[show abstract][hide abstract] ABSTRACT: To compare the diagnostic X-ray spectra derived by different methods for a constant potential dental X-ray unit.
Five methods of deriving X-ray spectra for a constant potential dental X-ray unit were compared: measurement by spectrometer using cadmium-zinc-telluride (CZT) detector, calculation by Monte Carlo simulation, calculation by two different, semi-empirical methods and estimation from transmission data. The dental X-ray set was a Heliodent MD unit (Sirona, Charlotte, NC, USA) operable at 60 or 70 kV. A semiconductor detector was used in the spectrometer measurements and an ionization chamber dosimeter in the transmission measurements. From the five methods, photon-fluence spectra were derived. Based on the photon-fluence spectra, average energies and transmission curves in aluminum were calculated.
For all five methods, the average energies were within 2.4% of one another. Comparison of the transmission curves showed an average difference in the range of 1 to 6%.
All of the five methods of deriving spectra are in extremely good agreement with each other.
[show abstract][hide abstract] ABSTRACT: The MCNP Monte Carlo radiation transport code was modified for diagnostic medical physics applications. In particular, the modified code was thoroughly benchmarked for the production of polychromatic tungsten x-ray spectra in the 30-150 kV range. Validating the modified code for coupled electron-photon transport with benchmark spectra was supplemented with independent electron-only and photon-only transport benchmarks. Major revisions to the code included the proper treatment of characteristic K x-ray production and scoring, new impact ionization cross sections, and new bremsstrahlung cross sections. Minor revisions included updated photon cross sections, electron-electron bremsstrahlung production, and K x-ray yield. The modified MCNP code is benchmarked to electron backscatter factors, x-ray spectra production, and primary and scatter photon transport.
Medical Physics 01/2001; 27(12):2680-7. · 2.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: Pulse-height distributions of two constant potential X-ray tubes with fixed anode tungsten targets were measured and unfolded. The measurements employed quantitative alignment of the beam, the use of two different semiconductor detectors (high-purity germanium and cadmium-zinc-telluride), two different ion chamber systems with beam-specific calibration factors, and various filter and tube potential combinations. Monte Carlo response matrices were generated for each detector for unfolding the pulse-height distributions into spectra incident on the detectors. These response matrices were validated for the low error bars assigned to the data. A significant aspect of the validation of spectra, and a detailed characterization of the X-ray tubes, involved measuring filtered and unfiltered beams at multiple tube potentials (30-150 kVp). Full corrections to ion chamber readings were employed to convert normalized fluence spectra into absolute fluence spectra. The characterization of fixed anode pitting and its dominance over exit window plating and/or detector dead layer was determined. An Appendix of tabulated benchmark spectra with assigned error ranges was developed for future reference.
Radiation Research 12/2000; 154(5):564-81. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: Two methods for determining ion chamber calibration factors (Nx) are presented for polychromatic tungsten x-ray beams whose spectra differ from beams with known Nx. Both methods take advantage of known x-ray fluence and kerma spectral distributions. In the first method, the x-ray tube potential is unchanged and spectra of differing filtration are measured. A primary standard ion chamber with known Nx for one beam is used to calculate the x-ray fluence spectrum of a second beam. Accurate air energy absorption coefficients are applied to the x-ray fluence spectra of the second beam to calculate actual air kerma and Nx. In the second method, two beams of differing tube potential and filtration with known Nx are used to bracket a beam of unknown Nx. A heuristically derived Nx interpolation scheme based on spectral characteristics of all three beams is described. Both methods are validated. Both methods improve accuracy over the current half value layer Nx estimating technique.
Health Physics 11/2000; 79(4):402-6. · 1.02 Impact Factor