The Road to the Hydrogen Bomb

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Original concept of nuclear weapons was due to H G Wells

Aided and abetted by Leo Szilard 25 years later!

The problem which was already being mooted by such scientific men as Ramsay, Rutherford, and Soddy, in the very beginning of the twentieth century, the problem of inducing radio-activity in the heavier elements and so tapping the internal energy of atoms, was solved by a wonderful combination of induction, intuition, and luck by Holsten so soon as the year 1933

Wells wasn’t too far out in his fiction of 1914

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Atomic Structure in the 1930s

Bombarding matter with high energy particles- accepted means of smashing the atomic nucleus.Natural sources of e.g ∝ particles – limited energy

Rutherford saw need to accelerate +ve particles Through HV field – 8MV calculated.

Atom consisted of neutrons and protons in a small nucleus surrounded by electrons equal in number to protons

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Gamow was Russian and a loner when it came to research;many applying wave mechanics to electrons –applied it

to atomic nucleus.

QM tunneling

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Cockroft used these ideas to calculate the probability of a proton tunnelling into a nucleus; calculated this at 0.6% at 300kV – but 𝟏𝟎𝟏𝟒 protons bombarding the target per second!

1012 protons could penetrate nucleus

Best guess at what would be observed was that a γ ray would be emitted.

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14th April 1932 –– Walton used scintillation screen and lithium metal as target ;– many flashes of light - identified as ∝ particles.

37𝐿𝑖 + 1

1𝐻 → 224𝐻𝑒 + 17.7MeV

Energy of the ∝ particles – arises from mass difference and E=mc2

Li 7.016 amuH 1.008Total 8.024He 4.0026Total 8.005Difference = 0.019 amu

E=0.019×1.66×10-27×(3×108)2 = 2.84×10-12J = 17.7MeV (17.7×106eV)A trillion electron volts, or 1.602×10−7 J, about the kinetic energy of a flying mosquito.Artificial transmutation of elements was possible. Cockcroft and Walton received the Nobel Prize in Physics in 1951

‘We might in these processes obtain very much more energy than the proton supplied, but on the average we could not expect to obtain energy in this way. It was a very poor and inefficient way of producing energy, and anyone who looked for a source of power in the transformation of the atoms was talking moonshine’ Rutherford BA speech 1932

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Artificially Induced Radioactivity

Bad luck of the Joliot- Curies – twice

Third time lucky! – bombarded light elements with ∝ particles ;radioactivity observed; obeyed half life laws of naturally occurring radioactive

elements Nobel Prize for Chemistry 1935

1934 began experimental work on neutron bombardment of matter – shelf physics; light elements transmuted to lighter elements by ejecting either a proton or an alpha. Heavy elements lost energy by emission of a gamma ray and formation of a heavier isotope; Uranium – emitted a beta ray (electron) – several different half-lives observed – suggestion that new man made element Z= 93 formed.

Awarded 1938 Nobel Physics prize "for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons".

Emigrated to US in 1938 – wife was Jewish and Italy had passed racial laws- joins Columbia University.

Enrico Fermi (1901-1954)

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Nuclear Fission

Otto Hahn(1879-1968) Lise Meitner (1878 – 1968)Otto Frisch (1904-1979)

In 1938 Hahn and Meitner had been collaborating for 30 yrs - transmutation products of uranium.

Hahn repeated Curie expts α bombardment of Uranium- 3 new isotopes thought to be of radium.

Repeated again – the radium was in fact barium – but how?

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Otto Frisch visited Meitner– Christmas 1938;‘liquid drop’ model of atomic nucleus due to Bohr and Gamow;

Exptl paper from Hahn appeared January 1939. 1944 Nobel Prize in Chemistry

Frisch – additional expts in Bohr institute confirmed energy release estimates– theory paper to Nature Feb 1939;

Frisch tells Bohr in early January 1939– on his way to conference in USA.

