Understanding MCNP Output File: Mean Alpha Energy and Particle Distribution

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Discussion Overview

The discussion focuses on understanding the output from an MCNP simulation, specifically regarding mean alpha energy, error estimation, and particle distribution in energy bins. Participants explore the implications of the output file and seek clarification on specific aspects related to the simulation results.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant questions whether the output file provides an error estimate for the mean alpha energy and suggests using the central limit theorem to calculate it.
  • Another participant points out that the mean alpha energy reported may be misleading since the input file is in photon transport mode, indicating that the results pertain to fluence rather than energy.
  • There is a discussion about the interpretation of energy bins, with one participant expressing uncertainty about the average energy of each bin and seeking to plot a histogram based on the output data.
  • A participant confirms that MCNP assumes dimensions are in centimeters when inputting cell and surface card data.
  • Another participant suggests that plotting fluence against detector distance may be appropriate for modeling detection efficiency, but also notes that it could be interesting to analyze the response as a function of energy.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of the mean alpha energy and the nature of the output data. Some agree on the assumptions regarding dimensions, while the overall discussion remains unresolved regarding the correct interpretation of the MCNP output.

Contextual Notes

There are limitations regarding the assumptions made about the nature of the output data, particularly concerning the distinction between fluence and energy. Additionally, the discussion reflects uncertainty about the appropriate measurements for plotting results related to detector efficiency.

Who May Find This Useful

This discussion may be useful for individuals learning to use MCNP, particularly those interested in simulation outputs related to alpha particle detection and the interpretation of energy distributions.

zincsulphide
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Hi there,

I would like some help understanding the attached MCNP output file.

The file tells me that the mean alpha energy is 7.1931E-04 after a million simulations.

I have two questions:
  1. Does the file tell me anywhere what the error in the mean value is (+/-)? Or can I simply work this out from the central limit theroem (1/sqr(number of samples)) = 0.001 MeV
  2. Does the output file list the number of particles in each energy bin, or the average energy of each energy bin? I would like to plot a histogram of number of particles in each bin (or mean bin energy) vs. energy bins. I think the mean energy of each bin is as follows, but I am not sure.
2.0000E-01 1.97533E-04 0.0018
4.0000E-01 1.88478E-04 0.0018
6.0000E-01 3.33297E-04 0.0010
8.0000E-01 0.00000E+00 0.0000
1.0000E+00 0.00000E+00 0.0000
1.2000E+00 0.00000E+00 0.0000
1.4000E+00 0.00000E+00 0.0000
1.6000E+00 0.00000E+00 0.0000
1.8000E+00 0.00000E+00 0.0000
2.0000E+00 0.00000E+00 0.0000
total 7.19309E-04 0.0002

https://docdro.id/M06dhFQ

Thank you for any help
 
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Hi,
you write:
"The file tells me that the mean alpha energy is 7.1931E-04 after a million simulations."
remember that MCNP normalized the results with one particule. The result does not depend of the number of particle simulated.
Second you write "alpha Energy" but your input file is in mode p (only photons are transported no alpha !)
you write " mean alpha Energy" but you used a F4 tally : it is not not a energy but a fluence !
etc.
 
PSRB191921 said:
Hi,
you write:
"The file tells me that the mean alpha energy is 7.1931E-04 after a million simulations."
remember that MCNP normalized the results with one particule. The result does not depend of the number of particle simulated.
Second you write "alpha Energy" but your input file is in mode p (only photons are transported no alpha !)
you write " mean alpha Energy" but you used a F4 tally : it is not not a energy but a fluence !
etc.

Yes I'm self-teaching MCNP and finding it quite difficult.

Can I ask two questions:

1. When inputting dimensions into the cell and surface cards, does MCNP assume they are in centimeters?

2. For my task I am modelling the detection of efficiency of zinc sulphide for detecting alpha particles. If I am altering detector distance, is it appropriate to plot a graph of fluence against detector distance, or is there a more appropriate measurement?

Thanks again
 
hi
1 yes in cm
2it is interesting to have the response in function of energy
 

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