7.
Cockcroft and Walton's Accelerator
Rutherford
agreed that Cockcroft's calculations were correct and
that the idea was worth pursuing. Rutherford suggested
that Walton should abandon his linear accelerator and
team up with Cockcroft to work on producing protons
and a vacuum tube to accelerate them through 300,000
volts. Rutherford got a University grant of £1000
to buy a 300 kV transformer, and Allibone designed rectifiers
that could convert this alternating current into a steady
direct current. The equipment was too large to fit through
doors, so two Metropolitan-Vickers engineers had to
come to the Cavendish to finish building it!
By
the end of 1929 Cockcroft and Walton had constructed
a discharge tube that could withstand 300 kV, and the
apparatus was ready for testing in March 1930. Low energy
protons were produced in a small glass chamber above
the tube, and they were then accelerated down the tube
in a fine beam. This beam of high-energy particles came
out of the bottom of the tube and could bombard a target
placed beneath it.
Cockcroft
and Walton began by bombarding the light elements lithium
and beryllium, and the much heavier element lead. They
expected to see gamma rays, but found no convincing
evidence of a nuclear transformation even at 280 kV,
the limit of the apparatus. Disappointed, Cockcroft
tried to think of ways to get a higher voltage.
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