Atomic nuclei have an energy-level structure somewhat similar to 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 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. High frequency >1019 Hz; high energies >100KeV and short wavelengths <10 picometers Produced by the decay from high energy states of atomic nuclei (gamma decay), Natural sources of γ rays : radioactive decay or interaction with high energy cosmic rays Thorium concentrations on the Moon, as mapped by Lunar Prospector The first excited energy level of Si is at 1.779 MeV, when this level decays to the ground state, a gamma ray with 1.779 MeV is produced. When a detector detects Gamma ray of this energy, it can be interpreted that they are produced by decay of the first excited level of Si The intensity of the 1.779 MeV Gamma Ray would depend upon the proportion of 28Si – hence Gamma ray can also be used for elemental abundance mapping Mass number = number of protons + number of neutrons A X Z Element symbol Atomic number = number of protons Number of neutrons = Mass Number – Atomic Number There are many types of uranium: 235 238 U 92 U 92 A 235 A 238 Z 92 Z 92 Number of protons 92 Number of protons 92 Number of neutrons 143 Number of neutrons 146 Isotopes of any particular element contain the same number of protons, but different numbers of neutrons. Decay of radioactive isotopes Excitation by cosmic ray particles The process by which a nucleus of an unstable atom loses energy by emitting particles of ionizing radiation. A material that spontaneously emits energetic alpha particles, beta particles, and gamma rays is considered radioactive. A stochastic process at the level of single atoms – impossible to predict when an atom will decay. But the chance that a given atom will decay is constant over time. For a large number of atoms, the decay rate is computable from the measured decay constants of the nuclides (or from the halflife periods) Radioactive decay results in the emission of : • an alpha particle (a), • a beta particle (b), or • a gamma ray(g). Sources of Planetary Gamma Rays: (1) Radioactive decay: Terrestrial GRS αlpha emission An alpha particle is identical to that of a helium nucleus. It contains two protons and two neutrons. Sources of Planetary Gamma Rays: (1) Radioactive decay: Terrestrial GRS αlpha emission A X Z A-4 4 Y He + Z-2 2 unstable atom alpha particle more stable atom Sources of Planetary Gamma Rays: (1) Radioactive decay: Terrestrial GRS αlpha emission 222 Rn 86 226 Ra 88 4 Radium 226 Ra 88 He 2 Radon 222 Rn + 86 4 He Sources of Planetary Gamma Rays: (1) Radioactive decay: Terrestrial GRS βeta emission Beta decay occurs when a neutron changes into a proton and an electron. The atomic number, Z, increases by 1 and the mass number, A, stays the same. Sources of Planetary Gamma Rays: (1) Radioactive decay: Terrestrial GRS 218 At βeta emission 85 218 Po 84 0b -1 Polium 218 Po 84 Astatine 218 At + 85 b -1 0 Sources of Planetary Gamma Rays:(1) Radioactive decay: Terrestrial GRS Gamma emission Unlike a and b particles, Gamma rays are not charged particles . High frequency >1018 Hz; high energies >100KeV and short wavelengths <10 picometers When atoms decay by emitting a or b particles to form a new atom, the nuclei of the new atom formed may still have too much energy to be completely stable. This excess energy is emitted as gamma rays (gamma ray photons have energies of ~ 1 x 10-12 J). Sources of Planetary Gamma Rays: (1) Radioactive decay: Terrestrial GRS Alpha radiation stopped by a sheet of paper. Beta radiation halted by an Al plate. Gamma radiation is dampened by lead. Sources of Planetary Gamma Rays: (1) Radioactive decay: Terrestrial GRS Half-life period: The time required for one-half of the radioactive (parent) isotopes in a sample to decay to radiogenic (daughter) isotopes. Several elements have isotopes with half-lives long enough that they have not decayed completely since they formed by nucleosynthesis over 4.6 Ga ago Sources of Planetary Gamma Rays: (1) Radioactive decay: Terrestrial GRS Natural potassium contains 0.012 % of the radioactive isotope K-40 (half-life 1.27 Ga) 90 % of K-40 undergo a β transition into the ground state of Ca-40, 10 % decay into an excited state of Ar-40, either by β+ emission or by electron capture. In the transition to its ground state, Ar-40 emits a gamma quantum of 1.460 MeV. 19 20 40Ar 18 Sources of Planetary Gamma Rays: (1) Radioactive decay: Terrestrial GRS Only three are common enough within earth materials to make them geologically useful - Bi214, Tl208, and K40. Bi214 comes from the decay of U238 and is, therefore, an indication of the concentration of uranium in the earth materials that lie within the range of the detector. Tl208 comes