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This article comes from the official account of Wechat: back to Park (ID:fanpu2019), author: Liu Hang

To detect the elementary particles that make up matter, you don't need a super-giant detector. You may not believe it, but we can build a simple detector at home-we can observe the trajectories of particles from distant universes.

Do you have any trouble doing scientific experiments for your children? Now you have a chance to make your child the prettiest boy in the class. Do it yourself, do a particle physics detection experiment, and you can see the tracks of elementary particles shuttling through the living room with a less complicated operation!

Although invisible to the naked eye, elementary particles are always around us as background radiation. Natural background radiation (natural radiation) sources include cosmic rays, radioactive decay of elements in rocks, and decay of radioactive elements in organisms (such as potassium-40 in bananas). Cosmic rays are extremely high-energy subatomic particles (mainly protons and nuclei accompanied by electromagnetic radiation) that travel through space and fly to Earth. When they hit the earth's atmosphere, they scatter with particles in the atmosphere, producing secondary particles that pass through the atmosphere to the earth's surface. When cosmic rays pass through the clouds, they produce ghostly particle tracks visible to the naked eye.

When it comes to particle detection, what is most often mentioned in the science news in recent years is the large Hadron Collider (Large Hadron Collider) of CERN, which is currently the top scientific device in the field of particle physics. The size of the probe is beyond our imagination: it weighs thousands of tons, contains millions of detection units, and is a research project supported by an international partnership of thousands of scientists.

But particle detectors are not always that big and complex. Some particle detectors can be very simple, and the kiloton LHC has been developed from the simplest detector. The cloud chamber is one of the oldest particle detectors, related to many discoveries in the history of particle physics, and led directly to the birth of two Nobel Prizes. British physicist Charles Wilson (Charles Thomson Rees Wilson,1869-1959), who wanted to study the formation and optical phenomena of clouds in moist air, inadvertently invented the cloud chamber, the first particle detector. He perfected the first (expanded) cloud chamber in 1911 and won the Nobel Prize in 1927. American physicist Carl Anderson (Carl David Anderson,1905-1991), who discovered positrons and muons using expansion cloud chambers in 1932 and 1936, won a Nobel Prize in 1936 for his discovery of positrons.

The study of early subatomic particles in the cloud chamber is very important, and with the continuous development of observation technology and accelerators, the original detectors remain in the long history. However, the method of studying the properties of particles by observing the trajectories of particles is still used in large accelerators and is one of the most important research methods in the field of particle physics. Following in the footsteps of history, Wilson was able to build a cloud room alone, which means that we also have the opportunity to build a cloud room in our own home, where we can observe the trajectory of particles.

Today, we introduce the method of making a home version of a cloud chamber detector, which uses evaporated alcohol to create a "cloud" to observe particles.

The working principle of the cloud chamber first, let's take a look at the specific working principle of the cloud room.

When a charged particle moves in the air, it will collide with the atmospheric molecules and ionize the air molecules. Gas molecules are ionized to attract polar molecules (water and alcohol are both polar molecules). At this time, if it is surrounded by supersaturated vapor of polar molecules, polar molecules can be liquefied with ionized gas molecules as condensation nuclei. In this way, polar molecules continue to gather and form cloud-like droplets that can be seen by the naked eye. So, if we see traces of "clouds", it means we see the trajectory of the particles.

In order to observe the above phenomena in the experiment, we need to build our "cloud chamber" by creating a "cloud" in an airtight container filled with supersaturated steam of polar molecules. Supersaturated steam refers to the vapor that exceeds the density of saturated steam at a certain temperature but is not liquefied or condensed. It is characterized by instability and liquefaction or sublimation if condensation occurs. Figuratively speaking, like dew on a blade of grass on a cool autumn morning, supersaturated steam in a container forms cloud droplets on anything it can stick to.

The cloud chamber can be achieved by different types of supersaturated vapors, which we use here with alcohol or isopropanol. In an airtight container, keep the top warm and the bottom cold. The warm top vaporizes the alcohol / isopropanol, and the vaporized vapor is liquefied at the bottom, so it is easy to form supersaturated vapor at the bottom of the container. When charged particles pass through vapor, they ionize air molecules. Alcohol / isopropanol in the vapor are polar molecules that are attracted to the ionized particles. It then condenses into a cloud and drops to the bottom of the container. The trajectory formed in the cloud chamber may look like the trajectory of an airplane-the slender lines mark the path of particles through the cloud chamber.

Building a cloud chamber requires the following materials and several simple steps:

Materials required:

A transparent plastic or glass container, such as a fish tank. The container must be kept airtight to ensure that the alcohol is supersaturated.

