmed, and in addition, silver ions have been shown to be effective in eliminating dose rate effects. The temperature coefficient during irradiation has been determined as −0.2% per degree and post-irradiation storage effects are shown to be negligible. The precision of the system is about 0.5% at a dose of 25 kGy, and a suggested composition for a working dosimeter is given.     

Application of jade samples for high-dose dosimetry using the EPR technique

The dosimeter characteristics of jade samples were studied for application in high-dose dosimetry. Jade is the common denomination of two silicates: jadeite and actinolite. The EPR spectra of different jade samples were obtained after irradiation with absorbed doses of 100 Gy up to 20 kGy. The jade samples present signals that increase with the absorbed dose (g-factors around 2.00); they can be attributed to electron centers. The EPR spectra obtained for the USA jade samples and their main dosimetric properties as reproducibility, calibration curves and energy dependence were investigated.     

A polymer-alanine film for measurements of radiation dose distributions

A film dosimeter (0.35 mm thick) composed of polyethylene-vinyl acetate and microcrystalline l-α-alanine has been prepared and investigated with respect to dosimetric properties using electron spin resonance spectroscopy. The useful absorbed dose range is ∼ 25 to 10⁵ Gy and no dose-rate dependence of the response is observed between 1 and 10⁷ Gy s⁻¹ within the dose range up to 50 kGy. With irradiation temperature increasing from 25 to 80°C, the response increases at most by 10%. The response is stable, within experimental uncertainty, at least up to 2500 h after irradiation. The suitability of the polymer-alanine film for measurements of ionizing photon and electron dose distributions is demonstrated.     

Application of solutions of Rhodamine B in dosimetry

Aqueous solutions of the organic dye Rhodamine B were found to be useful in measuring radiation doses in the range 0.1–2 kGy. Either spectrophotometric or spectrofluorometric measurements can be used. The dosimeter readout is unchanged for at least 60 days after irradiation if the solution is kept in the dark at room temperature. Effects of pH and the dye concentration on the dosimeter response were investigated. Radiation chemical yield and fluorescence quantum yield were also calculated.     

Dosimetry commissioning for an industrial cobalt-60 gamma-radiation facility

In order to establish plant operational parameters of the industrial ⁶⁰Co irradiation facility, UTR GAMA-Pi, a commissioning dosimetry procedure was carried out at the National Laboratory for Engineering and Industrial Technology (LNETI), Lisbon, Portugal. Detailed dose mapping, with and without industrial product, was used to measure the distribution of absorbed dose in the radiation field inside the irradiation cell. “Dummy” and “dosimetry” boxes were prepared, each having 200 kg m⁻³ bulk density, using a combination of cardboard sheets and folded and crumpled newspapers. Four dosimeter systems were used for measurements: red and amber Perspex, radiochromic dye films, ethanol-monochlorobenzene solutions and potassium dichromate solutions. All systems were calibrated against the Fricke dosimeter solution. The dosimetry commissioning procedures allowed the establishment of the setting of conveyor dwell times to reach prescribed minimum doses, as well as the computation of a source utilization efficiency of approx. 19.5% and a dose uniformity ratio of approx. 1.25. Using these data, the capacity throughput of the irradiation plant was evaluated to be 4265 m³ yr⁻¹ or 853 tonnes yr⁻¹ for the ⁶⁰Co plaque source with an initial activity of 1.095 × 10¹⁶ Bq (295 kCi) (November 1988), at an operational working time of 7000 h yr⁻¹, and a specified product dwell time setting of 12.6 min, for a minimum absorbed dose of 25 kGy and a product bulk density of 200 kg m⁻³.     

Comparison of the TL fading characteristics of Ge-doped optical fibres and LiF dosimeters

Fading is important in choosing appropriate thermoluminescence (TL) materials for particular applications. Comparison is made herein of changes due to fading in the TL yield of Ge-doped fibres and lithium fluoride (LiF) dosimeters, for varying temperature and dose. The fading is independent of dose for all investigated dosimeters while the loss in TL yield reduces for lower storage temperatures. At room temperature and for 133 days of storage, a maximum signal loss of 5% has been observed for both forms of LiF dosimeter, while 9 and 50 μm core diameter Ge-doped fibres produced a loss of 11% and 8%, respectively.     

Examination of beam profile measurement using an imaging plate by the light exposure fading method for quality assurance of external radiation therapy

High-resolution film dosimetry has been used for several decades to check and to measure two-dimensional dose distributions. However, in recent years, the automatic processor has been replaced by the spread of computed radiography, or has been little used hospitals. In this study, we measured the off-center ratio (OCR) of the open field, after an irradiating radiation beam was delivered to the imaging plate (IP) under conditions in which the IP was exposed to a fixed amount of light with fading, and compared these data with the OCR measured by an ionization-chamber dosimeter, which is the standard method used for measuring radiation dose. Profile measurement using IP could be achieved by performing light fading, even at a range of more than 100 MU. Further, by using a metallic filter, we succeeded in demonstrating that the profile measurement of IP in an open irradiation field could approximate the values of those obtained by an ionization chamber dosimeter. This method can serve as a simple, easy-to-use method for evaluating the QA of dose distribution in radiation therapy.     

