Defining the particle source is an important element of setting up a Monte Carlo radiation transport model. A number of features are available to aid in the definition of source terms. These features are useful for creating new sources and understanding and modifying existing sources. Display of source volumes verifies proper placement. Interactive editing or by use of familiar coefficients on a dialog insulates the uses from the non-intuitive MCNP syntax that is based on the mechanics of sampling points within the volume. The source property pages and interactive features are executed from the Source submenu of the Data menu.
Variable and Distribution EditingThe Source Variables and Source Distribution property pages show and offer editing of those source items. Variables include such information as location, direction, particle type, and energy. Explicit values (including default values) or distributions can be assigned to source variables.
Distributions are used when a variable is sampled from a range of values rather than assuming a single value. Examples include energy sampled from a spectrum or volumes sampled from multiple locations (such as the brain and bladder volumes in the example). Values in distributions can be edited. When a distribution is shown, the source variable or other distribution that references it is also shown. The data can be shown with or without line numbers. Line numbers are useful for lengthy data such as energy spectra.
Source Volumes and PositionsMCNP sources are sampled within a volume or on a surface. The volumes are restricted to points, lines, rectangles, boxes, cylinders, circles, and spheres. The Cartesian objects are aligned with the X, Y, and Z axes. The latter three may have inner radii greater than 0. Interactive creation and editing by mouse clicks and motion in the 2D windows is similar to that for boxes, spheres, and cylinders. (The interactive capability is under development as of Moritz version 1.11.) The volumes can also be edited and created on the Source Volumes property page.
When a source reflects radioactive emission from an object, the object must be within the source volume. It is always prudent to verify the correct placement of the source. The source volumes can be shown in the 2D and 3D views. Because flat objects-points, lines, rectangles, and circles-will rarely coincide exactly with the plane of a 2D view, they are drawn if their plane and the window's plane are closer than the distance corresponding to the size of a pixel in that window. The drawing styles and visibilities for the source volumes can be set independently of other objects on the Source Display Style property page.
We use as an example a simplified model of a patient with a radioactive agent concentrated in the brain and bladder. A 2D view of spherical source volumes surrounding the brain and bladder is shown above. The same volumes are shown in 3D at the left. The locations are taken directly from the MCNP input file or from a new definition within Moritz.
Several other interactive features are under development. A common source configuration is emission from a point with directions restricted to a range of angles. This opening cone can be defined using the same mechanism for interactive definition of a cone surface. The operation can be to define only the opening angle or to set both the angle and position. Other interactive operations define directions and axis vectors.
Tabular DataTabular data can be imported with flexible options that obviate the need to edit data to a specific format. The user can choose which columns are assigned to which distributions and options for those distributions. A number of header lines can be skipped. A specified number of lines can be read rather than the entire file. The Tabular Data property page is used to set the options and read the data file.
GammaDef for Radioactive Photon SourcesThe GammaDef program provides a convenient method for defining photon sources from the decay of radioactive isotopes. It is a tool for the selection of isotopes and gamma-ray lines from a database of all radioactive isotopes. Rather than incorporate this tool into Moritz, we provide it as a separate program that runs in conjunction with Moritz. The GammaDef program prepares lists of gamma-ray line emissions from radioactive elements for use as an energy source specification in MCNP and MCNPX.
GammaDef works with a data set of over 40,000 lines, extracted from several data bases maintained at the National Nuclear Data Center at Brookhaven National Laboratory. The user interface offers convenient selection of which elements, isotopes, and lines to include. The yields can be adjusted to reflect an interval of radioactive decay. GammaDef can also filter the lines by energy and line yield and filter isotopes by half-life. Working libraries consisting of a subset of isotopes contained in the master library can be created and used. When multiple isotopes are present, the user must enter the fraction for each isotope. A source energy distribution in MCNP format using the selected isotopes and lines can be written to a file or sent directly to Moritz.
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