Answer to Question #4716 Submitted to "Ask the Experts"

How do I figure out the thickness of a lead syringe shield based on HVL (half-value layer) and the attenuation coefficient of lead? How do you tell how much shielding a person will get based on the amount of thickness of lead and attenuation coefficient?

A random review of a commercial vendor's lead and tungsten syringe shields indicates that most of the shields come in fixed thicknesses and are sized according to commercially available syringe sizes. If the vendor does not specify the thickness or lead-equivalent thickness, then you will have to contact the vendor for additional information or obtain a syringe shield and measure the thickness. The half-value layer in lead will vary depending on radionuclide that is being shielded. Half-value layers can be obtained from radiopharmaceutical package inserts. For example, the half-value layer of ^99mTc (the most commonly used radionuclide in nuclear medicine) is 0.2 mm of lead. 0.95 mm lead will attenuate 90% of the gamma rays, while 1.8 mm of lead will attenuate 99% of the gamma rays. Typically, commercially available syringe shields contain more lead than listed above, therefore, providing adequate protection for the user.

The amount of exposure can be calculated outside the syringe shield based on the syringe shield thickness, the amount of activity in the syringe, and the gamma ray constant for the radionuclide; however, it is much easier to measure the dose rate outside the shield with the syringe in the shield. This will also allow the user to observe that the dose rates are much higher at the ends of the shield, where there is no way to shield the radiopharmaceutical dosage.
Ken "Duke" Lovins, CHP