Isopropanol or alcohol (concentration 90% or higher). Wear goggles when handling, please avoid being touched by children

Felt, you can also use sponge or disposable paper towels and other materials that can absorb water. Alcohol and isopropanol are easily volatilized by heat. In order to fill the airtight container with enough supersaturated steam, blankets are required to absorb sufficient amounts of alcohol or isopropanol.

Fixture. To fix the felt in an airtight container, choose according to your actual container. Because there is isopropanol or alcohol steam in the container, it will dissolve and invalidate the glue. You can use gum or clay.

Dry ice (solid carbon dioxide, available online). This is to keep the bottom of the container at a low temperature. Wear thick gloves when handling dry ice. (note: if you want to try methods without dry ice, see Resources [5]).

A flat container and lid for holding dry ice. But be careful not to go too deep to block the view.

A black metal plate or paper large enough to cover the lid of a container. Black absorbs light and eliminates reflection, helping to observe the trajectories produced by particles. The metal plate has a better heat conduction effect to keep the lower end of the closed container at a low temperature.

lighting

The steps of the experiment:

1. Cut the felt to the size of the bottom of the fish tank and fix it at the bottom of the fish tank. After fixing the felt, soak it completely with isopropanol / alcohol, but do not leave liquid on it. Don't drink! )

two。 Put dry ice in a flat box.

3. First put the lid on the dry ice and put the black metal plate / paper on the lid.

4. Buckle the fish tank upside down on the lower box with the bottom facing up and the cover tight.

Now, our home version of the homemade cloud room has been completed! To maintain the upper temperature (heating alcohol to form alcohol vapor and supersaturated vapor in the cloud chamber), a cup of warm water can be placed at the top of the cloud chamber. For better observation, we can shine a flashlight on a transparent container in a dark environment. Everything is ready, and we only need to wait for about 10 minutes to see the trajectory left by the particles.

Particles through the cloud chamber you may see many different shapes of tracks, they are not left by the same kind of particles. There are several common types of tracks.

A short, thick track.

A short, thick trajectory (about 5cm length), indicating that the particles do not come from cosmic rays. It is most likely left by the decay of a radon atom in the atmosphere and the release of a helium nucleus. Radon is a naturally occurring radioactive element, but its concentration in the air is very low and its radioactivity is lower than that of peanut butter. Helium nuclei released from radon atoms are large and low in energy, so they leave short, thick tracks.

A long and straight track

The long and straight tracks come from secondary cosmic particles, such as the muon of the particle and its antiparticle positive μ. Pairs of positive and negative muons are produced when cosmic rays hit atmospheric molecules high in the atmosphere. They ionize the surrounding air molecules because they are so massive that they scatter from the air molecules, leaving a clean, straight track. Of course, these trajectories may also be the trajectories of high-speed electrons.

Crimp path

If the observed trajectory looks like the path of a lost tourist, you may have observed slow electrons or positrons. When cosmic rays hit atmospheric molecules, they produce the most pairs of electrons and positrons. It may also be slow electrons derived from the photoelectric effect. Slow electrons and positrons are lighter particles that bounce when they hit air molecules, leaving jagged and curly tracks.

A bifurcated trajectory

If the observed trajectory is bifurcated, you probably happen to see the decay of a particle. Many particles are unstable and decay into more stable particles. Of course, it could also be other processes (such as electron mu scattering).

There are more common advanced games in the cloud room, such as nuts, bananas and clay, which are all sources of radiation in our daily life. Please put them near the cloud chamber to observe the impact of increased radiation.

The cloud chamber provides an excellent opportunity to test the method of shielding radiation. Place different materials between the radioactive source and the cloud chamber: water, a piece of paper, sheet metal, our hands, and so on. Which material is more effective against radiation?

Try to add a magnetic field to the cloud chamber. Place the magnet next to the cloud chamber and observe that the positively and negatively charged particles will bend in the opposite direction.

More attempts, let us know more about physical phenomena. Is it too interesting to stop?

Particle detector is not only very important to the development of particle physics, but also extends to science, industry and even life, from drug development, medical imaging, analysis of ancient cultural relics, testing new materials, protecting astronauts and so on. This not only makes our lives safer and healthier, but also enriches our knowledge.

So go ahead and experience it for yourself!

references

[1] https://aip.scitation.org/doi/10.1063/1.1745504

[2] https://scoollab.web.cern.ch/sites/scoollab.web.cern.ch/files/documents/20200521_JW_DIYManual_CloudChamber_v7.pdf

[3] https://www.symmetrymagazine.org/article/january-2015/how-to-build-your-own-particle-detector

[4] https://www.symmetrymagazine.org/article/september-2014/detectors-in-daily-life

[5] https://www.youtube.com/watch?v=gt3Ad5_Z5IA

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