RADIOACTIVITY AND NUCLEAR REACTIONS

Chapter 18

The NucleusProtons and neutrons are packed tightly in the nucleus, where you find the majority of the atom’s mass.

The Strong Force Since protons repel each other, the strong force allows protons and neutrons to be attracted to each other. This is 4 x stronger than electric force! Unlike electricity, the strong force is a short-range force. When protons and neutrons move away from each other, the force weakens.

Strong Force vs. Large NucleiBecause there are so many protons and neutrons in large nuclei, the electric repulsion of all the protons is stronger than then short-range strong force. Therefore, protons and neutrons are held together less tightly in a large nucleus.

Instability and Radioactivity When the strong force is not strong enough to hold the nucleus permanently together, the nucleus begins to decay and give off matter and energy (radioactivity.) All nuclei containing more than 83 protons are radioactive (although there are smaller radioactive nuclei.) Synthetic elements, produced in a lab, contain a large number of protons (92+), so they decay soon after they are created.

Radioactivity

Isotopes

You already know that an isotope is an atom with a different number of neutrons to protons. Isotopes of heavier elements are stable when the ratio of neutrons to protons is about 3 to 2. If the ratios are different (less or greater), than the nucleus is considered unstable. This makes those isotopes radioactive.

You can tell if an atom is radioactive by comparing the mass number (p + n) to the atomic number (p):

mass number 12

C mass number 14

C atomic number 6 atomic number 6

Which atom of carbon is radioactive?

The Discovery of Radioactivity

In 1896, Henri Becquerel left uranium salt in a desk drawer with a photographic plate. When he opened the plate later, he found an outline of clumps of the uranium salt. He realized that the saltss must have emitted some unknown invisible rays, or radiation, that darkened the film (it was an x-ray of the salt!)

In 1898, Marie and Pierre Curie discovered 2 new elements (polonium and radium), that were also radioactive. They were able to obtain .1 g of radium for several tons of the mineral pitchblende after more than 3 years of experiments.

Nuclear Decay

The 3 types of nuclear radiation are alpha, beta and gamma radiation. Alpha and beta are particles, and gamma radiation is the resulting electromagnetic wave.

Nuclear (Radioactive) Decay

Alpha Particles

An alpha particle is made up of 2 protons and 2 neutrons that are emitted from the decaying nucleus. It is the same as a He nucleus and has a charge of +2 and an atomic mass of 4. It actually

becomes 4 He 2

Alpha particles are the largest with the most charge. They lose energy quickly when reacting with matter. When they go through matter, the electrons in the matter’s atoms react and are pulled away. This leaves behind positively charged ions. Because alpha particles quickly lose energy, they are the least penetrating form of nuclear radiation. They can be stopped by a sheet of paper.

Alpha ParticlesAlpha decay occurs when the nucleus of an atom is so

heavy it needs to lose both protons and neutrons.

Alpha particles can be dangerous inside the human body, as a single alpha particle can damage fragile biological molecules.

Alpha particles can be used in some smoke detectors. The detectors ionize the surrounding air. An electric current flows through this air to form a circuit, unless smoke particles break the circuit, causing the alarm to sound.

Transmutation

An atom loses 2 protons when it emits alpha particles, so it forms into a different element. Transmutation is the process of changing one element to another through nuclear decay.

The new element has 2 fewer protons and decreases the mass number by 4. The charge of the original nucleus = the sum of the charges of the nucleus and the alpha particle that are formed.

Transmutation of Polonium into LeadPolonium (Po) loses 2 protons (out of 84) and the mass

number is decreased by 4:

210Po 206Pb + 4He+84 +82 +2

Beta Particles

Sometimes an unstable nucleus in a neutron decays into a proton, and then it emits an electron (this is what can damage cells.) This electron is called a beta particle. Beta decay is caused by a weak force.

Now the atom has one more proton than it did before the decay. This means that it goes through transmutation. However, the mass number doesn’t change during beta decay, so the mass number is the same as the original element.

When can Beta Decay Occur?• When an atom has far greater neutrons than protons.

When an atom has far greater protons than neutrons.

When an atom has far greater electrons than protons.

When an atom has far greater protons than electrons.

When an atom has far greater electrons than neutrons.

Beta Decay: Iodine Changing into Xenon

131 131 I + Xe +53 -1 +54

Beta Particles (with a charge of -1) are faster and more penetrating than alpha particles. They can go through paper (but not a sheet of aluminum foil.) They can also cause damage to biological cells.

Gamma Rays

The most penetrating form of nuclear radiation (EM wave with highest frequency, shortest wavelength.)

They are energy (no mass or charge.) They are emitted from the nucleus when alpha or beta decay occurs. They can penetrate almost all solids except for exceptionally dense materials such as lead or concrete. However, because they produce no charge, they can actually do less damage than alpha particles.

Nuclear Decay Song

• http://www.youtube.com/watch?v=J8p7OIdyt54&safety_mode=true&persist_safety_mode=1&safe=active

Radioactive Half-Life and DatingA measure of the time required by the nuclei of an isotope to

decay is called a half-life. The half-life of a radioactive isotope is the amount of time takes for half the nuclei in a sample of the isotope to decay.

Carbon Dating: Used to date once-living things. C14 has a half-life of 5,730 years, found in molecules of CO2. This is found in plants and in plant-eating animals.

