A four-electrode ion chamber for gamma and pulsed x-ray monitoring

The performance of two indigenously developed gamma-sensitive cylindrical ion chambers has been evaluated for area monitoring applications. The ion chambers have outer diameter of 178 mm, overall length of 310 mm and 4.5 litre sensitive volume. When filled with high purity argon gas at 10 atmospheres the chambers have gamma sensitivity of 4.6nA/R/h. One of the chambers has the conventional two-electrode design while the other has four electrodes connected to the HT and signal terminals alternately. The introduction of additional electrodes reduced the electrode spacing from 5cm to 1.55cm and 100% collection efficiency was achieved for 1 kV at gamma exposure of 3380R/h. The four-electrode high-pressure ion chamber can be used as a wide-range gamma-monitoring device from 0.1 mR/h up to 3000R/h. In comparison the two-electrode chamber can not measure exposure levels above 1 R/h. Tests in a beam dump experiment conducted at the electron accelerator facility at CAT, Indore showed that the four-electrode high-pressure chamber can monitor pulsed x-ray background of about 20R/h (average) corresponding to a peak dose-rate of 2x107R/h. Further tests at one atmosphere argon filling pressure showed that the four-electrode ion chamber can monitor gamma background ranging from 10mR/h to 105R/h encountered in isotope laboratory hot cells. (author)