Module 6: Chapter 25 - Radioactivity Flashcards
How was radioactivity discovered?
Radioactivity was first discovered by Henri Bequerel when he noticed that photographic film wrapped in light proof paper in a dark room became fogged by some nearby uranium.
Tests carried out revealed the following had no effect on the nuclear reaction which was occuring:
- Heating the mineral to extreme temperatures
- Cooling the mineral
- Dissolving it in acid
- Altering the pressure
- Putting it in strong magnetic fields
- Electrically charging it or putting it in an electric field
What is ionising radiation?
Any form of radiation that can ionise atoms by removing an electron to leave a positive ion
What is a cloud chamber?
A cloud chamber can be used to detect the presence of ionising radiation. It contains air saturated with vapour at a very low temperature. When the air molecules are ionised, liquid condenses onto the ions to leave tracks of droplets marking the path of the radiation.
What is the nature of alpha radiation?
- Symbol = α
- Particle
- It consists of positively charged particles. Each alpha particle comprises 2 protons and 2 neutrons (a helium nucleus)
- It has a charge of +2e
- It is stopped by a sheet of paper / 3cm of air
- It is very strongly ionising
What is the nature of beta radiation?
- Symbol = β⁻/β⁺
- Particle
- It consists of fast-moving electrons (β⁻) or fast-moving positrons (β⁺)
- A beta-minus particle has charge -e and a beta-plus particle has charge +e
- It is stopped by a sheet of aluminium foil
- It is strongly ionising
What is the nature of gamma radiation?
- Symbol = γ
- High frequency EM wave
- It consists of high-energy photons with wavelengths less than about 10⁻¹³m. They travel at the speed of light
- They have no charge
- It is stopped by a sheet of lead or several metres of concrete
- It is weakly ionising
How are alpha, beta, and gamma forms of radiation formed?
All are emitted from the nuclei of atoms as a result of changes wihtin unstable nuclei
What can be used as a detector for radiation?
Zinc Sulphide, it will create small flashes if it encounters a form of radiation
Explain why alpha radiation is the most ionising but the least penetrating
The most ionising form of radiation (alpha) has the largest electric charge and largest mass. Therefore it loses energy the quickest and is the most ionsing. However, since it loses energy the quickest, it is the least penetrating
What is the wavelength of Gamma radiation?
Less than 10⁻¹³m
What is radioactive decay?
The emission of a radioactive particle from an unstable nucleus
What is the effect of a uniform electric field on different types of radiation?
- The negative Beta-minus particles are deflected towards the positive plate
- The positive alpha and beta-plus particles are deflected towards the negative plate. However, alpha particles are deflected less than either beta particle due to a far greater mass. The paths of beta-minus and beta-plus particles are mirror images
- Gamma rays are not deflected as they are uncharged
Why is it not possible to carry out an experiement with a Geiger-muller tube for beta-plus decay to determine the absorption of beta-plus particles by different materials?
Beta-plus particles (positrons) would immediately be annihilated by the large quantity of electrons available in all matter
What is the range in air of alpha particles?
A few centimetres
What is the range in air of Beta particles?
A few metres
What is the ranger in air of gamma radiation?
Several km
What is the penetration ability of alpha particles?
0.2mm of paper
What is the penetration ability of beta particles?
A few mm of aluminium
What is the penetration ability of gamma radiation?
a few centimetres of lead / a few metres of conrete
What precautions are taken when dealing with a radioactive source?
- They are stored in lead-lined storage containers
- When handling the radioactive sources, a pair of tongs with long handles must be used to keep the source as far from your body as possible
What is nuclear transmutation?
The process by which radioisotopes are constantly changing to try and stabilize and form stable isotopes. In the process, they will release energy and matter from their nucleus and often transform into a new element. The radioactive decay and transmutation process will continue until a new element if formed that has a stable nucleus and is not radioactive
What are the dangers of radioactivity?
All forms of nuclear radiation cause ionisation, therefore they can cause significant damage to living cells
What is a radioisotope?
A radioactive isotope - it has an excess numbers of neutrons or protons, giving it excess nuclear energy, and making it unstable
What is a parent nucleus?
A nucleus before the occurence of radioactive decay
What is a daughter nucleus?
A new nucleus formed following a radioactive decay
What are the conservation rules of nuclear reactions?
- Nucleon number A and proton number Z must be conserved
- Conservation of mass and energy are interchangeable as the energy released in nucler reactions is produced from mass
What is the nuclear transformation equation for alpha decay?
Energy is also released in the decay
What is the nuclear transformation equation for Beta-minus decay?
What is the nuclear transformation equation for beta-plus decay?
When are gamma photons emitted?
Gamma photons are emitted if a nucleus has surplus energy following an alpha or beta emission
What is the nuclear decay equation for gamma decay?
When is beta-minus decay likely to happen?
When the radioactive nuclei has too many neutrons for stability (neutron-rich)
When is beta-plus decay likely to happen?
When the radioactive nuclei has too many protons for stability (proton-rich)
What type of nuclear decay is caused by the weak nuclear force?
Beta decay
When is alpha decay likely to occur?
When the radioactive nuclei has more than 82 protons
What is a decay chain?
