<s>
Photomultiplier	B-Algorithm
tubes	I-Algorithm
(	O
photomultipliers	O
or	O
PMTs	O
for	O
short	O
)	O
are	O
extremely	O
sensitive	O
detectors	O
of	O
light	O
in	O
the	O
ultraviolet	B-Application
,	O
visible	O
,	O
and	O
near-infrared	O
ranges	O
of	O
the	O
electromagnetic	O
spectrum	O
.	O
</s>
<s>
These	O
detectors	O
multiply	O
the	O
current	O
produced	O
by	O
incident	O
light	O
by	O
as	O
much	O
as	O
100	O
million	O
times	O
or	O
108	O
(	O
i.e.	O
,	O
160	O
dB	O
)	O
,	O
in	O
multiple	O
dynode	B-Algorithm
stages	O
,	O
enabling	O
(	O
for	O
example	O
)	O
individual	O
photons	B-Application
to	O
be	O
detected	O
when	O
the	O
incident	O
flux	B-General_Concept
of	O
light	O
is	O
low	O
.	O
</s>
<s>
The	O
combination	O
of	O
high	O
gain	O
,	O
low	O
noise	O
,	O
high	O
frequency	O
response	O
or	O
,	O
equivalently	O
,	O
ultra-fast	O
response	O
,	O
and	O
large	O
area	O
of	O
collection	O
has	O
maintained	O
photomultipliers	O
an	O
essential	O
place	O
in	O
low	O
light	O
level	O
spectroscopy	O
,	O
confocal	O
microscopy	O
,	O
Raman	B-Algorithm
spectroscopy	I-Algorithm
,	O
fluorescence	O
spectroscopy	O
,	O
nuclear	O
and	O
particle	O
physics	O
,	O
astronomy	O
,	O
medical	O
diagnostics	O
including	O
blood	O
tests	O
,	O
medical	B-Application
imaging	I-Application
,	O
motion	O
picture	O
film	O
scanning	O
(	O
telecine	B-Application
)	O
,	O
radar	O
jamming	O
,	O
and	O
high-end	O
image	O
scanners	O
known	O
as	O
drum	O
scanners	O
.	O
</s>
<s>
Research	O
that	O
analyzes	O
light	O
scattering	O
,	O
such	O
as	O
the	O
study	O
of	O
polymers	B-Language
in	O
solution	O
,	O
often	O
uses	O
a	O
laser	O
and	O
a	O
PMT	O
to	O
collect	O
the	O
scattered	O
light	O
data	O
.	O
</s>
<s>
Photomultipliers	O
are	O
typically	O
constructed	O
with	O
an	O
evacuated	O
glass	O
housing	O
(	O
using	O
an	O
extremely	O
tight	O
and	O
durable	O
glass-to-metal	O
seal	O
like	O
other	O
vacuum	O
tubes	O
)	O
,	O
containing	O
a	O
photocathode	O
,	O
several	O
dynodes	B-Algorithm
,	O
and	O
an	O
anode	O
.	O
</s>
<s>
Incident	O
photons	B-Application
strike	O
the	O
photocathode	O
material	O
,	O
which	O
is	O
usually	O
a	O
thin	O
vapor-deposited	O
conducting	O
layer	O
on	O
the	O
inside	O
of	O
the	O
entry	O
window	O
of	O
the	O
device	O
.	O
</s>
<s>
The	O
electron	O
multiplier	O
consists	O
of	O
a	O
number	O
of	O
electrodes	O
called	O
dynodes	B-Algorithm
.	O
</s>
<s>
Each	O
dynode	B-Algorithm
is	O
held	O
at	O
a	O
more	O
positive	O
potential	O
,	O
by	O
≈100	O
Volts	O
,	O
than	O
the	O
preceding	O
one	O
.	O
</s>
<s>
A	O
primary	O
electron	O
leaves	O
the	O
photocathode	O
