Thursday, March 29, 2012

Neutrons Made It Possible

Neutrons are everywhere, in everything, hiding in plain sight. Historically, they had to be imagined before they were detected because they were so hard to find. Rutherford had thought special "neutral pairs" of protons and electrons were inside atomic nuclei in order to explain why atomic weights varied and also to explain isotopes. To him, an alpha particle had four protons and two electrons instead of two protons and two neutrons and no electrons.

Certainly bearing a charge--being polarized--had made the earlier detection of electrons and protons easier because electromagnetic fields could deflect them. Their flight paths could be swayed. They could be attracted or repelled. Their charges gave them away and allowed them to be counted because they ionized things. Not so neutrons. Even today, neutrons are detected only indirectly via their effects on other signalling atoms.

At first, neutron radiation was confused with a more powerful type of gamma ray because it passed right through matter and with such ease--outdoing even gamma rays. But the new radiation did weird things to hydrogen-containing molecules like paraffin. And, they didn't do something that gamma rays did: they didn't spring electrons from metals like X-rays (the photoelectric effect). But the clincher was proving that the radiation had heft...mass. James Chadwick did that and won the Physics Nobel Prize the very next year.*

If neutrons have no polarity, no "handle," to steer them or deflect them, how are they channeled?  Mainly by projecting them through tunnels of neutron absorbing materials. Those that make it through have directional velocity. It sounds crude, but that's all there is.

At Los Alamos National Laboratory (LANL), there used to be an underground experimental station which resembled a spoked wheel. At the center, the hub, was a block of tungsten (I was told--of course I couldn't see it).  A proton beam impinged this target from above, creating a spallation of neutrons. Numerous tunnels led outwards from the hub like spokes on wheel. The tunnels were surrounded by paraffin and borax--materials with good neutron capture cross-sections. Each tunnel created a beam of neutrons terminated out at a "rim" of connected experimental stations.
*The defining experiments are briefly described here.
Chadwick wrote in 1940 (after reviewing progress on the atomic bomb):
[I] realised that a nuclear bomb was not only possible, it was inevitable. I had to then take sleeping pills. It was the only remedy.

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