Page 25 - AAS & AES & FES 01082016_Neat

t
that easily emits electrons. Often the cathode is coated with hat easily emits electrons. Often the cathode is coated with oxide of cesium,oxide of cesium, although the oxide
of other metals from group IAIA or group IIA is used. The energy from the collision of a photon or group IIA is used. The energy from the collision of a photon

with photocathode is sufficient to result in ejection of an electron. The photocathodeith photocathode is sufficient to result in ejection of an electron. The photocathodeith photocathode is sufficient to result in ejection of an electron. The photocathode serves as a
w w
t t transducer that converts electromagnetic radiation into electricity. The emitted electrons aransducer that converts electromagnetic radiation into electricity. The emitted electrons aransducer that converts electromagnetic radiation into electricity. The emitted electrons are

attracted to dynode 1 by the relatively positive ttracted to dynode 1 by the relatively positive potential that is maintained on the dynode as potential that is maintained on the dynode as
a
compared to the potential on theompared to the potential on the photocathode. After acceleration by the potential difference After acceleration by the potential difference
c
between the photocathode andetween the photocathode and the first dynode, the electrons strike the dynode. The dynodes are ike the dynode. The dynodes are
b
coated with a compound that emits several electrons for each impinging electron from thecompound that emits several electrons for each impinging electron from thecompound that emits several electrons for each impinging electron from the
photocathode, as illustrated in Fig.in Fig. Usually the dynodes are coated withthe dynodes are coated with either CsSb, BeO, or

GaP. Upon striking the first dynode each aP. Upon striking the first dynode each electron causes emission from the dynode of about four ssion from the dynode of about four
G
or five electrons. The result is an r five electrons. The result is an amplification of the electric current that flows through the PM of the electric current that flows through the PM
o
tube. The electrons that are emitted from The electrons that are emitted from
dynode 1 are attracted to the moreynode 1 are attracted to the more
d
P
Positive dynode 2. At dositive dynode 2. At dynode 2 each
impinging electron results in emission ofmpinging electron results in emission of
i
four or five electron ve electron and further

amplification of the electric current that of the electric current that
f
flows through the PM tube. The power lows through the PM tube. The power
supply of the potential divider typically upply of the potential divider typically
s
supplies about 900V and resistor are upplies about 900V and resistor are
s
about 100000 ohm..

In general the amplification of a PM tube is the average number he amplification of a PM tube is the average number n of electroof electron emitted at each

dynode raised to the power that equals the number ynode raised to the power that equals the number d of dynodes in the PM tube the PM tube :
d
d
Amplification = n

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