title: Technical Note: Effect of explicit M and N-shell atomic transitions on a low-energy x-ray source.
abstract: OBJECTIVE
  In EGSnrc, atomic transitions to and from the M and N-shells are treated in an average way by default. This approach is justified in which the energy difference between explicit and average M and N-shell binding energies is less than 1 keV, and for most applications can be considered negligible. However, for simulations of low energy x-ray sources on thin, high-Z targets, characteristic x-rays can make up a significant portion of the source spectra. As of release V4-2.4.0, EGSnrc has included an option to enable a more complete algorithm of all atomic transitions available in the EADL compilation. In this paper, the effect of M and N-shell averaging on the calculation of half-value layer (HVL) and relative depth dose (RDD) curve of a 50 kVp intraoperative x-ray tube with a thin gold target was investigated.
  
  
  METHODS
  A 50 kVp miniature x-ray source with a gold target (The INTRABEAM System, Carl Zeiss, Germany) was modeled with the EGSnrc user code cavity, both with and without M and N-shell averaging. From photon fluence spectra simulations, the source HVLs were determined analytically. The same source model was then used with egs_chamber to calculate RDD curves in water.
  
  
  RESULTS
  A 4% increase of HVL was reported when accounting for explicit M and N-shell transitions, and up to a 9% decrease in local relative dose for normalization at 3 mm depth in water.
  
  
  CONCLUSIONS
  The EGSnrc default of using averaged M and N-shell binding energies has an observable effect on the HVL and RDD of a low energy x-ray source with high-Z target. For accurate modeling of this class of devices, explicit atomic transitions should be included.
authors.affiliation: Medical Physics Unit, McGill University, Montreal, Quebec H4A 3J1, Canada.
authors.forename: Peter G F
authors.initials: PG
authors.lastname: Watson
authors.affiliation: Medical Physics Unit, McGill University, Montreal, Quebec H4A 3J1, Canada.
authors.forename: Jan
authors.initials: J
authors.lastname: Seuntjens
mesh_terms: D017785:Photons / Q000627:therapeutic use; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D014965:X-Rays