The phenomenon of radioactivity was discovered by the French scientist Henri Becquerel, in 1896. He noted that crystals of potassium uranyl sulphate, placed over a wrapped photographic plate even in total darkness, could produce background marks on the plate after its development. Marie and Plerre Curie in 1898 isolated the elements radium and polonium. Other such elements are uranium and thorium. Such elements are known as naturally occurring radioactive elements. The man-made elements, which are radioactive and exhibit artificial radioactivity. These are technetium, neptunium, curium and californium. The phenomenon of radioactivity is due to decay of unstable nuclei Naturally occurring radioactive elements undergo decay by emitting - alpha, beta, and gamma rays, thus producing stable, non-radioactive daughter element. When a radioactive nucleus emits an alpha particle, its atomic number is reduced by 2 and its mass number is reduced by four. When uranium 238 nucleus is bombarded with alpha particle, it produces throrium-234 nucleus. The activity of a radioactive element is measured by the rate at which it changes into its daughter element.

Half-Life

Half-life period is the time required for disintegration of one half of atoms of radioactive species initially present. The decay of some substances, such as uranium-238 and thorium-232, appears to continue indefinitely without detectable diminution of the decay rate per unit mass of the isotope (specific-decay rate). Other radioactive substances show a marked decrease in specific-decay rate with time. Among these is the isotope thorium-234 (originally called uranium X), which, after isolation from uranium, decays to half its original radioactive intensity within 25 days. Each individual radioactive substance has a characteristic decay period or half-life; because their half-lives are so long that decay is not appreciable within the observation period, the diminution of the specific-decay rate of some isotopes is not observable under present methods. Thorium-232, for example, has a half-life of 14 billion years.

Radioactive Decay Series

When uranium-238 decays by alpha emission, thorium-234 is formed; thorium-234 is a beta emitter and decays to form protactinium-234. Protactinium-234 in turn is a beta emitter, forming a new isotope of uranium, uranium-234. Uranium-234 decays by alpha emission to form thorium-230, which decays in turn by alpha emission to yield the predominant isotope, radium-226. This radioactive decay series, called the uranium-radium series, continues similarly through five more alpha emissions and four more beta emissions until the end product, a nonradioactive (stable) isotope of lead (element 82) of mass 206 is reached. Every element in the periodic table between uranium and lead is represented in this series, and each isotope is distinguishable by its characteristic half-life.

An interesting application of knowledge of radioactive elements is made in determining the age of the earth. One method of determining geologic time is based on the fact that in many uranium and thorium ores, all of which have been decaying since their formation, the alpha particles have been trapped (as helium atoms) in the interior of the rock. By accurately determining the relative amounts of helium, uranium, and thorium in the rock, the length of time during which the decay processes have been going on (the age of the rock) can be calculated. Another method is based on the determination of the ratio of uranium-238 to lead-206 or of thorium-232 to lead-208 in the rocks (that is, the ratios of concentration of the initial and final members of the decay series). These and other methods give values for the age of the earth of between 3 billion and 5 billion years.

The rate of radioactive decay can be measured by counting the number of particles emitted per unit using several instruments-scintillation counter, Wilson cloud chamber and Geiger Muller counter.

Transmutation is the conversion of one element into another, and is a dream of alchemists. A large number of transmutations have been carried out using the alpha particles, protons, deutrons and some heavier nuclei. Transmutant elements produced are carbon from beryllium, magnesium from sodium, carbon from nitrogen.

The first man-made radioisotopes are phosphorous silicon (14Si), nitrogen (7N). Several types of particle accelerators have been constructed to impart high energies to the sub atomic particles.

Synthetic elements are produced by the particle bombardment to the parent element to synthesize artificial elements such as technetium (from molybdenum), neptunium (from uranium), curium (from plutonium), and californium (from uranium).

Nuclear fission is a reaction, in which a heavy nucleus is broken up into two fragments of lighter nuclei and several neutrons. Atomic Bomb is based on the nuclear chain reaction, in nuclear fission.

In an atomic bomb, two or more pieces of fissionable material (uranium-235 or plutonium-239), each less than the critical mass, are brought together rapidly (*by means of a conventional explosive) so that, making one piece. Reactions starts with releasing large amount of energy.

In nuclear reactors, the nuclear fission reaction is a controlled one, and the energy released is harnessed.

A nuclear reactor consists of

• (a)   Fissionable material, (uranium enriched in U-235 (2-3%)
• (b)   Moderator (graphite or heavy water) to slow down the neutrons, thereby increasing the efficiency of their capture, and to bring about a fission reaction.
• (c)   Control rods made up of boron steel or cadmium is used to capture the neutrons, and to restrict the reaction and thus it remains a controlled one.

The large amount of energy released in the form of heat is converted into electrical energy.

Breeder reactors, is a facility in which fissionable isotope, which is enriched and used as a fuel in a nuclear reactor.

Ex:

• Uranium-238 is bombarded with fast neutrons to produce uranium-239, and later plutonium-239.
• Naturally more abundant thorium-232 breeds a fissionable isotope uranium-233

Nuclear fusion- two or more light nuclei combine to form a heavy nucleus, of helium. On the basis of fusion reaction, hydrogen or thermonuclear bomb is designed. The energy of the sun or other stars is believed to arise from the fusion of hydrogen nuclei to form helium.

