Why Does Fission Chamber React to Neutron Fluxes Over Source Range?

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Discussion Overview

The discussion centers on the operational characteristics of fission chambers in response to neutron fluxes, particularly why they are effective beyond a certain source range despite being ionization chambers sensitive to gamma rays. The scope includes technical explanations and conceptual clarifications regarding detector functionality in nuclear environments.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that the primary purpose of a fission chamber is to measure neutron radiation through induced fissions, rather than gamma or beta radiation.
  • It is suggested that to enhance sensitivity to neutrons, fission chambers may be shielded with high Z materials that absorb beta particles and scatter gamma rays while allowing neutrons to pass through.
  • One participant mentions that in light water reactor (LWR) in-core detectors, there are significantly more neutrons at full power compared to when the reactor is shut down, complicating the design of fission chambers for a wide operational range.
  • Another participant indicates that excore detectors might function as ion chambers, while in-core detectors are described as solid state devices.
  • The signal-to-noise ratio is discussed, with a larger ratio at power compared to the source range, where the neutron levels are low relative to gamma radiation.

Areas of Agreement / Disagreement

Participants express varying views on the effectiveness and design considerations of fission chambers, indicating that multiple competing perspectives exist regarding their operational characteristics and the challenges faced in different reactor states.

Contextual Notes

Limitations include the dependence on specific reactor conditions and the complexity of detector design across different operational ranges. The discussion does not resolve the technical challenges or assumptions related to the effectiveness of fission chambers in varying neutron flux environments.

chivasorn
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why does fission chamber start to work from neutron fluxes more than source range in spite of being a kind of ionization chamber that is sensitive to gamma rays?
 
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chivasorn said:
why does fission chamber start to work from neutron fluxes more than source range in spite of being a kind of ionization chamber that is sensitive to gamma rays?
Well, the purpose of a fission chamber is to measure neutron radiation (by virtue of induced fissions), and not gamma or beta radiation. If a detector measures the radiation fields of different types when one is interested in a specific type, the one has to have another detector sensitive to the type which one must exclude.

The goal of a fission chamber is to have it be more sensitive to neutrons than gamma or beta particles. One way would be to shield it with a thick high Z material (which would absorb betas (electrons) and scatter (absorb) gammas (by Compton and photoelectric effects), while having a low absorption cross-section for neutrons, such that it responds primarily to the neutron radiation. I believe such a detector also has a lower voltage than a gamma or beta detector.
 
If you are talking about a LWR incore detector, there are 10 orders of magnitude more neutrons at full power than shut down. It would be hard to make a fission chamber detector capable of working over such a large range. That is why there are different kinds of detectors to use while at low power or shut down.
 
Excore detectors possibly would be ion-chambers. In-core detectors are solid state.

http://www.freepatentsonline.com/3872311.pdf (1973)

http://www.freepatentsonline.com/4123658.pdf (1977)


In the power range, one has a large signal-to-noise ratio as compared to the source range where the signal-to-noise ratio is very low due to the low level of neutrons compared to gammas.

See also - V and Rh detectors are most common.
http://www.tpub.com/content/doe/h1013v2/css/h1013v2_75.htm
http://www.tpub.com/content/doe/h1013v2/css/h1013v2_76.htm (see discussion on wide range fission detectors)
 

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