Low Light Detection with PMT

There is no shortage of photomultipliers in the PhysicsOpenLab laboratory !
For this reason, we decided to use one of these instruments to measure physical phenomena characterized by low intensity light emission such as phosphorescence and bioluminescence. We have already made measurements of this type, but with different instruments, based on the photon counting technique. Photon counting is a technique used for extremely low brightness measurements, when the light intensity is not so low the photon counting technique is no longer usable because the frequency of the pulses quickly becomes too high to be measured.
In these cases, the pulses produced by the PMT overlap each other and produce a continuous current which can be measured with suitable amplifiers.

Instrumentation

A commercial Matsusada HV generator was used to power the PMT. The instrument is capable of generating a stabilized and adjustable high negative voltage, up to a maximum of 3kV. These generators are found quite easily on the eBay market. For this realization, it was chosen to put the anode at ground (zero) potential and the cathode to negative potential, which must therefore be suitably isolated. The image below shows the voltage divider diagram and an image of the Matsusada J4-3N-LX generator.

The signal is taken directly from the anode, without the decoupling capacitor, since the anode has zero potential. The advantages of this type of connection are greater “cleanliness” of the signal and the possibility of operating even in continuous mode.
The photo-current generated by the photomultiplier is sent to an amplifier that converts the current signal into a voltage signal which is acquired and recorded by the Logger based on the Raspberry Pi.

The image below shows the complete setup, including HV power supply, light-tight box with PMT, amplifier and RasPi Logger.

Photomultiplier

The photomultiplier that we adopted – shown in the following image – is the R647 Hamamatsu model.

PMT R647 Hamamatsu Datasheets

Type : Head-On
Size : 13mm
Photocathode : Bialkali
Max Voltage : 1250V
Bias Voltage : 1000V
Dynodes : 10
Rise Time : 2.1ns
Dark Current : 15nA
Gain : 1 x 10⁶

The photomultiplier tube was inserted inside a light-tight metal container : the “dark box”. , the BNC connectors have been positioned on one side of the box, to supply the PMT tube and to take the signal from the anode. In the following image you can see the inside of the “dark box”:

Amplifier

The current signal produced by the photomultiplier is converted and amplified by a suitable charge amplifier. It is an element based on a low noise operational amplifier, in which the current signal is converted into a voltage signal. The conversion modes are essentially two: through a feedback resistor (Rf) or by charging a feedback capacitor (Cf).
In the first case we obtain what is known as a Trans-Impedance Amplifier, while in the second we obtain a Charge Sensitive Preamplifier.

In the case of the trans-impedance amplifier, the voltage signal faithfully reproduces the current signal and the Cf capacitor only has the function of stabilizing the output and preventing unwanted oscillations, this configuration can be used for both impulse and continuous signals.
In the case of the charge preamplifier, the current pulse is “integrated” by the capacitor Cf, while Rf has the function of “discharging” Cf to prevent the amplifier output from immediately going into saturation, in this configuration the input signal shape is not preserved. This configuration is mainly used in the case of pulse signals.

In our case we used a “hybrid” configuration. The photomultiplier generates a series of close pulses that overlap and give rise to a continuous photo-current with random variations. Our amplifier converts the input current with a feedback resistance Rf = 150KΩ, and the voltage signal is filtered by the capacitor Cf = 330pF. Together these two components make up a low pass filter with cutoff frequency f = 3KHz.

Depending on the type of signal under examination, we can insert an additional low-pass RC filter in order to obtain an even more leveled signal.
The image below shows our amplifier, with the power supply section which produces the bipolar voltage +5V/-5V and with the RC filter. Note that the feedback components of the amplifier and the filter are placed on connectors so that they can be easily changed according to the specific application.
The output signal is acquired through ADC with a Raspberry Pi, as shown in the post RasPI ADC Logger.

Phosphorescence Measures

Phosphorescence is the phenomenon of radiative emission by some materials/substances as a result of the absorption of energy through ultraviolet rays (very energetic) and the subsequent re-emission in the form of visible light (at lower energy). Phosphorescent materials continue to emit light up to many hours after the end of external illumination. When all the stored energy is exhausted, the material no longer emits light.

