Transcript Chapter 29 – Nuclear Physics

Lecture Outline
Chapter 29
College Physics, 7th Edition
Wilson / Buffa / Lou
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29.1 Nuclear Structure and the
Nuclear Force
Early models of the atom had it as a kind of
“plum pudding,” with the negatively charged
electrons strewn through a diffuse positive
charge.
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29.1 Nuclear Structure and the
Nuclear Force
Rutherford measured scattering of alpha
particles (helium nuclei) from gold. If the
plum pudding model were correct, the
scattering angles should be small.
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29.1 Nuclear Structure and the
Nuclear Force
What Rutherford observed were some very
large-angle scatters; these could happen only
if the positive charge in the atom were packed
into a very tiny space.
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29.1 Nuclear Structure and the
Nuclear Force
As small as the atom is—about 10–10 m across—
the nucleus is about 10,000 times smaller.
If the atom were the size of a football field, the
nucleus would be about the size of a grape.
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29.1 Nuclear Structure and the
Nuclear Force
The nucleus is not just a ball of positive
charge; it is made of protons (positively
charged) and neutrons (uncharged).
In order to hold these positive charges
together in a very small volume, the
repulsive Coulomb force must be
overwhelmed by another force. This force is
called the strong nuclear force.
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29.1 Nuclear Structure and the
Nuclear Force
The nuclear force is very strong, but also very
short-range—it acts only over the distance of
a few proton or neutron diameters.
The nuclear force is the same between
neutrons and protons as it is between
protons and between neutrons. The attraction
between neutrons (and the lack of repulsion)
is essential for keeping the nucleus together.
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29.1 Nuclear Structure and the
Nuclear Force
Nuclear notation
includes the element
symbol and the mass
number, and may
include the atomic and
neutron numbers. These
are not strictly
necessary, though, as
the element determines
the atomic number.
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29.1 Nuclear Structure and the
Nuclear Force
Isotopes are nuclei of the same element that
have different numbers of neutrons.
Hydrogen isotopes have their own names—
the hydrogen nucleus is a single proton,
deuterium is a proton plus a neutron, and
tritium is a proton plus two neutrons.
An isotope of carbon, carbon-14, is used to
date formerly living materials.
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29.2 Radioactivity
Three types of
radioactivity are indicated
here. Alpha rays are
helium nuclei, beta rays
are electrons, and gamma
rays are photons.
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29.2 Radioactivity
Radioactivity originates in the nucleus; it is not
affected by chemical reactions or
environmental conditions.
Nuclear decays of a given isotope are
observed to occur at a fixed rate. This is an
indication that quantum mechanics is at work;
classically, if it is energetically favorable for a
nucleus to decay, it will do so immediately.
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29.2 Radioactivity
An alpha particle is a helium nucleus, with two
protons and two neutrons.
When a nucleus emits an alpha particle, its
neutron number and proton number each
decrease by 2, and its mass number by 4.
Parent Nucleus and Daughter Nucleus
Example:
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29.2 Radioactivity
Two conservation laws hold for radioactive
decay:
1. The total number of nucleons (protons and
neutrons) is constant.
2. Electric charge is conserved.
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29.2 Radioactivity
• A 239Pu nucleus undergoes alpha decay.
What is the daughter nucleus?
29.2 Radioactivity
Beta decay is the emission of an electron.
This electron is not one of the atomic
electrons, nor is it sitting around in the
nucleus; it is created during the decay itself.
In the process, a proton becomes a neutron,
or vice versa.
Example:
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29.2 Radioactivity
Beta decay can also occur by the emission of a
positron, or by the capture of an inner orbital
electron.
Example of positron decay:
Example of electron capture:
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29.2 Radioactivity
Gamma decay occurs when an excited
nucleus decays to its ground state. Nuclei
have energy levels similar to those of atoms,
although the energy levels and separations
are about 1,000,000 times larger. Excited
nuclei are usually produced as a daughter
nucleus from either alpha or beta decay.
Example:
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29.2 Radioactivity
• Worksheet with examples
29.2 Radioactivity
Alpha, beta, and gamma rays react quite
differently when interacting with matter.
Alpha particles are doubly charged and
relatively massive; they do not penetrate
very deeply at all. A sheet of tissue paper is
enough to stop them.
Beta particles are light and singly charged;
they will penetrate farther. Still, a few
millimeters of aluminum will stop them.
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29.2 Radioactivity
Gamma rays are massless and uncharged,
and can be very penetrating. A sufficiently
thick layer of lead will stop them, but it would
need to be more than a centimeter thick.
Radioactive particles stopping in the human
body can cause considerable damage.
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29.3 Decay Rate and Half-Life
The half-life is related to the decay constant; it
is the time for half the initial nuclei to decay.
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