with	O
the	O
energy	O
of	O
the	O
incoming	O
photon	B-Application
,	O
or	O
about	O
3eV	O
for	O
"	O
blue	O
"	O
photons	B-Application
,	O
minus	O
the	O
work	O
function	O
of	O
the	O
photocathode	O
.	O
</s>
<s>
A	O
small	O
group	O
of	O
primary	O
electrons	O
is	O
created	O
by	O
the	O
arrival	O
of	O
a	O
group	O
of	O
initial	O
photons	B-Application
.	O
</s>
<s>
The	O
primary	O
electrons	O
move	O
toward	O
the	O
first	O
dynode	B-Algorithm
because	O
they	O
are	O
accelerated	O
by	O
the	O
electric	O
field	O
.	O
</s>
<s>
Upon	O
striking	O
the	O
first	O
dynode	B-Algorithm
,	O
more	O
low	O
energy	O
electrons	O
are	O
emitted	O
,	O
and	O
these	O
electrons	O
are	O
in	O
turn	O
accelerated	O
toward	O
the	O
second	O
dynode	B-Algorithm
.	O
</s>
<s>
The	O
geometry	O
of	O
the	O
dynode	B-Algorithm
chain	O
is	O
such	O
that	O
a	O
cascade	O
occurs	O
with	O
an	O
exponentially-increasing	O
number	O
of	O
electrons	O
being	O
produced	O
at	O
each	O
stage	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
at	O
each	O
stage	O
an	O
average	O
of	O
5	O
new	O
electrons	O
are	O
produced	O
for	O
each	O
incoming	O
electron	O
,	O
and	O
if	O
there	O
are	O
12	O
dynode	B-Algorithm
stages	O
,	O
then	O
at	O
the	O
last	O
stage	O
one	O
expects	O
for	O
each	O
primary	O
electron	O
about	O
512	O
≈	O
108	O
electrons	O
.	O
</s>
<s>
This	O
large	O
number	O
of	O
electrons	O
reaching	O
the	O
anode	O
results	O
in	O
a	O
sharp	O
current	O
pulse	O
that	O
is	O
easily	O
detectable	O
,	O
for	O
example	O
on	O
an	O
oscilloscope	O
,	O
signaling	O
the	O
arrival	O
of	O
the	O
photon(s )	O
at	O
the	O
photocathode	O
≈50	O
nanoseconds	O
earlier	O
.	O
</s>
<s>
The	O
necessary	O
distribution	O
of	O
voltage	O
along	O
the	O
series	O
of	O
dynodes	B-Algorithm
is	O
created	O
by	O
a	O
voltage	O
divider	O
chain	O
,	O
as	O
illustrated	O
in	O
Fig	O
.	O
</s>
<s>
The	O
capacitors	O
across	O
the	O
final	O
few	O
dynodes	B-Algorithm
act	O
as	O
local	O
reservoirs	O
of	O
charge	O
to	O
help	O
maintain	O
the	O
voltage	O
on	O
the	O
dynodes	B-Algorithm
while	O
electron	O
avalanches	O
propagate	O
through	O
the	O
tube	O
.	O
</s>
<s>
Besides	O
the	O
different	O
photocathode	O
materials	O
,	O
performance	O
is	O
also	O
affected	O
by	O
the	O
transmission	O
of	O
the	O
window	O
material	O
that	O
the	O
light	O
passes	O
through	O
,	O
and	O
by	O
the	O
arrangement	O
of	O
the	O
dynodes	B-Algorithm
.	O
</s>
<s>
The	O
first	O
demonstration	O
of	O
the	O
photoelectric	O
effect	O
was	O
carried	O
out	O
in	O
1887	O
by	O
Heinrich	O
Hertz	O
using	O
ultraviolet	B-Application
light	O
.	O
