12—5.
AIR EQUIVALENT AND STOPPING POWER 269
in the gure and With only one radius (N 8 X 10"13 cm.) for the
nuclei of different radioactive substances.
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Distance d from the center
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PIG. 12—8.
The potential barrier of a nucleus. -
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=
12—5.
Air Equivalent and Stopping Power.'—The air eguz'w—
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lent (l‘a) is dened as that thickness of air under standard condi- '
tions which produces the same loss of energy of the alpha particles
'
-
as does
given thickness (L‘) of material placed in their path. If
the energy loss is suiciently
such as occurs when a. very
,
thin foil of the material is used, it Will be directly pr0portional to
the corresponding reduction in range of the particles
_
In order to measure ta, determine the range (d) of the alpha
particles emitted by
radioactivesubsta1äçe3 say by the
scintillation method.
Then, insert the thinf0il in the path of the
rays and re—deterfnine the distance
*sou‘rce— to the
'
screen at Which
The reductiônin
range (d —— d’ ) is
taken as 15°
Thus,ïî
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Where» T = 288KP=76œntlmetersad T=273+ C
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