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 Uis 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

92U235

92UExamples

- 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 nucleus,

X-rays associated with outer electrons

Source

50keV < Gamma rays < 40MeV

15 keV < Diagnostic X-rays < 150 keV

2 eV < Visible Light Photons < 4 eV

Energy Ranges

Radioactivity

Nuclear equations must balance

Conservation laws of physics

must be satisfied

226 222 4

88 86 2Ra Rn He g

Conserved:

•Total number of nucleons (A) (protons + neutrons)

•mass + energy

In all nuclear decays small quantity of mass

destroyed

E = mc2

Laws of Conservation

Nuclear reactions

observed in all nuclear decays

•Conservation of mass plus energy

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

Conservation •charge •total number of nucleons

239 235 4

94 92 2Pu U + He + energy

239

94 Pu

Write the decay equation for the following:

decaying by emission

Nuclear reactions

Unit of energy is the Joule

But unit of energy used in

atomic and nuclear physics is the

Electron volt (eV)

The electron volt is equal to the amount of

energy gained by an electron as it accelerates

through a potential difference of one volt

Definition

Energy (Joules) = 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

Range

Radioactivity

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

Type Mass kg Structure Charge

Range

(in air)

Damage

Alpha

() 6.6 x10-27 +2e mm High

Beta

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

Gamma

(g) 0 EM 0 m Low

4

2 He