P7 Flashcards
Radioactivity
What piece of equipment can be used to detect radiation levels ?
Geiger counter
What are the three types of radioactive radiation from least to most penetrating ?
- alpha radiation
- beta radiation
- gamma radiation
How did Ernest Rutherford use alpha radiation to prove the existence of the nucleus ?
- alpha radiation is made of positively charged particles
- atomic nuclei are positively charged
- like charges repel
- in an evacuated chamber (vacuum so no air particles collided with the radioactive particles), an alpha radiation emitter and a thin strip of gold foil were set up, with a detector outside that allowed reflected radiation to be observed as spots of light
- narrow beam of alpha radiation fired and lifht spots on the detector were counted from multiple places around the chamber
- most went through, whilst some rebounded and were picked up by the detector
- findings:
[] 1 in 10 000 alpha particles deflected by angles greater than 90 degrees
[] most passed straight through the foil
[] deflection per minute decreased as angle of deflection increased - Rutherford posited:
[] nucleus is extremely small because most particles passed through and weren’t deflected
[] nucleus is positively charged
[] nucleus is where most mass of the atom is concentrated
What is radioactive decay and when does it happen ?
- the release of radioactive radiation from an unstable nucleus
- RANDOM - cannot be influenced, induced, predicted or controlled
Why was Rutherford’s work regarding the nucleus so quickly accepted ?
- his theoretical calculations of the nucleus’ size matched Madsen and Geiger’s (Rutherford’s students who actually carried out the experiment) experiment exactly
- explained radioactivity in terms of changes that happen to an unstable nueus when radioactive radiation is emitted
- predicted the existence of the neutron (mass thing)
What was the plum pudding model and why was it disproved by Rutherford and his students ?
- said that electrons embedded in a sphere of positive charge
- discovery of nucleus meant that the nucleus was concentrated positive charge at the centre, not a sphere with electrons embedded
What did Niels Bohr discover and how ?
- energy levels
[] posited that electrons orbit the nucleus at specific distances and energy levels - showed electrons in orbit can move to another energy level via absorption of electromagnetic radiation (moves away from nucleus as gains energy) or emission of EM radiation (moves closer to nucleus as loses energy)
- Bohr’s theoretical calculations matched experimental calculations done once observing light emission from particular atoms
Why does radioactive decay occur ?
so the unstable nucleus can stabilise itself
What is an alpha particle made up of (and thus what is its relative mass, charge and atomic number) ?
- 2 protons and 2 neutrons
- atomic number is 2
- charge is +2
- relative mass is 4
- identical to the nucleus of helium so sometimes represented as such in nuclear equations
What happens when an alpha particle is emitted from a nucleus ?
nucleus of original atom loses 2 neutrons and 2 protons (may become an isotope of a different element altogether)
What is a beta particle made up of (and thus what is its relative mass, charge and atomic number) ?
- it is an electron created and emitted by a nucleus with too many neutrons compared with the proton number
- relative mass = 0.0005
- relative charge = -1
What happens when a beta particle is emitted from a nucleus ?
- a neutron in the nucleus changes into a proton and a beta particle, the latter of which is emitted
- atomic number of nucleus of atom increases by 1 because of the new proton
[] mass number remains constant - charge of nucleus increases
What is gamma radiation and what happens when it’s emitted from a nucleus ?
- ELECTROMAGNETIC radioactive particles that have no mass or charge
- emission does not affect the relative charge or mass of an atom’s nucleus
Why are neutrons sometimes emitted as radioactive particles from a nucleus ?
- alpha particles collide with unstable nuclei within a substance
- destabilises the nuclei even further, so emit neutrons
Why can gamma and neutron radiation pass through substances more easily (more penetrative) than alpha or beta radiation ?
they are uncharged
List the three main radiation types from most to least ionising (and thus most to least harmful to living cells)
- alpha
- beta
- gamma
What is the range of each key type of radiation in air ?
- alpha = 5cm
- beta = 1m
- gamma = unlimited
[] spreads out in air without being absorbed
What are each key type of radiation absorbed/stopped by ?
