Simulation of detector calibration using the Monte Carlo method is very convenient. The computational calibration procedure using the MCNP code was validated by comparing results of the simulation with laboratory measurements. The standard source used for this validation was a disc-shaped filter where fission and activation products were deposited. Some discrepancies between the MCNP results and laboratory measurements were attributed to the point source model adopted. In this paper, the standard source has been simulated using both point and surface source models. Results from both models are compared with each other as well as with experimental measurements. Two variables, namely, the collimator diameter and detector-source distance have been considered in the comparison analysis. The disc model is seen to be a better model as expected. However, the point source model is good for large collimator diameter and also when the distance from detector to source increases, although for smaller sizes of the collimator and lower distances a surface source model is necessary.
Quality control of X-ray tubes for medical radiodiagnostic services is very important. Therefore, it is convenient 10 develop new procedures to characterise the X-ray primary beam, obtaining an accurate assessment of the actual photon spectrum. The Compton scattering technique is very useful to determine X-ray spectra, avoiding the pile-up effect in the detector, as usually a large room is not available to apply other techniques. The Compton scattering procedure has been simulated using the Monte Carlo method. Several simplified models have been developed for the scattering assernbly, considering the X-ray focus as a point source.1. Introduction The development of new procedures to characterise the primary photon beam from medical X-ray tubes under normal work conditions is very important for quality control of those devices.In X-ray spectra measurements, the number of photons per time unit that reaches the detectcr shall he limited since detectors cease to work properly at high count rates. At high fluence rate a 'pile-up' effect can be observed. In the pile-up, an accumulation of pulses produces a distortion of the X-ray spectrum measured. To obtain appropriate count rates for X-ray tubes with high fluence rate avoiding the pile up effect, a collimator with.small diameter and long tube-detector distance should be used. Nevertheless, this equipment assembly is hard 10 use in a laboratory or X-ray room a cause of the small space available. Therefore, a Compton scdttering technique is proposed in order to avoid the pulse pile-up [I J. Computer simulation is always a useful tool that permit?, to check various dispositions and components of an cxperimental technique in order to improve it. In this work, the Compton scattering technique is simulated using the MCNP 4C code [Z], based on the Monte Carlo method. Different models are dcveloped in order to compare the effect on the obtained spectrum of each cumponent of the Compton scattering equipment. 0-7803-7939-X/03/$17.00 0 2003 IEEE 228The MCNP code is a well established computer code, very useful to simulate the calibration of a HPGe detector [3]. It was validated for monoenergetic sources [4], but it can be also applied when an energy spectrum is provided. In particular, it has been used to obtain pulse height distributions from gamma sources [SI. Compton spectrometerThe Compton spectrometer includes a shielding chamber, a scattering chamber containing the scattering material (PMMA or lucite) and a spectrometer tube with lead collimators [6]. The assembly can be seen in figure 1. Shielding and scattering chambersThe main purpose of those chambers is to line up the primary beam coming from the X-ray tube and also to direct the 90" scattered beam towards the spectrometer tube. Moreover, this, assembly avoids the undesired scattered photons to reach the spectrometer tube. Collimators A, B and C define the direction of the primary beam. Furthermore, collimator C reduces the backscattering effect produced by photons that pass across the scattering chamber without being...
The quality control of mammography units is necessary to reduce the dose imparted to women as much as possible. An accurate characterisation of the primary X-ray spectra is very useful for this purpose. Primary spectra can be obtained using Compton spectrometry techniques. In this work, a commercial spectrometer used to characterise a mammography X-ray tube has been simulated using the Monte Carlo method by means of the MCNP code. Using the developed model, a Response matrix is obtained. Owing to the fact that this matrix is ill-conditioned, the inversion is not a simple process. This problem has been solved using the truncated singular value decomposition method. Results obtained when this methodology was applied have been compared with theoretical X-ray spectra.
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