Measurement of linear accelerator spectra, reconstructed from percentage depth dose curves by neural networks.

Purpose Linear accelerator (linac) spectra, used to improve the accuracy of dose calculation and to produce a complete description of beam quality, among other aspects, are relevant in radiotherapy and linear accelerator physics. Methods In this work we apply neural networks in solving an ill-conditioned system of linear equations, to indirectly measure the linear accelerator spectra via the percentage depth dose curves. The photon beam spectra are related to radiation doses through a Fredholm integral equation. To address our problem we measured the percentage depth dose curve in water and solved a discretized Fredholm equation using artificial neural network. After analysing the typology of our physical problem we selected a MultiLayer Perceptron Neural Network and designed the most suitable neural network architecture. Results The reconstructed spectra were compared with spectra from three linacs, two of them obtained by us through simulations and the third produced by another author, achieving a concordance between 92 % and 96 %. Conclusions Therefore, the spectrum of any accelerator can be quickly and easily reconstructed from measured percent depth dose curves, applying a trained artificial neural network.

A novel and fast methodology to calculate doses in LDR brachytherapy.

We present the concept of a new methodology for faster simulation of the doses in brachytherapy with permanent implants, based on the knowledge of the seeds arrangement, adding previously simulated doses in an equivalent medium in terms of the atomic composition of the organ in question. To perform the doses calculations we use Monte Carlo simulations. We simulated a cylindrical I-125 seed and compared our results against published data. Our proposal is to have the doses simulated previously in different arrangement of seed-absorbents, and then, considering the spacial positions of the seeds after the implants, these doses can be directly added, obtaining a very fast computation of the total dose. Two phantoms of prostates with permanent implant seeds in 2D and 3D arrangements were simulated. The results of the proposed methodology were compared with two complete Monte Carlo simulations in 2D and 3D designs. Differences in doses were analysed, obtaining statistical discrepancies of less than 1% and reducing the simulation time by more than 4 orders of magnitude. With the proposed methodology, it is possible to perform rapid dose calculations in brachytherapy, using laptop or desktop computers.

Influence of the sample density in γ spectrometry.

The determination of γ activities of radionuclides in the environment is of great interest and good accuracy is required in the results of these measurements. However, one of the main sources of error in this technology is the difference in the densities of the samples, because the effects of self-absorption, within the samples, become significant. We have produced Monte Carlo simulations for samples with different densities and for photons with energies in the range of interest, obtaining the peak efficiency as a function of density and energy. We fitted it to obtain a two-dimensional mathematical representation of this function, so we can use this fitted function saving the time of Monte Carlo simulation. To verify the validity of our proposal, the natural radioactivity of soil samples were determined using a GeHP solid state detector and very good results were obtained.

A methodology to determine natural radioactivity by γ spectrometry without using calibrated standard samples.

When determining the radioactivity in materials by γ spectra, different processes are involved difficulting the fit. Besides, calibrated standard samples are necessary. In this work we propose a methodology to determine natural radioactivity using simulated γ spectra by Monte Carlo and without the use of calibrated standard samples. We applied this methodology to determine radioactivity of K 40 isotopes from a light salt sample. Then we applied this process in soil samples and the results where compared with the values obtained by an other laboratory. In this comparison our results are on average a 13% smaller.

Analysis of X-ray absorption spectra of the K and L2,3 edges of GaN within the FP-LAPW method.

Gallium nitride, GaN, is a semiconductor material with several technological applications. In this work we obtain ab initio XANES spectra using FP-LAPW method within the DFT formalism using different potentials (LDA, PBE and TB-mBJ) in order to study the electronic properties of the system. The spectra calculated using the effect of the fractional core hole were compared with experimental data obtaining a very good agreement.

L X-ray fluorescence cross sections experimentally determined for elements with 45 < Z < 50 at 9 keV.

Experimental determination of L fluorescence cross-sections for elements with 45