• The halo counter is a special scintillation counter with holes around the beam passage.
  • It is use to identify beam particles that are too far away from the axis.
  • The halo counter reject/flag the particle that is large in angle, and is rejected by the collimater filter.
  • It is useful for flagging particles that interacted with certain absorber and underwent scattering.

external image Diagram-of-a-scintillation-counter.jpg
It is an instrument used to measure ionizing radiation. It all starts when the charged particle hits the scintillator. The atom would be excited and emit photons. The photo goes to the photocathode, which then releases electron. The electrons are focused and accelerated to hit the first dynode. Then the dynode releases second electron from the first electron, focus and accelerate it, then hits the second dynode. It keeps going so a current is amplified. Then at the end, it is released at the anode as a measurable pulse at each photo and is passed to the processing electronic. That way, the machine can tell origin incident radiation. It has to be used in complete darkness.