A VERY WOBBLE UNSTABLE DROP
A
C O:~IPAC T
HISTORY OF NUCLE A R FISSIOX
:\L\ H C' O POLO:'\ I
by Irene Curie (7t) and Frederic Joliot (7t) in Paris of a m ysterious penetrating neutral p article - w hich the couple
wrongly interpreted as a fast recoil proto n - exclaimed
»idiots! They have not understood that it is the neutron! « 2
Majorana did not bother to publish his own interpretation
of the experiment . The concept of an electrically neutral
particle was first proposed by Rutherford in 19 20 . In alecture b efor e the Royal Societ y of London , Rutherford speculated on the possibility of a third constituent of matter .
H e rightly theorized that it would have no electrical charge ,
but mistakenly thought it would be the combination of an
electron and a proton strongly bound together, and not a
new elem entary particle . It is this lecture that set James
Chadwick (7t), Rutherford's assistant, on the hunt for the
elusive particle . It took Chadwick twelve years t o find evidence of the neutron's existence and to announce its experim ental confirmation at Cambridge in 193 2, a few months
after Majorana's derisive r em ark . 3 Because the neutron is
heavy and electrically inert, it is able to get through the
electrical barrier around the atomic nucleus and r ock it, as
in a billiard strike. The discover y of the neutron opened a
new and exciting research field into nuclear physics.
Szilard's insight into the neutro nic chain r eaction narrowly preceded the first intuition, by G erman chemist Ida
Noddack, of the possibility of nuclear fission under neutron
bombardment. Noddack, in her now fam ou s September
1934 paper Ub er da s Element 93, 4 writes: »When heavy nuclei are bombarded by neutrons, it is conceivable that the
nucleus breaks into several large fragments, which w ould
of course be isotopes of known elem ents but would not b e
neighbors of the irradiated elem ent. « Her article sharply
criticized the interpretation that Enrico Fermi (7t) - the
father of the atomic bomb - provided of his ow n neutron
b ombardment experiments in Rome , and called for the
proper r eading of his results. The article w as completely
ignor ed by the scientific community. This is because the
widely held idea, an alchem y of sorts proposed by Fer m i,
was that under neutron irradiation uranium transmutated
into adjacent elem ents called »transuranics«. These are elem ents in the periodic t able with an atomic number greater than 9 2, that of uranium, which is the heaviest elem ent
H .G . W ells begins his 1914 novel Th e World Set Free with a
dedication to Frederick Soddy (1t), whose work inspired the
book. Soddy was a brilliant chemist w ho worked w ith Ernest
Rutherford (7t), the first man to visualize the planetary
structure of the atom. In a 1904lecture to the British Co rps
of Royal Engineer s, Soddy had r emarked with prescience
that »it is probable that all heavy matter possesses - latent
and bound up with the structure of the atom - a similar
quantity of energy to that possessed by radium [a recently
discovered elem ent). If it could b e tapped and controlled,
what an agent it would be in shaping the world's destiny!
The man who puts his hand on t he lever by which a parsim o niou s nature r egulates so jealously the output of this
store of energy would possess a w eapon by which he could
destroy the earth if he chose .« The World Set Free provides
the first fictio nal depiction of the atomic bomb and its de struction of major cities, and eeril y prefigur es the development of the nuclear weapons that ended World W ar II
and inaugurated the Cold War era. At the end of chapter 2,
secti on 4, W ells describes the dropping over Berlin of the
first atomic bomb, made of Carolinum, a fi ctional elem ent.
»Once launched , the bomb was absolutely unapproachable
and uncontrollable until its forc es wer e nearly exhausted,
and from the crater that burst open above it, puffs of heavy
incandescent vapour and fragments of viciously punitive
rock and mud, saturated with Carolinum , and each a cen tre of scorching and blistering energy, were flung high and
far. Such was the crowning triumph of militar y science ,
the ultimate explosive that was to give the de cisive touch to
war. «
Altho ugh W ells w as perso nall y acquainted with the Hun garian physicist Leo Szi lard, it r emains unclear w hether his
novel actually inspired Szilard to envision - in a sudden
illumination as he walked through London on a morning
in September r 9 3 3 - the core m echanism of the atomic
bo mb. »As the light changed to green and I cr ossed the
street«, Szilard recall s, »it ... sudd enl y occurred to m e that
if w e could find an elem ent w hich is split by neutrons and
which would emit two neutrons when it absorbs one neu tron, such an elem ent, if assembled in sufficiently large
m ass , could sustain a nuclear chain reaction .« 1
Ettore Majorana's name is not associated with the history
of nuclear fi ssion , but it is notable that many recognize him
as one of the first physicists to have had a clear vision of the
neutron's existence. There isn't, though, any scientific paper
to prove thi s cl aim . It is said that Majorana, immediat ely
upon getting the news in Rom e in 1932 of the obser vation
Richa rd Rhodes, The Making of the Atomic Bomb, London 19 86 .
