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Results for “Monte Carlo Method”

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At least 19 recordsLinked to original sources

PubMed2006

Correction factors for the INER-improved free-air ionization chambers calculated with the Monte Carlo method.

Monte Carlo method was used to simulate the correction factors for electron loss and scattered photons for two improved cylindrical free-air ionization chambers (FACs) constructed at the Institute of Nuclear Energy Research (INER, Taiwan). The method is based on weighting correction factors for mono-energetic photons with X-ray spectra. The newly obtained correction factors for the medium-energy free-air chamber were compared with the current values, which were based on a least-squares fit to experimental data published in the NBS Handbook 64 [Wyckoff, H.O., Attix, F.H., 1969. Design of free-air ionization chambers. National Bureau Standards Handbook, No. 64. US Government Printing Office, Washington, DC, pp. 1-16; Chen, W.L., Su, S.H., Su, L.L., Hwang, W.S., 1999. Improved free-air ionization chamber for the measurement of X-rays. Metrologia 36, 19-24]. The comparison results showed the agreement between the Monte Carlo method and experimental data is within 0.22%. In addition, mono-energetic correction factors for the low-energy free-air chamber were calculated. Average correction factors were then derived for measured and theoretical X-ray spectra at 30-50 kVp. Although the measured and calculated spectra differ slightly, the resulting differences in the derived correction factors are less than 0.02%.

Journal Article↗

PubMed2003

Efficiency transfer in the calibration of a coaxial p-type HpGe detector using the Monte Carlo method.

Monte Carlo simulation was applied to the efficiency transfer exercise described in the EUROMET428 project (Appl. Radiat. Isot. 55 (2001) 493), evaluating the peak efficiencies in the energy range 60-2000 keV for a typical coaxial p-type HpGe detector and several types of source configuration: point sources located at various distances from the detector and a cylindrical box containing three matrices. The efficiency values were derived in two ways: (a) by direct calculation taking into account the physical dimensions of the detector provided by the supplier, and (b) by means of relative computation (efficiency transfer) taking also into consideration the known efficiency values for a reference point source. As expected, some significant discrepancies between the calculated and experimental values were found when a direct computation was made using the data provided by the supplier. On the contrary, the results for the peak efficiency derived by relative calculation by means of an efficiency transfer were in good agreement with the experimental values. The deviations found with this last procedure were generally below 5% for all the geometries considered, which is entirely satisfactory for the purposes of routine measurements.

Calibration↗

PubMed2002

Absorbed dose calculations in a brachytherapy pelvic phantom using the Monte Carlo method.

Monte Carlo calculations of the absorbed dose at various points of a brachytherapy anthropomorphic phantom are presented. The phantom walls and internal structures are made of polymethylmethacrylate and its external shape was taken from a female Alderson phantom. A complete Fletcher-Green type applicator with the uterine tandem was fixed at the bottom of the phantom reproducing a typical geometrical configuration as that attained in a gynecological brachytherapy treatment. The dose rate produced by an array of five (137)Cs CDC-J type sources placed in the applicator colpostats and the uterine tandem was evaluated by Monte Carlo simulations using the code PENELOPE at three points: point A, the rectum, and the bladder. The influence of the applicator in the dose rate was evaluated by comparing Monte Carlo simulations of the sources alone and the sources inserted in the applicator. Differences up to 56% in the dose may be observed for the two cases in the planes including the rectum and bladder. The results show a reduction of the dose of 15.6%, 14.0%, and 5.6% in the rectum, bladder, and point A respectively, when the applicator wall and shieldings are considered.

Brachytherapy↗

PubMedDate not supplied

[The possible role of rare tautomers of DNA bases in mutagenesis: study of the effect of hydration on tautomeric equilibrium by the Monte-Carlo method].

Monte Carlo simulation of hydration of keto and enol tautomers of 9-methylguanine (G) and 1-methyluracil (U) has been performed in relation to a possible role of tautomer transitions of DNA bases in mutagenesis. The comparison of the simulation results with published experimental data and results of quantum mechanical computations leads us to conclusion about difference in the origins of keto-form prevalence in water solutions of G and U. Such a reason for uracil (thymine) is a greater intrinsic stability of keto tautomer while the more preferable hydration of keto form of G is the reason of its predominance. As a consequence, during biosynthesis less hydrophylic environment (as compared to water solution) can be a reason for guanine transition into enol form and can result in mispair formation. Such a mutation mechanism is of a minor probability through thymine (uracil) tautomerization.

