J.C. Humphreys

The response of plastic dosimeters to gamma rays and electrons at high absorbed dose rates

Abstract Several clear plastics and dyed plastics are commercially available for dosimetry in intense radiation fields, particularly for radiation processing applications using gamma rays from intense cobalt-60 sources and electron beams from accelerators running at potentials from 0.1 to 10 MeV. Some of these widely used dosimeters are: colorless cellulose triacetate, polymethylmethacrylate, or polyvinylchloride, showing induced absorption bands in the ultraviolet spectrum; radiochromic dyed plastics, such as nylon, polychlorostyrene, polyvinylbutyral, polyvinylpyrrolidone, showing induced absorption bands in the visible spectrum; blue cellophane, showing a bleaching in the visible absorption spectrum. A major source of error in making dose interpretations by spectrophotometric analysis of these dosimeters comes from dose-rate dependence of response. The present work shows measurements at high dose rates (e.g. 100 and 1050 rad · s -1 with gamma radiation, 10 6 rad · s -1 with a scanned beam of 2-MeV electrons, 10 10 rad · s 1 with a pulsed beam of 10-MeV electrons and 10 11 to 10 14 rad · s -1 with single pulses of 2-MeV electrons). For calibrations with gamma rays, only those plastics containing the radiochromic triphenylmethane cyanides or methoxides, i.e. nylon, polychlorostyrene, polyvinylbutyral, or polyvinylpyrrolidone, can correctly interpret dose determinations from electron beam irradiation. The other systems show a marked rate dependence of response. Of radiochromic dye systems, only polyvinylpyrrolidone as host matrix can be read with precision immediately after electron irradiation, since a slow build-up of the absorption band during the first hours after irradiation occurs in the other radiochromic plastic systems.

Radiochromic dosimetry for validation and commissioning of industrial radiation processes

Abstract Radiochromic dosimeters in various forms, thin films, coated films and papers, optical waveguides, liquid solutions, and small pellets, serve as useful dosimeters for many radiation processing applications. While they cover wide dose ranges (10 0 – 10 6 Gy) as routine dosimeters, if properly calibrated and controlled, they also may be used accurately and reproducibly for dose mapping, dose setting, and process commissioning procedures, as well as for transfer calibration of other routine dosimeters in photon and electron radiation fields.

The gamma-ray response of radiochromic dye films at different absorbed dose rates☆

Abstract In leucotriphenylmethane radiochromic dye systems, using polymeric host materials, low-intensity rate dependence of gamma-ray response at doses >10 kGy and at dose rates

Calorimetry of electron beams and the calibration of dosimeters at high doses

Abstract Graphite or metal calorimeters are used to make absolute dosimetric measurements of high-energy electron beams. These calibrated beams are then used to calibrate several types of dosimeters for high-dose applications such as medical-product sterilization, polymer modification, food processing, or electronic-device hardness testing. The electron beams are produced either as continuous high-power beams at approximately 4.5 MeV by d.c. type accelerators or in the energy range of approximately 8 to 50 MeV using pulsed microwave linear accelerators (linacs). The continuous beams are generally magnetically scanned to produce a broad, uniform radiation environment for the processing of materials of extended lateral dimensions. The higher-energy pulsed beams may also be scanned for processing applications or may be used in an unscanned, tightly-focused mode to produce maximum absorbed dose rates such as may be required for electronic-device radiation hardness testing. The calorimeters are used over an absorbed dose range of 10 2 to 10 4 Gy. Intercomparison studies are reported between National Institute of Standards and Technology (NIST) and UK National Physical Laboratory (NPL) graphite disk calorimeters at high doses, using the NPL 10-MeV linac, and agreement was found within 1.5%. It was also shown that the electron-beam responses of radiochromic film dosimeters and alanine pellet dosimeters can be accurately calibrated by comparison with calorimeter readings.

