Light Speed: It’s not Just a Good Idea

[Kerry Wong] took apart a PM2L color analyzer (a piece of photography darkroom gear) and found a photomultiplier tube (PMT) inside. PMTs are excellent at detecting very small amounts of light, but they also have a very fast response time compared to other common detection methods. [Kerry] decided to use the tube to measure the speed of light.

There are several common methods to indirectly measure the speed of light by relating frequency to wavelength (for example, using microwave ovens and marshmallows). However, measuring it directly is difficult because of the scale involved. In only a microsecond, light travels almost 1000 feet (986 feet or 299.8 meters).

Continue reading “Light Speed: It’s not Just a Good Idea”

Creating a Scanning Monochromator

If you need a specific wavelength of light for research purposes, the naïve way of obtaining that is a white source light, a prism, and a small slit that will move across your own personal Dark Side of the Moon album cover. This is actually a terrible idea; not only won’t you have a reference of exactly what wavelength of light you’re letting through the optical slit, the prism itself will absorb more of one wavelength of light than others.

The solution is a monochromator, a device that performs the same feat of research without all the drawbacks. [Shahriar] got his hands on an old manual monochromator and decided to turn it into a device that performs automatic scans.

The key of a monochromator is a diffraction grating, a mirrored surface with many fine parallel grooves arranged in a step pattern. Because of the surface of the diffraction grating, it’s possible to separate light according to its spectrum much like a prism. Unlike a prism, it’s effectively a first surface mirror meaning all wavelengths of light are reflected more or less equally.

By adding a stepper motor to the dial of his monochromator, [Shahriar] was able to automatically scan across the entire range of the device. Inside the monochromator is a photomultiplier tube that samples the incoming light and turns it into a voltage. By sampling this voltage and plotting it with MATLAB, [Shahriar] was able to plot the intensity of every wavelength of light within the range of the device. It’s all expertly explained in the video below.

Continue reading “Creating a Scanning Monochromator”

Cooling a photomultiplier tube


A photomultiplier tube is a device used to measure very low levels of light. It’s a common tool of particle physics when trying to detect just a few photons. It turns out that running a tube at room temperature will not provide the best results. To improve the accuracy and sensitivity of his equipment [David Prutchi] built this thermoelectric photomultiplier tube cooling rig.

You can’t actually see the tube in this image but it looks similar to a vacuum tube or Nixie tube. The difference being that the components inside the glass dome make up the detector instead of an amplifier or filament display. To make a physical interface with the glass [David] wrapped it in magnetic shielding and finished with a layer of aluminum foil tape. This cylinder was then snugly fit inside of an aluminum heat sync. two Peltier coolers were attached to the outside of the heat sync, using Arctic Silver thermal compound to help transmit heat. A thermocouple was also added to monitor the temperature of this first stage of cooling. All of this fits into an aluminum enclosure which was filled with expanding spray foam before having a trio of fan-cooled heat syncs attached to it.

Large area x-ray detector

This is an x-ray detector built by [Ben Krasnow]. It’s an interesting combination of parts working with an oscilloscope. The result is an audible clicking much the same as you would hear from a Geiger counter

He’s measuring backscatter, which is the reflection of x-rays on other objects. Because the signal will be quite weak compared to waves emitted directly from an x-ray source he needed a large collector to measure them. He started by gutting an x-ray image intensifying cassette. This has a phosphor layer that glows when excited by x-rays. The idea is that the glowing phosphors do a better job of exposing film than direct x-rays can. But [Ben’s] not using film. He built that pyramid-shaped collector with the phosphor material as the base. At the apex of the pyramid he mounted a photomultiplier tube (repurposed from his scanning electron microscope) which can detect the excited points on its surface. His oscilloscope monitors the PMT, then issues a voltage spike on the calibration connector which is being fed to an audio amplifier. Don’t miss his presentation embedded after the break.

[Ben] mentions that this build is in preparation for a future project. We’d love to hear what you think he’s working on. Leave your guess in the comments section.

Continue reading “Large area x-ray detector”

Imploding Vacuum tubes for science

The researchers at Brookhaven National Laboratory are looking for a way to harden photomultiplier tubes. In order to make a more durable tube the researchers decided it would be a good idea to first observe how the tubes are failing. So they got their hands on an old torpedo test bay and smashed some bulbs inside of it. Check in after the break for some high fps bulb smashing.

Photomultiplier tubes are used in massive quantities to detect the highly elusive neutrino particle. The problem is when you have 50,000 photomultipliers submerged in pressurized water the the collapse of just a single bulb can cause a shock wave of destruction. This is what happened in japan in 2001 when a maintenance worker unknowingly compromised a single bulb in a 11,000 bulb array. When the tank was repressurized that single compromised bulb caused them to lose 7,000 more.

[via wired]

Continue reading “Imploding Vacuum tubes for science”