Radioactivity Nuclei that are unstable decay; many such decays are governed by another force called the weak nuclear force. Radioactive rays were observed to be of three types: α Alpha radiation are helium nuclei and can barely penetrate a piece of paper β Beta radiation are electrons and can penetrate 3 mm of aluminum γ Gamma radiation are electromagnetic radiation and can penetrate several centimeters of lead

Alpha and beta rays are bent in opposite directions in a magnetic field, while gamma rays are not bent at all.

Alpha Decay In general, alpha decay can be written: Alpha decay occurs when the strong nuclear force cannot hold a large nucleus together. The mass of the parent nucleus is greater than the sum of the masses of the daughter nucleus and the alpha particle; this difference is called the disintegration energy. Alpha decay is so much more likely than other forms of nuclear disintegration because the alpha particle itself is quite stable.

Example of alpha decay: Radium-226 will alpha-decay to radon-222

Alpha Radiation α: helium nucleus He Positive charge, speed 10% of c 4 2 4 He 2+ Positive charge, speed 10% of c

Alpha decay Reaction Reactants Products Polonium 212 decays into lead 208 by emitting an alpha particle. The rest masses of these three are: 3.51986 x 10-25 kg 3.45324 x 10-25 kg 6.646 x 10-27 kg Reactants Products (a) Calculate the difference between the initial rest mass and the total final rest mass. 1.6 x 10-29 kg (b) This mass decrease appears in the form of Kinetic energy. Calculate the kinetic energy released in this reaction. 1.44 x 10-12 J (c) Assume the Alpha particle, being much less massive than the lead nucleus, gets almost all this energy. Calculate its speed just after the decay. 2. 1x 107 m/s

Beta Decay Beta decay occurs when a nucleus emits an electron. An example is the decay of carbon-14: The nucleus still has 14 nucleons, but it has one more proton and one fewer neutron. In general, beta decay can be written: The fundamental process is a neutron decaying to a proton, electron, and neutrino:

e Beta Radiation Negative charge, speed 90% of c - 1 See Beta emission applet Negative charge, speed 90% of c - 1 e Beta Radiation

Beta decay can also occur where the nucleus emits a positron rather than an electron: The nucleus still has 19 nucleons, but it has one more neutron and one fewer protons. In general, positron emission can be written: The fundamental process is a proton decaying to a neutron, positron, and neutrino:

And a nucleus can capture one of its inner electrons: The nucleus still has 7 nucleons, but it has one more neutron and one fewer protons. In general, positron emission can be written: The fundamental process in an electron capture:

Gamma Decay Gamma rays are very high-energy photons. They are emitted when a nucleus decays from an excited state to a lower state, just as photons are emitted by electrons returning to a lower state. In general, gamma emission can be written:

Gamma Radiation γ electromagnetic radiation, high frequency, high energy, no charge, speed is c

Conservation laws observed in Radioactive Decay Atomic # (Z) & Mass # (A) Energy (includes mass ~ E=mc2) alpha production: beta production: gamma ray production:

Problem 1 What is the maximum KE of the emitted β particle during the decay of The reaction is

Half-Life and Rate of Decay Nuclear decay is a random process; the decay of any nucleus is not influenced by the decay of any other. The number of decays in a short time interval is proportional to the number of nuclei present and to the time: Here, l is a constant characteristic of that particular nuclide, called the decay constant.

Maths not required in AP Physics 1 This equation can be solved, using calculus, for N as a function of time: The half-life is the time it takes for half the nuclei in a given sample to decay. It is related to the decay constant: Maths not required in AP Physics 1

Decay Series A decay series occurs when one radioactive isotope decays to another radioactive isotope, which decays to another, and so on. This allows the creation of nuclei that otherwise would not exist in nature.