Can Positrons Be Manufactured Through Neutron Bombardment?

[Eric Muller]

Hi everyone,

We know that positrons are emitted from certain isotopes and that isotopes form as a by-product of nuclear fission in reactors. So, I'm curious if positrons could be manufactured by the neutron bombardment of a stable isotope, such as Na, and then using the resulting beta-emitting isotope as a positron source? The positrons could then be channeled into a Penning Trap for storage. Sodium is cheaper than high energy lasers and gold discs. Neutrons are plentiful in fission reactors, if not neutron tube devices. And positrons have charge and so can be contained with EM fields. Of course there are always details, but would this idea generally work?

Thanks to everyone in advance.

 

[mfb]

Yes, but the amount of positrons you can store is limited by the charge density they give.
Also, neutron rich isotopes (=everything you get frequently in nuclear reactors) are the wrong side, you need proton-rich isotopes.

 

[Matterwave]

Sounds like your scheme would induce perhaps some beta decay. You'd spend all that work and then trap some electrons which you could have done by just combing your hair or something. :D

 

[e.bar.goum]

Why not just use naturally occurring positron emitters? Get some 22Na, and you have a great beta+ source. No need to bombard anything with protons.

Moderate the positrons, pulse them, etc, and you have a great positron beamline. Such things are used in positron studies.

See: http://iopscience.iop.org/0957-0233/21/8/085702/pdf/0957-0233_21_8_085702.pdf

 

[Eric Muller]

@mfb: Ah, charge densities and proton rich isotopes. I will explore these topics. I pieced together my pet idea this morning after reading about beta decay. One can only get so far in two hours, and then its lunchtime. I figured there had to be more science to learn. Thanks for the leads.

@Matterwave: Lol. "Man Powers House with Sodium Block, News at 11."

@e.bar: Very true. And I dare to wonder if a little tiny itty bitty beam emitter might be made in a garage for a hobby. A few grams of a safe/cheap isotope, some electromagnets, a vacuum chamber and so forth. Fun with science. Really though, I am more curious about how positrons could be made and stored in larger quantities. Not that I want to try that in my garage. I'm simply curious why no one has put together an antimatter factory yet, given what is known about the stuff.

Thanks for the replies.

 

[Matterwave]

Positrons would very quickly annihilate with surrounding electrons. Vacuum chambers are only so good, they can't remove everything, and traps are also only so good at trapping them. It seems to me that you'd have a hard time keeping any macroscopic amount of anti-matter around. But I am not an expert in this area, so I can't say for sure whether it is possible or not.

 

[e.bar.goum]

An antimatter factory has been running at CERN since 2000. http://physicsworld.com/cws/article/news/2000/aug/11/antimatter-factory-opens-at-cern
Antiprotons, not positrons, but still an "antimatter factory".

What Matterwave has said is really true. It's all about the quality of your vacuum and your trap. I'm not an antimatter person, but I've got a friend who is, and he seems to spend an awful large portion of his time fixing his vacuum. Looking at the paper I linked above, the beamline is pretty much like what you've described, (but much more sophisticated), and the trapping efficiency ≈3×10^-5%. Which is fine, but only if you've got a lot of counts coming off your source.

ETA: A few grams of any radioisotope suitable for what you want to do isn't going to be safe nor cheap. Radiation safety is serious business. Don't attempt without knowing exactly what you are doing.

 

[snorkack]

Why not just use naturally occurring positron emitters? Get some 22Na, and you have a great beta+ source.

Na-22 does not occur naturally.
It is possible to produce proton rich radioactive isotopes by adding neutrons to even more proton-rich yet stable and natural isotopes. Some of them decay only by electron capture. But for example, the lightest isotope of this description, chlorine 36, should have the energy to emit positrons.

 

[mfb]

I'm simply curious why no one has put together an antimatter factory yet, given what is known about the stuff.

Positron beams are common in colliders, and antiproton beams are possible to make as well.
Positrons and antiprotons can be slowed down and stored in small quantities (thousands of particles) - up to several months in Penning traps. It is also possible to make antihydrogen, but this is neutral and harder to contain (order of minutes).

 

[Vanadium 50]

Na-22 does not occur naturally.

K-40 does.

And while you can't find Na-22 in a mine, you can find it in a catalog.

 

[snorkack]

K-40 does.

So does V-50. Which also seems to have the energy to emit positrons.

 

[e.bar.goum]

Na-22 does not occur naturally.

