28.04.2010
Metrology for radiation protection has, to date, been based on dosemeters which were developed and tested for continuous radiation. In the last few years, however, a change to pulsed fields has occurred in all areas, i.e. in medicine, industry and research, whereby at the same time, the use of electronic dosemeters has established itself in this field of metrology. The metrological problems it causes in pulsed fields of ionising radiation have meanwhile become obvious and have caused the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) to restrict the use of electronic dosemeters in pulsed radiation fields.
As a reaction to this metrological issue, PTB has set up a facility for the defined generation of pulsed X-ray radiation (Fig. 1). This pulsed X-ray facility was developed in close cooperation with Siemens on the basis of a modern medical standard product and will for the first time enable the testing of dosemeters for measurements in pulsed radiation fields.
In this facility, a special rotating anode X-ray tube (Fig. 2) is used. Thereby, the tube current - and thus the ionising radiation - is switched on and off by a grid, and the high voltage is applied continuously; this is called "grid-controlled pulsing". This has made it possible to generate radiation pulses having very short rise and fall times in the range of approx. 50 µs. Up to now, for the generation of pulsed radiation, the high voltage has basically always been switched on and off without using a grid; this method is called "input side generator pulsing". The problem with this method is that the rise and fall times for the tube high voltage lie in the range from approx. 300 µs to a few ms due to, e.g., wire capacity.
In the case of the special rotating anode X-ray tube used here, a small modification enabled grid-controlled pulsing application up to a high voltage (accelerating voltage) of 125 kV. This allows, on the one hand, very short pulses in the range of approx. 200 µs and, on the other hand, avoids passing through an undesirable voltage or energy ramp during the rise and fall time of the pulse. This is necessary since the testing only takes into account the dependence of the response on the duration of the pulse, not however on the energy.
Figure 1 : The pulsed X-ray facility consists of the generator (on the left), the beamgenerator (in the centre) with the rotating anode X-ray tube, the seal, the filterwheel and various slide-in diaphragms, as well as the slidable irradiation table (on the right). Here, the secondary standard ionisation chamber for the personal dose equivalent, Hp(10), has been established as the test item.
The adjustable tube high voltage lies in the range from 40 kV to 125 kV; the tube current is in the range from 0.1 mA to 800 mA. At electrical powers of up to 4 kW, continuous operation is possible; beyond that, only pulsed operation is possible. At the maximum possible power of 80 kW, the maximum pulse length is still 300 ms. The ability to generate continuous radiation is necessary to be able to connect the device to previous testing facilities. The pulse repetition rate can be up to 100 Hz, whereby, besides periodic pulse series, also arbitrary unperiodic pulse series can be adjusted. The main advantage this facility offers in contrast to medical standard products is the possibility to adjust all physical parameters of the radiation pulse individually, nearly independent of each other. The irradiation distance can be selected within a range from 30 cm to 300 cm. Thanks to the filter wheel and the adjusting of different tube high voltages, it is possible to select different radiation qualities.
Figure 2 : Grid-controlled rotating anode X-ray tube used in the facility. By slightly modifying the standard product, grid-controlled pulsing using tube voltages of up to 125 kV can be achieved.
The facility has been conceived both for the testing of radiation protection dosemeters with regard to their suitability for measurements in pulsed radiation fields and for research and development purposes for dosimetry in pulsed fields.
In the future, new electronic dosemeters and suitable test procedures will have to be developed so that radiation protection can also be guaranteed in pulsed fields of ionising radiation.