Question about counting rate in a detector

[Alexander Camargo]

Hello everyone. I am simulating a Cesium-137 source with an energy of 0.662 MeV and an activity of 225 mCi. When I use the "T: tally time bins" card, for example:

F24:P 1
E24 20
T24 0 1000 25I 3600 196I 200600

I understand that I am asking the program to give me the average flux in this cell, for an energy of 20 MeV, over the defined time intervals (Tremor). My question is: can this time be used to determine how long it will take for my detector to count a certain number of particles? Or is this time on the T card only used for the simulation time?

Would it make more sense to use the tally result (F4 * activity) to obtain the flux rate (Bq/cm².s) to determine the time?

 

[Alex A]

MCNP doesn't really understand macroscopic time in a simulation. It keeps track of flight time, and the BURN card understands MW.days but I don't think there is any way of telling it the activity of a source. Most tally results are per source particle, so multiplying by the source activity afterwards is the normal process (and you can multiply by arbitrary values so you can force the program to do the maths for you if you want). T card is in shakes, so it's counting time between the emission of the particle and the event being tallied. This is really useful if the simulation is of a time of flight spectrometer, for example.

E24 20, btw, would be a single bin including everything from 0 to 20 MeV.

F4 can work but then you need to determine flux response for different energies if you want to know what a detector would do. I'm assuming that is the goal because it's in the title of the thread. If your detector is a scintillator, or a germanium crystal, I would put that in the simulation and use an F8 tally. I don't know if F8s are per volume or not by default so I would be sure to check that. If this is a scintillator without an MCA you can still use energy bins to simulate the threshold setting.