Time between events, Impact Parameters and Activation Rates

Hello FLUKA wizards,

I am hoping to run a simulation of a muon veto detector to determine a few things:

  1. The time between the muon entering the veto tank and muon-induced neutrons activating Xenon in an inner chamber

  2. The muon impact parameters vs. the activation rate of the Xenon. (RESNUCLE ??)

  3. The muon impact parameter vs. energy deposits above a certain threshold within the Xenon (USRBIN ??)

First, I’m unsure which of these are possible to estimate- I’ve spent considerable time reading the forums and the manual. Can the impact parameter or distance of closest approach of a particle track be logged in FLUKA? Also, are secondaries propagated into “tertiaries”- can the beam of muons induce neutrons and actually activate the Xe in the detector volume?

For other information, I’ve already defined the detector geometry and material properties. This part of FLAIR is very intuitive and impressive.

Thank you for reading! I’d be grateful for any assistance.


Hello Regan,

thank you for the intriguing question. Before trying to fullly answer it, could you please describe the source term? (i.e. are the muons entering in the detectors primary particles, or are they generated in interactions?)

Ideally, could you please provide the inputfile and any user routine you are implementing for the simulation?

Hi Daniele,

I appreciate your response. The muons are to be the primaries. Once I figure out how to write a proper user routine, the muons will have a particular zenith angle and energy distribution as the entire experiment is to be underground. For now, they might as well be vertically downward for testing purposes.The only other particles I care about are the secondary neutrons.

I’ve attached the input file with the geometry- I’ve commented out the scoring cards as I don’t think I’m using them properly anyway.

Thanks again!

nEXO_Test.inp (10.1 KB)

Hi Regan,

thank you for the inputfile. Let’s start from the physics: there are two main ways to generate secondary neutrons from the muon beam:

  1. Via muon photonuclear interaction, where a muon interacts directly with a nucleus.
  2. Generating a photon which induce a photonuclear interaction. This may happens if the muons undergo a bremsstrahlung event, or due to the interaction of the electron produced in the muon decay. Please notice that the photonuclear interaction are not activated by default, you should control them with PHOTONUC card.

Now, I seem to understand that the main aspect you would like to control better is the scoring. Let’s try to answer to the three questions:

  1. Typically, when computing activation in a material, FLUKA provide us the capability to simulate easily the induced radioactivity after a certain irradiation profile (e.g. https://indico.cern.ch/event/1123370/contributions/4716073/attachments/2447835/4194554/18_Activation_2022_ULB.pdf). However, I think that you are more interested in a “time of flight” measure. If this is the case, you will be forced to use an advanced scoring (i.e. https://agenda.infn.it/event/20624/contributions/105895/attachments/68619/85014/AdvancedScoring2019.pdf).
    In particular, I would suggest you to use an mgdraw.f user routine. In this case, you would need to save the ATRACK variable when the muon enters the veto tank, propagate it with the stupre.f and stupre.f routines to the secondary particles (i.e. the neutrons). Finally, with mgdraw you can score the quantity of your interest (with the relative time) and the time of crossing as a list.

For questions 2 and 3, I am now fully sure of what you are trying to do. The impact parameter is a microscopic quantity, while the activation rate and the energy deposition are macroscopic ones.
Moreover, the impact parameters could describe a single interaction between a muon and a nucleus, while the energy deposited/activation rate are induced by a multitude of interactions.

For the other questions, one could not easily derive the impact parameter, but I suggest you to look into all the variables you could score in the mgraw.f user routine.

Finally, all the particles are propagated until they are below threshold, you will certainly have tertiaries. If you want to access to that information, you can use the variable LTRACK, which contains the generation number.

PS: Since you are working with mononenergetic muons with the same trajectory, you could also compute directly the time of crossing (which is the same for all muons) and just score the time of the event you are interested in.


Hi Daniele,

Thank you! This is very helpful. I will look into these routines.

I should mention that I’m not referring to the “impact parameter” in the Rutherfordian sense- I mean the distance of closest approach of the muon to the centre of the TPC vessel within the water tank of the geometry. So basically the origin or some translation thereof. So perhaps it is possible to pull these data.

In any case, I’m very grateful for the response. I will be reading these slides and giving it a shot.