Mesh Detector measures 5000x more than Superposed detector

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senar.m
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Mesh Detector measures 5000x more than Superposed detector

Post by senar.m » Wed Jan 16, 2019 6:37 pm

Hello
i am measuring the neutron flux that leaves the main channel of our model, and i am doing it by placing 2 detectors, at the same spot.
The measured results differ way to much and i dont know why. To be precise one detector measures a ~5000x bigger number.

The first detector is a mesh detector defined as:

det myMeshDet de 1 dx 100 160 60 dy -30 30 60

The second detector i use is a superposed one (material void) defined as follows:

surf z5 cylz 130 0 25 120 124
cell c_z5 -1 void -z5
det d_z5 dc c_z5 de 1

They have different shapes/sizes i do understand that the results cannot be identical. Nevertheless when i add the values of the mesh cells together i get 6000 more counts as the other detector.

What am i missing?

Thx

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Jaakko Leppänen
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Re: Mesh Detector measures 5000x more than Superposed detector

Post by Jaakko Leppänen » Wed Jan 16, 2019 10:10 pm

Did you divide the results with detector volume? Also, your mesh detector is not bound in the axial dimension.
- Jaakko

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Re: Mesh Detector measures 5000x more than Superposed detector

Post by senar.m » Thu Jan 17, 2019 4:16 pm

Dear Jaakko,

you were right, the mesh detector was not bound in the z axis resulting in a massive boost in counts. Thanks for that.

However, there are still relevant differences. I ran the simulation again and the superposed detector gets 4 more counts than the mesh detector now, although the mesh surface is almost twice as big.

I changed the definition of the mesh detector to:

Code: Select all

surf s_Mesh pz 122
det myMeshDet de 1 dx 100 160 60 dy -30 30 60 ds s_Mesh -2
The definition of the superposed detector did not change.

How can that be?

Also I do not get the difference between the option -2 (flux) and 0 (net current) of the ds restriction.

Thanks

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Jaakko Leppänen
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Re: Mesh Detector measures 5000x more than Superposed detector

Post by Jaakko Leppänen » Thu Jan 17, 2019 4:29 pm

Did you divide the results by volume? Serpent doesn't do this automatically. The result that you get is always the integral value.

Do you mean that the result for the super-imposed detector is 4 times higher than that of the mesh detector with the ds entry?

The surface detector (ds) measures flux over surface, which is not equivalent with flux over volume.
- Jaakko

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Re: Mesh Detector measures 5000x more than Superposed detector

Post by senar.m » Thu Jan 17, 2019 4:53 pm

I do not divide by the volume, as its the integral value the one that interests me.

I do understand that the super-imposed detector has a volume and therefore a divergence to a surface detector.
But in this case the depth of the volume detector is quite narrow.
These surfaces (of the detectors) measure the flux leaving the channel of the reactor so the neutrons coming from other sides should never be grater in number than the ones coming directly from the fuel.
Do you mean that the result for the super-imposed detector is 4 times higher than that of the mesh detector with the ds entry?
Yes

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Re: Mesh Detector measures 5000x more than Superposed detector

Post by Jaakko Leppänen » Fri Jan 18, 2019 6:42 pm

When you integrate flux over volume, you cannot expect to obtain the same result unless the volumes are the same.

The surface flux detector integrates flux over the surface, which is not the same thing as the volume-integrated flux.
- Jaakko

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Re: Mesh Detector measures 5000x more than Superposed detector

Post by senar.m » Sun Jan 27, 2019 9:38 pm

I tried some things out and this is my situation:

I want to measure all the neutrons that leave a channel with a superimposed detector.
The shape of the volume is a thin disc.

Turns out it measures twice as much if its 0,2mm thick instead of 0,1mm (radius = 25 cm).

Obviously it is somehow bound to a reaction rate, although the superimposed detector is declared as void material.

Is there a way to just measure all the neutrons that pass through a surface without using a mesh detector?

Gladly appreciate

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Re: Mesh Detector measures 5000x more than Superposed detector

Post by Ville Valtavirta » Mon Jan 28, 2019 10:47 am

The results of superimposed detectors are integrals over the surface area of the detector. If you double the surface area, you will generally double your result.

Similarly, results of normal volumetric collision detectors are integrals over volumes.

-Ville

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