3)
})
total number of neutrons accompanying the fission of Uranium
tion. It has been determined that
at this distance about a half of the
gamma ray dosage is received during the first second.
Taking shelter quickly behind a
235 or plutonium are released al-
most immediately, probably within
one hundred-millionth second of
the explosion. These are referred
to as the prompt neutrons. In addition, somewhat less than 1 per
cent, called the delayed neutrons,
are emitted subsequently. The lat-
convenient building or in a slit
trench, an act that is conceivable
within a second of seeing the bomb
flash, might thus mean the differ-
ence between life and death to a
ter
human being at a point where the
unprotected dosage would be near
the median Jethal value,
are
actually
expelled
some of the fission products.
from
It is estimated that the lethal
If the energy release of the bomb
were doubled from 20 to 40 kiloton
TNT equivalent, the median lethal
range, at which the dosage is 400 r.
would be increased from 4,200 feet
to 4,750 feet. This means that the
jethal area of the initial gamma
radiation would be much less than
double.
Consequently, the thickness of
shielding necessary to attenuate
to be increased greatly. For 4
forty kiloton TNT equivalent bomb
the dosage at 2,100 feet would be
20,000 r and about twenty-five
inches of concrete would reduce it
to 400 r, the median lethal dose.
This mav be compared with twenty
inches required at the same dis-
Effect of Neutrons
In general, concrete may repre-
sent a fair compromise for neutron
of the energy of the atomic explosion. Of this amount, perhaps less
than 1 per cent appears outside
because of the loss of energy to
the components of the exploding
bomb. Like the gamma rays neutrons can penetrate considerable
distances through air, and since
they are a physiological hazard,
they are a significant aspect of an
atomic explosion.
considerable thickness, the main
function of concrete is to slow
down the fast neutrons and so
shielding. However, unless used in
make them less of a biological
hazard. Better results would be
obtained by using a modified concete made by adding a considerable
proportion of iron (oxide) ore, such
as limonite or magnetite, to the
cement. Smal! pieces of iron, such
as Steel punchings,
incorporated.
More than 99 per cent of the
radiations
may also
he
was thought that the whole body
emitted
Any of the radioactive material
reaching the inhabited surface of
The neutrons emitted in the fis-
sion process carry about 3 per cent
nuciear
All the neutrons from the bomb
would reach a point 2,000 feet
distant within less than a second.
It would appear, therefore, that
most of the neutrons reaching the
earth would do so within such a
short period of time after the explosion that evasive action would
not be possible.
Increasing the energy of the
increase of less than 400 feet in
the lethal distance of the neutrons.
ton TNT energy equivalent bomb.
The
after one minute from the instant
of an atomic explosion, namely the
bomb by two would lead to an
tance for the nominal twenty kilo-
~
range of neutrons from a nominal
atomic bomb would be 1,800 feet
for fast and slow neutrons, while
for neutrons of intermediate energy the distance would probably
be increased to 2,400 feet.
In
other words, neutrons from an
atomic bomb would be lethal to
unshielded persons at distances
not greater than half a mile from
ground zero.
the radiation to Jess than the lethal
value at any point would not have
World-Wide Ruin by Contamination
Held Doubtful
could absorb up to 0.1 of radiation
per working day for long periods
without permanent harm.
This rate of absorption was ac-
residual radiations, arise mainly
from the fission products.
To a
lesser extent they also come from
the
uranium
2385
or
cepted
atoms that had escaped fission,
and,
in
certain
circumstances,
in various elements present in the
earth and in the sea.
larly exposed to radiations anaio-
gous to gamma rays there is no
authenticated case of injury where
the exposure has been kept down
to 0.1 r per day over extended
periods.
It should be understood that this
safe dose applies to absorption
over the whole body and for repeated and protracted exposures
over long periods of time. Small
The problem of dosage emitted
in a very short period of time,
namely, the ‘‘one-shot"’ dose, de-
areas car, be exposed to very much
larger quantities of radiation with
scribed yesterday, 15 quite differ-
ent from that arising in the case
of the residual nuclear radiations
no more than lceal injury being
experienced.
in addition, there is
a difference between acute, that is,
brief and occasional, exposure and
days,
weeks or months. A human being
receiving a total of 400 r (roentgen
the chronic exposure to which the
units of radiation) of the initial
nuclear radiation, that is, over a
period ot a minute or so, would
have
a
50
per
cent
chance
tolerance limit applies.
Thus, a dose of 5,000 r can be
used to treat a small skin cancer,
leaving a scar but no other permanent effect. Even the whole body
may absorb 50 r in one day without any apparent harm.
Somewhat larger single doses may have
unpleasant consequences, but will
not prove fatal unless repeated on
successive days.
of
survival, but, if the same amount
of radiation was absorbed over a
period of a month, the probability
of death would be considerably
less.
Human Tolerance Doses Set
The United States Committee on
Fission Brings Sixty Fragments
The fission of uranium 235 or
X-rays and Radium Protection
concluded in 1936 that the maximum human tolerance dose of
X-rays or nuciear radiation, which
could be taken up on successive
plutonium (they very seldom split
whole
(twins)
in equa] parts) results in the for-
mation of at least sixty atomic
fragments. representing isotopes
days was 0.1 r per day over the
30
or
reduced to 0.3 r per week,
Among X-ray technicians regu-
not be ignored, that radioactive
Material might be used deliberately, apart from an atomic explosion, for the purpose of making
certain areas uninhabitable.
for
dose
in the United States has now been
highly probable, must nevertheless
persist
tolerance
the accepted permissible dose rate
the earth in appreciable amounts
may represent a serious physiclogical hazard. In addition, there is
the possibility, which, although not
might
the
an adequate factor of safety for
personnel exposed to radiations
every working day for many years
from activity induced by neutrons
which
as
permissible dose of nuclear radiation. However, in order to insure
plutonium
body.
In
other
words,
it
ar
of
probably
thirty-four