19- Nuclear Flashcards
What is the ionising and penetrating power of alpha particles?
Strongly ionising, low penetrating.
What is the ionising and penetrating power of Beta radiation?
Weakly ionising, can penetrate several cm of aluminium, or a few meters of air.
What is the ionising and penetrating power of Gamma radiation?
Very low ionising, Highly penetrating and can go through several mm of lead and through several meters of concrete.
Structure and charge of alpha particle?
2 protons and 2 neutrons, 2+
What is beta- decay
When a neutron turns into a proton and releases an electron
What is beta+ decay?
When a proton turns into a neutron
Explain gamma decay
A way of unstable nuclei releasing extra energy in the form of a electromagnetic wave from a spontaneous process.
Explain the effects of ionising radiation:
If it hits an atom it can knock of electrons/ ionise them. These can cause chemical changes, which in the body can damage or kill living cells.
How would you investigate the absorption of different types of ionising radiation?
Set up Geiger counter aimed at an absorber (made of aluminium for example). Place counter close to absorber so air particles affect on alpha radiation is minimised. Measure background radiation for a set amount of time. Then place radioactive sample on other side of absorber. Measure activity for set time, then repeat with increasing thicknesses of absorber. Then repeat with each different radioactive source.
What is Half thickness?
The thickness of absorbing material needed to half, on average, the number of photons that get through.
Define Absorbed Dose and what is it’s units?
The number of Joules per Kilo absorbed by a tissue. Measured in Gray (Gy)
Define Effective Dose and what is it’s units?
The Absorbed Dose multiplied by various quality factors that takes into account the type of radiation absorbed and the tissue that has been exposed to it. Measured in Sieverts, Sv.
What are the two equations for radioactive risk?
(equivalent dose) * (incidence per Sievert)
and
(probability of event occurring) * (consequence)
What is the allowed dose per year for a person?
1000 micro Sieverts per year.
Explain the trend of stability for stable isotopes as mass number increases:
Smaller nuclei have equal numbers of protons and neutrons. As mass increases, heavier stable nuclei have more neutrons than protons too overcome electronic propulsion which is negated by the strong nuclear force.
What is the strong nuclear force?
A fundamental force which holds nuclei together and opposes the electronic repulsion of protons.
What is binding energy?
The amount by which the rest energy of a nucleus is less than the rest energy of the protons and neutrons completely seperated.
What is the equation for rest energy?
E[rest] = mc^2
What is the name for the difference in mass between a nucleus and the sum of the masses of all of its protons and neutrons completely separated?
Mass defect
Equation for binding energy per nucleon?
(Binding energy)/ (no. of nucleon)
Explain the trend between the tightness of the bind of a nucleus and the mass of its nucleons:
The tighter the nucleus is bound, the smaller the mass of the nucleons.
Explain the Nuclear valley:
Between Hydrogen and Iron (which is the strongest nuclear bonded element) binding energy per nucleon becomes more negative. Atoms “fall down” the valley via nuclear fusion. Elements heavier than iron tend to fall down the valley via alpha decay.
Explain why unstable nuclei have less negative binding energies per nucleon:
The nucleons are less tightly bound. Therefore they have a greater mass. This means the difference in mass between these nucleons and nucleons which are completely unbound is less neagtive.
What is the definition of an atomic mass unit (u)?
the weighted mean mass of approximately one proton or neutron compared to 1/12th of the mass of a carbon-12 atom.
Define nuclear fission:
The splitting of heavy nucleus into smaller nuclei, fast moving neutrons, and energy.
Define a chain reaction:
The process in which the products of one nuclear fission go on to start one or more further reactions.
What is a nuclear moderator?
A material (usually water) which is used to slow down the speed of neutrons in a reactor, so that they are more likely to be absorbed/ captured into the nuclei instead of being deflected.
Why does the use of a moderator and thereby increasing the Volume to Surface area ratio increase the rate of fission?
Fewer neutrons will escape from the large mass of fuel without being absorbed.
Define when a reaction “goes critical”.
When the products from each fission are self sustaining and go on to trigger on average one other event.
What is the use of control rods in a reactor?
To be able to be lowered and raised into a reactor to moderate the number of neutrons being absorbed, so that the rate of fission can be increased or decreased.
Define activity (A) and state its units
The number of nuclei decaying per second. Becquerel, Bq
Define half life
The time taken for the number of nuclei in a radioactive source to fall to half the original value. Alternatively, it is the time taken for the activity of a sample to fall to half the original value.
Explain how you would determine the half life of a radioactive source:
Measure background count. Place the radioactive source in front of the detector. Record the count of activity every 10 seconds for about 3 mins. Count rate is found by dividing the number of counts in each period by 10. The corrected count rate is found by subtracting the total count rate by the background count rate.
How do you determine half life from a graph of count rate against time?
…
What is the decay constant, λ, and measure its units.
The probability that a nucleus will decay in a given time.
Equation to model radioactive decay?
ΔN/Δ t = -λN
Equation linking half life and decay constant?
T[1/2] = (ln2)/λ
