14—14; DISCOVERY OF— ARTIFICIAL RADIOACTIVITY 321
%” 5
'
-
_
counter tube whose window consisted of a moderately thin foil of
aluminum. They found that the counting rate gradually increased
‘
from the moment when the alpha source was rst brought up, and
became essentially constant after fteen minutes. The shape of the
curve
(gure 14—10) was the same as the formation curve of
natural radioactive substances.
Furthermore, when the alpha
particle source was removed,_ thetube continued to giye counts, the
number decreasing as shown on the right of
.
the vertical dotted line
in the gure. By means of an expansion chamber, they found that
the activity was due to the‘aluminum window, which
,
positrons. The positrons had an energy distribution similar to that
of disintegration electrons from natural ”radioactive substances
_
(gure 12—14) with an upper limit of 3 Mev and a most probable
%
_
$
5
.
.
‘
a.
|
|
—
,
15
,
T=l95
.
TIME++
FIG. 14—10.
Positrons ‘eniittéd
alpha PaÎtÎCÏ€S'
energy
10f‘5 times
the number0f alphapart1cles,le,y1dd
:
10—6
_
_'
_
'
That
um.t
.
after removal
_
Bymeans@fc°«quatwnïï'“12"maïbe‘“mVædlmthehalfhf€
Twæfoundtaethreemmutesandftænsecondswhenthe
’
WhentheenerY@fthelnüdetalPhaPartlde5wasdecreasedî “
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