I need a bit of a hand in simulating an AmBe source.
The issue I am seeing is that I get significantly different neutron counts between the two primary particles I’m testing. I’m looking at how many neutrons are being generated in total and also how many exit the AmBe geometry.
I’ve created the geometry, which is a cylinder roughly 12 cm in height and 2.5 cm in diameter and made of AmO2 and Be (~80% AmO2 and ~20% Be) and in a steel housing.
For the primary particles I tested both alpha particles (5.486MeV) and Am241 Ion. The particles were spawned in the center of the AmBe geometry. Each run below was 10 000 000 events. I’m using QGSP_BIC_HP. GEANT4 version 11.0.1
Here are my test results primary Alpha Particles
Total generated neutrons: 1858 - Emean = 3.1487 MeV ( 1.4638 keV → 10.792 MeV)
Exiting world neutrons: 1216 - Emean = 2.1423 MeV ( 3.8754 meV → 10.792 MeV)
Here are my test results primary Am241
Total generated neutrons: 16502 - Emean = 3.1606 MeV ( 22.508 eV → 16.947 MeV)
Exiting world neutrons: 10766 - Emean = 2.095 MeV ( 3.1727 meV → 16.947 MeV)
I expected some variations this is almost an order of magnitude difference.
Should these approach give roughly the same number of neutrons, thus meaning there could be a bug in my code?
Or is there some physics here I’m missing?
Yea, that looks to likely be the solution here.
I was focused on the first decay and forgot the full process occurs in simulation.
Since most of the alphas are generated from the decay of Am241, would it be a good idea to halt the decays afterwards to more accurately simulate an AmBe source (can GEANT4 do this?)? since an experiment may be over the course of hours or days?
EDIT:
Another question: When using the Alpha particles as primaries, the neutrons spawn at a rate of roughly 1/6000. Does this seem a bit too high?
There are other ways to “halt” the decay of AmBe and its daughters. The easiest is to just use:
/process/had/rdm/nucleusLimits Amin, Amax, Zmin, Zmax
Here that would be:
/process/had/rdm/nucleusLimits 240, 242, 94, 96
Americium 241 would be the only thing allowed to decay.
And yes, I’d probably just focus on Am241 to start and not its daughters since Np237 has a half life on the order of 10^6 years.
Another question: When using the Alpha particles as primaries, the neutrons spawn at a rate of roughly 1/6000. Does this seem a bit too high?
The neutron production rate depends heavily on the ratios of Be/O2 and if not in literature is probably locally optimized for neutron rate.
Hello,
I tried modelling an accurate AmBe neutron source in Geant4 about two years ago but stopped at getting the neutron yield right but it was just a side project and this might be my solution:
And yes, I’d probably just focus on Am241 to start and not its daughters since Np237 has a half life on the order of 10^6 years.
It states: Specific source emission rate of an 241AmBe soruce is 6x10-5 neutrons/second/bequerel
(bequerel refers the activity of the 241Am)
with some math you can get to the ratio of neutrons per alpha.
And the normalized neutron energy spectrum should look like this:
I ended up simulating primary alpha particles at energies and probabilities (from Am241 decay) according to what the IAEA states inside my AmBe volume. This plot is where I’m at. Simulated NaI Detectors with detector resolution implemented.
I’m happy i can see the 4.44MeV peak and the 2 escape peaks. Something there is probably Iron from the steel housing too.