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Ionising Radiation - media.cns

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Canadian Nuclear Society
Ionising Radiation Workshop
CNS Team
Naturally
Occurring
Radioactive
Material
Doug De La Matter
Peter Lang
Bryan White
Jeremy Whitlock
Rolly Meisel
Be Aware of NORM
Short Version
2011-04-08
www.cns-snc.ca
1
The ionising radiation workshop kit…
Geiger
counter
USB
Interface
Computer
If your table has a computer, please don’t
disturb it -- we’ll get to it shortly.
2
The CNA Nuclear Science & Technology High School
Curriculum website
•
•
Reference material
Curriculum-approved lesson plans
www.cna.ca
3
4
So why does the CNS provide this workshop?
•
We believe that students will benefit from
simple, personal, practical demos /
experiments that enrich their classroom
experience.
•
We’re convinced that investing in science
teachers is the best way we can help
improve public understanding of ionising
radiation.
5
Program for Today:
• Electromagnetic Radiation
• Particle Radiation
• Ionising vs Non-ionising Radiation
• Radioactive Decay and background
• Detecting Radiation
• Experiments with a Geiger Counter
6
What is Radiation?
• Energy emitted by a source travelling through
space away from the source.
• Most radiation we encounter is Electro-Magnetic
radiation and behaves like visible light.
I’m a wave!
I’m a particle!
(0 rest mass)
Just call me a photon.
7
Particle Radiation
• Radiation can also refer to sub-atomic particles:
– most have finite “rest mass”
– Electrons, protons, neutrons, alpha particles, muons,
pions, neutrinos, …?
• Particles may be released from an atomic nucleus
undergoing radioactive decay, or fission, or by an interaction
such as “scattering”.
• Particles may be produced by interactions of other particles
-- or may be produced by a particle accelerator.
8
How about a microwave oven?
• Popular slang sometimes refers to cooking food in
a microwave oven as “Nuking”.
• Microwaves are electromagnetic radiation with
photons having miniscule energies compared to
the binding energy of atomic electrons.
• You can’t “Nuke” anything in a microwave oven.
NONIonising!
9
Electromagnetic Radiation
non-ionising
ionising
Figure copied from “Radiation Awareness” PowerPoint File
by Health Physics Society, crediting NASA/JPL-Caltech
Notice that cell phone radiation falls well into the
“non-ionising” region of electromagnetic radiation.
10
Radioactive Decay
• A radioactive atom has excess energy in its
nucleus, but not quite enough to change to a lower
energy state, and then...
…spontaneously it changes to a lower energy state.
• It does this by emitting sub-atomic particles…
…and/or electromagnetic energy in the form of
gamma radiation…
…through quantum-mechanical
tunnelling and other
mechanisms.
• One decay per second is known
as one becquerel (Bq) of activity.
11
Online Interactive Chart of the Nuclides
http://www.nndc.bnl.gov/chart/
12
Alpha Radiation
• Heavy nuclei that have “2 too many protons” will
emit particles made up of 2 protons and 2 neutrons.
• These are known as “alpha particles”
(the nucleus of a helium atom, 4He+2)
• After alpha emission, there is a nuclide with a
different atomic number (a different element):
this is known as transmutation.
• The resulting nuclide may or may not be
radioactive itself.
13
Beta Radiation - 1
• A nucleus that has “1 too many neutrons” will
emit an electron – a beta-minus particle
– A neutron changes into a proton, an electron
and an anti-neutrino
– The electron and anti-neutrino are emitted –
along with a photon (gamma) in many cases
• After beta emission, there remains a nuclide with
a different atomic number – a different element.
• The new nuclide may or may not be radioactive
itself.
14
Beta Radiation - 2
• A nucleus that has “1 too many protons” will
capture an orbital electron, or emit an antielectron – a positron – a beta-plus particle
– A proton changes into a neutron by:
• combining with an electron and emitting a
neutrino
• OR by emitting a positron and a neutrino
– This form of beta decay also emits a photon
(gamma) in most cases.
• There remains a nuclide with a different atomic
number – a different element.
• The new nuclide may or may not be radioactive
itself.
15
Gamma Radiation
• Highest energy EM radiation
• Interaction with matter similar to X-rays
• “Collision” with an electron can ionise the atom,
breaking a chemical bond.
16
Gamma Radiation
• Easily penetrates the body
• Intense sources (Co-60, Cs-137 and high energy
electron accelerators) are used to irradiate
tumors
• Absorbed by large
thickness of water, lead
metal or concrete
• The atmosphere over
your head provides
shielding equivalent to
10 m of water
17
Radioactive Decay
• If we start with 100 atoms of a particular nuclide,
after a certain time we will have 50 of those atoms left.
• This is known as the “half-life” of the nuclide.
• After another “half-life”, we will have 25 of those
atoms left .
100%
90%
80%
70%
60%
50%
40%
30%
20%
0.39%
10%
0%
0 1
2 3
4
5
6
7
8
18
Non-Ionising Radiation
• Does not displace electrons from atoms.
• Can break chemical bonds due to heating effects.
• Includes radio waves, microwaves, infrared
radiation, visible light, and some UV.
• Microwaves couple to molecular vibrations and
rotation.
• Visible light couples to electron quantum state
transitions.
19
Ionising Radiation
• Able to displace electrons from atoms, often
breaking chemical bonds.
• Includes ultraviolet light, x-rays, and gamma rays
from the electromagnetic spectrum.
• Includes alpha particles, beta particles, neutrons,
protons and (extremely rarely)
neutrinos.
