Basic Metrology: Dose/Dose-Rate Effects in Alanine Dosimetry

Summary

Check standards are used by the NIST Radiation Physics Division to monitor the performance of the alanine dosimetry system that is central to its high-dose transfer dosimetry service.

Description

Check standards are used by the NIST Radiation Physics Division to monitor the performance of the alanine dosimetry system which is central to its high-dose transfer dosimetry service. These measurements are performed to confirm the operational readiness of the calibration curve. Deviations from the expected check standard values could result from a wide range of sources that include manufacturing abnormalities in a dosimeter and spectrometer-related changes. In 2005, check-standard measurement deviations unveiled a previously unknown rate effect for the alanine dosimetry system (Desrosiers et al., 2008). This rate-effect study characterized a complex relation between the radiation chemistry of crystalline alanine and the applied dose rate that was also dependent on the absorbed dose. That the rate effect only becomes significant above 5 kGy likely contributed to it only recently being discovered despite decades of research in alanine dosimetry. It was learned that the effect is intrinsic to alanine and is not dependent on the chemical form or manufacturing formulation of the alanine dosimeter. The study postulated that the production of one (or more) of the radiation-induced alanine radicals is dependent on the dose rate.

A follow-up study (Desrosiers and Puhl, 2009) aimed to investigate the influence of irradiation temperature on the dose rate effect. No increase in the effect was found with increasing temperature, but the dose rate effect appeared to be nonexistent at irradiation temperatures of -10 °C and -40 °C.

More information can be found here. In summary, it is known that:

• the rate effect is estimated to be
  • zero at dose rates above 2 Gy/s
  • significant at some value below 2 Gy/s, and
  • clearly measurable at 1 Gy/s
• the rate effect depends on absorbed dose, it
  • is not measurable at 1 kGy or less
  • becomes significant above 5 kGy
  • reaches a maximum effect at about 30 kGy
• for doses above 5 kGy, the magnitude of the effect is not dependent on the relative values of the high and low rates compared, but rather only whether the two rates compared fall above 2 Gy/s and below 1 Gy/s. If dosimeters irradiated with dose rates from either of these two categories are compared the effect is measured and if the dose is above 30 kGy the effect is maximized. 

Ongoing measurements continue to map the dose rate effect across the 1 Gy/s to 2 Gy/s transition range.