- alpha = paper
- beta = metal foils
- gamma = thick lead sheet (several cm thick) or concrete (>1m thick)
Describe briefly how to test absorbing materials of radioactive radiation as well as the range of radioactive particles in air using a Geiger counter
FOR BOTH METHODS, FIRST:
- measure count rate with Geiger counter WITHOUT any radioactivity present
[] this is to measure background radioactivity to account for the 0 error
- in calculations, subtract this background count rate from the total count rate with radioactivity present to get the count rate solely of the radioactive source
ABSORPTION
- measure and record base count rate without radioactivity present
- place the absorbing material in front of a sealed container with the source inside
[] the container should have a narrow hole for radiation to be beamed through to the material
- place a Geiger tube behind this absorbing material (sends radiation to electronic counter)
- take away base count rate from total count rate
- when count rate without base count rate is 0, radiation has been fully absorbed by the material
RANGE IN AIR
- measure and record base count rate without radioactivity present
- place the Geiger tube in front of a sealed container with the source inside
[] the container should have a narrow hole for radiation to be beamed through to the material
- slowly move the tube away from the source along a long tape measure
[] when the count rate is 0, out of range of the radiation
- take away base count rate from total count rate
What is irradiation ?
When something is exposed to radioactivity but does not become radioactive
What is radioactive contamination ?
unwanted presence of materials containing radioactive atoms on other materials
How do workers who use ionising radiation reduce their exposure to it ?
- keeping as far away from the radioactive source as possible
[] includes using long handled tools - spending as little time as possible in contaminated/high-risk areas
- working behind barriers of thick concrete or lead plates
Give two uses of radioactive radiation
- smoke alarms
[] use alpha particles (which ionise in air) to generate an electric current between the air and the alarm
[] when alpha particles are absorbed by smoke, the current drops and the alarm sounds
[] beta/gamma particles don’t ionise enough in air to do this - automatic thickness monitoring in foil production
[] beta particles emitted and detected by a detector behind the foil sheets
[] if the foil is too thick number of particles passing through drops
[] detector sends signal to increase pressure of rollers and roll the metal more thinly
What is the half-life of a radioactive isotope ?
time taken for the number of nuclei of the isotope in a sample to halve
OR
the time taken for the count rate (activity) of a sample containing a radioactive isotope to fall by half
in both of these:
- count rate = number of nuclei decaying per second as detected by a Geiger counter
Describe how to determine one and two half-lives of a sample of 1000 undecayed nuclei FROM A GRAPH
- first find 500 on the y-axis (where undecayed nuclei are shown), then trace it to the line horizontally, then vertically down to the x-axis to find the time
- to find the second half-life, halve 500 to get 250 and repeat the process
A radioactive isotope has a half life of 15 days and an initial count rate of 200 counts/s. Find the count rate after 45 days.
- one half life of this isotope = 15 days
- 45/15 = 3
[] so 3 half lives, meaning to half the initial count rate three times also - 200/2 = 100
- 100/2 = 50
- 50/2 = 25
count rate after 45 days = 25 counts/s
What is the unit for activity ?
Bq
What are the main ways in which radioactivity is used in medicine ?
- radiotherapy for cancer treatment
- medical imaging
- tracers to monitor organ (function)
How and why are radioactive tracers used to monitor organ function ?
- patient is given small amount of radioactive isotope
- isotope emits gamma radiation that is detected by monitors
- for monitoring kidneys for example:
[] patient given water with small amounts of radioactive iodine
[] if kidney is healthy, reading goes up as the iodine enters the kidney, then down as it exits
[] if kidney is blocked, reading doesn’t go down after entering the kidney
why:
- radioactive substances used here like radioactive iodine have half lives that are long enough for the tests to be carried out, but short enough that they decay almost completely soon enough so that the patient is not harmed
- radioactive substances used here emit gamma radiation which pass through all body tissues and can be detected by the detector and displayed on the monitor
- radioactive substances used here decay into stable, harmless products
How are medical images taken using gamma rays ?
- patient injected with solution with gamma-emitting isotope
[] half life should be long enough to take the image but short enough that it almost completely decays afterwards - solution absorbed by nearby organ
- gamma camera detects gamma radiation through holes in thick lead grid in front of detector
- detector signals used to build up image of the radiation in the organ and thus of the organ itself
How are gamma rays used to treat cancer ?