from the decay of Th232 and is an indicator of thorium content K40 is one of the minor natural isotopes of potassium and the only isotope of K that is radioactive. It makes up only 0.012% of the total potassium in rocks and soils, but because this fraction remains quite constant, even during weathering and metamorphism, the gamma radiation from it is a good indicator of changes in the amount of potassium in rocks. – 2.82 MeV – 1.76 MeV 214Bi – peak energy 2.82 MeV 214Bi – peak energy 1.76 MeV; 40K – peak energy of 1.46 MeV. 206Tl – 1.46 MeV 40K 208Tl •Bismuth 214, represents the Uranium window at 1,760 KeV THORIUM URANIUM POTASSIUM Gamma Ray Spectrum (Typical Felsic rocks – 120 m height) All three of these energies are constant; they never change, they therefore constitute well defined peaks in the energy spectrum emanating from rocks. •Thallium 208, represents the Thorium window at 2,620 KeV •Potassium has a single emission energy at 1,460 KeV Detectors: • Geiger-Muller tubes, • Scintillation counters, • Semiconductor detectors, • Thermoluminiscence detectors etc. Scintillation counter GM Counter A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic (usually containing anthracene), or organic liquid (see liquid scintillation counting) that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube (PMT) measures the light from the crystal. The PMT is attached to an electronic amplifier and other electronic equipment to count and possibly quantify the amplitude of the signals produced by the photomultiplier. K-R U-G Th-B K-R K-R U-G U-G Th-B Th-B Sources of Planetary Gamma Rays: (2) Cosmic rays: Planetary GRS Gamma-ray photons in the energy region 0.1-10 MeV are emitted by excited nuclei and have discrete energies characteristic of that element. The excitation of nuclei can come from the radioactive decay of very-long-lived radioisotopes in planetary materials, such as 40K, 238U, and 232Th. For other elements in space, excitation is provided by cosmic-ray bombardment. Except for periods of time during and immediately after major solar particle events, this excitation is caused mainly by galactic cosmic rays (GCR) Sources of Planetary Gamma Rays: (2) Cosmic rays: Planetary GRS Nuclear particles, mainly protons and helium nuclei (alpha particles) , accelerated to very high energies outside solar system Energetic particles (typically of order a GeV) Nearly isotropic flux of about 2-3 particles/(cm2-s). Generate nuclear cascades on striking dense matter (~9 neutron/incident energetic particle). The cascade particles most effective in generating gamma rays are neutrons. The high-energy neutrons produced in this manner can undergo further nuclear collisions. Sources of Planetary Gamma Rays: (2) Cosmic rays: Planetary GRS Elastic scattering – GCR particle hits a nucleus and scatters elastically. The target nucleus is left in the ground state with some added kinetic energy - No Gamma ray is emitted Non-elastic scattering – GCR collision with a stable nucleus produce a nucleus which is different from the initial one – either by exciting to a higher energy state or by creating a new nucleus (change the number of protons/neutrons) If a GCR collision with a stable nucleus excites it to a higher energy state and the return to normal state produces a gamma ray, it is called Inelastic scattering. An inelastic scatter is represented as (n, n γ) In an Inelastic scattering, GCR particles with energy greater than an excited state of the nucleus may excite the nucleus to that level and leave with a lower energy. The excited level decays with its characteristic half-life (10-12 sec), producing a Gamma ray of equivalent energy Sources of Planetary Gamma Rays: (2) Cosmic rays: Planetary GRS Neutrons with 0.85 MeV react with a 56Fe nucleus, excite it to the first excited level of 56Fe nucleus at 0.8467 MeV, which in turn de-excites to produce a 0.8467 MeV Gamma ray. The reaction is represented as 56Fe (n,nγ) 56Fe (=> a nuclear reaction of GCR particles with 56Fe nucleus in which a neutron (n) enters and a neutron and a gamma ray (n γ) exit, leaving the nucleus unchanged (56Fe). Sources of Planetary Gamma Rays: (2) Cosmic rays: Planetary GRS In non-elastic scattering other than inelastic scattering, the incident neutrons have energies higher than the excited level, in which case, in addition to excitation, other reactions can occur, for example, a neutron is emitted, and a new nucleus is formed: 56Fe(n,2nγ) 55Fe (=> a reaction with 56Fe nucleus in which a neutron (n) enters and two neutrons (2n) exit and a new nucleus (55Fe) is produced. You can have other reactions in which a radioactive nucleus is formed that emits one or more gamma rays before it decays. For example, 27Al(n,α) 23Na However for