Name: ______________________________
New Zealand Institute of Physics
PHYSICS 2007
Level 2
90526 Demonstrate understanding of atoms and radioactivity
Credits: Two
Answer ALL the questions in the spaces provided.
If you need more space for any answer, use the pages provided at the back of this booklet and clearly
number the question.
For all numerical answers, full working should be shown and the answer should be rounded to the
correct number of significant figures and given with an SI unit.
For all ‘describe’ or ‘explain’ questions, the answer should be in complete sentences with all logic fully
explained.
YOU MUST HAND THIS BOOKLET TO THE SUPERVISOR AT THE END OF THE ASSESSMENT.
For Assessor’s use only
Achievement Criteria
Achievement
Identify or describe aspects of
phenomena, concepts or principles.
Achievement
with Merit
Give descriptions or explanations in
terms of phenomena, concepts,
principles and/or relationships.
Achievement
With Excellence
Give concise explanations that
show clear understanding, in terms
of phenomena, concepts, principles
and/or relationships.
Overall Level of Performance
90526 • Demonstrate understanding of atoms and radioactivity • Question and Answer Booklet 2007
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It is recommended that you take 20 minutes to complete this assessment.
QUESTION ONE: ATOMIC MODELS
In the early part of the 20th century Rutherford investigated the paths taken by positively
charged alpha particles into and out of a very thin piece of gold foil. The diagram shows the
paths of three alpha particles.
A
B
C
Gold nucleus
(a)
Explain the different paths A, B and C of the alpha particles.
Path A Explanation: _______________________________________________
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Path B Explanation: _______________________________________________
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Path C Explanation: _______________________________________________
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90526 • Demonstrate understanding of atoms and radioactivity • Question and Answer Booklet 2007
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Before Rutherford’s work, Thomson had devised a model of the atom.
(b)
Explain how that the discovery that atoms can emit electrons allowed Thomson to
devise his model.
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90526 • Demonstrate understanding of atoms and radioactivity • Question and Answer Booklet 2007
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QUESTION TWO: NUCLEAR REACTIONS
(a)
60
Co )
Cobalt-60 undergoes beta decay. Show that the beta decay of cobalt-60 ( 27
60
Ni ).
results in nickel-60 ( 28
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(b)
Radon-212 ( 212
88 Rn ) is a radioactive gas. Show that when radon-212 undergoes
alpha decay, polonium-208 ( 208
86 Po ) is formed.
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90526 • Demonstrate understanding of atoms and radioactivity • Question and Answer Booklet 2007
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QUESTION THREE: SMOKE DETECTORS AND HALF-LIFE
(a)
A smoke detector contains radioactive americium241 which emits radiation.
Complete the following equation to identify the radiation emitted.
Am 
241
95
Diagram 1
237
93
Np  ........
Diagram 2
Smoke detector
Warning: This device contains
radioactive americium
Diagram 1 shows the position of a smoke detector inside a house.
Diagram 2 shows the warning label at the back of the smoke detector.
(b)
Explain why the radiation given out by the americium is unlikely to do any harm to
the people living inside the house.
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90526 • Demonstrate understanding of atoms and radioactivity • Question and Answer Booklet 2007
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(c)
The graph below shows how the number of americium atoms inside a source goes
down slowly.
10000
9000
8000
Number of 7000
6000
americium
5000
atoms
4000
3000
2000
1000
0
0
200
400
600
800
1000
Time in years
Estimate the half-life of americium-241.
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(d)
Radon-212 has a half-life of 24 minutes. Show that if you start with 96 mg of radon212 that you will have 91 mg of polonium-208 two hours later (and not 93 mg).
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90526 • Demonstrate understanding of atoms and radioactivity • Question and Answer Booklet 2007
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QUESTION FOUR: ALPHA, BETA AND GAMMA RADIATION
The table below gives information about six radioactive isotopes.
Isotope
Type of radiation emitted
Half-life
technetium-99
iridium-192
polonium-210
polonium-213
uranium-239
hydrogen-3
gamma ray
gamma ray
alpha particle
alpha particle
beta particle
beta particle
6 days
74 days
138 days
less than 1 second
24 minutes
12 years
(a)
State what an alpha particle is.
