Distance Factor

The distance factor means that the further an individual remains from a radiation source, the less exposure that individual will receive. The intensity of a radiation field decreases as the distance from the source increases.

The figure shows the effect of distance on gamma exposure rates. If the exposure rate at one foot (30.5 cm) away from the source is 1,000 mR/hr, the exposure rate at two feet (61 cm) away will be 250 mR/hr. The 250 mR/hr is 1/4 of the exposure rate at one foot.

Effect of Gamma Exposure Rate and Distance On Total Exposure. At 1 foot, the exposure rate is 1,000 mR/hr. At 2 feet, it is 250 mR/hr. At 3 feet, it is 111 mR/hr. At 4 feet, it is 62 mR/hr.