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Standard 11 Nuclear Processes

This standard requires a knowledge of chemical and physical concepts and sufficient mathematical skills to describe the nucleus and its subatomic particles. Topics covered are nuclear reactions and their accompanying changes in energy and forms of radiation and quantification of radioactive decay as a function of time. Recall that the mass and charge of the proton is 1 amu and positive, respectively; and the mass and charge of the neutron are 1 amu and no charge. Remember that an element’s average atomic mass is based on the abundance and the mass number for individual isotopes. The nucleus of atoms are held together strong nuclear force which overcomes the repulsion between charged protons at very close distances.

Nuclear processes are those in which an atomic nucleus changes, including radioactive decay of naturally occurring and human-made isotopes, nuclear fission, and nuclear fusion. As a basis for understanding this concept:

11 a. Students know protons and neutrons in the nucleus are held together by nuclear forces that overcome the electromagnetic repulsion between the protons.

The nucleus is held together by the strong nuclear force. The strong nuclear force acts between protons, between neutrons, and between protons and neutrons but has a limited range comparable to the size of an atomic nucleus. The nuclear force is able to overcome the mutual electrostatic repulsion of the protons only when the protons and neutrons are near each other as they are in the nucleus of an atom.

Atomic Structure Atoms are made up of three tiny particles called protons, neutrons and electrons.

Particle / Position in atom / Charge / Mass
Proton / Nucleus / Positive / 1
Neutron / Nucleus / No charge / 1
Electron / In energy levels (shells)
outside the nucleus / Negative / Negligible

Atoms have no overall electrical charge because the number of positive protons and negative electrons are equal, so they cancel each other out.

The number of protons in the nucleus of an atom determines what element it is. Atoms of the same element have the same number of protons.

At the centre of an atom is the nucleus, which is very small. It has a positive charge because it contains protons. It is very dense because it contains protons and neutrons which make up most of the mass of an atom. The total number of protons and neutrons (nucleons) in an atom is called its mass number, or nucleon number.

11 b. Students know the energy release per gram of material is much larger in nuclear fusion or fission reactions than in chemical reactions. The change in mass (calculated by E = mc2 ) is small but significant in nuclear reactions.

Two major types of nuclear reactions are fusion and fission. In fusion reactions two nuclei come together and merge to form a heavier nucleus. In fission a heavy nucleus splits apart to form two (or more) lighter nuclei. The binding energy of a nucleus depends on the number of neutrons and protons it contains. A general term for a proton or a neutron is a nucleon. In both fusion and fission reactions, the total number of nucleons does not change, but large amounts of energy are released as nucleons combine into different arrangements. This energy is one million times more than energies involved in chemical reactions.

Nuclear energy is the energy that is trapped inside each atom. One of the laws of the universe is that matter and energy can't be created nor destroyed. But they can be changed in form. Matter can be changed into energy. The world's most famous scientist, Albert Einstein, created the mathematical formula that explains this. It is:

E = m c2

This equation says:

E [energy] equals m [mass] times c2[c stands for the velocity or the speed of light. c2 means c times c, or the speed of light raised to the second power -- or c-squared.]

You can listen to Einstein's voice explaining this at:

Scientists used Einstein's famous equation as the key to unlock atomic energy and also create atomic bombs.

Nuclear Fission[1]

An atom's nucleus can be split apart. When this is done, a tremendous amount of energy is released. The energy is both heat and light energy. Einstein said that a very small amount of matter contains a very LARGE amount of energy. This energy, when let out slowly, can be harnessed to generate electricity. When it is let out all at once, it can make a tremendous explosion in an atomic bomb.

A nuclear power plant (like Diablo Canyon Nuclear Plant shown below) uses uranium as a "fuel." DiabloCanyon is located in San Luis ObispoCounty and is scheduled for permit renewal in 2024.

Uranium is an element that is dug out of the ground many places around the world. It is processed into tiny pellets that are loaded into very long rods that are put into the power plant's reactor. The word fission means to split apart. Inside the reactor of an atomic power plant, uranium atoms are split apart in a controlled chain reaction.

Inside a nuclear reactor, unstable atoms with large nuclei are bombarded with neutrons. This causes the nuclei of the atoms to split into two smaller nuclei. This is called nuclear fission. Further neutrons are released which can hit other atoms causing further nuclear fission. This is called a chain reaction.

The new atoms formed are also radioactive. Many of these have very long half-lives, so will be dangerous for many years. This material could hurt people if released, so it is kept in a solid form. The very strong concrete dome in the picture is designed to keep this material inside if an accident happens.Large amounts of energy are released during radioactive decay or nuclear fission. This energy can be transferred into electrical energy in a nuclear power station, or used destructively as with a nuclear bomb.

In a chain reaction, particles released by the splitting of the atom go off and strike other uranium atoms splitting those. Those particles given off split still other atoms in a chain reaction. In nuclear power plants, control rods are used to keep the splitting regulated so it doesn't go too fast.

