Lecture 22




Alpha, Beta & Gamma

Decay equations

Conservation laws

Radioactivity Discovery 1896 – Antoine Henri 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 )

Energetic rays: • had no apparent source No energy needed to create the rays? – Violates the law of conservation of energy!!!!

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 Radioactivity

1898 – Marie and Pierre extracted new and highly radioactive elements Polonium and from pitch-blend.

Both Shared Nobel Prize in Physics, 1903 With

Other elements including Uranium were later found to be radioactive

Certain elements had nuclei that were unstable and would “decay” causing emission of penetrating, highly energetic “rays” Radioactivity

Independent of Chemical State Radiation independent of chemical state of radioactive element

chemical reaction •nucleus unchanged •only orbital participate

Radioactivity Nothing to do with orbital electrons! unaffected by chemical, physical conditions

Radioactivity disintegration or decay of an unstable nucleus.

3 distinct types of radiation discovered: Named, α, β and g The Nucleus

Atomic Structure ’s experiment 1911 Alpha particles directed at a very thin film of gold foil

Most passed through the foil with no deflection Indicated that the is mostly empty space

A few particles were scattered at very large angles Indicated that the nucleus is a concentrated mass within the atom

Electrons do not deflect alpha particles •to small and

Conclusion: inside orbits is mostly empty space with an dense nucleus at its center


Nucleus Atom _ Nucleons { _ _ _ + Approximate diameters • Atom10-10 m _ • 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 Nuclear force (Attractive) between nucleons • and protons, • neutrons and neutrons (short range • neutrons and protons force) repulsive force •between protons Nuclear force > Coulomb repulsive force . result stable nuclei • Responsible for large energy associated with nuclear radiation • High energy in nuclear power Radioactivity

Nucleus Atom _ Neutrons nucleons { _ Protons _ _ + _ _ Z is the atomic number (number of protons in the nucleus)

Z = 1 Z = 2 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

Radioactivity Nuclear force (Attractive) •between nucleons Coulomb repulsive force •between protons Nucleus not stable if number of protons is large relative to number of neutrons Extra neutrons mitigate the effect of the repulsive forces between the protons Stable Nuclei

Large nuclei stable only if they contain more neutrons than protons Radioactivity

Nuclear notation Element whose symbol is X can be denoted protons +neutrons A Z X protons


238 is called Uranium 238. It has 92 protons and (238-92) 92 U = 146 neutrons

235 is called Uranium 235. It has 92 U 92 protons and (235-92) = 143 neutrons Radioactivity Examples

1 1 H a hydrogen nucleus (or just a proton)

4 a helium nucleus (or an ) 2 He

235 92 U Z is often not written (i.e. 235U)

Notation can be used for particles other than nuclei Examples 1 A is denoted by 0 n

An electron or beta particle denoted by 0 1 

Nuclei with the same charge but different masses are called isotopes of the element

Same number of protons but Isotopes different number of neutrons Different isotopes • Same element (same chemical properties) • Same number of protons • Different nuclear properties

Examples 238 235 92U 92U 1 2 3 1 H 1 H 1 H

12 6 C - most abundant in nature

14 - used in radioactive dating 6 C Radioactivity 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

• Undeflected beam • 2 beams deflected in different directions 

Source g (,,g)

Strong 

Magnetic Field Bin Radioactivity

α particles, α radiation Characteristics 2 neutrons and 2 protons (Helium nucleus 4 2 He +ve charge twice that of electron mass of 7000 times that of an electron

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

Typical decay equation AA44 ZZX22 Y He

Parent X → Daughter Y + α Radioactivity

Example: Decay— particle emitted

238 234 4 92U 90 Th  2 He  energy

• 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

Energy of atom (mass) less than individual parts Radioactivity

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

Created and ejected from nucleus

Neutron splits to form an electron and a proton

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

Energy carried by beta radiation is kinetic

•Moves much faster than alpha particle • at greater than half speed of light Radioactivity  Decay A notation Can be used for Z X neutrons and electrons AA0 ZZX11 Y  e 

1 1 0 0n 1 p  1 e 

14 14 0 6C 7 N  1   antineutrino Atomic mass stays the same Number of protons increases

As if one neutron has changed to a proton Antineutrino created in and ejected from the nucleus (all  decay) • Mass-less particle? • Travels at the speed of light ? • No effect to biological tissues • So penetrating that it deposits no energy Radioactivity Gamma Decay Excited nucleus returns to non-excited state by releasing gamma radiation Emission of a high frequency (wave) Gamma rays: only generated in the nucleus No Charge No Mass Move at the speed of light Like all electromagnetic waves ()

No change to the identity of the nucleus 40  40 * indicates excited state 20Ca 20 Ca g

Something must excite the nucleus • Often preceded by another type of decay where nucleus is left in an excited state 60 60 0 27Co  28 Ni* + -1 + antineutrino Followed by 60 60 28Ni*  28 Ni + gg 1 + 2 Radioactivity

Gamma Decay

• EM radiation. Very high energy • Uncharged • Source is often excited nuclear state occurring after alpha and . • Excited state may remain for some time. Metastable state


gamma rays associated with nucleus, X-rays associated with outer electrons Energy Ranges 50keV < Gamma rays < 40MeV 15 keV < Diagnostic X-rays < 150 keV 2 eV < Visible Light Photons < 4 eV Radioactivity

Nuclear equations must balance

Conservation laws of physics must be satisfied 226 222 4 88Ra 86 Rn  2 He g

Conserved: •Total number of nucleons (A) (protons + neutrons) •mass + energy

In all nuclear decays small quantity of mass destroyed

E = mc2 Nuclear reactions

Laws of Conservation observed in all nuclear decays Conservation •charge •total number of nucleons

•Conservation of mass plus energy

Energy produced in nuclear decay is the result of a small amount of mass being destroyed E = mc2

Write the decay equation for the following:

239 94 Pu decaying by  emission

239 235 4 94Pu  92 U + 2 He + energy Nuclear reactions

Unit of energy is the But unit of energy used in atomic and is the Electron volt (eV) Definition The electron volt is equal to the amount of energy gained by an electron as it accelerates through a potential difference of one volt Energy () = qV Charge on an electron = 1.6 x10-19 C eV = 1.6 x10-19 C x 1volt =1.6 x 10-19 Joules

1eV = 1.6 x 10-19J

15 keV < Diagnostic X-rays < 150 keV 2 eV < Visible Light Photons < 4 eV Radioactivity Range Distance radiation can travel in a given material before dissipating all of its energy Depends on the material Greater electron density stops radiation most effectively Range depends on radiation type

Equal energies and same material •Alpha radiation - Smallest range •Beta radiation - Middle range •Gamma radiation - Largest range

Radiation interacting with electrons in the material results in energy dissipation Summary of Radiation

Range Type Mass kg Structure Charge (in air) Damage

Alpha 6.6 x10-27 4 +2e mm High () 2 He

Beta 9.11 x 10-31 electron -e cm Med ()

Gamma 0 EM 0 m Low (g)