Chemistry Project Reports on “Radio-Activity”
Definition of radioactivity: the property possessed by some elements (such as uranium) or isotopes (such as carbon 14) of spontaneously emitting energetic particles (such as electrons or alpha particles) by the disintegration of their atomic nuclei; also: the rays emitted.
Definition of radioactivity for Students
1: the giving off of rays of energy or particles by the breaking apart of atoms of individual elements (as uranium)
2: the rays or particles that are carried off when molecules break apart
What is radioactivity?
Radioactive decay occurs in unstable atomic nuclei – that is, ones that don’t have enough binding energy to hold the core together due to an excess of either protons or neutrons.
It comes in three main types – named alpha, beta, and gamma for the first three letters of the Greek alphabet.
An alpha particle is identical to a helium nucleus, being made up of two protons and two neutrons bound together.
It initially escapes from the nucleus of its parent atom, invariably one of the most substantial elements, by quantum mechanical processes and is repelled further from it by electromagnetism, as both the alpha particle and the nucleus are positively charged.
The process changes the original atom from which the alpha particle is emitted into a different element.
Its mass number decreases by four and its atomic number by two. For example, uranium-238 will decay to thorium-234.
Sometimes one of these daughter nuclides will also be radioactive, usually decaying further by one of the other processes described below.
Beta decay itself comes in two kinds: β+ and β-.
β- emission occurs by the transformation of one of the nucleus’s neutrons into a proton, an electron and an anti neutrino. Byproducts of fission from nuclear reactors often undergo β- decay as they are likely to have an excess of neutrons.
β+ decays is a similar process but involve a proton changing into a neutron, a positron and a neutrino.
After a nucleus undergoes alpha or beta decay, it is often left in an excited state with excess energy.
Just as an electron can move to a lower energy state by emitting a photon somewhere in the ultraviolet to infrared range, an atomic nucleus loses energy by emitting a gamma ray.
Gamma radiation is the most penetrating of the three and will travel through several centimeters of lead.
Beta particles will be absorbed by a few millimeters of aluminum, while alpha particles will be suspended in their tracks by a few centimeters of air, or a sheet of paper – although this type of radiation does the most damage to materials it hits.
NATURAL RADIOACTIVITY :
The phenomenon of spontaneous emission of certain kinds of radiation by some elements is called radioactivity or natural radioactivity.
ARTIFICIAL OR INDUCED RADIOACTIVITY :
The phenomenon in which the synthetic disintegration of a stable nucleus leads to the formation of a radioactive isotope is called artificial radioactivity.
NATURE AND CHARACTERISTICS OF RADIOACTIVE SUBSTANCES
It complies that on applying the field, the rays emitted from the radioactive substances are separated into three types called ALPHA, BETA, GAMMA rays.
The ALPHA rays are turned in a direction which shows that they carry positive charges the BETA rays are turned in the opposite direction, and the GAMMA rays are not turned at all explaining that they carry no charge.
PROPERTIES OF ALPHA RAYS
The direction of deflection of the ALPHA rays in the electric and magnetic field showing that they carry positive charge. It is found that each ALPHA particles carries two units of positive charges and has mass nearly four times that of hydrogen atom.
The velocity of ALPHA rays is found to be nearly1/10th to 1/20th of that of light, dependent on the nature of source.
ALPHA rays ionize the gas through which they pass.
ALPHA rays have low penetrating power. They can penetrate through the air only to a distance of about 7 cm.
ALPHA rays affect photographic plate and produce luminescence when they strike zinc sulphide screen.
PROPERTIES OF BETA RAYS
The direction of deflection of BETA rays in the electric and magnetic field shows that they carry a negative charge. These particles possess the same charge and mass as that of the electron.
The velocity of BETA rays depends upon the nature of the source. The speed of particles varies from 3% to 99% of that of light.
The ionizing power of BETA particles is about 1/100th of that of ALPHA particles.
Their penetrating power is about 100 times greater than that of ALPHA rays.
Like ALPHA rays, BETA rays affect a photographic plate, and the effect is much higher. However, there is no significant effect on a zinc sulfide screen become of their lower kinetic energy.