DNA↗

PubMed2005

Accounting for both random errors and systematic errors in uncertainty propagation analysis of computer models involving experimental measurements with Monte Carlo methods.

A Monte Carlo method is presented to study the effect of systematic and random errors on computer models mainly dealing with experimental data. It is a common assumption in this type of models (linear and nonlinear regression, and nonregression computer models) involving experimental measurements that the error sources are mainly random and independent with no constant background errors (systematic errors). However, from comparisons of different experimental data sources evidence is often found of significant bias or calibration errors. The uncertainty analysis approach presented in this work is based on the analysis of cumulative probability distributions for output variables of the models involved taking into account the effect of both types of errors. The probability distributions are obtained by performing Monte Carlo simulation coupled with appropriate definitions for the random and systematic errors. The main objectives are to detect the error source with stochastic dominance on the uncertainty propagation and the combined effect on output variables of the models. The results from the case studies analyzed show that the approach is able to distinguish which error type has a more significant effect on the performance of the model. Also, it was found that systematic or calibration errors, if present, cannot be neglected in uncertainty analysis of models dependent on experimental measurements such as chemical and physical properties. The approach can be used to facilitate decision making in fields related to safety factors selection, modeling, experimental data measurement, and experimental design.

Journal Article↗

PubMed1984

The computation of scatter in mammography by Monte Carlo methods.

A Monte Carlo computer program has been used to calculate the ratio of the scatter-to-primary radiation recorded by a range of image receptors used in mammography. The dependence of this ratio on breast size and photon energy has been investigated and the contributions of different sources of scattered radiation examined. The use of magnification and grid techniques have been included in the program and the performance of the Philips mammographic grid examined in detail. The results of the calculations are in good agreement with the limited experimental data available.

Female↗

PubMedDate not supplied

[Study of the flexibility of complementary dinucleoside phosphates by the Monte-Carlo method].

Monte-Carlo calculations have revealed a significant flexibility of the double stranded dinucleoside phosphates AA: TT, AT: AT, TA: TA, GG: CC, GC: GC and CG: CG within the limits of the B-family of forms. Fluctuations of the dihedral angles and of the phase angle of pseudorotation, P, have been found to be 5-20 degrees and 10-20 degrees respectively. The r.m.s. deviation of the helical winding angle varies from 3 degrees to 6 degrees for different dimers. The G: C-containing dimers proved to be more rigid that the A: T-containing ones, though the methyl groups of thymines take part in stabilization of AA: TT and AT: AT dimers. The propeller angles in A: T and G: C pairs are of opposite sing. The pyrimidine-(3'-5')-purine dinucleoside phosphates demonstrate the least stability. All the dimers proved to be more flexible when opened into both grooves than in perpendicular directions. Due to the greater value of the glycosidic angle in the 3'-monomer the base pairs are opened into the wide groove. A connection between opening of the base pairs and bending of the double helix for different nucleotide sequences has been discussed. This might be important for the DNA package in nucleosomes.

Base Composition↗

PubMed1990

[Estimation of scatter component in SPECT planar image using a Monte Carlo method].

A Monte Carlo simulation was performed to estimate quantitatively the scattered photons in planar images of SPECT. As a phantom we used a water-filled cylinder with a line source and calculated the energy spectra of primary and multiple scattered photons separately at each pixel of the planar images. The energy spectra of primary and scattered photons were studied on following parameters: the size of the phantom; the location of the source; the width of the energy window centered at 72.3 (Tl-201), 141 (Tc-99m), and 159 keV (I-123); and the view angle of the planar images. Obtained results were; (1) the energy spectra of Compton scattered photon varied with the phantom size, the source location, and the photopeak energy, (2) the scattered photons within energy windows of 10-30% centered at the photopeak energy were mainly composed by Compton scatter of the first order, (3) the higher order scattering component of the Compton photons did not represent the location of the line source on the planar image, and (4) the scatter fraction defined by the ratio of the scattered photons to the primary photons increased with increasing the size of the phantom and the width of energy window at the low photopeak energy. From the results, we discussed on the scatter subtraction methods.