Fermi corrected his Nobel lecture on learning of Hahn and Strassman’s results

When a neutron is captured by a nucleus, rearrangement of its structure happens.

The nucleus is no longer stable; sheds excess energy by splitting into two pieces.

The fragments, have excess neutrons - shed very rapidly by the fragments ; more neutrons are produced on average than are consumed .

The Nature Of The Fission Process

If the uranium is large and compact enough, it becomes a "critical mass", in which a self- sustaining chain reaction occurs.

For U235 this is about 52kg in the absence of reflectorsFor Pu239 about 10kg.

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Simulation

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10n + 92

235𝑈 → 56141𝑩𝒂 + 3692𝑲𝒓 + 3 10n

Mass (amu)U235 235.04Ba141 140.91Kr92 91.926n 1.009

Total Mass Before fission 236.049After fission 235.863Difference 0.186

Equivalent energy released = 0.186×931 MeV = 2.77×10-11J for 1 atomIn 1kg of U235 we have (1000/235)×6×1023 atoms

Total energy released is 7×1013J –

about 20 million kWh equivalent to burning ͠ 3 million tonnes of coal

1, Otto Stern; 2, Enrico Fermi; 4, Niels Bohr; 6, Harold Urey; 8, Gregory Breit; 10, Isidor I Rabi; 11, George Uhlenbeck; 12, George Gamow; 13, Edward Teller; 16, Hans Bethe; 27, Charles L., Critchfield; 29, Aage Bohr; 45, Murle Tuve

Theoretical Physics Conference GWU January 1939

Bohr – talks to Fermi; Wigner; Szilard – US press reports seen by Oppenheimer; Lawrence and Alvarez on west coast of USA.

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John Wheeler (1911 – 2008)

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The ‘Hungarian Conspiracy’

Edward Teller (1908 – 2003)

Jahn–Teller effect distortions; BET isotherm; Monte Carlo computational modelling

Gene Wigner (1902-1995)

Together with Fermi designed the first nuclear reactors; director of Oak Ridge aspect of Manhattan Project

Leo Szilard (1898 – 1964)1934 filed for a patent on the concept of the neutron-induced nuclear chain reaction -. - ‘if thickness of material is sufficient an explosion can be produced

‘In the course of the last four months it has been made probable — through the work of Joliot in France as well as Fermi and Szilard in America — that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future. This new phenomenon would also lead to the construction of bombs, and it is conceivable — though much less certain — that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transportation by air.’

‘Einstein’s’ letter to FDR

Verbally delivered to Roosevelt by Alexander Sachs on October 11. Roosevelt stated that the letter required action, and authorized the creation of the ‘Uranium Committee’

August 1939

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Peierls and Frisch Memorandum

Frisch invited to Birmingham Univ in summer of 1939 – trapped by outbreak of war

Prof of mathematical physics since 1937. As aliens both were banned from working on radar – so collaborated on nuclear fission!

Calculated that critical mass of 235U was ~ 1 kg; T > sun and P > earth centre after a few millionths of a second. Natural uranium –

–0.7% 235U

Wrote up as a memo which Oliphant passed to Sir H Tizard – chief govt science advisor

MAUD committee first met 10th April 1940

Rudolf Peierls (1907 – 1995)

Mark Oliphant (1901 – 2000)

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Sir Francis Simon (1893-1956)

1933 Lindemann invites him to Oxford

prevented from working on radar.

In 1940 in collaboration with Nicholas Kurti he was commissioned by the MAUD Committee to investigate the feasibility of separating uranium-235 by gaseous diffusion.

Initial experiments used his wife's wire kitchen strainer hammered to make smaller holes

Conclusions on the separation of uranium isotopes was transferred to the Manhattan Project and was the basis of the process that produced sufficient U235 to make the atomic bomb.

MAUD committee;G P Thomson (chair); Mark Oliphant;

P M S Blackett; James Chadwick;Philip Moon; John Cockcroft.