non-elastic scattering where a new nucleus is formed, high energy GCR particles are required, whose flux is low. Therefore most gamma ray flux on planetary surfaces are because of inelastic scattering, Sources of Planetary Gamma Rays: (2) Cosmic rays: Planetary GRS • The intensity of gamma rays emitted by a particular element and for a particular process depends: concentration of that element, the reaction cross-section for the process, and the number of particles available with the appropriate energy. • The inelastic scatter reactions have a large cross section for all common nuclei. • Therefore, all the most abundant elements give a useful yield of gamma rays from inelastic scattering process Sources of Planetary Gamma Rays: (2) Cosmic rays: Planetary GRS The observed flux of gamma rays at the sensor is a function of • Rate at which the flux is produced i.e., the rate of the inelastic scattering reactions • Transportation through the atmosphere of the planet Sources of Planetary Gamma Rays: (2) Cosmic rays: Planetary GRS Calculation of emitted gamma ray flux 2 F /2 d Sin d dx [ S ( x ) / 4 ] sec exp( x sec ) 0 0 0 S ( x ) Source strength at depth x Exponential mass attenuation coefficient for that gamma ray (that is over a path length of 1/μ, the intensity of the gamma ray reduces by 1/e), drerived from Lambert-Beer Law, I = Ioe-t , where I0= g-ray intensity at zero path length, t = path length, I = g-ray intensity transmitted and = attenuation coefficient When S(x) is constant: F S 2 , where S is the production rate of the gamma ray at any depth. = This equation means that the observed flux can be directly converted to elemental abundance, because production rate is directly related to the elemental abundance (higher the abundance, the higher the rate of production of gamma rays) = Also higher energy gamma rays would have stronger flux because of lower mass attenuation coefficient Sources of Planetary Gamma Rays: (2) Cosmic rays: Planetary GRS Transport of Gamma rays On the way from the surface to the detector, the Gamma rays may Vanish completely (give energy to an electron, the photoelectric effect) Scatter such that it loses some of its energy (Compton scattering), or Or may arrive at the detector without any attenuation Only the fraction of Gamma rays that arrive at the sensor with the intial energy unchanged are used for mapping elements. The detector is assumed to be directly above the surface Sources of Planetary Gamma Rays: (2) Cosmic rays: Planetary GRS Transport of Gamma rays - Spatial resolution If we assume that the source is constant with depth, then a remote Gamma ray detector would detect equal fluxes of gamma rays from all parts of the surface, even near the horizon where only gamma rays produced very near the surface will reach the detector. • Thus fairly large areas of the planet will sampled by an orbiting detector, if it is isotropic. However, the gamma ray flux per unit surface area on the planet will be strongest at the nadir (because of the r-2 effect), so the planetary surface at the detector’s nadir is best sampled. Rough rule: Spatial resolution is equal to one-half of the height of the detector Sources of Planetary Gamma Rays: (2) Cosmic rays: Planetary GRS Transport of Gamma rays - Spatial resolution 30 metres Gamma-ray detection systems have no collimation or focusing system, so gamma rays from all directions are recorded. Boynton et al, 2004 Planetary GRS Data Processing Collection and Processing of Gamma-Ray Spectra For Mars Gamma-ray spectra are collected at a rate of 360 spectra per orbit by GRS aboard Mars Odyssey, for a duration of about 19.75 s each. Each spectrum consists of 16,384 channels of discrete counts spanning the energy range from 0 to 10 MeV. The spectra are stored in a large database from which they are subsequently retrieved for a particular region and/or time period of interest, summed together to increase the statistical precision, and processed to determine the intensity of the emission lines in the resultant spectra [Evans et al., 2006]. Planetary GRS Data Processing Collection and Processing of Gamma-Ray Spectra Why accumulation of gamma ray spectra? The intensities of the gamma rays coming from a planetary surface are very low. The count rate is low enough that each photon is counted and individually sorted by energy to generate the spectrum. As an example of the low counting rate of the gamma-ray photons, the count rate for the Si line at 1779 keV is 0.035 per second. Roughly a third of the 19.75-s spectra will not have a single count in the Si line peak. Clearly one needs long accumulation times to achieve a reasonable signal-tonoise ratio. Planetary GRS A GS spectrum of Mars taken from June 10 through July 16, 2002.