- narrow beam of radiation from cobalt fired at tumours/cancer cells to destroy them
[] however may harm surrounding tissue
[] half life of 5 years - used due to penetrative power - radioactive implants
[] sometimes also emit beta radiation
[] come as small seeds/tiny rods
[] permanent implants half half lives long enough that they irradiate and thus damage/kill tumours and that they decay shortly after doing so
Give the percentages of background radiation from different sources (8)
- 50% = radon gas in air
- 10% = CMBR
- 11.5% = food + drink
- 14% = ground
- 14% = medical purposes
- 0.2% = nuclear weaponry
- 0.2% = air travel
- 0.1% = nuclear reactors
What is nuclear fission ?
the splitting of an atom’s nucleus into 2+ smaller DAUGHTER nuclei ROUGHLY EQUAL IN SIZE
- RELEASES ENERGY (in form of gamma radiation and kinetic energy of released neutrons and fragment nuclei)
- RELEASES TWO OR THREE (fission) NEUTRONS
What is induced fission vs spontaneous fission ?
induced = when nuclear fission is induced via a neutron colliding with the nucleus and being absorbed
- most commonly in reactors, when uranium-235 or plutonium-239 absorb neutrons
spontaneous = when nuclear fission happens without a neutron striking the nucleus
How do nuclear fission reactors take advantage of the stable release of energy from nuclear fission ?
- one nuclear fission releases other neutrons which can induce more nuclear fission in fissionable nuclei
- causes CHAIN REACTION of nuclear fission
- steady release of energy as such
Where can you find controlled vs. uncontrolled fission (chain) reactions ?
controlled = nuclear reactor
uncontrolled = nuclear bomb
How do nuclear reactors control the rate of fission ?
- use water (at high pressure) in pipes through reactor core as a coolant and moderator
[] slows down rate of reactions by slowing fission neutrons via collisions with water molecules and thus taking energy so can’t trigger more fission - use control rods in reactor core which absorb surplus neutrons
[] depth of rods adjusted to maintain steady reaction as necessary - thick concrete walls enclose reactor so gamma radiation doesn’t escape
What is nuclear fusion ?
- when two smaller nuclei are forced near enough that they form one heavier nucleus
- releases energy (nuclear radiation and kinetic energy of particles emitted and the new nucleus)
- creates and emits some particles
- only happens if nuclei used are LESS THAN OR EQUAL TO RELATIVE MASS OF 55
[] can happen above this, but energy must be supplied
Why must high temp/pressure be used in nuclear fusion ?
gives particles increased kinetic energy and forces them closer together
[] collide with more force, helping to overcome electrostatic repulsion due to both having like charges
How is the high temperature of particles in nuclear fusion maintained throughout the reaction ?
- heated by using a large electric current through the plasma
- plasma contained with a magnetic field around the edges of the reactor so they don’t touch the walls and go cold
Give the benefits and disadvantages of nuclear fusion as an energy source
benefits:
- uses heavy hydrogen as fuel, which is naturally abundant in seawater unlike the radioactive fuels for nuclear fission
- reaction product, helium, is a harmless gas
- releases a lot of energy, so could easily meet electricity and energy demands
disadvantages:
- hard to make them economical, as the energy used to sustain the magnetic field and heat the plasma via electric currents currently outweighs the profit of the energy released by the reaction
- nuclear fusion reactors are still in early stages of development and could take years to become practical
What is the unit for radiation ?
sieverts/Sv
- 1 Sv = 1000 mSv
Give some of the main issues of nuclear fission/fusion
- produce nuclear waste
[] has to be stored safely underground for many many years so that the surrounding areas are not contaminated by radioactivity
[] unused uranium and plutonium from the waste are chemically removed for future use - nuclear meltdowns like Fukushima and Chernobyl
[] impact economy and health across the world pretty much - risk of radiation on human and environmental health
- at the moment, have a relatively short operating life (20 years) with high construction and decommissioning costs
How will new nuclear (fission) reactors improve on the current ones ?
- longer operating lives (60 years instead of 20)
- lower costs of commission and decommission
- less effect on environment
- more safety features
[] eg. convection of outside air through cooling panels along reactor walls
