Prog Med Phys.  2016 Dec;27(4):272-276. 10.14316/pmp.2016.27.4.272.

Effect of Target Angle and Thickness on the Heel Effect and X-ray Intensity Characteristics for 70 kV X-ray Tube Target

• Kim G
• Lee R

• KMID: 2376563
• DOI: http://doi.org/10.14316/pmp.2016.27.4.272

Abstract

To investigate the optimum x-ray tube design for the dental radiology, factors affecting x-ray beam characteristics such as tungsten target thickness and anode angle were evaluated. Another goal of the study was to addresses the anode heel effect and off-axis spectra for different target angles. MCNPX has been utilized to simulate the diagnostic x-ray tube with the aim of predicting optimum target angle and angular distribution of x-ray intensity around the x-ray target. For simulation of x-ray spectra, MCNPX was run in photon and electron using default values for PHYS:P and PHYS:E cards to enable full electron and photon transport. The x-ray tube consists of an evacuated 1 mm alumina envelope containing a tungsten anode embedded in a copper part. The envelope is encased in lead shield with an opening window. MCNPX simulations were run for x-ray tube potentials of 70 kV. A monoenergetic electron source at the distance of 2 cm from the anode surface was considered. The electron beam diameter was 0.3 mm striking on the focal spot. In this work, the optimum thickness of tungsten target was 3 µm for the 70 kV electron potential. To determine the angle with the highest photon intensity per initial electron striking on the target, the x-ray intensity per initial electron was calculated for different tungsten target angles. The optimum anode angle based only on x-ray beam flatness was 35 degree. It should be mentioned that there is a considerable trade-off between anode angle which determines the focal spot size and geometric penumbra. The optimized thickness of a target material was calculated to maximize the x-ray intensity produced from a tungsten target materials for a 70 keV electron energy. Our results also showed that the anode angle has an influencing effect on heel effect and beam intensity across the beam.

Keyword

Anode angle; Target thickness; MCNPX

MeSH Terms

Aluminum Oxide
Copper
Electrodes
Heel*
Radiography
Strikes, Employee
Tungsten
Aluminum Oxide
Copper
Tungsten

Figure

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  Fig. 1. Geometry for Monte Carlo simulation of x-ray intensity and assessment of anode heel effect. The position of detectors for calculation of x-ray beam intensity is also shown.

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  Fig. 2. X-ray intensity for different thicknesses of tungsten target material.

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  Fig. 3. The x-ray spectrum without Alumina envelope for target angle 12 degree at 10 cm distance from the target position.

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  Fig. 4. The angular distribution of the x-ray intensity emitted by the tungsten target for 3 μm target thickness.

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  Fig. 5. The x-ray intensity variation with anode angle for 3 μm target thickness.

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  Fig. 6. Illustration of anode heel effect for different anode angles and target thicknesses.

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  Fig. 7. The x-ray spectra with 1 mm alumina envelope for the target angle 12 and 35 degree at 10 cm distance from the target position.

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  Fig. 8. Off-Axis x-ray intensity for target angle 12 (a) and 35 degree (b).

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