Transcript Chapter 30

Chapter 30:
Nuclear
Physics and
Radioactivity
Radioactivity
• Radioactivity is the
discentigration of an
unstable nuclei.
• when the nuclei decays the
nucleus emits alpha rays, beta
rays, and gamma rays.
Henri Becquerel
Becquerel was a French
physicist that came from a
family of scientists.
He used potassium uranyl
sulfate, K2UO2(SO4)2, and
exposed it to sunlight by
placing it on photographic
plates and wrapping it in black
paper.
This method revealed uranium
crystals.
Becquerel’s Results
More Becquerel Results
• He concluded that the
phosphorescent substance in
question emitted radiation which
penetrated the paper.
• He demonstrated that radiation
emitted by uranium shared certain
characteristics with x-rays, but
could be deflected by a magnetic
field, thus it must consist of charged
particles.
Becquerel’s
Accomplishments
• He was awarded the Nobel prize for
physics in 1903 for his discovery of
spontaneous radioactivity.
• The SI unit of radioactivity was named
after him, the Becquerel (Bq), which is
one transformation (or decay) per
second.
Marie and Pierre Currie
• They investigated radioactivity in
uranium after Becquerel made his
discovery.
• Pierre died and Marie finished the
work they started.
• After Marie finished her chemical
extraction, she said the compound
she was working with was more
radioactive then the uranium.
• This in turn led to the discovery of Po
and Ra (polonium and radium).
Marie’s Recognition
• In 1903 she and her husband were
awarded the Nobel prize in physics for
spontaneous radiation.
• In 1904 she was awarded the Nobel
prize in chemistry for the discovery of
two elements.
• She was the first person to receive two
Nobel prizes.
Observation of Radioactive
Rays
• Alpha Rays which barley penetrate
a piece of paper.
• Beta Rays which can penetrate
3mm of aluminum.
• Gamma Rays which can penetrate
several centimeters of lead.
We now know:
• Alpha rays are helium nuclei
• Beta rays are electrons
• Gamma rays are
electromagnetic radiation
Rays reacting to magnetic field
A
Z
X
X= Chemical symbol for element
A= Atomic mass
Z= Atomic Number
Radioactive Decay Law
N = Ne
- λt
0
No = initial amount of Substance
λ = decay constant (different for every substance)
t= time
e= natural exponential who’s value is 2.718…
14
6
C
∆N
∆t
The number of decays per second is called the activity.
The previous formula can also be written as:
∆N
∆t
∆N
∆t
=( ) e
- λt
Half-life Formula
The half-life is the time it takes for half the nuclei
in a given sample to decay. This formula is
derived from the previous formula we looked at.
Example
•
A radioactive material is known to produce 3000
decays per minute at one time, and 4.6 hours later it
produces 750 decays per minute. What is its half
life?
3000 1500  750
2.3 hrs
Example
• What fraction of a sample whose half life is
6 months will remain after 2 years?
• 24 = 4 ½ life: ½ after 6 months
6
¼ after 12 months
1/ after 18 months
8
1/
16 after 24 months
Section 30.13
Detection of Radiation
Detection of Radiation

Individual particles such as electrons,
protons, α particles, neutrons, and γ rays
are not detected directly by our senses.

Several instruments have been developed
to make up for this.
Geiger Counter
•
A cylindrical metal tube filled with
a certain type of gas (usually
helium, neon, or argon) with a wire
running down the center. The wire
is kept at a very high positive
voltage (slightly less than that
required to ionize the gas) with
respect to the cylinder.
•
Charged particles passing through
the window ionize a few gas
atoms. The freed electrons
accelerate toward the wire ionizing
more atoms along the way. When
this “avalanche” of electrons hits
the wire a voltage pulse is
produced which can be amplified
and displayed in the form of
audible clicks or by a needle
meter.
Scintillation Counter

A scintillator is a material that emits visible
light when struck by charged particles.
Typically crystals of NaI or certain plastics.

The scintillator is attached to a
photomultiplier tube which converts the
energy of the scintillator-emitted photons
into an electrical signal.

The photons emitted strike a photoelectric
surface called a photocathode. This emits
electrons which travel through the tube
striking several electrodes of successively
higher voltages along the way. When each
electrode is struck, more electrons are
ejected. By the time they reach the end they
have multiplied into a large number of
electrons, close to 106 or more. These
electrons produce an electric signal which
can be sent to a counter just like the Geiger
counter.
Bubble Chamber

In addition to detecting the presence of charged particles, some devices can be
used to determine the path.

The bubble chamber uses a superheated liquid kept close to boiling point (usually
liquid hydrogen). The bubbles characteristic of boiling form around ions produced by
the passage of charged particles. Photos can determine the path of the particles.

A magnetic field is usually applied across the chamber and the momentum can be
determined from the radius of curvature of the particle paths.
Wire Drift Chamber

The wire drift chamber is a more modern
way of detecting the paths of charged
particles. It can be likened to a very large
and more complex Geiger counter.

It consists of many thin wires immersed in
gas. Some are grounded while others are
kept at a high voltage. Charged particles
passing through the chamber ionize the gas.
Electrons drift toward the high voltage wires
and produce an “avalanche” along the way.
These produce voltage pulses.

The positions of the particles are determined
electronically by the position of the wire and
by the time it takes for the pulses to reach
the sensors at the end of the wires. These
are used by computers to reconstruct the
paths.
Homework
Problems 36 & 39
Concept
Radioactivity is unaffected by…
a.
Heating
b.
Cooling
c.
Chemical reagents
d.
a&b
e.
All of the above
The decay constant is…
a.
The same for all isotopes
b.
Different for different isotopes