248
11.
SPONTANEOUS DISINTEGRATION
'
and smaller amounts as time goes on. This is illustrated by the alpha
ray activity of the thorium active deposit. Upon disintegration,
/-1a
/
B, 7
/{5
10”6 sec.
,
Th A——>Th B——>Th C———>Th C’
0.145 sec.
10.6 hr.
60.5
‘
'
-
Th C”———-+Th D (Lead)
Isolate
-
3.1 min.
5
°°
‘ ?
thorium emanation or thoron (a gas) becomes a solid called
thorium A and may be collected on a negatively charged metal
plate nea‘rby.. When the metal plate is rst placed in an alpha ray
electroscope,* the activity is practically zero. If the rate at
‘
which the electroscope leaf collapses is observed at regular time
intervals, a curve like that in gure 11—5 Will be obtained. This ‘
_
|
_
.
-------…------.---…--.….”
,.
;
.
'.
g
w
'
î
:
a
*
.
<
.L
.
——
»
…
°
:
T
2T
“
'
0
'
10
20
30
37
lime "m hours
…
FIG. 11—5. Activity curve of a substance having a comparatively—short-lived parent.
alpha ray activity curve is a result of the combined formation and
deCay of
C from thorium B.
From the active depos
'
thorium (above), it may be seen that the thOrium A
quickly
whose'life is much longer
The abeta'_ and‘ gamma rays from
B, however, do not
apprec1ably affect
electroscope so that theobserved,
@; f;_ actmtymust resultfrom the? alpha r'äèys
thorium @ andC
: The alpharaysfrom the l‘attervproduce“rthemaj or activity} butthe