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RADIOACTIVE DECAYAtomic Physics

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Becquerel

• In 1896 Henri Becquerel discovered thatcertain uranium compounds would fogphotographic plates as if exposed to light.

• He discovered that a magnetic field could deflect the radiation that caused the fogging.

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Pierre and Marie Curie

Marie and Pierre Curie investigated further.

• They discovered that thorium is also radioactive, and discovered two new elements: radium and polonium (named for Marie’s native Poland).

• Marie coined the term radioactivity.

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Three Radiation Types

• Alpha: the emission of a helium nucleus (an alpha particle):

• Beta: the emission of a high energy electron:

• Gamma: the emission of a high energy photon:

4201

γ

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Radiation in magnetic field

Heads Up!

Guaranteed exam Question

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Conservation In Nuclear Reactions

Charge: Net charge remains constant: total charge of the reactants = total charge of the products.

Atomic mass number: The total atomic mass number for the products = total atomic mass number for the reactants.

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Example

A. Is this reaction possible?230 226 4

90 88 2Th Ra

1. Charge: (p+ are +)+90 = +88 + +2 (charge is balanced)

2. Atomic mass number:230 = 226 + 4 (N is balanced)

The reaction is possible.

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Example

B. Is this reaction possible?

60 60 127 28 0Co Ni n

neutron

1. Charge:+27 ≠ +28 + 0 (charge not balanced)

2. Atomic mass number: 60 = 60 + 1 (N not balanced)

The reaction is not possible.

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1. alpha (α) – spontaneous emission of an alpha particle by a large, unstable nucleus. 2 4 2

2, ,He He

• consists of 2 protons and 2 neutrons

• cannot penetrate more than a sheet of paper

• ionize atoms they collide with

• low biological hazard

Alpha Decay

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Note:

In a large nucleus, the electrostatic force pushing p+ apart is nearly as strong as the attractive strong force, making the nucleus unstable.

An alpha particle (2 p+ and 2 n°) is a very stable configuration, so alpha particles are emitted not individual p+or n°.

In alpha decay, Z decreases by 2, and A decreases by 4.

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Alpha reactions

In general:

Example:

4 42 2

A AZ ZX Y

238 234 492 90 2U Th

Parent element Daughter element

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Example

Write the alpha decay process for polonium-210.

Name the parent and daughter elements.

210 206 484 82 2Po Pb

Parent element:polonium

Daughter element:lead

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Energy in alpha decay

Mass defect, and mass-energy equivalence can be used to determine the maximum Ek an emitted alpha particle will have.

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Example

Write the alpha decay process for the decay of radium-226.

Use the data on page 881 to determine the mass defect in the reaction, and its energy equivalence. This energy will be the maximum Ek an alpha particle could have in the reaction.

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Decay reaction

Parent element: daughter element:radium radon

226 222 488 86 2Ra Rn

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Mass defect

Note: if Δm were negative, this means no energy was released, so the decay would not happen.

222 486 2

22688

= ( )

parent products

Rn

m m m

Ra m m

226.025410 (222.017578 4.002603 )

0.005229

u u u

u

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Energy equivalence

Most of this energy will be Ek of the alpha particle.

-27

-30

m = 0.005229 u 1.660539 10 kg / u

m = 8.62829... 10 kg

2

-30 8 2

E = mc

= 8.66829... 10 kg (3.00 10 m / s) -13= 7.8146... 10

= 4.88

J

eV

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2. beta (β) – spontaneous emission of a high energy electron

A neutron emits a beta particle (an e¯) and becomes a proton. penetrate up to 3mm of aluminum, 500

sheets of paper produces burns to skin or tissue

01e or

the antiparticle of beta particle is a positron

01e or

Beta decay,

Z increases by 1, and A does not change. The transformation of a neutron into a

proton involves the weak nuclear force, a fundamental force

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Beta particle01

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3. gamma(γ) – emission of a high energy photon

most harmful because it penetrates through most matter

penetrates the living cells, breaks down DNA bond resulting in cancer growths, mutations in offspring

uses – radiation therapy, irradiate foods, fruits

E and B

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Uranium-238

146 n92 p

Gamma radiation

n = 1 n = 2

e

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Note:

-stable isotopes have the same number of protons and neutrons

-elements with more than 83 protons are unstable and transmutate into stable nuclei with higher binding energies

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Text: p. 796 Binding energy VS atomic mass No. (see that the binding energy decreases)

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1. Alpha Decay occurs when the electromagnetic force

within a nucleus is as great or slightly greater than the strong nuclear force

during the decay, momentum, energy ( mass energy) and charge are conserved

238 234 492 90 2U Th He Eg)

Mass is conserved 238=238

Charge is conserved 92=92

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Decay chain for Uranium

14 decays lead (stable)

238 234 492 90 2U Th He 234 226 4

90 88 2Th Ra He

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2. Beta DecayA. Beta negative decay A nucleus decays emitting an electron,

beta negative or β decay A neutron within a nucleus transforms

into a proton

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The daughter nucleus has more mass than the electron and the energy released goes into the kinetic energy

The β particle has less Ek than expected and this missing energy is carried away by a neutral particle called a neutrino, v, and its mass and size are unknown

β decay involves a weak nuclear force acting on electrons and neutrinos and antimatter

Note: Antimatter – matter that has a charge opposite to that of ordinary matter

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Antimatter

The antimatter of the electron is a positron,

The antimatter of a neutrino (v) is an antineutrino ( )

01e or

01e or

v

Eg)an electron

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B. Beta positive decay

β+ decay0

1

54 54 025 24 1

-during decay, a proton transmutates into a neutron

eg)

positron neutrino

A AZ Z neutrinoX Y v

Mn Cr v

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Gamma Decay (γ-decay)

Nuclei energy levels change, which correspond to different configurations of nucleons within a nucleus

Excited states of a nucleus, nucleons are farther apart, binding energy is less, but the total energy of a nucleus is less than the ground state

When the nucleus makes a transition to a lower energy level, it emits a gamma ray photon

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Example:

12 126 6

12 12 05 6 1

atomic mass and number are the same

)

A AZ ZX X

C C

eg decay and then decay

B C v

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What are the electrical properties of the rays in the diagram below?