Mobile measuring system for dose measurement with ionization chambers in pulsed radiation fields

To determine the dose applied when persons and goods are examined with X-ray scanners, a transportable measuring system was developed at Department 6.3 This system allows charge pulses from ionization chambers to be measured with a resolution of 12 fC (12 x 10^-15 C) and a maximum measuring rate of 100 Hz. The measuring system is equipped with additional sensors to apply all necessary corrections to the measurement values.


Measurement of the dose applied by X-ray scanners requires a measuring system with the aid of which small doses can be measured at a high dose rate in short pulses. For this purpose, ionization chambers are well-suited detectors. The problem involved in the measurement with ionization chambers is that the charge collected in the X-ray pulse often is of the same order of magnitude as the charge from the chamber leakage and input bias current of the amplifier used which is generated in usual integration times of many seconds. The measuring system described allows the signal from the X-ray pulse to be separated from the background signals by performing measurements at short time intervals, see figure 1.

Figure 1: Example of a measured charge curve from an X-ray pulse

In the transportable measuring system, a charge measuring device developed at the Department is used. The charge measuring device is composed of an integrator amplifier, a chamber voltage source and a timing unit. The integrator amplifier has an input bias current of less than 3 fA. Six charge measuring ranges from 20 pC to 2 µC are available. The output of the integrator amplifier is connected to a 12-bit analog/digital converter (ADC). This ADC allows the collected charge to be recorded with a maximum clock pulse of 100 Hz. Air pressure, temperature and air humidity are measured to correct the change of the air density in the ionization chamber volume. The charge measurement with all corrections of the measurement values is controlled with a notebook. All measurement data recorded and the values calculated from them are stored in the computer.

Since early in 2005, the measuring system, see figure 2, has been successfully used on different X-ray scanners.

Figure 2: Mobile dosimetry measuring system with ionization chamber and climatic sensor

Contact person:

G. Buchholz, Dep. 6.3, WG 6.35, E-Mail: gerd.buchholz(at)ptb.de

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