Sudbury Neutrino
Observatory (SNO)
20
Transmutation of Elements
222
86
Rn decays to
218
84
Po
via О±-emission
Atomic number
2
Mass number
4
220
84
Po decays to
220
85
At
via ОІ-emission
Atomic number
1
Mass number is unchanged
21
Background Radiation
We are all exposed to ionising radiation – and
most of that is natural background radiation
Natural Background 73%
Medical Sources 25%
Living at the boundary of a nuclear station 1%
Other Sources 1 %
The dose we absorb each year in Sieverts (Sv)
from background varies with geology &
geography by a factor of 100
Inhalation (Radon) - 1.2 mSv
External Terrestrial - 0.48
Cosmic Radiation - 0.4
Ingestion - 0.3
22
23
Detecting X-rays, Gamma Rays and Particles
• Photochemical films
• Cloud Chambers (track detectors)
• Scintillators (NaI – Li, liquid)
• Solid state detectors (GeLi, thermoluminescent)
• Gas discharge (Geiger detector)
PLEASE
Don’t Break the Window!
24
The Geiger Detector
• Ionising radiation scatters off atoms in
the detector, removing electrons from
their atoms.
• Free electrons are accelerated toward a
positively charged anode (~500V DC).
• These electrons ionise additional atoms in the gas
space, leading to an avalanche discharge.
• Electronics detect the discharge current pulse.
• The counter can detect ONE event at a time.
• It cannot distinguish between one ionising event
and many events occurring within the dead-time
interval.
25
Experiment 1: Background Radiation
• Ionising radiation is everywhere.
• Background “measurements” can be tricky and
time consuming.
• Short counting intervals give small average
numbers of counts leading to unreliable statistics.
• Long counting intervals
can be tedious.
• The effect of shielding is
easy to show.
• A container of water
provides shielding to reduce
background count
26
Experiment 1: Background and Shielding
Counting for 10 minutes may not produce
consistent results
27
Experiment 2
• You can make a simple set of
measurements with weak sources
such as NoSaltВ®
• Potassium chloride (KCl) is a
convenient source of K-40 available
in any grocery store
• Place the KCl near
the Geiger window
• Note the jump in
counts per minute
5 kBq of 40K for $5.99 in
grocery stores everywhere
28
To get a Geiger Kit donated to your school you
must ask the CNS. You might borrow one?
Check the list on the CNS website to
find a school nearby.
29
Experiment 3: Th-232 in vintage camera lenses
From about 1950 through to 1980,
several consumer cameras were made
using thorium oxide in the glass lens to:
• enhance the refractive index of the glass
• keep the dispersion low
Such “bright” sources provide counting rates at or
above 5000 counts per minute.
•
•
•
•
many measurements can be made in a short time
acceptable level of statistical errors
students are more likely to remain engaged
cameras can be found on sources such as EBay
30
Experiment 3: Th-232 in vintage camera lenses
• For high school demonstration
experiments, these lenses are a
conveniently “bright” source of
particles.
• The radioactive material is
embedded inside the glass of the
lens, and most of the particle
emissions are absorbed by air.
Kodak
Signet 40
camera lens
31
Experiment 4: The balloon experiment NEW
• The balloon is an electrostatic precipitator
collecting dust particles from the air.
• The α decay of
222Rn
(3.82 day) to:
7.
218Po
(3.1 min) – α
8.
214Pb
(26.8 min) – β-
9.
214Bi
10.
214Po
(19.9 min) – β-
These
dominate the
balloon data
(164 µs) – α
11. 210Pb (22.2 year) – β12. 214Bi (5.012 day) – β13.
210Po
(138.4 day) – α
14.
206Pb
(stable)
32
Blue Balloon charged with microfibre cloth
4000
deflated balloon on
Geiger
3500
paper disk atop Geiger window to
Counts per minute
3000
2500
2000
1500
1000
inflated
balloon
on
500
0
14:52
16:04
17:16
18:28
19:40
20:52
33
Purple Balloon, 32 minute half-life
5000
Counts per minute
4000
3000
2000
1000
0
19:12
19:40
20:09
20:38
21:07
21:36
22:04
22:33
23:02
23:31
Time
purple balloon
selected half-life
Balloon deflated, under plastic disk
34
Purple Balloon & paper, 31 minute half-life
5000
Counts per minute
4000
3000
2000
1000
0
09:36
10:48
12:00
13:12
14:24
15:36
16:48
18:00
Time
Purple balloon, paper
Selected half-life
Balloon deflated, under plastic disk, paper
disk on Geiger window
35
Purple Balloon & paper, 31 minute half-life, log scale
10000
Counts per minute
1000
100
10
09:36
10:48
12:00
13:12
14:24
15:36
16:48
18:00
Time
Purple balloon, paper
Selected half-life
36
Plastic Food Wrap Roll side up
180
160
Counts per minute
140
120
100
80
60
40
20
0
16:19
16:48
17:16
17:45
18:14
18:43
19:12
19:40
20:09
20:38
Time
37
Vintage Vaseline Glass: a uranium source
• Uranium compounds added to glass give it a
green-yellow hue and it fluoresces under UV light.
• It provides alpha, beta, and gamma radiation,
but is not as intense as the vintage camera
lenses.
38
Vintage Fiestaware: a uranium source
•Uranium compounds added to the ceramic glaze
give these saucers a red-orange hue (and no they
don’t fluoresce under UV light)
•The maximum count rates at minimum separation
with an RM-80 are about 30000 cpm and 20000 cpm
for these two samples
39
Canadian Nuclear Society
Ionising Radiation Workshop
Be Aware of NORM
CNS Team
Thanks
for Your
Attention
www.cns-snc.ca
Bryan White
Doug De La Matter
Peter Lang
Jeremy Whitlock
Rolly Meisel
Additional photographs
copyright R. Meisel
used with permission
40
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