The seemingly ordinary "little light bulb" is actually an amazing "big technology"

Produced by: Science Popularization China

Author: Tan Nian (PhD in Physics)

Producer: China Science Expo

According to a recent report by CCTV News (October 11, 2024), the world's largest single plexiglass sphere was built in China. This large glass sphere with a diameter of 35.4 meters is located 700 meters underground in Jiangmen, Guangdong, and is a key component of the detector of the Jiangmen Neutrino Experiment Center.

In the photos taken by CCTV News, the most eye-catching thing is that the inside of the big glass ball is filled with "small light bulbs". What are the uses of these seemingly "ordinary" small light bulbs?

In fact, their true identity is "photomultiplier tubes" , which are vacuum devices that can detect single photons and are used for neutrino detection.

The exterior of the plexiglass sphere of the Jiangmen Neutrino Experiment

(Photo source: CCTV News)

Rows of "small light bulbs" inside the plexiglass ball

(Photo source: CCTV News)

“Ordinary” Photomultiplier Tube

The ordinary-looking photomultiplier tube looks like a light bulb that we usually buy in stores for a few dollars, but the international price is $3,000 each! Because it is not actually a light bulb, but a 20-inch photomultiplier tube, which is full of technological content and much more expensive than a light bulb.

Photomultiplier tubes waiting to be tested

(Image source: Institute of High Energy Physics, Chinese Academy of Sciences)

A photomultiplier tube (PMT) is a vacuum electronic device that can convert light signals into electrical signals.

Simply put, it can convert incident photons into electrons through the photoelectric effect (the effect that won Einstein the Nobel Prize in Physics). A small number of electrons will undergo secondary electron emission under the action of the multiplying electric field, which can be multiplied and amplified to obtain more electrons. These amplified electrons are collected through the anode and then output [1].

When incident photons hit the cathode, photoelectric effect occurs to produce electrons. Under the action of the multiplying electric field, the electrons continuously fly to the multiplying electrode to produce secondary electrons. After multiple multiplications, the signal is amplified.

(Image source: Wikipedia)

Imagine that you are in a pitch-black environment and your eyes can't see anything, but the magical photomultiplier tube has sharp eyes. It can sensitively and quickly capture those extremely weak light signals and convert them into stronger electrical signals, helping you to accurately see these extremely weak light signals.

Because of their unique advantages such as high sensitivity and high response speed, photomultiplier tubes are widely used in space technology, archaeology, medicine, geology, biology, astronomy, metallurgy, chemistry, agriculture, and military fields. They are used in equipment such as photon counting, weak light detection, chemiluminescence, bioluminescence, cosmic ray detectors, spectrophotometers, colorimeters, and biochemical analyzers [2].

Neutrinos that come and go without a trace

In the standard model of particle physics that explains the composition of everything in the world, neutrinos are one of the fundamental particles that make up our material world. The most notable feature of neutrinos is that they hardly interact with matter and have very strong penetrating power, which makes it very difficult for scientists to detect neutrinos in experiments.

There are 12 basic particles that make up the material world, including 6 quarks, 3 charged leptons and 3 neutrinos.

(Image source: Institute of High Energy Physics, Chinese Academy of Sciences)

You might think that neutrinos should be rare, but the fact is that there are actually a lot of neutrinos in our lives, almost everywhere. 300 trillion solar neutrinos pass through your body every second, the bananas you eat are constantly producing neutrinos, and even the human body itself is constantly producing neutrinos through the decay of potassium 40. So why don't you notice it?

Because these neutrinos have very strong penetrating power, they can not only pass through the human body almost unimpeded, but can even pass through the earth and the sun. This extremely strong penetrating power makes neutrinos "come and go without a trace" and difficult to capture, which also earns neutrinos the name "ghost particles."

Scientists, however, do not believe in superstition and hope to detect such particles through neutrino experiments. Although neutrinos themselves cannot be detected directly, when a large number of neutrinos pass through the detector, a small number of them have a chance to be captured by the working material of the detector and react to generate observable photons, which are then collected and amplified by photomultiplier tubes, which is equivalent to indirectly seeing the neutrinos.

Detecting neutrinos is very difficult. Modern large-scale neutrino experiments often require tens of thousands of tons of material, leaving a large amount of working material waiting to be detected. A few of the neutrinos in a large number are captured by the detector by chance. Taking the Jiangmen Neutrino Experiment as an example, 20,000 tons of liquid scintillator can only detect 60 reactor neutrinos, 4 atmospheric neutrinos, 1 earth neutrino, and 90 boron-8 solar neutrinos per day. In comparison, there are 100,000 cosmic rays as background, and this is the result after the detector is placed 700 meters underground and the cosmic ray flux is reduced by 200,000 times [3].