Emilio Seg re and Gian Carl o Wick's reco llections. Ettore Majorana's name
is generally associated with the most elusive of all particles, the neutrino, detected
in '99) by Frederick Reines (7t), almost sixty years after Majorana's vanishing.
2
15
3
J.Chadwick , Possible Existence cif a Ne utron , Nature
4
I. Noddack, Uber das E/ement9 3, Angewandte Chemie 37, 6)3; '934·
1 2 9,
3 1 2; 1 9 3 2.
found in nature. Noddack 's intuition of the splitting of the
uranium nucleus was too wild for the scientific orthodoxy
of the time. Emilio Segre (7t), a close collaborator of Fermi,
later wrote that »the possibility offission, however, escap ed
us although it was called specifically to our attention by Ida
Nod dack . The reason for our blindness is not clear«. It took
five more years and the intuition of another woman, Lise
Meitner, to prove her right .
It is r emarkable that the idea of nuclear fission came from
two women, neither of whom was ultimately awarded a
Nobel Prize. It is speculated that a number of factors are
res ponsible for the neglect ofNoddack's prescient I934 pap er. One is a controve rs y over a premature report with her
husband Walter Noddack of the discovery of e lem ent 4 3,
Masurium, in I925. Their ex perimental results could not
be r eproduced. This, added to the quite inappropriate patriotic name they chose for it - from Masuria, the region
in Eastern Prussia where Noddack 's famil y came from together with her husband 's affinities with Nazism, had
damaged her reputation. However, she was criticized for
not having p erformed the chemical analyses of the reaction
products of neutron irradiated uranium to substantiate her
intuition of nuclear fission. It was such experimental work
that fiv e years later produced thi s ev idence.
In I935, Lise Meitner, a brilliant Austrian chemist of Jewish
origin, began work with her German colleague Otto Hahn (7t)
at the Kaiser -Wilhelm-Institut in Berlin, to chemically iso late the transmutation products of the heaviest natural e lements under neutron irradiation . They were assisted by a
younger German chemist, Fritz Strassmann. With the ad vent of the Third Reich, from I 933 onward s many Jewish
scientists were discharged or forc ed to resign from their
academic positions. With the Anschluj3 of Austria in I938,
their situation becam e critica l. Meitner was forced to fl ee
into exile to Stockholm in I9 3 8. From there she established
a secret scientific correspondence with Hahn who continued their ex periments in Berlin. By bombarding uranium
with neutrons Hahn and Strassmann were abl e to disintegrate it into r adioactive isotop es chemically inseparable
from barium, an elem ent slightl y more than half as heavy
16
as uranium - that is to say, radioactive barium . The ir exp erimental data puzzled them as it contradicted all the
available knowledge about transuranic b ehaviour. In D ecember I9 38 Hahn wrote to Me itner seeking help to inter pret the ir strange findings: »Perhaps yo u can suggest some
fantasti c explanation. W e understand that it rea ll y can' t
break up into barium .. . So try to think of some other pos sibility.« Soon after, Meitner and her nephew Otto Frisch,
a young Jewish experimentalist who had found refuge in
Copenhagen, travelled to Kungalv, a village in west Sweden,
to spend Christmas together. They took walks in the snow
during which they discussed Hahn's results. There, sitting
on a tree stump, or a tree log - biographe rs di ve rge on this
detail - Meitner came to the true interpretation of the experiment : what Hahn and Strassman n had achieved with out under standing it was to split the uranium nucleus into
sm a lle r e lem ents, in what is now regarded as one of the
few pivotal experiments of the 2oth centur y. The liquid drop model of the nucleus, according to whi ch neutrons
and protons behaYe like the molecules in a drop of liquid ,
develop ed in 1938 by Niels Bohr (7t), the famous Danish
physicist, greatly helped Meitner and Frisch to reach this
conclusion: when the nucleus was hit by a neutron, it b ehaved as a droplet that we nt wobbly befor e splitting, somehow like a dividing living cell.
Soon after Christmas Frisch Yisited Bohr in Copenhagen to
share their findings with him . He re members that Bohr exclaimed »Oh, what idiots we have been! Oh , but thi s is wonderful. Thi s is just as it must be !« As Hahn and Strassmann
published their experimental proof of the di sintegration of
uranium into barium in Die Naturwissenschciften, ' Frisch and
Meitner publi she d its correct interpret ation in Nature. 6
Shortly thereaft er, Frisch published a pape r that produced
experimental confirm ation of nuclea r fission, t he te rm he
0. Hahn and F. Strassmann, Ubcr den Xachn ·eis und das l'erh ahen Jer bei Jer
Besrrahlung des Uram
\\'issenschaften 2 7,
miHels
1
Xeutronen entscehcnden Erdalkalimeca!le, Di e Na tu r-
9 3 9.
