Gamma ray spectrometer
Overview
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
—see figure) of the intensity of gamma radiation versus the energy of each photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
.
Gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s are electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
at the short-wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
, or high-frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, end of the electromagnetic (EM) spectrum, which ranges from gamma rays (short) to radio (long).
The study and analysis of gamma-ray spectra for scientific and technical use is called gamma spectroscopy
Gamma spectroscopy
Gamma-ray spectroscopy is the quantitative study of the energy spectra of gamma-ray sources, both nuclear laboratory, geochemical, and astrophysical. Gamma rays are the highest-energy form of electromagnetic radiation, being physically exactly like all other forms except for higher photon energy...
, and gamma-ray spectrometers are the instruments which observe and collect such data.
Because the energy of each photon of EM radiation is proportional to its frequency, gamma rays have sufficient energy that they are typically observed by counting individual photons.
Atomic nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
have an energy-level structure somewhat analogous the energy levels of atoms, so that they may emit (or absorb) photons of particular energies, much as atoms do, but at energies that are thousands to millions of times higher than those typically studied in optical spectroscopy.
(Note that the short-wavelength high-energy end, of the atomic spectroscopy energy range (few eV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
to few hundred keV
Kev
Kev can refer to:*Kev Hawkins, a fictional character.*Kevin, a given name occasionally shortened to "Kev".*Kiloelectronvolt, a unit of energy who symbol is "KeV".* Krefelder Eislauf-VereinKEV can refer to:...
), generally termed X rays, overlaps somewhat with the low end of the nuclear gamma-ray range (~10 MeV
MEV
MeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...
to ~10 keV) so that the terminology used to distinguish X rays from gamma rays can be arbitrary or ambiguous in the overlap region.)
As with atoms, the particular energy levels of nuclei are characteristic of each species, so that the photon energies of the gamma rays emitted, which correspond to the energy differences of the nuclei, can be used to identify particular elements and isotopes.
Distinguishing between gamma-rays of slightly different energy is an important consideration in the analysis of complex spectra, and the ability of a GRS to do so is characterized by the instrument's spectral resolution
Spectral resolution
The spectral resolution of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum...
, or the accuracy with which the energy of each photon is measured.
Semi-conductor detectors, based on cooled germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
or silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
detecting elements, have been invaluable for such applications.
Because the energy level spectrum of nuclei typically dies out above about 10 MeV, gamma-ray instruments looking to still higher energies generally observe only continuum spectra, so that the moderate spectral resolution of scintillation
Scintillation
Scintillation can refer to:*Scintillation , atmospheric effects which influence astronomical observations*Interplanetary scintillation, fluctuations of radio waves caused by the solar wind...
(often sodium iodide (NaI) or caesium iodide, (CsI) spectrometers), often suffices for such applications.
A number of investigations have been performed to observe the gamma-ray spectra of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and other astronomical sources, both galactic
Galactic
Galactic is a funk and jazz jam band from New Orleans, Louisiana, United States.-Origins and background:Originally formed in 1994 as an octet and including singer Chris Lane and guitarist Rob Gowen, the group was soon pared down to a sextet of: guitarist Jeff Raines, bassist Robert Mercurio,...
and extra-galactic.
The Hard X-ray/Low-Energy Gamma-ray experiment (A-4) on HEAO 1, the Burst and Transient Spectrometry Experiment (BATSE) and the OSSI (Oriented Scintillation Spectrometer Experiment) on CGRO,
the C1 germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
(Ge) gamma-ray instrument on HEAO 3,
and
the Ge gamma-ray spectrometer (SPI) on the ESA INTEGRAL
INTEGRAL
The European Space Agency's INTErnational Gamma-Ray Astrophysics Laboratory is an operational Earth satellite, launched in 2002 for detecting some of the most energetic radiation that comes from space. It is the most sensitive gamma ray observatory ever launched.INTEGRAL is an ESA mission in...
mission
are examples of cosmic spectrometers, while the GRS on the SMM
Solar Maximum Mission
The Solar Maximum Mission satellite was designed to investigate solar phenomenon, particularly solar flares. It was launched on February 14, 1980....
and the imaging Ge spectrometer on the RHESSI satellite have been devoted to solar observations.
Gamma-ray spectrometers have been widely used for the elemental and isotopic analysis of airless bodies in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, especially the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
.
These surfaces are subjected to a continual bombardment of high-energy cosmic rays, which excite nuclei in them to emit characteristic gamma-rays which can be detected from orbit.
Thus an orbiting instrument can in principle map the surface distribution of the elements for an entire planet.
Examples include the mapping of 20 elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
observed in the exploration of Mars, the Eros asteroid
433 Eros
433 Eros is a near-Earth asteroid discovered in 1898, and the first asteroid to be orbited by a probe . It is an S-type asteroid approximately 34.4×11.2×11.2 km in size, the second-largest NEA after 1036 Ganymed, and belongs to the Amor group.Eros is a Mars-crosser asteroid, the first known...
and the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
.
