Physics
Radioactivity: an introduction
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ATOMIC STRUCTURE
- Atoms have a tiny central nucleus made up of protons
and neutrons around which there are electrons.
A proton has a mass that is very similar to that of a neutron, but is nearly
2000 times that of an electon.
- The number of electrons in an atom is equal to the number of protons in
the nucleus. The atom as a whole has no electrical charge i.e. it is electrically
neutral. If it loses one or more electrons, the atom becomes a positive ion.
|
Mass |
Charge |
Proton |
1 |
+1 |
Neutron |
1 |
0 |
Electron |
≈ 1/2000 |
-1 |
The relative masses of protons, neutrons and electrons and their relative charges
are as shown to the right
- All atoms of a particular element have the same number of protons
- but they may have differing numbers of neutrons and are called isotopes
of that element. Atoms of different elements have different numbers of protons.
- The number of protons in an atom is called the proton number
or atomic number Z. The number of neutrons in an atom is
called the neutron number N
- The total number of protons and neutrons (i.e. nucleons)
in an atom is called its nucleon number or mass
number A. (The term nucleon means
either a proton or neutron.)
DISINTEGRATION OF UNSTABLE NUCLEI
If a nucleus contains too many or too few neutrons, it is unstable
and eventually disintegrates into smaller fragments. The time it takes for different
nuclei to suddenly disintegrate can vary from billions of years down to the
tiniest fraction of a second.
Unstable nuclei can disintegrate in many ways. The two most common
ways that unstable nuclei disintegrate are when
- the nucleus breaks in two: one small cluster of 2 protons and 2 neutrons
(i.e. a helium nucleus) and another much larger cluster of
nucleons.
- an electron is ejected from the nucleus.
Very short wavelength electromagnetic radiation is usually
emitted (as a photon) almost immediately after the helium nucleus or electron
is formed.
Both the helium nucleus and the electron fly off at high speed and the electromagnetic
radiation travels at the speed of light, so all three of these can be thought
of as rays. The first scientists investigatinf these rays didn't know
what they were made of, so they called them by the following names:
- the fast helium nuclei were called alpha rays;
- the even faster electrons were called beta rays;
- the electromagnetic radiation (travelling at the speed of light) was called
gamma rays.
The terms alpha rays, beta rays and gamma rays are
still used for convenience today. Each individual Helium nucleus ejected is
often referred to as an alpha particle. Each electron emitted
from the nucleus is often referred to as a beta particle.
The emission of these radiations by unstable nuclei is referred to as radioactivity.
The breaking up of unstable nuclei is called radioactive decay
or radioactive disintegration.
A radioactive decay is associated with:
- an emission of radiation;
- the radioactive element changes into a completely different element
- a lighter element in fact. This is because the proton number changes.
In a radioactive decay, the nucleus before the decay is called the parent
nucleus. This decays into the daughter nucleus.
To be more specific:
- For each alpha particle emitted, the daughter nucleus has two protons and
two neutrons less than the parent nucleus; i.e. the atomic number is 2 less
and the mass number is 4 less.
- For each beta particle emitted, a neutron in the nucleus becomes a proton;
ie the atomic number is 1 more, but the mass number is unchanged.
- Gamma radiation in itself causes no change in the structure of the nucleus.
When a parent nucleus emits an alpha or beta particle, a gamma is often emitted
as well.
Sometimes daughter nuclei are themselves unstable and so decay, forming part
of a whole radioactive series of decays. For
example, Uranium isotopes, which have a very long half-life, decay via a series
of relatively short-lived radioisotopes to produce stable isotopes of lead
as the end-products.
You should understand how to complete balanced nuclear equations.
Radioactive emissions are random (sometimes called spontaneous).
HISTORY
Radioactivity
was discovered by Becquerel.
TYPES OF RADIOACTIVITY
Radioactivity is the emission of radiations
resulting from the disintegration of atomic nuclei.
