Technical Note: Effect of explicit M and N-shell atomic transitions on a low-energy x-ray source

Area of Science:

• Medical Physics
• Computational Physics
• Atomic Physics

Background:

• EGSnrc software typically averages M and N-shell atomic transitions, which is generally acceptable due to small energy differences.
• However, for low-energy X-ray sources interacting with high-Z targets, characteristic X-rays can be a significant component of the emitted spectrum.

Purpose of the Study:

• To investigate the impact of M and N-shell averaging on Half-Value Layer (HVL) and Relative Depth Dose (RDD) calculations.
• To evaluate the necessity of explicit atomic transition modeling in EGSnrc for specific X-ray source applications.

Main Methods:

• Modeled a 50 kVp miniature X-ray source with a gold target using EGSnrc, comparing simulations with and without M and N-shell averaging.
• Determined source HVLs analytically from photon fluence spectra.
• Calculated RDD curves in water using the egs_chamber user code.

Main Results:

• Including explicit M and N-shell transitions increased the HVL by 4%.
• A decrease of up to 9% in local relative dose was observed when normalizing at 3 mm depth in water.

Conclusions:

• The default averaging of M and N-shell binding energies in EGSnrc has a measurable effect on HVL and RDD for low-energy, high-Z X-ray sources.
• Explicit modeling of atomic transitions is recommended for accurate simulations of such devices.