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- Otto Frisch was distressed when he found that Nature
did not preclude the construction of an 'atomic bomb'. Robert Matthews
is distressed he didn't listen to him
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- ABOUT 20 years ago, I was one of a party of sixth-formers
who sat in on one of the last lectures given by the great Austrian-born
physicist Otto Frisch.
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- The trip had been organised by our physics teacher, who
was always keen to show that there was life and real people beyond the
A-level syllabus. But I'm afraid it was a case of pearls before adolescent
swine. I recall wondering why we had been bussed miles to listen to this
garden gnome in a suit, when we could have been watching Top of the Pops.
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- Only years later, while researching an article about
the first atomic explosion - which took place 54 years ago this week -
did I discover the true stature of the tiny old man who stood before us
that evening.
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- Frisch, a refugee from the Nazis, was a physics lecturer
at Birmingham University in the early war years. Picking his way through
the blacked-out streets of Birmingham one night in February 1940, Frisch
was reflecting on the success of the Luftwaffe when he started to have
bad feelings about some calculations he had done a few months earlier.
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- He had been investigating claims that a suitably large
mass of uranium might support a "chain reaction", where particles
unleashed by the break-up of one uranium atom smash into other atoms, unleashing
yet more particles and so on in a runaway sequence that would release large
amounts of energy.
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- Worried about the obvious military potential of such
a reaction, Frisch had estimated how much uranium would be needed - and,
like many physicists, he had concluded that the amount necessary for a
chain reaction would be way too large to pack into a bomb.
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- Frisch had published his negative findings and, again
like many physicists, felt relieved that Nature had contrived to forbid
the construction of an "atomic bomb".
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- But on that walk home, Frisch was worried that perhaps
he had overlooked some loophole in the otherwise ineluctable argument against
atomic bombs. He soon realised that he had - and could only hope that his
Nazi counterparts had too.
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- Attention had focused on two isotopes of uranium - U-235
and U-238 - and the effects on them of two types of particles: so-called
"fast" and "slow" neutrons. That gave four permutations,
and all had been studied and deemed effectively useless as the basis of
a weapon except for U-235 and fast neutrons.
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- Frisch "just sort of playfully", as he later
put it, plugged some rough calculations into a formula that gave the corresponding
mass of uranium needed to create a bomb. "To my amazement," he
recalled, "it was not a matter of tons, but something like a pound
or two."
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- Frisch took the results to his Birmingham colleague and
fellow refugee Rudolf Peierls, who had devised the bomb formula. He, too,
said he was "staggered" by the finding. It seemed Nature had
not forbidden the making of an atomic bomb after all, and Frisch and Peierls
reasoned that, if they knew it, the chances were the Nazis' own scientists
did too - or at least soon would.
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- The pair approached their boss, Mark Oliphant, who took
their findings to the Air Ministry's scientific committee. The memorandum
that the pair had produced ultimately became the blueprint for the Manhattan
Project, the Allied effort to build the first atomic bomb.
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- We now know that the Nazis never got close to making
their own atom bomb - essentially because their top scientist muffed the
calculation performed by Frisch. What is less well-known is that, just
weeks after Frisch made his calculation, the Japanese also started to look
at the possibility of atomic bombs - and, unlike the Nazis, also discovered
that just a few pounds of U-235 would suffice. Astonishingly, they were
still trying to mass-produce the isotope as late as April 1945, when their
bomb-making centre was destroyed by the US bombers.
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- By then, the Allies were carrying out final tests on
the first working atom bomb. These included a nightmarish experiment in
which two blocks of uranium, just less than the mass needed for a nuclear
explosion, were rushed past each other, to give crucial data on the likely
release of energy from the actual bomb. It was a procedure nervously dubbed
"tickling the dragon's tail" by Manhattan Project scientists,
and its daring architect was Frisch.
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- And to think that I passed up the chance to talk to the
dragon-tickler himself, just so I could get home in time to watch Dave
Lee Travis introduce Abba.
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