The three main types of radioactive radiation (and these were
the ones investigated by the original discoverers) are:
Alpha (α) particles
are positively charged and relatively heavy and slow |
Beta (β) particles
are negatively charged, light and fast |
Gamma (γ) rays
are electromagnetic radiation with a very short
wavelength |
An alpha particle is a small cluster
of 2 protons + 2 neutrons i.e. it is a helium nucleus. |
A beta particle is an electron,
but unlike other electrons you have studied, it comes from inside
the parent nucleus. |
A gamma ray is a photon - like
a light photon but with much higher energy.
|
These radiations all - to varying degrees - ionise matter that they travel
through.
THE PENETRATING POWER OF THE RADIATIONS
Alpha particles are easy to stop, gamma rays are hard to stop, beta particles
are in between
- Particles that ionise other atoms strongly have a low penetrating power,
because they lose energy each time they ionise an atom.
- Radioactive decay is not affected by external conditions.
IONISATION
A neutral atom has a cloud of electrons round the nucleus.
The total negative charge of this electron cloud exactly balances the positive
charge of the nucleus.
An ion is:
- a neutral atom or molecule that has lost one or more electrons
to form a positively charged ion, or
- a neutral atom or molecule that has gained one or more
electrons to form a negatively charged ion.
When this happens to an atom/molecule or a group of atoms/molecules, we say
that it has become ionised and that ionisation
has occurred.
Ionizing radiation is radiation that can eject one or more electrons from a
neutral atom, thereby creating two charged particles (an
ion-pair): (1) the negatively charged electron and (2) the positively
charged partially-stripped atom that is left behind. In order to be ionizing,
the radiation must be able to deliver enough energy to a single atom to remove
an electron.
Each of the three types of radiation - alpha, beta and gamma - cause atoms
and molecules to lose electrons from the cloud of electrons
round the nucleus, leaving behind positive ions.
Particles that ionise other atoms strongly have a low penetrating power,
because they lose energy each time they ionise an atom.
Ionising radiation can be detected using:
- a photographic film or
- a Geiger-Müller detector (GM detector/tube)
PROPERTIES OF RADIOACTIVE RADIATIONS
ALPHA PARTICLES |
BETA PARTICLES |
GAMMA RAYS |
Symbol:
Mass:
Charge:
Speed:
Ionising ability:
Penetrating power:
Stopped by: |
α
4 - big, heavy
+2
slow
high
low
paper |
|
Symbol:
Mass:
Charge:
Speed:
Ionising ability:
Penetrating power:
Stopped by: |
β
1/2000 - small, light
-1
fast
medium
medium
thin aluminium |
|
Symbol:
Mass:
Charge:
Speed:
Ionising ability:
Penetrating power
Stopped by: |
γ
0
0
speed of light
weak
high
thick lead |
|
Alpha particles are Helium nuclei, because they
are made of 2 protons and 2 neutrons. |
|
WHAT ARE THE RADIATIONS?
A beta particle is an electron. So it has a charge
of -1, and a mass of just under1/2000th that of a proton. |
|
Gamma rays are electromagnetic waves, like light
and radio waves, but with a much shorter wavelength. Thus they have
no charge. [ The only real difference between gamma rays and
X-rays is that gamma rays originate from within
the nucleus, whereas X-rays don't. ] |
|
When an alpha particle passes an atom, it tends to attract the
oppositely charged outer electrons away from the atom.
Alpha particles have a large charge (+2), so they
strongly and easily ionise other atoms that they pass. Ionising
atoms requires energy, so alpha particles lose energy and slow down
rapidly and as they travel. This is one reason that alpha particles
have such a low penetrating power. |
|
HOW DOES RADIATION CAUSE IONISATION?
When a beta particle passes an atom, it tends to push (repel) outer
electrons off the atom.
Beta particles affect atoms' outer electrons less than do beta
particles, so cause less ionisation, lose less energy, and thus
have a longer range - typically a few metres in air. |
|
Gamma rays, like alpha and beta particles, ionise atoms and molecules
- but much more weakly. They do not 'push or pull' outer electrons
from atoms like the alphas and betas do: they give some of the electrons
surrounding the nucleus some of their energy, causing the electron
to be ejected from the atom.
