Answer to Question #4440 Submitted to "Ask the Experts"

I would like to know the HVL (half-value layer) and TVL (tenth-value layer) values for I-131 radiation (364 keV) in mm Pb and in cm concrete. Thank you.

While reviewing literature for this information, I found a number of different values, but most agreed within reason. For ¹³¹I in lead (Pb), the half-value layer (HVL) is approximately 3.0 mm and the tenth-value layer (TVL) is approximately 1.0 cm. In concrete (density = 2.2 g/cm3), the HVL is approximately 5.5 cm and the TVL is approximately 18 cm. These values were taken from Figure 6.12 and Table 6.12 in the Handbook of Health Physics and Radiological Health.

There are other references that either provide this information or part of it, such as half-value layer only. It is helpful to know that TVL = 3.33 HVL. Keep in mind that the variation in values for HVL may be due to choice of data used in the calculations to determine the values. ¹³¹I has a number of gamma-ray emissions of different energies, mass absorption coefficients are often taken from a graph (choosing a value has some uncertainty), and concrete densities may vary significantly depending on the type of concrete being used.

Due to possible variations in values for HVL and TVL, you should always indicate the reference for the values that are used in any calculation.

Ken "Duke" Lovins, CHP

References

• Schleien B, Birky B, Slaback Jr L. Handbook of health physics and radiological health, 3rd ed. Lippincott Williams & Wilkins; pp 6-41 and 6-61.
   
• Bureau of Radiological Health, US Dept. of Health, Education and Welfare, Radiological Health Handbook, Revised Edition. US Government Printing Office: Washington, DC; p 155; 1970.