Activation in gas at elevated pressure

Geometry and Materials
1

Dear Fluka experts

I am getting some unexpected results when increasing the pressure of a fixed volume of Nitrogen gas.

I am interested in the amount of activation generated in the nitrogen gas. However, when I increase the pressure from 1bar to 10bar (a factor of 10 density increase - 0.0011g/cc to 0.011g/cc) I get a factor of 100 increase in the total activity.

I have tried with and without the MAT-PROP card and set the pressure to 10 bar although this seems to make little difference to the results.

I have attached my input file although this includes lots of other details as I was previously using it to look at the foils separating the gas.

Thanks,
Mike

He_10atm.inp (31.2 KB)

He_10atm.flair (46.2 KB)

Table of results below

Pressure 1bar 10bar
Decay time 1 sec 1 sec
Window gap 1.34E+06 2.35E+08
Gap 1 1.28E+06 2.42E+08
Gap 2 1.59E+06 2.53E+08
Gap 3 1.73E+06 2.65E+08
Gap 4 2.28E+06 2.81E+08
Gap 5 2.90E+06 2.98E+08
Gap 6 3.50E+06 3.11E+08
Gap 7 4.77E+06 3.35E+08
Gap 8 5.07E+06 3.48E+08
Gap 9 4.25E+06 3.64E+08
Gap 10 2.94E+06 3.23E+08
Gap 11 2.22E+06 2.44E+08
Gap 12 2.03E+06 1.93E+08
Gap 13 1.97E+06 1.96E+08
Gap 14 5.03E+06 4.23E+08
TOTAL (Bq) 4.29E+07 4.31E+09
TOTAL (MBq) 42.88 4310.77
3

Dear Michael,

I am looking into this and getting back to you.

Cheers,

Markus

4

Dear Michael,

sorry for this delay to reply.

For gases at higher pressure than 1 atm, one needs to set to their actual density in the MATERIAL card WHAT(3) AND the gas pressure must be given in the MAT-PROP card WHAT(1), and that’s what you correctly did.

In the manual you might have seen this:

MATERIAL card, WHAT(3)
Density in g/cm3. Note that if the density is lower than 0.01, the material is considered to be a gas at atmospheric pressure unless set otherwise by MAT-PROP.

MAT-PROP card, WHAT(1)
Gas pressure in atmospheres.

Default (option MAT-PROP not given):
If the density of the default material or that assigned by a MATERIAL card is > 0.01, the material is not assumed to be a gas. Otherwise it is a gas at a default pressure of 1 atmosphere.

Now, concerning the scaling of residual activity:
In the output file one finds that the star density scales very well with the pressure, so the number of inelastic interactions increases as expected with higher material density.

1Region # name  ALL-PART Star Density   BEAMPART Star Density   ENERGY        Density    
                Stars/cm**3             Stars/cm**3               GeV/cm**3             
                /one beam particle      /one beam particle      /one beam particle      
     15 Gap1           1.165000000D-06         1.165000000D-06         1.120537306D-06  
     15 Gap1           1.269520000D-05         1.267485000D-05         1.120995845D-05  
     15 Gap1           1.239833964D-04         1.238936515D-04         1.126321296D-04

However, the RESNUC yield or activity does not scale with different pressure. It took me a moment to figure this out … and actually essential credits go to @cesc, @vasilis and @ceruttif .

The RESNUC scoring accounts for all residual isotopes that stop in the scoring region (and not as one might be mistaken where they are produced). For solids, that usually is the same region, but in your case the production and scoring volume is a gas. Although the residual isotopes are produced in the gap, there is a chance that they still have sufficient kinetic energy to escape from the region. This is evidently more relevant at lower gas densities.

Deactivating the IONTRANS Full transport card will disable the heavy ion transport of the residual nuclei, and then the results for RESNUC activity scale indeed with increasing pressure.

For illustration:

with IONTRANS card:

1 bar:       Bq          1.08777000E+06   +/-   11.6004210      %
10 bar:      Bq          2.40818848E+08   +/-  0.824882746      %
100 bar:     Bq          4.63271526E+09   +/-  0.237412408      %

without IONTRANS card:

1 bar:       Bq          4.92219840E+07   +/-   1.60599887      %
10 bar:      Bq          4.94479360E+08   +/-  0.659861267      %
100 bar:     Bq          4.99488205E+09   +/-  0.210247964      %

Kind regards,

Markus

2 Likes
5

Hi Markus

Thanks for your reply and explanation. So as I am interested in the isotopes and residual activity “in the gas” its best to keep the IONTRANS card on I think. Do you agree?

Many thanks to you and the team for looking into this,

Mike

6

I’d certainly agree. The migration of the radioactive species, which breaks the scaling one would naively expect as discussed above, is a physical effect that shall better be taken into account.

7

Agreed as well.

Consider that in reality you might have diffusion processes (and other migration or adsorbance processes), which are not modelled at all here and which may be of importance depending on your setup.

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