284
—
12.
ALPHA, BETA AND GAMMA RAYS
(about 20 cm. long, 3.5 cm. diameter). The pressure in the tube is
determined With the open—tube mercury manometer C'. The pressure in centimeters of mercury is equal to the difference between
the barometer reading and the dilÏerence in height of the mercury
_
in the two arms. Air may be admitted to the tube slowly through
the capillary D and stop cock E.
'
‘
The active material is on a small metal plate (F), xed With
soft wax to a second plate supported by a long metal tube. The
_
front of the source is covered With a brass disc pierced With a
large number of holes in order to secure an approximately parallel
bundle of alpha-particles. The source and its collimator may
_
D
a:
_,_
B
G
H
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.
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‘: F
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.
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-FIG. 12—19.
Scintillatioh method of measuring the range of alpha particles.
…beslid in and out thr0ugh a rubber stopper. There is a small
the end of the tube to allow evacuation of B. The act1V6
"
any substance ‘emittig alpha particles but,be—
causeo‘f theïcon‘tamination of the walls of the tube by d1s1ntegra—
' _j _f _f
‘
, Çtiôh;_p1‘oductsor‘by recOil atoms,only certain substances shouldbe
0f10mum
bemad
fr0mthe@l,Crushed
the active deposit of radium usedln
if ' ;=1i medlcalwm‘kTheseneedles werexoiiginally lled
rad1um
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Withacompara