How to split up a photon-neutron beam?

[Alex Tintor]

I need urgent help! we have a beam made of some neutrons and some photons. I thought about the beryllium polycrystaline but I'm not really sure it could work. The beam has an energy of 2 MeV moreless. Sorry for my spoken english.

 

[mfb]

Both photons and neutrons have 2 MeV? Which component do you want to keep, or do you want to keep both?

 

[Alex Tintor]

Well I want to keep the photons if it's possible.

 

[ChrisVer]

how about using a mirror ?

 

[Alex Tintor]

What i want is to absorb or reflect the neutron with something and let the photons pass, i don't know if with a mirror it could work.

 

[ChrisVer]

In general, neutrons are absorbed easily by light elements (such as hydrogen or hydrogen-based matterial like water)... But I don't know what might happen to the photons then...probably nothing....

The mirror was a bad idea for the 2MeV photons, since they need extremely good mirror-alignment and large scale-mirrors to not absorb the gamma rays. Plus its smoothness might be "destroyed" by the neutrons...

 

[Vanadium 50]

You're stuck, I'm afraid. Neutrons interact with nuclei, and photons interact with electrons. You won't find nuclei without electrons. ChrisVer's "probably nothing" is off the mark.

The best you can do with separation is to try and filter the neutrons out with a material with a large neutron cross-section like boron. This will affect the photons, though, and it's up to you to see if you can find a compromise you can live with. If your source is pulsed, you might think about timing: a 2 MeV neutron is pretty slow: every nanosecond the photons get another 30 cm in front of the neutrons.

 

[Alex Tintor]

Thank you about the information! I will start immediatly trying to find some information about boron. But if it does not work I will think about rebuilding the circuit.

 

[Vanadium 50]

There are other options besides boron like gadolinium or cadmium. You need to see what's best at the energies you have.

 

[ChrisVer]

Well I am saying probably nothing because in general light atomic masses don't interact so much with high energetic photons (gamma rays). That's why they say that in order to shield the neutron radiation you have to put several layers of heavier and lighter nuclei - the light will shield the neutrons and the heavy will shield the produced [by neutron interactions] gamma photons. If water was enough, they wouldn't put many layers, right?

 

[mfb]

All those neutron absorbers work better with slower neutrons.

I see some different approaches that could be interesting:

• Use properties of the initial beam - is it pulsed (see V50)? Is the beam divergence different for photons and neutrons?
• Start earlier: what is producing this beam and how can you influence it?
• A thin layer of (amorphous?) material with low Z - neutrons will either pass through nearly unaffected or get a large deflection angle, while most photons will pass through roughly in their initial direction. This does not give a nice neutron-free beam of photons but it leads to some separation.
• Bragg reflection of photons with a really tiny angle.

 

[ChrisVer]

• Bragg reflection of photons with a really tiny angle.

For the reflection of gamma rays, I checked here:
http://www.newton.dep.anl.gov/askasci/mats05/mats05275.htm

In principle you can try to reflect photons by using appropriate "mirror" matterials, but the mirror should be very large and smooth . For 0.1MeV photons for example and the certain matterial, they give the critical angle to be 0.04 degrees. Obviously sending the photon energies 1 order of magnitude above this, will make your critical angle less and so a much smoother mirror is needed to achieve this parallel-ness. However, I believe that neutrons can destroy the smoothness and so the mirror's reflection-ability to your gamma rays.

• A thin layer of (amorphous?) material with low Z - neutrons will either pass through nearly unaffected or get a large deflection angle, while most photons will pass through roughly in their initial direction. This does not give a nice neutron-free beam of photons but it leads to some separation.

That's what I tried to to give at a second glance over the problem. However V50 pointed out that this is not helpful because the matterial's electrons will absorb the photons. It is true that neutrons radiation will not be completely gone, but you can suppress it somehow against the photons if my initial point is correct (that photons won't interact that much with the small Z atoms). However the interacting neutrons will generate further gamma-radiation.
For all this suggestion of mine, I used the last paragraph here:
http://www.thomasnet.com/articles/custom-manufacturing-fabricating/radiation-shielding-materials
and that's why I also did my last comment about small Z. If it was enough to shield the photons as well, then they wouldn't write:

However, low density materials can emit gamma rays when blocking neutrons, meaning that neutron radiation shielding is most effective when it incorporates both high and low atomic number elements.

