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11 Radioactivity
1 a What do you understand by ‘background radiation’?
State two sources of this radiation.
b State three safety precautions to be observed when using
radioactive sources.
c Describe briefly two uses of radioactive sources.
d How would you test to distinguish between two radioactive
sources, one of which emits only alpha particles and the other
that emits only beta particles?
2 Uranium 238 and uranium 235 are ‘isotopes’ of uranium and have
the same atomic number, 92.
a What do the numbers 238 and 235 represent?
b i What does the number 92 tell you about the nucleus of
either of these two atoms?
ii What else does the number 92 tell you about the atom as
a whole?
c In what way does the nucleus of uranium 238 differ from the
nucleus of uranium 235?
3 What changes, if any, occur in the atomic number of a radioactive
atom if the nucleus emits
a an -particle
b a -particle
c a -ray?
4 Atomic bombs and atomic reactors both provide large quantities
of energy through chain reactions.
a Explain what is meant by a chain reaction.
b Explain how the reaction, which is violent in the case of a
bomb, is slowed down in the reactor.
5 In an experiment to find the half-life of radioactive iodine, the
count-rate falls from 200 counts per second to 25 counts per
second in 75 minutes. What is its half-life?
6 What are the particles you would expect to find in an atom? Give
some idea of their relative masses and state what electrical charge,
if any, each kind has.
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7 a electron proton neutron nucleus nucleon
Which of these:
i orbits the nucleus?
ii is a particle with a positive charge?
iii is uncharged?
iv is lighter than the others?
v contains neutrons and protons?
vi is a name for 2 types of particles?
vii has a negative charge?
b Complete the table.
electrons protons neutrons nucleon symbol
number
sodium-23 11
aluminium-27 13
strontium-90 38
c Which type of radiation, alpha, beta, or gamma, from
radioactive materials:
i has a positive charge?
ii is the most penetrating?
iii is easily deflected in a magnetic field?
iv consists of waves?
v causes the most intense ionization?
vi has the shortest range in air?
vii has a negative charge?
viii is not deflected by an electric field?
ix travels at the speed of light?
x has the same mass as an electron?
xi is stopped by a thin sheet of aluminium?
xii is the most massive?
8 If the half-life of a radioactive gas is 2 minutes, then after
8 minutes the activity will have fallen to a fraction of its initial
value. This fraction is:
1
a __ 1
b __ 1
c ___ 1
d ___
4 8 16 32
9 a What are isotopes?
b Why are isotopes difficult to separate by chemical methods?
10 Why aren’t gamma rays affected by an electric field?
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11 Plot the data in the table on a graph and determine the half-life
of the radioactive isotope (polonium-218).
Time / mins 0 1 2 3 4 5 6 7
Count rate / 260 205 160 129 104 82 64 51
(counts/sec)
12 A sample of a radioactive substance was emitting 4000 alpha
particles a second at the start of an experiment. Ten minutes
later it was emitting 2000 particles per second.
a What is the half-life of the substance?
b How long after the start would you expect to measure a
count of 500 particles a second?
c How much activity (in counts per second) would you expect
after 10 half-lives?
d The natural ‘background’ count of radiation is about
2 counts per second in most places. How long would it take
for radiation from the radioactive waste to be just less than
this background radiation?
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