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August 03, 2010


Science Glossary Pages

Nuclear Science: selected terms

See Natural Abundance

Alpha Decay
Nuclear decay by emission of an alpha particle (4He nucleus ).

Atomic Mass (Atomic Weight)
The mass of a neutral atom of a nuclide. The atomic weight of an atom is the weight of the atom based on a scale where 12C = 12. The atomic weight of an element is the weighted average of each isotope.

Atomic Number
The number of protons in the nucleus

Beta Decay(click here for animation)
Nuclear decay by emission of an electron or a positron. Positron decay is always accompanied by electron capture decay.

Beta-Delayed Particle Emission(click here for animation)
When a large amount of decay energy is available, the nucleus may emit:

X-rays produced when fast electrons pass through matter. The bremsstrahlung (German for "slowing-down radiation") energy varies from 0 to the energy of the electron.

Carbon-Nitrogen-Oxygen Cycle (see also hot CNO cycle)
In stars more massive than the sun (>1.1 Solar masses), this cycle is the primary process which converts hydrogen into helium. 12C serves as a catalyst, an ingredient which is necessary for the reaction but is not consumed.
Listing of the steps

Compton Scattering
Collision process between a gamma ray and a bound atomic electron where only part of the gamma-ray energy is transferred to the electron. The probability for Compton scattering is approximately proportional to Z, and for energies greater than 500 keV approximately proportional to 1/Egamma

Conversion Electron
An alternate process to x-ray emission during the de-excitation of an excited atom.

Decay Branching %
The nuclide decay rate by a particular decay mode. Some nuclides decay by only one mode (100%), and others by more than one mode. For example, 187Pb decay by beta decay (98%) and alpha decay (2%).

Decay Mode
Disappearance of a radioactive substance due to nuclear emission of an alpha or beta particle, capture of an atomic electron, neutrinos, spontaneous fission, and the emission of bremsstrahlung, x-rays, and conversion electrons. In rare instances proton, neutron, or light element (for example 14C) emission can occur. When a large amount of decay energy is available, beta-delayed emission of neutrons, protons, and other particles may occur.

Decay Scheme
A drawing depicting the decay of a parent nucleus to a daughter nucleus. The betas or alphas are shown as arrows from the parent level to daughter level(s). Gamma rays de-exciting daughter levels are shown on the decay scheme.

Electromagnetic radiation
Radiation consisting of electric and magnetic waves that travel at the speed of light. Examples: light, radio waves, gamma rays, x-rays.

An elementary particle with a unit electrical charge and a mass 1/1837 of the proton. Electrons surround the atom's positively charged nucleus and determine the atom's chemical properties. In our diagrams, an electron is represented by this: 

Electron Capture Decay
Nuclear decay by capture of an atomic electron. If the decay energy is greater than 1022 keV, positron emission can also occur in competition with electron capture.

Energy Scale
The energy scale used by most nuclear scientists is electron volts (eV), thousands of electron volts (keV), and millions of electron volts (MeV). An electron volt is the energy acquired when an electron falls through a potential difference of 1 volt. 1 eV=1.602*1012ergs. Masses are also given by their "mass-equivalent" energy (E=mc2). The mass of the proton is 938.27231 MeV.

Where: e is energy, m is mass, and c is the speed of light. Einstein's famous equation describes how energy and mass are related. In our animated decays, mass is lost. That mass is converted into energy in the form of electromagnetic waves. Because the speed of light is so great, a little matter can transform into large amount of energy.

The Evaluated Nuclear Structure Data File is evaluated by an international collaboration of nuclear scientists. ENSDF is a database of nuclear structure and decay data.

Gamma Rays
A highly penetrating type of nuclear radiation, similar to x-rays and light, except that it comes from within the nucleus of an atom, and, in general, has a shorter wavelength. Gamma rays emission is a decay mode by which excited state of a nucleus de-excite to lower (more stable) state in the same nucleus. In our diagrams, a gamma ray is represented by this: 

Geiger Counter
A radiation detector consisting of two electrodes with a low-pressure gas in between. A voltage is maintains such that if radiation passing through the counter ionizes the gas, an avalanche of electrons will occur. Geiger counters can count radiation but cannot distinguish either the energy or kind of radiation.

Ground State
A lowest energy state of the nucleus.

Used to measure the rate of radioactive decay of disintegration. The time lapse during which a radioactive mass loses one half of its radioactivity.

Helium Burning (triple alpha process)
When temperature in the core of a star reaches 100 million degrees, three colliding helium nuclei fuse to form a carbon nucleus. This process occurs when the star is a red giant.

Hot Carbon-Nitrogen-Oxygen Cycle (see also C-N-O cycle)
Complete Listing of the Steps

Hydrogen Burning
Hydrogen burning is the fusion of four hydrogen nuclei (protons) into a single helium nucleus (two protons and neutrons.) The process is a series of reactions. The type of reactions depend on the mass of a star and its core temperature and density. In our Sun, the process is a proton-proton chain. In more massive stars, the C-N-O cycle (Carbon-Nitrogen-Oxygen) serves to fuse hydrogen into helium.