Computer Simulation↗

PubMed1993

The application of correlated sampling to the computation of electron beam dose distributions in heterogeneous phantoms using the Monte Carlo method.

Although the Monte Carlo method is capable of computing the dose distribution in heterogeneous phantoms directly, there are some advantages to computing a heterogeneity correction factor. If this approach is adopted there are savings in time using correlated sampling. This technique forces histories to have the same energy, position, direction and random number seed as incident on both the heterogeneous and homogeneous water phantom. This ensures that a history that has, by chance, travelled through only water in the heterogeneous phantom will have the same path as it would have through the homogeneous phantom, resulting in a reduced variance when a ratio of heterogeneous dose to homogeneous dose is formed. Metrics to describe the distributions of uncertainty, efficiency, and degree of correlation are defined. EGS4 Monte Carlo calculation of the dose distribution from a 20 MeV electron beam on water phantoms containing aluminum or air slab heterogeneities illustrate that this technique is the most efficient when the heterogeneity is deep within the phantom, but that improved efficiency can be realized even when the heterogeneity is at or near the surface. This is because some correlation between the two histories is retained despite passage through the heterogeneity.

Electrons↗

PubMed1995

Modelling the distribution of laser light in port-wine stains with the Monte Carlo method.

Using the Monte Carlo method, we have calculated the distribution of absorbed light in skin during the laser treatment of port-wine stains. Our model includes the effect that the blood capillaries and epidermis have on the propagation of light through skin. It is more complete than those used by previous workers. In this paper, we change the number of scattering dimensions, the spot diameter, and the capillary separation and diameter, and we use the wavelengths 577 nm and 585 nm. One-dimensional scattering calculations are misleading but two-dimensional results suffice for large spot diameters. The model shows that changing the position or diameter of the capillaries has a larger effect on the distribution of absorbed light in the skin than changing the wavelength of the laser beam from 577 nm to 585 nm. Changing the wavelength does increase absorption in the deeper capillaries, but this is not significant. From our results, we discuss the optimal treatment of port-wine stains.

Animals↗

PubMed2001

Ab initio structure determination of monoclinic 2,2-dihydroxymethylbutanoic acid from synchrotron radiation powder diffraction data: combined use of direct methods and the Monte Carlo method.

The crystal structure of 2,2-dihydroxymethylbutanoic acid (C(6)H(12)O(4)) in monoclinic form has been determined ab initio from synchrotron radiation powder diffraction data. Two O and five C atoms were first derived by direct methods. Two missing O atoms and one C atom were found by the Monte Carlo method without applying constraint to their relative positions. Positional and isotropic displacement parameters of these non-H atoms were refined by the Rietveld method. Molecules are linked by hydrogen bonds and they make sheet-like networks running parallel to the (010) plane. The Monte Carlo method is demonstrated to be a powerful tool for finding missing atoms in partially solved structure.

Journal Article↗

PubMedDate not supplied

Some adaptive monte carlo methods for Bayesian inference.

Monte Carlo methods, in particular Markov chain Monte Carlo methods, have become increasingly important as a tool for practical Bayesian inference in recent years. A wide range of algorithms is available, and choosing an algorithm that will work well on a specific problem is challenging. It is therefore important to explore the possibility of developing adaptive strategies that choose and adjust the algorithm to a particular context based on information obtained during sampling as well as information provided with the problem. This paper outlines some of the issues in developing adaptive methods and presents some preliminary results.

Algorithms↗

PubMed1999

Searching for alcoholism susceptibility genes using Markov chain Monte Carlo methods.

Markov chain Monte Carlo (MCMC) methods offer a rapid parametric approach that can test for linkage throughout the entire genome. It has an advantage similar to nonparametric methods in that the model does not have to be completely specified a priori. However, unlike nonparametric methods, there are no limitations on pedigree size and MCMC methods can also handle relatively complex pedigree structures. In addition MCMC methods can be used to carry segregation analysis in order to answer questions on the genetic components of a disease phenotype. Segregation analysis gave evidence for between two and eight alcoholism susceptibility loci, each having a modest effect on the phenotype. MCMC methods were used to map alcoholism loci using the phenotypes ALDX1 (DSM-III-R and Feighner criteria) and ALDX2 (World Health Organization diagnosis ICD-10 criteria). There was mild evidence for quantitative trait loci on chromosomes 2, 10, and 11.

