Board Work

1. What is the electrical interaction betweena. a proton and another proton in a nucleus?

b. a proton and a neutron in a nucleus?

c. a neutron and another neutron in a nucleus?

Nuclear Physics

Chapters 33–34

Objectives

• Describe the different types of nuclear reactions and identify the properties of nuclei undergoing them.

• Identify reactions and products in the nuclear power cycle and nuclear weapons.

Nucleons

Source: Griffith, Physics of Everyday Phenomena

Poll Question

The electric force tends to

A. make nucleons stick together.

B. push nucleons apart.

Nuclear Forces

• Electrostatic force (Coulomb’s Law)– Protons repel protons

• Strong nuclear force– Nucleons attract at short distances

Identifying Nuclei

• Number of protons = atomic number Z– determines the element

• Number of neutrons N– Isotope: same Z, different N

• N + Z = mass number A

• Isotope name: Name-A (e.g. Carbon-12)

Board Work

2. Find the elements with these atomic numbers:

a. 11

b. 23

3. Find the atomic numbers of the following elements:

a. Chlorine

b. Magnesium

Board Work

4. What isotope has 16 protons and 16 neutrons?

Nuclear Reactions

Parent Daughter(s) + “particles”

Decay Timescale

• 50% chance a nucleus will decay in one half-life

Source: Griffith

Poll Question

The fraction of parent nuclei remaining after two half-lives is

A. None — it has all decayed.

B. 1/2.

C. 1/4.

D. 1/8.

Nuclear Reaction Types

• Alpha decay

• Beta decay

• Gamma emission

• Fission

• Fusion

Main Radioactive Emissions

Source: Griffith

Alpha Decay

• Nucleus emits an alpha particle = 2 p + 2 n(helium-4 nucleus)

• Daughter nucleus has Z – 2, N – 2, A – 4

Poll Question

After alpha decay, the atomic number of a nucleus is

A. larger

B. smaller

C. the same

as before.

Poll Question

After alpha decay, the mass number of a nucleus is

A. larger

B. smaller

C. the same

as before.

Beta decay

• n = neutron• p+ = proton• e– = electron (beta particle)• e = electron anti-neutrino

• e–, e escape, p+ remains in nucleus• Daughter has same A, Z + 1, N – 1

n p+ + e– + e

Poll Question

After beta decay, the atomic number of a nucleus is

A. larger

B. smaller

C. the same

as before.

Poll Question

After beta decay, the mass number of a nucleus is

A. larger

B. smaller

C. the same

as before.

Gamma Emission

• Excited nucleus emits a high-energy photon (gamma-ray)

• Occurs only after another decay type

Source: Griffith

Poll Question

After gamma emission, the atomic number of a nucleus is

A. larger

B. smaller

C. the same

as before.

Board Work

5. Find the daughters of the following nuclear decays:

a. 239Pu +

b. 99Mo +

Nuclear Stabilities

Source: Hodgson et al., Introductory Nuclear Physics

Think Question

Which kind of atomic nuclei tend to undergo alpha decay?

A. Nuclei with large mass number A.

B. Nuclei with too many neutrons.

C. Very light (low Z) nuclei.

D. Nuclei with too many protons.

Think Question

Which kind of atomic nuclei tend to undergo beta decay?

A. Nuclei with large atomic number Z.

B. Nuclei with too many neutrons.

C. Very light (low Z) nuclei.

D. Nuclei with too many protons.

Fission

• Nucleus breaks into two large fragments plus several neutrons

Source: Griffith

Fusion

• Two nuclei combine to make a larger nucleus

• Neutrons usually released as well

• Requires extremely high energies (temperature) to overcome nucleus-nucleus electric repulsion

Binding Energy

Source: Georgia State U., Hyperphysicshttp://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html

fission

fusion

Think Question

What sort of nuclei release energy by fission?

A. Nuclei lighter than iron.

B. Nuclei about as heavy as iron.

C. Nuclei heavier than iron.

Summary

• Nuclei contain positive protons and neutral neutrons.

• Nuclear stability is a balance between electrostatic repulsion and the attractive strong force.

Summary

• Radioactive atoms can emit alpha particles, beta particles, and gamma rays.

• Nuclei can also split apart (fission) and combine (fusion).

• Reaction types depend on nuclear characteristics.

Reading for Next Time

• Nuclear fission and fusion– characteristics of reacting nuclei– conditions when reactions occur