radioactivity nucleus isotopes alpha, beta & ... radioactivity no energy needed to create the rays?

Download Radioactivity Nucleus Isotopes Alpha, Beta & ... Radioactivity No energy needed to create the rays?

Post on 31-May-2020

0 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

  • Radioactivity

    Nucleus

    Isotopes

    Alpha, Beta & Gamma

    radiation

    Decay equations

    Conservation laws

    Lecture 22

  • Radioactivity

    No energy needed to create the rays?

    – Violates the law of conservation of energy!!!!

    Discovery

    1896 – Antoine Henri Becquerel 1852-1908, discovered nuclear radiation. (Shared Nobel Prize in Physics, 1903)

    Observed that a photographic plate was darken by invisible penetrating rays emitted from pitch blend (mineral containing uranium)

    Energetic rays:

    • had no apparent source

    1905 – Einstein

    Energy can be created by the destruction

    of a small amount of mass: E = mc2

    Law of conservation of energy modified

    to conservation of energy + mass

  • 1898 – Marie and Pierre Curie extracted

    new and highly radioactive elements

    Polonium and Radium from pitch-blend.

    Both Shared Nobel Prize in Physics, 1903

    With Henri Becquerel

    Other elements including Uranium were

    later found to be radioactive

    Radioactivity

    Certain elements had nuclei that were

    unstable and would “decay” causing

    emission of penetrating, highly energetic

    “rays”

  • Independent of Chemical State

    Radioactivity

    Radioactivity

    disintegration or decay of an unstable nucleus.

    3 distinct types of radiation discovered:

    Named, α, β and g

    Radiation independent of chemical state of

    radioactive element

    chemical reaction

    •nucleus unchanged

    •only orbital electrons participate

    Radioactivity

    Nothing to do with orbital electrons!

    unaffected by chemical, physical conditions

  • The Nucleus

    Atomic Structure

    Indicated that the nucleus is a concentrated

    mass within the atom

    Conclusion:

    inside electron orbits is mostly empty

    space with an dense nucleus at its center

    Most passed through the foil with no deflection

    Rutherford’s experiment 1911

    Alpha particles directed at a

    very thin film of gold foil

    Indicated that the atom is mostly empty space

    A few particles were scattered at very large angles

    Electrons do not deflect alpha particles

    •to small and light

  • Radioactivity

    Nucleus Atom

    • Nucleus 10-14 to 10-15 m

    Nucleus has most of the mass • Density of about 1017 kgm-3

    Extremely large forces in the nucleus

    Approximate diameters

    • Atom10-10 m

    Nuclear force (Attractive) between nucleons

    • proton and protons,

    • neutrons and neutrons

    • neutrons and protons

    Coulomb repulsive force

    •between protons

    • Responsible for large energy associated with nuclear radiation

    • High energy in nuclear power

    Neutrons

    Protons Nucleons {

    + _

    _

    _

    _

    _

    _

    Nuclear force > Coulomb repulsive force

     result stable nuclei

    (short range

    force)

  • Radioactivity

    Nucleus

    Neutrons

    Protons

    Atom

    Z is the atomic number

    (number of protons in the nucleus)

    Z = 1 Hydrogen

    Z = 2 helium

    Z = 3 Lithium etc

    Mass number A = Z + N

    where N is number of neutrons

    Many combinations of nucleons are possible – only some are stable Unstable combinations result in nuclear decay to a stable nucleus

    nucleons { + _

    _

    _

    _

    _

    _

  • Nuclear force (Attractive)

    •between nucleons

    Coulomb repulsive force

    •between protons

    Nucleus not stable if number of protons is

    large relative to number of neutrons

    Radioactivity

    Stable Nuclei

    Large nuclei stable

    only if they contain

    more neutrons than

    protons

    Extra neutrons mitigate the effect of the repulsive

    forces between the protons

  • Radioactivity

    Element whose symbol is X can be denoted

    A

    Z X

    Examples

    238

    92 U is called Uranium 238.

    It has 92 protons and (238-92)

