US Dept of Labor

Occupational Safety & Health AdministrationWe Can Help

Example Problems


  1. The half-life of Po-210 is 138.4 days. An anti-static plate is obtained with an activity of 300 µCi. What is the activity 3 years later?

    3 yr   =   3(365)   =   1095 days

    At   =   A0e • A0e-2t   =    0.693 / τ1 / 2

    At   =   (300)e  -0.693•1095 / 138.4 =  (300)e-5.48

    =   (300)(0.00416)   =   1.25µCi

    An alternative method of calculating the answer is to calculate the number of half lives that have passed and used An = A0 (1/2)n.

    n = 1095days / 138.4 days/ half - life =  791 half lives

    An   =   300(1/2)7.91 = 300 (0.00416)   =   1.25 µCi

  2. A source has an intensity of 500 mr/hr at 6 feet. What is the intensity at 50 feet?

    I1 / I2 = [d1 / d2]2  I1 = 500    d1 = 6    d2 = 50   

    I2 = [I1 / d2 / d1]2 = 500 / (50/6)2 = 500 / (833)2 = 500 / 69.4 = 7.2 mR/hr

  3. The half-value layer for a shielding material is 5 mm. If the intensity of the radiation beam striking the shield is 100 R/hr and the shield is 2 inches thick, what is the intensity is mR/hr on the other side of the shield?

    I = I0e  -0.693•x / HVL = (100)e  -0.693(2")(25.4 mm/inch) / 5mm  

    =   (100)e -7.04 =  100 (0.000876)  =  0.088 R/hr

    = 88 mR/hr

  4. Calculate the absorbed dose rate produced in bone (f = 0.922) by 1 MeV gamma radiation which produced an exposure rate of 0.5 mR/hr.

    D = 0.869 ƒ X (R) rads

    = (0.869)(0.922)(0.5 x 10-3 R/hr)

    = 0.4 x 10-3 rads/hr

    = 0.4 m rad/hr

  5. Calculate the absorbed dose rate in soft tissue (f = 0.965) for the condition in problem 4.

    D = (0.869)(0.965)(0.5 X 10-3 R/hr)

    = 0.419 m rad/hr

Back to Top

Thank You for Visiting Our Website

You are exiting the Department of Labor's Web server.

The Department of Labor does not endorse, takes no responsibility for, and exercises no control over the linked organization or its views, or contents, nor does it vouch for the accuracy or accessibility of the information contained on the destination server. The Department of Labor also cannot authorize the use of copyrighted materials contained in linked Web sites. Users must request such authorization from the sponsor of the linked Web site. Thank you for visiting our site. Please click the button below to continue.

Close