I am using geant4.10.3p2.
When I generated neutron, Beta-decay occurred at ~ 50 us after generation.
I confirmed the energy of the neutron at the post step is 0.
However, the decay products (electron, anti-neutrino, proton) is generated at ~200 s
I don’t know why this timing gap appears, do you know the reason?
The neutron stopped at 50 us (I’m guessing you generated a neutron beam into some target material?). The G4 decay process is set up to assign a production time to the daughters based on the registered half-life.
For instance, if you produced a beam of U-238 ions, you would see the ion stop in the target with a post-step time of 500 ns or something, but you would see the alpha particle and Th-234 daughter nucleus assigned creation times of maybe +3 billion years (1e+17 seconds) or so.
Thank you for your reply,
(I’m guessing you generated a neutron beam into some target material?).
Yes, I generated a neutron in the pure water material.
You mean, “Stopping is [us] order but the production time of the daughter particles follows its lifetime ([s] order)”, right?
In that case, I am wondering why thermal neutron capture by hydrogen does not occur until Beta decay occurs.
(Of course, it is probabilistically possible, but it is extremely low)
That doesn’t sound right. Thermal neutron capture on hydrogen (H-1) should make a deuteron, with an immediate 2.3 MeV gamma emission. If you’re capturing on deuterium (H-2), then you’ll get tritium with another prompt gamma emission. The beta decay from tritium should be substantially delayed, by the tritium half-life.
How do you see the timing you describe? Are you using a step function? If so, you can see the time stamp for the neutron capture by looking for a post-step where the particle is the neutron, and the post-step process name is “nCapture.” With that, you should see a triton secondary particle, along with a gamma. The post-step time stamp, and the creation times of the two secondaries, should be equal.
In a separate call to the step, you should see the tritium have a decay-at-rest, where the daughters of the decay have a much later time stamp. If that’s not what you’re seeing, you might consider posting a bug report.
That doesn’t sound right. Thermal neutron capture on hydrogen (H-1) should make a deuteron, with an immediate 2.3 MeV gamma emission. If you’re capturing on deuterium (H-2), then you’ll get tritium with another prompt gamma emission. The beta decay from tritium should be substantially delayed, by the tritium half-life.
I’m sorry but I think I couldn’t tell you what I am wondering correctly…
In my case, almost all events have a “nCapture” process for the neutron final state interaction and produce a deuteron and a gamma-ray at the same time.
However, very few events have “Decay” and the Global time is ~50 [us] for the neutron final state, and produce anti-neutrino, electron, and proton. That looks like a beta-decay of the neutron. I can not understand why the global time of the initial step for these daughter particles is ~ 100 [s].
You mean, “Stopping is [us] order but the production time of the daughter particles follows its lifetime ([s] order)”, right?
If my understanding of your mention is correct, the neutron with Ekin = 0, wait for ~100 s until beta-decay occurs. I am also wondering why the neutron is not captured by proton until beta-decay occurs…
How do you see the timing you describe?
I am using the step function and see the Global time.
Yes, that’s exactly right! The neutron can scatter and lose it’s energy. If it doesn’t capture (and there’s certainly a non-zero chance that it won’t!), then it will beta decay (n → e- anti-nu p), with a half-life around 600 seconds.
It’s not “captured by proton”; it becomes a proton, along with an electron and anti-neutrino.
Thermal neutrons have an energy of well under 1 eV. The binding energy of the deuteron is 2.3 MeV, so almost any neutron-proton interaction should end up as a capture. The capture rate in an “infinite” water tank is something like 440 Hz (cross-section * velocity * number density of H), compared to the neutron half-life of 10.2 minutes, so the relative rates are around 270,000 to 1.
In your simulation runs, how many captures do you see compared to how many neutron decays?
Also, what physics list are you using, and what material? For thermal neutrons, you should be including NeutronHP in your physics list, have thermal-neutron physics enabled, and be using custom materials defined with the appropriate “thermal neutron” material names. See the Hadr04 example.
I’m sorry for very late reply.
I am using NeutronHP list, and material is the pure water.
The capture rate in an “infinite” water tank is something like 440 Hz (cross-section * velocity * number density of H), compared to the neutron half-life of 10.2 minutes, so the relative rates are around 270,000 to 1.
My result is consistent with your calculation. I think that is reason. Thank you very much.