Leo Szilard was a Hungarian-American physicist and inventor. He con...
### TL;DR
This seemingly short and simple paper describes an exp...
H. L. Anderson was an American nuclear physicist who was Professor ...
Enrico Fermi was an Italian physicist and Nobel Prize winner. He em...
This research lead to the design of the first nuclear reactor capab...
**Thermal Neutron**
Thermal neutrons **are free neutrons, they a...
A nuclear chain reaction occurs when one single nuclear reaction ca...
> "From this result we may conclude that a nuclear chain reaction c...
AUGUST
1,
1939
PHYSICAL
REVIEW
Printed
in
U.
S.
A.
VOLUME
56
Neutron Production and
Absorption
in Uranium*
H.
L.
ANDERSON,
E.
FER.liH AND
LEo
SziLARD
Columbia
University,
New York,
New
York
(Received
Jul
y
3
,
1939
)
I
T
has
been found
1
-
3
that
there
is
an
abundant
emission
of
neutrons
from
uranium
under
the
action
.
of
slow
neutrons,
and
it
is
of
interest
to
as-
ce
rt
ain
whet
h
er
and
to
what
extent
the
number
of
neutrons
e
mitted
exceeds
the
number
absorbed.
This
question
can
be
investi
ga
ted
by
placing
a
photo-neutron
source in
the
center
of
a
large
water
tank
and
comparin
g,
with
and
without
uranium
in
the
water,
the
number
of
thermal
neutrons
present
in
the
water.
In
the
previous
experiments
of
this
type
1
•
3
it
was
attempted
to
have
as
closely
as
possible
a spherically
sym-
metrical
distribution
of
neutrons.
The
number
of
thermal
neutrons
present
in
the
water
was
de-
termined
by
measurin
g
along
one
radius
the
neutron
density
p
as
a
function
of
the
distance
r
from
the
center,
and
then
calculating
fr
2
pdr.
A
difference in
favor of
uranium
of
abo
ut
five
percent
was
reported
by
von
Halb
an,
Joliot
and
Kovarski.
4
Since one
has
to
measure
a small
difference,
s
li
ght
deviations
from a
sp
h
er
ically
symmetrical
distribution
might
g
iv
e misleading
results.
The
present
experiments
which
are
based
on
the
same
ge
neral
principle
do
not
require
such
symmetry.
In
order
to
measure
the
number
of
therma
l
neutrons
in
the
water
we
filled
the
tank
with
a
ten-percent
solution
of
MnS04.
The
activity
in-
duced
in
manganese
is
proportional
to
the
number
of
thermal
neutrons
present.
A
physical
aver-
aging was performed
by
stirring
the
solut
i
on
before
m
eas
urin
g
the
activity
of
a
sample
with
an
ionization
chamber.
To
obtain
an
effect
of
suffi-
cient
magnitude,
about
200
kg
of
U30s
was
used.
Th
e
expe
rim
enta
l
arrangement
is
shown
in
.
Fig.
1.
A
photo-neutron
so
urc
e,
consist
in
g of
about
2
g
of
radium
and
250
g
of
beryllium was
*Pub
li
cation
assisted
by
the
Erne
s
t
Kempton
Adams
Fund
for
Physical
Rese
a
rch
of
Co
lumbi
a U
niversity.
tv.
Halban,
J
o
li
ot
and
Kovarski,
Natur
e
143,
470
(1939).
2
L.
Szilard
and
W.
H.
Zinn,
Ph
ys
. R
ev
.
55
,
799
(
1939
)
.
a
And
er
so
n,
F
ermi
and
Han
ste
in,
Phy
s.
R
e
v. 55,
797
(1939)
.
4
v. Halban, Joliot
and
Kovar
ski,
Nature
143,
680
(1939).
pla
ced
in
the
center
of
the
tank.
The
geo
metry
was
such
that
practically all
neutr
ons
emitted
by
the
source
and
by
the
ur
a
nium
oxide
were
s
l
owed
down
a
nd
absorbed within
the
tank.
Each
irr
a
diation
exten
d
ed
over
several half-life periods
of
radioman
ga
nese
and
the
observed
activity
of
the
solution
was
about
four
times
the
back-
ground
of
the
ionizati
o
n
chamber.
Alt
e
rnatin
g
measurement
s
were
taken
with
the
cans
filled
with
uranium
oxide,
and
with
e
mpty
cans
of
the
same
dimen
s
i
o
ns.
The
activity
pr
ove
d
to
be
about
ten
perce
nt
hi
g
h
er
with
uranium
oxide
than
without
it.
