Dosimetry with electronic dosemeters in pulsed photon radiation fields - 3608S02002
urn:nbn:de:0221-2009082197 BfS-RESFOR-17/09
Summary
In this report, the influence of pulsed radiation fields on electronic personal dosemeters (EPD) based on Silicon semiconductor detectors has been studied. Due to physical effects in the detector and an unavoidable delay during data processing, there is a dead time after each measured event. This causes a loss of pulses – the so-called dead time loss –, especially for high dose rates.
This effect is amplified in pulsed radiation fields. Even if the average dose rate is equal to that of a stationary radiation field, the dose rate during a pulse can be up to several magnitudes higher. Consequently, the dead time loss increases.
In case of a rectangular pulse shape, the dead time loss for a given dose rate can be calculated explicitly via a given empirical formula. For arbitrary pulses, a Monte-Carlo code has been developed. If the dead time and the calibration factor of the dosemeter are known, it is possible to obtain the operational range for any pulsed radiation field.
This calculation has been carried out for two exemplary pulsed radiation sources. For the Dosilab EDM-III and a model dosemeter based on approximated parameters, the responsivity was calculated. The evaluated dose range is deemed sufficient for radiation protection purposes. Two other dosemeters, namely the MGP DMX 2000X and Thermo EPD N2 resp. Mk2 could not be rated as the necessary parameters have not been disclosed by the manufacturers. However, judging by experimental reports on these dosemeters in comparison to the EDM-III, the operational ranges should be similar.
In general, if the maximal pulse dose rate does not exceed 4 Sv/h, then any of the studied dosemeters are usable both in stationary and in pulsed radiation fields with dead time loss no more than 20%. From the perspective of radiation protection, the EPD can be deployed without restrictions in such radiation fields. The dosemeters may still be used for higher pulse dose rates. In this case however, an individual analysis using the means compiled in this report is recommended.
There are applications where the pulse width is shorter than the dead time of the dosemeter. In this case, the dose per pulse is the relevant quantity to evaluate the operational range of the EPD. The dosemeters are usable as long as the dose per pulse does not exceed 45% of the calibration factor that describes the average dose per detector count.
