Arne Miller

Radiochromic plastic films for accurate measurement of radiation absorbed dose and dose distributions

Abstract Thin radiochromic dye films are useful for measuring large radiation absorbed doses (105–108 rads) and for high-resolution imaging of dose patterns produced by penetrating radiation beams passing through non-homogeneous media. Certain types of amino-substituted triarylmethane cyanides dissolved in polymeric solutions can be cast into flexible free-standing thin films of uniform thickness and reproducible response to ultraviolet and ionizing radiation. The increase in optical density versus energy deposited by radiation is linear over a wide range of doses and is for practical purposes independent of dose rate (1–1014 rad s−1). Upon irradiation of the film, the profile of the radiation field is registered as a permanent colored image of the dose distribution. Unlike most other types of dyed plastic dose meters, the optical density produced by irradiation is in most cases stable for periods of at least one year. Methods have been developed for casting various types of thin radiochromic plastic films and combinations of plastics (cellulose acetate, polyvinyl butyral, polyvinyl acetate, polyvinyl pyrrolidone and polyvinyl chloride) having radiation absorption characteristics corresponding to those of many polymeric systems in industrial radiation processing. The result is that errors due to energy dependence of response of the radiation sensor are effectively reduced, since the spectral sensitivity of the dose meter matches that of the polymer of interest, over a wide range of photon and electron energies (0.01–10 MeV).

Calorimetry at industrial electron accelerators

Calorimetry is a convenient way to measure doses at industrial electron accelerators, where high absorbed doses (1–100 kGy) are delivered at dose rates of 102−105 Gy s−1 or even higher. Water calorimeters have been used for this purpose for several years, but recently other materials such as graphite and aluminum have been used. These materials offer advantages of simpler construction and higher sensitivity. Dose estimates using these different types of calorimeters differ by ~ 5%, and the possible reasons for these discrepancies are discussed.

Temperature, humidity and time. Combined effects on radiochromic film dosimeters

Abstract The effects of both relative humidity and temperature during irradiation on the dose response of FWT-60-00 and Riso B3 radiochromic film dosimeters have been investigated in the relative humidity (RH) range 11–94% and temperature range 20–60°C for irradiation by 60 Co photons and 10-MeV electrons. The results show that humidity and temperature cannot be treated as independent variables, rather there appears to be interdependence between absorbed dose, temperature, and humidity. Dose rate does not seem to play a significant role. The dependence of temperature during irradiation is +0.25 ± 0.1% per °C for the FWT-60-00 dosimeters and +0.5 ± 0.1% per °C for Riso B3 dosimeters at temperatures between 20 and 50°C and at relative humidities between 20 and 53%. At extreme conditions both with respect to temperature and to humidity, the dosimeters show much stronger dependences. Whenever possible one should use dosimeters sealed in pouches under controlled intermediate humidity conditions (30–50%) or, if that is impractical, one should maintain conditions of calibration as close as possible to the conditions of use. Without that precaution, severe dosimetry errors may result.

Megagray dosimetry (or monitoring of very large radiation doses)

Abstract A number of suitably calibrated plastic and dyed films and solid-state systems can provide mapping of very intense radiation fields with high spatial resolution and reasonable limits of uncertainty of absorbed dose assessment. Although most systems of this type suffer from rate dependence and temperature dependence of response when irradiated with charged particle beams at high dose rates, a few are stable, easily calibrated, and capable of faithful imaging of detailed dose profiles, even at doses up to 10 6 Gy and dose rates up to 10 8 Gy·s −1 . Candidates include certain undyed plastic films (e.g., polyhalostyrenes), some dyed cellulosics, radiochromic dye films (e.g., Nylonbase “Trogamid” films) and pure LiF crystals.

Application of the Sunna dosimeter film in gamma and electron beam radiation processing

Abstract The OSL (optically stimulated luminescence) based Sunna film containing a microcrystalline dispersion of LiF in a polymer matrix has been recently introduced for high-dose dosimetry. Our previous investigations revealed the applicability of the system in the dose range of 0.01–100 kGy, but irradiation temperature and dose rate effects above 5 kGy reduced its usefulness. The recent discovery of the use of spectrophotometric analysis in the UV range for measuring doses above 5 kGy is a suitable option, while the OSL analysis can be applied for measuring lower doses due to the lack of temperature effect on the response of the film below 1 kGy. The film can be applied both in gamma and electron fields, although its response to the two types of radiation is somewhat different. No significant effects of UV light and humidity have been found, and effects of irradiation temperature are only significant in the case of OSL analysis for doses above 5 kGy. The applicability of two types of Sunna films in electron and gamma radiation processing is discussed in the paper.

