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. 1
20
Embed
1 Radioactivity Outcomes - Lawless Teachinglawlessteaching.eu/.../nucleus/radioactivity_1page.pdfA sample of radium-226 contains 2.6×1021nuclei and is emitting 3.5×1010particles
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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.
1
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.
2
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.
3
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
4
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
5
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
6
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.
7
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.
8
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.
9
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.
10
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.
11
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
12
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.
13
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.
14
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.
15
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.
16
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
Hig
he
r Le
ve
l
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.
17
Hig
he
r Le
ve
l
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?
18
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