What is radioactivity and how it works.ppt

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1. Teaching radioactivity

2. Cloud chamber
Practical Physics photos - cloud chamber tracks

3. An overview
•recognising & overcoming teaching challenges
•ionising radiation: measuring activity & health effects
•radiological protection
•class experiments
•nuclear decay
•contexts for teaching radioactivity
•support, references
•energy from the nucleus, nuclear decommissioning

4. Teaching challenges
Atoms are unimaginably small and cannot be seen (though
‘pictures’ of atoms are created by techniques such as scanning
tunnelling microscopy).
All the evidence that we have about atoms is indirect; scientists
create models to explain the observations.
The random nature of radioactive decay is hard to grasp (though it
can be heard using a GM tube and counter) and seems even
harder to reconcile with the notion of predictable half-life.
(Statistics of extremely large numbers of atoms -> predictability.)

5. Fear of radiation
• usually undetectable by human senses
• serious consequences
–cancers (time-delayed)
–contamination long-lasting
• unaware of background radiation
• media scares - especially after Chernobyl
• secrecy - industrial, military & political interests

6. Teaching order is crucial
Education research shows that
•Basic misconceptions are widespread.
•A conventional approach that puts theoretical ideas first can be
a barrier to understanding.
Tried and proven effective:
•Start with macroscopic phenomena before moving to
microscopic descriptions and explanations.
•Use a range of examples to illustrate the relative scale sizes of
atoms and nuclei.
Robin Millar et al (1990) ‘Teaching about radioactivity and ionising
radiation: an alternative approach’ Phys. Educ. 25 338-342

7. Contexts for radioactivity

Medical physics: radioactive materials for diagnosis & treatment
Nuclear electric: how nuclear (fission) reactors work; might fusion be
a future energy source?
Food irradiation: reliable info on the Food Standards Agency website
Other uses of radioactive materials: industry, agriculture,
estimating age of Earth, archaeology, domestic smoke detectors
Disposal of nuclear waste: an unavoidable problem to solve
History of nuclear weapons: bomb designs, espionage &
international politics
Science in the news: e.g. depleted uranium, polonium-210
Note: Controversial issues require a clear & unbiased treatment.

8. Source v radiation
Common descriptions:
‘a cloud of radiation from Chernobyl’
‘water unfit to drink because it contains radiation’
so …
Carefully distinguish
•radioactive material from the radiation it produces.
•concepts of activity and dose.
Radioactive materials produce ionising radiation (e.g. ).
Use a source – journey – detector model of radiation.

9. Contamination v irradiation
Radiation absorbed: Many students believe that objects that
have been irradiated (e.g. sterilised syringe or dressing, or
food) will themselves become radioactive – that they can re-
emit the radiation some time later.
In effect, they seem to think that radiation is somehow ‘conserved’.
In everyday language, when we say that a sponge has
absorbed water, we assume that it can release the water later.

so …
Distinguish between contamination & irradiation.

10. More formal thinking
Becoming quantitative: Focus on
•the ‘strength’ of radioactive materials (their activity)
•the rate of change of this strength (half-life)
•radiation damage possibly done to a person (radiation dose).
Under a microscope: Consider
•What actually happens when radiation is emitted?
•Is the material left behind still radioactive?
•What happens when radiation is absorbed?
•How is it possible that radiation can cause, as well as cure
cancer?
Atom and nucleus. Nature of . Damaging DNA.
Many random decays make a pattern.

11. Ionisation – the key concept
1896: Becquerel fortuitously discovered radioactivity,
while investigating phosphorescence in uranium salts.
Invisible rays (ionising radiation) from a fluorescent substance,
potassium uranyl sulfate, were detected by a photographic plate.
1903: Becquerel shared a Nobel Prize with Pierre &
Marie Curie for discovering radioactivity.
The becquerel (Bq) is the SI unit of activity of a
radioactive sample. A sample of radioactive material with
activity 1 Bq has one nucleus decay per second. 1 Bq = 1 s
-1
.
An adult human has an activity of ~4000 Bq.