A series of radioactive decays of different radioactive daughter nuclei as a sequential series of transformations. The typical radioisotope does not decay directly to a stable state, but rather it decays into another radioisotope. Therefore, there is usually a series of decays that take place until the atom has become a stable isotope (meaning that the nucleus of the atom has reached a stable state)
What are the 2 characteristics of radioactive decay?
- It is random
- It is spontaneous
Explain what is meant by “radioactive decay is random”
- You cannot predict when a particular nucleus in a sample will decay or which one will decay next
- Each nucleus within a sample has the same chance of decaying per unit time
What is meant by “radioactive decay is spontaneous”?
The decay of nuclei is not affected by:
* The presence of other nuclei in the sample
* Any external factors (pressure, temperature etc)
What is the half-life of an isotope?
The average time it takes for half the number of active nuclei in the sample to deca
What is the activity, A, of a source?
The rate at which the nuclei decay in a radioactive source. It is the number of alpha particles, beta particles, or gamma photons emitted from the source per unit time. It is measured in Becquerels (Bq) or decays per second
What is the decay constant, λ?
The probability of decay of an individual nucleus per unit time. It has the unit s⁻¹
What is the equation for the activity of a radioactive source?
With reference to the number of nuclei
A = λN
A = activity, λ = decay constant, N = number of nuclei
What is the relationship between the number of radioactive nuclei and the time?
Exponential decay
What is the count rate of a radioactive source?
The number of counts per second detected by a GM (Geiger-Muller) tube
What is the difference between count rate and activity?
The count rate is only a fraction of the activity as count rate is just the rate at which radioactive emissions are detected, as not all of the decay events will be detected they are not the same
What is the exponential decay equation for radioactive decay?
λ = decay constant, t = time, X = multiple variables such as number of nuclei, Activity, Mass
Derive the equation for half life in terms of the decay constant
What is the equation linking half life and decay constant?
λ = decay constant, t½ = half life
Caesium-137 has a half-life of 30 years. how many Caesium-137 nuclei will be needed to give an activity of 2.0x10⁵ Bq
2.73x10¹⁴
A newly-prepared radioactive source contains 5.0g of Stronium-90, which has a half-life of 28 years.n Calculate the time taken for the mass of strontium-90 to be reduced to 1g
65 years
2.14x10²⁴
Calculate the power still available after 100 years
214W
What is carbon dating?
A method for determining the age of organic material, by comparing the activities, or the ratios of carbon-14 to carbon-12 nuclei, of the dead material of interest and similar living material
How is carbon-14 produced in the Earth’s atmosphere?
High speed protons in cosmic rays from space collide with atoms in the upper atmosphere to produce neutrons. These neutrons in turn collide with nitrogen-14 nuclei in the atmosphere to form carbon-14 nuclei (with the emission of a proton). These carbon-14 nuclei are radioactive and will eventually emit beta-minus particles (electrons) to form Nitrogen-14 again, repleneshing the amount of carbon-14 in the atmosphere
What is the half life of carbon-14?
5700 years
What are the equations for the formation and decay of carbon-14 in the atmosphere?
Explain how carbon dating works
All living things on earth contain carbon atoms. Through the carbon cycle all living organisms take in carbon from the atmosphere. A small amount of atmospheric carbon is the radioisotope carbon-14. Carbon-14 has a half life of 5700 years and is produced continuously in the upper atmosphere by cosmic rays. The ratio of carbon-14 to carbon-12 in atmospheric carbon is 1.3x10⁻¹², and it is the same in all living organisms. Once the organism dies, it stops taking in carbon, whilst the carbon-14 it currently contains continues to decay, therefore this ratio of carbon-14 to carbon-12 decreases over time. The activity of carbon-14 in a sample of organic material is proportional to the number of undecayed carbon-14 nuclei. The time since the organism has died can therefore be determined by comparing the activities, or ratios of carbon-14 to carbon-12 nuclei, of the dead material and similar living tissue.
A wooden axe fouund in an egyptian tomb is found to have an activity of 0.38Bq. The activity of an identical mass of wood cut from a living tree is 0.65Bq. Calculate the age of the wood used to make the axe
4400 years
What is an assumption made when carbon dating?
The radio of carbon-14 to carbon-12 in the atmosphere/living organisms has stayed constant over long periods of time
What are limitations to carbon dating?
The ratio of carbon-14 to carbon-12 atoms in the atmosphere/living organisms has not remained completely constant as:
* Increased emission of carbon dioxide due to the burning of fossil fuels may have reduced this ratio
* Natural events such as volcanic eruptions and solar flares from the sun
* Testing of nuclear bombs
The tiny amounts of carbon-14 present can also be a limitation as the activities are extremely small, comparable to the background rate
Why can’t you use carbon dating to determine the age of rocks?
Many rocks on the Earth/meteors were formed during the creation of the Solar System. The half-life of carbon-14 is only 5700 years and is therefore not long enough to use for the oldest rocks which can be dated at 3.7-3.8 billion years old.
How can you determine the ages of rocks?
As you cannot use carbon dating, the decay of Rubidium-87 is instead used to date ancient rocks. Nuclei of rubidium-87 emit beta-minus particles and decay into stable nuclei of Strontium-87. The half life of the isotope of Rubidium-87 is about 49 billion years, so it is sufficiently long enough to date ancuent rocks.
