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

From: Helmut Vincke <Helmut.Vincke_at_cern.ch>
Date: Wed, 2 Oct 2013 09:57:33 +0000

Dear colleagues

On the 12th of September Reiner Geyer asked a couple of very interesting questions concerning the isotope decay handled in FLUKA. Details can be found in the email below.
As far I know, up to now there was no answer provided. Could I ask you to comment on the email and provide the FLUKA community with an answer.

Many thanks in advance

Best regards

-----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.

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 Wed Oct 02 2013 - 13:21:39 CEST

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