If the reaction is not controlled, you could have an atomic bomb. But in atomic bombs, almost pure pieces of the element Uranium-235 or Plutonium, of a precise mass and shape, must be brought together and held together, with great force. These conditions are not present in a nuclear reactor.

This chain reaction gives off heat energy. This heat energy is used to boil water in the core of the reactor. So, instead of burning a fuel, nuclear power plants use the chain reaction of atoms splitting to change the energy of atoms into heat energy. A lot of water is required for nuclear plants.

This water from around the nuclear core is sent to another section of the power plant. Here, in the heat exchanger, it heats another set of pipes filled with water to make steam. The steam in this second set of pipes turns a turbine to generate electricity. Below is a cross section of the inside of a typical nuclear power plant.

Nuclear Fusion

Another form of nuclear energy is called fusion. Fusion means joining smaller nuclei (the plural of nucleus) to make a larger nucleus. The sun uses nuclear fusion of hydrogen atoms into helium atoms. This gives off heat and light and other radiation.

In the picture to the right, two types of hydrogen atoms, deuterium and tritium, combine to make a helium atom and an extra particle called a neutron.

Also given off in this fusion reaction is energy! The same energy that fuels the sun and stars.[2] Scientists have been working on controlling nuclear fusion for a long time, trying to make a fusion reactor to produce electricity. But they have been having trouble learning how to control the reaction in a contained space.

What's better about nuclear fusion is that it creates less radioactive material than fission, and its supply of fuel can last longer than the sun.

11. c. Students know some naturally occurring isotopes of elements are radioactive, as are isotopes formed in nuclear reactions.

Sometimes atoms with the same number of protons in the nucleus have different numbers of neutrons. These atoms are called isotopes of an element. Both naturally occurring and human-made isotopes of elements can be either stable or unstable. Less stable isotopes of one element, called parent isotopes, will undergo radioactive decay, transforming to more stable isotopes of another element, called daughter products, which can also be either stable or radioactive. For a radioactive isotope to be found in nature, it must either have a long half-life, such as potassium-40, uranium-238, uranium-235, or thorium-232, or be the daughter product, such as radon-222, of a parent with a long half-life, such as uranium-238.

Isotopes Atoms of the same element which have different numbers of neutrons are called isotopes. Carbon exists in three forms:

12 / 13 / 14
C / C / C
6 / 6 / 6
number of protons / 6 / 6 / 6
number of electrons / 6 / 6 / 6
number of neutrons / 6 / 7 / 8

Some isotopes are radioactive. These are radioisotopes or radionuclides. Their nuclei are unstable and can spontaneously split up, emitting radiation and producing a new atom, with a different number of protons. This is called radioactive decay and is a random process – you don’t know which atoms may suddenly undergo a nuclear change. Radioisotopes called tracers are used determine complex metabolic pathways – How photosynthetic reactions discovered.

Examples of isotopes

Element / Stable isotopes / Unstable isotopes / Where found
Carbon / Carbon-12
Carbon-13 / Carbon-14 / Air, plants and animals
Potassium / Potassium-39
Potassium-41 / Potassium-40 / Rocks, plants and sea water
Uranium / Uranium-234
Uranium-235
Uranium-238 / Rocks

Simple hydrogen, deuterium, and tritium nuclei illustrate and define isotopes. Recall that isotopes have the same number of protons but different numbers of neutrons and thereby atomic mass.

11. d. Students know the three most common forms of radioactive decay (alpha, beta, and gamma) and know how the nucleus changes in each type of decay. Radioactive isotopes transform to more stable isotopes, emitting particles from the nucleus. These particles are helium-4 nuclei (alpha radiation), electrons or positrons (beta radiation), or high-energy electromagnetic rays (gamma radiation).

In 1896, Henri Becquerel discovered radioactivity. Radioactivity is the spontaneous emission of energy and particles by atoms of certain elements producing new elements. Materials that emit this kind of radiation are said to be radioactive and to undergo radioactive decay.In 1899, Ernest Rutherford discovered that uranium compounds produce three different kinds of radiation. He separated the radiations according to their penetrating abilities and named them:

alpha,βbeta, and γgammaradiation, after the first three letters of the Greek alphabet.

Alpha decay

In alpha decay, the nucleus emits an alpha particle; an alpha particle is essentially a helium nucleus, so it is a group of two protons and two neutrons. A helium nucleus is very stable. Alpha radiation can be stopped by a sheet of paper.

An example of an alpha decay involves uranium-238:

Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. This type of smoke detector is more common because it is inexpensive and better at detecting the smaller amounts of smoke produced by flaming fires. Inside an ionization detector is a small amount (perhaps 1/5000th of a gram) of americium-241. The radioactive element americium has a half-life of 432 years, and is a good source of alpha particles.

Beta decay

A beta particle is an electron. In beta decay an electron is involved. The number of neutrons in the nucleus decreases by one and the number of protons increases by one. Six millimeters of aluminum are needed to stop most beta particles

An example of such a process is:

Gamma decay

The third class of radioactive decay is gamma decay, in which the nucleus changes from a higher-level energy state to a lower level. Several millimeters of lead are needed to stop gamma rays.

