Physics - Radioactivity Flashcards
Radioactivity
How big is an atom?
What is the structure of an atom?
Atoms are about 1 x 10-10m in diameter.
Atoms are made up of a positively charged nucleus containing protons and neutrons surrounded by negatively charged electrons.
The radius of a nucleus is less than 1/10,000 of the radius of an atom. Most of the mass of an atom is concentrated in the nucleus.
Where are the electrons?
The electrons are arranged at different distances from the nucleus (different energy levels).
The electron arrangements may change with the:
● absorption of electromagnetic radiation (move further from the nucleus; a higher energy level)
● emission of electromagnetic radiation (move closer to the nucleus; a lower energy level)
What is an atom?
In an atom the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge.
What notation do we use to
describe atomic nuclei?
All atoms of a particular element have the same number of protons (Atomic number). The total number of protons and neutrons in an atom is called its mass number.
What is an isotope?
Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element. They have a different mass.
How is an ion formed?
Atoms turn into positive ions if they lose one or more outer electron(s).
Why do scientists look for new evidence?
New experimental evidence may lead to a scientific model being changed or replaced.
How was our current model of the atom developed?
Before the discovery of the electron, atoms were thought to be tiny spheres that were indivisible.
The discovery of the electron led to the plum pudding model of the atom (J.J. Thomson). The plum pudding model suggested that the atom is a ball of positive charge with negative electrons scattered throughout it.
The results from Rutherford’s alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged (positively). This
nuclear model replaced the plum pudding model.
Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances.
The theoretical calculations of Bohr agreed with experimental observations.
Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton
was given to these particles.
The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.
What is radioactive decay and what does it result in?
Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable.
This is a random process called radioactive decay.
Activity is the rate at which a source of unstable nuclei decays.
Activity is measured in becquerel (Bq)
Count-rate is the number of decays recorded each second by a detector (e.g. Geiger-Muller tube).
Count-rate is proportional to activity (but not identical because it is only measuring from one single direction when activity from a source spreads in all directions).
The nuclear radiation emitted may be:
● an alpha particle (α) – this consists of two neutrons and two protons and has a charge of +2, it is the same as a helium nucleus
● a beta particle (β) – a high speed electron ejected from the nucleus as a neutron turns into a proton
● a gamma ray (γ) – electromagnetic radiation from the nucleus
● a neutron (n).
What are the penetrative
properties of nuclear radiation?
Alpha particles are strongly ionising. They have a short range in air (a few cm) and are stopped by a thin sheet of paper.
Beta particles are less ionising and have a greater range in air. They are stopped by a few mm of aluminium.
Gamma waves ionise weakly. They have an unlimited range in air. They can be stopped by thick
concrete/lead.
What are the common uses for
nuclear radiation?
Alpha particles can be used in smoke detectors. They are very good at ionising the air so an electric current can flow. If smoke gets into the detector, not as much ionisation takes place and the current changes. This makes the alarm sound.
Beta particles can be used to check the thickness of paper/thin metals. A source of beta particles is directed towards the material. A detector on the other side counts the number of particles that get through. If more get through, the material is too thin and the rollers being used to thicken/thin the material are moved apart to allow the material to thicken. As it thickens the count on the detector will drop.
Gamma rays have various uses, most of which involve the destroying of cells in the sterilisation of medical
equipment or on food. They can be used to detect and treat cancer. They can be used to find leaks in pipes underground.
What is the half-life of a substance?
Radioactive decay is random.
The half-life of a radioactive isotope is the time it takes for the number of nuclei of the isotope in a sample to halve, or the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level.
How would you determine half-life from a graph?
Find half of the original value on the y-axis. Draw a line horizontally across until it touches the curve then draw a line vertically down to the x-axis, this gives the half life.
The activity/count rate/number of undecayed nuclei remaining after a number of half-lives, n can be found by: original value x 1/2^n
What is the difference between
contamination and irradiation?
Contamination: occurs if an object has a radioactive source introduced onto it, the object becomes radioactive.
Irradiation: the exposure of objects to beams of radiation, the object doesn’t become radioactive.
What are the hazards associated with contamination and irradiation?
Contamination risks the transporting of radioactive materials to other places and potentially contaminating and irradiating others.
Irradiation risks DNA damage, leading to the mutation of cells which can lead to cancer.
What precautions must be taken to protect against contamination?
Contamination can be managed by the wearing of protective clothing such as gloves and haz-mat suits.
Using tongs to make sure the material is kept at a distance from the user would be helpful.
What precautions must be taken to protect against irradiation?
Irradiation can be managed by:
● Keeping a radioactive source at a safe distance using tongs
● Using concrete/lead shielding to prevent particles/waves reaching body tissues
● Not pointing a source in the direction of people
● Spending as little time as possible in at-risk areas (and using a dosimeter to measure exposure)
What is the point in peer review in science?
It is important for the findings of studies into the effects of radiation on humans to be published and shared with other scientists so that the findings can be checked and verified by peer review.
What is background radiation?
Background radiation is around us all of the time. It comes from:
● natural sources such as rocks (radon gas emitted) and cosmic rays from space
● man-made sources such as the fallout from nuclear weapons testing and nuclear accidents, medical sources, food and drink and air travel.
The level of background radiation and radiation dose may be affected by occupation and/or location.
What is radiation dose?
Radiation dose is measured in sieverts (Sv)
1000 millisieverts (mSv) = 1 sievert (Sv)
You are not expected to remember the unit of radiation dose.
How is radiation used in medicine?
Nuclear radiations are used in medicine for the:
● exploration of internal organs
○ Radioactive tracers can be injected into a person’s bloodstream and it can be used to identify blockages/tumours
○ Tracers should have a half-life that is short enough to be gone after a reasonable amount of time to limit the risk of damage to the patient, but also long enough that a useful image
can be provided.
○ The tracers should also result in the emission of gamma radiation as it can be detected
outside the body.
○ Gamma radiation in a narrow beam is used to destroy cancerous tumours (radiotherapy).
○ Radioactive implants are used to destroy cancer cells in some tumours (beta or gamma
emitting isotopes)
What is nuclear fission?
Nuclear fission is the splitting of a large and unstable nucleus (e.g. uranium-235 or plutonium) into two smaller daughter nuclei. In the process energy is released in large amounts and excess neutrons (usually 2 or 3) are emitted (fission neutrons).
Usually, for fission to occur the unstable nucleus must first absorb a neutron. Spontaneous fission can occur but is too rare to use for sustained energy release.
All of the fission products have kinetic energy. The neutrons may go on to start a chain reaction. Where they are absorbed by another nucleus.
How is fission controlled in a
nuclear power station?
The chain reaction is controlled in a nuclear reactor (whilst monitoring the temperature of the reactor) with control rods that absorb some of the neutrons (so they cannot cause further fissions) to control the
energy released.
The explosion caused by a nuclear weapon is caused by an uncontrolled chain reaction.
A moderator (often simply water) is used in a nuclear reactor to ensure that fission neutrons slow down to the correct kinetic energy to be absorbed by further uranium nuclei.
What is nuclear fusion?
How would it be done on Earth?
Nuclear fusion is the joining of two small nuclei to form a heavier nucleus. Huge amounts of energy are released in the process.
High pressure and temperatures are needed to overcome electrostatic repulsion. These are achieved with strong magnetic fields and electric currents/lasers which in cases increases the temperature to near 100 million Kelvin.
