Scoring energy spectrum Cs-137 in LaBr3

Dear FLUKA experts,
I have the same question as in this post, however I do not see an answer to the last question:

I simulated a LaBr3 crystal and I want to score the energy spectrum of a Cs-137 source and compare it with the result I got experimentally. I simulated the source using the ISOTOPE option and HI-PROPE card as a cylindrical source in a plastic encapsulation (as in the linked post). The LaBr3 is covered by an aluminum layer and it has a window since it is hygroscopic. I do not know the exact design of the scintillator, but I also assumed it’s SiO2 the window and 3 mm thick.

I scored the number of events inside the LaBr with the card DETECT and applied a GEB assuming FWHM = a +b*(E+cE^2)^(1/2) where a,b,c were obtained from the experimental spectra.

I set both the transport and production energy threshold of electrons and photons to 10 keV which is the threshold of my detector.

I show here the simulation results (after the GEB) and experimental results. The photoelectric peak and the Compton edge are well reproduced, differences in the backscattered peak are expected since the actual crystal is enclosed in case which I did not simulate. However I do not understand the peak at about 65 keV. I see from the previous post that the same peak was detected (is it at the same energy or am I wrong?).

I also attached my input and flair file. You can see that I also add some USRTRACK and USRBDX cards to check the photons and electrons entering the scintillator region, however they did not help me in understanding the origin of that peak.
LaBr3_Cs137_cover+window.flair (25.3 KB)
LaBr3_Cs137_cover+window.inp (3.8 KB)

Can you help me?

Thank you in advance

I would like to add that the peak at 65 keV disappears if I place the source at 5 cm from the crystal.

Kind regards

Dear Francesca,

Thanks for your post.

The origin of the peak you witness at around 65 keV in the DETECT spectrum can be traced back to additional gamma lines between roughly 32 keV and 36 keV from the relaxation of 137mBa following the beta decay of 137Cs. You see them summed up in your DETECT spectrum, since it does an event-by-event energy deposition.

You do not see the peak corresponding to the sum of the two lines anymore when you put the source 5 cm in front of the geometry because the solid angle subtended by your detector drops drastically and the chance to capture both of these gammas drops as well.

Hope this helps,


Dear @cesc,

thanks a lot for you reply, yes it clarifies my question.
I just have a last doubt: the sum of the 32 keV and 36 keV is not physical though, right? They come from the Kalpha and Kbeta line respectively and these two process are exclusive. Am I wrong?

If I want to write my own routine to emit the 32 keV + 662 keV (or 36 keV + 662 keV) which routine should I use?


Dear Francesca,

Am I wrong?

No, you are right!

Decaying radioisotopes as source terms in FLUKA are treated in so-called semi-analogue mode. This implies that decay products (e-,e+,gamma,alpha) are explicitly sampled and transported, but they are sampled from inclusive database information: on average you get the right number, kind, and energy of decay products, but one loses possible correlations among them (hence the attribute “semi” above) as turned out to be the case in your example: the Ka and Kb lines should be indeed exclusive, but they are actually sampled independently.

Sorry that I missed pointing this out in last evening’s reply.

Now, in practical terms, a viable solution is indeed to use the source.f routine as your intuition suggests, manually introducing the energy and branching ratios of the lines you care about and steering the sampling accordingly.

With kind regards,


Thanks a lot for your clear explanation.
I will try with the source.f routine.

With best regards,

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