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Lizard
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The standard output waveform of a photomultiplier tube has a "negative" pulse, like below. Why is the amplitude negative?
Why does a photomultiplier tube produce a negative output pulse?
In summary: The question is why is the pulse negative?In summary, the standard output waveform of a photomultiplier tube is typically negative due to the surrounding circuitry and the negative charges of electrons. This is because the anode current is converted to a voltage by a grounded resistor, causing the pulse to be negative. This is the result of the photo-cathode and multiplier dynodes being at a negative potential relative to the anode. The design of PMTs was first invented in the 1930s and has since been used in various applications such as TV cameras.
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Show the surrounding circuitry.
- #3
Baluncore
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The photo-cathode and multiplier dynodes are at negative potential relative to the anode. The pulse is negative because it is the anode current being converted to a voltage by a grounded resistor.
See fig. 2. here; https://en.wikipedia.org/wiki/Photomultiplier_tube#Structure_and_operating_principles
See fig. 2. here; https://en.wikipedia.org/wiki/Photomultiplier_tube#Structure_and_operating_principles
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hutchphd
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And the electrons are negative !
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sophiecentaur
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Best answer so far.hutchphd said:
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Vanadium 50
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I think @phinds was better. It's not impossible to design a circuit where the pulse is positive, even though the electrons are negative. A PMT is but one element in a circuit.sophiecentaur said:
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sophiecentaur
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Agreed but, pour a load of electrons on something and its Potential will go in the negative direction. The OP was a bit too simple, of course, but electrons are at the coal face, here.
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Baluncore
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The conventional current flows from ground, down throught the current to voltage resistor to the anode. That is why the pulse is negative.hutchphd said:
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hutchphd
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But it does make a practical difference where ground potential is chosen. These were developed in the 1930's. I think I'll stick with door #1...Vanadium 50 said:
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tech99
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I have used a PMT tube with grounded cathode as part of a TV camera. But grounded cathode not useful for DC outputs referenced to ground.hutchphd said:
I think the PMT concept was invented by Philo Farnsworth as part of his Image Dissector TV camera.
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Vanadium 50
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Hamamatsu says PMTs were invented by Iams et al. in 1935 and the modern multidynode tubes in 1936 by Zworykin.
As far as the negative signal, had it been positive and someone said, "oh, it's common to have an odd number of inverting amplifiers in the base", we all would have nodded sagely.
As far as the negative signal, had it been positive and someone said, "oh, it's common to have an odd number of inverting amplifiers in the base", we all would have nodded sagely.
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hutchphd
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When I nod sagely it means I've dozed off...just give me a little jab..
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sophiecentaur
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I took the clue for the 'no output' level of zero. The question wording refers to the tube itself afaics. It's clearly been connected into a circuit that establishes a bias voltage of 0V. Then, along come the negative charges.
FAQ: Why does a photomultiplier tube produce a negative output pulse?
What is a photomultiplier output pulse?
A photomultiplier output pulse is a short burst of electrical current that is generated by a photomultiplier tube when it is exposed to light. This pulse is an amplified version of the original signal and is used to measure the intensity of the light that was detected.
How does a photomultiplier output pulse work?
A photomultiplier output pulse works by converting light into an electrical signal. When light enters the photomultiplier tube, it strikes a photocathode, which releases electrons. These electrons are then accelerated and multiplied by a series of dynodes, creating a larger electrical signal that is measured as the output pulse.
What factors affect the amplitude of a photomultiplier output pulse?
The amplitude of a photomultiplier output pulse is affected by several factors, such as the intensity of the incident light, the efficiency of the photocathode, and the gain of the dynodes. Other factors that can influence the amplitude include the temperature and voltage applied to the photomultiplier tube.
How is the time duration of a photomultiplier output pulse determined?
The time duration of a photomultiplier output pulse is determined by the time it takes for the electrons to travel from the photocathode to the last dynode. This time, also known as the transit time, is typically in the nanosecond range and can vary depending on the design of the photomultiplier tube.
What is the typical shape of a photomultiplier output pulse?
The typical shape of a photomultiplier output pulse is a fast rise followed by a slower decay. This shape is due to the time it takes for the electrons to reach the final dynode and the subsequent discharge of the dynode. The shape can also be affected by the circuitry used to measure the pulse and any external factors that may influence the signal.