12. Detecting ionising radiation
•cloud chamber
•spark counter, with related animation
•gold leaf electroscope
•Geiger-Müller tube
•ionization chamber
•photographic film
•bubble chamber
•scintillation counter
•semiconductor detectors
•multi-wire proportional chamber etc…
www.darvill.clara.net/nucrad/detect.htm

13. UK background radiation

14. Radiation dose
Absorbed dose
The amount of energy that cells absorb,
measured in grays (Gy).
1 gray = 1 joule absorbed per kg of tissue
Equivalent dose
A measure of possible harm from radiation,
also taking account of the radiation type,
measured in sieverts (Sv).
Type of Radiation Factor
gamma rays 1
beta particles 1
neutrons & protons 10
alpha particles 20

UK annual average dose is 2.6 mSv.
Maximum allowable dose for employees is 20 mSv.

15. Health effects of radiation
Several things can happen when an ionising radiation
penetrates a cell:
•The cell is unaffected.
•The cell is damaged but is able to repair itself.
•The cell is killed.
•The cell’s DNA is damaged but remains able to reproduce itself, in
its modified form. This cell could become cancerous.
If a sex cell is hit, ionisation may cause a genetic mutation.

16. An analogy...
Here's a way to think about measures of radiation:
Imagine that you're out in a rainstorm.
• The amount of rain falling is measured in becquerels.
• The amount of rain hitting you is measured in grays.
• How wet you get is measured in sieverts.

17. Radiological protection
Three principles:
justification - Show that the benefits outweigh the harm that the
radiation might cause.
optimisation - Keep all exposures as low as reasonable
achievable (control measures involve increasing distance,
using shielding materials, and/or reducing exposure time).
dose limitation - Keep the total dose for workers below
specified limits.
These principles apply to potential accidental exposures as well
as predictable normal exposures.

18. Sealed radioactive sources
Currently available from education suppliers:
•cobalt-60: pure gamma (if low energy betas are filtered out)
•strontium-90: pure beta
•americium-241: alpha and some gamma
•caesium-137: beta, then gamma (from its decay product,
metastable Ba-137)
Other sources you may have in your school
•radium-226: alpha, beta and gamma
•plutonium-239: pure alpha

19. Ionising Radiations Regs (1999)
The employer must appoint a qualified Radiation
Protection Adviser.
Schools and colleges must
–account for, store properly, handle safely, & monitor
radioactive substances
–have standard operating procedures, with a designated
Radiation Protection Supervisor
–ensure suitable risk assessments in advance of practical
work
CLEAPSS booklet L93 Managing Ionising radiations &
radioactive substances

20. School-based training
•security & storage arrangements
•record keeping
•safe handling of each type of source
•correct use of associated equipment, monitoring
•action if source is dropped or a spill occurs
•when to seek help & advice from the RPS
No source should ever be left unattended by the teacher
in charge.

21. A useful comparison
In the UK
annual whole body dose from background radiation ranges
from 1 - 10 mSv
diagnostic medical radiation gives average dose 0.37 mSv
A teacher's hand receives dose of 0.01 mSv during a
standard school demonstration. (Dose to students is far
lower because of their distance.)

22. Experiments
•investigating natural radioactivity
•detectors of ionising radiation: cloud chamber, spark
counter, electroscope, GM tube
•ionising radiations and their properties
•simple model of exponential decay (100s of dice)
A-level: measuring the half-life of Pa-234 or Ba-137

23. Penetration and absorption
Alpha & beta lose Ek
in ionising encounters with atoms of the
absorbing medium - see IOP animation
Gamma may interact with an electron or with a nucleus (in several
ways), producing 1 or more ‘secondary electrons’ which ionise.
type of
radiation
number of ion-pairs formed per
cm of path in air
thickness of aluminium to
reduce beam intensity by
half /cm
alpha 10
5
0.0005
beta 10
3
0.05
gamma depends on energy of the secondary
electrons
8.0

24. A nuclear atom
Rutherford, Geiger &
Marsden, 1909-11

25. Inside the nucleus
‘Nucleons’ (protons and neutrons) are held together by
the strong force.
Z = proton number (atomic number)
A = nucleon number (atomic mass)
N = neutron number, A - Z
isotopes: same element, different mass
e.g.
H H, H,
3
1
2
1
1
1

26. Nuclear disintegrations
Alpha decay
Beta decay
Gamma decay: Nucleus left in an excited state after
emission of alpha or beta. No change in A or Z.