Well, that's embarrassing. Of course it doesn't. According to the mass tables, it's "trace" in natural samples. I would hope I meant "get" as in "buy", but then the rest of my statement says otherwise.

But yes, there are plenty of other options.

 

[Eric Muller]

An antimatter factory has been running at CERN since 2000. http://physicsworld.com/cws/article/news/2000/aug/11/antimatter-factory-opens-at-cern
Antiprotons, not positrons, but still an "antimatter factory".

What Matterwave has said is really true. It's all about the quality of your vacuum and your trap. I'm not an antimatter person, but I've got a friend who is, and he seems to spend an awful large portion of his time fixing his vacuum. Looking at the paper I linked above, the beamline is pretty much like what you've described, (but much more sophisticated), and the trapping efficiency ≈3×10^-5%. Which is fine, but only if you've got a lot of counts coming off your source.

ETA: A few grams of any radioisotope suitable for what you want to do isn't going to be safe nor cheap. Radiation safety is serious business. Don't attempt without knowing exactly what you are doing.

Thanks for the link. I read it over. I think I'll hold off on the radioisotope thing. As I said earlier, this was all speculation. And in the process I have learned quite a bit more than I knew before.

 

[Eric Muller]

Positron beams are common in colliders, and antiproton beams are possible to make as well.
Positrons and antiprotons can be slowed down and stored in small quantities (thousands of particles) - up to several months in Penning traps. It is also possible to make antihydrogen, but this is neutral and harder to contain (order of minutes).

So a big challenge seems to be keeping the stuff around. If that could be a mastered it would be a big leap forward. I am curious is why all of the labs seem to use beams, instead of masses of beta+ emitting isotopes?

 

[e.bar.goum]

I am curious is why all of the labs seem to use beams, instead of masses of beta+ emitting isotopes?

Except that's not quite true, because like I said above, some labs also use masses of beta+ isotopes, for example, at the facility I linked to the paper about above. They do materials analysis with positrons. It's not true at all to say "all the labs seem to use beams".

But if you want to look at e+ e- collisions for particle physics problems, you'll want high energy positrons, at very very high luminosity. So you're immediately in trouble using a lump of radioactive isotopes. Low energy positrons aren't so much of a problem, you can always accelerate them afterwards, but the emittance is going to be bad. But if you want high intensity, you're going to have to have a really really hot source. That's a terrible plan from a safety point of view. With an accelerator, once you turn off the beam, the radiation is (mostly) gone. (save whatever you have activated, but you can mitigate this)

So, if you want high luminosity, high energy positrons, you're much better off producing it as a beam. See slide 36 of this lecture to see how positrons can be produced: http://www.desy.de/f/students/lectures2011/timmermans_summerstudent2011_lc.pdf

In short, you get a high energy electron beam, slam it into a thin target, and get electron positron pairs via pair production. Much more convenient and controllable than a lump of radioactive isotopes.

Further, once you've produced them anyway, you can always decelerate them if you want to do antimatter studies, as they do at CERN. So the setup is generic.

Basically, it depends on the science you want to do.

 

[mfb]

So a big challenge seems to be keeping the stuff around. If that could be a mastered it would be a big leap forward.

In which way?

I am curious is why all of the labs seem to use beams, instead of masses of beta+ emitting isotopes?

Beams are a way to store then, beta+ emitters are a way to produce them, you can't compare them.

High-energetic collisions produce positrons that are easier to capture in relevant amounts because they all fly in nearly the same direction.
And for antiprotons, you don't have a choice.

 

[Eric Muller]

@e.bar.goum: Thank you for the detailed explanation. I'm not familiar with luminosity or intensity as I am learning all of this via wikipedia, and not through formal academic training. I thought beta+ was all the same be it from isotopes or beams, so why not isotopes? I follow your point that the experiment determines the source of the antimatter and the parameters of said antimatter are source dependent.

@mfb: I figure shelf-stable and inexpensive containment would improve access to scientists in government, industry, and academia. Imagine if antimatter could be stored with the safety and reliability as say, a commonly manufactured chemical. Increased antimatter R&D would result in these sectors, generating new science and new applications (and many new job openings for physicists :D) . I have no doubt that many a knowledgeable physicist can explain to a layperson such as myself the many and various difficulties of this prospect. Bear in mind that history has shown, time and again, that somewhere a researcher will stumble upon a solution. It could be an unexpected outcome in an experiment, a sudden stroke of ingenuity, or a long process of trial and error. My own hope is that improvements in antimatter tech. would lead to improved or lower-cost medicine, and of course FTL travel.

