Photon detection and counting system

A photo multiplier tube is connected through a wave shaping and coupling cuit to an evaluation circuit; to provide for simple evaluation, for example, in an integrator, the coupling and wave shaping circuit includes an amplifier and a limiter, preferably a Zener diode connected through suitable coupling resistors to the integrator. Switches are provided to close the circuit from the photomultiplier tube (PMT) to the integrator, connected with a photon emission event trigger circuit so that, under non-event producing conditions, the integrator is disconnected from the PMT to prevent evaluating and possibly indicating spurious dark current noise pulses.

Cross reference to pertinent literature: "How to Make Every Photon Count" 
by M. R. Zatzick, "Electro-Optical Systems Design", June 1972, pp. 20-27. 
The present invention relates to a photon detection and counting system 
including a photo multiplier tube (PMT), the output of which is connected 
through a pulse and wave shaping circuit to an evaluation circuit which 
includes a measuring instrument, print out apparatus, or the like. 
The outputs from the PMT have to be normalized or standardized so that they 
can be efficiently and readily counted. One system of this type is 
described in the cross referenced literature. The output signal of a PMT, 
which includes pulses corresponding to incidence of photons, or photon 
emissive events sensed by the PMT are conducted to an 
amplifier-discriminator circuit, which standardizes the output signal of 
the PMT to a stable base line. These signals are then applied to a 
threshold amplifier, which, for each pulse applied to its input which 
exceeds the threshold set by the threshold amplifier, provides an output 
pulse of constant amplitude and duration which is independent of the 
amplitude, or magnitude, and duration of the input signal itself. These 
standardized output pulse signals are then applied to a digital indicator, 
to the input circuit of a computer, or to an oscilliscope which is used to 
calibrate the entire system. Of course, the outputs can be selectively 
connected to more than one of these output-evaluating devices. 
It has previously been proposed to connect the output signal of the PMT 
directly to an electrometer in order to integrate the output signals 
therein over a time base. The output then will be an analog indication 
which, however, is relatively inaccurate since the energy content of the 
separate photon events, which cause pulses to be emitted by the PMT 
changes by changes arising in the PMT itself by changes in the focusing 
conditions, secondary emission threshold values and other temperature 
effects arising within the PMT itself. 
It is an object of the present invention to provide a photon detection and 
counting system which is accurate but, in contrast to systems proposed in 
the prior art, is simple and requires but little by way of apparatus, and 
especially, does no require expensive highly accurate and rapid 
sophisticated electronic circuitry. 
Subject matter of the present invention: Briefly, a wave shaping and 
coupling circuit is provided to connect the output of the PM tube to an 
evaluation circuit which, includes a summing integrator, in which the wave 
shaping and coupling circuit includes an amplifier and a limiter connected 
to the amplifier; the amplifier may have an amplification factor of 1,000 
or so, preferably not less, and should be essentially immune to overload, 
and have a rapid overload-recovery time. 
In accordance with a feature of the invention, the integrating circuit is 
disabled during the time when no photon events are expected. This can 
readily be effected by connecting a programmable trigger circuit to a 
relay, or the like; the programmer will trigger a circuit programming the 
photon-emissive event, and, at the same time, connecting the integrator to 
the output of the P M tube only when the photo-emissive event has been 
commanded; at other times, the integrator is separated from the PMT so 
that spuriously arising dark current pulses will not be sensed or counted. 
The system is simple and highly accurate; disconnecting the PMT from the 
evaluation circuit unless a photon-emissive event has been triggered 
additionally increases the accuracy of measurement.

