Subatomic Particles Homework Pass

Neutron, neutral subatomic particle that is a constituent of every atomic nucleus except ordinary hydrogen. It has no electric charge and a rest mass equal to 1.67493 × 10−27 kg—marginally greater than that of the proton but nearly 1,839 times greater than that of the electron. Neutrons and protons, commonly called nucleons, are bound together in the dense inner core of an atom, the nucleus, where they account for 99.9 percent of the atom’s mass. Developments in high-energy particle physics in the 20th century revealed that neither the neutron nor the proton is a true elementary particle; rather, they are composites of extremely small elementary particles called quarks. The nucleus is bound together by the residual effect of the strong force, a fundamental interaction that governs the behaviour of the quarks that make up the individual protons and neutrons.

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>neutrons. But these basic atomic components are by no means the only known subatomic particles. Protons and neutrons, for instance, are themselves made up of elementary particles called quarks, and the electron is only one member of a class of elementary particles that also includes…


The neutron was discovered in 1932 by the English physicist James Chadwick. Within a few years after this discovery, many investigators throughout the world were studying the properties and interactions of the particle. It was found that various elements, when bombarded by neutrons, undergo fission—a type of nuclear reaction that occurs when the nucleus of a heavy element is split into two nearly equal smaller fragments. During this reaction each fissioned nucleus gives off additional free neutrons, as well as those bound to the fission fragments. In 1942 a group of American researchers, under the leadership of the physicist Enrico Fermi, demonstrated that enough free neutrons are produced during the fission process to sustain a chain reaction. This development led to the construction of the atomic bomb. Subsequent technological breakthroughs resulted in the large-scale production of electric power from nuclear energy. The absorption of neutrons by nuclei exposed to the high neutron intensities available in nuclear reactors has also made it possible to produce large quantities of radioactive isotopes useful for a wide variety of purposes. Furthermore, the neutron has become an important tool in pure research. Knowledge of its properties and structure is essential to an understanding of the structure of matter in general. Nuclear reactions induced by neutrons are valuable sources of information about the atomic nucleus and the force that binds it together.

A free neutron—one that is not incorporated into a nucleus—is subject to radioactive decay of a type called beta decay. It breaks down into a proton, an electron, and an antineutrino (the antimatter counterpart of the neutrino, a particle with no charge and little or no mass); the half-life for this decay process is 614 seconds. Because it readily disintegrates in this manner, the neutron does not exist in nature in its free state, except among other highly energetic particles in cosmic rays. Since free neutrons are electrically neutral, they pass unhindered through the electrical fields within atoms and so constitute a penetrating form of radiation, interacting with matter almost exclusively through relatively rare collisions with atomic nuclei.

Neutrons and protons are classified as hadrons, subatomic particles that are subject to the strong force. Hadrons, in turn, have been shown to possess internal structure in the form of quarks, fractionally charged subatomic particles that are thought to be among the fundamental components of matter. Like the proton and other baryon particles, the neutron consists of three quarks; in fact, the neutron possesses a magnetic dipole moment—i.e., it behaves like a minutemagnet in ways that suggest that it is an entity of moving electric charges.

DIY Subatomic Particle Detector

March 19, 2015

You and everything around you is made of subatomic particles; they’re the smallest and most fundamental building blocks of matter. They’re everywhere, yet you’ve never seen them. Here’s a way to reveal them at home.

All you need is: a jar, a sponge, rubbing alcohol, a flashlight, a black marker, and some dry ice.

Now we just need a subatomic particle. But on Earth, most subatomic particles, like electrons, are bound inside atoms. Lucky for us, when stars die, they can explode, shooting pieces of atoms across the universe. Some eventually reach Earth’s atmosphere. Here they collide with molecules in our air, bursting into showers of electrons and other subatomic particles, like muons. They’re shooting invisibly—all around you, and through you. Here’s how to actually see them fly by.

First stuff the sponge in the bottom of the jar and pour some alcohol on it.

Then color the inside of the lid black and place it on the jar.

Pour out a pile of dry ice.

Turn the jar upside down on the dry ice.

Wait several minutes for the lid to cool down.

Shine your flashlight over the lid, turn off all other lights, and look closely for a line of small droplets to appear.

Here’s what’s happening. As alcohol gas vapor floats down from the sponge toward the freezing-cold lid, it becomes supersaturated, meaning any disturbance will cause the gas to condense into a cloud of liquid droplets. So, if an electron or muon from a cosmic shower shoots through the vapor, a cloud of droplets forms along its path.

As amazing as it may seem, every line that you see in your jar comes from a subatomic particle. You've revealed the smallest building blocks of the universe, as they shoot past your eyes, all the way from distant star.


Produced, animated, and edited by
Greg Kestin
A special thanks to:
Anna Rothschild
Lauren Aguirre
Ari Daniel
Demonstration and lighting technician:
Allen Crockett
Original Footage
© WGBH Educational Foundation 2015
Media Credits:
“Microscopy” APM Music, Music, SFX


(main image: DIY experiment)
©WGBH Educational Foundation 2015


U.S. Dry Ice Distributor Directory
Outside the U.S.
Some grocery stores also carry dry ice.


Materials: Jar & lid, sponge, 91% rubbing alcohol (or greater % purity; not 70%), permanent black marker, flashlight, and dry ice.

  • Step 1: Stuff the sponge in the bottom of the jar so it doesn’t fall down when the jar is turned upside-down.

  • Step 2: Pour a bit of alcohol on the sponge, but not too much that it will drip or fall when turned upside-down.

  • Step 3: Color as much of the inside of the lid black as you can. This is so there is contrast with the droplets, which will appear white as they reflect the flashlight’s light. Thin black paper attached to the lid may also work, but it must be thin enough so the cold lid still cools down the bottom of the jar.

  • Step 4: Put the lid on the jar.

  • Step 5: Put on well-insulated gloves and pour out a pile of dry ice.

  • Step 6: Place the jar upside-down on the dry ice. You may want to surround the edge of the lid with dry ice, so it cools down faster.

  • Step 7: Shine your flashlight over the lid (try various angles to get best visibility of droplets).

  • Step 8: Turn off all other lights.

  • Step 9: Look for lines of droplets. You should be able to see droplets in the vapor falling down to the lid like snow, but a few times a minute (or more frequently) you will see a line of droplets appear and fall down to the lid. When you see a line, it was from a subatomic particle shooting through your jar!


  • The setup here is a simple version of a cloud chamber.

  • Subatomic particles include both elementary particles, such as electrons and muons, as well as nuclei such as protons or alpha particles. "The smallest and most fundamental building blocks of matter" refers to the former.

  • The “pieces of atoms” from exploded stars include predominantly protons, alpha particles, and some heavier nuclei.

  • The showers of subatomic particles are called “air showers.” While the subatomic particles that eventually reach the earth are mostly muons, electrons, positrons and photons, higher up in the shower protons, neutrons, pions, and kaons are also present.

  • “Cosmic rays” are the high energy particles that bombard the earth’s atmosphere, creating “air showers."

  • When a charged particle passes through the alcohol vapor, some of it is ionized (becomes electrically charged), and the neighboring vapor is attracted to the ions, forming visible droplets. This is the “disturbance” that muons or electrons create as they pass through the vapor.
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