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Construction and manufacture of a photon irradiation facility |
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Scientific news from division 5 | ||||
| Construction and manufacture of a photon irradiation facility |
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| As replacement of an old irradiation facility, the Scientific Instrumentation Department constructed and manufactured - in cooperation with the “Radiation Protection Dosimetry” Department - a new photon irradiation facility for the calibration of standards and dosimeters. In this facility, an X-ray tube with an accelerating voltage of max. 120 kV is used. The facility is used to realize, as a matter of routine, 40 reference radiation fields by changing the tube voltage and filtering the direct X-ray. 30 filters of different thickness (0.1 mm to 12 mm) and different metal alloys which generate a different spectrum are accommodated in a filter wheel. The beam cross section is adjusted via lead diaphragms in two diaphragm wheels. The angular positions of filter and diaphragm wheels are determined via absolute angle encoders. Between the diaphragm wheels, a stationary monitor chamber is situated for beam power measurement. To avoid that persons who are present in the measurement room are exposed to inadmissible X-radiation and that the measurement results are influenced by scatter radiation, the X-tube is installed in a housing made of lead (5 mm) and steel (1 mm). Via adjusting screws, the tube housing is suspended in all axes so that it can be adjusted. A seal (lead gate) in front of the tube opening releases the beam for irradiation. Two compressed air cylinders which are mounted in series move this gate valve by 2 x 50 mm. Moving of the seal in the same direction after radiation ensures that the irradiated field always stays uniform. In the case of a failure of the compressed air network, the seal is automatically closed by force of gravity and a compressed air reserve. The lead thickness of the seal is dimensioned in such a way that persons may stay in the measurement room even when the X-tube is in operation. From an adjoining room, the dosemeters are run over a rail, guided to the measuring installation and fixed on a circular table. Two measurement tables, which can alternatively be moved to a table (displaceable by 4 000 mm in beam direction) via guide rails, are available for different measuring arrangements. The table has a position repetition accuracy of 1 mm. The position is determined via magnetic tapes and magnetic sensors. Two line lasers are arranged on the housing to represent the course of the X-ray. The centre of the X-ray is where the two laser beams intersect. All movements of the table and revolutions of the filter and diaphragm wheel are controlled via a computer and documented. |
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| Figure 1: Photon irradiation facility The dosemeter can be seen on the right of the translation stage. In the background, the diaphragm and the filter wheel can be seen; behind them are the seal and the X-tube. |
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