PROPERTIES OF GAMMA RAYS
They are not reflected in the electric and magnetic fields showing these by that they do not carry any charge.
They travel with some velocity as that of light.
As they do not have any mass, their ionizing power is inferior.
Their penetrating power is about 100 times more than that of BETA rays. Thus they can penetrate through lead sheets as thick as 150mm.
GAMMA rays have minimal effect on the photographic plate or zinc sulphide screen.
Half-lives and probability
Radioactive decay is determined by quantum mechanics – which is inherently probabilistic.
So it’s impossible to work out when any particular atom will decay, but we can make predictions based on the statistical behavior of large numbers of particles.
The half-life of a radioactive isotope is the time after which, on average, half of the original material will have decayed. After two half-lives, half of that will have decayed again and a quarter of the original material will remain, and so on.
Uranium and plutonium are only weakly radioactive but have very long half-lives – in the case of uranium-238, around four billion years, roughly the same as the current age of the Earth, or the estimated remaining lifetime of the Sun. So half of the uranium-238 around now will still be here when the Sun dies.
Iodine-131 has a half-life of eight days, so, once fission has stopped, less than 1% of iodine-131 produced in a nuclear reactor will remain after about eight weeks. Other radioisotopes of iodine are even shorter-lived.
Caesium-137, however, sticks around for longer. It has a half-life of about 30 years, and, because of this and because it decays via the more hazardous beta process, is thought to be the most significant health risk if leaked into the environment.
Although some radioactive materials are produced artificially, may occur naturally and result in there being a certain amount of radiation in our environment all the time – the “background radiation”.
In the background
There is a natural level of radiation all around us, which comes from several sources.
Some gamma radiation comes from space as cosmic rays. Another pollution comes from sources in the atmosphere, such as radon gas and some of its decay products.
There are also natural radioactive materials in the ground – and as well as the distinct elements such as uranium there are also dangerous isotopes of common substances such as potassium and carbon.
To understand how much background radiation is around, it helps to distinguish between effects on ordinary matter and the human body.
The amount of radiation absorbed by non-biological matter is measured in grays, a unit equivalent to a joule of energy per kilogram of mass. For biological tissue, a dose equivalent is measured in sieverts (Sv) depending on the type of radiation involved and how much damage that radiation does to the particular cells affected.
The dose equivalent in sieverts is the dose in grays multiplied by some “quality factor” for the type of tissue irradiated and for the kind of radiation – for electrons or gamma-rays, 1; for alpha particles such as those given off by the radioactive decay of uranium, 20.
The average amount of radiation received from background sources in the UK is around 2–2.5 mSv per year. Because of the preponderance of granite, which contains higher than average levels of uranium, in areas such as Cornwall or Aberdeen shire it can be twice this level – not high enough to cause any concern, but high enough that nuclear facilities can’t be built there as the background level already exceeds the maximum allowed radiation limit. In some parts of the world, such as northern Iran, the background radiation is as high as 50 mSv per year.
There are a variety of other natural and routine artificial causes of low doses of radiation.
A dental x-ray will give you a dose of under one mSv; a full-body CT scan, ten mSv.
As fewer cosmic rays are stopped by the atmosphere the higher you go, the crew of a passenger jet flying between the US and Japan once a week for a year would receive an additional a dose of around nine mSv.
Under normal conditions, the dose limit for workers in the nuclear industry is 50 mSv per year.
The effects on human health
There are two main health effects caused by radiation, which act over the short- and long-term and also at shorter and greater distances.
Radiation causes health problems by killing cells in the body, and the amount and type of damage done depend on the dose of radiation received and the time over which the disease is spread out.
The dose limits for emergency workers in the event of a nuclear accident are 100 mSv if protecting property or 250 mSv in a life-saving operation.
In conventional chemical combinations, only the electron present in the outermost orbital’s are involved, i.e., they are transferred from one atom to the other molecule remain unaffected. However, there is a particular event in which the nucleus of an atom is involved. “The branch of chemistry dealing with the phenomena involving the nucleus of the atoms is known as NUCLEAR CHEMISTRY”. In fact, the only phenomena involving the nucleus of an atom are RADIOACTIVITY both natural and artificial.