Norman V. Klassen

GafChromic MD‐55: Investigated as a precision dosimeter

GafChromic MD-55 is a fairly new, thin film dosimeter that develops a blue color (lambda max = 676 nm) when irradiated with ionizing radiation. The increase in absorbance is roughly proportional to the absorbed dose. In this study, GafChromic MD-55 was irradiated with 60Co gamma rays. A double irradiation method was used in which a dosimeter is given an unknown dose followed by a known, calibration dose. With this method, GafChromic MD-55 was used to measure doses in the vicinity of 6 Gy with an uncertainty of less than 1%. It was found that the measured optical density of GafChromic MD-55, as presently fabricated, is affected by the polarization of the analyzing light, an important consideration when using GafChromic MD-55 as a precision dosimeter. GafChromic MD-55 was found to consist of seven layers. The response to polarized light was measured for the whole dosimeter and for the three Mylar films which form part of the dosimeter.

Fricke dosimetry: the difference between G(Fe3+) for 60Co gamma-rays and high-energy x-rays.

A calibration of the Fricke dosimeter is a measurement of epsilon G(Fe3+). Although G(Fe3+) is expected to be approximately energy independent for all low-LET radiation, existing data are not adequate to rule out the possibility of changes of a few per cent with beam quality. When a high-precision Fricke dosimeter, which has been calibrated for one particular low-LET beam quality, is used to measure the absorbed dose for another low-LET beam quality, the accuracy of the absorbed dose measurement is limited by the uncertainty in the value of G(Fe3+). The ratio of G(Fe3+) for high-energy x-rays (20 and 30 MV) to G(Fe3+) for 60Co gamma-rays, G(Fe3+)MV(Co), was measured to be 1.007(+/-0.003) (confidence level of 68%) using two different types of water calorimeter, a stirred-water calorimeter (20 MV) and a sealed-water calorimeter (20, 30 MV). This value is consistent with our calculations based on the LET dependence of G(primary products) and, as well, with published measurements and theoretical treatments of G(Fe3+).

Two-stage cell shrinkage and the OER for radiation-induced apoptosis of rat thymocytes

Suspensions of rat thymocytes were given 0.09-100 Gy using 60Co gamma-rays. The radiation-induced changes in the thymocytes were examined from minutes to hours post-irradiation using electron microscopy, agarose gel electrophoresis, staining and Coulter Counter sizing. Sizing by Coulter Counter showed, for the first time, that thymocytes which undergo apoptosis shrink in two distinct stages, first by a sudden decrease from an original volume of 99 microns 3 to a volume of 76 microns 3, followed by a gradual decrease to 57 microns 3 over the space of a few hours. The oxygen enhancement ratio for apoptosis was measured to be about 3.5, similar to the value for reproductive death for many mammalian cells.

An experimental and Monte Carlo investigation of the energy dependence of alanine/EPR dosimetry: II. Clinical electron beams

The energy dependence of alanine/EPR dosimetry for 8, 12, 18 and 22 MeV clinical electron beams was investigated by experiment and by Monte Carlo simulations. Alanine pellets in a waterproof holder were irradiated in a water phantom using an Elekta Precise linear accelerator. The dose rates at the reference point were determined following the TG-51 protocol using an NACP-02 parallel-plate chamber calibrated in a (60)Co beam. The EPR spectra of irradiated pellets were measured using a Bruker EMX 081 EPR spectrometer. Experimentally, we found no significant change in alanine/EPR response to absorbed dose-to-water over the energy range 8-22 MeV at an uncertainty level of 0.6%. However, the response for high-energy electrons is about 1.3 (+/-1.1)% lower than for (60)Co. The EGSnrc Monte Carlo system was used to calculate the ratio of absorbed dose-to-alanine to absorbed dose-to-water and it was shown that there is 1.3 (+/-0.2)% reduction in this ratio from the (60)Co beam to the electron beams, which confirms the experimental results. Alanine/EPR response per unit absorbed dose-to-alanine was also investigated and it is the same for high-energy electrons and (60)Co gamma-rays.

