[fluka-discuss]: RE: Isotope decays in fluka

From: Reiner Geyer <Reiner.Geyer_at_cern.ch>
Date: Fri, 4 Oct 2013 15:18:47 +0000

Dear Joachim,
Thank you very much for your answer. You are of course right with the mono energetic gamma line of Ba-137m where I missed the internal conversion.
But my main issue is still there.
I repeated the simulation with your proposed options (semi-analogue decay). Unfortunately the problem with the detect card persists.
This is best illustrated with the decay of Ba-137m. The attached plot shows:
Top left :The electron flux into a sphere from a Ba-137m source being in its centre.
Top Right: The photon flux into a sphere from a Ba-137m source being in its centre.
Bottom Right: The result of the "DETECT" card (the energy deposited in the sphere).
The "DETECT card" shows several gamma lines instead of one while the USRBDX spectra seem to be reasonable. The lines are partially at too high energies.
The simple geometry is shown in the attachment "Geometry.pdf". The material between the source and the sphere is vacuum.
Can the DETECT card be used at all with isotope decays? Is the simulation "analogue" enough or is it just some wrong setting in my input file? Where is my mistake?
Any advice by FLUKA experts would be very welcome,
Reiner Geyer

-----Original Message-----
From: Joachim Vollaire
Sent: 03 October 2013 11:25
To: Reiner Geyer; fluka-discuss_at_fluka.org
Cc: Chris Theis; Helmut Vincke; Stefan Roesler
Subject: RE: Isotope decays in fluka

Dear Reiner,

Your question is rather long so it might be difficult to address all remarks (I just looked at USRBDX spectra, maybe with the suggestions below you can have hints already to look at the DETECT card...). First note that to look at the decay products of a single radionuclides I would not make use of an irradiation profile, but you could just activate decay like this (what I did) :

 RADDECAY 2.0
*
DCYSCORE -1. PHOT-FL PHOT-FL USRBDX
*
USRBDX 1.0 PHOTON -55.0 INBOX SCORE 1.0PHOT-FL
USRBDX 0.003 0.0 300.0 &
*
DCYSCORE -1. ELEC-FL ELEC-FL USRBDX
*
USRBDX 1.0 ELECTRON -55.0 INBOX SCORE 1.0ELEC-FL
USRBDX 0.003 0.0 300.0 &
*
DCYSCORE -1. POSI-FL POSI-FL USRBDX
*
USRBDX 1.0 POSITRON -55.0 INBOX SCORE 1.0POSI-FL
USRBDX 0.003 0.0 300.0 &
*


In general from which database do you get your decay information ? In case have a look at the attached plot, which could explain some of the structures you observe that you did not succeed to relate to your FLUKA results....

*********************************
?Cs-137 (see attached plot): the USRBDX beta spectrum (top left) shows an electron line at around 0.662 MeV, which I don't understand. The USRBDX gamma spectrum(top right) with one line looks ok. The detector card has several maxima, while only one is expected with a lower edge at 0.662 MeV. Since the gamma should actually be coming from the decay of Ba-137m, I had as a next step a look to Ba-137m itself.
?Ba-137m (see attached plot):: the USRBDX beta spectrum (top left) shows again an electron line at around 0.662 MeV, which I don't understand. The USRBDX gamma spectrum(top right) with one line looks ok. The detector card has several lines at larger energies, while I would have expected only one at 0.662 MeV.
*********************
See plot in attachment besides the g line there are discrete electrons at 624 keV and 655 keV (not 662 you need maybe to increase your spectrum resolution, I see both when I run with a good resolution....) and the maximum beta energy for Cs-137 is 1.175 MeV so what you observe seems reasonable ?

Hoping that this first hints will help you to better interpret your results. Once again I did not look into the DETECT results myself....

Greetings
Joachim


-----Original Message-----
From: owner-fluka-discuss_at_mi.infn.it [mailto:owner-fluka-discuss_at_mi.infn.it] On Behalf Of Reiner Geyer
Sent: 12 September 2013 17:58
To: fluka-discuss_at_fluka.org
Subject: [fluka-discuss]: FW: Isotope decays in fluka


Dear colleagues,
I am trying to simulate realistic detector responses for isotope decays within the frame work of Fluka. The goal is to obtain  the right Monte Carlo corrections for  measurements in the context of radiation protection. In the simulation, the self-absorption of the emitted radiation inside the object, the geometry of the setup and the properties of the detector should be taken into account. The measured quantities of the detector should be parameterized in terms of the energies deposited inside its active part.


In order to validate the approach and to understand the way, in which isotope decays are described in Fluka, I started with a simplified setup (please see also the attached input file).  Here, I observe some problems for certain isotope decays, which I do not fully understand and are discussed below.