Operation and methodology of an alanine ion-selective electrode dosimetric system: II. Dose-response

Dissolution of irradiated α-alanine in an aqueous solution results in the liberation and formation of ammonia, which can be quantified for dosimetric purposes by means of an ion-selective gas sensing electrode. The operational principles, readout procedure as well as data analysis were presented and discussed in a preceding paper. Here the dose-response (0.5–1000 kGy) by direct concentration measurements is evaluated. A total ammonia yield decreasing from 6.5 (±0.3) × 10⁻⁷ mol J⁻¹ (i.e. 6.3 (±0.3) molecules per 100 eV) at 1 kGy, to 2.9 (±0.1) × 10⁻⁷ mol J⁻¹ (2.8 (±0.1) per 100 eV) at 1000 kGy is observed. No significant fading at ambient temperature is found during the first 2 months, even at 1000 kGy. Irradiation temperature coefficients of +0.34 to +0.47%°C⁻¹ were measured, dependent on dose level, The precision in concentration measurement for one calibration is within 1%; long-term precision varies from 2 to 4% (95% conf. lim.) for doses above 1 kGy.     

ESR response of bulk samples of clear fused quartz (CFQ) material to high doses from 10 MeV electrons: Its possible application for radiation processing and medical sterilization

Clear fuse quartz (CFQ) could be a good candidate for dose measurements in the radiation processing of food material and in the sterilization of disposable medical and pharmaceutical products. In this study, bulk samples of CFQ materials were exposed to 10 MeV electrons at doses between 5–15 and 25–35 kGy; the dose ranges normally used for processing and sterilization. After irradiation, the samples were subjected to electron spin resonance analysis to test the intensity of their signal against the applied doses. The reproducibility of dose curve of the material to 10 MeV electrons was also studied, in which the dosed samples were annealed at 800 °C for 20 min and reused for further dose curve reproducibility investigation. The dose response curves of the reused samples showed a good linearity between the absorbed dose and the ESR intensity of the signals. They also indicated that the ESR intensity in the dose response curve, in both dose ranges, decreased by ∼37% for the first reuse process and ∼41% for the second reuse process of the samples in comparison to the first measurement. The thermal fading of the cylindrical samples of the CFQ, at room temperature, showed a sharp decay of ∼20% over a period of 15 days, after which the decay-rate was negligible for a measured period of 60 days.     

A comparison of the performance characteristics of four film dosimeters in a 10-MeV electron beam.

Performance characteristics of four routine film dosimeters (CTA, B-3, PVC and LiF (Sunna)) in a 10-MeV electron beam were investigated. Dose response curves for the range 5-40kGy are presented. The dose response curve is linear for the CTA film, supralinear for the LiF (Sunna) film and sublinear for the B-3 and PVC films. The low sensitivity of the CTA film to ionizing radiation restricts its use to high doses, while the supralinear response curve of the LiF (Sunna) dosimeter allows this film to be used only in measurements of low doses. The optical absorption temperature coefficients for the CTA, B-3 and LiF (Sunna) films were found to be at the level of +/-0.1%/( composite function)C. The temperature coefficient for the PVC film decreased from +0.35%/( composite function)C immediately after irradiation to +0.04%/( composite function)C three months later. Because of the significant time dependence of the PVC film signal, a careful standardization of the heating regimen and of the time between the irradiations and measurements is necessary.     

Response of thermoluminescent dosimeter CaF2:Dy to ionizing and non-ionizing radiations

Thermoluminescent dosimeter CaF₂:Dy or TLD-200 was studied for its response to both gamma-ray ionizing radiation and u.v. non-ionizing radiation. Emphasis was placed on high temperature treatment. For gamma-ray irradiation, heat treatment is limited to 400°C; beyond that the sensitivity of CaF₂:Dy decreases. However, for u.v. irradiation, high-temperature treatment at 900°C is required for CaF₂:Dy to become an intrinsic dosimeter. Major studies involve TL output, changes in peak height of glow curves, optical density changes after high temperature treatment, and reproducibility.     

Irradiation of Langmuir-Blodgett multilayer preparations of phospholipids and a fatty acid. 2: Effect of X-radiation.

