If you are interested in having your dosimetry system calibrated in the PTB's reference radiation fields, the line of action to be followed is described here.
For the calibration of a dosemeter the proportionality factor between the reading of the dosemeter and the absorbed dose to water under well defined reference conditions is determined. This is achieved by irradiating the dosimeter probe - usually an ionization chamber - with a well known dose in the water phantom while the signal, i.e. the electrical charge generated in the ionization chamber, is measured simultaneously. The calibration factor is the ratio of the signal measured under reference conditions to the value of the absorbed dose to water.
Because not all reference conditions can be met during such a measurement (for instance the air pressure or the temperature deviate from their reference values), the dosimeter signal has to be corrected so that the calibration factor is valid for reference conditions. Therefore a number of auiliary quantities, as for instance the air pressure, the temperature of the water in the phantom, the humidity or the polarizing voltage have to be recorded.
Detailed information about the calibration procedure and the influence quantities can be found in the technical report TRS-469 "
Calibration of Reference Dosimeters for External Beam Radiotherapy" issued by the International Atomic Energy Agency.
For absorbed dose measurements based on the German dosimetry protocol DIN 6800-2 the use of ionization chambers is recommended, which are calibrated in the 60Co beam in terms of the absorbed dose to water.
Furthermore the PTB offers calibrations in the high-energy photon beams of the 
clinical linear accelerators. Dose measurements based on such direct calibrations may have the advantage of a smaller uncertainty which could be important for research projects which need a very precise dosimetry.
All calibrations in terms of absorbed dose to water are traceable to the primary standard for absorbed dose to water, the water calorimeter.