Thi
s
r
es
ult
s
hows
that
in
our
ar
r
angement
mor
e n
e
utron
s
are
emitted
by
ur
anium
than
are
absorbed
by
uranium.
In
order
to
find
the
average
number
of
fast
neutrons
emitted
by
uranium
for
each
thermal
neutron
absorbed
by
u
rani
um
,
we
have
to
d
e
-
termine
what
fraction
of
the
total
number
of
neutrons
emitted
by
the
photo-n
e
utron
so
urce is,
in
o
ur
exper
im
ent,
absorbed in
the
thermal
re
g
ion
by
uranium.
The
number
of
photo-neutrons
0000
000000
00000000
0
0
0
0
-s
oooo
oooo
00000000
000000
0000
IOcm
FIG.
1.
Horizonta
l
sect
i
o
n
through
center
of
cylindrical
tank
which
is
fill
ed
with
540
li
ters
o
f
10-p
er
cent
MnSO,
solution.
A
Photo-n
e
utr
o
n
so
ur
ce composed
of
2.3
grams
of r
ad
ium
250
gram
s of beryllium. B,
One
of
52
cylin-
drical cans 5 em in diamet
er
and
60
em in height, which
are
either
empty
or
fill
ed with
uranium
oxide.
284
285
NEUTRON
PRODUCTION
AND
ABSORPTIO
emitted
by
the
source
is
indicated
by
the
activity
of
the
solution
in
the
tank
when
the
irradiation
is
carried
out
with
e
m
pty
cans
s
urr
o
undin
g
the
source.
Vve
a
m
easure of
th
i
s
numb
er
by
taking
into
acco
un
t
that
in
our
sol ution
abo
u
t
20
percent
of
the
ne
u
trons
are
capt
ur
ed
by
manganese
and
the
rest
by
hydrogen.
In
order
to
obta
in
,
in
the
same
units
, a
measure
of
the
num-
ber
of
neutrons
absorbed
by
uranium
we
pro-
ceeded
in
the
followin
g
way:
A
mixture
of
sand
and
manganese
powder,
having
the
sa
m
e
thermal
ne
u
tron
absorption
as
uranium
oxide replaced
the
u
ranium
oxide
in
t
of
the
cans
which
were
distributed
uniformly
among
the
other
uranium
ox
id
e-filled
cans.
After
irradiation,
all
this
powder
was
mixed
toget
h
er,
a
ten-pe
r
cent
MnS04
solu-
t
i
on
was
prepared
from
a
samp
l
e,
and
i
ts
activity
was
m
easured
w
i
th
o
ur
ionization
cha
mb
er.
In
this
way
we
found
that
abo
u
t
SO
percent
of
t
h
e
neutr
ons e
mitt
e
d
by
the
so
urc
e
are
absorbed
as
t
hermal n
e
utrons
by
uranium
in
our
arrange-
ment.
I
t
follows
that,
if
ur
an
ium
absorbed
only
the
rm
al
neutrons,
the
observed
ten-percent
in-
c
rea
se
in
activity
obtaine
d
with
ur
a
nium
present
would correspond
to
an
average
emission
of
a
bout
1.2
neutrons
per
thermal
neutron
absorbed
by
uranium.
Thi
s
number
should
be
increased
,
to
perhaps
1.5,
by
tak
in
g
int
o
account
the
neutrons
wh
ich
, in
our
p
art
i
c
ular
a
rran
ge
ment
,
are
absorbed
at
r
eso
nanc
e
in
t
h
e
nonthermal
region
by
uranium
,
without
ca
u
s
in
g
neutr
on
e
mi
ssiOn.
From
this
result
we
m
ay
conclude
that
a
nuclear
cha
in
r
eact
ion
could
be
maintained
in
a
syste
m
in
which neu
t
rons
are
slowed
down
w
i
th-
o
u
t
mu
c
h
absorption
until
t
hey
re
ac
h
thermal
e
ner
gies
a
nd
are
then
mo
st
ly
absorbed
by
ura-
nium
rather
than
by
another
el
e
m
e
n
t.
I
t
remain
s
a
n
open
question, however,
whet
h
er
this
holds
for
a
syste
m in w
hi
ch
hydrogen
is
u
se
d
for
s
lowing
d
ow
n
the
neutrons.
