Transcript 4.4-Radioactivity

Radioactivity: the process by which
atoms emit energy in the form of
electromagnetic waves, charged
particles, or uncharged particles.
• In 1896, Henri Bequerel discovered that
uranium and other elements emitted
invisible rays that can penetrate solid
material. These materials are now called
“radioactive”
• The most common unit for
radiation is counts per second
(known as a Becquerel, Bq)
CBC archives - radioactivity
Natural Sources
• Exposure to radiation is unavoidable because radioactive
elements occur in nature.
- some forms of carbon and potassium are absorbed
by your body are radiactive.
C  600 Bq/kg of body mass
K  110 Bq/kg “ “
“
Alps Iceman: 5,300 years old
-Cosmic rays: high energy
radiation coming from space.
- higher exposure than
normal when flying at high
altitudes
- Radioactive uranium and radium are found in soil and
rocks. When they disintegrate, the produce another
radioactive atom: radon gas.
Uranium
deposits
around the
world
Artificial Sources
• Nuclear power
- Electricity
- Submarines
- Space probes
February 1, 2005—The U.S. Navy released this
photograph last Thursday of the nuclear submarine
San Francisco, which crashed headlong into an
uncharted undersea mountain near Guam on
January 8. Standing more than three stories high
and with classified technology veiled by a tarp, the
fast-attack submarine is shown awaiting repairs in a
Guam dry dock.
The impact shredded the submarine's nose, killed
one sailor, and injured 60 more. The sailors were
largely protected by the vessel's reinforced inner
hull, which did not rupture. After the wreck, the crew
quickly ascended and sailed along the ocean's
surface back to their base in Guam.
The Cassini space probe is powered by energy released
from 28.8 g of radioactive Pu. The radiation is absorbed by
ceramic surronding the Pu and the heat is converted ot
electricity. Each Kg of Pu emits 556 J each second.
• There is a lot of
radiation released
inside nuclear
reactors and by the
spent fuel (but still
less than is emitted
by x-ray machines)
• Some coal-fired power
plants emit more
radioactivity than
nuclear plants (uranium
in coal ash)
- Nuclear bombs
-Medical applications:
- X-rays are used for
diagnosis
- Cancer treatment
In medicine: we
use a unit called
Sieverts
(10 Sv is a lethal
dose for most
tissues)
Effects of Radiation
- Ionizing radiation carries energy values on the order of
1000’s of eV.
- Typical chemical bonds can be broken by radiation energy
of 5 or less eV.
- Cells do have repair
mechanisms, but they
are not perfect and they
can be overwhelmed.
- Large particle radiation
(such as α particles) can
do more damage per unit
of energy.
Effects of Cell Damage:
1) Cell dies: organelles or
enzymes can no longer
function
2) Cell survives:
Damage is
passed on to
daughter cells in
the form of
mutations (some
mutations can
lead to caner).
• Cells undergoing
division are more
susceptible to
damage
Radiation Strength
Depends on three factors:
1) The kind of particles/EMR emitted
2) Amount of radioactive material present
3) The rate at which atoms disintegrate to emit
radiation (1 count/second = 1Bq) – depends on
the isotope.
Structure of the Nucleus - Review
Which elements are these?
(protons are shown in red
and neutrons in white.)
They are both carbon. Both have 6 protons. i.e. they
both have an atomic number of 6.
These are two isotopes (varieties) of carbon.
- same chemical properties, but different physical properties
(e.g. how they behaving in nuclear reactions)
- different number of neutrons, therefore different atomic
masses
In nuclear physics, we often call atoms nuclides.
Carbon-12
Carbon-14
12
6
C
Mass number = 12
14
6
C
Mass number = 14
p+ = 6
p+ = 6
n0 = 6
n0 = 8
Mass number = #p+ + #no 12
C
6
+
Atomic number = #p
ISOTOPES
NAME
hydrogen-1
hydrogen-2
(deuterium)
hydrogen-3
(tritium)
SYMBOL
1
1
H
2
1
H
3
1
H
ISOTOPES
NAME
SYMBOL
lithium-6
6
3
Li
lithium-7
7
3
Li
The Strong Nuclear Force
• Using
accelerators,
scientists have
discovered the
forces that hold
nuclei together
The big circle marks the location of the Large Hadron Collider
(LHC) at the European particle physics laboratory in CERN. The
tunnel where the particles are accelerated is located 100 m (320
ft) underground and is 27 km (16.7 mi) in circumference. The
smaller circle is the site of the smaller proton-antiproton
collider. The border of France and Switzerland bisects the
CERN site and the two accelerator rings.
• Nuclear forces act over very small ranges. (3 x 10-15 m)
• Over 100 times greater than the electrostatic force.
• The strong nuclear force is independent of the charge
• The attraction is the same between:
p+ - p+
n0 - n0
n0 – p+
Unstable (Radioactive) Nulcides
• Unstable nuclides tend to disintegrate causing:
 A different nuclide is to be produced
 Energy to be released as radiation
• Unstable nuclides have too few neutrons in relation to the
number of protons.
 In general, the more protons in a nucleus, the more
neutrons that are required to overcome the electrostatic
repulsion.
• All elements with atomic numbers greater than 82 exist
only as unstable nuclides.
Types of Radiation
• Rutherford discovered
three types of radiation
• Also discovered that
elements transform into
different elements
during the process
(called transmutation).
• The original element is called the
parent nuclide. The newly formed
element is called the daughter nuclide.
Alpha Decay
• Alpha particles (α) are helium- 4
• They are ejected at high speeds
but can be stopped by aluminum
foil
222 Rn
86

