The design, performance and use of fission counters

The basic design criteria for electron-collection fission counters are discussed; data are presented concerning the effect of thickness of fissile material on sensitivity and the effect of electrode spacing and gas pressure on pulse height. High detection efficiencies are obtained with electrode coatings of 1 mg/cm2, and thicknesses up to 2mg/cm2 may be used when high sensitivities are required. When argon is used as the filling gas the product of gas pressure and electrode spacing (pd, in atmosphere-cm) should lie in the region 0.4¿1.0, the higher value being preferable if it does not give rise to difficulties in ionization collection. The operating level of the discriminator bias is usually determined by the effect of ¿-activity in the fissile material. Typical values of the operating bias level at which a high detection efficiency is possible correspond to 5¿20 MeV particle energy. The designs of several types of fission counter are described, illustrating experimental applications and the use of this type of detector in reactor instrumentation systems.