The simplified setup is as follows:
1. A source(Region b2) of radioactive decays,  which is a plate with 40x40 cm and 1 cm thickness. The material, for test reasons, is just vacuum.
2. A detector (Region s2) is  a hollow sphere, filled with lead. The thickness of lead is10 cm. The source is centred inside the sphere. The material between the object and the sphere is vacuum. In this setup, any radiation produced inside the sphere should be seen by  it.
3. The kind of isotope being used in the simulation is defined via the  beam card with the option "ISOTOPE" and  the  card "HIPROPE". Co-60, Cs-137, Ba-137, Sr-90, Mn54, Cl-36, Sc-44 were chosen for different runs.
4. The decay of the isotope is controlled by the cards "RADDECAY", "IRRPROFI", "DCYTIMES".  The number of replicas is switched to 1. In my example the irradiation lasts 1 s with 1e15 particles (isotopes). The decay time is set to 10 s.
5. The particle currents into the sphere are scored by two USRBDX definitions BDXCS1S2 (for photons only) and BDXES1S2 (for electrons only) together with their "DCYScore" card - see also attached pictures. From my understanding, the number of particles X per second, which are crossing the  sphere is : ln(2)/T_1/2 * number of Isotopes*BR. Here T1/2 is the half life and BR is the branching ratio of the isotope to the final state X. The units should be [particles/s/[cm2]]. This quantity should to be independent from the number of primary particles of the "START" card.
6. In order to measure the energy deposited in the detector region s2, the detect cards is used.  The number of entries in  this histogram  is identical with   the number of particles of the "START" card. The DETECT histo seems to have one entry per simulated event. Since there is no "DCYScore" card for  "DETECT", the interpretation  of the decay time is not applicable. I  believe,  that the histo might be interpreted as the full decay chain of the isotope with an infinitive large decay time, summed up in energy over all particles per event. But this is just guessing.

In the following, the results of short simulations for Co-60, Cs-137, Ba-137, Sr-90, Mn54, Cl-36, Sc-44 are discussed and the problems are summarized:

Co-60 (see attached plot) : both USRBDX card and the DETECT card seem to have the right result. The USRBDX Beta spectrum (top left) shows both transitions with roughly the right ratio. The USRBDX gamma spectrum (top right) shows both gamma lines. The DETECT (Bottom right)card shows the energy sum of the decay particles seen by the detector.
        
?Cs-137 (see attached plot): the USRBDX beta spectrum (top left) shows an electron line at around 0.662 MeV, which I don't understand.  The USRBDX gamma spectrum(top right)  with one line looks ok.  The detector card has several maxima, while only one is expected with a lower edge at 0.662 MeV.  Since the gamma should actually be coming from the decay of Ba-137m, I had as a next step a look to  Ba-137m itself.

?Ba-137m (see attached plot):: the USRBDX beta spectrum (top left) shows again an electron line at around 0.662 MeV, which I don't understand. The USRBDX gamma spectrum(top right)  with one line looks ok.  The detector card has several lines at larger energies, while I would have  expected only one at 0.662 MeV.

?Sr-90 (see attached plot):: the USRBDX beta spectrum (top left) looks ok. The USRBDX gamma spectrum might be ok, also I don't understand the details.  The detector card (bottom right) shows energies up to 2.5 GeV, which  obviously includes the beta decays of Y-90, which are missing (or suppressed)  on the top left. This can be understood, if the USRBDX weights the decay products of Y-90 with   ln(2)/T_1/2(Y90) * number of Isotopes*BR.

?Mn-54 (see attached plot)::  The USRBDC histogram in the top looks more or less ok. But the gamma line from the detect card has a substructure, which I don't understand. May it come from the x-ray decays?

Sc-44 (see attached plot)::  The beta+ decay, the annihilation of the positron with two gammas and  the gamma of the Ca-44 seem to be correctly described. Also the electron captures looks correct.


Summary:
C0-60, Sc-44 like other isotopes seem to work perfectly well.
Mn-54: shows a sub structure in the gamma line from the "DETECT" card.
Ba-137m:  has too many gamma lines from the "DETECT" card but looks ok in USRBDX.
Ba-137-m: shows a mono energetic  electron line in USRBDX.
Sr-90: looks fine except that the decay time is not taken into account in the Detect card .
Sc-44; looks fine in all aspects.

                It can easily be that I have used some cards in the wrong way.  Any kind of help or advice would be welcome.


Thank you very much,
Reiner Geyer

PS. For the simulation, I would have to supress secondary decays of the isotope, which come later than several micro-second after the first one. Alternatively or event better, the delayed decays could be counted as new events instead of summing up energies over minutes. This can probably be done only by an new user routine?
















Received on Fri Oct 04 2013 - 18:16:06 CEST

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