The effective alpha annual dose is usually evaluated by using a K-value system. But the measured value of K is strongly dependent on the energy of the alpha particles used for measurement. This drawback is avoided in the a-value system

which is based on the experimental fact that the TL per unit length of track is approximately independent of particle energy. Thus

we can specify alpha sensitivity in terms of TL per unit length of track and calibrate alpha source in terms of the length of track delivered per unit volume of sample. In a-value system the effective alpha annual dose is obtained directly from alpha count rate.