Help understanding neutron induced event

[lobis]

Hello,

I am doing cosmic neutron simulations. I am generating neutrons from outside the detector and recording the events where energy is deposited in the sensitive volume (gas).

After doing some analysis I am puzzled by the fact that there are some events with the observable “neutronTrackCounter” with value 0 (in Geant4Analysis). As I understand it takes into account all tracks so the first track (which is always a neutron) should always be taken into account.

Sample event with 0 neutron tracks:

After taking a closer look I found that the “primary generator” in this case is an excited nucleus of lead (Pb206[1683.99keV], stable isotope of lead) that emits some gammas.

Is this something that is supposed to happen? Because when I choose to simulate N neutron events I will use this N (total neutrons launched) to compute my statistics but if some proportion of events are not really neutron events, my statistics need to be corrected. Shouldn’t this event be a part of the parent event where this nucleus was excited instead of an event of its own?

Also I am trying to understand how the energy is deposited in the gas in this particular case but I cannot trace back (using the viewer) which particle generated the electron tracks inside the gas (and the energy deposits), shouldn’t there be a visible track going from the lead atom to the gas? (how do we explain those unconnected points?)

Thanks,

Luis.

[jgalan]

@lobis

“neutronTrackCounter”

Hi Luis, the source of the problem might be that TracksCounter observable reads in plural.

→ neutronTracksCounter

We should try to find a solution to identify misspelled observables in our RMLs.

@lobis

“primary generator” in this case is an excited nucleus of lead (Pb206[1683.99keV], stable isotope of lead)

If you are simulating neutrons I would expect that your PrimaryGenerator is not other thing than neutrons.

I can imagine that your neutron activates a Pb atom, and this one decays after some time which is larger than subEventTimeDelay.

<parameter name="subEventTimeDelay" value="100" units="us" />

Then, the event will be splitted in two independent sub-events. I.e., they will behave as completely independent events, sharing a common eventId.

@lobis

Also I am trying to understand how the energy is deposited in the gas in this particular case but I cannot trace back (using the viewer) which particle generated the electron tracks inside the gas (and the energy deposits), shouldn’t there be a visible track going from the lead atom to the gas? (how do we explain those unconnected points?)

Probably the best is to simulate again this event, using the same seed stored in TRestG4Metadata::fSeed and activate all the volumes, so energy deposits are visible at any volume.

That will help to understand the event. I believe you just have a Pb206 decay, emitting a 516keV gamma that might do a small Compton energy deposit in the gas volume, other cross markers could be in other volumes than the gas, it is hard to see only with one viewer projection. But I guess. The best is to attach always your PrintEvent, so that others can try to understand the event. Looking only to the viewer is hard.

[lobis]

@jgalan

If you are simulating neutrons I would expect that your PrimaryGenerator is not other thing than neutrons .

I can imagine that your neutron activates a Pb atom, and this one decays after some time which is larger than subEventTimeDelay .

<parameter name="subEventTimeDelay" value="100" units="us" />

Then, the event will be splitted in two independent sub-events. I.e., they will behave as completely independent events, sharing a common eventId .

This is probably what is happening, thanks, I did not know of this possibility.