One of the most used phosphorus is zinc sulfide, doped with silver and also with copper. Zinc sulphide is an interesting material : ZnS is a semiconductor, ie a material with an electron-filled valence band and an empty conduction band. It is known that the energy gap between the two bands is, in pure ZnS, about 350 kJ/mol (3.6 eV). Given the large energy gap, only a very small concentration of charge carriers is present in normal temperature conditions. Doping with copper, silver or manganese introduces intermediate electronic energy levels within the forbidden band. In this situation, lighting with UV light or even ambient light excites the electrons, taking them from the valence band to the conduction band. The subsequent electron-hole recombination mechanism, through the intermediate energy levels introduced with doping, leads to the emission of phosphorescence.

The phenomenon of phosphorescence has been the subject of numerous experiments, conducted with the photon counting technique, as described in the posts : Glowing in the Dark, Zinc Sulphide Phosphorescence and Preparation of some Phosphorescent Compounds.

The phosphorescence is often of rather high intensity, this makes it necessary to partialize the photocathode of the PMT photon counter (for example with a pin-hole) otherwise the instrument goes into saturation. For this reason it can be useful to use the photo-current measurement technique instead of the photon counting one.
The test we did with our setup was positive, we acquired the brightness data of the zinc sulphide phosphorescence with a rate of 100Hz, obtaining the decay curve shown in the following graph. The result is in agreement with what was obtained in the previous measurements.

Bioluminescence Measures

With our instrumentation we investigated the phenomenon of bioluminescence. It is a fascinating physical phenomenon which consists in the emission of light by living organisms. Bioluminescence has been observed in numerous organisms : from the classic fireflies to particular species of mushrooms, passing through numerous marine organisms such as crustaceans, cephalopods and abyssal fish. Bioluminescence is a case of chemiluminescence (studied in the post : Luminol (ENG)), however it has characteristics that make it unique and interesting.

It is known that the light emission originates from the oxidation reaction of the Luciferin protein, in the presence of ATP, this reaction is catalyzed by the enzyme Luciferase. The fundamental aspect of this reaction is that it is an enzyme catalyzed reaction. In simplified form the reaction can be described as follows :

Luciferin + O₂ + ATP – (with Luciferase) -> OxyLuciferin + AMP + CO₂ + photon (hν) 

The reaction continues until one of the reagents (Luciferin, ATP) is exhausted, while the luciferase enzyme is not consumed by the reaction.

The bioluminescence we studied is that produced by ostracods (Sea Fireflies), tiny crustaceans with dimensions around the millimeter. We found (on the online market) dried samples of these crustaceans, in the image below you can see the crustaceans inserted in a test tube.

For the measurement we finely ground the crustaceans and we inserted the powder obtained in a test tube, we added water, stirred and put the test tube in the dark box to measure the intensity of the bioluminescence.

The graphs below show the measurement made with our instrumentation. The signal produced by the PMT and subsequently amplified is sampled with 100 Hz frequency, the measurement continued for a few minutes.

Some interesting features emerge from the graph. The first is that the initial light intensity is quite high, in fact the graph is “cut” for the first 20 seconds because the amplifier has gone into saturation. The second characteristic is that the maximum emission is not reached instantly but a time of about 0.5 seconds is required from the moment of adding water to the powder, as can be seen in the graphic detail below.

The other interesting feature is the trend of brightness decay over time. By drawing the graph on a logarithmic scale, two sections are noted, the first one is characterized by a constant slope – which means an exponential decrease – the second section, on the other hand, has a different and non-constant slope.

For the first part of the curve, the exponential fitting is perfect. Exponential decay is characteristic of first order systems (radioactive decay is an example) in which the reaction rate – which is proportional to the intensity of the light emitted – is proportional to the concentration of the reagent, in our case luciferin. This is in agreement with the initial hypothesis that the luciferase enzyme did not consume during the reaction.

Of course the situation – and the chemical reaction – is more complex, because it sees the participation of other reagents (ATP, O₂) and intermediate products. In fact, the overall trend cannot be approximated only with an exponential decrease curve.

Following these tests we can say that our instrumentation has proven to be suitable for making quantitative measurements on very low intensity light sources.

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