The Nucleus and Radioactivity Radioactivity: Spontaneous changes in the nucleus that emit energy as radiation (particles or rays) Nuclei contain protons and neutrons; some combinations of these particles are unstable Examples of Radioactive Nuclei Include: Uranium, Plutonium Hydrogen-3 Potassium-38

Types of Radiation Include: Radioactive Decay: Emission of radiation produced by unstable nuclei changing to a more stable state Types of Radiation Include: Alpha rays a: positive charge Beta rays b: negative charge Gamma rays g: no charge a and b rays consist of streams of particles g rays consist of electromagnetic radiation

A positron has a +1 charge and is called a “positive electron.” positron: an antiparticle of a β particle (their charges are opposite, but their masses are the same) A positron has a +1 charge and is called a “positive electron.” +1 e positron: β+ or A positron is formed when a proton is converted to a neutron. 1 p 1 n +1 e + proton neutron positron 3

a particle: contains 2 protons and 2 neutrons identical to helium nucleus travel only short distances b particle: electrons produced in the nucleus, then emitted travel greater distances than a particles

g Ray: High-energy ray similar to an X ray Travel great distances Daughter Nuclei: New nuclei that result from unstable nuclei undergoing radioactive decay Example: Uranium-238 gives up an a particle, resulting in a daughter nucleus of a different element, Thorium (Th)

Summary of Radiation Types

Alpha Decay When a radioactive nucleus emits an alpha particle, a new nucleus results. The mass number of the new nucleus is 4 less than that of the initial nucleus. The atomic number is decreased by 2.

Nuclear Reactions: Alpha Emission Alpha emission is the decay of a nucleus by emitting an a particle. 8

In a balanced nuclear equation, the sum of the mass numbers and the sum of the atomic numbers for the nuclei of the reactant and the products must be equal. 251Cf 247Cm + 4He 98 96 2 Am + Np 241 4 He 237 95 2 93

Write an equation for the alpha decay of Rn-222. 222Rn new nucleus + 4He 86 2 Mass number: 222 – 4 = 218 Atomic number: 86 – 2 = 84 Symbol of element 84 = Po 222Rn 218Po + 4He 86 84 2

Beta Decay The unstable nucleus converts a neutron into a proton (emitting an electron from the nucleus) The mass number of the new nucleus remains the same The atomic number of the new nucleus increases by 1 1n 0e + 1H 0 -1 1

Nuclear Reactions: Beta Emission Beta emission is the decay of a nucleus by emitting a β particle; 1 neutron is lost and 1 proton is gained. 12

Example: Potassium - 42 is a beta emitter. 42K new nucleus + 0e 19 -1 Mass number : (same) = 42 Atomic number: 19 + 1 A = 20 Symbol of element 20 = Ca 42K 42Ca + 0e 19 20 -1

Learning Check Write the nuclear equation for the beta decay of Co-60. 27

Solution Write the nuclear equation for the beta decay of Co-60. 60Co 60Ni + 0e 27 28 1

Nuclear Reactions: Positron Emission Positron emission is the decay of a nucleus by emitting a positron, β+; 1 proton is lost and 1 neutron is gained. 16 16

Gamma  Radiation Gamma radiation is energy emitted from an unstable nucleus indicated by m. In a nuclear equation for gamma emission, the mass number and the atomic number are the same. 99mTc 99Tc +  43 43

Summary of Radiation

Some radioactive isotopes are more stable than others, and therefore decay more slowly Half-Life: Time required for half of the unstable nuclei in a sample to decay Example: A Potassium-38 sample weighs 100 grams. 8 minutes later, the sample is weighed again and found to weigh 50 g. The half-life of potassium-38 is 8 minutes

Note: The half-life of a radioactive isotope is a property of a given isotope and is independent of the amount of sample, temperature, and pressure. 20

Half-Lives Vary Dramatically Between Elements

Half-Life Calculations After one half-life, 40 mg of a radioisotope will decay to 20 mg. After two half-lives, 10 mg of radioisotope remain. 40 mg x 1 x 1 = 10 mg 2 2 1 half-life 2 half-lives Initial 40 mg 20 mg 10 mg

Determine how many half-lives occur in the given amount of time. Practice: If the half-life of iodine-131 is 8.0 days, how much of a 100. mg sample remains after 32 days? Determine how many half-lives occur in the given amount of time. 1 half-life 8.0 days 32 days x = 4.0 half-lives 23

For each half-life, multiply the initial mass by one-half to obtain the final mass: 1 2 1 2 1 2 1 2 100. mg x x x x = 6.25 mg initial mass final mass The mass is halved four times. 24

Learning Check The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 26 hours?

Solution Half life = 13 hrs Number of half lives = 2 Amount remaining = 64 mg x 1 x 1 = 16 mg 2 2 13 hrs 13 hrs 64 mg 32 mg 16 mg

Radiation and Health Free Radicals: Very reactive compounds that can cause mutations, cancer; usually caused by long-term exposure to low-level radiation Radiation Sickness: Illness and symptoms caused by short-term exposure to intense radiation

Uses of Radioisotopes Medical: diagnosing and disease (cancer, thyroid, brain scans)

Common Imaging Techniques PET Scans (Positron Emission Tomography): gamma rays create a 3D image of organs, used to analyze blood flow, metabolic activity and brain function CT (Computed Tomography): X-rays are used to create series of images of the brain, identifying brain damage and hemorrhaging MRI (Magnetic Resonance Imaging): H protons in magnetic field are used to create color images of soft tissue

Health/Agriculture: food irradiation Radioactive dating: determine age of fossils

Nuclear Power Plants: Alternative energy source

Units of Radiation Curie (Ci): number of disintegrations per second per gram of radium; 3.7 x 1010 disintegrations per second Rad (Radiation Absorbed Dose): amount of material able to deliver 2.4x10-3 cal of energy to 1 kg of tissue Rem (Radiation Equivalent in humans): amount of biological damage caused by different types of radiation

In 1934 Radioactivity was Artificially Induced for the first time!! High-energy particles (such as neutrons) can create unstable nuclei that then undergo radioactive decay (Cyclotrons and Linear Accelerators)

Nuclear Fission: Process in which large nuclei split into smaller nuclei when bombarded with neutrons, releasing large amounts of energy Example: When a neutron bombards U-235, an unstable nucleus of U-236 forms smaller nuclei such as Kr-91 and Ba-142.

Chain Reaction: Nuclear reaction in which the products of a reaction cause that reaction to occur repeatedly Nuclear Fusion: Process in which small nuclei combine (fuse) to form larger nuclei Example: Hydrogen nuclei combine to form Helium nuclei