MCNP6.2 - SDEF position related keywords

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In summary: The comment in the manual is that none of the position-related keywords (i.e., CEL, SUR, RAD, AXS, EXT, X, Y, Z, and CCC) can be a dependent distribution of POS. This means that expressions like "EXT=FPOS=D1" or "RAD=FPOS=D1" are not legit. However, there are examples of this use. For instance in the example 10 (paragraph 5.8.6..10 at page 402) of the manual, the source is defined as SDEF PAR=SF CEL=D1 POS=D2 RAD=FPOS=D3.
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19matthew89
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It's unclear whether SDEF position related keywords could be dependent on POS
Hi everyone,

I have started delving into the "nightmare" of source definition and in my opinion, the MCNP manual here sounds even more cryptic and obscure than in other sections.

By the way, I have a question related what is reported in paragraph 5.8.1 of the MCNP® Code Version 6.3.0 Theory & User Manual. It says:
"None of the position-related keywords (i.e., CEL, SUR, RAD, AXS, EXT, X, Y, Z, and CCC) can be a dependent distribution of POS."
So from what I understand, none of the position-related keyword could be defined as dependent on POS, so namely expressions like "EXT=FPOS=D1" or "RAD=FPOS=D1" are not legit.

However, there are examples of this use.

For instance in the manual itself: the Example 10 (paragraph 5.8.6..10 at page 402) defines the source as
Code:
SDEF PAR=SF CEL=D1 POS=D2 RAD=FPOS=D3
.

So, according to your experience, is the command allowed and works? I ask because, I plan to define a pretty complicated surface source and at the moment the most intuitive way of defining it would be exactly via a FPOS.

Thanks a lot in advance
 
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My suggestion is to give it a whirl and use ptrac to see where the first thousand or so particles are created. Maybe you don't have to use the complete complicated source you intend for your final situation, just enough to demo that it will or will not work as you need.
 
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@19matthew89
yes you can use Fpos with MCNP "parameters depend of the position"
for example you want to simulate a source whose energy depends on the position you can write:

sdef pos=d1 par=p erg=fpos=d2
si1 L 0 0 0 0 0 10
sp1 1 1
DS2 S 3 4 $ energy is function of position
SI3 L 1.17 1.33 $ in (0,0,0) energy= 1.17 and 1.33 MeV
SP3 1 1
SI4 L .511 $ in (0,0,10) energy=511 keV
SP4 1

It is a simple example. You can imagine fpos for rad, cel, ...
hope your supervisor is ok with that :wink:
 
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PSRB191921 said:
@19matthew89
yes you can use Fpos with MCNP "parameters depend of the position"
for example you want to simulate a source whose energy depends on the position you can write:

sdef pos=d1 par=p erg=fpos=d2
si1 L 0 0 0 0 0 10
sp1 1 1
DS2 S 3 4 $ energy is function of position
SI3 L 1.17 1.33 $ in (0,0,0) energy= 1.17 and 1.33 MeV
SP3 1 1
SI4 L .511 $ in (0,0,10) energy=511 keV
SP4 1

It is a simple example. You can imagine fpos for rad, cel, ...
hope your supervisor is ok with that :wink:
Thanks a lot! I will try it straightaway!
I'll check anyway the origin of particles via the Table 110 to be sure!
Absolutely...my supervisor is totally ok with that ;)

So...what about the comment in the manual then? What does it mean?
I ask also because, poring over the topic, I found this paper too (https://www.sciencedirect.com/science/article/pii/S0306454907001788) where the author caustically complains (section 3 and 4) that FPOS is misbehaving and not working.
 

FAQ: MCNP6.2 - SDEF position related keywords

What is the purpose of the SDEF card in MCNP6.2?

The SDEF card in MCNP6.2 is used to define the source of particles in a simulation. It specifies the properties of the source particles, such as their type, energy, position, direction, and time distribution.

How do I specify a point source position using the SDEF card?

To specify a point source position using the SDEF card, you can use the POS keyword followed by the coordinates of the point. For example, "SDEF POS=1 2 3" sets the source at the position (1, 2, 3) in the simulation space.

Can I define a distributed source using the SDEF card?

Yes, you can define a distributed source using the SDEF card by specifying a distribution for the position. This can be done using the SI and SP cards in conjunction with the POS keyword to define a spatial distribution.

What is the difference between POS and X, Y, Z keywords in the SDEF card?

The POS keyword specifies a single position for the source, while the X, Y, Z keywords allow you to define distributions or ranges for each coordinate separately. This is useful for defining sources that are not confined to a single point.

How do I combine multiple position-related keywords in the SDEF card?

To combine multiple position-related keywords in the SDEF card, you can use them in sequence, each followed by their respective parameters. For example, "SDEF POS=1 2 3 X=d1 Y=d2 Z=d3" defines a point source at (1, 2, 3) with distributions d1, d2, and d3 for the X, Y, and Z coordinates, respectively.

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