Isotope Definition

Geometry and Materials
1

Hello,

How does one define a material composed of isotopes of an element? I have read the documentation and also looked through the forum and this training lecture:

but it’s still ambiguous as there’s no explicit example of a material with isotopes.

If we take an element that’s not one of the 23 built in ones, Molybdenum for example with 5 stable isotopes over 1% abundance. Can you just only specify the Z and the abundances are already included in FLUKA? (if so, is it documented which istopes and abundances are implemented in the code?). Or how do we specify them completely?

MATERIAL ,  42 , , 10.28 , , 94 , MO94
MATERIAL ,  42 , , 10.28 , , 95 , MO95
MATERIAL ,  42 , , 10.28 , , 96 , MO96
MATERIAL ,  42 , , 10.28 , , 97 , MO97
MATERIAL ,  42 , , 10.28 , , 98 , MO98
MATERIAL , , , 10.28 , , MOLY
COMPOUND , -0.0919 , MO94 , -0.159 , MO95 , -0.167 , MO96 , MOLY
COMPOUND , -0.0958 , MO97 , -0.243 , MO98 , , , MOLY

or

MATERIAL, 42, , 10.28 , , , MOLY

and this already has the isotopes internally defined despite not being a built-in material?

And you have to specify the density each time for every material card?

For the MATERIAL card, WHAT(1) says " Atomic number (meaningful only when not coupled to a COMPOUND card; otherwise set to 0.)," so I should not be specifying this as I’m using a COMPOUND card?

I want to both use built-in defaults if they exist and are listed what they are but also be able to implement explicit material definitions to ensure they are the same as NIST ones.

I found an example about Uranium in the forum but the comments say the solution is wrong and won’t behave as expected but don’t give a written out example.

Any help would be really appreciated,
Thanks,
Laurie

3

If the density has to be specified for an isotope do I have to redefine isotopes when I’m using them in different materials with different densities?

4

Yes. Natural abundances are automatically assumed.

You can retrieve them from the isotop.inc variables.

Any MATERIAL card needs a density value.
Any MATERIAL card defining an element - with natural (default) or single isotope composition - needs the respective atomic number. Any MATERIAL card defining a compound, as further specified by a COMPOUND card carrying the same material name, does not expect any atomic number (this is also the case of user’s redefinition of the isotopic composition of an element).

When a material is used in a compound, its density is relevant to this purpose only if the latter is defined in terms of volume fractions.

5

Hello Francesco,

Thank you for the information - that helps a lot. So then, both my examples are ok then…?

MATERIAL ,  42 , , 10.28 , , 94 , MO94
MATERIAL ,  42 , , 10.28 , , 95 , MO95
MATERIAL ,  42 , , 10.28 , , 96 , MO96
MATERIAL ,  42 , , 10.28 , , 97 , MO97
MATERIAL ,  42 , , 10.28 , , 98 , MO98
MATERIAL , , , 10.28 , , MOLY
COMPOUND , -0.0919 , MO94 , -0.159 , MO95 , -0.167 , MO96 , MOLY
COMPOUND , -0.0958 , MO97 , -0.243 , MO98 , , , MOLY

MATERIAL, 42, , 10.28 , , , MOLY

Volume fraction is a little easier so if I assume that, then I have to redefine each element and its isotopes for each compound material they’re finally used in?

e.g. if in the above example I wanted, say, MoGr with a different density < 10, I would need to redefine the isotopes and element material / compound, then add another material / compound with graphite all with the final density of MoGo?

I guess I’m asking if the density number is really used from the MATERIAL card when defining an istotope?

Many thanks,
Laurie

6

Dear Laurie,
the resulting physical quantity that is used in the simulation is the number density of each type of atoms/molecules to estimate mean free path/collision rate for the interaction. The neutral number densities depends both on the structure of material and mass of a single particle. So, the density of material is used to calculate absolute volume number density of material. Giving the mass number of isotope only you do not define the material. Also density of the material is important for the material phase selection (gas/solid). So yes, MATERIAL card densities are used by FLUKA algorithms for different reasons.

Also pay attention, that mass density of isotopes will be different. However, assuming that isotopes of the same material have same spatial structure, it will be proportional to the atomic weight.

The answer for the second question is “yes”, you have to “assemble” your material compound from other materials and/or compounds.

Kind regards,
Illia