How Photomultipliers Detect Single Photons

September 12, 2024 by Maya Posch 12 Comments

If you need to measure the presence of photons down to a very small number of them, you are looking at the use of a photomultiplier, as explained in a recent video by [Huygens Optics] on YouTube. The only way to realistically measure at such a sensitivity level is to amplify them with a photomultiplier tube (PMT). Although solid-state alternatives exist, this is still a field where vacuum tube-based technology is highly relevant.

Despite being called ‘photomultipliers’, these PMTs actually amplify an incoming current (electron) in a series of dynode stages, to create an output current that is actually easy to quantify for measurement equipment. They find uses in everything from Raman spectroscopy to medical diagnostics and night vision sensors.

The specific PMT that [Huygens Optics] uses in the video is the Hamamatsu R928. This has a spectral response from 185 nm to 900 nm. The electrode mesh is where photons enter the tube, triggering the photo cathode which then ejects electrons. These initial electrons are then captured and amplified by each dynode stage, until the anode grid captures most of the electrons. The R928 has a gain of 1.0 x 10⁷ (10 million) at -1 kV supply voltage, so each dynode multiplies the amount of electrons by six, with a response time of 22 ns.

PMTs are unsurprisingly not cheap, but [Huygens Optics] was lucky to find surplus R928s on Marktplaats (Dutch online marketplace) for €100 including a cover, optics and a PCB with the socket, high-voltage supply (Hamamatsu C4900) and so on. Without documentation the trick was to reverse-engineer the PCB’s connections to be able to use it. In the video the components and their function are all briefly covered, as well as the use of opamps like the AD817 to handle the output signal of the R928. Afterwards the operation of the PMT is demonstrated, which makes clear just how sensitive the PMT is as it requires an extremely dark space to not get swamped with photons.

An interesting part about the demonstration is that it also shows the presence of thermionic emissions: anode dark current in the datasheet. This phenomenon is countered by cooling the PMT to prevent these emissions if it is an issue. In an upcoming video the R928 will be used for more in-depth experiments, to show much more of what these devices are capable of.

Thanks to [cliff claven] for the tip.

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Russell Kirsch: Pixel Pioneer And The Father Of Digital Imaging

August 20, 2020 by Kristina Panos 28 Comments

It’s true what they say — you never know what you can do until you try. Russell Kirsch, who developed the first digital image scanner and subsequently invented the pixel, was a firm believer in this axiom. And if Russell had never tried to get a picture of his three-month-old son into a computer back in 1957, you might be reading Hackaday in print right now. Russell’s work laid the foundation for the algorithms and storage methods that make digital imaging what it is today.

Russell reads SEAC’s last printout. Image via TechSpot

Russell A. Kirsch was born June 20, 1929 in New York City, the son of Russian and Hungarian immigrants. He got quite an education, beginning at Bronx High School of Science. Then he earned a bachelor’s of Electrical Engineering at NYU, a Master of Science from Harvard, and attended American University and MIT.

In 1951, Russell went to work for the National Bureau of Standards, now known as the National Institutes of Science and Technology (NIST). He spent nearly 50 years at NIST, and started out by working with one of the first programmable computers in America known as SEAC (Standards Eastern Automatic Computer). This room-sized computer built in 1950 was developed as an interim solution for the Census Bureau to do research (PDF).

Standards Eastern Automatic Computer (SEAC) was the first programmable computer in the United States. Credit: NIST via Wikimedia

Like the other computers of its time, SEAC spoke the language of punch cards, mercury memory, and wire storage. Russell Kirsch and his team were tasked with finding a way to feed pictorial data into the machine without any prior processing. Since the computer was supposed to be temporary, its use wasn’t as tightly controlled as other computers. Although it ran 24/7 and got plenty of use, SEAC was more accessible than other computers, which allowed time for bleeding edge experimentation. NIST ended up keeping SEAC around for the next thirteen years, until 1963.

The Original Pixel Pusher

This photo of Russell’s son Walden is the first digitized image. Public Domain via Wikimedia

The term ‘pixel’ is a shortened portmanteau of picture element. Technically speaking, pixels are the unit of length for digital imaging. Pixels are building blocks for anything that can be displayed on a computer screen, so they’re kind of the first addressable blinkenlights.

In 1957, Russell brought in a picture of his son Walden, which would become the first digital image (PDF). He mounted the photo on a rotating drum scanner that had a motor on one end and a strobing disk on the other. The drum was coupled to a photo-multiplier vacuum tube that spun around on a lead screw. Photo-multipliers are used to detect very low levels of light.

