270
'
12.
ALPHA, BETA AND GAMMA RAYS
duce the same energy loss as one centimeter of air.
Then, for
small energy loss,
.
a = t/ta.
(12—8)
Near the end of
of the alpha particles from polonium,
one
cèntimeter of air is the equivalent of approximately 0.006
millimeter or
mg./cm.2 of aluminum and of 0.002 millimeter
or 4 mg./cm.2 of gold.
,
-
The energy
lost by an alpha particle in travelling unit distance
in a substance is called the stopping power of that substance. It
‘
varies With the velocity of the particle. On the other hand, the
relative stopping power (S); ie, the ratio of the energies lost in unit
distance in the substance and in standard air, is approximately
_
independent of the velocity of the particle. Again, if the energy
'
losses are small, they are proportional to the range reductions so
that
_
S = :../t = 1/t..
(12-9)
Inasmuch as 8 does vary somewhat With the velocity of the alpha
particles, the values given in table 6 at the end of the book are
to be taken as averages over the range of energies of the natural
radioactive ‘alpha rays. The mass stopping power is the
pOWer divided by the density of the substance.
__
The atomic stopping power is the stopping power divided by the
'
number (71) of atoms per
cubic centimeter of the substance.
.
For
a
substance of, density (p), area (a), thickness (t‘) and atom1C
»
n = paN/a,
'
(12—10)
'fvvhere' N is Avogadro’s number. The relative atomic stopptng
power—(s) is the ratio of the energy lost per
atom of the substance
…
tothat lost—per atom of a standard gas. Usually, airat 15°C
-;pfes,sure composed of average atoms ofatmlc
‘
*
? s