Nuclear Reactions

Alpha, Beta, and Gamma Decay

CS 4.2

CS 4.3

State what is meant by alpha, beta and gamma decay of radionuclides.

Identify the processes occurring in nuclear reactions written in symbolic form.

The AtomThe atom consists of two parts:

1. The nucleus which contains:

2. Orbiting electrons.

protonsneutrons

All matter is made up of elements (e.g. carbon, hydrogen, etc.).

The smallest part of an element is called an atom.

Atom of different elements contain different numbers of protons.

The mass of an atom is almost entirely due to the number of protons and neutrons.

The Atom

XA

Z

Mass number

Atomic number

Element symbol

= number of protons + number of neutrons

= number of protons

XA

ZA = number of protons + number of neutrons

Z = number of protons

A – Z = number of neutrons

Number of neutrons = Mass Number – Atomic Number

U235

92U

238

92

There are many types of uranium:

A

Z

Number of protons

Number of neutrons

A

Z

Number of protons

Number of neutrons

U235

92U

238

92

There are many types of uranium:

Isotopes of any particular element contain the same number of protons, but different numbers of neutrons.

A 235

Z 92

Number of protons 92

Number of neutrons 143

A 238

Z 92

Number of protons 92

Number of neutrons 146

Most of the isotopes which occur naturally are stable.

A few naturally occurring isotopes and all of the man-made isotopes are unstable.

Unstable isotopes can become stable by releasing different types of particles.

This process is called radioactive decay and the elements which undergo this process are called radioisotopes/radionuclides.

Radioactive decay results in the emission of either:

• an alpha particle (),

• a beta particle (),

• or a gamma ray

Radioactive Decay

An alpha particle is identical to that of a helium nucleus.

It contains two protons and two neutrons.

Alpha Decay

XA

ZY

A - 4

Z - 2 + He4

2

Alpha Decay

unstable atom

more stable atom

alpha particle

Alpha Decay

Ra226

88

Rn222

86

He4

2

XA

ZY

A - 4

Z - 2 + He4

2

Ra226

88Rn

222

86+ He

4

2

Alpha Decay

Rn222

86He

4

2+Po218

84He

4

2

Rn222

86 +Y A

ZHe

4

2

Alpha Decay

He4

2U

234

92 +Th230

90He

4

2

X A

Z +Th 230

90He

4

2

Alpha Decay

Th 230

90 +Y A

ZHe

4

2

Alpha Decay

He4

2+Ra226

88He

4

2Th

230

90

X A

Z +Pb 214

82He

4

2

Alpha Decay

He4

2+Pb214

82He

4

2Po

218

84

Beta DecayA beta particle is a fast moving electron which is emitted from the nucleus of an atom undergoing radioactive decay.

Beta decay occurs when a neutron changes into a proton and an electron.

Beta DecayAs a result of beta decay, the nucleus has one less neutron, but one extra proton.

The atomic number, Z, increases by 1 and the mass number, A, stays the same.

Beta Decay

Po218

84

0

-1

At218

85

XA

ZY

A

Z + 1 + 0

-1

Beta Decay

Po218

84Rn

218

85 + 0

-1

Th234

90Y

A

Z + 0

-1

Beta Decay

Th234

90Pa

234

91 + 0

-1

X A

ZPb

210

82 + 0

-1

Beta Decay

Tl210

81Pb

210

82 + 0

-1

Bi210

83Y

A

Z + 0

-1

Beta Decay

Bi210

83Po

210

84 + 0

-1

X A

ZBi

214

83 + 0

-1

Beta Decay

Pb214

82Bi

214

83 + 0

-1

Positron Decay

• Positron decay is like a mirror image of beta decay. These points present a simplified view of what positron decay actually is:

• 1) Something inside the nucleus of an atom breaks down, which causes a proton to become a neutron.2) It emits a positron and a neutrino which go zooming off into space.3) The atomic number goes DOWN by one and mass number remains unchanged

Positron Decay

Positron decay

Try these positron decays

Electron capture

Electron capture is not like any other decay - alpha, beta, or position. All other decays shoot something out of the nucleus. In electron capture, something ENTERS the nucleus.

Electron Capture

• The atomic number goes DOWN by one and mass number remains unchanged

Electron capture

Try these electron captures

Gamma DecayGamma rays are not charged particles like and particles.

Gamma rays are electromagnetic radiation with high frequency.

When atoms decay by emitting or particles to form a new atom, the nuclei of the new atom formed may still have too much energy to be completely stable.

This excess energy is emitted as gamma rays (gamma ray photons have energies of ~ 1 x 10-12 J).