radguide

34 Radiation for Radionuclide Users It follows, then, that any reduction in the time (t) spent in radiation field ( ) will decrease the total dose received. Example 3: A biochemist is working with a microcap vial of 50 mCi of 32P in a 1 ml solution. The dose rate to the researcher’s thumb when holding the vial (a source to tissue distance of 1 cm) is 3.2 mrem/ sec. If the researcher works 5 days per week all year with this type of sample, what is the maximum time per day she can handle the vial and not exceed her shallow dose equivalent limit to her hand? daily maximum = 50,000 mrem/yr divided by 250 work days/yr daily maximum = 200 mrem/day t = 200 mrem/day divided by 3.2 mrem/sec t = 62.5 sec/day This example clearly illustrates the substantial doses which an individual’s hand can receive in a short period of time when he or she is working with relatively high specific activity 32 P solutions. If the individual added some shielding or just increased the distance by a few centimeters the dose would be greatly reduced as we shall see in the next examples. Maximizing Distance from the Source If one assumes that a source of gamma or x-rays occupies a single point, it can be shown mathematically that the dose rate at distance d from the source { (d)} varies inversely with the square of the distance from the source: Eq. 9 where (d) is the dose rate at distance d from the source and is the dose rate at a distance of 1 unit (meters, feet, etc.) from the source. From this equation, it is apparent that doubling the distance from a radiation source decreases the dose rate by a factor of four. Increasing the distance by a factor of three decreases the dose rate by a factor of nine, etc. The inverse square equation can also be used to approximate the dose rate from high energy beta particle emitters over short distances (e.g., less than 0.5 meters). At distances greater than this, beta particle dose rates decrease even more rapidly than predicted by the equation due to attenuation in air. Example 4: The same researcher as in the previous example wants to increase