Development of Large-area Ultra-low Background Alpha Ionization Chamber

An ionization chamber was developed for measuring alpha particle emissivity at ultra-low background levels. The ionization chamber is filled with high purity gas and operated in flow mode. Consequently the ionization chamber is insensitive to muon showers in cosmic ray and gamma ray from environment. After removing the radon by flowing gas, the alpha background mostly emanates from different surfaces inside the chamber. The pulse shape analysis method was employed to distinguish the location of emanation of each alpha particle. While alpha particles from chamber surfaces are rejected, only alpha particles emanating from the sample are finally counted at ultra-low levels. The chamber has a 1 521 cm² sample area and a background of 3×10^-4 cm^-2•h^-1.