Response of radiochromic film dosimeters to gamma rays in different atmospheres

The high-dose gamma ray response (103 - 5×105 Gy) of radiochromic film dosimeters, with ten kinds of plastic matrices (polychlorostyrene containing 1 or 25% C, polybromostyrene containing 2 or 43% Br, nylon, polyvinyl chloride, cellulose triacetate, and an aromatic polymide) were investigated when irradiated under certain conditions in vacuum and in different atmospheres (air, oxygen, nitrogen, and nitrous oxide). In addition, the stability of the films was studied for storage periods up to one month after irradiation under these conditions. The responses and stabilities of the polyhalostyrene and nylon films were only slightly affected by the different atmospheres of irradiation, but there were marked differences of response for the other film types. The dyed cellulose triacetate films and polyvinylchloride films are generally more sensitive in N2O and O2-deprived atmospheres than in air or in O2, but the opposite is true for the dyed polyvinyl butyral and aromatic nylon films. The dyed cellulose triacetate and dyed polychlorostryrene with 1% C are the most stable films for all conditions or irradiation. For accurate routine radiation processing dosimetry, it is important to know the conditions of irradiation so that appropriate dosimetry systems and procedures may be used and so that suitable correction factors can be applied. Emphasis must be given to differences in atmospheric conditions encountered by dosimeters in practical industrial situations, which may cause marked differences in ultimate response factors.

Calorimeters for calibration of high-dose dosimeters in high-energy electron beams

Abstract Graphite calorimeters in both single-slab and modular multiple-slab disk geometries have been designed and used for measuring absorbed doses in graphite in intense electron beams of 2 to 12 MeV energy. The modular system has also been used with two to eight graphite elements to measure electron-beam depth-dose curves and to calibrate the response functions of radiochromicfilm and alanine-pellet dosimeters. An advantage of this new design is that such calibrations can be accomplished without requiring electron-beam current or charge monitoring, since the calorimeter modules adjacent to the dummy graphite element used to hold the dosimeters during calibration provide simultaneous calorimetric dose monitoring. Temperature-dependence corrections for dosimeter response are also simplified in this way.

Measurements of dose distributions in various materials irradiated by 10-MeV electrons

Abstract Calibrated radiochromic dye films were used to measure depth-dose profiles and lateral absorbed dose distributions in several irridiated media. Measurements were made in carbon, aluminum, polystyrene, and beef muscle irradiated by incident electrons of about 10 MeV. The dye-film dosimeters were placed within the media in several geometrical configurations. In one arrangement, the dosimeters were used to integrate the dose distribution within the volume of irradiated beef packages. These measurements show a significant variation on dose near the lateral edges of the medium. The experimental depth-dose distributions are compared with Monte Carlo calculations and with reported experimental measurements. These data are of interest in radiation effects studies, radiation processing, and diagnostic and therapeutic radiology.

Calibration of high-energy electron beams by use of graphite calorimeters

Abstract A multi-body graphite calorimeter has been designed for the absolute calibration of high-intensity electron beams in the energy regime of 2 to 12 MeV. This novel calorimeter consists of eight thermally and electrically insulated disks of high-purity graphite that serve as the active calorimetric bodies, arranged in a stacked array and oriented so that the flat surfaces are perpendicular to the electron-beam axis. Calibrated thermistors imbedded in the disks act as temperature sensors. The temperature of each disk is measured in real-time during irradiation by a scanning multichannel digital meter interfaced with a computer-based data acquisition system. The resultant data provide a depth-dose profile from which the electron energy can be calculated. Calorimeters of this type would be useful in standardizing the absorbed dose to passive routine dosimeters in the range of 100 Gy to 50 kGy, typical of that delivered by industrial processing electron beams.

The role of the office of radiation measurement in quality assurance

Abstract Over the past ten years the National Institute of Standards and Technology has, through its Office of Radiation Measurement, developed several national Secondary Laboratories systems in the field of ionizing radiation. These Secondary Laboratories systems provide the necessary calibrations and quality assurance testing to support and affirm the caliber of the measurements in special areas of ionizing radiation. The areas that are supported by the program include State Radiation Protection, Personnel Dosimetry, Survey Instrument Calibration, High-level Dosimetry, Radiation Therapy, Bioassay, Survey Instrument Testing, Ionizing Radiation, Environmental Radioactivity, Radioactivity Standards, and Radon.

ASTM dosimetry activities: A progress report

Abstract Radiation dosimetry standards are under development in ASTM on the selection of dosimetry systems for use in the operation of gamma ray or electron beam food processing facilities, on the dosimetry in gamma ray facilities used for radiation-hardness testing of electronic devices, and on the use of dosimetry systems for general applications such as the Fricke and ceric-cerous sulfate dosimeters.