When a particle traverses a medium, the particle's path is usually not a straight due to scattering. The CSDA range is a very close approximation to the average path length traveled by a charged particle as it slows down to rest, calculated in the continuous-slowing-down approximation (CSDA). In this approximation, the rate of energy loss at every point along the track is assumed to be equal to the total stopping power. Energy-loss fluctuations are neglected. The CSDA range is obtained by integrating the reciprocal of the total stopping power with respect to energy.
The projected range is the distance between the point where the particle enters the stopping medium and the point where the particle is absorbed (or comes to rest), projected onto the original direction of travel. The CSDA range is always higher than projected range .