Friday, January 19, 2007

The Gamma Laser

Originally written July 27, 2006

I saw Sharon Weinberger on the Daily Show, last night, touting her book, Imaginary Weapons. The book is her expose of weird DOD projects involving fringe science, etc. Amid the talk about psychic espionage and mind control rays, she mentioned the “atomic hand grenade” and hafnium. I know a lot of the background of that one, so there’s an excuse.

In the early 60s, when I was barely a teenager, there was an article in Scientific American about the gamma ray laser, graser, gaser, call it what you will. I read the article, talked about it with my science buddies, then put it in the back of my mind for a while. Then I went to RPI and joined the Rensselaer Engineer, the school’s student engineering magazine, and wrote a lot of articles, so many that some had to be under pseudonyms. One of them was on the gamma laser.

RPI’s library at the time was under fire for being inadequate, but it was good enough to get me a copy of the paper by Lev Rivlin describing the gamma laser, and I was young and cocky and indulged in a bit of speculation of my own in the article. So let me give some technical background.

Lasers work by a quantum trick. Light is typically emitted from an atom that is in an “excited state,” i.e. one or more of its electrons is not in its lowest possible energy level. The situation is symmetric, in that an atom in its lowest energy state (the ground state) will also absorb a photon to put it up into the excited state. The probability of the atom emitting a photon is related (actually, with caveats, it’s identical) to its likelihood of absorbing the photon in the reverse reaction.

It turns out (this was an Einstein thing), that you can get the atom to emit its photon “prematurely” if you hit it with a photon of exactly the same energy as the one it will emit. Thus the “stimulated” part of the light amplification through stimulated emission of radiation, and since you now have two identical photons, you also get the “amplification.”

In order to get real amplification, you need what is called an “inverted population,” where the number of atoms in the excited state is greater than the number of atoms in the ground state, otherwise the ground state atoms absorb all the photons you can make. Inversion is usually done by “pumping” the ground state into much higher energy states, which then decay into a “metastable state,” one that hangs around for a much longer time. Pumping can be done optically, chemically, or electrically, and all three are used in lasers.

The gamma ray laser does all this with energy shells in the atomic nucleus rather than electrons in the outer atomic shell. Also, because gamma rays are more energetic than regular light, you get a problem called “dynamic line broadening.” What happens there is that, because gamma rays pack a lot of energy, they have a “kick” that causes the emitting nucleus to recoil. But that recoil lowers the energy of the emitted photon, so it’s no longer at the right energy to stimulate the emission of the next atom. So the lasing action becomes very inefficient.

What Rivlin proposed was to make use of the Mossbauer effect. In the ME, the atom is embedded in a crystal matrix, and said crystal matrix allows the atom to vibrate only at certain fixed energies, so-called “phonon resonances.” That’s another quantum effect. If the “kick” from the gamma emission doesn’t match one of these resonances, then the entire crystal matrix is what rebounds. Well, the difference in masses between a macrocystal and a single atom is so great that all the energy goes into the photon and essentially none is lost to the matrix.

That left two problems for the gamma laser. The first is how to get the inverted population. The second is how to make an “infinite medium” i.e. get a long enough path in the lasing medium to obtain a lot of amplification. In most lasers, you put mirrors at both ends to create a “long path,” for the photons and lasing medium to do their thing.

Rivlin suggested that with a properly metastable isotope of high purity, only a few centimeters would constitute an effectively long path and no mirrors would be necessary. Others have suggested specially created crystal diffraction mirrors (which can reflect even low energy gamma rays if they are properly tuned to the correct frequency). In my little article, I suggested that low angel reflection might be sufficient, so you’d have maybe dozens of rods arranged in a polygon, each only a couple of degrees off the next, with a low angle metal surface in between. A similar trick is used for x-ray astronomy.

Pumping was something else again. I don’t think that either Rivlin or the Scientific American article suggested nuclear transmutation via neutrons, but that was something that I also speculated about.

Nothing much happened on the nuclear laser for another decade or more, but it became a hot topic for a little while during the SDI (“Star Wars”) period. There was even an underground bomb test that was briefly touted as having achieved amplification. Later that result was said to be a measurement error by some, while others hinted darkly at fraud. It appeared like the design was an attempt to “brute force” the matter (and there’s no brute like a thermonuclear bomb), but I could never figure out how they were going to solve the line broadening problem, and, by all reports, they didn’t.

In 1987 there was an experiment reported involving a metastable isotope of tantalum (Ta180m) exposed to high energy x-rays, with the result being a fluorescence that seemed to indicate some quantum stimulation was occurring. For a variety of practical and theoretical reasons, a lot of attention was then given to hafnium-178m2, the second metastable isotope of hafnium, having a half-life of about 30 years.