In
s
uch
a
syste
m
the
abso
rp
t
ion
of
n
e
utron
s
ta
k
es
place
in
th
r
ee
diff
erent
ways:
Th
e
neutrons
are
absorb
ed
at
ther
m
a
l
energ
i
es,
both
by
hydro-
gen
and
ur
ani
um
,
and
they
a
re
also
absorbed
by
uranium
at
re
so
nance
b
efore
they
are
slowed
down
to
thermal
ener
g
ies.
Our
result
is
inde-
pendent
of
the
ratio
of
the
concentrations
of
hydro
gen
and
uranium
, in sofar
as
it
shows
that,
for
thermal
neutrons, the
ratio
of
the
cross
section
for ne
u
tron
production
and
neutron
ab-
sorption
in
uranium
is
greater
t
h
an
one,
and
probably
abo
u
t
1.5.
\
Nhat
fraction
of
the
neu-
trons
will
reach
t
h
e
rmal
energ
i
es
w
i
t
h
o
ut
being
absorb
ed will, h
oweve
r,
depend
on
the
ratio
of
the
average
concent
ra
t
ions
of
h
yd
rogen
and
ur
ani
um
.
Since
there
is
an
apprec
i
ab
l
e
absorption
even
far
from
the
center
of
the
resonance
band,
it
follows
that
the
fraction
of
neutrons
abso
rb
ed
by
uranium
at
resonance will
increase
with
d
e-
creasi
ng
hydrogen
concentrat
ion.
This
has
to
be
ta
k
e
n
into
account
in
discussing
t
h
e
possibility
of
a nuclear
chain
reaction
in
a
syste
m
composed
essent
i
a
ll
y
of
ur
a
nium
and
hydrogen.
A
chain
reaction wou
ld
require
that
more
neutrons
be
produced
by
uranium
than
absorbed
by
uranium
and
hydro
gen
together.
In
our
exper
iment
the
ratio
of
the
average
concentration
of
h
y
dro
ge
n
to
ur
a
nium
atoms
was
17
to
1
,
and
in
the
experi-
ment
of
von
Halban,
Joliot
and
Kovarski
this
r
at
io was
70
to
1.
At
such
conce
ntr
at
i
ons
the
absorption
o
f
hydro
gen in
the
t h
e
rmal
region will
prevent
a
chain reaction.
By
reducing
the
con-
centration
of
hydro
gen
one would
obtain
the
followin
g e
ffect:
On
the
one
h
a
nd
a
lar
ger
fraction
of
those
neutrons
which
reach
thermal
energies
will
be
absorbed
by
uranium;
on
the
other
hand
fewer
neutrons
reach
t
h
e
thermal
r
eg
ion
du
e
to
an
incr
ease
d
absorption
by
uranium
at
re
sonance.
Of
t
h
ese
two
counteracting
factors
the
first is
more
importan
t
for
hig
h h
y
dro
gen
co
ncentra-
tions
and
the
second
is
mor
e
important
for
lo
w
hydro
gen
concentrations.
Starting
with
high
hydrogen co
n
ce
ntrations
,
the
ratio
of
neutron
produ
ct
ion
to
t
ota
l
neutr
on
absorption
will
thus
fir
st
rise
,
pass
through
a
maximum,
and,
as
the
hydro
gen
concentration
is d
ec
rea
se
d,
there-
after
decrea
se.
We
attempted
to
es
timate
the
quantities
involv
e
d
from
the
information
avail-
able
about
resonanc
e
absorption
in
uranium
•-
7
and
from
the
observed
net
ga
in
of
0.2
in
the
number
of
neutrons
in
our
experiment.
The
effect
of
the
absorption
at
resonance
t
urn
s
out
to
be
so
5
Meitner,
Hahn
a
nd
Strassman,
Z
eits.
f.
Physik 106,
2
49
(
1937
).
s v.
Hal
ban,
Kovar
s
ki
and
Savitch,
Compt
es
rend
u
s
208, 1396 (1939).
7
H . L. Anderson
and
E.
F ermi,
Ph
ys.
Rev. 55, 1106
(1939).
ANDERSON,
FERMI
AND
SZILARD
286
large
that
even
at
the
optimum
concentration
of
hydrogen
it
is
at
present
quite
uncertain
whether
neutron
production
will
exceed
the
total
neutron
absorpt-ion.
More
information concerning
the
resonance
absorption
of
uranium
as
well
as more
accurate
measurement
of
some
of
the
values
which
enter
into
our
calculation
are
required
before
we
can
conclude
whether
a
chain
reaction
is
possible
in
mi.xtures
of
uranium
and
water.
We
wish
to
thank
Dr.
