Measuring facility at the high-energy photon field

The measuring facility at the high-energy photon field uses two Geiger-Müller (GM) counting tubes to determine the dose rate. Prior to a measurement, these counting tubes are calibrated by means of a secondary standard ionisation chamber. The measuring facility contains a charge measuring device which has been developed in this department for this calibration.

The charge measuring device consists of electrometers to measure the ionisation chamber currents, a chamber voltage module, and a time measurement device to determine the integration times. As the ionisation currents are, due to the low dose rate and the, in part, small chamber volumes, in the range of a few femtoamperes, an electrometer from the Keithley company (Type 642) is utilised besides the electrometer developed in this department.

For the GM counting tubes, counters along with high-voltage modules are provided in the measuring facility. Commercially available devices are used as counters. The high-voltage modules were developed in this department.

High-energy photons are generated by nuclear reactions which occur when suitable target materials are bombarded with accelerated protons. As an additional reference during calibration and testing, pulses with a pulse rate proportional to the proton current are available. These impulses are counted and recorded like the impulses from the GM counting tubes.

Software in the Delphi programming language was developed in the department to control and evaluate the measurements.

Because of the large distance between radiation hall and operating room, the small ionisation chamber currents have to be measured in the radiation hall. For this reason the entire measuring facility was mounted into a mobile trolley. Control of the measuring facility takes place using remote control via a TCP/IP network connection from the operating room.

Figure : Measuring facility at the high-energy photon field