How to Calculate Half-Value Layers for Evaluation

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The half-value layer, abbreviated as HVL, is a measurement used in modern imaging. It represents the thickness of a material that will reduce a particular radiation by one-half intensity. The HVL is not only unique to the material, but it is also unique to the type of radiation. For example, the HVL for lead is different than that of steel. Similarly, the HVL for gamma rays is different than that of x-rays. HVL can be determined experimentally or mathematically, using its inverse relationship to the attenuation coefficient.


    Position an x-ray source so it radiates on an exposure meter.

    Turn on the x-ray source.

    Read the exposure level on the exposure meter. This value with no absorbers between the devices is your 100 percent reading.

    Turn off the x-ray source and place an absorber between the x-ray source and the exposure meter. Turn the source back on.

    Read the exposure meter. If the exposure is more than 50 percent of the intensity of the x-rays from the source, turn off the source and add another absorber. Then turn the source back on.

    Repeat Step 5 until the exposure is 50 percent of your initial value. This total thickness of the absorbers is the half-value layer.


    Determine the attenuation coefficient of a material. This can be found in a table of attenuation coefficient or from the manufacturer of the material.

    Divide 0.693 by the attenuation coefficient to determine the HVL.

    Multiply your answer by 10 to express your HVL in millimeters. This is necessary because many attenuation coefficients are given with the units cm^-1, and some HVLs are expressed in mm. Your answer can also be multiplied by 0.39 to convert centimeters to inches.


    • Overexposure to x-rays and other radiation can be harmful. When using radiation in the lab, take proper precautions to protect yourself.


About the Author

Kevin Carr has been writing for a variety of outlets and companies since 1991. He has contributed to McGraw-Hill textbooks for middle school and high school, written for the Newspaper Network of Central Ohio and has been a featured film critic for online publications including 7M Pictures and Film School Rejects. Carr holds a Bachelor of Science in education.

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