Because they don't interact much with matter, gamma rays usually
don't lose much energy as they travel. So they have a high penetrating
power and a very long range. It takes a thick sheet of metal such
as lead or concrete to reduce them significantly. |
|
TYPES OF RADIOACTIVE DECAY
In alpha and beta decay, the parent nucleus decays to form
a daughter nucleus.
- nucleus splits in two - usually one oif this pieces is a Helium nucleus
(alpha particle).
- a nucleon splits in two - usually a neuton splitting to become a proton
+ electron.
- de-excitation to produce gamma.
ALPHA DECAY |
BETA DECAY |
Gamma rays |
|
Alpha-decay occurs in very heavy elements, like
Uranium. If their nucleus has too many protons (i.e.
it is proton-rich), it is unstable. Such a nucleus can
become more stable by emitting an alpha particle. |
|
WHEN DOES RADIOACTIVITY OCCUR?
Beta decay occurs in nuclei, such as Strontium-90, that are unstable
because they have too many neutrons in their nuclei
(i.e. they are very neutron-rich). They become more stable
by emitting a beta particle. |
|
• After a nucleus has emitted an alpha- or beta-particle,
it may still have too much energy: we say it is in an "excited
state".
• It can get rid of this surplus energy by emitting
a photon of very high frequency electromagnetic radiation, i.e.
a gamma ray.
• Gamma rays are given off by most
alpha- and beta-emitters alongside the particles
themselves. They are not emitted just on
their own. |
|
Alpha particles carry away some of the energy that was in the nucleus
before it disintegrated.
|
|
WHAT CHANGES TAKE PLACE?
Beta particles (as with alpha particles) carry away some of the
energy that was in the nucleus.
|
|
Gamma rays carry away usually much more nuclear energy than do
alphas or betas.
Gamma rays are not only produced in radioactive decay. They arise
from all sorts of high-energy nuclear reactions. e.g. in particle
accelerators. They also form a small part of cosmic rays (which
are mostly particles). |
|
Alpha decay leaves a daughter nucleus that is of a different element
to the parent one. When a nucleus emits an alpha-particle:
- atomic mass decreases by 4.
- atomic number decreases by 2
e.g. Americium-241 (an α-source
used in smoke detectors), which has an atomic number of 95 and an
atomic mass of 241, will decay to Neptunium-237 (which has an atomic
number of 93 and an atomic mass of 237).
The equation would look like this:
| 241 |
Am |
→ |
237 |
Np |
+ |
α |
| |
|
| 95 |
93 |
Now an alpha particle is the same as the nucleus of a Helium atom
(2 protons and 2 neutrons). So it helps to balance equations if
we write He instead of α
as follows:
| 241 |
Am |
→ |
237 |
Np |
+ |
4 |
He |
| |
|
|
| 95 |
93 |
2 |
|
|
NUCLEAR EQUATIONS FOR RADIOACTIVE DECAY
Beta decay also leaves a daughter nucleus that is of a different
element to the parent one.
A beta particle is just an electron. What is an electron doing
coming out of a nucleus? Under certain conditions, a neutron can
decay, turning into a proton plus an electron. The proton remains
in the nucleus, whilst the electron (the beta particle) flies off
at high speed.
This means that when a nucleus emits a beta-particle:
- atomic mass is unchanged
- atomic number increases by 1
e.g. Strontium-90 undergoes beta decay and forms Yttrium-90
Because a beta particle is just an electron, we could write e
instead of β in the equation as follows:
| 90 |
Sr |
→ |
90 |
Y |
+ |
0 |
e |
| |
|
|
| 38 |
39 |
-1 |
The following is NOT on the GCSE syllabus:
- Really there are two types of beta decay. The
more common kind (and the one that is meant in the GCSE
syllabus) is beta-minus decay, in which electrons
are ejected from the nucleus. The less common kind is beta-plus
decay, in which positrons are emitted.