</s>
<s>
At	O
the	O
time	O
the	O
maximum	O
first-stage	O
gain	O
that	O
could	O
be	O
achieved	O
was	O
approximately	O
10	O
(	O
very	O
significant	O
developments	O
in	O
the	O
1960s	O
permitted	O
gains	O
above	O
25	O
to	O
be	O
reached	O
using	O
negative	O
electron	O
affinity	O
dynodes	B-Algorithm
)	O
.	O
</s>
<s>
Numerous	O
visits	O
were	O
made	O
by	O
RCA	O
scientific	O
personnel	O
to	O
the	O
USSR	O
in	O
the	O
1930s	O
,	O
prior	O
to	O
the	O
Cold	B-Application
War	I-Application
,	O
to	O
instruct	O
the	O
Soviet	O
customers	O
on	O
the	O
capabilities	O
of	O
RCA	O
equipment	O
and	O
to	O
investigate	O
customer	O
needs	O
.	O
</s>
<s>
During	O
one	O
of	O
these	O
visits	O
,	O
in	O
September	O
1934	O
,	O
RCA	O
's	O
Vladimir	O
Zworykin	O
was	O
shown	O
the	O
first	O
multiple-dynode	O
photomultiplier	O
,	O
or	O
photoelectron	O
multiplier	O
.	O
</s>
<s>
By	O
October	O
1935	O
,	O
Vladimir	O
Zworykin	O
,	O
George	O
Ashmun	O
Morton	O
,	O
and	O
Louis	O
Malter	O
of	O
RCA	O
in	O
Camden	O
,	O
NJ	O
submitted	O
their	O
manuscript	O
describing	O
the	O
first	O
comprehensive	O
experimental	O
and	O
theoretical	O
analysis	O
of	O
a	O
multiple	O
dynode	B-Algorithm
tube	O
—	O
the	O
device	O
later	O
called	O
a	O
photomultiplier	O
—	O
to	O
Proc	O
.	O
</s>
<s>
The	O
caesium-antimony	O
photocathode	O
had	O
a	O
dramatically	O
improved	O
quantum	O
efficiency	O
of	O
12%	O
at	O
400nm	O
,	O
and	O
was	O
used	O
in	O
the	O
first	O
commercially	O
successful	O
photomultipliers	O
manufactured	O
by	O
RCA	O
(	O
i.e.	O
,	O
the	O
931-type	O
)	O
both	O
as	O
a	O
photocathode	O
and	O
as	O
a	O
secondary-emitting	O
material	O
for	O
the	O
dynodes	B-Algorithm
.	O
</s>
<s>
In	O
subsequent	O
years	O
other	O
products	O
were	O
added	O
,	O
such	O
as	O
"	B-Device
cathode-ray	I-Device
"	I-Device
tubes	I-Device
,	O
photomultiplier	B-Algorithm
tubes	I-Algorithm
,	O
motion-sensing	O
light	O
control	O
switches	O
,	O
and	O
closed-circuit	O
television	O
systems	O
.	O
</s>
<s>
High	O
dark	O
current	O
;	O
used	O
mainly	O
in	O
near-infrared	O
,	O
with	O
the	O
photocathode	O
cooled	O
;	O
GaAs:Cs	O
,	O
caesium-activated	O
gallium	O
arsenide	O
,	O
flat	O
response	O
from	O
300	O
to	O
850nm	O
,	O
fading	O
towards	O
ultraviolet	B-Application
and	O
to	O
930nm	O
;	O
InGaAs:Cs	O
,	O
caesium-activated	O
indium	O
gallium	O
arsenide	O
,	O
higher	O
infrared	O
sensitivity	O
than	O
GaAs:Cs	O
,	O
between	O
900	O
–	O
1000nm	O
much	O
higher	O
signal-to-noise	O
ratio	O
than	O
Ag-O-Cs	O
;	O
Sb-Cs	O
,	O
(	O
also	O
called	O
S11	O
)	O
caesium-activated	O
antimony	O
,	O
used	O
