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

[chivasorn]

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?

 

[Astronuc]

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.

 

[QuantumPion]

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.

 

[Astronuc]

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)