218Po  4 He (or could be
84
2
written as  )
For all nuclear reactions: NUCLEONS AND CHARGE ARE
CONSERVED
i.e. The sum of the mass numbers on both sides of the
arrow must be equal and the sum of the atomic numbers
on both sides of the arrow must be equal
222 nucleons
222 Rn
86
charge = +86
222 nucleons

218Po  4 He
84
2
charge = +86
Beta Decay
• A neutron decays into a
proton and an electron.
• The electron is ejected
from the nucleus at a high
speed – called a beta
particle (β).
• β particles can penetrate
several mm of lead.
228 Th  228Pa  0 e (can also be written :   )
90
91
1-
228 nucleons
228 nucleons
228 Th
90
charge = +90

228Pa  0 e
91
1charge = +90
• Gamma rays can be
emitted along with an
alpha or beta particle.
• When a nucleus emits
only a gamma (γ) ray,
the energy of the
nucleus is reduced but
the mass number and
the atomic numbers stay
the same.
• γ rays can penetrate
many cm of lead.
Gamma Decay
60 Co *
27
exited

60 Co
27

unexcited
• Often, the same nuclide can undergo different decay modes…
Decay Series
… or go through a series of decays.
Example 1: Complete the balance equation:
210
83
Bi  He  ____
4
2
Nuclear charge: 83 – 2 = 81
According to my periodic table, that must be
Nucleons: 210 – 4 = 206
What type of radiation
is this?
Alpha Decay
TI
Example 2: Complete the balanced equation and identify the
radiation type.
Np
U  e  ____
237
92
0
1
237
93
Neptunium-237
Beta Decay
Other Decay Modes
• Some radionuclides can transmutate by capturing an
electron from the lowest energy level.
 A proton is converted into a neutron
41Ca
20
41K  neutrino
1 0e  19
• Positron emission: (same mass as an electron, but a
positive charge)
111Sn
50
0e
 111
In

