Nuclear Reactions

Alpha, Beta, and Gamma Decay

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

Identify the processes occurring in nuclear reactions written in an equation form.

The Atom

The atom consists of two parts:

1. The nucleus which contains:

2. Orbiting electrons.

protons neutrons

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

X

A

Z

Mass number

Atomic number

Element symbol

= number of protons + number of neutrons

= number of protons

X

A

Z A = Mass number =

number of protons + number of neutrons

Z = Atomic Number = number of protons

Number of neutrons = Mass Number – Atomic Number

U

235

92 U

238

92

There are many types of uranium:

Mass #

Atomic #

Number of protons

Number of neutrons

Mass #

Atomic #

Number of protons

Number of neutrons

U

235

92 U

238

92

There are many types of uranium:

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

Mass # 235

Atomic # 92

Number of protons 92

Number of neutrons 143

Mass # 238

Atomic # 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),

•  a beta particle (b),

•  or a gamma ray(g).

Radioactive Decay

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

It contains two protons and two neutrons.

Alpha Decay

Alpha Particle

•  Symbol: •  Mass: Heavy •  Change to Nucleus: Decreases the mass# by

4 and the Atomic # by 2 •  Penetration: Low •  Protection provided by: Skin •  Danger level: Low

X A

Z Y

A - 4

Z - 2 + He 4

2

Alpha Decay

unstable atom

more stable atom

alpha particle

Alpha Decay

Ra 226

88

Rn 222

86

He 4

2

X A

Z Y

A - 4

Z - 2 + He 4

2

Ra 226

88 Rn

222

86 + He 4

2

Alpha Decay

Rn 222

86 He

4

2 + Po 218

84 He

4

2

Rn 222

86 + Y A

Z He

4

2

Alpha Decay

He 4

2 U

234

92 + Th 230

90 He

4

2

X A

Z + Th 230

90 He

4

2

Alpha Decay

Th 230

90 + Y A

Z He

4

2

Alpha Decay

He 4

2 + Ra 226

88 He

4

2 Th

230

90

Beta Decay

A 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 Decay

As 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

Po 218

84

b 0

-1

At 218

85

Beta Particle

•  Symbol: •  Mass: Light •  Change to Nucleus: Converts a neutron to a

proton and increases atomic # by 1 •  Penetration: Medium •  Protection provided by: Paper, clothing •  Danger level: Medium

X A

Z Y

A

Z + 1 + b 0

-1

Beta Decay

Po 218

84 At

218

85 + b 0

-1

Th 234

90 Y

A

Z + b 0

-1

Beta Decay

Th 234

90 Pa

234

91 + b 0

-1

X A

Z Pb

210

82 + b 0

-1

Beta Decay

Tl 210

81 Pb

210

82 + b 0

-1

X A

Z Bi

214

83 + b 0

-1

Beta Decay

Pb 214

82 Bi

214

83 + b 0

-1

Gamma Decay

Gamma rays are not charged particles like a and b particles.

Gamma rays are electromagnetic radiation with high frequency.

When atoms decay by emitting a or b 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).

Gamma Rays

•  Symbol: •  Mass: No mass •  Change to Nucleus: None •  Penetration: High •  Protection provided by: Lead •  Danger level: High

Stable Nuclei

The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

Neutron-Proton Ratios

For smaller elements stable nuclei have a neutron-to-proton ratio close to 1:1.

Neutron-Proton Ratios

As elements get larger, it takes a greater number of neutrons to stabilize the nucleus.

Stable Nuclei

•  Nuclei above this belt have too many neutrons.

•  They tend to decay by emitting beta particles.

Stable Nuclei

•  Nuclei below the belt have too many protons.

•  They tend to become more stable by positron emission or electron capture.

Nuclear Fission •  How does one tap all that energy? •  Nuclear fission is the type of reaction carried out

in nuclear reactors.

Nuclear Reactors In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

Nuclear Reactors •  The reaction is kept in check

by the use of control rods. •  These block the paths of

some neutrons, keeping the system from reaching a dangerous supercritical mass.

Nuclear Fusion

•  Fusion would be a superior method of generating power.

–  The good news is that the products of the reaction are not radioactive.

–  The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.

–  Tokamak apparati like the one shown at the right show promise for carrying out these reactions.

–  They use magnetic fields to heat the material.