Thick-wall ionization chamber

The specific features and main characteristics of a thick-walled ionization chamber designed for measurements of low-intensity fluxes of high-energy photons passing through a hydrogen bubble chamber are presented. The high-voltage electrodes of the chamber are two copper disks 25 and 15 mm thick and 140 mm in diameter. The total thickness of the from wall of the chamber and of a high-voltage electrode is chosen in such a way as to ensure the maximum of an electron-positron shower generated by photons in the gas volume of the chamber. The collecting electrode 3 mm thick is between the high-voltage electrodes at a distance of 11 mm from each of them. The electrodes are fixed to flanges inside a copper cylinder with an earth potential. High-voltage supply and pulsed voltage output are accomplished by means of brass rods insulated from the flanges with fluoroplastic seals. After pumping, the chamber has been filled with a mixture of 90% Ar and 10% CO2 at a pressure up to 4 ata. The chamber is calibrated in the enery range 600 to 1300 MeV against a quantometer. The measurement results show that the chamber has a sensitivity of (9.5 +- 0.5) 10-19 C/MeV in integral operation and 3.10-9 V/MeV in pulsed operation. The highest sensitivity is achieved at the total thickness of the front wall and high-voltage electrode equal to 30-35 mm, and it does not practically depend on photon energy when being 42 mm thick