Download II Radioactive isotopes

Radioactive isotopes
Isotopes
Variants of different (atomic) mass numbers
(A) of a single chemical element are called
as isotopes;
Nucleons of different isotopes of a single
chemical element have the same numbers of
protons (atomic number – Z) and different
number of neutrons (N) – {Z+N=A};
Nucleons of isotopes of a single chemical
elements can be stable or radioactive;
Several stable and radioactive isotopes can
form a single chemical element;
Isotope geology: Use of isotopes for
description and interpretation of geological
processes;
Isotope geochemistry: Study of isotopic
compositions of terrestrial and extraterrestrial
materials, covering age, origin, evolution and
history of the studied geological formation;
- Geochronology: age determination, based
on the radioactive decay;
- Explanation of geological processes (genetic
geochemical approach: change/variation
of isotopic compositions (ratios) in time;
Geochemical aspect of radioactivity – history
•H. Becquerel (1896): discovery of radioactivity of U-salts
– revolution in physics;
•Marie & Pierre Curie, E. Rutherford, F. Soddy (until 1902):
discovery of process of radioactivity, radiation, stable
and not stable nucleus (decay, emission - number of
radioctive atoms);
•Curie (1903): radioactivity is an exoterm process, idea of
Kelvin (Earth formed from a cooling melt system and its
age 94 m.y.) cannot be held (first geochemical link);
•Rutherford (1905): based on He-content of U ores, age of the
Earth is 500 m.y.;
•A. Holmes (1913) “The age of the Earth”, U-Pb ages were
used;
•Analytical developments (MS, sensitive detectors, ionizing radii)
- history -
Nucleus transformations – Nuclear reactions
Rutherford’s experiment (1919): element transmute
14
7
N  He 
4
2
17
8
O H
1
1
Irene Joliot-Curie (1934): artificial radioactivity
27
13
Al  He 
4
2
Tight supercription:
14
7
30
15
P n
1
0
N α,p O
17
8
Alα,n P
27
13
mass number (A) – atomic number (Z)
30
15
Radioactivity: decay processes of radioactive nucleus (large energy,
ionizing radiation).
Main types of radioactive decay:
α-decay
•from nucleus of an atom He nuclide (strongly bound 2
protons and 2 neutrons) spits out,
•strongly ionizing, large energy (several 1 million
kJ/mol, effective range in air < 1 cm,
•velocity:19 cm/s, it can be stopped by paper, skin
β-decay
•in nucleus of an atom its neutron ejects an electron
(electron emission), meanwhile the neutron turns into
proton,
•moderately ionizing, energy 1-1 million kJ/mol,
effective range in air some 10 cm,
•beta radiation can be stopped by low mass number
elements (Al) or wood
γ-decay
•large energy gamma rays (electromagnetic radiation),
originating along with alpha and beta particle ejection,
•slightly ionizing,
•effective range in air endless, speed of light,
•it can be stopped by high mass number elements (Pb),
Properties of radioactive decays
type
mass charge
(aam)
(e)
alpha
4,0026
+2
He-nuclides
beta
1/1837
-1
electrons
0
electromagnetic
rays
gamma
aam: average atom mass
0
radiation
Decreasing energy of the electromagnetic radiation →
Increasing wavelength of the electromagnetic radiation →
Natural radioactivity
Nucleosynthesis: >2200 nuclides (stable, slow decay,
radionuclides)
Generally, atomic number of the natural radioactive
elements > 80 (Tl-tól).
Exceptions: 40K, 87Rb, 138La, 147Sm, 176Lu, 187Re,
cosmogene isotopes (3H, 10Be, 14C, 26Al, 32Si, 36Cl)
Terrestrial decay series presently:
• Thorium series (Th-232, a radioactive nuclide)
• Uranium series (U-238, a radioactive nuclide)
• Actinium series (U-235, a radioactive nuclide)
190Pt
+
Stable isotopes
Cosmogen isotopes
Natural radioactive
isotopes
Z (atomic
number)
N (neutron)
Radioactive Decay Systems of Geochemical Interest
&

Terrestrial decay series presently:
• Thorium series (Th-232, a radioactive nuclide) -> Pb-208
• Uranium series (U-238, a radioactive nuclide) -> Pb-206
• Actinium series (U-235, a radioactive nuclide) -> Pb-207
(these Pb nuclides are called as radiogenic stable isotopes)
•Neptunium series (Np-237, a former radioactive nuclide) > Bi-209
Activity (Bq):
One Bq is defined as the activity of a quantity of
radioactive material, in which one nucleus decays
per second (after H. Becquerel),
Bq/l – activity concentration,
Bq/kg – specific activity
Radioactive decay is a stochastic (random) process
at the level of single atom. It is impossible to predict
when a particular atom will decay. For a large
collection of atoms, however, the decay rate for that
collection can be calculated from their measured
decay constants () or half-lives (t1/2).
This is the basis of radiometric dating.
The radiometric dating provides “only” the age of occurrence
of some physical or chemical processes.
The radiometric dating on minerals and rocks defines the time
when the last precipitation/crystallization happened or
determines the age of cooling process below a certain
threshold of temperature.
Blocking (closure) temperature: a given temperature value
in a decreasing T system; below this T diffusion stops acting.
Figure 2.1- Nominal closure
temperatures of various
geochronometers and
thermochronometers (from
Gwilym, 2005). Systems are
ordered by closure
temperature on the Y-axis;
the red dashed line
indicates the
thermochronometers used
in this work.
Benedetta, 2013
Geochronology
•
•
•
•
•
•
•
Rb-Sr
Sm-Nd, Lu-Hf, La-Ce
K-Ar
40Ar-39Ar
K-Ca
U-Th-Pb
Re-Os
Figure 1.2-Tectonic sketch map of Europe (from Artemieva et al., 2006)
Benedetta, 2013