- An almost massless particle called an anti-neutrino
is emitted alongside the beta-minus particle. In beta-plus
decay, the corresponding particle is a neutrino.
|
|
|
Unlike alpha and beta decay, the nucleus retains the same composition
- i.e. it is still the same element.
In gamma emission:
- atomic number unchanged
- atomic mass unchanged.
|
|
ISOTOPES and RADIOISOTOPES
You can think of different isotopes of an atom being different "versions"
of that atom.
Consider a usual carbon atom. It has 6 protons and 6 neutrons - we call it
carbon-12 because it has an mass number of 12 (6 protons plus 6 neutrons).
If we add a neutron, it's still a carbon atom (carbon-13), but it's
a different isotope of carbon. One useful isotope of carbon is carbon-14, which
has 6 protons and 8 neutrons. This is the atom we look for when we're carbon
dating an object.
So isotopes of an atom have the same number of protons, but a different number
of neutrons.
Just because something is called an "isotope" doesn't necessarily
mean it's radioactive. Those that are radioactive are called "radioactive
isotopes" (or radioisotopes for short). Carbon-14 is a
radioisotope, but carbon-12 is not.
[ The terms nuclides and radionuclides are often used instead
of isotopes and radioisotopes. They are nearly always interchangeable.
]
ACTIVITY and HALF-LIFE
- The activity of a radioactive source
is measured in becquerels (Bq)
- A radioactive source emits less and less radiation as it gets older (i.e.
its 'activity' decreases with time)
- This decrease in activity is due to the corresponding decrease in the number
of radioactive nuclei
- Different radioactive substances decay at different rates
The half-life of a radioactive substance:
- is the average time it takes for the number of parent
radioactive atoms in a sample to halve;
- is the time it takes for the count rate from the original
substance to fall to half its initial level.
Furthermore:
- The half-life is different for different radioactive isotopes
- Use an activity-time graph (e.g. diagram on the left) to determine the half-life
of a material
- Describe how the half-life of a material can be measured
- Apply an understanding of half-life to explain why different sources are
suited to different purposes
Activity-Time graph
|
Half lives of some radioisotopes
Radioactive Parent |
Stable Daughter |
Half life |
Rubidium-87
| Strontium-87
| 49 billion years |
Thorium-232
| Lead-208
| 14 billion years |
Uranium-238
| Lead-206
| 5 billion years |
Potassium-40 |
Argon- 40 |
1 billion years |
Uranium-235
| Lead-207
| 704 million years |
Carbon-14
| Nitrogen-14
| 5730 years |
| Americium-241
|
| 432 years |
| Cobalt-60
|
| 5 years |
| Iodine-131
|
| 1 week |
| Sodium-24
|
| 15 hours |
| Technetium-99m
|
| 6 hours |
| Bismuth-212
|
| 1 minute |
| Polonium-215
|
| 2 milliseconds |
|
NUCLEAR FISSION
Fission means splitting in two.
Sometimes some nuclei do this of their own accord (this is called spontaneous
fission). In situations such as a nuclear reactor, 'induced' fission occurs
as a result of when large stable nuclei are bombarded with neutrons. when this
hapens:
- if it has absorbed a neutron, there are now to many neutrons to be stable;
- the unstable nucleus splits into two smaller nuclei;
- further neutrons are released which may cause further nuclear fission resulting
in a chain reaction;
- the new atoms which are formed are themselves radioactive.
Nuclear reactors use fission to produce nuclear energy.
The energy released by an atom during radioactive disintegration or nuclear
fission is very large compared to the energy released when
a chemical bond is made between two atoms. Thus a nuclear power station produces
much more energy than, say, a coal-fired power station.
The opposite of nuclear fission is nuclear 'fusion', where two nuclei are made
to fuse together.
BACKGROUND RADIATION
There are radioactive substances all around us, from:
- the air (the radon in the air accounts for over half the
total)
- the rocks, soil and hence building materials
- seawater
- plants and hence in food.
- space
The radiation from all these sources is called background radiation.