    = 146 neutrons

    protons +neutrons

    protons

    is called Uranium 235. It has

    92 protons and

    (235-92) = 143 neutrons

    235

    92 U

    Nuclear notation

  • Radioactivity

    1

    1H

    4

    2 He

    1

    0 n

    0

    1

    Examples

    a hydrogen nucleus (or just a proton) a helium nucleus (or an alpha particle)

    Z is often not written (i.e. 235U)

    Notation can be used for particles

    other than nuclei

    Examples

    A neutron is denoted by

    An electron or beta particle

    denoted by

    235

    92 U

  • Nuclei with the same charge but different

    masses are called isotopes of the element

    Same number of protons but

    different number of neutrons Isotopes

    Different isotopes

    • Same element (same chemical properties)

    • Same number of protons

    • Different nuclear properties

    Isotopes

    238

    92U 235

    92U Examples

    - most abundant in nature

    - used in radioactive dating

    12

    6C

    14

    6C

    1

    1H 2

    1H 3

    1H

  • Radioactivity

    Radioactive decay

    Alpha, beta, and gamma

    radiation may be emitted

    Can be distinguished experimentally

    Beam of radiation containing all three types

    passed through a strong magnetic field

    Beam separates into three distinct parts

    Source

    (,,g)

    g

     Strong

    Magnetic Field Bin

    • Undeflected beam

    • 2 beams deflected

    in different directions

  • α particles, α radiation

    Radioactivity

    Characteristics

    2 neutrons and 2 protons (Helium nucleus

    Most of the energy carried by alpha radiation is in the form of kinetic energy

    Parent X → Daughter Y + α

    +ve charge twice that of electron

    4 4

    2 2

    A A

    Z ZX Y He 

     

    mass of 7000 times that of an electron

    Typical decay equation

    4

    2 He

  • Radioactivity

    Example: Decay— particle emitted

    • Daughter nucleus (Thorium) has 2 less protons

     Z = 92 – 2 = 90  Th

    • Daughter nucleus has lost atomic mass of 4

     A = 238 – 4 = 234

    • Energy is always released in a nuclear reaction

    238 234 4

    92 90 2U Th He energy  

    Energy of atom (mass) less than individual parts

  • Beta Decay

    Emission of an electron

    Created at the time of decay

    •Not one of the orbital electrons •Not existing in the nucleus prior to decay

    Neutron splits to form an electron and a proton

    Created and ejected from nucleus

    Beta particle (electron) •Charge (-1.6 * 10-19 C) •mass (9.11 * 10-31 kg)

    Radioactivity

    Energy carried by beta radiation is kinetic

    •Moves much faster than alpha particle • at greater than half speed of light

  • 1 1 0

    0 1 1n p e   

    • Mass-less particle?

    • Travels at the speed of light ?

    • No effect to biological tissues

    • So penetrating that it deposits no energy

    notation A

    Z X Can be used for

    neutrons and electrons

    14 14 0

    6 7 1C N antineutrino  

    Atomic mass stays the same

    Number of protons increases

    As if one neutron has changed to a proton

    Radioactivity

     Decay

    0

    1 1

    A A

    Z ZX Y e    

    Antineutrino created in and ejected from

    the nucleus (all  decay)

  • Gamma Decay

    Radioactivity

    Emission of a high frequency (wave) photon

    Gamma rays: only generated in the nucleus No Charge No Mass

    Move at the speed of light Like all electromagnetic waves (photons)

    Excited nucleus returns to non-excited state

    by releasing gamma radiation

    Something must excite the nucleus

    • Often preceded by another type of decay

    where nucleus is left in an excited state

    Followed by

    No change to the identity of the nucleus

    * indicates excited state 40 40 20 20Ca Ca g

      

    60 60 0

    27 28 -1Co Ni* + + antineutrino

    60 60

    28 28 1 2Ni* Ni + + g g

  • • EM radiation. Very high energy

    • Uncharged

    • Source is often excited nuclear state occurring after alpha and beta decay.

    • Excited state may remain for some time. Metastable state

    Gamma Decay

    Radioactivity

    gamma rays associated with nucle

Recommended

View more >