-Eric

 

[e.bar.goum]

Thank you for the detailed explanation. I'm not familiar with luminosity or intensity as I am learning all of this via wikipedia, and not through formal academic training. I thought beta+ was all the same be it from isotopes or beams, so why not isotopes? I follow your point that the experiment determines the source of the antimatter and the parameters of said antimatter are source dependent.

Sorry, luminosity and intensity is just the same as "brightness" - or the number of positrons coming out per unit time. So if I want a hell of a lot of positrons in my beam, like you want for a particle physics experiment (more positrons = more chances to see what you want to see), you either would need a hell of a lot of radioactive material, which is problematic for the reasons I outlined above, or you need a nice strong electron beam, which is easy. Of course, all positrons are the same, but their energies are not. The energy of a positron emitted by nuclear decay is determined by the particular decay, but will be on the order of MeV. If I produce them in my accelerator, they will have energies on the order of GeV (or whatever I want, above 1022 KeV).

I figure shelf-stable and inexpensive containment would improve access to scientists in government, industry, and academia. Imagine if antimatter could be stored with the safety and reliability as say, a commonly manufactured chemical.

To contain antimatter you need very strong electric and magnetic fields, and very hard vacuums. The best number I've seen is 8 days for confinement, but that's a very sophisticated setup, and used laser cooling. The storage of antimatter will always require serious amounts of infrastructure, not a jar on a shelf.

 

[snorkack]

Of course, all positrons are the same, but their energies are not. The energy of a positron emitted by nuclear decay is determined by the particular decay, but will be on the order of MeV.

No, it is not.
In alpha and gamma decay, one particle, respectively alpha and gamma particle, takes the whole energy of decay, save the recoil of the nucleus which is also determined by the fixed decay energy. Therefore all alpha or gamma particles from same decay have same energy.
In beta decay and positron emission, two particles are emitted at the same time: electron and antineutrino, or positron and neutrino. And the fixed decay energy is divided between two particles in a random manner. So positrons emitted by nuclear decay have all energies from zero to maximum energy determined by that particular decay.

If I produce them in my accelerator, they will have energies on the order of GeV (or whatever I want, above 1022 KeV).

Again, they will have all energies - and no minimum.
If you aim at a target cathode rays of fixed energy 1023 keV, some of the electrons just bounce around target. Some produce X-rays - of all energies, up to the maximum 1023 keV and down to visible, infrared and radio waves. Because the electron still exists and has energy after collision that emits the photon.

And these 1023 keV electrons will also produce positrons - with maximum energy 1 keV. Because 1022 keV is expended on creating positron and electron.

If you target something with 1 GeV electrons, you get everything. 1 GeV photons, 1 eV photons, positrons up to several hundred MeV and again down to no energy, and also muons, pions and kaons, again with all energies from zero to some maximum.

 

[e.bar.goum]

I really didn't intend to imply that the positrons were of fixed energies. Or that they were the only things produced in high energy experiments.

 

[mfb]

Imagine if antimatter could be stored with the safety and reliability as say, a commonly manufactured chemical.

Then you are still limited by the amount you can produce.. Sure, it would make handling it easier.

My own hope is that improvements in antimatter tech. would lead to improved or lower-cost medicine, and of course FTL travel.

I don't see applications in medicine. And it has absolutely nothing to do with FTL travel. Large amounts of antimatter would allow very compact energy storages and maybe spaceships reaching some significant fraction of the speed of light, but not FTL. You would have to produce all the antimatter first, however. If you had the total amount of antimatter that has been trapped so far (in several decades!) at once, it would be enough to cook coffee with it, but not more.

 

[Eric Muller]

Then you are still limited by the amount you can produce.. Sure, it would make handling it easier.I don't see applications in medicine. And it has absolutely nothing to do with FTL travel. Large amounts of antimatter would allow very compact energy storages and maybe spaceships reaching some significant fraction of the speed of light, but not FTL. You would have to produce all the antimatter first, however. If you had the total amount of antimatter that has been trapped so far (in several decades!) at once, it would be enough to cook coffee with it, but not more.

Darn. And here all these pop sci articles have me thinking about antimatter for hypothetical FTL drives (Alcubierre/White). I get that the possibility of FTL is very controversial and that's a topic for another thread. But I was raised on Star Trek. The Alcubierre/White stuff has me very excited, even if it is a remote remote remote possibility.

I will say this thread has been great. I think I've learned more from these conversations than I would have just clicking around the internet.