for	O
reflective	O
mode	O
photocathodes	O
;	O
response	O
range	O
from	O
ultraviolet	B-Application
to	O
visible	O
,	O
widely	O
used	O
;	O
bialkali	O
(	O
Sb-K-Cs	O
,	O
Sb-Rb-Cs	O
)	O
,	O
caesium-activated	O
antimony-rubidium	O
or	O
antimony-potassium	O
alloy	O
,	O
similar	O
to	O
Sb:Cs	O
,	O
with	O
higher	O
sensitivity	O
and	O
lower	O
noise	O
.	O
</s>
<s>
can	O
be	O
used	O
for	O
transmission-mode	O
;	O
favorable	O
response	O
to	O
a	O
NaI:Tl	O
scintillator	O
flashes	O
makes	O
them	O
widely	O
used	O
in	O
gamma	O
spectroscopy	O
and	O
radiation	O
detection	O
;	O
high-temperature	O
bialkali	O
(	O
Na-K-Sb	O
)	O
,	O
can	O
operate	O
up	O
to	O
175°C	O
,	O
used	O
in	O
well	O
logging	O
,	O
low	O
dark	O
current	O
at	O
room	O
temperature	O
;	O
multialkali	O
(	O
Na-K-Sb-Cs	O
)	O
,	O
(	O
also	O
called	O
S20	O
)	O
,	O
wide	O
spectral	O
response	O
from	O
ultraviolet	B-Application
to	O
near-infrared	O
,	O
special	O
cathode	O
processing	O
can	O
extend	O
range	O
to	O
930nm	O
,	O
used	O
in	O
broadband	O
spectrophotometers	O
;	O
solar-blind	O
(	O
Cs-Te	O
,	O
Cs-I	O
)	O
,	O
sensitive	O
to	O
vacuum-UV	O
and	O
ultraviolet	B-Application
,	O
insensitive	O
to	O
visible	O
light	O
and	O
infrared	O
(	O
Cs-Te	O
has	O
cutoff	O
at	O
320nm	O
,	O
Cs-I	O
at	O
200nm	O
)	O
.	O
</s>
<s>
Ultraviolet	B-Application
glass	O
transmits	O
visible	O
and	O
ultraviolet	B-Application
down	O
to	O
185nm	O
.	O
</s>
<s>
Magnesium	O
fluoride	O
transmits	O
ultraviolet	B-Application
down	O
to	O
115nm	O
.	O
</s>
<s>
Photomultiplier	B-Algorithm
tubes	I-Algorithm
typically	O
utilize	O
1000	O
to	O
2000	O
volts	O
to	O
accelerate	O
electrons	O
within	O
the	O
chain	O
of	O
dynodes	B-Algorithm
.	O
</s>
<s>
Voltages	O
are	O
distributed	O
to	O
the	O
dynodes	B-Algorithm
by	O
a	O
resistive	O
voltage	O
divider	O
,	O
although	O
variations	O
such	O
as	O
active	O
designs	O
(	O
with	O
transistors	B-Application
or	O
diodes	O
)	O
are	O
possible	O
.	O
</s>
<s>
If	O
used	O
in	O
a	O
location	O
with	O
strong	O
magnetic	O
fields	O
,	O
which	O
can	O
curve	O
electron	O
paths	O
,	O
steer	O
the	O
electrons	O
away	O
from	O
the	O
dynodes	B-Algorithm
and	O
cause	O
loss	O
of	O
gain	O
,	O
photomultipliers	O
are	O
usually	O
magnetically	O
shielded	O
by	O
a	O
layer	O
of	O
soft	O
iron	O
or	O
mu-metal	O
.	O
</s>
<s>
Photomultipliers	O
with	O
large	O
distances	O
between	O
the	O
photocathode	O
and	O
the	O
first	O
dynode	B-Algorithm
are	O
especially	O
sensitive	O
to	O
magnetic	O
fields	O
.	O
</s>
<s>
Photomultipliers	O
are	O
typically	O
used	O
as	O
the	O
detectors	O
in	O
