## Simulating a neutron beam?

Discussion on physics, models and calculation methods
eugene171
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### Simulating a neutron beam?

How might one simulate a neutron beam? I have MCNP code for the beam in question:

mode n
nps 10000000 \$ Number of histories to be run
imp:n 1 1 0 \$ importance of materials
sdef erg=2 \$ monoenergetic neutrons at 16.5 MeV
sur = 6
dir = 1 \$ cosine of angle
pos -.75 0 0
vec = 1 0 0
si1 .499
sp1 -21 1
c

But I'm having trouble finding in the manual how to make neutron sources other than those diffuse in the material

Jaakko Leppänen
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### Re: Simulating a neutron beam?

What kind of surface do you have in the mcnp input. The problem with Serpent is that it can only handle spherical and cylindrical surfaces (Sec. 9.2.1 in the Manual), and doesn't have the option to apply cosine distributions (other than mono-directinal and isotropic). One option is to generate a source file (Sec. 9.2.4) consisting of discrete points with position, energy and direction given separately for each point.
- Jaakko

eugene171
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### Re: Simulating a neutron beam?

The surface in question is a rectangular cell, bounded by six planes (2 px, 2 py, 2 pz).

Before trying to do anything too complicated with the source, I decided to try using

src 1 sd 0 0 1 se 2

for isotropic neutrons at 2 MeV coming from the Z direction.

I am getting the following error:

Code: Select all

``````Starting the transport calculation cycle...

***** Mon Aug 12 15:52:44 2013 (seed = 1376340764)

Fatal error in function ExternalSource:

Coordinate sampling routine missing

Simulation aborted.``````
Is this because of the geometry of the target?

Jaakko Leppänen
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### Re: Simulating a neutron beam?

You get the error because you are missing the spatial distribution (see Sec. 9.2.1). In fact, you should be able to define the source planes using the sx, sy and sz entries (just set the width of the volume small enough to approximate a planar surface).

To get 6 planar sources, you need 6 separate src cards.
- Jaakko

eugene171
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### Re: Simulating a neutron beam?

I seem to be doing things wrong despite trying to follow the manual.

I keep getting the error:
Fatal error in function ExternalSource:

Coordinate sampling routine missing

Simulation aborted.

Here is my input file thus far; can you give some input on what I'm doing incorrectly?

Code: Select all

``````%Eugene O'Donnell
% --- Test Thorium file for a 1.5x1x1 cm plate -------------------

set title "Thoriumed"

% --- Plate bounds

surf 1 cuboid -0.75 0.75 -0.5 0.5 -0.5 0.5
surf 2  sph  0 0 0 50
surf 3  sph  0 0 1.5 0.5  %eventually to be used as source

% --- Cell definitions

cell  1  0  thorium    -1 2    % inside the plate
cell  2  0  air  1 -2 -3
cell  3  0  air  -3  %is sphere, r of 0.5
cell 99  0  outside  2

% --- Fuel materials:

mat thorium -11.7
90232.06c   1  %.06c from jeff311u

mat air -0.001184 %at 25C
8016.06c 0.2 %.06c from jeff311u
7014.06c 0.8 %.06c from jeff311u

% --- Cross section data library file path:

set acelib "/xs/sss_jeff311u.xsdata" %not sure if jeff311u ought to be used

% --- black boundary condition assumed:

set bc 1

% --- group constant generation:

% universe = 0 (homogenization over all space) assumed, commented out
% symmetry = 0 no symmetry assumed, not input
% number of energy groups commented out. I don't think it's needed here

set gcu  0
set sym  0
% set nfg  4  0.625E-6  5.5E-3  0.821

% --- Neutron source

%src 1 sx -0.75 0.75 sy -0.5 0.5 sz -0.5 0.5 sd 0 0 1 se 2
src 2 %most basic case to test functionality

% ------------------------------------------------------------[code]``````
[/code]

Jaakko Leppänen
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### Re: Simulating a neutron beam?

It looks like you have some geometry errors (overlapping cells) in the input. I think your cell definitions should be:

Code: Select all

``````cell  1  0  thorium    -1     % inside the plate
cell  2  0  air  1 -2 3
cell  3  0  air  -3  %is sphere, r of 0.5
cell 99  0  outside  2
``````
You can get rid of the error message by adding "sc 1" in the source definition (sample source points in cell 1). But then, because cell 1 is small compared to the geometry dimensions, you'll get another error about low sampling efficiency. This problem is solved by putting x-, y- an z-boundaries around the source cell, as you have done with your other source card. I got the calculation going with definition:

Code: Select all

``````src 1 sc 1 sx -0.75 0.75 sy -0.5 0.5 sz -0.5 0.5 sd 0 0 1 se 2
``````
For a source in cell 3, try:

Code: Select all

``````src 1 sc 3 sx -0.5 0.5 sy -0.5 0.5 sz 1.0 2.0 sd 0 0 1 se 2
``````
- Jaakko