P7: Radioactivity Flashcards
What is the basic structure of an atom?
Central nucleus containing neutral neutrons and positive protons, with negative electrons that orbit in energy levels around it
Nearly all of the mass of the atom is concentrated in the nucleus
What is the scale/size of an atom?
Radius = ~ 1x10-¹°
Nucleus radius = 1/10 000 radius of atom
How does the energy in energy levels differ depending on their distance from the nucleus?
Energy levels which are further away have higher energy
How can electrons change energy levels?
If an atom gains electromagnetic radiation, an electron can move to a higher energy level - the atom can now emit the electromagnetic radiation, and the electron returns to its original energy level
What do the numbers next to the elements mean?
Small number = proton and electron number (atomic number)
Big number = relative atomic mass (protons + neutrons)
What is an isotope?
Different versions of the same element with different numbers of neutrons
What is an ion?
An atom that has gained or lost an electron to gain a positive (lose) or negative (gain) charge
After the electrons were discovered, what was the new atomic model presented?
The plum pudding model - a ball of positive charge with negative electrons embedded in it
What experiment proved the existence of a positive nucleus?
The alpha-scattering experiment
Describe the alpha scattering experiment
A piece of gold foil was rolled to just a few atoms thick (that’s why it was chosen).
Then, positive alpha particles were fired at it.
Observations deducted that most alpha particles passed through the gold foil without changing direction, sometimes they were slightly deflected and other times they were directly deflected backwards
Why did the alpha-scattering experiment disprove the plum-pudding model?
Most of the alpha particles went through the foil - meaning the atom is mostly empty space.
As some of the atoms were deflected, the centre of the atom must have a positive charge as it deflected the positive particles.
Some particles bounced straight back - meaning the mass of the atom is concentrated in the centre
What was the new atomic model proposed?
The nuclear model - a positive nucleus surrounded by mostly empty space with tiny orbiting electrons
What were other additions to the nuclear model?
Energy levels, proposed by Bohr
Neutrons, proposed by Chadwick
What is the process of radioactive decay?
The process of an unstable nucleus giving out radiation to stabilise itself
What is the nature of radioactive decay?
It’s random - scientist cannot predict when it will happen
What is activity and what are its units?
The rate at which a source of unstable nuclei decay.
Its measured in Becquerel (Bq), where 1 Bq = 1 decay per second
How is activity measured?
Using a Geiger-Muller tube - the count-rate is the number of decays recorded each second by a detector
What are the 4 types of radiation?
Alpha radiation
Beta radiation
Gamma radiation
Neutron
What is the structure of an alpha particle?
A single helium nucleus - 2 protons and 2 neutrons
What is the structure of a beta particle?
A single electron that has been ejected from the nucleus at high speed
How is a beta particle made?
It is formed when a neutron in the nucleus changes into a proton and an electron - which is then released
What is the structure of gamma radiation?
Gamma rays - a type of electromagnetic radiation from the nucleus
What is the range of alpha particles in air?
5cm
They are large, meaning they can’t travel far in air before colliding with it and being stopped
What is the range of beta particles in air?
1 m
What is the range of gamma rays in air?
~ 1km
What material stops alpha particles?
A sheet of paper
What stops beta particles?
A few mm of aluminium
What stops gamma rays?
Several cm of lead
What is ionising power?
When radiation collides with atoms, it causes them to lose electrons (form ions)
Describe the ionising power of each of the forms of radiation
Alpha - strongly ionising
Beta - relatively ionising
Gamma - weakly ionising
What are the rules for alpha decay?
Atomic number decreases by 2
Mass number decreases by 4
What are the rules for beta decay?
Atomic increases by 1
Mass number remains the same
What is the half-life of a radioactive isotope?
The time it take for the number of nuclei of the isotope in the sample to halve
sample =! a single atom
It is also the time it takes for the count rate (activity) from a sample containing the isotope to fall to half its initial level
What is irradiation?
Exposing an object to nuclear radiation (it does not become radioactive)
e.g. gamma radiation is used for sterilisation
What is radioactive contamination?
When unwanted radioactive isotopes end up on other materials
What are the dangers of ionising radiation?
The risk of cancer is increased
What precaution can be taken against alpha radiation?
Wear gloves
What precautions can be taken against beta and gamma radiation?
Lead apron
Shielding (using lead walls and lead-glass screen)
What precaution can be taken against all types of radiation?
Radiation monitor to measure how much radiation has been received - if it’s too much, then the person will know not to work with radioactive isotopes
What precaution can be taken against all types of radiation?
Radiation monitor to measure how much radiation has been received - if it’s too much, then the person will know not to work with radioactive isotopes
How dangerous is alpha contamination?
Very - alpha radiation is very strongly ionising, but easily stopped by dead cells on the skin surface. It can be dangerous if inhaled or swallowed,
(thus they cannot escape the body and crash into living cells, damaging their DNA)
How dangerous is beta contamination?
Relatively - it’s quite ionising and enter the body through the skin
How dangerous is gamma radiation?
Meh - Weakly ionising, passes straight through the body
How dangerous is gamma radiation?
Meh - Weakly ionising, passes straight through the body
What is background radiation?
A measure of the level of the ionising radiation in the environment of a particular location
What are the natural sources of background radiation?
Radioactive rocks e.g. granite
Cosmic rays e.g. supernovae
What are the man-made source of background radiation?
Nuclear accidents e.g. at power stations
Nuclear testing
What can affect your exposure to nuclear radiation?
Your location and occupation
What are the units for dose of radiation?
Sieverts (Sv)/ millisievert (mSv)
What are some uses of radiation in medicine?
To explore internal organs and
To control or
Destroy unwanted tissue
How is radioactive iodine used to explore the thyroid gland?
The patient drinks radioactive iodine which emits gamma radiation. The thyroid gland is found in the neck and it absorbs iodine to make hormones.
The gamma radiation leaves the body and can be detected. If a scan shows that the gland has absobed too much or too little, doctors can diagnose the condition.
What is a tracer?
Injected radioactive sources that can make tissues show up through a medical imaging process
What are the issues/ specifics of using radioactive tracers?
They must use weakly ionising radiation (beta/ gamma)
They must emit radiation that can be detected outside the body (beta/ gamma)
It must not decay into another radioactive isotope
Must have a short half life so it’s not present in the body for a long time
What is radiotherapy?
Destroying cancer tissue with ionising radiation
Gamma radiation is used, they pass through the body and destroy the tumour
What is the problem with radiotherapy?
It can also destroy healthy tissue
What is nuclear fission?
The splitting of a large, unstable nucleus by the absorption of a neutron
What happens during nuclear fission?
When a neutron hits a nucleus, it forms 2 daughter nuclei of roughly equal size, 2-3 neutrons and a gamma ray + energy, all fission products have kinetic energy.
The released neutrons can now hit more uranium nuclei and trigger fission again, and again, releasing enormous energy.
What is a chain reaction?
When the fission of one nucleus results in the fission of other nuclei, and theirs results in the fission of more nuclei.
How is energy generated in a nuclear reactor?
Using a controlled chain reaction
(explosion in a weapon is caused by an uncontrolled chain reaction)
What is nuclear fusion?
Two light nuclei joining together to form one heavier nucleus
- some of the mass of the nuclei is converted to energy and released as radiation and heat
