1 About photomultiplier tube
A photomultiplier tube is used to detect weak light signal. It’s a vacuum tube consisting of an input window, a photocathode, focusing electrodes, an electron multiplier and an anode usually sealed into an evacuated glass tube.
Light which enters a photomultiplier tube is detected and produces an output signal through the following processes:
(1) Light passes through the input window
(2) Light excites the electrons in the photocathode so that photoelectrons are emitted into the vacuum (external photoelectric effect)
(3) Photoelectrons are accelerated and focused by the focusing electrode onto the first dynode where they are multiplied by means of secondary electron emission. This secondary emission is repeated at each of the successive dynodes.
(4) The multiplied secondary electrons emitted from the last dynode are finally collected by the anode.
(5) An output current signal is produced in the Anode.
(6) A conditioning circuit and a data acquisition module is used to acquire the signal in the output.
Photomultiplier tubes are widely used to detect particle radiation, ionizing radiation and measure the intensity and spectrum of light-emitting materials.
For example, it can be a detector of spectrophotometers for the measurements of photons or a part of monitoring instruments for the measurements of various radiation in the atmosphere.
2 Challenges of testing a photomultiplier tube
It is important to accurately measure the amplitude of the output signal of a photomultiplier tube and capture all the signals without loss. Assuming that the output is a pulse, there will be the following challenges to do the test of a photomultiplier tube.
1). Low amplitude
Normally, the amplitude of the output pulse of the photomultiplier tube is very weak. It requires a test instrument with not only high bandwidth and sampling rate but also high resolution.
2). Small gap between two neighbor pulses
Sometimes the time interval between the two output pulses is relatively small, which needs the test instrument to be fast enough to capture all the pulses without loss.
3). Capability to count the number of the pulses occurred in a period
In order to analyze the probability of the event like particle radiation occurring, we sometimes need to count the number of pulses captured over a period of time. It requires the software for a test instrument to have a feature of counting.
4). Be able to sort out the pulses
As the output pulses can be generated from different sources or different photomultiplier tubes, we need to sort out them. Using advanced triggers are always a good way to get it. The software of a test instrument which is always an oscilloscope should include various of advanced triggers.
5). High voltage environment
A photomultiplier tube is powered by a high voltage with a few KVs. The environment of high voltage can cause more interference. It needs the instrument to be able to do the measurements reliably under the environment of high voltage.
3 Test a photomultiplier tube with PicoScope 6000E
Pico’s PicoScope 6000E series oscilloscopes can meet well the challenges of testing a photomultiplier tube addressed above.
The PicoScope 6000E Series fixed-resolution and FlexRes oscilloscopes provide 8 to 12 bits of vertical resolution, up to 3 GHz bandwidth and 10 GS/s sampling rate. These specifications allow the pulse signal with low amplitude to be tested accurately.
Working under rapid trigger mode, PicoScope 6000E Series oscilloscopes can capture continuous pulses with the gap between two pulses down to 300 ns typically. This helps reduce the loss of pulses dramatically.
The PicoScope 6000E Series features hardware-based trigger time-stamping. Each waveform acquired with PicoScope 6000E Series oscilloscopes in PicoScope 7 software is timestamped with time from previous waveform. This feature makes it easy for users to count the number of pulses in a period of time.
The PicoScope 6000E Series offers a set of advanced trigger types including pulse width, runt pulse, windowed, rise/fall time, logic and dropout. They can be used to sort out the pulses we are interested in.
Figure 7 below show the output pulses from photomultiplier tubes tested in PicoScope 7. PicoScope 6424E oscilloscope is used. There are three waveforms showed in the pictures as three photomultiplier tubes work simultaneously.
The amplitude of the pulses is low to a few millivolts. The scope works with 2.5 GS/s and rapid mode. The logic “OR” trigger is used. It acquires totally 10000 waveforms. The user is able to browse the waveforms one by one after the scope stops acquiring data. The capture time and trigger timestamp are also provided so that the user can know the occurring time of each pulse and how many pulses occurred in a period of time.
Figure 8 below shows the output pulses from photomultiplier tubes tested in Python with PicoSDK.
4 Why PicoScopes?
PicoScope 5000D series and 6000E series are popularly used for testing the outputs of photomultiplier tubes in standalone or system integration.
The customer develops their own application software in programming languages like LabVIEW, Python, C#, based on Pico SDK, when PicoScopes work as a part of the testing system.
The main reasons that people like to integrate PicoScopes into their testing systems are the unique advantages and powerful capabilities of PicoScopes.
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High reliability
Established in 1991, Pico Technology has been focusing on the development and manufacture of PC-based test instrument and data acquisition equipment. Decades of product iteration and stringent quality controls make PicoScopes highly reliable.
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Cost-efficiency
Different from benchtop instruments, the fact that PicoScopes don’t have PC in the box gives them more unique hardware advantages due to the simple structure, including high bandwidth/sampling rate, deep memory, high/flexible ADC resolution, multiple analog and digital inputs, fast speed, etc. The use of full capability of an external PC allows PicoScopes more software capabilities, including 30+ serial decoders, advanced math functions (FFT, filters, measurements plotting, etc). Moreover, dozens of PicoScope models make the customers easier to pick one affordable and suitable for their applications.
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High/Flexible ADC resolution
PicoScopes offer a wide range of vertical resolution options from 8 to 16 bits. The higher the resolution, the greater the vertical accuracy and the dynamic range. The flexible resolution feature in PicoScopes is based on Pico’s breakthrough ADC technology which allows users to switch from 8 to 16 bits in one unit.
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Fast and powerful SDK
PicoScopes provide a level of interconnectivity and customization that is not usually available on most benchtop oscilloscopes. The SDK (Software Development Kit) allows users to create custom applications for their particular projects. That makes PicoScopes go beyond just being a regular oscilloscope.
PicoScopes running under SDK have much better performance. For example, it can acquire and transmit data continuously to PC with the speed up to 312 MS/s; The memory can be segmented up to 2 million; The users can set the advanced triggers and generate waveforms programmatically.
Programming with SDK is simple and easy. Professional technical support is always ready and lots of code examples can be found in github.com/picotech.
All Pico products including PicoScopes come with a free-of-charge SDK. The SDK includes drivers for Windows, macOS, Linux and Raspberry Pi (ARM7). It allows users to write their own software to control the instruments with popular languages such as C, C#, C++, Python, MATLAB, LabVIEW and Microsoft Excel.
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Compact and portable units
Unlike traditional benchtop instruments, PicoScopes are compact, light and portable. When used with a laptop computer, a PicoScope allows you to carry a complete electronics toolset in one bag with your PC. And small size also makes PicoScopes easier to be integrated into the systems with almost no increase in weight or size.
All the opinions in this article are of Derek Hu | Pico Technology. The Volt Post takes no responsibility for the opinions, figures, and statistics mentioned in the column.*