Photoelectron escape from a silicon X-ray detector: Monte Carlo study

[Measured X-ray Spectra and Absorbed Dose Conversion Factors in a Water Equivalent Phantom Irradiated with Diagnostic X-rays]

The diagnostic X-ray spectra in a water equivalent phantom have been measured. From these measured spectra, the absorbed dose conversion factors of water were derived. The primary X-ray spectra were also measured and the scattered X-ray spectra were calculated by subtraction. The measurements were made at the depths of 0, 5, 10, 15, and 20 cm in a 20 cm-thick phantom and at the X-ray tube voltages 60, 90, and 120 kV by using a small silicon diode detector. The radiation field size was 30×30 cm² at the phantom surface. In the obtained spectra, the fraction of the scattered photon number is increased with the depth. The X-ray qualities of the spectra in the phantom were near the qualities of primary X-rays when the depth is 0 cm, and became near the qualities of scattered X-rays as the depth increases. The changes of the X-ray qualities due to the depth change were small; photon mean-energy changes were within 4.6 keV. The changes in the absorbed dose conversion factors were also small (within 0.68%). These conversion factors were 0.4-2.3% larger than those obtained from the effective energy of incident X-rays and only -0.3 to 0.5% larger than those obtained from the X-ray spectra calculated from the aluminum half value layer and the tube voltage of incident X-rays. This study shows experimentally that the absorbed dose in a water-like phantom can be calculated with good accuracy by using the absorbed dose conversion factor obtained from the incident X-rays.

Experimental and Monte Carlo-simulated spectra of standard mammography-quality beams

A spectrometric study of standard mammography-quality beams by using experimental and Monte Carlo simulation methods was carried out in this work. The qualities of these beams are described according to the International Electrotechical Commission 61267 standard and the Technical Report Series 457 International Atomic Energy Agency report. Specifically, the non-attenuated RQR-M beam series was studied.

METHODS

A Si-PIN diode-based spectrometer and the PENELOPE Monte Carlo code (v. 2008F1) were used for experiments and simulations, respectively. In addition, an ionization chamber was used to determine the half-value layers (HVLs) of each beam quality. The measurements were done in the mammography dosimeter calibration setup of our laboratory, and the Monte Carlo simulations reproduced such conditions.

RESULTS

The relative differences between the HVLs calculated from experimental and simulated spectra were lower than 2.4% for all the beam qualities studied. These differences are 1.2% and 3.1% when comparing the HVLs calculated from the experimental and simulated spectra to those determined by using the ionization chamber, respectively. A semi-empirical relation was found to obtain the nominal tube potential from the effective tube potential.

CONCLUSION

According to our results, the mammography beams used in this work have energy spectra similar to clinical beams.

[Absorbed dose conversion factors obtained from X-ray spectra measured at water phantom surface]

The absorbed dose conversion factor for X-rays at the water phantom surface has been obtained from the measured spectra. These measurements have been made at tube voltages of 60 kV to 120 kV and field sizes ranging from 5 x 5 cm(2) to 30 x 30 cm(2) with and without additional 2 mm aluminium filtration. A small silicon diode detector with little angular dependence was used for this measurement. The absorbed dose conversion factor obtained was 0.03-0.43% smaller than that obtained from the primary X-ray spectrum. The difference was large for high-voltage and heavily filtered X-rays. As field size increases, the conversion factor decreases, but the decrease is slight when field size exceeds 20 x 20 cm(2). The absorbed dose conversion factor obtained from the primary or surface X-ray spectrum is 0.4-1.8% larger than that obtained from the effective energy of primary X-rays. The difference is large in high-voltage X-rays and decreases slightly with increases in field size.

X-ray spectroscopy in mammography with a silicon PIN photodiode with application to the measurement of tube voltage

In this work a silicon PIN photodiode was employed in mammographic x-ray spectroscopy under clinical and nonclinical conditions. Measurements have been performed at a constant potential tungsten anode tube, adapted in this work with molybdenum filters to produce a beam like that used in mammography, and at a clinical equipment with a molybdenum anode tube by using an additional aluminum filtration. The corrected x-ray spectra were in full agreement with those generated by theoretical models published in the literature and agree well with those measured with a CdZnTe detector for tube voltages less than 30 kV. The half value layer and the relative exposure values calculated from the corrected silicon PIN photodiode spectra were in agreement with those measured with an ionization chamber. These results indicate that a silicon PIN photodiode are very suitable for mammographic x-ray spectroscopy. As an application, the voltage (kV) applied to mammographic x-ray equipment has been measured through the evaluation of the spectra high energy cut off. Uncertainties evaluated for the voltage values calculated from the measured spectra are less than 0.13% for voltages in the range 20-35 kV. The low uncertainties associated with the obtained results in this work point out that the method employed can be accurately used for calibration of noninvasive mammographic kVp meters.

Measurement of backscattered x-ray spectra at the water surface in the energy range 60 kV to 120 kV

Backscattered x-ray spectra at the water surface have been measured by using a small silicon diode detector. The measurements have been made at tube voltages 60 kV to 120 kV (HVL 2.4-6.1 mm Al) and field sizes 5 x 5 cm2 to 30 x 30 cm2. The measured spectra are corrected for detector distortion and for the angular dependence of detector efficiency. The obtained backscattered spectrum has a lower mean energy and a narrower shape than the primary spectrum. The ratio of the mean energy of the backscattered spectrum to that of the primary spectrum is between 0.83 and 0.94. The ratio of the spectrum width at 10% of the continuous spectrum maximum is between 0.65 and 0.78. The change of spectral shape due to the field size is slight. In the high-voltage spectra, the peak due to the Compton scattering of tungsten Kalpha x-rays is observed. The backscatter factors (BSFs) calculated from the obtained spectra show a satisfactory agreement with other studies. The difference between the BSF defined as the ratio of air kerma and the BSF defined as the ratio of water kerma is also calculated; the maximum difference is 0.43%. The empirical equation showing the relation between the two BSFs is presented.

A silicon diode in a thimble-type mount for measurement of diagnostic x-ray spectra

A silicon detector which can be used like a thimble chamber has been constructed. The silicon diode chip used here is very small (1.7 x 1.7 x 0.1 mm3), and the construction materials of the detector mount are thin and have low atomic numbers. Thus, the characteristic x-rays from the mount give no distortion on the obtained spectra, and the photons incident on the back surface of the silicon chip can be measured. Furthermore, the perturbation of the x-ray field is small. The energy resolution of the detector is 2.0 keV (full width at half maximum) for 59.5 keV photons at room temperature. Bremsstrahlung spectra (60-100 kV, 1-2 mA) have been measured without using a pinhole collimator; the obtained spectra are in good agreement with those obtained with a Ge detector. The spectrum of the radiation in a water phantom irradiated with 90 kV x-rays has also been measured by inserting the detector in the phantom; the obtained scattered spectrum has been corrected for the angular dependence of detector efficiency. The increase in total photon number due to this correction has been found to be below 2%.