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(b)
Two isotopes of polonium are given in the table. How do the nuclei of these two
isotopes differ?
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(c)
Describe how alpha particles ionise atoms in a medium.
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(d)
A doctor needs to monitor the blood flow through a patient’s heart. The doctor injects
a radioactive isotope into the patient’s bloodstream. Explain why the doctor would
choose technetium-99 instead of the other isotopes listed in the table above.
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90526 • Demonstrate understanding of atoms and radioactivity • Question and Answer Booklet 2007
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Question
Number
If you need more space for any answer, continue here. Clearly number the question.
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90526 • Demonstrate understanding of atoms and radioactivity • Question and Answer Booklet 2007
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9
New Zealand Institute of Physics
ASSESSMENT SCHEDULE
Physics Level 2
90256 Demonstrate understanding of atoms and radioactivity
Note: Minor computational errors will not be penalised. A wrong answer will be accepted as correct provided there
is sufficient evidence that the mistake is not due to a lack of understanding. Such evidence includes:


the last written step before the answer is given has no unexpanded brackets or terms and does not require
rearranging.
the power of any number that is multiplied by a power of 10 is correct.
Correct units and significant figures are required only in the questions that specifically ask for them.
Evidence Statement
Question
Evidence
One
(a)
The gold atom is mainly empty space so
most of the alpha particles pass straight
through since they are a long way from the
nuclei.
Achievement
Mentions empty
space or a long
way from nuclei
The gold nucleus has the same charge
(positive) as the helium nucleus (alpha
particle). When the alpha particle comes
close to the gold nucleus strong repulsive
forces exist and a deflection is possible.
Merit
Excellence
Correct
explanation
Correct
explanation
The gold nucleus has the same charge
(positive) as the helium nucleus (alpha
particle). When the alpha particle comes
directly at the gold nucleus strong repulsive
forces exist and a large angle deflection is
possible.
Correct
explanation
(b)
Thomson devised that the atom must
comprise electrons and since atoms had to
be neutral overall the rest of the atom must
be positive. His model did not consider the
possibility of discrete positive particles.
Correct
explanation
Question
Evidence
Co 
Two
(a)
60
27
(b)
212
88
Rn 
60
28
Ni 
0
1

Th  24 He
208
86
Achievement
Merit
Correct
Correct
90526 • Demonstrate understanding of atoms and radioactivity • Question and Answer Booklet 2007
Excellence
10
Question
Three
(a)
Evidence
241
95
Rn 
237
93
Np  24 He
Achievement
Excellence
Correct
(b)
Since it is alpha particles they only travel a
short distance in air so they will never be
ingested by the people in the house.
Alphas only travel a
short distance
(c)
450+/-20 years
Correct
(d)
2 hours equates to 5 half-lives. This means
that the amount of radon left with be 96/25 =
3 mg. Therefore there is 93 mg of products
left. The products are polonium and helium
nuclei. Po will take 208/212 part of 93 mg
which is approximate 91 mg.
Question
Evidence
Four
(a)
Merit
Correct
explanation
Correctly
calculate 3 mg of
Radon left.
Achievement
Helium nucleus
Correct
(b)
Different number of neutrons
Correct
(c)
A positively charged alpha particle applies a
strong attractive force to the outer electrons
of an atom of the medium. This force is
sometimes sufficiently strong to remove the
electron from the atom creating a positively
charged ion.
(d)
Half-life needs to be short so that it can be
removed quickly from the body avoiding
over-exposure Gamma rays are needed as
the detector will be some distance from the
source of radiation (the heart) and a
gamma source can pass through without
being absorbed.
Correct
explanation
Merit
Excellence
Correct
explanation
Indication of short
half life or gamma
ray non-absorption.
Correct
explanation
Sufficiency Statement
Total opportunities:
Sufficiency:
Achievement
Merit
Excellence
9
6
3
6A
4A+3M
4A+3M+2E
90526 • Demonstrate understanding of atoms and radioactivity • Question and Answer Booklet 2007