v The nuclear density is calculated as 1.816 x1017 kgm-3, which is an extremely very high value compared to the density of any other material available on earth. If one cubic metre box is packed with a material of 1.816 x 1017 kilogram, then it is the density of the nucleus and hence nuclear matter is in an extremely compressed state.
v The negative beta particle is emitted when a neutron changes into a proton, an electron and an antineutrino. The positive beta particle is emitted when a proton changes into a neutron, positron and neutrino. Thus, proton and neutron are mutually convertible. The proton and neutron are considered to be two different charge state of the same particle and in common they are called as nucleon.
v The experimental values of magnetic moments of a free neutron have some support to the meson theory of nuclear forces. A free neutron dissociates into a proton and negatively charged meson. This combination will have a negative magnetic moment. It follows that though uncharged, a neutron will have a negative magnetic moment.
v Nuclear forces are the strongest ever known forces. They are stronger than other known forces like electrostatic force and gravitational force. Nuclear forces are 1040 times stronger than gravitational force.
v Theoretically infinite time is required for the atoms of the radioactive element to lose all its radioactive property and all radioactive elements are the same in this respect.
v The age of earth is estimated from the relative abundance of the two isotopes of uranium 92U238 and 92U235 as 593 x 107 years.
v Larger doses of radiations produce irrecoverable effects within few weeks. When excessive doses are absorbed, the first noticeable disorder is a drop in white blood cell count. This is followed by radiation sickness pattern of diarrhoea, vomiting and fever. The more serious is the damage done to the bone marrow and to other cells which leads to the production of cancerous cells and become malignant tumours.
v Classical physics fails to explain the emission of alpha particles. Quantum mechanics provides a successful explanation for the problem of alpha emission. According to the quantum wave mechanics, the alpha particles are in constant motion inside the nucleus and bounces back and forth from the walls of the potential barrier. In short, the alpha particles behave like a wave form inside the nucleus. In each collision with the walls of the potential barrier, there is a probability that the particle leak through the barrier. This effect is known as tunneling effect.
v A king drew a line on the floor and asked his wise men to shorten the line without modifying the original line. Everyone was puzzled, how it is possible to shorten a line without modifying the original line. At that time, one of his ministers came forward and drew another longer line near the first line. The minister asked the king, look at the two lines and say which of the lines is shorter? The king at once replied, the line drawn first is shorter than the line drawn latter. This shows that the length is relative.
v Suppose that we have an observer who is at rest with respect to several fixed charges. This observer will clearly see only an electric field due to these charges. If we have another observer who is moving with respect to the first observer, then the second observer will see a group of moving charges and will consequently see a magnetic field in addition to an electric field.
v Quantum physics is the study of the behaviour of matter and energy at the molecular, atomic, nuclear, and even smaller microscopic levels. We need quantum mechanics to explain the behaviour of electrons in atoms or solids or the behaviour of atoms in molecules. In the early 20th century, it was discovered that the laws that govern macroscopic objects do not function the same in such small realms.
v Classical mechanics is completely definite theory in the sense that the computational procedures do not introduce any statistical uncertainties into the system themselves. Quantum mechanics on the other hand is fundamentally a probabilistic theory.
v Exclusively for biomedical purposes, many number of electron accelerators are also in operation. For instance, a 42 MeV Betatron at Christian Medical College Hospital, Vellore, a 20 MeV Linear Accelerator at All India Institute of Medical Sciences, New Delhi, a 12 MeV Linear Accelerator at the Department of Radiology, Srinagar, a 10 MeV Linear Accelerator at Tata hospital, Bombay and a 8 MeV Microtron in the Department of Physics, University of Pune.
v On August 6, 1945, a uranium fission bomb was detonated over the Japanese city of Hiroshima. The bomb, called little boy was a gun-type device which used an explosive charge to force two sub-critical masses of 92U235 together. It was 28 inches in diameter and 120 inches long, a relatively small package to deliver an explosive force of some 20,000 tons of TNT. Casualties included both direct blast victims plus those who died from radiation-induced cancer in subsequent years. The bomb was triggered to explode at a height of 550 meters (1800 ft), a height calculated to cause the widest area of damage. In the detonation of the uranium fission bomb over Hiroshima, about 130,000 people were reported killed, injured, or missing. Another 177,000 were made homeless.
v On August 9, 1945 a plutonium fission bomb was detonated over the Japanese city of Nagasaki, three days after a uranium fission bomb was dropped on Hiroshima. The bomb, called fat man, was 128 inches long and had a diameter of 60.5 inches. It used implosion to compress the sub-critical assembly of plutonium. This kind of device had been tested less than a month before the drop, and was the subject of several other weapons tests after World War II. The explosive yield was about 20,000 tons of TNT, generated in about a microsecond. The bomb was triggered to explode at a height of 550 meters (1800 ft), a height calculated to cause the widest area of damage.
v Nuclear Power Corporation of India Ltd. (NPCIL) is the public sector company which owns, constructs and operates nuclear power plants in India. NPCIL plans to put up a total installed nuclear power capacity of 20,000 MW by the year 2020. India’s nuclear power programme has 46 units of reactors in operation. There are 43 units of pressurised heavy water reactors (PHWR) and 3 units of boiling water reactors (BWR) with a total power generation capacity of 4560 MWe.
v Unlike most other particles, neutrinos are able to escape from dense regions such as the core of the sun or the Milky Way and they can travel long distances from far-away galaxies without being absorbed, carrying information about these areas. In this sense, neutrinos are cosmic messengers, and neutrino astronomy is becoming increasingly important. So far, only two sources of extraterrestrial neutrinos have been observed: the sun and supernovae. On earth, both natural and artificial neutrino sources exist: radioactive materials from inside earth can undergo beta decay, producing geo-neutrinos. In addition, nuclear fission reactors produce neutrinos, and particle accelerators are being used as neutrino sources for research. Finally, when cosmic rays hit Earth’s atmosphere, atmospheric neutrinos are emitted as decay products of pions and muons. Neutrinos are very useful for studying astronomical and cosmological phenomena, and neutrino detectors are being built worldwide, deep underground to filter out the ‘noise’ of other particles.
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