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**Thermal Neutron**
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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