Office of Research

XIV. Basic Protection Measures

Earlier in this manual the "ALARA Concept" was mentioned.  This idea that radiation exposure should be kept as low as reasonably achievable is based on a linear extrapolation model.  This concept holds that any exposure no matter how small has some negative effect.  When attempting to limit personnel exposure to ionizing radiation, controlling the variables of Time, Distance, and Shielding will have the greatest effect.

A. Time

For a source of given strength, the absorbed dose is proportional to the duration of the exposure.  Experiments should be carefully planned to minimize exposure time.  This is one reason it is a good practice to use trial runs without the radionuclide present to increase your efficiency, identify problem areas, and possibly reduce the time needed to complete a task. 

B. Distance 

The exposure rate from a point source of radiation is inversely proportional to the square of the distance.  In other words, increasing the distance by a factor of 2 will decrease the exposure rate by a factor of 4.

C. Shielding

Shielding is any material that is used to absorb or attenuate radiation before it reaches a point of interest.

1. High energy beta Particles such as those from the decay of P-32 (E Max. 1.73 MeV), will be shielded using low density material such as lexan sheeting to avoid x-ray production from Bremstrahlung radiation. 

2. Gamma or X-ray shields will be composed of high atomic number materials such as lead, concrete, or water. When more that one thickness of shielding is used, the joints of the shielding should be overlapped.

3.         Neutron shields should be constructed of hydrogenous materials such as water, paraffin, or plastic.

4.         Half value / Tenth Value Layer shielding (gamma):

The thickness of shielding material required to reduce the exposure rate to one half or one tenth of its original value respectively.  A table of some measured HVL and TVL values for various shield materials is shown below.

 

             Half value layer                      Tenth value layer

 

Radiation

 

Lead

 

Concrete

 

Lead

 

Concrete

 

125 kVp X-ray

 

0.28 mm

 

2.0 cm

 

0.93 mm

 

6.6 cm

 

Cs-137

 

6.5 mm

 

4.8 cm

 

21.6 mm

 

15.7 cm

 

Co-60

 

12 mm

 

6.2 cm

 

40 mm

 

20.6 cm