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6—9.
ELECTRON MULTIPLIER TUBES
127
4.‘ 8, in general, is reduced by heat treatment of a surface
_
and by contaminations, especially byelectro—Ïxositive impurities.
S depends greatly on
the condition of the Surface.
'
5. Moderate temperature Changes have but little effect on .
—
the value ofS.
‘
'
6. The velocity of the secondary electrons are usually only
_a few volts, even when 1900 volts primary electrons areused.
'
7. The secondary electrons
leave the surface at
'
random in all directions.
'
Î
8. Secondary emission can occur
from insulators, such as
Ï
With high velocity primary electrons, S‘ may be greater .
,
than unity, in which caSe the surface of the insulator acquirés a
positive instead of a negative charge.
6—9.
Electrbn Multiplier
phenomenon of secondary emission is used in electron multiplier _‘tubes to gain large
current amplication. Figure 6—15shows thepr‘inciple o_operation.7
The photoelectric or thermi0nic cathode K ‘ _
"
'
—*
emits electrons Which are directed against the plate .
P
P1 by electric”, magnetic or combined
The
‘
/
plate may consièt of oXidi2ed silver, zirconium or ' Pa
_
"
beryllium
a
layer of .cesium so as to '
P
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be a good emitter of secondary
The
'
_
'
secondary electrons fromÆ-i
a
-
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_
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