Announcements
Binding Energy
Radioactive Decay
Nuclear Physics and Radioactivity
Sections 30.2 - 30.6
Nuclear Physics and Radioactivity
Final Questions
Announcements
Binding Energy
Reading Assignment
Read section 30.7 - 30.11
Homework Assignment 14
Homework for Chapter 30 (due by Thursday, December 9)
Q: 1, 5, 11, 17
P: 10, 16, 26, 40
Nuclear Physics and Radioactivity
Radioactive Decay
Final Questions
Announcements
Binding Energy
Radioactive Decay
Final Questions
Nuclear binding energy
The nuclear binding energy is the total energy required to tear a nucleus apart into its constituent protons
and neutrons
If we divide a nucleus’ binding energy by its mass number A, we get the binding energy per nucleon
The drooping of the binding energy curve at high mass numbers tells us that nucleons are more tightly
bound when they are assembled into two middle-mass nuclides than into a single high-mass nuclide
In other words, energy can be released by nuclear fission (or splitting) of a single massive nucleus into two
smaller fragments
The drooping of the binding energy curve at low mass numbers tells us that energy will be released if two
nuclides of low mass number combine to form a single middle-mass nuclide (nuclear fusion)
Nuclear Physics and Radioactivity
Announcements
Binding Energy
Radioactive Decay
Final Questions
Nuclear forces
The nuclear force (or residual strong force) is the force responsible for binding the protons and neutrons
into the atomic nucleus
The nuclear force must be strong enough to overcome the repulsive force between the positively charged
nuclear protons
The nuclear force must also be short-range, because its influence does not extend very far beyond the
nuclear surface
If two nucleons are separated by less than 10−15 m, the attractive nuclear force between them is
very strong
If two nucleons are separated by a distance greater than this, the nuclear force is essentially zero
The present view is that the nuclear force is not a fundamental force of nature but is, instead, a secondary,
or “spillover”, effect of the strong force (or strong interaction) which is the attractive force that binds
particles called quarks together to form the nucleons
Nuclear Physics and Radioactivity
Announcements
Binding Energy
Radioactive Decay
Final Questions
Radioactive decay
Most of the nuclides that have been identified are radioactive
Such a nuclide (a radionuclide) spontaneously emits a particle, transforming itself in the process into a
different nuclide
Radioactive decay provided the first evidence that the laws that govern the subatomic world are statistical
There is absolutely no way to predict whether any given nucleus in a sample will be among the number of
nuclei that decay at any given instant
For a radioactive sample that contains N0 nuclei at t = 0, the number N of nuclei remaining at any
subsequent time t is given by
−λt
N = N0 e
where λ is the decay constant (or disintegration constant)
The number of decay per second R = −dN/dt, is called the decay rate (or the activity) and is given by
R = R0 e
−λt
where R0 is the decay rate at t = 0
The SI unit for activity is the becquerel (1 becquerel = 1 Bq = 1 decay per second)
A quantity of special issue is the half-life τ , defined as the time after which both N and R are reduced to
one-half their initial values
ln 2
τ =
λ
Nuclear Physics and Radioactivity
Announcements
Binding Energy
Radioactive Decay
Final Questions
Alpha decay
A nucleus that decays by emitting an alpha particle (a helium nucleus) is said to undergo alpha decay
For example, the radionuclide 238 U decays according to the scheme
238
U→
234
4
Th + He
with a half life of 4.47 × 109 y
When α decay occurs, the daughter nucleus (234 Th in this case) is different from the parent nucleus (238 U
in this case)
Nuclear Physics and Radioactivity
Announcements
Binding Energy
Radioactive Decay
Final Questions
Alpha decay
A nucleus that decays by emitting an alpha particle (a helium nucleus) is said to undergo alpha decay
For example, the radionuclide 238 U decays according to the scheme
238
U→
234
4
Th + He
with a half life of 4.47 × 109 y
When α decay occurs, the daughter nucleus (234 Th in this case) is different from the parent nucleus (238 U
in this case)
Beta decay
A nucleus that decays by emitting an electron or a positron (a positively charged particle with the mass of
an electron) is said to undergo beta decay
Two examples of this decay are
32
and
64
P→
32
Cu →
64
S+e
−
+ν
+
Ni + e
+ν
where ν (ν) represents a neutrino (antineutrino), a virtually massless, neutral particle that is emitted
during the process
Nuclear Physics and Radioactivity
Announcements
Binding Energy
Reading Assignment
Read section 30.7 - 30.11
Homework Assignment 14
Homework for Chapter 30 (due by Thursday, December 9)
Q: 1, 5, 11, 17
P: 10, 16, 26, 40
Nuclear Physics and Radioactivity
Radioactive Decay
Final Questions