.0000000000001 in.

Subatomic investigators find neutrons elusive little things to deal with. Unlike electrons, protons and positrons, they have no electric charge. Slippery as wrestlers covered with oil they slide through the electric fields of atoms, are not deflected until they collide squarely with a nucleus. Nevertheless their mass (about 1,800 times that of an electron) has been established within fairly precise limits. And last week three Columbia physicists announced the size of the neutron as slightly less than .0000000000001 (one ten-trillionth) of an inch.

Before they could reach that conclusion even with the powerful wave mathematics developed by Germany's Erwin Schrodinger, Drs. George Braxton Pegram, John R. Dunning and Isidor Isaac Rabi had to lead their particles like circus animals through a complex series of hoops & hurdles. Beryllium powder was placed in a glass tube containing the radioactive gas radon. Alpha particles from the radon knocked neutrons out of the beryllium. First hurdle was a metal ring which deflected part of the neutron beam toward a cylindrical detection chamber less than an inch across, a half-inch deep. The chamber's door was guarded by a paraffin screen from which the neutrons evicted protons. Having positive charges, the protons ripped through the chamber, freeing ions which were collected on an insulated electrode so that their flow could be amplified, detected, measured.

Reduced to its simplest terms:

Just as one could establish the size of small boats anchored off-shore by the way that they reflected or scattered the incoming waves, so the three physicists estimated the size of atomic nuclei by the way in which they scattered the neutrons.

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