A thermonuclear fusion reactor to generate a vast source of energy is still a technological challenge. The reaction is considered possible by the fusion of Deutrium nuclei into helium. Deutrium is available in plenty.

Hazards of Nuclear radiations

• The discharge of volatile radioactive isotopes into the atmosphere and the leakage of radioactive material into the water stream are hazardous.
• The radiations from radionucleides and neutrons have harmful biological effects. They may cause genetic mutations leading to skin cancer and leukemia.
• The radionucleides, such as iodine-131, ingested with food accidentally organs like thyroid glands, and get concentrated in the gland, & may cause severe damage to the thyroid gland.
• The extreme care is taken while working with radioisotopes.

Rocket Propellants

The rocket engines of the space vehicles are powered by the chemical propellants.

A propellant is a combination of an oxidizer and a fuel, which when ignited undergoes combustion to release the hot gases, through the nozzle of the rocket motor.

The passage of the gases through the nozzle provides the required thrust for the rocket to move forward.

The propellants are of three types:

Solid propellant is a blend of polymeric binder such as polyurethane or polybutadiene as fuel and ammonium perchlorate as oxidizer; with some other additives. Another solid propellant is a mixture of nitroglycerine and nitrogellulose. Nitrocellulose gel and nitroglycerine sets in as a solid mass. Solid propellants, once ignited, would burn with a pre-determined rate and cannot be regulated to stop or start again.

Liquid propellants have an oxidizer such as liquid oxygen, nitrogen tetroxide or nitric acid and a fuel such as kerosene, alcohol, hydrazines or liquid hydrogen. The liquid propellants give higher thrusts than solid propellants and it can be controlled by switching on and off the flow of propellant.

The hybrid rocket propellant consists of a solid fuel and a liquid oxidizers (e.g. liquid nitrogen tetroxide and acrylic rubber).

Some important rockets/booster propellants are given below-

• The Saturn booster rocket uses a combination of kerosene and liquid oxygen as the propellant for initial stage and liquid oxygen and liquid hydrogen for the upper stages.
• Titan ballastic missiles use hydrazine as fuel and nitrogen tetroxide as oxidizers.
• The US Space Shuttle uses liquid hydrogen-liquid oxygen with solid boosters at lower stages.
• The Proton rocket of Russia used liquid propellants made up of kerosene and liquid oxygen.
• Our rockets like SLV-3 and ASLV use composite solid propellants.
• The PSLV rocket uses a solid propellant in the first and the third stages, while the second and fourth stage uses liquid propellants.
• The liquid propellants are nitrogen tetroxide & UDMAH and nitrogen tetroxide and MMH for second and fourth stages of rockets respectively.

Superconductivity, phenomenon displayed by certain conductors that demonstrate no resistance to the flow of an electric current. Superconductors also exhibit strong diamagnetism; that is, they are repelled by magnetic fields. Superconductivity is manifested only below a certain critical temperature Tc and a critical magnetic field Hc, which vary with the material used. Before 1986, the highest Tcwas 23.2 K (-249.8° C/-417.6° F) in niobium-germanium compounds. Temperatures this low were achieved by use of liquid helium, an expensive, inefficient coolant. Ultra low-temperature operation places a severe constraint on the overall efficiency of a super-conducting machine. Thus, large-scale operation of such machines was not considered practical. But in 1986 discoveries at several universities and research centers began to radically alter this situation. Ceramic metal-oxide compounds containing rare-earth elements were found to be super conductive at temperatures high enough to permit using liquid nitrogen as a coolant. Because liquid nitrogen, at 77K (-196° C/-321° F), cools 20 times more effectively than liquid helium and is 10 times less expensive, a host of potential applications suddenly began to hold the promise of economic feasibility. In 1987 the composition of one of these super conducting compounds, with Tcof 94K (-179° C/-290° F), was revealed to be (Y0.6Ba0.4)2CuO4. It has since been shown that rare-earth elements are not an essential constituent, for in 1988 a thallium-barium-calcium copper oxide was discovered with a Tc of 125K (-148° C/-234° F). Superconductivity was first discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes, who observed no electrical resistance in mercury below 4.2 K (-268.8° C/-451.8° F).

Applications

Because of their lack of resistance, superconductors have been used to make electromagnets that generate large magnetic fields with no energy loss.

Superconducting magnets have been used in studies of materials and in the construction of powerful particle accelerators. Using the quantum effects of superconductivity, devices have been developed that measure electric current, voltage, and magnetic field with unprecedented sensitivity.

The discovery of better superconducting compounds is a significant step toward a far wider spectrum of applications, including faster computers with larger storage capacities, nuclear fusion reactors in which ionized gas is confined by magnetic fields, magnetic suspension of high-speed (“Maglev”) trains, and perhaps most important of all, more efficient generation and transmission of electric power.

Superconductivity at 77 Kelvin in liquid nitrogen is of great potential in technological application.

Possible applications are in electronics, building magnets, levitation transportation and power transmission.

Now research is more concentrated to do superconductivity at the room temperature.