Transcript The History of Radioactivity

Radiation
Electromagnetic Radiation
X-rays
Roentgen and famous
picture of his wife’s hand
William Roentgen
German Physicist - 1895
• Was working with cathode ray
tubes when he noticed that a
phosphorescent material in his
lab was glowing several
meters away.
• He made a phosphorescent
screen and was shocked when
he put his hand in front of the
screen – he could see the
outline of his bones!!
• He did not know what kind of
rays were responsible for this
phenomenon so he called
them X-rays
Electromagnetic Spectrum
(of waves)
• (x-rays were part of EM spectrum)
• Light, heat (infra-red), microwaves, ultraviolet, x-rays, gamma rays and radio
waves are all just energy waves of
different frequencies.
• The sun emits radiation across the EM
spectrum
Anatomy of a Wave
Frequency and Wavelength
• Frequency (f ) - the number of waves that
go by each second (Hertz – Hz)
• Wavelength (λ) - the length of one
complete wave.
(eg. Radio waves from station 95.3 MHz, 95
300 000 waves go by each second and a
wave length of about 3m)
Electromagnetic Spectrum
Low Frequency
Low Energy
High Frequency
High Energy
Summary of EM radiation
• All EM radiation travels at the speed of
light (3.0X108 m/s)
• Frequency is proportional to Energy
f E
• Frequency is inversely proportional to
wavelength
f 
1

• EM radiation has NO MASS and can travel
without particles
Particle Radiation
Discovery of Radioactivity
Henri Becquerel
French Physicist
• Becquerel put different elements in the
sun and then placed them on
photographic plates in dark drawers to
study phosphorescence.
• One day in 1896 there was no sun and
he put Uranium on a photographic
plate in a dark drawer. The next day
the plate was cloudy! Energy was
coming from Uranium itself!
Discovery of Radium and Polonium
• Marie and Pierre
Curie spent years
purifying radioactive
elements.
• They discovered new
radioactive elements
Radium and Polonium
in1898.
Marie and Pierre Curie of
Poland and France
Three types particle radiation
• Rutherford found three
different types of
particles were emitted.
• He called them alpha(),
beta() and gamma(γ)
particles
Ernest Rutherford
New Zealand
Penetrating ability of particles
• alpha particles stopped by paper
• beta particles stopped by 5 mm of aluminum
• gamma rays stopped by 30 cm of dense lead
Charge on three types of radiation
In a magnetic field
• Alpha particles deflected one way,
• Beta particles deflected the other way
• Gamma rays not deflected at all.
γ(0)
(+)
(-)
Spontaneous Radiation
Summarized
+
+
-
Type of Radiation
Alpha particle
Beta particle
Gamma ray (EM)
What is it?
Helium Nucleus
Electron
Photon
Symbol


g
Mass (atomic mass
units)
4
1/2000
0
Charge
+2
-1
0
Speed
slow
fast
speed of light
Ionizing ability
high
medium
indirectly
low
(paper)
medium
(5mm of aluminum)
high
(30 cm lead)
Penetrating power
Where does all of this radiation come from?
Radioactive Atom
Ionizing Radiation
alpha particle
X-ray
beta particle
gamma ray
Radiation Types Clarified
+
• Particle radiation
+
consists of particles that have mass.
ie. alpha or beta particles.
• Electromagnetic Radiation
consists of massless waves/photons of
higher and lower energies.
ie. X-rays, gamma rays, light, radio
waves, UV, IR, microwaves
Structure of the Atom (review)
• Atoms all contain a dense nucleus with protons
and neutrons.
• Elements are arranged on the periodic table
ONLY by the number of protons in the nucleus
• Electrons travel around the nucleus and
attempt to balance the charge from the
protons.
Isotopes
• Isotopes are elements with the same number of
protons and a different number of neutrons
eg. Carbon-12 and Carbon-14
(this is also the charge
on the nucleus)
Stable Isotopes
• Elements can only accept certain
numbers of neutrons. Too many or
too less, and elements become
unstable and will decay (orange) or
not form at all (white)
Radioactive Decay
• Radioactive decay is the spontaneous
decay of atoms by emitting alpha, beta or
gamma particles.
• New elements are always formed during
alpha and beta decay
• (This is the sort of thing that happens with
radioactive waste)
Alpha Decay
Beta Decay
137
55
Cs Ba e
137
56
0
1
• Beta decay involved the ejection of a beta particle (could
either be an electron or a positron)
• Electrons come from a neutron and change it to a proton!!
Gamma Decay
• The “*” denotes high energy
• Gamma rays are emitted when a particle has too much
energy. No new elements are formed.
Sources of Gamma Rays
• Supernova explosions, neutron stars, and black
holes are all sources of celestial gamma-rays
Balancing Nuclear Equations
• Both MASS and CHARGE must be conserved in
any nuclear reaction
• This means that the sum of masses and atomic
numbers on the right and left sides of the
equations must be equal!
eg. The decay of Uranium-238:
U  He Th
238
92
4
2
234
90
Balancing examples:
226
88
Ra He  222
86 Rn
4
2
42
19
241
95
K e 
0
1
42
20
Ca
K  He 2 n 
4
2
1
0
243
97
Bk
Half Life
• The half-life is the amount of time it takes for half
of the unstable atoms in a sample to decay.
• The half-life for a given isotope is always the
same no matter how many particles you have or
what happened in the past.
• For example, if an element has a half life of 4 days and
starts off with 16g of unstable particles, then after the
first four days, 8g will remain. After the next four days,
4g will remain. After four more days, 2g will remain etc.
Half Life
18
16
14
12
10
8
6
4
2
Decay applet
24 days
20 days
16 days
12 days
8 days
4 days
0
0 days
radioactive
grams
remaining
Carbon-14 Dating
• Cosmic rays change Nitrogen-14 in
to Carbon-14 in the atmosphere.
This radioactive form of Carbon-14
is absorbed into organisms through
carbon dioxide.
• Once the organism dies, the
carbon slowly decays to nitrogen14
• The half-life of carbon-14 is about
5730 years.
How it works
Uses of Radiation
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Pet Scans – Uses positrons to get moving 3d image
by reacting with radioactive injections
Cat Scans – 3D X-ray image
X Rays (do not include Cat Scans)
Food irradiation
Tracers, leakage, and wear in industry, density and
thickness measurements
Cancer Treatment
Activation analysis, crime solving – composition
determining using spectrometry
Smoke detectors
Microwave ovens
Cell/mobile phones
The dangers of radiation
Ionizing radiation
The Dangers of Radiation
• Both high frequency electromagnetic radiation
and particle radiation can ionize atoms (give
them a charge).
• Ionized atoms can change the DNA leading to the
reproduction of cancerous cells
• Types of ionizing radiation include alpha, beta
(particle), gamma, x-rays and neutrons (EM).
Genetic Changes
How do we measure radiation?
Geiger Muller Tube (Geiger Counter)
• Has a large cylindrical cathode (positive electrode) and a
tungsten anode (negative electrode) with a voltage across them.
• When alpha, beta or gamma radiation ionizes a gas particle, the
positive ion moves to the cathode and the electron moves to the
anode, thus sending a signal to the counter.