X-irradiation of Langmuir-Blodgett (LB) preparations of stearic acid with doses up to a few thousand Gy produced no change of measured electrical conductance in the direction perpendicular to the stacked monolayers. However, irradiation of LB preparations of phospholipids resulted in increased conductance. The effect depended on dose, but not on dose rate and, unlike the corresponding effect of UV-radiation, did not reverse at room temperature. For doses up to about 2 kGy the increased conductance fell away over some tens of minutes if the temperature was raised above 45 degrees C. For doses between 2 and 60 kGy the conductance increased linearly, but less rapidly than the initial rise and the increase was only partly reversible by heating. The rate of increase of conductance rose again for doses above about 60 kGy and for these doses the increase could not be reversed on heating. It is suggested that X-irradiation left molecules in a damaged but reversible state similar to that found after UV irradiation; and that subsequent excitation and ionization damaged the molecules irreversibly.     

Modern dosimetric tools for 60Co irradiation at high containment laboratories

Abstract

Purpose: To evaluate an innovative photo-fluorescent film as a routine dosimetric tool during ⁶⁰Co irradiations at a high containment biological research laboratory, and to investigate whether manufacturer-provided chamber exposure rates can be used to accurately administer a prescribed dose to biological specimens.

Materials and methods: Photo-fluorescent, lithium fluoride film dosimeters and National Institutes of Standards and Technology (NIST) transfer dosimeters were co-located in a self-shielded ⁶⁰Co irradiator and exposed to γ-radiation with doses ranging from 5–85 kGy. Film dose-response relationships were developed for varying temperatures simulating conditions present when irradiating infectious biological specimens. Dose measurement results from NIST transfer dosimeters were compared to doses predicted using manufacturer-provided irradiator chamber exposure rates.

Results: The film dosimeter exhibited a photo-fluorescent response signal that was consistent and nearly linear in relationship to γ-radiation exposure over a wide dose range. The dosimeter response also showed negligible effects from dose fractionization and humidity. Significant disparities existed between manufacturer-provided chamber exposure rates and actual doses administered.

Conclusion: This study demonstrates the merit of utilizing dosimetric tools to validate the process of exposing dangerous and exotic biological agents to γ-radiation at high containment laboratories. The film dosimeter used in this study can be utilized to eliminate potential for improperly administering γ-radiation doses.     

EPR dosimetric properties of formates.

As a part of a program to develop an electron paramagnetic resonance (EPR) dosimeter suited for clinical use (doses in the cGy range), polycrystalline samples of lithium formate monohydrate (HCO2Li.H2O), magnesium formate dihydrate (C2H2O4Mg.2H2O), and calcium formate (C2H2O4Ca) have been examined. L-Alanine was included for comparison and reference. Samples were irradiated with 60Co gamma-rays and 60-220 kV X-rays. The dosimeter response was assessed using the peak-to-peak amplitude of the first-derivative EPR spectrum. Dose-response curves for the 60Co gamma-irradiated samples were constructed, and the dependences of the response on the photon energy, microwave power, and modulation amplitude were studied. Stability of the irradiation products upon storage (signal fading) was also investigated. Lithium formate monohydrate is by far the best candidate of the tested formates, suitable for measuring doses down to approximately 0.1 Gy. Lithium formate monohydrate is more sensitive than alanine by a factor of 5.6-6.8 in the tested photon energy range, it exhibits no zero-dose signal and shows a linear dose response in the dose range from 0.2 to 1000 Gy. Its EPR signal was found unchanged in shape and intensity 1 week after irradiation to 10 Gy. Various less favorable properties rendered the other formates generally unsuitable, although calcium formate exhibits some interesting EPR dosimetric properties.     

High-level gamma dosimetry using phototransferred thermoluminescence in quartz

Phototransferred thermoluminescence (PTTL) in natural quartz, separated from sand, has been studied for its application in high-level gamma dosimetry. The paper reports dose-vs.-PTTL response of 110°C and 180°C peaks in the dose range of 10 Gy to 6 kGy. Using this method, the dose rate delivered to the sewage sludge during irradiation at SHRI facility, Vadodara, India, was estimated to be 0.35±0.02 kGy/h.     

Cyanocobalamin solutions as potential dosimeters in low-dose food irradiations

Potential of aqueous solutions of cyanocobalamin in gamma radiation dosimetry was investigated. The solutions are inexpensive, nontoxic and easy-to-prepare dosimeters, which could be useful for measuring gamma radiation doses in various applications, such as quarantine treatment of fruit or insect disinfestation of grains and pulses. The optical absorbance of cyanocobalamin solutions of the optimal concentration 0.08 mM decreases with increasing radiation dose. The reproducible dependence of the absorbance decrease on the dose can be described with a polynomial. Pre- and post-irradiation stability of the solution absorbance, as well as effects of the irradiation temperature and dose rate, were studied. The response is not significantly affected by storage of the irradiated dosimeters under ambient conditions for 20 days. The performance characteristics of this chemical dosimetry system suggest that it can be useful to measure doses in irradiations of food.     

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.