A PMT 10 is supplied with power from a stabilized power supply source (12). 
The anode of the PMT is connected over a working resistor 14 to ground, or 
chassis, and, further is connected across a high gain, broad band 
amplifier 16 to a resistor 18 of about 1,000 ohms to a limiter circuit L. 
The amplifier 16 should, preferably, have high tolerance to overload and 
rapid overload-recovery characteristics. The limiter L is formed by a 
Zener doide 20 connected between the junction of the output side of 
resistor 18 and ground, or chassis. The output of the limiter circuit is 
connected through the working contacts of a relay 22, which is bridged by 
a manually operated push switch S1 through a coupling resistor 24 of about 
100 kilo ohms to the input of an integrator 26. Integrator 26, preferably, 
includes an operational amplifier having a capacitor C in its feedback 
circuit. The integrator integrates the amplified clipped, or limited, 
pulses from the PMT 10 and provides an integrated signal which corresponds 
to the number of the pulses applied to the integrator 26. This signal can 
be indicated by a cumulative indicator 28, or can be further processed by 
being taken off an output connection line 30, for example, by being 
applied to an output printer, to a computer, input terminal, or the like. 
The feedback capacity C connecting the output of the operational amplifier 
of integrator 26 back to its input, preferably, is variable, that is, can 
be switched between various capacitor values in order to provide for a 
plurality of measuring ranges. 
The integrator has a reset, or discharge terminal 32 which is connected 
through the working contact of a second relay 34, bridged by switch S2 to 
ground, or chassis. Both relays 22, 34, preferably are reed relays, sealed 
into a tube filled with a protective gas. The connection between resistor 
18 and the Zener diode 20 of limiter L is further connected through a 
switchable resistor 36 to an electrometer 38. Typical resistance value for 
switchable resistance value for switchable resistor 36 is about 100 kilo 
ohms. The electrometer will then indicate the number of the photo events 
per unit of time, that is, the total photon energy. 
Operation: The PMT 10 provides current pulses at each photon event, which 
have a current value of about 10.sup.-8 A. The circuit, with the 
resistance values above referred to then will store in the integrator 26 
for each photon event a charge of about 10.sup.-11 Coulomb. The measuring 
range of the integrator can be adjusted by suitable selection of the 
feedback capacitor C, for example, to provide at full scale deflection of 
indicator 28 an indication when the integrator has stored 10.sup.-9 
Coulomb, corresponding to about 100 photon events at its setting of 
maximum sensitivity; at a position of lesser sensitivity, full scale 
deflection can correspond to 10.sup.-4 Coulomb, corresponding to about 10 
.sup.7 photon events. 
The system permits analog, or digital counting of photon events with very 
few, and inexpensive, components. The voltage pulses across the resistor 
14 have a value of about 10.sup.-2 V and are amplified by amplifier 16 
which, preferably, has an amplification factor a equal to 1,000 or so. 
This is an amplification factor which can be readily attained by 
commercially available, inexpensive operational amplifiers. The Zener 
diode 20 of the limiter L then limits the voltage pulses to an amplitude 
of 1V; the limited pulses are applied through relay 22, then closed, over 
resistor 24 to integrator 26 which, for any typical photon event, receives 
a charge of 10.sup.-11 Coulomb. This charge value depends on the time 
constant of the amplifier 16 and its recovery time after overload. In 
actual practice, it has been found that the variation of charge for each 
photon event will average out already after counting of about 10 photon 
events. It is thus not necessary to provide an expensive pulse wave 
shaping circuit in which a standardizing pulse generator is used, 
controlled by primary current pulses. This very expensive - since very 
rapidly operating - component can be omitted. 
The relays 22 and 34 which, preferably are reed relays, but may be other 
types of control switches, further increase the sensitivity of the system 
when measuring photon or other light pulses. Closing of the circuit 
through relay 22 is controlled to occur only during the time that the 
light pulse is in effect. This is commanded by means of a program control 
40. Program control 40 has three outputs. Output 42 is connected to 
trigger the event which causes the light pulse to be emitted; output 44 is 
connected to the control coil, or control terminal of the relay 22 to 
close the relay and hold it closed only during the time period when the 
light pulse occurs. A third output 46 is delayed, for example, by being 
triggered by the trailing flank of the pulse from output 42 to reset the 
integrator 26, by discharging the integrator. The indicator 28, of course, 
is then arranged to hold its previous indication until the next pulse 
occurs. 
The temporal position of the control signals at the outputs 42, 44 insures 
that the contact of relay 22 is closed as soon as the light pulse begins 
and that the contacts of relay 22 open after the light pulse has 
terminated. Since the relay may have some operating time, the pulse on 
terminal 44 may occur just slightly before the event trigger pulse on 
terminal 42 is initiated. For example, when investigating cathodic 
electroluminescence in non-aqueous solutions, the closing time of the reed 
relay 22 was set to be about 2 milliseconds; the control signal at the 
output 44 was programmed to occur about 0.2 milliseconds in advance of the 
trigger signal at terminal 44, since luminescence occurs practically 
without delay, in order to take into consideration the response time of 
the relay 22. The PMT was cooled to dry ice temperature during the 
measuring step. Only a few ten or so dark current pulses per second were 
fund under such conditions. It is thus possible to measure short light 
flashes without normally counting a single dark current pulse, since the 
dark current pulses are distributed, based on statisticaly distribution. 
The control signal at output 46 closes the terminals of relay 34 which 
discharge the integrator and reset the integrator for a new measurement 
for a subsequent pulse from program control 40, trigerring a subsequent 
photon-emissive event. 
Electrometer 38 permits continuous indication of the pulse rate. It permits 
supevision of the dark current, as well as of the light current, or pulse 
current. In the practical embodiment, resistor 36 has a value of about 100 
kilo ohms, and a full scale deflection of 10.sup.-8 A scale of the 
electrometer corresponding to about 1,000 pulses per second. A d-c 
amplifier with a subsequently connected indicator instrument may be used 
instead. If the resistor 36 can be switched between various resistance 
values, different measuring ranges for pulse rate can be obtained. The 
programming control 40 may, of course, be set to provide pulses at 
terminal 46 to the relay 34 only after a plurality of photon events have 
been triggered, so that the number of integrated light pulses, integrated 
in the integrator 26 and indicated at indicator 28, or transmitted over 
line 30 can be suitably selected. 
Various changes and modifications may be made within the scope of the 
inventive concept.