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Simulating Scintillation Pulses with an LED Light Pulser Simulating Scintillation Pulses
with an LED Light Pulser
LED stands for Light Emitting Diode. It
is a special type of diode that emits
light when an electric current flows
through it. It can be used to simulate
a scintillation pulse to test the
electronic circuits in the measurement
system.
A diode is a device that has high
resistance to the flow of electric
current in one direction and a low
resistance to the flow of electric
current in the other direction. The
symbol for a diode is an arrow
pointing to a short line.
Electric current can flow through the
diode easily in the direction of the
arrow. The wire lead that is negative
(cathode) is marked on the diode with
a metal tab, flat spot, short lead or
colored dot. With a positive potential
(voltage) applied on the positive lead
and the current return connected to
the other lead, a very large current will
flow. (This is assuming, of course, that
it is above a threshold level. For an
LED, the threshold is about 1 or 2
volts.) This is called forward biasing;
and since the diode has little resis-
tance in this direction, the current will
be quite large. If the positive and
return leads are switched, there will be
Technical Information Note
Document #504
very little current flow. The diode has
high resistance in the reverse polarity
(or reverse bias) mode.
An LED is a diode that emits light when
it is forward biased. A typical LED has a
forward resistance of a few 100 ohms
and a reverse resistance of 5 mego-
hms. Most LEDs are operated at 10 to
20 milliamperes of current. In the
forward biased mode of operation, too
much current can damage or destroy
an LED. To make sure that the current
is limited, a series resistor is used. The
resistors value is calculated by Ohms
law, V = IR, where V is in volts, I is in
amperes and R is in ohms. For a 1 volt
supply, R=100 ohms to get the 10
milliampere current and for a 10 volt
supply R=1000 ohms. The symbol for
an LED is that of a diode but with a
photon of light leaving the arrow.
To simulate a scintillation pulse, an LED
is operated in a pulsed mode. The
light being emitted by the LED is
proportional to the current flowing
through the diode. The current is
proportional to the area of the exciting
voltage above the LED threshold level.
Consider first a trapezoidal voltage
pulse. The chart below shows the
applied voltage above and the
resulting current flow below.
The instantaneous current is propor-
tional to the voltage above threshold
divided by the total resistance. The
total light produced by the LED is
proportional to the area of this
trapezoid which is mathematically
equal to the total charge passing
through the LED. A photomultiplier
tubes output would be proportional
to the percentage of this light that it
collects and hence to the total charge
passing through the LED.
A square pulse can be used also. If the
sensing electronics is sensitive to the
total charge output of the PMT only,
then any shaped pulse can be used.
Square pulse generators are available
and the area under the current curve
(the total charge) is simply the product
of the current (in amperes) times the
duration (in seconds) to yield the total
charge in coulombs.
(continued on back) USA
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Scintillation Products
Technical Information Note
LED Light Output, per HP
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1.0
1.1
20
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40
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Light E
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If preamplifiers and shaping amplifiers are
used, the square pulse can generate pulses that
do not match the scintillator pulses shape. In
these cases, a tail pulse generator can be used
to excite the LED and to simulate a scintillation
pulse shaped event. Again, only the area of the
exciting voltage pulse above the diode thresh-
old can produce current flow in the diode.
It should be noted that the light output of LEDs
is exponentially dependent on temperature
(note the chart below for a Hewlett Packard
LED). Since most LEDs are encased in an
insulator and the only heat sinks are through
the two leads, they can get quite hot rapidly,
changing the output as it operates. For this
reason, LEDs are not used for gain stabilizing
scintillation detectors. (They are, however,
used in many devices as a light source to check
functionality.) There are gain stabilizing
systems and devices available that employ
LEDs. These have internal feedback loops to
control the stability of light output. Berkeley
Nucleonics Corporation manufactures such
systems, and Integrated Photomatrix Limited
manufactures devices called self-monitoring
emitters.
For further information on a specific LED, the
manufacturer should be contacted.