flying-spot	B-Algorithm
scanners	I-Algorithm
.	O
</s>
<s>
Photomultiplier-amplified	O
photocurrents	O
can	O
be	O
electronically	O
amplified	O
by	O
a	O
high-input-impedance	O
electronic	O
amplifier	O
(	O
in	O
the	O
signal	O
path	O
subsequent	O
to	O
the	O
photomultiplier	O
)	O
,	O
thus	O
producing	O
appreciable	O
voltages	O
even	O
for	O
nearly	O
infinitesimally	O
small	O
photon	B-Application
fluxes	B-General_Concept
.	O
</s>
<s>
The	O
aforementioned	O
refers	O
to	O
measurement	O
of	O
light	O
fluxes	B-General_Concept
that	O
,	O
while	O
small	O
,	O
nonetheless	O
amount	O
to	O
a	O
continuous	O
stream	O
of	O
multiple	O
photons	B-Application
.	O
</s>
<s>
For	O
smaller	O
photon	B-Application
fluxes	B-General_Concept
,	O
the	O
photomultiplier	O
can	O
be	O
operated	O
in	O
photon-counting	O
,	O
or	O
Geiger	O
,	O
mode	O
(	O
see	O
also	O
Single-photon	O
avalanche	O
diode	O
)	O
.	O
</s>
<s>
In	O
Geiger	O
mode	O
the	O
photomultiplier	O
gain	O
is	O
set	O
so	O
high	O
(	O
using	O
high	O
voltage	O
)	O
that	O
a	O
single	O
photo-electron	O
resulting	O
from	O
a	O
single	O
photon	B-Application
incident	O
on	O
the	O
primary	O
surface	O
generates	O
a	O
very	O
large	O
current	O
at	O
the	O
output	O
circuit	O
.	O
</s>
<s>
In	O
either	O
case	O
,	O
the	O
photomultiplier	O
can	O
detect	O
individual	O
photons	B-Application
.	O
</s>
<s>
The	O
drawback	O
,	O
however	O
,	O
is	O
that	O
not	O
every	O
photon	B-Application
incident	O
on	O
the	O
primary	O
surface	O
is	O
counted	O
either	O
because	O
of	O
less-than-perfect	O
efficiency	O
of	O
the	O
photomultiplier	O
,	O
or	O
because	O
a	O
second	O
photon	B-Application
can	O
arrive	O
at	O
the	O
photomultiplier	O
during	O
the	O
"	O
dead	O
time	O
"	O
associated	O
with	O
a	O
first	O
photon	B-Application
and	O
never	O
be	O
noticed	O
.	O
</s>
<s>
A	O
photomultiplier	O
will	O
produce	O
a	O
small	O
current	O
even	O
without	O
incident	O
photons	B-Application
;	O
this	O
is	O
called	O
the	O
dark	O
current	O
.	O
</s>
<s>
Photon-counting	O
applications	O
generally	O
demand	O
photomultipliers	O
designed	O
to	O
minimise	O
dark	O
current	O
.	O
</s>
<s>
Nonetheless	O
,	O
the	O
ability	O
to	O
detect	O
single	O
photons	B-Application
striking	O
the	O
primary	O
photosensitive	O
surface	O
itself	O
reveals	O
the	O
quantization	O
principle	O
that	O
Einstein	O
put	O
forth	O
.	O
</s>
<s>
Photon	B-Application
counting	O
(	O
as	O
it	O
is	O
called	O
)	O
reveals	O
that	O
light	O
,	O
not	O
only	O
being	O
a	O
wave	O
,	O
consists	O
of	O
discrete	O
particles	O
(	O
i.e.	O
,	O
photons	B-Application
)	O
.	O
</s>
