Photomultiplier tube facts for kids
Photomultiplier tubes (or PMTs for short) are super sensitive tools. They can detect even tiny amounts of light. This light can be ultraviolet (like from the sun), visible light (what we see), or near-infrared (heat light). PMTs are a special type of vacuum tube. They can make a tiny light signal up to 100 million times stronger! This helps scientists see very faint light, even single particles of light called photons.
PMTs are very useful because they can make signals much stronger (high gain). They also don't add much unwanted signal (low noise). They react very quickly to light and can collect light from a large area. Because of this, PMTs are important in many fields. These include studying light, confocal microscopy (for detailed images), Raman spectroscopy and fluorescence spectroscopy (for analyzing materials), nuclear and particle physics (studying tiny particles), and astronomy (looking at stars). They are also used in medical diagnostics like blood tests and medical imaging. You might find them in old movie film scanners (telecine) or special image scanners called drum scanners. Some parts of PMT technology are even used in night vision devices.
Newer devices like silicon photomultipliers and avalanche photodiodes can do similar jobs. However, PMTs are still the best choice for detecting very faint light that isn't perfectly focused.
Contents
How Photomultipliers Work
PMTs are usually made from a glass tube with all the air removed. Inside, there's a photocathode, several special plates called dynodes, and an anode.
Detecting Light
When light particles (called photons) hit the photocathode, they knock out electrons. This happens because of something called the photoelectric effect. The photocathode is often a thin, shiny layer inside the tube's window.
Multiplying Electrons
These first electrons are then guided by an electrode towards the electron multiplier. This part is made of many dynodes. Each dynode is set at a slightly higher electrical voltage than the one before it. This voltage difference pulls the electrons from one dynode to the next.
When an electron hits a dynode, it knocks out more electrons from that dynode. These new electrons then speed towards the next dynode, where they knock out even more electrons. This process creates a chain reaction, making more and more electrons at each step. For example, if each electron creates 5 new ones, and there are 12 dynodes, one original electron can become about 100 million electrons!
Getting the Signal
All these multiplied electrons finally reach the last stage, called the anode. When this huge number of electrons hits the anode, it creates a strong electrical pulse. This pulse is easy to detect, for example, on a device like an oscilloscope. It tells us that light hit the photocathode about 50 nanoseconds earlier.
To make the dynodes work, a special circuit called a voltage divider is used. This circuit makes sure each dynode has the correct voltage. Often, the photocathode is at a very negative voltage (like -1000 Volts). The anode, where the signal comes out, is usually close to zero volts.
Different Designs
PMTs come in different shapes. Some are "head-on" or "end-on," where light enters the flat top of the tube. Others are "side-on," where light enters from the side. The materials used for the photocathode and the way the dynodes are arranged also change how the PMT works. Manufacturers provide guides to help choose the right PMT for different jobs.
History of Photomultipliers
The invention of the photomultiplier came from two important discoveries: the photoelectric effect and secondary emission.
The Photoelectric Effect
In 1887, Heinrich Hertz first showed the photoelectric effect using ultraviolet light. This effect means that light can make electrons jump off a material. A couple of years later, Elster and Geitel showed it could happen with visible light on metals like potassium and sodium. Adding caesium later helped PMTs detect red light too.
Albert Einstein famously explained the photoelectric effect in 1905. This helped him develop the idea of quantum mechanics, for which he won the Nobel Prize in 1921. He showed that light acts like tiny packets of energy, or "quanta."
Secondary Emission
Secondary emission is when electrons hitting a surface cause even more electrons to be released from that surface. This idea was first reported in 1899 by Villard. In 1902, Austin and Starke found that more electrons came off a metal surface than hit it. After World War I, in 1919, Joseph Slepian suggested using this effect to make electrical signals stronger.
The Race for Television Cameras
In the 1920s, scientists were trying to build practical television cameras. Early cameras weren't sensitive enough. So, researchers worked on PMT technology to make cameras like the iconoscope and orthicon much better. The goal was to combine the photoelectric effect (light making electrons) with secondary emission (electrons making more electrons) to create a useful photomultiplier.
First Photomultipliers
In early 1934, a team at RCA in New Jersey, including Harley Iams and Bernard Salzberg, built the first single-stage photomultiplier. It had a photocathode and one secondary emission stage. This device could make signals about 8 times stronger.
Later in 1934, scientists wanted even stronger signals. Since one stage had limits, they needed multiple stages. The challenge was to guide the electrons from one stage to the next. Early multi-stage PMTs used strong magnetic fields to bend the electrons' paths.