49
1
Fission and Fusion
Nuclear Fission
• The reaction used in all of the world’s nuclear power
plants. The fuel is usually uranium, put plutonium can
also be used.
• Can be used in nuclear
bombs.
Involves “splitting” an atom
into smaller nuclides.
• Initiated by a slow moving
neutron.
Fission Animation
More animations
Example 3: Predict the missing fission product.
Nuclear Fission Chain Reaction
• The emitted neutrons
strike more uranium
atoms, causing them to
undergo fission.
• This reaction is very
hard to control.
http://www.space
kid.net/nuclear/fis
sion.html
Canada’s
CANDU Reactor
• Canadian Deuterium
Uranium Reactor
Nuclear Fusion
• The process that
made the atoms that
make you.
• Two nuclide with
extremely high energy
collide to form a
bigger nuclide.
animation
Example 3: Predict the missing reactant.
Nuclear fusion as an energy source
on earth is still experimental
• A sample of radioactive material consists of vast number
of nuclei that don’t all decay simultaneously.
• We can’t predict when a single nucleus will decay (it is
governed only by probability)
• The decay from parent nuclide to daughter nuclide
follows a characteristic decay curve.
• Rutherford noticed that the radioactivity of
a sample of radon gas was reduced by half
every ~1 minute.
12.5%
•This called the half-life of the isotope.
half-lives can vary from 10-22s to 1028
s, depending on the isotope.
50%
25%
100%
Radioactivity
Radioactive Decay Curve
• Half-lives are always a uniform
interval of time for a
particular isotope.
Time
• More examples of half-lives:
- Polonium-214 ---1.6 x 10-4 s
- Carbon-14 --------5730 years
• If you have 10 g of
carbon-14 when an
organism dies, after
5730 years, you’ll have
5 g. After another 5730
years, you’ll have 2.5g.
• The age of a material
can be determined
using radioactive
dating
• An equation that describes half-life
1
N  N0  
2
Amount or
mass of the
parent nuclide
remaining
Original
amount of
parent
nuclide
n
Number of
half-lives
that have
passed
Example 1:
If a 2.00 g sample of strontium-90 is
produced in a reactor, how much will
remain after 10.0 years have passed.
(The half-life of Sr-90 is 29.1 years.)
Effect of Strontium-90 on
Squamous Cell Carcinoma in an
Eastern Box Turtle (Terrapene
carolina); Discussion of
Alternative Treatment Modalities
Cheryl B. Greenacre, DVM, Dipl.
ABVP - Avian and Royce Roberts,
DVM, MS, Dipl. ACVR
1.58 g
Example 2:
A baby mammoth
found frozen in a
glacier is found to
contain one quarter of
its original carbon-14.
Determine its age if the
half life for the
radioactive decay of
carbon-14 is 5.73 x 103
years.
1.15 x 104 years
Extension example:
A pregnant ichthyosaur
fossil is located just below
a volcanic ash layer
containing a ratio of
uranium-235 to lead-207
of 4:1. Determine the
minimum age of the fossil
in years. (The half-life of
U-235 is 7.13 x 108 a)
1
N  N0  
2
n
N 1
 
N0  2 
n
 N 
1

log 

n
log



N
2


 0
log N  log N 0
n
1
log
2
230 million years
• The mass of a nucleus is always less
than the mass of all the separate
nucleons (protons and neutrons)
• This difference in mass is called the
mass defect
• Energy is required to make a nucleus
(called the binding energy)
E = mc2
 The binding energy
is related to the
mass defect by the
equation E = mc2
Example 1
Determine the mass defect of an alpha particle.
alpha particle mass (2 protons, 2 neutrons) = 6.65 x 10-27kg
massprotons =2(1.67 x 10-27kg) = 3.34 x 10-27 kg
massneutrons = 2(1.67 x 10-27kg) = 3.34 x 10-27 kg
total mass of separate nucleons = 6.68 x 10-27 kg
mass defect =
-
= 0.03 x 10-27kg
• In nuclear reactions, mass is converted to energy or
energy is converted to mass
E = mc2
Example 2:
Calculate the energy produced in the reaction
mass2H = 3.34341 x 10-27 kg massα = 6.6463 x 10-27 kg
mass3H = 5.00661 x 10-27 kg massn = 1.6749 x 10-27 kg
masstotal = 8.35002 x 10-27 kg masstotal = 8.3212 x 10-27 kg
Mass defect = 8.35002x10-27 kg – 8.3212x10-27 kg
= 2.882 x 10-29 kg
E = mc2
E = (2.882 x 10-29 kg)(3.00 x 108 m/s)2
E = 2.59 x 10-12 J
CANDU
In a CANDU reactor, 1 kg
of fuel (natural uranium)
produces 3.4 x 105 MJ of
heat that is converted to
electricity.
CANDON’T
In oil and coal power
plants, 1 kg of fuel
produces about 4 MJ of
heat
• Energy may create matter through the process called pair
production. The process must produce 2 particles whose
total charge is zero, since charge must be conserved. Pair
production requires a very high energy photon.
• A particle and its antiparticle (antimatter) are often
produced. Example: an electron and anti-electron (positron)
have the same mass, but opposite signs.
Example 3:
A 8.50 x 1020 Hz photon produces an electron and an antielectron. Determine the total kinetic energy of the particles.
Law of Conservation of Energy:
Photon energy = energy to make 2 particles + Ek
Ephoton = Eelectron + Eantielectron + Ek
hf
= mc2 + mc2 + Ek
hf
= 2(mc2) + Ek
Ek
= hf – 2(mc2)
Ek
= (6.63 x 10-34 J•s)(8.50 x 1020 Hz) – 2(9.11 x 10-31kg)(3.00 x 108 m/s)2
Ek
= 4.00 x 10-13 J
Practice:
Mass-Energy:
P 907: Q 1-3
P 928: Q 27
Radioactivity in General
P 904, 905: Q 9-13, 15, 16