Neutron yield from different reaction channel in case of alpha on Sn 116

Dear experts,

For a simulation with 40 MeV alpha on Sn 116, while extracting neutron energy for different channels, in the plot for 116Sn (alpha, 2n)118Te, a double hump pattern is observed, which seems nonphysical. Can you please guide in this aspect?

I have added the files (please note, I have not introduced the correction factor regarding the particle history and number of stars and weight in the plot). Only one cycle data is plotted here.

Please pardon me if I have asked too many queries in this topic.



Alpha_40_Sn116.inp (2.3 KB)
mgdraw.f (12.4 KB)

Dear Riya,

thank you for reaching out and asking this question. What you are observing in your result is a very real physical effect, and is in fact expected.

In FLUKA the interaction of a 40 MeV alpha with 116Sn is handled by the BME model (see this FLUKA training lecture), which performs the complete fusion to form the compound nucleus 120Te* with about 39 MeV excitation energy.

The 120Te* passes through the following stages in FLUKA:

  • pre-equilibrium: driven by nucleon / light fragment emission in the continuum or nucleon-nucleon collisions. In this stage, it is possible (and actually very likely) for an emitted neutron to carry away a large amount of energy.
  • evaporation: after equilibrium is reached, we expect neutrons to be evaporated with much lower energies on average.
  • (any remaining excitation energy is emitted as gamma rays in the final gamma de-excitation stage).

What you are seeing in the case of 116Sn(a,2n)118Te is the net result of both types of neutron emission, as displayed in this plot:


As you can see, the double-humped spectrum you have simulated arises from the sum of these two contributions, and was indeed expected.

I hope this helps,


PS: A similar behavior is also seen in the other two reactions. In the case of a single neutron emission, 116Sn(a,n)119Te, the emitted neutron must necessarily take most of the available excitation energy (otherwise further particle emission could take place), hence the rather high neutron emission energies you witness (coming from the pre-equilibrium stage) and the lack of contribution from the evaporation stage:


In the case of three neutron emissions, 116Sn(a,3n)117Te, while it is still possible to emit one or two pre-equilibrium neutrons, and then emit a matching number of evaporation neutrons after that, it is far more likely that the nucleus emits three evaporation neutrons, as you can see in this plot:



Thank you @smarin for the detailed explanation. It was really helpful to get the clear picture of the reactions. Can you please let me know how you got separate neutron yield for evaporation and pre-equilibrium, so that if I ever stuck up in such condition, I could explore the output ? Do I have to switch on and/or off any card (may be EVAPORAT) and then use mgdraw routine to print those yield value ?



Unfortunately, such a disentangling is presently not accessible to the user, since it has to be performed at a deeper level in the core routines simulating the various reaction stages (note that we are talking about subsequent reaction stages, not alternative reaction paths).
The PHYSICS/EVAPORAT card has to be there anyway, and does not switch on/off evaporation, rather enables also the emission of heavy fragments in the evaporation stage.

1 Like

Thank you @ceruttif.



Dear @smarin,

To understand the BME model output as plotted by you, I tried Talys simulation for the above reaction, (with default parameters only, further adjustment may be required while comparing with experimental output). The binary emission spectra (after subtracting total pre-equilibrium part) are somewhat similar to 1n channel. What I understood little bit from Talys output is that if the compound nucleus emits 1 neutron even with lower energy, it is reflected as binary neutron output (see the attached image). However, in FLUKA, 1 n channel only refers to the process when there is no chance of emission of another neutron and that is why it is residing in the higher energy range and no contribution is present at lower energy.

I am aware that FLUKA is able to reproduce the overall emission spectra, however in terms of a particular channel, there is a difference in interpreting the outcome from FLUKA and other nuclear reaction codes.

It will be very helpful if you can provide your input in this regard.



c11.pdf (16.2 KB)

Iā€™m not sure I understand what you mean. Stefano referred to (alpha,1n) as the reaction channel producing 119Te, which by construction implies the emission of one neutron only (a pre-equilibrium one). By the way, this is not a nomenclature specific to FLUKA. On the other hand, in your TALYS plot, you do not seem to filter on the final residue and thereby consider a well identified channel. There the 1n spectrum seems rather to refer to the first evaporated neutron, which is not the only emitted neutron.

Dear @ceruttif ,

I felt Talys has filtered out the residue, since the output shows like this:


You should clarify with TALYS experts if 119Te is meant to be in its ground state, so as to be the actual final residue, or in an excited state prone to emitting further particles, which makes it just a transient state, as the second block indicates.
With the production of a 120Te composite nucleus of about 40 MeV excitation energy, the emission of one neutron of energy up to 15 MeV is likely to allow for further particle emission, generating a different residue and so ultimately populating a different channel.