Nuclear Decay NotesNuclear Decay Notes

Stability CurveStability Curve

Atomic number Z

Neu

tron

nu

mb

er

N

Stable Stable nucleinuclei

Z = Z = NN

20 40 60 80 100

40

100

140

20

60

80

120

Nuclear particles Nuclear particles are held together are held together by a nuclear strong by a nuclear strong force.force.

Nuclear particles Nuclear particles are held together are held together by a nuclear strong by a nuclear strong force.force.A stable nucleus A stable nucleus remains forever, but remains forever, but as the ratio of as the ratio of N/ZN/Z gets larger, the gets larger, the atoms decay.atoms decay.Elements with Z > Elements with Z > 82 are all unstable.82 are all unstable.Elements with Z > Elements with Z > 82 are all unstable.82 are all unstable.

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RadioactivityRadioactivity

As the heavier atoms become more unstable, particles and photons are emitted from the nucleus and it is said to be radioactive. All elements with Z > 82 are radioactive.

As the heavier atoms become more unstable, particles and photons are emitted from the nucleus and it is said to be radioactive. All elements with Z > 82 are radioactive. Examples Examples

are:are:Alpha particles Alpha particles particles particles

(electrons)(electrons)Gamma rays Gamma rays

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The Alpha ParticleThe Alpha Particle

An An alpha particlealpha particle is the nucleus of a is the nucleus of a helium atom (helium atom (44

22He)He) consisting of two consisting of two protons and two neutrons tightly bound.protons and two neutrons tightly bound.

Charge = +2Charge = +2

Mass = 4.001506 uMass = 4.001506 u

Relatively low speeds (Relatively low speeds ( 0.1c )0.1c ) Not very penetratingNot very penetrating

The Beta-minus ParticleThe Beta-minus Particle

A A beta-minus particlebeta-minus particle is simply an is simply an electron (electron (00

-1-1e) that has been expelled e) that has been expelled from the nucleus. from the nucleus.

Charge = Charge = ee- - = -1= -1-

High speeds (near High speeds (near cc))-

Mass = 0.00055 uMass = 0.00055 u-

Very penetratingVery penetrating-

The Gamma PhotonThe Gamma Photon

A A gamma raygamma ray has very high has very high electromagnetic radiation carrying electromagnetic radiation carrying energy away from the nucleus.energy away from the nucleus.

Charge = Zero Charge = Zero (0)(0)

Mass = zero (0)Mass = zero (0)

Speed = c (3 x 10Speed = c (3 x 1088 m/s) m/s)

Most penetrating Most penetrating radiationradiation

Radioactive DecayRadioactive DecayAs discussed, when the ratio of N/Z gets As discussed, when the ratio of N/Z gets very large, the nucleus becomes unstable very large, the nucleus becomes unstable and often particles and/or photons are and often particles and/or photons are emitted.emitted.Alpha decayAlpha decay results in the loss of results in the loss of two protons and two neutrons from the two protons and two neutrons from the nucleus. nucleus.

42

4 42 2

A AZ ZX Y energy

4 42 2

A AZ ZX Y energy

X X is is parent parent atom and atom and YY isis daughter daughter atomatomThe energy is carried away The energy is carried away

primarily by the K.E. of the alpha primarily by the K.E. of the alpha particle.particle.

Example :Example : Write the reaction that Write the reaction that occurs when radium-226 decays by occurs when radium-226 decays by alpha emission.alpha emission.

4 42 2

A AZ ZX Y energy

4 42 2

A AZ ZX Y energy

226 226 4 488 88 2 2Ra Y energy

From tables, we find Z and A for From tables, we find Z and A for nuclides. The daughter atom: Z = 86, nuclides. The daughter atom: Z = 86,

A = 222A = 222226 222 488 86 2Ra Rn energy 226 222 488 86 2Ra Rn energy

Radium-226 decays into radon-222.

Radium-226 decays into radon-222.

Beta DecayBeta Decay

Beta Beta decay decay results when a neutron results when a neutron decays into a proton and an electron. decays into a proton and an electron. Thus, the Z-number Thus, the Z-number increasesincreases by one. by one.

01 1

A AZ ZX Y energy 0

1 1A AZ ZX Y energy

X X is is parent parent atom and atom and YY isis daughter daughter atomatom

The energy is carried away The energy is carried away primarily by the K.E. of the primarily by the K.E. of the electron.electron.

-

Example :Example : Write the reaction that Write the reaction that occurs when Carbon-14 decays by occurs when Carbon-14 decays by Beta emission.Beta emission.

146C 14

7N + 0-1β + energy

01 1

A AZ ZX Y energy 0

1 1A AZ ZX Y energy