‘
266
12.
ALPHA, BETA AND GAMMA RAYS
‘>
i.e., inversely as the number of gas molecules passed through.
By obs'erving the range
of the alpha particles at different pressures
and temperatures
With a shallow chamber or With a uorescent
.
screen, one may
then compute the range under standard condi;
"
tions; i.e., 76 centimeters of mercury and 15° C. (0° C. is also
used). Another method, due to Geiger and Nutall, for measuring
'
the rangeof alpha particles from weak sources uses the apparatus
illustrated in gure 12—5. The source of alpha particles is located
‘
at 8 in the center of a charged
UT
metal sphere  whose radius
.
is greater than the maximum
range
at atmospheric pressure.
?
The battery B has sufcient
T,,
potential, say 700 volts, to draw
% Th&RdTh ,
over to the ball all ions proË
'
duced.
As the pressure
%
,,
ball is reduced, the alpha parâ
Ra
ticles are able to reach its wallS
“Î
and at still lower pressures
‘
‘Ë
would go beyond, were i,t not
5%
for the walls.
The 10n12atwn
-
Rd‘Th
::
current
as
measured by the
_
eléctroscope is fairly constant
as
the pressure is decreased,
cm.Range
'
until after the particles have
4
3
2
1
.
.
begun hitt1ng the wallsIt
52 48 4—4 4°
16
12
8
4
°
then falls
o" markedly Since
Fm.
Results from the apparatus
thePartldes are nOt produc1ng:
.,
‘
of gure 12—5.
as many ions as they arecapthekmks in the curves
indicate the pressures
at which thealpha
1nthecurveind1cate
sets of alpha particles of differentranges
Therange°fphapartlcles in various gases and hqmdshas
beenmeasuredbYthescmtlaüon method. (See table5)F°f
8911dsau0rescent screen andanactwe ‘
source
are
placed0n
°PP031œSidesofathmwedge 0fthemater1al The sant1atms
are'°“æn‘“-°œas€abl‘üWYoverthatthickness of materiaïwhœh