In 1999, a University of Texas group announced a stimulated emission result from hf178m2, triggered by a dental x-ray machine. It seemed like the Holy Grail was coming into view.
But then the criticisms began, the worst of which being that no researcher has ever replicated the original result, not even one of the UT group. Also, the original experiment did not have a control, so WTF?

Then came theoretical calculations that said that the process shouldn’t have actually reached breakeven, but a practical consideration was more important. The isotope does not occur in nature and is the product of an accelerator, which makes it hugely expensive. The idea of making a weapon out of it is ludicrous.

Well, so much for that.

But, as I say, I follow the field generally, and there is another story out in the hinterlands:

United States Patent 4,939,742 Bowman July 3, 1990 Neutron-driven gamma-ray laser

A lasing cylinder emits laser radiation at a gamma-ray wavelength of 0.87 .ANG. when subjected to an intense neutron flux of about 400 eV neutrons. A 250 .ANG. thick layer of Be is provided between two layers of 100 .ANG. thick layer of .sup.57 Co and these layers are supported on a foil substrate. The coated foil is coiled to form the lasing cylinder. Under the neutron flux .sup.57 Co becomes .sup.58 Co by neutron absorption. The .sup.58 Co then decays to .sup.57 Fe by 1.6 MeV proton emission. .sup.57 Fe then transitions by mesne decay to a population inversion for lasing action at 14.4 keV. Recoil from the proton emission separates the .sup.57 Fe from the .sup.57 Co and into the Be, where Mossbauer emission occurs at a gamma-ray wavelength.

This if very similar to some of the speculations I had back in 1969, but it gets around a big problem I noticed. I thought that the target nucleus would have to be of low atomic weight (low z) because otherwise the Compton effect would be too parasitic to allow amplification. This patent suggests that the production of the excited nucleus can be made to eject the atom from its normal substrate into a medium (in this case, beryllium foil) that is very low z, where the actual lasing would occur. I was thinking thermal neutrons, but the patent uses higher energy neutrons, so that’s how the recoil would occur. I also suspect that there may be some small angle gamma/x-ray reflection occurring in the device as well.

The patent holder, Charles Bowman, is someone I’ve noticed before; he was on of he scientists analyzing the Yucca Mountain nuclear waste site and who described a scenario where there might be a (low yield) nuclear explosion from the nuclear waste. That particular bit of work fit in with some other speculations that I’ve had, about highly moderated nuclear supercritical reactions, something that’s about half-way between a so-called “dirty bomb” and a real nuke. But that’s a topic for another time.


Anonymous said...

Thanks, lets build it! I need something to get rid of these pigeons outside my window.

Seriously, why wouldn't momentarily deep freezing the medium reduce the recoil loss?

James Killus said...

The gamma laser described by the Bowman patent almost certainly is to be run under cryogenic conditions.

If you mean "momentarily deep freezing" a bomb-based gamma laser, the "freezing" would not survive the absorption of the hard x-rays from the bomb; the crystal lattice would be destroyed by the absorption recoil of the atoms involved.

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Gids said...

If stimulated emission could be used on radioactive substances, would that be a good way to rapidly decommission nuclear waste?

For example if the waste were to be "enriched" by the same means as fuel is, until population inversion is achieved, and then arranged with reflectors so that amplification is possible, could that dramatically reduce the half-life of a gamma source?

Aaron said...

Dr. Kapale at WIU and several authors from the National Science Institute and Institute for Applied Science in Russia are looking at using the lasing without inversion (LWI) phenomenon to generate short bursts of coherent gamma wavelengths. It's all theoretical at this point but the phenomenon could lead to a working gamma laser or "graser". Which would be cool. Nothing like stripping something down to atomic fragments and having the leftovers annihilate everything around them with secondary emissions. Practical applications beyond the obviously military escape me at the moment but I'm sure there's something in quantum research that could use something like that. I also can't think of what you'd use as a backstop to prevent said graser from annihilating everything in a straight line in front of it if it fired for more than a few nanoseconds...

Anonymous said...

Actually I've pondered this awhile myself.
I believe it is possible. The laser should be constructed of a lot of grazing light pipes that collect all of the emissions from a central core and direct them back through the core again. The exit port(s) would be where there is no light pipe to collect emissions. The core could be any radioactive isotope that can be induced.
I see it as the holy grail for nudging an asteroid from a long distance away... though it would be a potent weapon system as well.

Anonymous said...

Your barking up the wrong tree. I'm throwing in my dice with BEC molecular positronium for the gamma laser.

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