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.

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.

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%.

A radiation accident at an industrial accelerator facility.

On 11 December 1991, a radiation overexposure occurred at an industrial radiation facility in Maryland. The radiation source was a 3-MV potential drop accelerator designed to produce high electron beam currents for materials-processing applications. This accelerator is capable of producing a 25 milliampere swept electron beam that is scanned over a width of 112.5 cm and which emerges from the accelerator vacuum system through a titanium double window assembly. During maintenance on the lower window pressure plate, an operator placed his hands, head, and feet in the beam. This was done with the filament voltage of the electron source turned "off," but with the full accelerating potential on the high voltage terminal. The operator's body, especially his extremities and head, were exposed to electron dark current. In an attempt to reconstruct the accident, radiochromic film and alanine measurements were made with the accelerator operated at two beam currents. Measured dose rates ranged from approximately 40 cGy s-1 inside the victim's shoe to 1,300 cGy s-1 at the hand position. Approximately 3 mo after the accident, it was necessary to amputate the four digits of the victim's right hand and most of the four digits of his left hand. Electron paramagnetic resonance spectrometry, which measures the concentration of radiation-induced paramagnetic centers in calcified tissues, was used to estimate the dose to the victim's extremities. A mean dose estimate of 55.0 +/- 3.5 Gy (95% confidence level) averaged over the mass of the bone was obtained for the victim's left middle finger (middle phalanx).