Traceability of the PTB low-dose rate photon calibration facility

PTB operates an Underground laboratory for DOsimetry and spectrometry (UDO) in the Asse salt mine at a depth of 490 m below the ground. The ambient dose equivalent rate at UDO is only 2 nSv h(-1). Therefore, UDO is well suited for dosimetry at low dose rates as well as for other low-level measurements. UDO provides the only photon calibration facility worldwide for calibrations at low dose rates (comparable to those in the natural environment and even below), which are traceable to primary standards. Photon fields are generated by using radioactive sources of the following nuclides: (241)Am, (57)Co, (137)Cs, (60)Co and (226)Ra. The dose rates and mean photon energy values at the reference point of this calibration facility were calculated analytically after performing calibration measurements using a secondary standard. This procedure ensures for the traceability of the reference dose rate values to PTB's primary standards. The mean photon energy of each spectrum was calculated with respect to the different quantities air kerma and ambient dose equivalent, respectively. The assessment of the uncertainties of the basic properties of the photon calibration fields will be described.