James M. Puhl

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.

Investigation of applicability of alanine and radiochromic detectors to dosimetry of proton clinical beams.

Cancer therapy studies using proton accelerators are underway in several major medical centers in the U.S., Russia, Japan and elsewhere. To facilitate dosimetry intercomparisons between these laboratories, alanine-based detectors produced at the National Institute of Standards and Technology and commercially available radiochromic films were studied for their possible use as passive transfer dosimeters for clinical proton beams. Evaluation of characteristics of these instruments, including the LET dependence of their response of proton energy, was carried out at the Institute of Theoretical and Experimental Physics. Results of absolute dose measurements were regarded as a preliminary step of dose intercomparison between ITEP and NIST. Measurements made in a number of experiments showed average agreement between the ITEP and NIST dosimetry standards to 2.5%.

An Absorbed-Dose/Dose-Rate Dependence for the Alanine-EPR Dosimetry System and Its Implications in High-Dose Ionizing Radiation Metrology.

NIST developed the alanine dosimetry system in the early 1990s to replace radiochromic dye film dosimeters. Later in the decade the alanine system was firmly established as a transfer service for high-dose radiation dosimetry and an integral part of the internal calibration scheme supporting these services. Over the course of the last decade, routine monitoring of the system revealed a small but significant observation that, after examination, led to the characterization of a previously unknown absorbed-dose-dependent, dose-rate effect for the alanine system. Though the potential impact of this effect is anticipated to be extremely limited for NIST’s customer-based transfer dosimetry service, much greater implications may be realized for international measurement comparisons between National Measurement Institutes.

Temperature and relative humidity dependence of radiochromic film dosimeter response to gamma and electron radiation

Abstract Nylon-base radiochromic films (FWT-60 TM and FWT-460 TM ) are commercially-available, thin dosimeters that are widely used in radiation processing. These films cover the following ranges of absorbed dose: 2 × 10 3 to 5 × 10 4 Gy for FWT-60 TM and 5 × 10 3 Gy to 10 5 Gy for FWT-460 TM . Based on some earlier studies, their response functions have been reported to be dependent on the temperature and relative humidity during irradiation. The present study investigates differences in response over practical ranges of temperature, relative humidity, dose, and for different recent batches of films of both types. It is observed that for each new batch of film to be used for radiation processing, the effects of such parameters on response to both gamma rays and electrons should be investigated. It is also suggested that the films should be packaged under controlled atmospheric conditions (relative humidity) and should be calibrated under environmental conditions (temperature) at which they will be used routinely.

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.

Radiation-field mapping of insect irradiation canisters

Abstract Dosimetry methods developed at NIST for mapping ionizing radiation fields were applied to canisters used in 137 Cs dry-source irradiators designed for insect sterilization. The method of mapping the radiation fields inside of these canisters as they cycled through the gamma-ray irradiators involved the use of radiochromic films, which increase in optical density proportionately to the absorbed dose. A dosimeter film array in a cardboard phantom was designed to simulate the average insect pupae density and to map the dose within the full volume of the canister; the calibrated films were read using a laser scanning densitometer. Previously used dosimetric methods did not allow for the spatial resolution that is possible with these films. Results indicate that this dose-mapping technique is a powerful method of evaluating a variety of radiation fields of commercial radiation sources, with promising applications as a means of dose validation and quality control.

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.

A new EPR dosimeter based on polyvinylalcohol

Abstract A new dosimetry system based on the EPR response of polyvinyl alcohol is presented. The dose response was measured from 10–10 5 Gy and the persistence of the signal was monitored over a period of 14 days.

A Comparison of Harwell & FWT Alanine Temperature Coefficients from 25 °C to 80 °C

The dosimeters used to monitor industrial irradiation processing commonly experience significant temperature rises that must be considered in the dose analysis stage. The irradiation-temperature coefficient for a dosimetry system is derived from the dosimeter’s radiation response to the absorbed dose and the irradiation temperature. This temperature coefficient is typically expressed in percent change per degree. The temperature rise in dosimeters irradiated with high-intensity ionizing radiation sources can be appreciable. This is especially true for electron-beam processing in which dosimeter temperatures can approach 80 °C. A recent National Institute of Standards and Technology (NIST) study revealed modest (0.5 % to 1.0 %) deviations from the predicted value at temperatures above 70 °C for absorbed doses of 1 kGy and 20 kGy. However, these data were inconsistent with a concurrent manuscript published by National Physical Laboratory (NPL) researchers that found a significant dose-dependent non-linear alanine response but used dosimeters from a different manufacturer and a different experimental design. The current work was undertaken to reconcile the two studies. Alanine dosimeters from each manufacturer used by NIST and NPL were co-irradiated over a wide range of absorbed dose and irradiation temperature. It was found that though there was a slight variation in the temperature coefficient between the two alanine dosimeter sources both systems were linear with irradiation temperature up to 70 °C and the NPL observations of non-linearity were not reproduced. These data confirmed that there is no fundamental difference in the two commercial alanine dosimeter sources and that temperature corrections could be made on industrial irradiations at the extremes of irradiation temperature and absorbed dose.