# Comparison of detector counts due to External Sources using MCNP & OpenMC (Subcritical system)

**URL:** <https://openmc.discourse.group/t/comparison-of-detector-counts-due-to-external-sources-using-mcnp-openmc-subcritical-system/4788>\
**Category:** User Support\
**Created:** [September 12, 2024, 12:30pm UTC](https://openmc.discourse.group/t/comparison-of-detector-counts-due-to-external-sources-using-mcnp-openmc-subcritical-system/4788 "2024-09-12T12:30:19Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![thilagam](https://avatars.discourse-cdn.com/v4/letter/t/e274bd/32.png) [@thilagam](https://openmc.discourse.group/u/thilagam)\
**Post date:** [September 12, 2024, 12:30pm UTC](https://openmc.discourse.group/t/comparison-of-detector-counts-due-to-external-sources-using-mcnp-openmc-subcritical-system/4788/1 "2024-09-12T12:30:19Z")

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Dear All,

In my simulations estimating count rates due to external sources at various subcritical levels, I have observed that the discrepancy between the MCNP and OpenMC simulated count rates increases as keffk\_{\text{eff}}keff​ approaches 1. This difference becomes particularly significant when keffk\_{\text{eff}}keff​ exceeds 0.95.

Could someone kindly review the results and provide clarification on the cause of this variation? I would greatly appreciate your support in resolving this issue.

With thanks & best regards,  
L Thilagam

 ![Comparison of Detector Counts due to External Sources](https://global.discourse-cdn.com/free1/uploads/openmc/original/2X/6/613467b4e322830f4759cc753556e804cf92fb2d.jpeg)