Lesson 5 – Using GoldSim’s Built-in Database of Radionuclide Decay Chains
In the previous Exercise we created the species and manually defined decay rates (in the form of half-lives), daughter products and molecular weights. For non-radioactive applications this is how you will enter species. If you happen to use the same species in multiple models, GoldSim also provides the ability to import and export species from and to a spreadsheet so you can easily reuse your list of species.
However, when modeling radionuclides, it is often the case that you will need to model a large number of species (many tens) with complex decay chains, such that entering the species manually would be complex and prone to error. To address this, GoldSim provides an option to utilize a large built-in database of nuclide decay data (species, decay rates and daughter products). In particular, the feature allows you to view over 1200 nuclides (including both stable and radioactive species) and to selectively include some of these species in your simulation.
Note: The data is based on the International Commission for Radiation Protection (ICRP)’s Publication 107 (data table ICRP107.NDX). The GoldSim implementation uses an updated data set provided by Dr. Keith Eckerman of Oak Ridge National Laboratory in February 2012. This data set corrected errors in the atomic weights of some radionuclides.
Within the RT Module, when you open the Species dialog for the first time, it looks like this (open a new model now so you can follow along):
The Display field provides access to the full ICRP database. This drop-list provides three options:
- Modeled species (the default): Displays only those species for which the Include box is checked.
- Modeled species + Daughters: Displays only those species for which the Include box is checked and all of their progeny.
- All species: Displays all species.
By default, Display is set to “Modeled species” and a single “user-defined” species (Species1) is listed (and the Include box is checked).
Note: The Include box is always checked (and cannot be cleared) for any “user-defined” species (the default species initially present in all new models, and any additional species added via the Add button).
So how do you use this database to select the species for your model? Typically, you would follow these steps (follow along with these steps now):
- Select “All species” from the Display field. When you do so, you will see that GoldSim will display a large number of species. However, the Include box will not be checked for any of these (except for the default “Species1”). Only species for which that box is checked will be included in your model. Although the ICRP database includes a very large number of species, any particular model will only use a small subset of these. The subset you select to use will be a function of the inventory present, the duration of the simulation, and the time scale of the processes being simulated. In most cases, the half-lives of the various species (and their daughters) will play a key role in determining which species are to be included in the model. So we must next take several steps to reduce the size of this list (so you can select some species for inclusion).
- First, you will likely want to reduce the number of species displayed by filtering by half-life. In particular, depending on the time frame of your simulation and the time scale of the processes being simulated, you may want to specify a Min half-life to show in order to hide species with very short half-lives that you know you will not explicitly include in your simulation. For example, if you selected 1 hr (do that now), GoldSim will hide any species with a shorter half-life.
- In most cases, you will have a list of key species you know must be included in the simulation (e.g., for a disposal facility, species you know will be in the disposal inventory). You should check the Include box for each of these species. For now, let’s select Include for U-238. Note that you need not consider the daughters for any of these key species that you include yet. As will be seen in the next step, GoldSim provides a method for automatically including these.
Note that GoldSim provides some tools to facilitate finding and selecting species for inclusion. In particular, in any column in the Species dialog, clicking on it sorts it (in ascending order). Clicking it again sorts it in descending order. Clicking a column and then typing in a key combination takes you immediately to that row (e.g., typing Th would take you to the first Thorium isotope in the list).
- All new models initially have a “user-defined” species called Species1. You cannot delete this until you have selected other species to be included (there must always be at least one species). Now that you have included U-238, delete Species1 (by selecting the row and pressing the Delete button).
- If you now select “Modeled species + daughters” from the Display list, GoldSim will show only the key species that you have manually included (in this case, U-238), as well the entire chain of daughters (whose half-life is greater than the selected Min half-life to show). In our example, the list will look like this:
You will note that the Include box for some (but not all) of these daughters is selected and grayed out. In this example, U-238 has been manually included (the Include box has an X). As a result, four other nuclides (which are progeny of U-238) have been auto-included (and their Include boxes are marked as dark gray). Th-234 (and a number of other nuclides) are listed, but excluded (i.e., the Include box is cleared). This is because the range for Auto-include ICRP daughters is set from 1 yr to 1e+12 years. As a result, all daughters with half-lives in that range are auto-included. Hence, auto-inclusion provides a quick and convenient way to include important daughters in the model without having to do so manually.
It should be noted that you cannot exclude (clear the Include box) for an auto-included species. It is always included, unless the parent that caused it to be auto-included is manually excluded).
The final column of the Species element, Modeled daughters (skipped intermediates), indicates which daughters are included in the simulation (and which are excluded). If a daughter (or other descendant) of the species is included, it is listed without parentheses. If it is not included, it is listed surrounded by parentheses. In this case, the daughters that are excluded all have half-lives of less than 1 yr. Note that not all of these excluded daughters are actually present in the list of species we see here (e.g., Th-234 is, but Pa-234m is not). This is because some daughters (those with half-lives less than 1 hr) are hidden, since Min half-life to show is set to 1 hr.
It is critical to note that although excluding radionuclides with short half-lives may be appropriate for computing concentrations of key species of interest, it is important to account for these “skipped” radionuclides when computing doses. The manner of doing so is beyond the scope of this Course, but is well-known to health physicists and risk assessment professionals.
Based on your auto-inclusion settings, some daughters of importance to your simulation may still be excluded. You may want to examine these to determine if you would like to include them for a special reason. For example, stable species at the end of the chain will not be auto-included, and you may want to include these if you think they may be present in significant quantities. Also, you may want to include short-lived species (e.g., Rn-222) that have special transport properties (e.g., they are volatile). In this example, let’s manually select for inclusion stable Pb and Rn-222:
- Finally, to simplify the display, you can set the Display setting to “Modeled species” so that only the species included in the simulation are shown:
In this case, we have three manually included species (U-238, Pb and Rn-222) and four auto-included daughters of U-238.
Note: You should recall from Unit 5, Lesson 5 that the Species element populates two Array Label sets: the Species set and the Elements set (we will talk more about this latter set in Lesson 7). The order of the species in the Species Array Label set is defined by the Row # in the Species element. You can control this order (via the Species set ordering drop-list) if you wish. The Elements Array Label set, however, is always alphabetical.
It is important to understand what it means to exclude a daughter in a chain. When we do so, the parents of those species “skip over” them and proceed to daughters that have been included. To understand this, consider the U-238 decay chain. The first part of the chain actually looks like this:
U-238 => Th-234 => Pa-234m => U-234 …
However, Th-234 and Pa-234m have short half-lives (less than 1 yr), so if we exclude those two species, GoldSim “skips” them in the decay chain such that U-238 is assumed to decay directly to U-234:
You cannot edit the properties of the ICRP species. As seen in this dialog, if you click on a species in the Species dialog, a dialog for that species will be displayed, but none of the properties can be edited (and the Species ID of an ICRP species is highlighted in green)