Work completed July 1941 – disbanded‘Use of Uranium for a bomb’‘Use of Uranium as a Source of Power’

UK Tube Alloys Programme 1942 -1952

In US agitation – reorganising of U comm

Oppenheimer collects theoreticians – mid 1942

Bombarding 238U by neutrons produces a new fissile element; Feather and Bretscher238U+n → 𝟗𝟑

𝟐𝟑𝟗𝑿→ 𝟗𝟒𝟐𝟑𝟗𝑿 Pu discovered March 1941 by Seaborg and Segre.

May 1941 detn cross section for slow neutron fission as twice that of 235U.20

After Washington conference Fermi– bombarded UO2 with neutrons – calculates 1.2 neutrons emitted per thermal neutron captured.

With Szilard starts to build a reactor ‘Columbia pile’ using graphite as a moderator to determine if a self-sustaining nuclear reaction possible.

Moved to Chicago in 1942 as Manhattan project gathers momentum.Final pile had 771 000lbs of graphite; 80 590 lbs of UO2 and 12 400lbs of uranium metal. Pile went critical on December 2nd 1942. SCRAM – safety control rod axe man.

Chain Reaction Feasibility

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Groves appointed 17 Sept 194218th Sept buys 1 250 tons of pitchblendeAuthorises buying 52 000 acres of Tennessee

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October 1942 Groves backs Oppenheimer to lead theory and bomb designNovember 1942 authorises construction of Los Alamos

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Plutonium production reactors Designed by Wigner and FermiBuilt by Du Pont

Weight 4 400kgTNT equivalent 16 kilotons

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X_ray_image

Weight 4600 kgTNT equivalent 21 kilotons

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https://www.youtube.com/watch?v=Ru2PWmGIoB8

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Cost of Manhattan projectTo 1 October 1945 was $1.845 billion, ($25 billion 2015), 90% of cost construction, production of fissile material

less than nine days of wartime spending.

Compare with war effort by the US in Europe alone;14 000 combat aircraft;

18 M tons of war material shipped to Europe;40 billion rounds of ammunition;

56 M grenades;fired 23 M Artillery rounds in Europe.

By 1945 US had built 66% of all ships afloatMaking 50% of all manufactured goods.

Total cost to US taxpayer $296 Bn- less than 33% of US GDP.

‘The Guns at Last Light’ Rick Atkinson

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British A Bomb Effort

Capenhurst – U enrichment plantSpringfields – U235 and Pu productionWindscale – reactors for prodn of Pu

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HMS Plym

Thermo nuclear Fusion

Fission weapons limited by critical mass of fissionable material.

Fusion weapons – in principle unlimited - explosive force determined by volume of fusion materials.

Fermi’s 1941 thoughts – use a fission bomb to heat D2 to get thermonuclear fusion.Teller’s efforts to progress ‘Super’ bomb

2D+2D →3He+n+3.27Mev2D+2D →3T + p+4.03Mev2D+3T →4He + n + 17.59MeV3T+3T →4He + 2n + 11.27MeV3He+2D →4He+p+18.35Mev

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TNT = 12Mtons

W80 -5 – 15KT

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Detonated on November 1, 1952 on the island of Elugelab in Enewetak Atoll, in the Pacific Ocean,.

Created a crater 1.9 km (6,230 ft) in diameter and 50 m (164 ft) deep where Elugelab had been

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Sources and Further Reading

The Neutron and the Bomb A Brown

The Making of the Atomic Bomb R Rhodes

Dark Sun R Rhodes

Oppenheimer: American Prometheus K Bird and Martin J SherwinGenius in The Shadows William LanquetteLise Meitner A Life in Physics Ruth Sime

Before the Fallout from Marie Curie Diana PrestonTo Hiroshima

Churchill’s Bomb Graham Farmelo

Serving the Reich: Philip Ball The Struggle for the Soul of Physics under Hitler

Test of Greatness: Brian CathcartBritain’s Struggle for the Atom Bomb

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