Radiation protection for medical staff

The X-rays used at workplaces for medical diagnostics are almost exclusively pulsed radiation with pulse durations in the millisecond range (partly as single pulses, partly as pulse sequences) with high pulse dose rates. Typical spectrometers and conventional measuring instruments used in dosimetry cannot process the high event rates implied by this type of radiation. The novel spectrometer is specifically designed for this type of pulsed radiation.

It is based on combining a CeBr3 scintillation crystal and a Geiger-mode avalanche photodiode array. An X-ray photon entering the scintillation crystal of the spectrometer generates scintillation light. Hereby, the number of scintillation photons generated depends on the energy of the X-ray photon striking the spectrometer. These scintillation photons continue on to the cells of a Geiger-mode avalanche photodiode array triggering a load avalanche in individual cells, similar to the process that occurs in a Geiger-Müller counter tube. As a consequence, the cumulative signal of all Geiger-mode avalanche photodiodes reflects the energy of the X-ray photon which first struck the spectrometer.

The signal generated by an X-ray photon in the spectrometer has a duration of approx. 100 ns. This short signal length is possible due to the short decay time of the novel scintillator material CeBr₃, which amounts to 20 ns. The fast data acquisition required to record one measurement value per nanosecond and the initial data processing are performed by a field programmable gate array (FPGA) (sampling rate: 1 GS/s). All in all, the event rate that can be processed amounts to 4 MHz. Measuring the signals of single X-ray photons allows the high pulse dose rates to be measured. The spectrometer is designed for the energy region of X-rays from 15 keV to 150 keV as used in medical applications.

The new device has been characterized in the reference measurement fields of PTB with regard to its energy measuring range, its dose rate measuring range and its angular resolution. The next step will consist in measurements performed at real medical workplaces in hospitals.