Uranium Dating: Some rocks contain uranium isotopes. These isotopes decay into lead isotopes. The ratio of the uranium isotopes and the daughter nuclei (lead isotopes) is measured and the number of half-lives since the rock was formed can be calculated.

Detecting Radioactivity

Cloud Chambers: filled with water or ethanol vapor, in which a radioactive sample is placed. It gives off charged alpha or beta particles, which travel through the chamber. The particle knocks electrons of the air atoms in the chamber, creating ions. The water/ethanol vapor condenses around the ions, creating a visible path of droplets along the track of the particle (alpha = short, thick trails; beta = long, thin trails.)

Bubble Chambers

A bubble chamber holds superheated liquid (pressure prevents boiling.) When a moving particle leaves ions behind, the liquid boils along the trail.

Electroscopes

The electroscope has leaves at the bottom. If the scope gets an electric charge, the leaves repel each other. When an positive charge is introduced, then the excess negative charge is neutralized.

Nuclear radiation moving through the air can remove electrons from some air molecules, causing other air molecules to gain electrons. These positive air ions come in contact with the electroscope and attract the electrons from the leaves, so the leaves move back together.

Geiger Counter

A Geiger counter measures the amount of radiation by producing an electric current when it detects a charged particle.

It has a tube with a positive charge running through the center of a negatively charged copper cylinder. The tube is filled with gas at a low pressure. When radiation enters the tube at one end, It knocks electrons from the atoms of the gas. Then causes a chain reaction among the gas atoms, creating an “electron avalanche”. When a large number of electrons reaches the wire, a current is produced. The energy is turned into sound (clicking) and flashing light. The intensity of these energies measures the intensity of the radiation.

Geiger Counter Sound

• http://www.youtube.com/watch?v=upPiJ9vOYiY&feature=related&safety_mode=true&persist_safety_mode=1&safe=active

Background Radiation

Low-level radiation is naturally emitted by Earth’s rocks, soil and atmosphere. Traces can be found in building materials, plants and animals.

Most of the background radiation comes from the decay of radon gas. High levels can be very dangerous when it is trapped in homes. Sometimes radiation from cosmic rays can infiltrate the Earth’s atmosphere. Living organisms contain C14. Background radiation can never be completely eliminated.

Nuclear Reactions

Nuclear Fission

• Enrico Fermi thought that, by bombarding nuclei with neutrons, the neutrons would be absorbed (creating larger nuclei.) Instead, when a neutron struck a uranium-235 nucleus, it split apart into smaller nuclei. Fission means to divide.

• Only large nuclei can undergo nuclear fission. The products of fission includes both smaller nuclei and random neutrons. Some of the mass after fission is missing, because it turns into extreme energy. Einstein was the first to realize that the Law of Conservation of Mass and the Law of Conservation of Energy are actually tied to each other.

Nuclear Fission

Fission: Mass and Energy

Einstein’s Theory of Relativity proposed that mass can be converted into energy, and energy could be converted into mass:

Energy (J) = mass (kg) x 300,000 km/s2 (speed of light)Or

E = mc2

If one gram of mass is converted into energy, then approximately 100 trillion joules of energy is released.

Fission: Chain Reactions

During nuclear fission, the neutrons emitted can strike other nuclei and cause them to split, thus releasing more neutrons. A series of repeated fission reactions cause the release of neutrons during each reaction.

Critical Mass is the amount of material required so that each fission reaction produces at least one more fission reaction. If there is not enough material, critical mass is not reached, and there is no chain reaction.

If the chain reaction isn’t controlled, then an enormous amount of energy is released. Chain reactions can be controlled by adding materials that absorb neutrons. Then the reaction continues at a constant rate.

How a Nuclear Power Plant Works

• http://www.youtube.com/watch?v=oMbfHwRdess&safety_mode=true&persist_safety_mode=1&safe=active

Nuclear Fusion

Even more energy can be released when 3 nuclei with low masses are combined to form one nucleus with a larger mass. Fusion fuses atomic nuclei together.

An example would be 2 hydrogen (H) atoms combining to form a helium (He) atom with a larger nucleus.

Fusion and Temperature

Nuclear fusion requires positively charged nuclei to get close together. If they are moving very fast, then they would have the kinetic energy to overcome the repulsive electrical force.

Only at temperatures of millions of degrees Celsius are nuclei able to get close enough to fuse. The example below is the fusing of deuterium and tritium:

Fusion and Fission

• http://www.youtube.com/watch?v=yMebQ1J5TA4&feature=related&safety_mode=true&persist_safety_mode=1&safe=active

Fusion in the Future?

• http://www.youtube.com/watch?v=L3xpKBHIVnE&safety_mode=true&persist_safety_mode=1&safe=active

Nuclear Reactions and MedicineWhen a radioactive atom is introduced into the body, it can

travel to specific parts and join other molecules, where it can be easily found.

These radioactive isotopes (radioisotopes) are called tracers. They can be followed through the body and show how particular organs are functioning. Examples are how problems are detected in the thyroid, heart or gall bladder.

Gamma rays can used with radioisotopes to target and destroy the fastest growing cells (tumors.)

What is Nuclear Medicine?

• http://www.youtube.com/watch?v=NcFJbAOgrl8&safety_mode=true&persist_safety_mode=1&safe=active