Radioactivity!

Standard: Differentiate among alpha and beta particles and gamma radiation

Radioactivity Radioactivity is the breaking down of atomic nuclei by releasing particles (primarily alpha and beta particles) or electromagnetic radiaition (primarily gamma rays).

Alpha Particles Alpha particles are positively charges particles. Alpha particles consists of two protons and two neutrons. An alpha particle is simply a helium nuclei (He) which is ejected with high energy from an unstable nucleus

An alpha particle is simply a helium nuclei (He) which is ejected with high energy from an unstable nucleus.

When an atom loses an alpha particle, the Z number (atomic number) is lower by two, so move back two spaces on the periodic table to find what the new element is. The new element has an A number (atomic mass number) that is four less than the original elements.

Example

Because alpha particles are large and heavy, paper or clothing or even dead skin cells shield form their effects.

Beta Particles Beta particles are electrons. They are negatively charged (-1). They are fast moving because they are actually electrons. It is produce when a neutron in the nucleus breaks up into a proton and an electron. The proton remains inside the nucleus, increasing its atomic number by one but the electron is given off.

Example – Carbon -14 They are written as an electron (along with a proton) which is emitted from the nucleus as a neutron decays.

The Z number (atomic number) actually adds one since its total must be the same on both the left and the right of the arrow and the electron on the right adds a negative one. The A (atomic mass)number is unchanged. The Z number determines the element so look for it on the Periodic Table.

Gamma Rays Gamma rays are electromagnetic waves with extremely short wavelengths. They have no mass or charge so the Z and A numbers are not affected. They travel at the speed of light. It takes a thick block of lead to stop them. Radioactive atoms often emit gamma rays along with either alpha or beta particles.

Examples

Alpha & Beta Decay http://www.kentchemistry.com/links/Nuclear/AlphaBetaGamma.htm

Gamma Radiation Due to beta decay, Cobalt-60 atom with 27 protons transforms to Nickle-60 which has 28 protons in its nucleus.

Penetration power of radiation

Positron A particle with the mass of an electron but a positive charge. Positron decay is like a mirror image of beta decay. These points present a simplified view of what positron decay actually is: 1) Something inside the nucleus of an atom breaks down, which causes a proton to become a neutron. 2) It emits a positron and a neutrino which go zooming off into space. 3) The atomic number goes DOWN by one and mass number remains unchanged.

Some points to be made about the equation: 1) The nuclide that decays is the one on the left-hand side of the equation. 2) The order of the nuclides on the right-hand side can be in any order. 3) The way it is written above is the usual way. 4) The mass number and atomic number of the neutrino are zero. 5) The neutrino symbol is the Greek letter "nu.

Reading Assignment Read the article on Radioactivity Complete the one-pager Discuss

Name Alpha Beta Gamma Greek letter  -  Symbol 2p+, 2n0 electron Composition 2p+, 2n0 electron EM wave Charge 2+ 1- Stopped by paper, skin, clothing wood, glass thick concrete or lead

SUMMARY of alpha, beta, gamma Alpha particles An alpha particle is simply a helium nuclei (He) which is ejected with high energy from an unstable nucleus This particle, which consists of two protons and two neutrons, has a net positive charge. Although emitted with high energy, alpha particles lose energy quickly as they pass through matter of air and therefore, do not travel long distances. They can even be stopped by a piece of paper or the outer layers of human skin. These slow moving particles are generally the product of heavier elements Example : 23892U ----> 42He + 23490Th. Beta particles Beta particles are identical to electrons and thus have a charge of (-1). This type of decay process leaves the mass number of the nuclei unchanged. The element is transformed to a new element A beta particle is minute in comparison to that of an alpha particle and has about one hundred times the penetrating ability. Where an alpha particle can be stopped by a piece of paper a beta particle can pass right through. It takes aluminum foil or even wood to stop a beta particle. The electron that is released was not present before the decay occured, but was actually created in the decay process itself. Example : 3215P ----> 0-1e + 3216S

Gamma Rays As the name implies, these are not particles but high energy photons and can be found on the electromagnetic spectrum They are very similar to x-rays but have a shorter wavelength and therefore more energy The penetrating ability of gamma rays is much greater than that of alpha or beta particles. They can only be stopped by several centimeters of lead or more than a meter of concrete. In fact, gamma rays can pass right through the human body. Gamma rays often accompany other processes of decay such as alpha or beta. process. 23892U ----> 23490Th + 200 + 42He A ramification of alpha or beta particle production is that the newly formed nucleus is left in a state of excess energy. A way for the nucleus to release this excess energy is by emitting gamma rays. Since gamma rays have no mass, and are waves rather than particles, the elements atomic number does not change after emission.

Standard: Explain the process of half-life as related to radioactive decay. During radioactive decay an unstable nucleus spontaneously decomposes to form a different nucleus, giving off radiation in the form of atomic particles or high energy rays. This decay occurs at a constant, predictable rate that is referred to as half-life. A stable nucleus will not undergo this kind of decay and is thus non-radioactive.

The Co-60 atom undergoes beta decay but remains excited and emits 2 gamma rays to release additional energy. An antineutrino is also released.

Standard: Describe nuclear energy, its practical application as an alternative energy source, and its potential problems. How nuclear power works video: http://www.neok12.com/php/watch.php?v=zX7e6c58536f7e4f45010763&t=Nuclear-Power See AC Science Nuclear Energy lesson for potential nuclear problems.

Radioactivity Questions 1. A radioactive substance has a half-life of 10 years. What fraction of a sample of the substance would be left after 30 years? A. 1/2 B. 1/3 C. 1/8 D. 1/9 2. Which type of radiation, from an external source, will penetrate deepest into the human body? A alpha B gamma C ultraviolet D x-ray 3. Which of the following is the LEAST likely reason for the popularity of fission as a way of producing electricity? A Spent uranium fuel is easier to dispose of than ashes from burned coal. B Nuclear energy is sometimes less expensive than other energy sources. C Uranium provides more energy than an equal amount of petroleum. D Nuclear fission produces less air pollution than burning fossil fuels. Answers C, B, A, C t (hr) Amount of Gold-191 Remaining (mg) 0 13.2 12.4 6.60 24.8 3.30 37.2 1.65 4. Gold-191 is a radioactive isotope that has a half-life of 12.4 hours. If a lab starts with a 13.2-milligram sample of gold-191, how much will remain after 37.2 hours? A 6.60 mg B 4.40 mg C 1.65 mg D 0.825 mg