  • Isotopes of elements that undergo alpha decay produce other isotopes with two less protons and two less neutrons than the original isotope. Uranium-238, for instance, emits an alpha particle and becomes thorium-234.
  • Isotopes of elements that undergo beta decay produce elements with the same number of nucleons but with one more proton or one less proton. For example, thorium-234 beta decays to protactinium-234, which then beta decays to uranium
  • Alpha and beta decay are ionizing radiations with the potential to damage surrounding materials. After alpha and beta decay, the resulting nuclei often emit high-energy photons called gamma rays. This process does not change the number of nucleons in the nucleus of the isotope but brings about a lower energy state in the nucleus.

11.e.Students know alpha, beta, and gamma radiation produce different amounts and kinds of damage in matter and have different penetrations. Alpha, beta, and gamma rays are ionizing radiations, meaning that those rays produce tracks of ions of atoms and molecules when they interact with materials. For all three types of rays, the energies of particles emitted in radioactive decay are typically for each particle on the order of 1MeV, equal to 1.6 × 10−13 joule, which is enough energy to ionize as many as half a million atoms.

Types of Nuclear Radiation

Alpha particles are helium nuclei, so they have two protons and two neutrons, but no electrons. They carry a 2+ charge. The nuclear emission that has the greatest mass and least penetration ability.

Alpha particleshave the shortest ranges, and matter that is only a few millimeters thick will stop them. They will not penetrate a thick sheet of paper but will deposit all their energy along a relatively short path, resulting in a high degree of ionization along that path.

Beta particles are high energy electrons emitted from the nucleus of an atom. They carry a negative charge.

Beta particleshave longer ranges, typically penetrating matter up to several centimeters thick. Those particles are electrons or positrons (the antimatter electron), have one unit of either negative or positive electric charge, and are approximately 1/2000 of the mass of a proton. These high-energy electrons have longer ranges than alpha particles and deposit their energy along longer paths, spreading the ionization over a greater distance in the material.

Gamma rays are very short wavelength electromagnetic waves which travel at the speed of light. They do not have a charge. Because there is no charge this type of radiation will pass directly through an electric field without being detected.

Gamma rays can penetrate matter up to several meters thick. Gamma rays are high-energy photons that have no electric charge and no rest mass (the structural energy of the particle). They will travel unimpeded through materials until they strike an electron or the nucleus of an atom. The gamma ray’s energy will then be either completely or partially absorbed, and neighboring atoms will be ionized.

Therefore, these three types of radiation interact with matter by losing energy and ionizing surrounding atoms.

Alpha radiation is dangerous if ingested or inhaled. For example, radon-222, a noble gas element, is a naturally occurring hazard in some regions. Living organisms or sensitive materials can be protected from ionizing radiation by shielding them and increasing their distance from radiation sources. Most common exposure.

The order of penetrating ability, from greatest to least, is gamma > beta > alpha, and this order is the basis for assessing proper shielding of radiation sources for safety.

There are a number of naturally occurring sources of ionizing radiation. One is potassium-40, which can be detected easily in potash fertilizer by using a Geiger counter. The other is background cosmic and alpha radiation from radon.

Workers who may be exposed to radiation have to wear a radiation badge, which monitors the amount of radiation the person has been exposed to over a period of time. The badge contains a piece of photographic film. The film darkens if exposed to radiation. The more radiation a worker has been exposed to, the darker the film goes.

11. f.* Students know how to calculate the amount of a radioactive substance remaining after an integral number of half-lives have passed.

Radioactive decay transforms the initial (parent) nuclei into more stable (daughter) nuclei with a characteristic half-life. The half-life is the time it takes for one-half of a given number of parent atoms to decay to daughter atoms. One-half of the remaining parent atoms will then decay to produce more daughter atoms in the next half-life period. It is possible to predict only the proportion, not the individual number, of parent atoms that will undergo decay. Therefore, after one half-life, 50 percent of the initial parent nuclei remain; after two half-lives, 25 percent; and so forth. The intensity of radiation from a radioactive source is related to the half-life and to the original number of radioactive atoms present.

Radioactive Dating

Radioactive materials gradually decay and form new atoms. The time it takes for half the atoms in a sample to decay is called its half-life, so older samples emit less radiation. This idea is used to work out how old plant, animal and rock specimens are.

  • Carbon-14 is used to date things that were once living.
  • Uranium isotopes have very long half-lives and decay via a series of short-lived radioisotopes to produce stable isotopes of lead. The relative amounts of uranium and lead in a sample of igneous rack can be used to date the rock.
  • Potassium-40 decays to form argon. The proportions of radioactive potassium and stable argon can be used to date igneous rocks from which the gaseous argon has been unable to escape.

The more unstable the nuclei the shorter the half-life. Substances with long half-lives are often the most dangerous because they stay radioactive for many years.