Adolescent↗

PubMed2005

Calculation of the entropy of random coil polymers with the hypothetical scanning Monte Carlo method.

Hypothetical scanning Monte Carlo (HSMC) is a method for calculating the absolute entropy S and free energy F from a given MC trajectory developed recently and applied to liquid argon, TIP3P water, and peptides. In this paper HSMC is extended to random coil polymers by applying it to self-avoiding walks on a square lattice--a simple but difficult model due to strong excluded volume interactions. With HSMC the probability of a given chain is obtained as a product of transition probabilities calculated for each bond by MC simulations and a counting formula. This probability is exact in the sense that it is based on all the interactions of the system and the only approximation is due to finite sampling. The method provides rigorous upper and lower bounds for F, which can be obtained from a very small sample and even from a single chain conformation. HSMC is independent of existing techniques and thus constitutes an independent research tool. The HSMC results are compared to those obtained by other methods, and its application to complex lattice chain models is discussed; we emphasize its ability to treat any type of boundary conditions for which a reference state (with known free energy) might be difficult to define for a thermodynamic integration process. Finally, we stress that the capability of HSMC to extract the absolute entropy from a given sample is important for studying relaxation processes, such as protein folding.

Biopolymers↗

PubMed2000

Lattice-switch monte carlo method

We present a Monte Carlo method for the direct evaluation of the difference between the free energies of two crystal structures. The method is built on a lattice-switch transformation that maps a configuration of one structure onto a candidate configuration of the other by "switching" one set of lattice vectors for the other, while keeping the displacements with respect to the lattice sites constant. The sampling of the displacement configurations is biased, multicanonically, to favor paths leading to gateway arrangements for which the Monte Carlo switch to the candidate configuration will be accepted. The configurations of both structures can then be efficiently sampled in a single process, and the difference between their free energies evaluated from their measured probabilities. We explore and exploit the method in the context of extensive studies of systems of hard spheres. We show that the efficiency of the method is controlled by the extent to which the switch conserves correlated microstructure. We also show how, microscopically, the procedure works: the system finds gateway arrangements which fulfill the sampling bias intelligently. We establish, with high precision, the differences between the free energies of the two close packed structures (fcc and hcp) in both the constant density and the constant pressure ensembles.

Journal Article↗

PubMed2004

Properties of selected diatomics using variational Monte Carlo methods.

Using variational Monte Carlo and highly accurate trial wave functions optimized by Filippi and Umrigar, we calculate a number of molecular properties for the ground state of Li2, Be2, B2, C2, N2, O2, and F2. This is the first time that many of these properties have been computed.

Journal Article↗

PubMed2004

A ground state potential energy surface for H2 using Monte Carlo methods.

Using variational Monte Carlo and a simple explicitly correlated wave function we have computed the Born-Oppenheimer energy of the H2 ground state (X 1Sigmag+) at 24 internuclear distances. We have also calculated the diagonal correction to the Born-Oppenheimer approximation and the lowest-order relativistic corrections at each distance using variational Monte Carlo techniques. The nonadiabatic values are evaluated from numerical derivatives of the wave function with respect to the nuclear coordinates. With this potential energy surface we have computed several of the lowest vibrational-rotational energies for this system. Our results are in good agreement with the best values found in the literature.

Journal Article↗

PubMed1989

[UV-spectrophotometric analysis of the component of antondin injection by the Monte Carlo method of area fitting].

The Monte Carlo method is a kind of unique calculation method. It can resolve not only the question of define evaluation, but also that of random evaluation. The Monte Carlo area fitting method was applied to UV-spectrophotometry of the multicomponent complex preparations without previous separation. The area of each component of complex preparation within a suitable wavelength range of standard spectrogram was multiplied by certain coefficients, and then the products were obtained to fit the area of the measured multicomponent complex preparations. The method was used to simultaneous determination of barbital, amidopyrine and antipyrine in antondin injection. The experimental data were measured and treated by UV-190 spectrophotometer and microcomputer IBM-PC. The average recoveries of barbital, amidopyrine and antipyrine were 99.1 +/- 1.1% (CV), 99.6 +/- 1.3% (CV), and 99.7 +/- 1.0% (CV), respectively.

Aminopyrine↗