0
1
A
1Z
A
Z
0
1-
14
7
14
6
YX
eNC



27. Random decay
Radioactivity is a chance process.
•The chance of decay for each nucleus is constant with time,
independent of temperature, pressure, other physical
conditions.
•The properties of random decay are best displayed if large
numbers of events are involved.
•The rate of decay is proportional to the number of undecayed
nuclei present.
•The half-life of a radioisotope is the average time for half the
nuclei present to decay (for the activity to fall to ½ its previous
value).

28. We know that the popcorn will go ‘pop’, but we don’t
know exactly which kernel will pop at any given time.
Popcorn!

29. Half-life
equal ratios in equal times

30. Teaching how science works
• process of scientific enquiry (how we know what we know)
• applications, implications, benefits and risks
• making decisions (health, social, economic &
environmental effects), including ethical issues
• uncertainties in science

31. Support, references
Example teaching scheme, from the Practical Physics website
IOP DVD Teaching Radioactivity
www.talkphysics.org join the Group “IoP Teacher Network, London”
David Sang (ed, 2011) Teaching secondary physics ASE / Hodder
Teachers TV - Demonstrating Physics: Radioactivity
www.peep.ac.uk controversy & ethics

32. Stability
Isotopes shown here in
black are stable.
Radioisotopes are
unstable.
As the proton number
increases, an increasing
fraction of neutrons is
needed to form a stable
nucleus.
http://prezi.com/hllfbv98zptq/p2-nuclear-decay/

33. Energy from the nucleus (1)
four natural radioactive decay series (start with thorium,
neptunium, uranium, or actinium)
•spontaneous nuclear reactions: mother, daughter, radiation
emitted
•(with time) all four series end at Pb (lead)
‘Mass defect’: mass of products is less than mass of reactants
•energy released as E
k
of fragments,
2
mcE

34. Energy from the nucleus (2)
Fission
•1932: neutron discovered by Chadwick
•1934 onwards: experiments done by Fermi et al in Rome - neutron irradiation of
elements, starting with lightest & working through the periodic table up to
uranium. Expected transuranic elements. Hahn, Strassman, Lise Meitner repeat
with U.
•with a critical mass of U, neutrons emitted give a ‘chain reaction’
Fusion
•small nuclei combine, releasing more energy than fission e.g.
energyn2XeSrnU
energyn3BaKnU
1
0
140
54
94
38
1
0
235
92
1
0
141
56
92
36
1
0
235
92


energy HeHH
3
2
1
1
2
1


35. Nuclear & radiological skills
Nuclear and radiological technology has key roles in
• the health sector
• national defence
• nuclear power stations
• the clean-up of nuclear legacy
• a wide spectrum of research, development and
manufacturing activities
• a shortage of skilled workers, getting worse
DTI report, December 2002
Nuclear decommissioning

36. Nuclear power in the UK
All existing UK nuclear power stations to be decommissioned by
2023, except for Sizewell B.
Road to 2010 strategy: "Nuclear power is a proven technology which
generates low carbon electricity.
  It is affordable, dependable, safe, and
capable of increasing diversity of energy supply. It is therefore an
essential part of any global solution to the related and serious challenges
of climate change and energy security.“
10 sites identified by the Government where new nuclear power
stations could be built
•streamlined planning process, so 10 stations can open by 2018
•providing 40% of the country’s electricity by 2025

37. The legacy …
‘Hazardous life’: after 20 half-lives, activity falls to a millionth

38. Nuclear Decommissioning Authority
Government has taken on liabilities from BNFL, UKAEA
•research, education and training
•operation, decommissioning of nuclear installations
•clean-up of 18 nuclear sites
•operation of facilities for treating, storing, transporting, disposing
of hazardous material
2005: estimated £1b a year for 10 - 15 years, total £48b.
Feb 2013 (Commons Public Accounts Committee): ‘total lifetime cost
of decommissioning [Sellafield] has now reached £67.5 billion and
there’s no indication of when that cost will stop rising.’