Why am I seeing a discrepancy in photon energy when doping ZnS:Ag in MCNP 6.1?

[nrat320]

TL;DR Summary

How to create material to produce photons

I am a new user of MCNP and I am trying to generate photons in ZnS:Ag through electrons as my source particle. My simulation as it is now creates photons however they are not right. For example ZnS:Ag should create a lot of photons with energy of around 3.1eV. However I see a spike around 4.8eV which is not right. Because of this discrepancy, I think I am not doping ZnS with Ag right. My material input can be seen below. Can someone tell me what I am doing wrong?

m1 30000 .45 47000 .05 1600 .45

 

[Alex A]

Welcome to physicsforums!

Default cut off is 1kev, so I'm guessing you are using a card to set that to 1ev? Btw I assume 1600 is a typo.

Thing is, MCNP has no real understanding of visible light. It has no way of telling it if the lattice is in the cubic or wurzite form, and no routines to work out scintillation if you could specify it.

The usual way to model a scintillator is to have a tally tell you how much energy a particle deposited into the cell of that composition and work out the pulse size from that. So it typically might be an F8 p,e with a large number of energy bins. You can tell it to blur the result a bit or you can do that in the interpretation step yourself, the FWHM resolution depends on how the light is collected of course.

 

[Astronuc]

TL;DR Summary: How to create material to produce photons

m1 30000 .45 47000 .05 1600 .45

So one has Zn, Ag, S, and the fractions add to 0.95?

 

[nrat320]

Welcome to physicsforums!

Default cut off is 1kev, so I'm guessing you are using a card to set that to 1ev? Btw I assume 1600 is a typo.

Thing is, MCNP has no real understanding of visible light. It has no way of telling it if the lattice is in the cubic or wurzite form, and no routines to work out scintillation if you could specify it.

The usual way to model a scintillator is to have a tally tell you how much energy a particle deposited into the cell of that composition and work out the pulse size from that. So it typically might be an F8 p,e with a large number of energy bins. You can tell it to blur the result a bit or you can do that in the interpretation step yourself, the FWHM resolution depends on how the light is collected of course.

thanks for the the reply!

I have used the commands below to indicated to MCNP I want to produce 1eV photons as well.
cut:P j 1.0e-6 $1eV

I have also narrow down my resolution of the tally bins the the range below.
e26 0 198i .000010

In have read in the report "LA-UR-12-21068" that is possible to create photons in the eV range through the use of plib. But from what I understand form your message, MCNP just does not have the right physics to create low energy photons.

 

[nrat320]

So one has Zn, Ag, S, and the fractions add to 0.95?

At some point, I was just playing around with the weight fraction to see it it had effect. But it did not. I did try fractions that added up to 1. but same result.

 

[Alex A]

LA-UR-12-21068 recommends a minimum cut off around 12ev because the energy loss mechanisms stop working under this. That might be what your peak is, a build up of photons at 4.8ev due to no loss mechanism. MCNP wasn't designed to do this stuff, so the physics models are missing.

 

[nrat320]

I thought the 12eV cutoff was for electrons. But I think you are right that the physics for low energy wavelength is not well structured in MCNP. Which is disappointing since I wanted to observe photon production in different scintillator/phosphorous materials.

thanks for your insight!