RADIOACTIVITY B Sc 5 th sem DEFINATION Radioactivity

RADIOACTIVITY B. Sc. 5 th sem

DEFINATION Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear instability. Because the nucleus experiences the intense conflict between the two strongest forces in nature, it should not be surprising that there are many nuclear isotopes which are unstable and emit some kind of radiation. The most common types of radiation are called alpha, beta, and gamma radiation, but there are several other varieties of radioactive decay.

CAUSE What is Radioactivity Atoms become unstable due to large neutron to proton ratio. Such unstable nucleus emitted some radiations and convert in to some other stable nucleus and known as radioactive elements. These radiations are termed as radioactive rays. Generally these radiations consist some particles like alpha and beta particle in some time charge less gamma rays emitted.

DECAYS Radioactive decay rates are normally stated in terms of their halflives, and the half-life of a given nuclear species is related to its radiation risk. The different types of radioactivity lead to different decay paths which transmute the nuclei into other chemical elements. Examining the amounts of the decay products makes possible radioactive dating. Radiation from nuclear sources is distributed equally in all directions, obeying the inverse square law.

Radioactive decay (also known as nuclear decay, radioactivity or nuclear radiation) is the process by which an unstable atomic nucleus loses energy (in terms of mass in its rest frame) by emitting radiation, such as an alpha particle, beta particle with neutrino or only a neutrino in the case of electron capture, or a gamma ray or electron in the case of internal conversion. A material containing such unstable nuclei is considered radioactive. Certain highly excited short-lived nuclear states can decay through neutron emission, or more rarely, proton emission.

The Nature Of Radioactive Emissions The emissions of the most common forms of spontaneous radioactive decay are the alpha (α) particle, the beta (β) particle, the gamma (γ) ray, and the neutrino. The alpha particle is actually the nucleus of a helium-4 atom, with two positive charges 4/2 He. Such charged atoms are called ions. The neutral helium atom has two electrons outside its nucleus balancing these two charges. Beta particles may be negatively charged (beta minus, symbol e−), or positively charged (beta plus, symbol e+). The beta minus [β−] particle is actually an electron created in the nucleus during beta decay without any relationship to the orbital electron cloud of the atom. The beta plus particle, also called the positron, is the antiparticle of the electron; when brought together, two such particles will mutually annihilate each other. Gamma rays are electromagnetic radiations such as radio waves, light, and X-rays. Beta radioactivity also produces the neutrino and antineutrino, particles that have no charge and very little mass, symbolized by ν and ν, respectively.

Alpha Particles

Alpha decay is one type of radioactive decay, in which an atomic nucleus emits an alpha particle, and thereby transforms (or "decays") into an atom with a mass number decreased by 4 and atomic number decreased by 2 amount.

In alpha decay, an energetic helium ion (alpha particle) is ejected, leaving a daughter nucleus of atomic number two less than the parent and of atomic mass number four less than the parent. An example is the decay (symbolized by an arrow) of the abundant isotope of uranium, 238 U, to a thorium daughter plus an alpha particle:

Beta Particles Beta particles are energetic electrons that are emitted from the nucleus. They are born when a neutron decays to a proton. Since neutrons are neutral particles and protons are positive, conservation of charge requires a negatively charged electron to be emitted. Some isotopes decay by converting a proton to a neutron, thus emitting a positron (an anti-electron). These particles can penetrate matter more than can alpha particles, and it takes a small aluminum plate to stop most beta particles.

In nuclear physics, beta decay (βdecay) is a type of radioactive decay in which a beta ray (fast energetic electron or positron) and a neutrino are emitted from an atomic nucleus. . For example, a neutron, composed of two down quarks and an up quark, decays to a proton composed of a down quark and two up quarks.

Beta decay conserves a quantum number known as the lepton number, or the number of electrons and their associated neutrinos (other leptons are the muon and tau particles). These particles have lepton number +1, while their antiparticles have lepton number − 1. Since a proton or neutron has lepton number zero, β+ decay (a positron, or antielectron) must be accompanied with an electron neutrino, while β−decay (an electron) must be accompanied by an electron antineutrino

An example of electron emission (β− decay) is the decay of carbon-14 into nitrogen-14 with a half-life of about 5, 730 years: In this form of decay, the original element becomes a new chemical element in a process known as nuclear transmutation. This new element has an unchanged mass number A, but an atomic number Z that is increased by one. As in all nuclear decays, the decaying element (in this case 14 C 6 is known as the parent nuclide while the resulting element (in this case 14 N 7) is known as the daughter nuclide.

Gamma rays are photons that are emitted from the nucleus. Often an atom in an excited state will de-excite by emitting a gamma ray. Gamma rays are similar to light waves and x-rays, except they are usually much higher frequency and consequently, more energetic. This radiation has no charge, and can penetrate most matter easily, requiring lead bricks for shielding.

A gamma ray or gamma radiation, is a penetrating electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves and so imparts the highest photon energy.

DETECTING GAMMA RAYS Unlike optical light and x-rays, gamma rays cannot be captured and reflected by mirrors. Gamma-ray wavelengths are so short that they can pass through the space within the atoms of a detector. Gamma-ray detectors typically contain densely packed crystal blocks. As gamma rays pass through, they collide with electrons in the crystal. This process is called Compton scattering, wherein a gamma ray strikes an electron and loses energy, similar to what happens when a cue ball strikes an eight ball. These collisions create charged particles that can be detected by the sensor.