MadSci Network: Physics
Query:

Re: Gamma Ray Penetration

Area: Physics
Posted By: Sharon Shriver, Instructor (faculty; Ph.D.), Dept. of Pharmacology (I do molecular genetics), University of Pittsburgh
Date: Thu Oct 23 15:29:13 1997
Area of science: Physics
ID: 875702787.Ph
Message:
Although I am not a physicist or a radiologist, I work with radioactive 
isotopes in my biomedical research and found a quick answer to your 
question in my handy Radiation Safety Training Manual (published by the 
Radiation Safety Office of the University of Pittsburgh, to give credit 
where it's due). The following is extracted and paraphrased from that 
manual:

Gamma rays are attenuated exponentially, therefore theoretically it is not 
possible to stop the radiation completely.  However, the exposure can be 
reduced by various amounts. For example, if a thickness of a particular 
shielding material reduces the radiation to one-half the initial amount (a 
"half-value layer", or HVL), then the thickness of three such layers will 
reduce the dose to one-eighth (1/2 x 1/2 x 1/2) the initial amount.  
Similarly, three "tenth-value layers"  (TVL) will reduce the dose to 1/1000 
of the initial amount (1/10 x 1/10 x 1/10).  Shielding materials of high 
density and high atomic number, such as lead, are generally the most 
effective absorbers or shields for gamma rays. Thus less thickness and 
weight per square foot is required for such materials. However, steel, 
brick, concrete, or other materials can provide the same degree of 
shielding if used in appropriately greater thicknesses.  Below are some 
examples of tenth- value layers for some gamma-emitting radionuclides of 
various energies:
	
									Tenth Value Layer
	Radionuclide	Energy (MeV)  lead (cm)	concrete(in)
____________________________________________________________
	Cs-137			0.662			2.1			6.2
	Co-60			1.33			4.0			8.1
	Cr-51			0.32			0.7			4.3
	I-125			0.035			0.13		1.0
	Na-22			1.27			4.0			8.1
	

As you can see, 10 cm of lead will reduce gamma radiation from these 
radianuclides to various extents, from almost completely for I-125 to about 
1/500 the initial amount for Co-60. 


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