the nucleus and radioactivity radioactivity: spontaneous changes in the nucleus that emit energy as...

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  • 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, PlutoniumHydrogen-3Potassium-38

  • 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

  • positron: an antiparticle of a particle (their charges are opposite, but their masses are the same)A positron has a +1 charge and is called a positive electron.positron:+ orA positron is formed when a proton is converted to a neutron.neutronproton+positron

  • 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 DecayWhen 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 EmissionAlpha emission is the decay of a nucleus by emittingan a particle.

  • 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 24195Am42He+23793Np

  • Write an equation for the alpha decay of Rn-222.

    222Rn new nucleus + 4He 86 2Mass number: 222 4 = 218Atomic number:86 2 = 84Symbol of element 84 = Po

    222Rn 218Po + 4He 86 84 2

  • Beta DecayThe 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

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

  • 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 CheckWrite the nuclear equation for the beta decay of Co-60.

    60Co 27

  • SolutionWrite the nuclear equation for the beta decay of Co-60.

    60Co 60Ni + 0e 2728 1

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

  • Gamma RadiationGamma 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.

  • Half-Lives Vary Dramatically Between Elements

  • Half-Life CalculationsAfter 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

    Initial40 mg20 mg

    10 mg

  • 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 thegiven amount of time.32 days1 half-life8.0 daysx=4.0 half-lives

  • For each half-life, multiply the initial mass by one-half to obtain the final mass:100. mginitial massx12x12x12x12The mass is halved four times.= 6.25 mg final mass

  • 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 = 2Amount remaining = 64 mg x 1 x 1 = 16 mg 2 2 13 hrs 13 hrs64 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

  • Medical: diagnosing and disease (cancer, thyroid, brain scans)

    Uses of Radioisotopes

  • 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



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