Posterminaries
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Moving the Center ofMass Have you ever been stymied by what seems to be a completely intractable obstacle? You can't go around it, you can't go through it, and it simply refuses to move despite your patience and persistence. Well, take heart! You may have been making more progress than you think. There is an elegant phenomenon at the intersection of solid-state and nuclear physics that has a lesson to offer in this regard. Recoilless resonance absorption of nuclear gamma rays, otherwise known as the Mössbauer Effect, has been around for over 40 years. Its significance to science was recognized early by the 1961 Nobel Prize in Physics. But, to our knowledge, its sociological signifi cance has yet to be recognized, that is until now. You see, there is this very excited n u c l e u s inside an atom bound to the lattice of a t o m s in some macroscopic chunk of material. It feels compelled to fire off a gamma ray as one of its principal methods to unwind (and we presume to feel more comfort able with itself in its g r o u n d State). It doesn't care in what direction it fires and pretty much uses all 4K steradians, relying on serendipity to find useful targets where its radiation might illuminate. From the perspective of the nucleus, that gamma ray packs a wallop. This is not a delusion of grandeur but is based on a very real kick usually feit by the emitter as it recoils in an attempt to conserve m o m e n t u m , a w o r t h y goal indeed. The offending nucleus itself may even be knocked back off its perch. This is quite unsettling to the immediate neighborhood through which repercussions of the recoil propagate, generating a rather disorganized potpourri of vibrational modes that quickly degenerate into just so much very local heat. It's an event that goes largely unnoticed by the vast majority (roughly Avogadro's Number) of other nuclei in the chunk. Of course, conservation of momentum on the "chunk scale" will have its way, and the whole mass does itself recoil.* On average, however, thanks to all those steradians, the net movement over time is nulled and we needn't worry This is no small effect! One 14.4 keV gammaray emitted by 57Co from a 1 gram Fe chunk imparts a velocity to the chunk of 7.7 x 10"21 cm/s or roughly 2.4 Ängstroms/Century.
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about the chunk flying off, or even inching its way off, a table in the lab. As if this miniscule impact were not bad enough, just think of the disappointment when our nucleus, having expended all the energy it had available at the time, sees its gamma ray make, at most
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equally localized and globally imperceptible impact on a neighboring chunk of similar material as it goes crashing into it. There, it can barely find a sympathetic nucleus w i l l i n g t o a b s o r b it all at o n c e . Target nuclei tend to be finicky about the precise energy of anything they are willing to assimilate. Therefore, the gamma ray fritters away its energy, dist u r b i n g as m a n y i n d i v i d u a l l y nonessential constituent elect
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