D
.
W.
Stewart,
of
the
Department
of
Chemistry,
and
[r.
S
.
E.
Krewer,
for
advice
and
assistance
in
carrying
out
some
of
these
experiments.
We
are
much
indebted
to
the
Eldorado
Radium
Corporation
for
enabling us
to
work
with large
quantities
of
uranium
oxide
in
our
experiments,
and
to
the
Association
for
Scientific
Collaboration
for
the
use
of
the
photo-
neutron
source
and
other
facilities.
Leo Szilard was a Hungarian-American physicist and inventor. He conceived the nuclear chain reaction in 1933, patented the idea of a nuclear fission reactor in 1934.
Szilard feared the implication that nuclear chain reaction could have for the development of nuclear weapons for national defense. He shared his fears with Einstein in the summer of 1939 and he wrote the now-famous Einstein Letter which was delivered to President Franklin D. Roosevelt in October of 1939. This letter resulted in the creation of the Manhattan Project that built the atomic bomb.
Learn more about Szilard here: [American Heritage Foundation - Leo Szilard](https://www.atomicheritage.org/profile/leo-szilard)
> "From this result we may conclude that a nuclear chain reaction could be maintained in a
system in which neutrons are slowed down without much absorption until they reach thermal
energies and are then mostly absorbed by uranium rather than by another element."
H. L. Anderson was an American nuclear physicist who was Professor of Physics at the University of Chicago. He was a member of the team which made the first demonstration of nuclear fission in the United States. He contributed to the Manhattan Project and participated in the first atomic bomb test.
Learn more about Anderson here: [America Heritage Foundation - Herbert L. Anderson](https://www.atomicheritage.org/profile/herbert-l-anderson)
**Thermal Neutron**
Thermal neutrons **are free neutrons, they are not bound within an atomic nucleus.** They have a kinetic energy of about 0.025 eV - corresponding to the average energy of the particles of the ambient materials. Relatively slow and of low energy, thermal neutrons exhibit properties, such as large cross sections in fission, that make them desirable in certain chain-reaction applications.
They are produced by slowing down more energetic neutrons in a substance called a moderator after they have been ejected from atomic nuclei during nuclear reactions such as fission.
A nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions. Isotopes of Uranium (ex: uranium-235) were selected by scientists because they readily undergo fission. Fission occurs when a neutron strikes the nucleus of an isotope and splits it into fragments thus releasing an enormous amount of energy. The **fission process becomes self-sustaining as neutrons produced by the splitting of atom strike nearby nuclei and produce more fission. This is known as a chain reaction and is what causes an atomic explosion.** A nuclear chain reaction releases several million times more energy per reaction than any chemical reaction.
This research lead to the design of the first nuclear reactor capable of maintain a self-sustaining nuclear chain reaction the Chicago Pile 1.
*The Chicago Pile 1 reactor consisted of uranium and uranium oxide lumps spaced in a cubic lattice embedded in graphite. On December 2, 1942 it achieved the first self-sustaining chain reaction and thereby initiated the controlled release of nuclear energy.*
Enrico Fermi was an Italian physicist and Nobel Prize winner. He emigrated to the United States where he worked on the Manhattan Project during World War II. Fermi led the team that designed and built Chicago Pile-1, which in 1942 demonstrated the first artificial self-sustaining nuclear chain reaction.
Fermi was one of the few physicists to excel both theoretically and experimentally and was known for his ability to make good approximate calculations with little or no actual data.
You can find more papers authored by Fermi on Fermat's Library:
- [My observations during the explosion at Trinity](https://fermatslibrary.com/s/my-observations-during-the-explosion-at-trinity)
### TL;DR
This seemingly short and simple paper describes an experiment made by Szilard, Fermi and and Anderson. They were interested in studying the fission of Uranium and it's associated neutron absorption and production. They were interested in Uranium's potential for self-perpetuating nuclear reactions.
Szilard was convinced of the viability of a nuclear chain reaction and became concerned about the possibility of German scientists working under the Nazi regime might attempt to exploit nuclear fission for bomb-making purposes.
He shared his fears with Einstein in the summer of 1939 and he wrote the now-famous Einstein Letter which was delivered to President Franklin D. Roosevelt in October of 1939. This letter resulted in the creation of the Manhattan Project that built the atomic bomb.
You can find the famous Einstein-Szilard Letter here: [Atomic Heritage Foundation - Einstein-Szilard Letter](https://www.atomicheritage.org/key-documents/einstein-szilard-letter)