Gamma ray irradiation and a shielded chamber question

[girts]

Hi I was reading an article about a gamma ray materials testing camera built into one of the research reactors that worked in my country a while ago.
Now I have a few questions regarding some of the issues the article wasn't informing about.
It says that they built a special enclosed and shielded "hot" camera which was located physically near the core of the test reactor, they also say they used stainless steel piping into which liquid metal "In-Ga-Sn" was flowing with the help of electromagnetic pumps.

Now as far as I know, one way of producing gamma rays is by nuclear decay, other is probably by some type of neutron bombardment and secondary emission (please correct if I'm mistaken) Given the facts that I know I can guess they used the liquid metal to circulate around the core of the reactor, the liquid metal got bombarded by neutrons and then as it traveled back through the piping by some way (please tell me which) it gives out gamma emissions with an energy probably corresponding to that of the neutrons that bombarded it?
Could I be correct in assuming that they simply circulate the liquid metal around the core and as it gets bombarded it then circulates back up to the camera where the materials are located for materials testing and they get irradiated by the gamma rays that emit from the liquid metal in the stainless steel piping?
I assume stainless steel is used here as it can whitstand sufficiently high temperatures and is also rather transparent to neutrons?


Thanks.

 

[mathman]

There are plenty of gamma rays coming from the reactor core. I am not familiar with the setup you describe.

 

[girts]

Ok , from what I have been reading so far is that the test reactor in question (IRT-2000) had multiple horizontal and some vertical test channels but since they go through the reactor core they had neutron radiation alongside gamma radiation etc, so they developed a means to achieve pure gamma radiation without neutron backgroud by the means of a liquid metal loop.
The basic idea is that you take a mixture of liquid metal, in this case Indium, Gallium and some Tin, you pump it around a stainless steel pipe loop using electromagnetic pumps, as the liquid metal flows through or around the active core it gets bombarded with neutrons which make the mixture radioactive then as it flows back it goes through a special chamber and by the time it is in that chamber I assume the reactions (n,y) are taking place where the nucleus of the liquid metals have absorbed a neutron and now release a photon of corresponding energy, so by this way they get gamma radiation with specific frequencies and intensities inside the irradiation chamber and so the test samples don't become activated because there is no neutron radiation present.


So if someone here knows more detail about this process and the half lives of the liquid metals once they are used in the core and other aspects I would be happy to hear about that.

Thanks

 

[Astronuc]

Now as far as I know, one way of producing gamma rays is by nuclear decay, other is probably by some type of neutron bombardment and secondary emission (please correct if I'm mistaken)

This is correct.

In a nuclear reactor, gamma rays involving neutron absorption come from three sources: 1) prompt gammas release upon fission, 2) capture gammas from (n,γ) reaction, and decay gammas from decaying nuclei (that have been 'activated' from the neutron absorption. Brehmsstrahlung gammas also form from high energy electrons induced by Compton scattering or photoelectrons, but that involves high energy gammas, not neutron absorption.

Given the facts that I know I can guess they used the liquid metal to circulate around the core of the reactor, the liquid metal got bombarded by neutrons and then as it traveled back through the piping by some way (please tell me which) it gives out gamma emissions with an energy probably corresponding to that of the neutrons that bombarded it?

The gamma rays would have characteristic energies for the radionuclei. The main neutron absorber is probably In, specifically In-114 and In-116, with half-lives of 71.9 s and 14.1 s, respectively. These nuclei come from the activation of In-113 and 115, respectively.
https://www.nndc.bnl.gov/chart/reCenter.jsp?z=49&n=64

Ga has a low melting point (~29.8°C), while In has a melting point of 156.6°C and Sn 231.9°C. I suspect there may be a eutectic mix involved. The gamma emission from (n,γ) reaction is relatively spontaneous. The gammas in the gamma port are from decay of the radionuclides formed from neutron capture in the core. The gamma intensity (number of gammas emitted per unit time) is proportional to the neutron flux and absorption cross-section. For indium, the primary absorption is in the resonance range.
https://www.nndc.bnl.gov/sigma/index.jsp (look for (n,γ)).

Could I be correct in assuming that they simply circulate the liquid metal around the core and as it gets bombarded it then circulates back up to the camera where the materials are located for materials testing and they get irradiated by the gamma rays that emit from the liquid metal in the stainless steel piping?

That is correct. It's mostly like the In that is activated. Some Ga will activate, but the half-lives are much longer (on the order of minutes or hours). Sn has numerous isotopes, most of which are stable.

It is important to have a high energy gamma source without neutrons. Co-60 (half-life, 5.27 years) is a common gamma source with E = 1.17 and 1.33 MeV, which are actually decay gammas from the daughter product, Ni-60.

Stainless steel (probably one of the AISI 300 series) is typically used in reactor environments. It is not as transparent to neutron as Zr-alloys, but is stronger at temperature, and perhaps resistant to liquid metal embrittlement. Sn is slightly soluble in Zr, and I believe Ga is detrimental to Zr.