Radioactivity – Outcomes Describe the experimental evidence for there being

three types of radiation.

Discuss the nature and properties of each type.

Solve problems about mass and atomic numbers in

radioactive decay.

Demonstrate ionisation and penetration of each type.

Give uses of radioisotopes.

Describe the principle of operation of a radiation

detector.

Demonstrate a radiation detector.

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Radioactivity – Outcomes Define the becquerel (Bq).

Interpret nuclear reactions.

HL: State the law of radioactive decay.

Discuss the concept of half-life.

HL: Discuss the decay constant.

Solve problems about rates of decay and half-lives.

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Radiation Radioactivity is the decay of unstable nuclei with the

emission of one or more types of radiation.

There are three types of radiation, evidenced by the

effect of an electric field.

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Alpha (𝛼) Radiation Alpha radiation consists of 2 protons and 2 neutrons.

Hence it is often called a helium nucleus.

To emit an alpha particle, an atom must therefore lose 2

protons and 2 neutrons.

Thus, the atom reduces its atomic number by 2 and its

mass number by 4.

e.g. 92238𝑈 → 90

234𝑇ℎ + 24𝐻𝑒

e.g. 84210𝑃𝑜 → 82

206𝑃𝑏 + 𝛼

generally, 𝑍𝐴𝑋 → 𝑍−2

𝐴−4𝑌 + 24𝐻𝑒

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Beta (𝛽) Radiation Beta radiation consists of electrons.

A neutron in the nucleus splits up into a proton and an

electron.

The proton stays in the nucleus and the electron is

emitted.

Hence, the atom increases its atomic number by 1.

e.g. 90234𝑇ℎ → 91

234𝑃𝑎 + −10𝑒

e.g. 83210𝐵𝑖 → 84

210𝑃𝑜 + 𝛽

generally, 𝑍𝐴𝑋 → 𝑍+1

𝐴𝑌 + −10𝑒

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Gamma (𝛾) RadiationGamma radiation is high frequency electromagnetic

radiation.

Particularly after alpha or beta decay, nuclei end up in

a high energy “excited” state, indicated by an asterisk.

Falling to the ground state requires emitting a high

energy photon.

The nucleus does not change composition in gamma

decay, so the nuclear reactions are much simpler:

e.g. 2860𝑁𝑖∗ → 28

60𝑁𝑖 + 𝛾

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Radioactive Decay e.g. Write the nuclear reaction for potassium-40

undergoing beta decay.

e.g. If actinium-225 decays to francium-221, what type

of radiation was emitted?

e.g. bismuth-214 has a decay chain (i.e. multiple decays

in a row) ending at stable lead-206. If lead, bismuth, and

polonium are the only elements in the chain, write out

each reaction in the decay chain.

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Ionisation and Penetration Radiation can knock electrons out of matter, ionising it.

Alpha is the best ioniser, beta is the second best, and

gamma is the worst at this.

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The opposite is true for

penetration:

gamma requires thick lead

or concrete to block it

beta will be blocked by a

thin sheet of aluminium

alpha will be blocked by a

sheet of paper, or a few

cm of air. by stannered, ehamberg – CC-BY-SA-3.0

Demonstrate the Ionising Ability of Radiation

1. Charge an

electroscope.

2. Bring a radioactive

source near the

electroscope.

3. Note that the leaves

collapse.

4. The radiation ionises the

air around the

electroscope and the

new charges neutralise

the electroscope.

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Demonstrate the Penetrating Power of

Radiation1. Turn on a GM tube and note the number of counts over

two minutes.

2. Aim a source of alpha radiation at the GM tube and

record the number of counts over two minutes.

3. Place a sheet of paper between the source and GM

tube. Record the number of counts over two minutes.

4. Repeat for sources of beta and gamma radiation, using

a thin sheet of aluminium and a thick sheet of lead

respectively.

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Demonstrate the Penetrating Power of

Radiation1. Alpha radiation will be

blocked by a sheet of

paper.

2. Beta radiation will pass

through paper, but be

blocked by a thin sheet of

aluminium.

3. Gamma radiation will pass

through paper and

aluminium, but be blocked

by a thick sheet of lead.

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RadiationRadiation Nature Charge Ionising

Ability

Penetrating

Power

Range

Alpha (𝛼) helium

nucleus

+2 greatest least a few cm of air,

a piece of paper

Beta (𝛽) electron -1 medium medium a few cm of

aluminium

Gamma

(𝛾)

photons 0 least greatest a few cm of lead,

thick concrete

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Uses of RadioisotopesMedical imaging – radioisotopes placed in organs can

be used to create an image of the organ.

Cancer treatment.

Irradiating food to kill bacteria.

Carbon dating – comparing the presence of 𝐶14 in

organisms to “the present” (1950, before nuclear tests).

Tracing movement – the movement of isotopes can be

tracked in organisms or agriculture.

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GM Tube A Geiger-Müller tube consists of an inert gas with a high

voltage across it.

Normally the inert gas does not conduct, but ionising

radiation will create ions and electrons.

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The high voltage accelerates these charges, which bump into neutral molecules, creating more charges.

Thus, a single ionisation can produce many charges.

Each electron hitting the anode will cause a small current, which is counted. b

y s

vjo

-2 –

CC

-BY

-SA

-3.0

Solid State Detector Solid state detectors consist of a reverse biasedp-n

junction which is sensitive to ionising radiation.

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Radiation creates

electron-hole pairs in the

depletion layer.

These charges move due

to the voltage across the

diode, creating a small

pulse of current which

can be counted.

Activity The activity, A of a radioactive isotope is the number of

decays it undergoes per unit time.

Activity depends on the type and number of nuclei

present.

The Becquerel (Bq) is the unit of activity. Activity is 1 Bq if

one nucleus decays in one second.

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The decay of a single nucleus is a random process, so

we cannot make predictions or calculations.

In a sample, the Law of Radioactive Decay states that

the activity, A is proportional to the number of nuclei

present, N.

Formula: 𝐴 ∝ 𝑁

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Activity The constant of proportionality is the decay constant, 𝜆.

Formula: 𝐴 = 𝜆𝑁

The decay constant is different for every material.

e.g. strontium-90 has a decay constant of 0.008 s-1. How

many atoms are present if it emits 5 × 104 beta particles

per second?

e.g. A sample of radium-226 contains 2.6 × 1021 nuclei

and is emitting 3.5 × 1010 particles per second. Find its

decay constant.

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Half-Life The half-life, 𝑇 Τ1 2

, of a radioactive isotope is the time

taken for half of the nuclei in a sample to decay.

e.g. After one half-life, half of the sample will remain;

after two half-lives, one quarter of the sample will

remain, etc.

e.g. If the half-life of an isotope is 2 years, what fraction

of a sample will have decayed after 10 years?

e.g. Technetium-99m is a radioactive isotope used in

tracing. If 1 𝜇g is injected into a patient and technetium-

99m has a half-life of 6 hours, how much of the isotope

will remain after 1 day?

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Half-Life Half-life and decay constant are related to each other:

Formula: 𝑇 Τ1 2=

𝑙𝑛2

𝜆≈

0.693

𝜆

e.g. technetium-99m has a half-life of 6 hours. What is its

decay constant?

e.g. A sample of a radioactive isotope has 2 × 105

atoms. If the half-life of the isotope is 86.625 s, find its

activity.

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Half-Life e.g. What is the half-

life of the isotope

depicted in the

graph if the t-axis

shows N?

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