EGS5 Bug Alert
TVSTEP

Status of Bug

This bug has been corrected in the latest
(Feb 12, 2010) distribution of the EGS5 code. (Version 1.0.4)
In an egs5 simulation using mesh tallies, Takeshi Sugita (SSL) noticed that for electrons, the sum of tvstep was 8 times greater than sum of ustep, which was not expected for small meshes.

Upon inspection of egs5_electr.f, Namito noted that ustep is the quantity which must be used to score absorbed dose, etc., and that tvstep was used only to define the multiple scattering step. Namito also noted that egs5_photon.f contains the line "tvstep=ustep" just before the call to ausgab(0), so that ustep and tvstep agree for photons.

The issue here is that because of the introduction of the random hinge transport mechanics for electrons in egs5, none of the previous (egs4) relationships between the variables tstep, tustep, ustep, vstep, and tvstep hold. Many of these variables are redundant in egs5 and have been removed. The manual, however, was not adequately updated to explain the changes, and in fact, had retained language from the egs4 manual that was misleading and inaccurate. For instance, table B.6 of SLAC-730/KEK 2005-8 stated:

USTEP: User (straight line) step length requested and granted.
TVSTEP: Actual total Multiple scattering step to be transported.

To address the discrepancy between the manual and the source code and to retain some of the egs4 usage of the electron step variable tvstep, we have modified the code as follows:

1. Renamed previous egs5 variable "tvstep" as "tmstep", since it is used only as the step in multiple scattering.
2. Equivalenced the variables "ustep" and "tvstep".

After this modification "tvstep" is just a dummy variable, but, as in egs4, it can be used in egs5 to score dose, fluence, etc. (Note that "ustep" can also be used for scoring in egs5.)

We have also updated SLAC-730/KEK 2005-8 to reflect these changes and to better explain the relationships between the electron step size variables in egs5 vs. in egs4.

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Last updated 02Mar2010