The level of background radiation is higher in some places than in others (depending,
for example, on rock formations). eg because of (radioactive) radon gas released
from rocks. Radon is also released from building materials made using rock.
Describe how to take background radioactivity into account when performing
experiments.
DANGERS OF RADIOACTIVITY
Exposure to ionising
radiation can be harmful.
- When radiation ionises molecules in living cells it can
cause tissue/cell damage, including cancer. The larger the dose of radiation
the greater the risk of cancer.
- Radiation can cause mutations in living organisms.
- Radioactive waste disposal causes problems because it has
to be dumped somewhere that isn't dangerous. Some of these materials have
half-lives lasting thousands of years.
When sources of radiation are inside the body:
- alpha radiation is the most dangerous because it is so strongly absorbed
by cells;
- beta and gamma radiation are less dangerous because cells are less likely
to absorb the radiation.
When sources of radiation are outside the body:
- alpha radiation is least dangerous because it is unlikely to reach living
cells;
- beta and gamma radiation are the most dangerous because they can reach
the cells of organs and may be absorbed by them.
Radiation sickness is illness induced by exposure to ionizing
radiation, ranging in severity from nausea, vomiting, headache, and diarrhoea
to loss of hair and teeth, reduction in red and white blood cell counts, extensive
haemorrhaging, sterility, and death.
Describe the precautions that should be taken when handling radioactive materials.
Explain how the effects of radiation depend on the energy and penetration of
the emission as well as the amount of exposure.
( Workers who are at risk from radiation often wear a radiation badge to monitor
the amount of radiation they have been exposed to over a period of time. The
badge is a small packet containing photographic film. The more radiation a worker
has been exposed to, the darker the film is when it has been developed. )
USES OF RADIOACTIVITY
Alpha radiation
The following aplications make use of alphas' low penetrating power. The source
must have a half life long enough to lst the appliance (a few years).
- Thickness monitoring and control. As radiation passes through
a material it can be absorbed. The greater the thickness of a material the
greater the absorption. The absorption of radiation can be
used to monitor/control the thickness of paper and metal sheets being manufactured.
- Smoke detectors. Americium-241 (in oxide form) emits alpha
particles that ionise the oxygen and nitrogen molecules in the air inside
the detector's ionisation chamber. A low-level voltage applied across the
chamber's electrodes is used to collect these ions, causing a small steady
electric current to flow. When smoke enters the space between the electrodes,
the alpha radiation is absorbed by smoke particles. This causes the rate of
ionisation of the air - and therefore the electric current - to fall, which
sets off an alarm.
Beta radiation
- Radiotherapy. Higher doses of ionising radiation can kill
cells; they are used to kill cancer cells.
- Radio-dating. Measurements of the amounts of radioisotopes
and their decay products in rocks can be used to calculate the age of materials
such as rocks and fossils / archeological specimens.
- The proportion of the radioisotope carbon-14 to its
stable decay product carbon-12 can be used to date organic
materials (i.e. ones that once formed living organisms.)
- The proportion of the radioisotope potassium-40 to
its stable decay product argon can be used to date igneous
rocks from which the gaseous argon has been unable to escape.
- The proportion of uranium to its stable decay product
lead can also be used to date rocks. (Uranium isotopes,
which have very long half-lives, decay via a series of relatively short-lived
radioisotopes to produce stable isotopes of lead).
Gamma radiation
- Radio tracers. As a result of a patient swallowing an appropriate
mildly radioactive material, the material acts as a tracer in that it passes
round the body, accumulates in various organs, and allows radio photographic
images to be taken of the organs for diagnostic purposes.
The half life of these radio-tracers is short.
- Sterilisation. The same radiation can used to kill
harmful microorganisms.
As regards nuclear weapons, there are strongly differing opinions as to how
'useful' nuclear weapons are to human welfare.
Apply knowledge of the different penetrating abilities to explain why different
emissions are suited to each of the above applications.
Candidates should be able to evaluate the appropriateness of radioactive
sources for particular uses, including as tracers, in terms of:
- the type(s) of radiation emitted;
- their half-lives.