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35 Radiation for Radionuclide Users the amount of time per day which she can handle the vial of 50 mCi of 32 P. She places the vial in a test tube rack and holds the rack at a distance of 10 cm from the vial for most manipulations. How much time per day can she now spend handling the vial without exceeding her annual dose limit to the hand? = 0.32. mrem/sec = 1.9 mrem/min = 105.3 min/day (as opposed to 62.5 sec for the previous example) This example illustrates the usefulness of small amounts of distance in dramatically reducing external hazards. Shielding the Source The dose from a particular exposure situation can also be reduced by decreasing the source strength (i.e., dose rate) through the use of shielding. Shielding is any material which is placed around or in front of a radiation source to reduce its accessible dose rate. The amount by which a given shield reduces the dose rate from gamma or x-rays is given by the following equation: Eq. 10 where: = dose rate from source shielded by thickness x = dose rate from source without shielding x = absorber (i.e., shield) thickness e = base of natural logarithms µ = attenuation coefficient Example 5: A researcher is temporarily storing several millicuries of 131I waste solution within the fume hood. The dose rate at the face of the hood is 20 mrem/hr. If the researcher wraps several sheets of lead (total thickness of 1 cm) around the container, what will be