As the drum slowly rotated, a photo-multiplier moved back and forth, scanning the image through a square viewing hole in the wall of a box. The tube digitized the picture by transmitting ones and zeros to SEAC that described what it saw through the square viewing hole — 1 for white, and 0 for black. The digital image of Walden is 76 x 76 pixels, which was the maximum allowed by SEAC.

Variable-Shaped Pixels

If Russell Kirsch had any regrets, it is that he designed pixels to be square. Ten years ago at the age of 81, he started working on a variable-shaped pixels with the hope of improving the future of digital imaging. He wrote a LISP program to explore the idea, and simulated triangular and rectangular pixels using a 6×6 array of square pixels for each.

Alternative pixel geometries. Image via Cloudseed Films

In in the video below, Russell discusses the idea and proves that variable pixels make a better image with more information than square pixels do, and with significantly fewer pixels overall. It takes some finagling, as pixel pairs of triangles and rectangles must be carefully chosen, rotated, and mixed together to best represent the image, but the image quality is definitely worth the effort. Following that is a video of Russell discussing SEAC’s hardware.

Russell retired from NIST in 2001 and moved to Portland, Oregon. As of 2012, he could be found in the occasional coffeehouse, discussing technology with anyone he could engage. Unfortunately, Russell developed Alzheimer’s and died from complications on August 11, 2020. He was 91 years old.

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Warwalking For Radiation

September 10, 2019 by Elliot Williams 24 Comments

Can’t find a recently updated survey of radioactivity in your neighborhood? Try [Hunter Long]’s DIY scintillation counter warwalking rig. (Video also embedded below.) What looks like a paint can with a BNC cable leading to an unassuming grey box is actually a complete kit for radiation surveying.

Inside the metal paint can is a scintillation counter, which works by attaching something that produces light when struck by ionizing radiation on the end of a photomultiplier tube, to make even the faintest hits “visible”. And the BNC cable leads to a Raspberry Pi, touch screen, GPS, and the high-voltage converters needed to make the photomultiplier do its thing.

The result is a sensitive radiation detector that logs GPS coordinates and counts per second as [Hunter] takes it out for a stroll. Spoilers: he discovers that some local blacktop is a little bit radioactive, and even finds a real “hot spot”. Who knows what else is out there? With a rig like this, making a radiation map of your local environment is a literal walk in the park.

[Hunter] got his inspiration for the paint-can detector from this old build by [David Prutchi], which used a civil-defense Geiger counter as its source of high voltage. If you don’t have a CD Geiger detector lying around, [Alex Lungu]’s entry into the Hackaday Prize builds a scintillation detector from scratch.
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Tearing Down A Darkroom Relic For Buried Treasure

January 6, 2018 by Dan Maloney 10 Comments

If your goal is to harvest unique parts from defunct devices, the further back in time you go, the better the pickings stand to be. At least that’s what [Kerry Wong] discovered during his tear-down of a darkroom color analyzer from the early 1980s.

For readers whose experience with photography has been solely digital, you need to understand that there once was a time when images were made with real cameras on real film, and serious amateurs and pros had darkrooms to process the film. Black and white processing was pretty straightforward in terms of chemistry — it was just developer, stop, and fixing. Color processes were much trickier, and when it came to enlarging your film onto color photo paper, things could get really complicated. [Kerry]’s eBay find, a Besler PM1A color analyzer, was intended to help out in the color lab by balancing the mix of cyan, blue, and yellow components in the enlarger.

The instrument, which no doubt demanded a princely sum back in the day, is actually really simple, with the object of [Kerry]’s desire, a PM1A photomultiplier tube and its driver, being the only real find. Still, it’s an interesting teardown, and we’re eager to see what [Kerry] makes of the gem. A muon detector, perhaps? An X-ray backscatter machine? Or perhaps repeating his old speed of light experiments is on the docket.

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Arduino Does Hard Science

June 28, 2017 by Al Williams 10 Comments

We don’t know why [stoppi71] needs to do gamma spectroscopy. We only know that he has made one, including a high-voltage power supply, a photomultiplier tube, and–what else–an Arduino. You also need a scintillation crystal to convert the gamma rays to visible light for the tube to pick up.

He started out using an open source multichannel analyzer (MCA) called Theremino. This connects through a sound card and runs on a PC. However, he wanted to roll his own and did so with some simple circuitry and an Arduino.

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