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Radiography

Introduction
History
Present State
Future Direction

Physics of Radiography
Nature of Penetrating Radiation
X-rays
Gamma Rays
Activity
Decay Rate
  -Carbon 14 Dating
Ionization
Inverse Square Law
Interaction of RT/Matter
Attenuation Coefficient
Half-Value Layer
Sources of Attenuation
  -Compton Scattering
Geometric Unsharpness
Filters in Radiography
Scatter/Radiation Control
Radiation Safety

Equipment & Materials
X-ray Generators
Radio Isotope Sources
Radiographic Film
Exposure Vaults

Techniques & Calibrations
Imaging Consideration
Contrast
Definition
Radiographic Density
Characteristic Curves
Exposure Calculations
Controlling Quality

Film Processing
Viewing Radiographs
Radiograph Interp-Welds
Radiograph Interp - Castings

Advanced Techniques
Real-time Radiography
Computed Tomography
XRSIM

References

Quizzes
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Half-Value Layer

The thickness of any given material where 50% of the incident energy has been attenuated is know as the half-value layer (HVL). The HVL is expressed in units of distance (mm or cm). Like the attenuation coefficient, it is photon energy dependant. Increasing the penetrating energy of a stream of photons will result in an increase in a material's HVL.

The HVL is inversely proportional to the attenuation coefficient. If an incident energy of 1 and a transmitted energy is 0.5 is plugged into the equation introduced on the preceding page, it can be seen that the HVL multiplied by m must equal 0.693.

 

If x is the HVL then m times HVL must equal 0.693 (since the number 0.693 is the exponent value that gives a value of 0.5).

Therefore, the HVL and m are related as follows:

The HVL is often used in radiography simply because it is easier to remember values and perform simple calculations. In a shielding calculation, such as illustrated to the right, it can be seen that if the thickness of one HVL is known, it is possible to quickly determine how much material is needed to reduce the intensity to less than 1%.

 

 

 

Approximate HVL for Various Materials when Radiation is from a Gamma Source

 
Half-Value Layer, mm (inch)
Source
Concrete
Steel
Lead
Tungsten
Uranium
Iridium-192
44.5 (1.75)
12.7 (0.5)
4.8 (0.19)
3.3 (0.13)
2.8 (0.11)
Cobalt-60
60.5 (2.38)
21.6 (0.85)
12.5 (0.49)
7.9 (0.31)
6.9 (0.27)

Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source

 
Half-Value Layer, mm (inch)

Peak Voltage (kVp)

Lead
Concrete
50
0.06 (0.002)
4.32 (0.170)
100
0.27 (0.010)
15.10 (0.595)
150
0.30 (0.012)
22.32 (0.879)
200
0.52 (0.021)
25.0 (0.984)
250
0.88 (0.035)
28.0 (1.102)
300
1.47 (0.055)
31.21 (1.229)
400
2.5 (0.098)
33.0 (1.299)
1000
7.9 (0.311)
44.45 (1.75)

Note: The values presented on this page are intended for educational purposes. Other sources of information should be consulted when designing shielding for radiation sources.