Intensity Branching(%)
The intensity of a radiation emitted during radioactive decay.

Nuclides of the same atomic mass but different atomic number.

A long-lived excited state of the nucleus. Arbitrarily defined in as the Table of Isotopes as having a half-life greater than 1 ms.

Two or more nuclides having the same atomic number, thus constituting the same element, but differing in the mass number. Isotopes of a given element have the same number of nuclear protons but differing numbers of neutrons. Naturally occurring chemical elements are usually mixtures of isotopes so that observed (non-integer) atomic weights are average values for the mixture.

Mass Number
The sum of the number of neutrons and protons in a nucleus.

Natural Abundance
Percentage of an element occurring on earth in a particular stable isotopic form.

An electrically neutral particle with negligible mass. It is produced in many nuclear reactions such as in beta decay. In our diagrams, it is represented by this: 

One of the basic particles which make up an atom. A neutron and a proton have about the same weight, but the neutron has no electrical charge. In our diagrams, a neutron is represented by this: 

Neutron Decay
Nuclear decay by emission of a neutron.

Neutron-Induced Fission
Bombardment with a neutron resulting in splitting the nucleus into two parts (fission fragments), neutrons, and gamma rays.
Neutron-induced fission movie by Encylopedia Brittanica 

Neutron Separation Energy
The energy required to remove a neutron from a nucleus.

Nuclear Reaction
Reaction between an energetic incident projectile (neutron, proton, or nucleus ) from a reactor or particle accelerator and a target nucleus producing product nuclides, gamma rays, particles, and other radiations.

A proton or neutron.

The core of the atom, where most of its mass and all of its positive charge is concentrated. Except for hydrogen, it consists of proton and neutrons.

The Nuclear Science Reference file is a compilation of about 160,000 references relevant to nuclear structure and decay.

Pair Production
A collision process for gamma rays with energies greater than 1022-keV (two electron masses) where an electron /positron pair is produced. A heavy nucleus must be present for pair production. For high-energy gamma rays the pair production process is proportional to Z2 and ln(gamma).

A nucleus or particle has odd (-) or even (+) parity according to whether or not its wave function changes sign when all of the space coordinates are changed.

Photoelectric effect
Collision process between an x-ray or gamma rays and a bound atomic electron where the photon disappears, the bound electron is ejected, and the incident energy is shared between the ejected electron and the remaining atom. The photon energy must be greater than the atomic binding energy. The probability for the photoelectric effect is approximately proportional to Z5 of the absorber and falls of by about E(gamma)3.5.

Positron Annihilation
Positron decay in matter by annihilation with an electron. Usually and "atom" of positronium (e+e-) forms which annihilates to produce two 511-keV photons. Occasionally, the positron will annihilate in flight to produce on or more photons sharing the total rest mass and kinetic energy of the positron and electron.

One of the basic particles which makes up an atom. The proton is found in the nucleus and has a positive electrical charge equivalent to the negative charge of an electron and a mass similar to that of a neutron. A proton is a hydrogen nucleus. In our diagrams, a proton is represented by this: 

Proton-Proton Chain
In the Sun and other less massive stars, this chain is the primary source of heat and radiation. The proton-proton chain converts hydrogen into helium releasing energy in the form of particles and gamma-rays. Hydrogen is converted into helium in a chain of reactions. The first reaction takes an average of 1 billion years to occur while the others are much shorter. One step is only 1 second long. In the Sun, there are so many hydrogen nuclei that the 1 billion year waiting period does not stop it from producing tremendous radiation.
List of the steps 

Proton Separation Energy
The energy required to remove a proton from a nucleus.

Proton Decay
Nuclear decay by emission of a proton. 

The energy available for decay. This energy is released by the nucleus mainly as gammas, betas, neutrinos, and/or alpha particles.

Scintillation Counter
A scintillation counter consists of a material that emits light when radiation passes through it. Various liquid, plastic, and crystalline materials have scintillation properties. Scintillation light is measured with photomultiplier tubes. In general the amount of scintillator light detected is proportional to the energy of the radiation.

Semiconductor Detector
Radiation striking very pure Ge and Si semiconductor detectors can excite a large number of electrons into the conduction band leading to a measurable current. This current is proportional to the energy of the radiation. Semiconductor detectors can be used to accurately measure the energy and intensity of radiation.

Used to describe the angular momentum of the nucleus.

Spontaneous Fission
Nuclear decay by splitting the nucleus into two parts (fission fragments), neutrons, and gamma rays. 

Triple Alpha Process (see helium burning)


A type of radiation of higher frequency (or energy) that visible light but lower that gamma rays. Usually produced by fast electrons going through matter or by the de-excitation of excited atoms. In our diagrams, a x-ray is represented by this: 

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Origin of Diamonds

Belgian scientists have found a way of determining the origin of individual diamonds, according to the Diamond High Council of Antwerp. The researchers found they could gain a unique chemical image of each diamond, by drilling a tiny hole in it with a laser beam. This allowed them to identify the source from which it came because each diamond has a chemical composition specific to an individual mine. But the diamond body said the scientists would first have to build up a global picture of all existing mines - a process that could take years.