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Chapter 19 - Using the Atom
Terms in this set (37)
What are the features of alpha radiation?
Strongly ionising and not very penetrating. Each ionisation transfers energy away from the alpha particle , so it rapidly loses energy. It has a range of only a few centimetres in air.
What is an alpha particle?
An alpha particle is a helium nucleus.
What happens during alpha decay?
During alpha decay, the nucleus releases 2 neutrons and 2 protons (so the charge number drops by 2 and the mass drops by 4).
What are the features of beta (+ and -) radiation?
Weakly ionising, but more penetrating than an alpha particle (with a range in air of around 1 metre). Single electrons and positrons produce less ionisations per metre compared to an alpha particle, so they can travel further with the same initial energy.
What are beta+ and - particles?
Beta+ is a positron,
Beta- is an electron.
What happens during beta- decay?
A proton turns to a neutron, and an electron and antineutrino are emitted. The mass remains the same, but the charge of the original nucleus increases by 1.
What happens during beta+ decay?
A neutron turns to a proton, and a positron and neutrino are emitted. The mass remains the same, but the charge of the original nucleus drops by 1.
What are the features of gamma radiation?
Far less ionising than alpha and beta, and follows the inverse square law for intensity from its source.
How are tracks formed in a cloud chamber?
When a particle of ionising radiation passes through the vapour in the chamber, it produces ions. These ions give a platform for the vapour to condense onto, leaving a visible trail if the ionising particles path.
Why do denser materials reduce the range of ionising radiation?
Denser materials reduce range as they have more atoms to interact with the ionising radiation per metre of travel.
What is needed to block certain types of radiation?
Alpha - Stopped by a few sheets of paper or thin metal foil.
Beta - Stopped by a few millimetres of aluminium.
Gamma - Not easily stopped by a few centimetres of lead.
What is the equation for intensity of radiation as it travels through a material?
I = I₀e-⁻ᵐˣ
Where I is intensity, I₀ = initial intensity, m (μ) = absorption coefficient and x = thickness of material.
What is the equation for half thickness?
Half-thickness = (ln2)/μ.
What is absorbed dose and how is it calculated?
Measured in Grays (Gy), it is the number of joules of energy absorbed per kilogram of material.
Absorbed dose = Energy Absorbed/Mass of Absorber.
(Energy = Energy per particle x No. of particles).
What is dose equivalent and how is it calculated?
The dose equivalent (or effective dose) is a measure of the effectiveness of a particular dose of radiation.
Dose equivalent = Dose in Gray x Quality factor.
How is risk calculated?
Risk = dose equivalent = incidence per sievert.
What does ALARA stand for and what principles does it bring?
As Low As Reasonably Achievable.
The ALARA principle is followed by those working with radioactive materials, to reduce the amount of radiation (and therefore risk) that they are exposed to.
What do stable nuclei tend to have?
Equal numbers of protons and neutrons.
Why do light (low nucleon number) nuclei tend to be unstable?
They tend to have more neutrons than protons, making them unstable (as the numbers of hadrons are uneven).
Why do stable heavy (high nucleon number) isotopes have more neutrons than protons?
They can have more neutrons than protons in their nuclei, as these neutrons dilute the effects of electrical repulsion between the protons in the nucleus, allowing the strong force to overtake (holding the nucleus together and making it stable).
What is the strong force?
This is the force that overcomes electrical repulsion within the nucleus, holding neutrons and protons together inside the nucleus.
Where, in the nuclear valley, do nuclei "want" to be?
Lower down and very strongly bound, closer to iron.
On the nuclear valley, what is the fusion hill?
The nuclei on the fusion hill go downhill by nuclear fusion. Large numbers of protons and neutrons on the fusion hill are more strongly bound than smaller numbers, as the strong nuclear force is larger (as more gluons are exchanged).
On the nuclear valley, what is the coulomb slope?
This contains nuclei that need fewer protons and more neutrons, in order to overcome the forces of electrical repulsion. These usually decay via alpha decay or nuclear fission.
On the nuclear valley, what are the sides of the valley?
The sides of the valley contain unstable nuclei, with a slight excess of neutrons or protons. They climb they valley due to the Pauli Exclusion Principle. These nuclei "fall back down" the sides via positron (B+) or electron (B-) decay.
Why are gamma rays emitted by radioactive material when they decay?
Nuclei emit photons when they fall from a higher, excited energy state to a lower energy state. These photons are high energy gamma photons that are detected.
What is the Pauli Exclusion Principle?
Fermions (electron, protons and neutrons) cannot come together in the same quantum state. This is because the amplitude of the phasor arrows add to zero when they come together (out-of-phase).
Why do photons not follow the Pauli Exclusion Principle?
Photons phasor arrows can add up when they come together, meaning they 2 can add up and occupy the same space.
How is binding energy calculated?
Binding energy = (Rest energy of individual nucleons) - (Rest energy of whole nucleus).
Energy is usually found from finding the difference in mass (or mass defect) and multiplying this by c².
What is true about the value of binding energy?
The more negative the binding energy, the more stable the nucleus (as it is more bound together).
What needs to be subtracted from the mass (when finding binding energy) if the mass of the atom is given instead of the mass of the nucleus?
The mass of its electrons need to be subtracted from its mass, as this leaves only the mass of the nucleus.
Does an unstable nucleus always decay into a stable one straight away?
No - Unstable nuclei can still decay into other unstable nuclei. In fact, it can take years (due to long half-lives) for a nucleus to decay into a stable isotope.
What is nuclear fission?
This is where a heavy nuclei is split into two lighter nuclei, by firing a neutron at the nucleus. The nucleus absorbs this neutron, and energy is released.
The neutron causes the nucleus to vibrate like a liquid drop, and eventually come apart.
What is critical mass?
A reaction with critical mass is one that is self sustaining (each fission event triggers, on average, one other fission event).
A reaction can be stopped reaching super-critical mass by using control rods, to absorb some of the neutrons.
How can a moderator be used to increase the chances of a chain reaction?
A moderator, such as water, can be used to slow down the fired neutrons, increasing the chance of them colliding with a nucleus and being absorbed.
What is nuclear fusion and what is needed for the reaction to occur?
This is when small nuclei collide and fuse at very high temperatures.
These high temperatures are needed to give the nuclei enough energy to overcome the electrostatic repulsion force.
What is plasma and when is it used?
Plasma is a super-hot state of matter where all of the electrons are ripped from atoms.
It is used in fusion reactions, so that the nuclei continuously have enough energy to fuse with one another.
Recommended textbook explanations
Jerry S. Faughn, Serway
Paul G. Hewitt
Paul G. Hewitt
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