O.R. Frisch and L. Mcitncr, Disint egration of Uranium
\ 'eL1 ·1jpe f!JI\.uclear Reacrion, ~ature 143: 2 39 240, 1939.
17
by Neutrons:
A
coined for this reaction. 7 Frisch and Meitner: »On the basis
[... ]of present ideas about the behaviour of heavy nuclei, an
entirely different and essentiall y classical picture of these
new disintegration processes suggests itself. On account of
their close packing and strong energy exchange, the parti cles in a heavy nucleus would b e expected to move in a collective way which has some resemblance to the movem ent of
a liquid drop. If the movem ent is made sufficiently violent
by adding energy, such a drop may di vide itself into two
smaller drops .«
material could be introduced to slow down the neutrons
that were released. 11 They were able to start a chain reac tion, but the process was not divergent and quickly died
out. Three yea rs later, in December 1942, Fermi, who had
recentl y moved to Chicago, was able to produce the first
controlled, self-sustaining chain reaction on a large scale.
His reactor, the Chicago Pile 1, used graphite bars as a
moderator. The full r ealization of the atomic bomb was
now less than three years away.
(7t) Recipient of a Nobel Prize.
0
A
0
B
c
00
D
»lt seems therefore possible that the uranium nucl eus has
only small stability of form, and may, after neutron capture, divide itself into two nucle i of roughly equal size ...
These two nuclei will repe l each other and should gain a
total kineti c energy of c. 2oo Mev., as calculated from nu clear rad ius and charge.« 8 The r eleased energy cam e from
a small amount of mass lost in the split of the uranium nu cleus into barium and kr ypton, and was calculated using
Einstein's mass-energy equival ence formulaE
m c 2 • 9 Its
magnitude was millions of times greater than that of any
known chemical r eaction . Meitner's decoding of nuclear
fission, and the tremendous energy it liberated, sent a shockwave through the scientific communit y. 10
Leo Szilard had found in uranium the element he was yearning for. After reading Me itne r 's paper, he wrote a letter to
hi s friend Lewis Strauss, a businessman and a major figure
in the development of nuclear weapons and nuclear power
in the U .S . »I see ... possibilities in another direction. These
might lead to large-scale production of e nergy and radioactive elements, unfortunate ly also perhaps to atomic bombs .
This new discover y reYives all the hopes and fears in this respect which I had in 1934 and 1935, and which I have as good
as abandoned in the course of the past two year s.«
It took just a couple of month s to produce the first exp eri m ental chain reaction. Fermi, freshly arrived in America
w ith his wife to fl ee the racial laws aga inst Jews in Italy,
repeated Hahn 's ex periment and demonstrated that the
fission of uranium emitted secondar y neutrons, thus opening the possibility of a chai n react ion, exactly as Szilard
had envisioned it six year s earlier. In Pari s, Frederic Joliot,
Hans von Halban and Lew Kowarski showed that a self-propagating chain reaction should be possible if a moderating
=
O.R. Frisch, Phy~·ical Eridence}Or che Di1·ision
if Hea vy Nuclei under Neutron
Bombardment, Nat ure 143: 276 276, '939·
0. R. Frisch and L. Meitner, loc . cit.
The reaction is 1on
+ 2 359 2U - >
23692 U* - > 14456 Ba
+
8936Kr
+
3 1on + 177 MeV.. where 2 3692 U* is a very un~tabl e fonn of uranium that im mediately dis integrates, Ba stands for barium ,Kr for krypton, a rare gas. eV is the
abbrcdation.of »electron \·oit «, a unit of e ne rgy equal
ro the rergy acguired by
an e lectron falling through a difference of potential of one ,·o1t. The superscript
indicates the mass numbe r A wh ic h gi,·cs th e total number of protons and neutrons and yields an id ea of we ight. The subscript in dicates th e atomic number Z
whi c h gives th e numbe r of protons, hence the el ectrica l c harge .
1o
In 1944 Hahn was a,,·arclecl the Nobel Prize fo r physics for his research into
nuclear fission. Meitner's geographical separation from her former co lleague
led to the Nobe l comm ittee's fa ilure to understand her rol e in this disco,·cry.
Apparently Hahn himself had dmmpla:·ed her role after she left Ge r many. The
»Nobe l Mistake«, nen~r acknmdedged, \\·as partly repai red in 1966, \\·hen Hahn,
Mcitner, and Strassmann "·ere a"·anJeJ the U.S. Enrico Fermi A\\·ard.
H . Yon Halban , F. Joliot , L. Km,·arski, l.iberarion cj.\"eutron:> in the J\"uclear
Explosion C?IUranium, Nature No. 362o: 470 471, 1939.
-.. ',
•
•
'
'
(!)
'\
0
''
18
--
'0
\
G)
'
--
......
~