Direct Reading Dosimeters (DRD)

  

Instead of having a steady voltage applied to it all the time, an ionisation chamber  could be charged up to a known voltage like a capacitor. When the chamber is exposed to radiation the ionisation process will discharge the chamber to a lower voltage. The drop in voltage is a measure of the amount of radiation exposure the chamber has received. A practical example of this type of ionisation chamber is the direct reading dosimeter or DRD  . Fig. 6.3 shows the internal construction of the device.

The wall thickness of the instrument is such that (with the exception of the highest energy beta particles ) it responds to gamma radiation  only. The built-in scale reads directly in mSv of gamma dose. The ionisation chamber portion has a double electrode in the centre; one is a fixed metal wire and the other is a metal coated quartz fibre  spring. The case is the other electrode  of the ionisation chamber.

Initially, the electrodes are charged to around 200 volts by connecting an external power supply to the charging pin. Both the centre electrodes will then be positively charged and a repulsive force will exist between them. This force drives the moveable fibre away from the fixed wire. The moveable fibre is observed through a simple microscope against a calibrated scale, and the voltage is adjusted until the fibre is on the zero mark. Fig. 6.3 shows a view of the scale.

 
Figure 6.3: Direct Reading Dosimeter  

When the chamber is irradiated, the newly created ions will discharge the electrodes to a lower voltage. This will cause the fibre spring to move proportionally. The new position is indicated by the image seen in the microscope and gives a reading of the exposure. DRDs are made to operate in different ranges. Those in use at TRIUMF are calibrated from , and respond to gamma rays with energy above 35 keV.

The DRD provides an instantly available reading of the total gamma exposure received since the time of charging. At TRIUMF, the DRDs are used by workers to measure the gamma dose from a particular job, or when entering areas with higher fields during a shutdown. They are worn with and at the same location as the TLD badge .

Visitors and temporary workers who do not have TLD badges are also issued with DRDs . The instruments are read before and after use and the difference gives the gamma dose.

Three points should be noted when using DRDs:

1. There is a slight leakage of charge  from the fibre (equivalent to a reading between and a day) which will slowly discharge the chamber over a period of weeks. The leakage error is negligible if the readings are recorded at the start and end of each day, or period of use, if less than a day.
2. The DRD is fairly rugged, but should not be dropped or banged because the quartz fibre may move and give a false reading.
3. The DRD scale should be vertical when read, and not inclined to either side.