LEARNING OBJECTIVE:

1.11.13 Calculate shielding thickness or exposure rates for gamma/x-ray radiation using the equations.

SHIELDING CALCULATIONS

The simplest method for determining the effectiveness of the shielding material is using the concepts of half-value layers (HVL) and tenth-value layers (TVL).

One half-value layer is defined as the amount of shielding material required to reduce the radiation intensity to one-half of the unshielded value.

 

 

The symbol m is known as the linear attenuation coefficient and is obtained from standard tables for various shielding materials.

One tenth-value layer is defined as the amount of shielding material required to reduce the radiation intensity to one-tenth of the unshielded value.

 

 

Both of these concepts are dependent on the energy of the photon radiation and a chart can be constructed to show the HVL and TVL values for photon energies.

Table 2. Half-Value Layers

Photon

Energy

(keV)

HVL (cm)

Lead

(11.35 g/cm3)

Iron

(7.86 g/cm3)

Concrete

(2.4 g/cm3)

Water

(1.0 g/cm3)

500 0.38 1.0 3.3 7.2
1000 0.86 1.5 4.5 9.8
1500 1.2 1.8 5.6 12.0
2000 1.3 2.1 6.4 14.0
3000 1.5 2.4 7.9 17.5

The basic calculational approach to photon shielding is to determine the existing exposure rate, decide on the desired exposure rate after shielding and then calculate how many HVL or TVL will be needed. The basic equation for using the HVL concept is:

 

 

where:

 

 

 

The basic equation for using the TVL concept is:

 

 

where:

 

 

 

 

Sample Problem 1.11-13

Calculate the shielded exposure rate from a 500 mR/hr Cs-137 source with 5 cm of lead shielding. The HVL for Cs-137 and lead is 0.65 cm.

 

 

Sample Problem 1.11-14

Calculate the shielded exposure rate from a 7.4 R/hr Cs-137 source with 4 cm of lead shielding. The HVL for Cs-137 and lead is 0.65 cm.

 

 

Sample Problem 1.11-15

Calculate the #TVL and the thickness of lead required to reduce the exposure rate from a 7.5R/hr Co-60 source to less than 100 mR/hr. One TVL for Co-60 and lead is 4.0 cm.

 

 

Sample Problem 1.11-16

Calculate the #TVL and the thickness of lead required to reduce the exposure rate from a 450 mR/hr Co-60 source to less than 5 mR/hr. One TVL for Co-60 and lead is 4.0 cm.

 

SAMPLE PROBLEMS ANSWERS

QUIZ|Previous Lession|Module Menu|Core Menu

 

REFERENCES:

1. ANL-88-26 (1988) "Operational Health Physics Training"; Moe, Harold; Argonne National Laboratory, Chicago

2. "Basic Radiation Protection Technology"; Gollnick, Daniel; Pacific Radiation Press; 1983

3. "Radiological Health Handbook"; Bureau of Radiological Health; U. S. Department of Health, Education, and Welfare; Washington, D.C.; 1970.