Some differences may exist between french and english regulations. Lessons learned are only a translation of those of french incident and are not adapt to english regulation.
Description of the incident
During a crystallography analysis, two people handled the sample when the x-ray diffraction device was on. The collimator and beam stop, which were normally used to provide protection, had been removed.
The two people did not realize that the device was on. The exposure time was estimated at 40 seconds and the distance between the x-ray source and the persons was approximately 40 cm. It is estimated that the hands crossed the x-ray beam on several occasions.
The handlers were not wearing personal dosemeters. A reconstruction of the incident with two film badges (one corresponding to exposure to the hands and the other to that of the chest of operators) revealed :
- an equivalent dose of 480 mSv (on a surface of 2 cm in diameter) to the hands; and
- an effective dose of less than 0.1 mSv at the chest.
The dose received by the hands is practically equal to the annual limit for workers (500 mSv/year). Given the high equivalent dose to the hands, and the uncertainties involved, the handlers underwent a clinical examination (of which we do not know the results).
Lessons to be learned from the incident
On this kind of equipment, the dose rate in the meian beam can be extremely high, and can give rise to very high localised exposures of the hands from handling the sample while the equipment is turned on. For this reason, it is expected that units will be fitted with a robust safety and warning system to physically prevent access to the beam. The removal of any component of this system should cause the x-ray beam to be terminated/shielded.
During certain maintenance operations, it may be necessary to override certain safety systems, thus allowing potential access to the beam. In such cases, personal dosimetry – especially for the hands/fingers – is recommended. Even then it should be noted that dosemeters may not be exposed to very narrow beams, and a radiation monitor should always be used to confirm the radiological conditions.