A direct comparison of water calorimetry and Fricke dosimetry

Considerable effort has been devoted to measuring the absorbed dose to water using water calorimetry. Most of these efforts have been hampered by a lack of adequate knowledge of the heat defect of water. We argue that there is now sufficient information to establish with considerable confidence the heat defect of high-purity water containing various dissolved gases. For the present work we used water saturated with a 50/50 mixture of H2 and O2 gases, for which the heat defect is calculated to be -2.1%. As a test of this assignment, we have compared the absorbed dose to water as measured using water calorimetry with that obtained from Fricke dosimetry. The water calorimeter consisted of a small sealed vessel containing 100 ml of stirred water saturated with a 50/50 mixture of H2 and O2 gases. It was irradiated with 20 MV x-rays at a dose rate of about 0.4 Gy s-1. The same vessel was then filled with Fricke dosemeter solution, and irradiated under identical conditions. Our Fricke dosimetry is based on the Svensson and Brahme value of epsilon G (3.515 x 10(-3) 1 cm-1 J-1) and agrees to within 0.2% with the dose to water for 60Co gamma-rays obtained via graphite calorimetry. We find that for 20 MV x-rays, the dose to water determined by water calorimetry is 1.006 +/- 0.004 times the dose determined by Fricke dosimetry. Within 0.6(+/- 0.4)%, this result supports the calculated heat defect of -2.1% for water saturated with a 50/50 mixture of H2 and O2 gases.

Pulse radiolysis studies of time dependent spectral shifts of the solvated electron in ethanol and deuterated ethanol glasses at 76 K

The initial (?200 nsec) spectra of the trapped electron are similar in C2H5OH, C2H5OD, C2D5OH, and C2D5OD glasses at 76 K with λmax ∼1300 nm. The kinetics of the spectral shifts to the stable values of λmax near 540 nm are similar for C2H5OH and C2H5OD and resemble the kinetics of a series of first order processes or of tunneling reactions. Structure seems to be present in the partially relaxed spectra and may result from specific geoemetrical orientations of first solvation shell molecules.

WATER CALORIMETRY : THE HEAT DEFECT

Domen developed a sealed water calorimeter at NIST to measure absorbed dose to water from ionizing radiation. This calorimeter exhibited anomalous behavior using water saturated with gas mixtures of H2 and O2. Using computer simulations of the radiolysis of water, we show that the observed behavior can be explained if, in the gas mixtures, the amount-of-substance of H2 and of O2 differed significantly from 50 %. We also report the results of simulations for other dilute aqueous solutions that are used for water calorimetry—pure water, air-saturated water, and H2-saturated water. The production of H2O2 was measured for these aqueous solutions and compared to simulations. The results indicate that water saturated with a gas mixture containing an amount-of-substance of H2 of 50 % and of O2 of 50 % is suitable for water calorimetry if the water is stirred and is in contact with a gas space of similar volume. H2-saturated water does not require a gas space but O2 contamination must be guarded against. The lack of a scavenger for OH radicals in “pure” water means that, depending on the water purity, some “pure” water might require a large priming dose to remove reactive impurities. The experimental and theoretical problems associated with air-saturated water and O2-saturated water in water calorimeters are discussed.

Pulse radiolysis of aqueous solids at 76 K. An absorption band in the infrared

Abstract An IR absorption band with a maximum beyond 2400 nm has been found and assigned to the electron in the following systems irradiated at 76 K: crystalline D 2 O, and D 2 O glasses of ethylene glycol, MgCl 2 and LiCl. The band is tentatively attributed to an electron captured in a D-defect.

Optical Measurements on Solvated Electrons in Pulse-Irradiated Liquid Propane,

Abstract A broad absorption spectrum with λ max ⩾ 2000 nm has been observed in pulse-irradiated liquid propane at low temperatures, and assigned to the solvated electron. The electron decays by geminate recombination with an initial half-life of ⩽ 95 nsec at −185°C.

The effect of various dissolved gases on the heat defect of water

Recent measurements of the absorbed dose to air-saturated water, made using water calorimetry and assuming a zero heat defect for irradiated water, gave results 2%-5% higher than those determined by more conventional means. According to the current radiation chemical model for air-saturated water, the dose measured by water calorimetry assuming a zero heat defect should actually be 2% too low because of the endothermicity of the radiolysis processes in water. In order to examine possible sources for this discrepancy, we have constructed a small calorimeter (holding 100 ml of water) with which to measure the temperature rise in irradiated water saturated with various gases. The gases used were air, oxygen, argon, nitrogen, and hydrogen/oxygen mixtures. Irradiations were carried out with 20-MV x rays at a dose rate of 0.41 Gy/s. Our results are consistent with model calculations, except for some differences for accumulated doses of less than 100 Gy. The discrepancies we find at low doses and the discrepancies observed by others using water calorimeters may arise from impurities in the water.