"High Z" here was used for the generated gamma-photons. Of course you will lose some photons too within any matterial.

On the same ground, in most of cases, people want to seperate neutrons from photons by damping the last component (get neutron beams for studying). They can't seperate those two at the same time, what they are trying to do is to suppress the photon component relative to neutrons [even if it means losing some neutrons meanwhile]. Eg. abstract here:
http://iopscience.iop.org/1748-0221/7/03/C03022/pdf/1748-0221_7_03_C03022.pdf

 

[mfb]

More information about the beam origin and the purpose of the separation would help, I guess. This is certainly not a problem with a standard solution.

 

[QuantumPion]

While neutrons have no electric charge, they do have a magnetic moment. With a strong enough magnetic field and large enough distance you could deflect the neutrons while leaving photons totally unaffected. I'm not sure how strong it would have to be for 2 MeV neutrons though, possibly unfeasible.

 

[ChrisVer]

While neutrons have no electric charge, they do have a magnetic moment. With a strong enough magnetic field and large enough distance you could deflect the neutrons while leaving photons totally unaffected. I'm not sure how strong it would have to be for 2 MeV neutrons though, possibly unfeasible.

Are you sure about it? I think magnetic moment in a magnetic field will only make the moment to oscillate. If you want to apply a magnetic field for this purpose, I'd suggest to try a strongly varying magnetic field (like the Stern Gerlach experiment for electrons) . But I still think that this can only dispress the beam and neutrons are much heavier than electrons (that's why you will need a much larger gradient for the mag.field).

Just for a funny comment, move far away enough, so that the neutrons will undergo beta decay.

 

[mfb]

Neutrons have a magnetic moment of ~10^-26J/T. If you somehow manage to get a field gradient of 1000 T/m, this gives a force of 10^-23 N or an acceleration of 6000 m/s². To get a deflection by 1° at ~20,000km/s neutron speed, you need ~300km/s or 50s acceleration time, which gives a convenient length of 1 million kilometers for your deflecting structure.

 

[QuantumPion]

Neutrons have a magnetic moment of ~10^-26J/T. If you somehow manage to get a field gradient of 1000 T/m, this gives a force of 10^-23 N or an acceleration of 6000 m/s². To get a deflection by 1° at ~20,000km/s neutron speed, you need ~300km/s or 50s acceleration time, which gives a convenient length of 1 million kilometers for your deflecting structure.

No problem, just stick your detector on a satellite at the L2 Lagrange point and you're all set! :)

 

[ChrisVer]

No problem, just stick your detector on a satellite at the L2 Lagrange point and you're all set! :)

It's not working that way, because you will have to apply that magnetic field for that length of path... and 1000T/m sounds a very large number ... A smaller gradient would make the distance much larger... and at 16mil km you will already get the neutrons decay....
Well that was a funny argument in the first place.The one who opened the thread should give more details about his beams.
For using magnetic field for the neutrons it's easier to bind them with protons (ionized hydrogen) to get deuterium and accelerate the deuterium instead. But is 2MeV neutrons enough for the job?

 

[mfb]

Oh, sure, neutron decays! Put your detector on Mars! :D Sorry, could not resist.

I used a large value on purpose to get an upper limit. 1000 T/m over 1mm requires .5 T at one side and -.5 T at the other. That could be possible with small permanent magnets, but it would be a very narrow gap and we don't know details about the beam.
=> magnetic fields do not work

 

[Evanish]

I think I should start by mentioning I'm not qualified to answer this question. Would it be possible to scatter the neutrons without effecting the photons by passing the beam through something like Radon gas?

 

[Vanadium 50]

I think I should start by mentioning I'm not qualified to answer this question. Would it be possible to scatter the neutrons without effecting the photons by passing the beam through something like Radon gas?

Look upthread.

Neutrons interact with nuclei, and photons interact with electrons. You won't find nuclei without electrons.

 

[Evanish]

Look upthread.

I see. Thanks.