where L is the light yield, S is the scintillation efficiency, dE/dx is the specific energy loss of the particle per path length, k is the probability of quenching, and B is a constant of proportionality linking the local density of ionized molecules at a point along the particle's path to the specific energy loss; "Since k and B appear only as a product, they act as one parameter, kB, called Birks' coefficient, which has units of distance per energy. Its value depends on the scintillating material."