lecture-5 Gamma Ray Spectroscopy

Report
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.

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