Stopping Powers
Mary Chin
I had EGSnrc run a simulation of 20MeV electrons impinging on a .25cm water
slab, so that it grabs E(NP), EDEP and TVSTEP each time IARG=0. Using these
values, it outputs the guess stopping power, which is calculated as
EDEP/TVSTEP. I then end up with a list of guess stopping power values
alongside the corresponding E(NP). This list, I expect, would agree with the
521icru pegs4 data. And it did agree very well - all the way down to the
point where E(NP) approaches AE, which is .010MeV. For E(NP)<=.0106MeV, the
guess stopping powers were found to be hundreds - way beyond the values
given by the pegs4 data.
Could someone help me with this please?
Cheers,
Mary Chin
(PhD Student)
Department of Medical Physics
Velindre Hospital
Whitchurch
Cardiff CF14 2TL
United Kingdom
Mary Chin
This is a follow-up to the message posted yesterday. The simulation
concerned was set with ECUT=0 which was due to my wrong assumption. I
wrongly assumed that ECUT defaults to 0.521 MeV whenever it is set to zero -
which is true for BEAM/DOSXYZ but not for EGS.
I repeated my simulations by specifying ECUT=0.521 MeV, hoping the problem
to disappear but the values of EDEP/TVSTEP, which I expect to be the
stopping power, remained as enormous as before. It only disappeared when
repeated with ECUT=0.531 MeV. I thus conclude that the problem was due to
ECUT being set too low.
However, it raises another question: what happens when BEAM/DOSXYZ defaults
to ECUT=0.521MeV? Is the value low enough to be safe enough? For this
particular simulation of 20MeV electrons on a 2.5 mm water slab,
ECUT=0.521MeV doesn't appear low enough.
Perhaps someone could enlighten me.
Cheers,
Mary Chin
(PhD Student)
Department of Medical Physics
Velindre Hospital
Whitchurch, Cardiff CF14 2TL, United Kingdom
Iwan Kawrakow
Mary,
when electrons attempt a step that makes their energy less than
AE(medium), the step length is set to
range(E(NP)) - range(AE(medium))
but the energy loss becomes E(NP) - electron rest energy.
That's why you see the big discrepancy between EDEP/TVSTEP and the
stopping power. The "correct" approach in EGSnrc would be to
1. set EDEP to E(NP) - AE(medium) and
TVSTEP to range(E(NP))-range(AE(medium)),
do the step, call AUSGAB with iarg=0
2. set EDEP to AE(medium)-rm and call AUSGAB with iarg=2
(discard due to E < AE)
But this approach would not change the outcome (the entire electron energy
deposited over a distance equal to range(E)-range(AE)) at the same time
making the code less efficient (2 calls to AUSGAB instead of one).
Hope this helps,
Iwan
--
---------------------
I. Kawrakow, PhD
Ionizing Radiation Standards
Institute for National Measurement Standards
National Research Council of Canada
Ottawa, ON, K1A 0R6
Canada
Tel. (613) 993 2197
fax (613) 952 9865
e-mail iw...@irs.phy.nrc.ca