In the USSR, Leonid A. Kubetsky built a multi-dynode photomultiplier in 1934. It could make signals 1000 times stronger! By 1935, Vladimir Zworykin, George Ashmun Morton, and Louis Malter at RCA also described their own multi-dynode tube.
Modern Photomultipliers
In the late 1930s, Jan A. Rajchman at RCA developed electrostatic photomultipliers. These didn't need magnetic fields and became the standard design. The first widely produced PMT, the Type 931, still uses this design today.
Also in 1936, P. Görlich found a much better photocathode material: Cs3Sb (caesium-antimony). This material was much more efficient at turning light into electrons. It was used in the first successful commercial PMTs.
Key Companies
For many years, RCA was a leader in developing and selling photomultipliers. They even published a helpful "Photomultiplier Handbook." After RCA broke up in the 1980s, their PMT business became an independent company called Burle Industries.
Later, in 2005, Photonis bought Burle Industries. However, in 2009, Photonis stopped making PMTs.
Since the 1950s, a Japanese company called Hamamatsu Photonics has become a major leader in the PMT industry. Hamamatsu also provides its own helpful handbook.
Photocathode Materials
Photocathodes can be made from different materials, each with unique properties. Some common ones include:
- Ag-O-Cs: Sensitive to light from near-infrared to visible. It can have high "dark current" (signal without light), so it's often cooled.
- Sb-Cs: Good for ultraviolet and visible light.
- Bialkali (Sb-K-Cs, Sb-Rb-Cs): Similar to Sb-Cs but more sensitive and with less noise. Great for detecting flashes from scintillators used in radiation detection.
- Multialkali (Na-K-Sb-Cs): Wide range of sensitivity from ultraviolet to near-infrared. Used in devices that measure light across many colors.
- Solar-blind (Cs-Te, Cs-I): Sensitive to ultraviolet light but not to visible light or infrared.
Window Materials
The glass "window" of the PMT also affects which wavelengths of light can get through.
- Borosilicate glass: Common for near-infrared to about 300 nm.
- Ultraviolet glass: Lets through visible and ultraviolet light down to 185 nm.
- Synthetic silica: Transmits even further into the ultraviolet, down to 160 nm.
- Magnesium fluoride: Transmits ultraviolet down to 115 nm.
Using Photomultipliers Safely
PMTs typically need high voltages, often 1000 to 2000 volts, to work. The most negative voltage goes to the cathode, and the most positive to the anode.
Protecting PMTs
When a PMT is turned on, it must be kept away from bright light. Too much light can damage or even destroy it. Sometimes, safety switches or covers are used to protect the tube when its compartment is opened. Another way is to have a circuit that lowers the high voltage if too much current is detected.
Magnetic Fields
Strong magnetic fields can cause problems for PMTs. They can bend the paths of the electrons inside, making the PMT less effective. Because of this, PMTs are often placed inside a special shield made of soft iron or mu-metal to block magnetic fields.
What Photomultipliers are Used For
PMTs were some of the first "electric eye" devices. They were used to detect when a beam of light was broken.
Detecting Radiation
Today, PMTs are often used with scintillators to detect Ionizing radiation (like X-rays or gamma rays). They are found in hand-held radiation detectors and in large physics experiments.
Scientific Research
Scientists use PMTs in labs to measure the brightness and colors of light from materials like compound semiconductors and quantum dots. They are also key parts of many spectrophotometers, which are instruments that measure how much light a substance absorbs.
Medical and Imaging Uses
PMTs are used in many medical devices. For example, flow cytometers use PMTs to analyze blood samples and find out what's in them. An array of PMTs is also used in a gamma camera for medical imaging. They are also used as detectors in flying-spot scanners for imaging.
High-Sensitivity Detection
Even though new electronic parts have replaced many vacuum tubes, PMTs are still very important. They are excellent at detecting very faint light signals. They can even detect individual photons (single particles of light). This is called photon counting.
When a PMT detects a single photon, it creates a very large electrical pulse. This shows that light is made of discrete particles, just as Albert Einstein explained.
PMTs can produce a small electrical current even when there's no light hitting them. This is called "dark current." For photon-counting jobs, PMTs are designed to have very low dark current.
Temperature Effects
Interestingly, at very cold temperatures, PMTs can sometimes show an increase in electron emission. Scientists are still trying to fully understand why this happens.
See also
- Lucas cell
- Scintillation counter
- Silicon photomultiplier
- Total absorption spectroscopy
