NPCIL Scientific Assistant key terms

The time required for half the atoms in a radioactive sample to decay into other nuclides, or for its activity to fall to half its initial value. It is a fixed property of each radionuclide.

The spontaneous emission of particles or electromagnetic radiation from unstable atomic nuclei as they transform toward a more stable configuration. It is unaffected by ordinary chemical bonding or external temperature and pressure.

The SI unit of radioactivity, equal to one nuclear disintegration per second. Multiples such as the megabecquerel are common in plant monitoring, while the older curie equals 3.7 × 10^10 becquerel.

A mode of radioactive transformation in which a nucleus emits a helium-4 nucleus, losing two protons and two neutrons. Alpha particles travel only a few centimetres in air and are stopped by a sheet of paper or the outer skin layer.

A fast electron or positron emitted when a neutron converts into a proton, or the reverse, inside a nucleus. Beta radiation penetrates a few millimetres of aluminium and is a common hazard near unshielded sources.

High-energy electromagnetic radiation emitted by an excited nucleus as it drops to a lower energy state. It has no mass or charge, so it is attenuated by dense materials such as lead or concrete rather than stopped by thin shields.

The probability per unit time that a given radioactive nucleus will disintegrate. It is related to half-life by the expression lambda equals 0.693 divided by the half-life, and it determines the activity of a sample.

A quantity that expresses radiation dose in terms of its biological effect, obtained by multiplying absorbed dose by a radiation weighting factor. It is measured in sieverts and lets different radiation types be compared.

The amount of radiation energy deposited per unit mass of material, measured in gray, where one gray equals one joule per kilogram. It describes physical energy transfer rather than biological harm.

A device worn or placed by a worker to measure the radiation dose received over a period. Common types include thermoluminescent badges and electronic personal dosimeters that give immediate readings.

An acronym meaning as low as reasonably achievable, the principle that radiation exposure should be minimised by practical measures even when limits are already met. It drives planning of shielding, distance and working time.

Material placed between a radiation source and people or equipment to reduce the intensity reaching them. Lead, concrete and water are typical, and the required thickness depends on radiation type and energy.

X-ray radiation produced when fast charged particles, usually beta particles, are slowed by the electric field of a dense nucleus. It is why beta shields are made of low atomic number material such as plastic.

The splitting of a heavy nucleus such as uranium-235 into two lighter nuclei after absorbing a neutron, releasing energy and further neutrons. The released neutrons can sustain a chain reaction in a reactor core.

A self-sustaining series of nuclear fissions in which neutrons released by one fission trigger further fissions. It becomes critical when exactly one of the released neutrons on average causes another fission.

A material such as light water, heavy water or graphite that slows fast neutrons to thermal energies so they are more likely to cause fission in uranium-235. Slowing occurs through repeated elastic collisions.

A movable rod made of neutron-absorbing material such as boron or cadmium, inserted into a reactor core to reduce reactivity. Withdrawing or inserting rods adjusts the neutron population and thus the power level.

Water in which the hydrogen atoms are the isotope deuterium instead of ordinary hydrogen. It is an efficient neutron moderator and allows natural uranium to sustain a chain reaction, as in pressurised heavy water reactors.

Radiation with enough energy to remove electrons from atoms, producing ions. It includes alpha, beta, gamma, X-rays and neutrons, and its biological harm comes largely from damage to DNA.

The statement that the net force on a body equals the rate of change of its momentum, reducing to force equals mass times acceleration when mass is constant. It links cause of motion to acceleration.

The product of a body's mass and its velocity, a vector quantity conserved in a closed system with no external force. Impulse equals the change in momentum produced by a force acting over time.

The principle that the net work done on a body equals the change in its kinetic energy. It offers a shortcut for problems involving variable forces and distances rather than time.

The relationship stating that current through a conductor is directly proportional to the potential difference across it, provided temperature stays constant. The constant of proportionality is the resistance.

Two ways of connecting components. In series the same current flows through each element and resistances add; in parallel the same voltage appears across each branch and conductances add.

The ratio of real power to apparent power in an alternating current circuit, equal to the cosine of the phase angle between voltage and current. A low value means more current is drawn for the same useful power.

A device with two coils linked by a magnetic core that changes alternating voltage from one level to another. It works on mutual induction, so it does not function on steady direct current.

The generation of an electromotive force in a conductor when the magnetic flux through it changes. It underlies generators, transformers and many sensors, and its direction follows Lenz's law.

The idea that one mole contains Avogadro's number of particles, about 6.022 × 10^23, and that molar mass in grams equals the numerical value of atomic or molecular mass. It converts between mass and particle count.

The number of moles of solute dissolved per litre of solution, expressed in mol per litre. It changes slightly with temperature because solution volume expands, unlike molality based on solvent mass.

A logarithmic scale of hydrogen ion concentration, defined as the negative logarithm to base ten of that concentration in mol per litre. A pH below seven indicates acidity at room temperature.

A solution that resists large changes in pH when small amounts of acid or base are added, typically made from a weak acid with its conjugate base. It works best near the acid's pKa value.

The calculation of quantities of reactants and products in a balanced chemical equation using mole ratios. It allows prediction of limiting reagent, theoretical yield and gas volumes.

A formal charge assigned to an atom in a compound by counting shared electrons with the more electronegative atom. Tracking changes in these numbers identifies oxidation and reduction in a reaction.

The gradual deterioration of a metal through chemical or electrochemical reaction with its environment, such as rusting of iron in moist air. It is an oxidation process accelerated by electrolytes and oxygen.

The comparison of two quantities by division and the statement that two such comparisons are equal. It is used to scale recipes, mix concrete, share amounts and solve direct and inverse variation problems.

A fraction expressed with denominator one hundred, written with the per cent sign. Converting between fractions, decimals and percentages is a frequent step in profit, loss, interest and data interpretation questions.

The sum of a set of values divided by the number of values, also called the arithmetic mean. It is used with weighted averages and in problems where one value changes the group mean.

A form of logical reasoning with two or more premises leading to a conclusion, common in verbal reasoning tests. Validity depends on the form of the argument, not on whether the statements are factually true.

A small, fast memory placed between the processor and main memory that stores recently used data and instructions. It reduces average access time because most programs reuse the same data repeatedly.

Two classes of main memory. RAM is volatile read-write working memory that loses contents when power is removed, while ROM is non-volatile memory holding firmware that normally cannot be rewritten.

System software that manages hardware resources, files, memory and processes and provides services to application programs. Examples include Windows, Linux and macOS, and it enforces user access control.