Realization of the domestically-made Jiangmen neutrino experiment

The detection of neutrinos is inseparable from photomultiplier tubes, especially large-sized photomultiplier tubes that can achieve higher photon detection efficiency and help scientists find weak neutrino signals.

In the international market, a Japanese company has a monopoly on photomultiplier tubes. It can be said that this company is truly the only one in the world in the field of photomultiplier tube manufacturing. Especially for large-sized photomultiplier tubes, before 2016, only Japanese companies in the world were capable of producing them. The world's largest 20-inch photomultiplier tube is priced at $3,000 each, and buyers can only bite the bullet and place an order because it is the only one in the world.

At that time, Japan's Super-Kamiokande neutrino experiment ordered more than 11,000 such large-size photomultiplier tubes from domestic companies. The cost of photomultiplier tubes alone was more than 30 million US dollars (according to the exchange rate at the time, it was more than 200 million RMB).

The 20-inch photomultiplier tube looks inconspicuous and ordinary, and looks like a light bulb. Although it is expensive, it is worth the money.

Scientists' neutrino experiments cannot do without it. From the Kamioka experiment to the Super-Kamiokande experiment, the photomultiplier tube "helped" the Japanese win two Nobel Prizes in Physics. One was in 2002 when Masatoshi Koshiba won the award for the discovery of supernova neutrinos, and the other was in 2015 when Takaaki Kajita and others won the award for the discovery of neutrino oscillations.

The "Jiangmen Neutrino Experiment" large scientific facility that Chinese scientists began to build in 2015 requires the use of 20,000 20-inch photomultiplier tubes. If all of them are purchased from Japanese companies, it will be a very large expense.

How to reduce experimental costs and achieve great results with little money?

It is better to make large-sized photomultiplier tubes than to buy them!

Therefore, before the Jiangmen neutrino experiment started, scientists from the Institute of High Energy Physics of the Chinese Academy of Sciences had already made sufficient preparations.

Chinese scientists successfully developed a 20-inch photomultiplier tube in the laboratory

(Image source: Institute of High Energy Physics, Chinese Academy of Sciences)

Led by the Institute of High Energy Physics of the Chinese Academy of Sciences, a collaboration group consisting of China North Industries Group Corporation North Night Vision, Xi'an Institute of Optics and Precision Mechanics of the Chinese Academy of Sciences, China National Nuclear Control Systems Co., Ltd. and Nanjing University worked together to overcome multiple technical difficulties such as high quantum efficiency photocathode preparation technology, microchannel plates, large-size glass shells, and vacuum optoelectronic device packaging technology, and finally developed a sample tube whose key technical indicators such as quantum efficiency, collection efficiency and single photoelectron peak-to-valley ratio reached the international advanced level.

This achievement has made a breakthrough in the manufacturing technology of 20-inch photomultiplier tubes. The new type of photomultiplier tubes has been domestically produced and has complete independent intellectual property rights, which has greatly enhanced the innovation ability and international competitiveness of domestic enterprises in ultra-large vacuum devices [4].

Wang Yifang, then director of the Institute of High Energy Physics of the Chinese Academy of Sciences, and the photomultiplier tube

(Image source: Institute of High Energy Physics, Chinese Academy of Sciences)

Northern Night Vision has established a special production line for producing large-size photomultiplier tubes. These 20-inch photomultiplier tubes can not only be used in the Jiangmen neutrino experiment, but also can be used to measure weak light signals. For example, the high-altitude cosmic ray observatory LHAASO in Sichuan also uses 20-inch photomultiplier tubes produced by Northern Night Vision to detect cosmic rays.

Our large-size photomultiplier tubes are the photomultiplier tubes with the highest photon detection efficiency in the world, breaking the international monopoly in this field and obtaining patent authorizations from the European Union, the United States, Japan, etc. This is a successful example of cross-border cooperation between the scientific community and the industrial community to promote industrial technological progress through large scientific facilities. We look forward to more such achievements in the future!

References:

1. The invention of the photomultiplier tube: http://www.ihep.cas.cn/kxcb/kjqy/201702/t20170204_4741583.html

2. Application of photomultiplier tubes: http://www.ihep.cas.cn/kxcb/kjqy/201702/t20170204_4741602.html

3. Cao Jun: The history and future of neutrino research, Modern Physics Knowledge 2015, 27(6): 4-8.

4. Breakthrough in Photomultiplier Tube Manufacturing Technology: http://news.sciencenet.cn/htmlnews/2016/11/361915.shtm