Dose determination with nitro blue tetrazolium containing radiochromic dye films by measuring absorbed and reflected light

Abstract Tetrazolium salts as heterocyclic organic compounds are known to form highly coloured, water insoluble formazans by reduction, which can be utilized in radiation processing dosimetry. Radiochromic films containing nitro blue tetrazolium dissolved in a polymer matrix were found suitable for dose determination in a wide dose range both by absorbance and reflectance measurements. The concept of measuring reflected light from dose labels has been discussed earlier and emerged recently due to the requirement of introducing semiquantitative label dose indicators for quarantine control. The usefulness of the method was studied using the newly developed radiochromic dye films as well as already existing ones.

Thin plastic radiochromic dye films as ionizing radiation dosimeters

Abstract Radiochromic dye films were fabricated by casting polyvinyl butyral (PVB) in weakly acidic solution with the leucocyanide of pararosaniline. Calibrated films of 10–25 μm thickness were useful over a response range of about 103–105 Gy, by applying spectrophotometric analysis at the wavelength of the maximum of the radiation-induced absorption band (550 nm). The effects of temperature, pressure, and humidity during curing of the films pointed to the need for carefully controlling these parameters. For casting films at the high altitude of Mexico City (≈ 2500 meters), the optimum conditions are 45–75% r.h. and 20–25° C for a drying period of 72 to 92 hours, when the solvent is a mixture of ethanol and 2-methoxyethanol. The response of films fabricated in this way were compared with those of commercially available PVB and Nylon films. The effects of temperature, humidity, and period of storage on the response of these films were studied in the range from −5 to 60° C and from 11.8 to 96.6% r.h. for up to four months between irradiation and spectral analysis, and within nominal experimental uncertainty (≈ 10%), we found that all the radiochromic films studied can be stored for extended periods under steady-state conditions in the temperature range from −5 to 30° C and from 11.8–75.6% r.h. without correction factors for instability, but under extreme conditions of moisture at elevated temperatures the radiochromic image showed a fading effect on storage.

Polystyrene calorimeter for electron beam dose measurements

Abstract Calorimeters made from polystyrene have been constructed for dose measurement at 4–10 MeV electron accelerators. These calorimeters have been used successfully for a few years, and polystyrene calorimeters for use at energies down to 1 MeV are being tested. Advantages of polystyrene as the absorbing material are the radiation stability of this material and the fact that identical phantoms can be made for irradiation of other dosimeters for calibration. This provides a precise tool for establishing traceability of dose measurements at industrial electron accelerators.

Dose distributions in electron irradiated plastic tubing

Abstract Plastic tubes have been crosslinked by irradiation at a 10 MeV linear electron accelerator and at a 400 keV DC electron accelerator at different irradiation geometries. The diameter of the different tubes was 20, 33 and 110 millimeters. Dose distributions have been measured with thin radiochromic dye films, indicating that in all cases irradiation from two sides is a necessary and sufficient condition for obtaining a satisfactory dose distribution.

Distribution of Energy Deposited in Plastic Tubing and Copper-Wire Insulation by Electron Beam Irradiation

Abstract Scanned electron beam treatment is used to improve the physical properties of certain polymers, such as shrinkable plastic tubing and insulated wire and cable. Tubing or wires are passed at high speed under the beam scanner, and the material is irradiated to absorbed doses of several Mrad as uniformly as possible, usually by means of a multipass arrangement. In the present study, using irradiation by a scanned 0.4 MeV electron beam, measurements were made of high-resolution distributions of absorbed dose in polyethylene tubing and copper wire coated with polyethylene, nylon, or polyvinyl chloride insulation. Radiochromic dye films equivalent to the insulating materials were used as accurate dosimeters having a response independent of dose rate. Irradiations were in various geometries, wire and plastic thicknesses, positions along the beam scan, and with different backing materials near the wire as it passed through the electron beam.