Measurement uncertainty of the secondary standard measuring facility for the water absorbed dose between 5 Gy and 25 Gy

The secondary standard measuring facility for the absorbed dose to water in high-energy radiation fields on the basis of Alanin/ESR could be improved by a new probes positioning device. At 5 Gy, the relative reproducibility standard deviation now amounts to 0.4%, and at 25 Gy, a deviation of 0.2% is achieved.

The reproducibility standard deviation of the secondary standard measuring facility for the absorbed dose to water, D_W, could be reduced by an improvement of the sample holder. In the case of the new holder (see Figure), the Alanin probe is supported on a small, pivotable quartz tube with a guide bushing made of Teflon. A quartz capillary with the reference substance, which is used for the relative determination of the amplitude of irradiated Alanin probes, is now located directly below the probe to be evaluated. This allows the reproducibility during positioning to be improved.

In the range between 5 Gy and 25 Gy, the reproducibility standard deviation for DW can be indicated as linear equation: u (D_W) = 0.0012 * D_W +0.0117 Gy. This corresponds to a relative reproducibility standard deviation of 0.4%, 0.3% and 0.2% for doses of 5 Gy, 10 Gy and 20 Gy. The positioning and temperature uncertainties during irradiation and the uncertainty of the probe mass increase the relative standard uncertainty by another 0.15%. Together with the standard uncertainty of the primary standard (water calorimeter) of 0.2% thus results a combined relative standard uncertainty of 0.5% at 5 Gy or 0.3% at 25 Gy.

These data are valid when the probes required to draw-up a calibration curve had not been irradiated more than one week before or after the measuring probes proper had been irradiated. Another prerequisite is that the conditions during irradiation and storage are identical for test and calibration probes. In the case of different ages, it may be necessary to include an additional uncertainty contribution of up to 0.3%. At present, the uncertainty contributions due to different ages or different storage conditions are still the subject of extensive investigations.

In addition to the activities related with the uncertainty budget, the first Alanin probes from in-house production have been subjected to sampling inspection. Moreover, the response of the Alanin dosemeter for ultra-hard X-radiation was investigated on the accelerator of Department 6.2.

Figure: Sketch of the newly developed sample holder