Centripetal force is an external force required to make a body move along a circular path with uniform speed.
When a body moves in a circle, its direction of motion at any instant is represented by a tangent at that instant. It is found that the direction of the moving body in a circle keeps changing continuously. But the magnitude of the velocity remains constant.
In 1928, Geiger and Muller in Germany developed a 'Particle detector' for measuring 'ionizing radiation'. They named it 'Geiger Muller Counter'. It has been one of the most widely used nuclear detectors in the early days of Nuclear physics. It is a gas filled counter that operates in the Geiger region. The main difference between 'proportional counter' and GEIGER-MULLER counter is that in the former, the avalanche is formed only at a point whereas it forms in the central wire in case of Geiger Muller Counter. The amplification does not therefore depend on the initial ionization produced by the ionizing particle.