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Radiochemistry
Dr Nick Evans
[email protected]
Why Radiochemistry?
Radioisotopes are widely used in:
1. Diagnostic and therapeutic nuclear medicine
2.
3.
4.
5.
•
500 000 per annum
Mechanistic and kinetic studies of reactions
Analysis
Agriculture
Industry
Why?
1. Ease and sensitivity of detection of radioisotopes
2. Automation of counting of radioisotopes
3. Availability of radioisotopes
Theory of Atomic Structure
• Atom = nucleus + extra-nuclear electrons
• Nucleus = neutrons and protons held together by
“strong interactions”
• Strong nuclear force (interaction) is a
fundamental force of nature which affects only
quarks, antiquarks, and gluons
• Range of force is about 10-15 m
• Strong enough to overcome Coulombic repulsion
of protons
Potential Energy of Proton near Nucleus
PE
Coulombic Repulsion
0
Distance from nucleus (r)
Range of attractive
Nuclear Force
Binding Energy of Nucleus
• Indication of how strongly the nucleus is bound
together
• Energy liberated in formation of nucleus from its
nucleons is a measure of its stability
• High binding energy = stable nucleus
• Sum of individual masses of nucleons
is different
16
O
to mass of nucleus, e.g. for 8
Binding Energy of Nucleus (2)
• On
12C
scale:
– Mass of proton
– Mass of neutron
– Mass of electron
= 1.007825 amu
= 1.008665 amu
= 0.0005485 amu
• Thus:
–
–
–
–
8 protons
8 neutrons
8 electrons
Sum
=
=
=
=
8.0626
8.06932
0.004388
16.136308
• Actual mass of 16O on 12C scale = 15.9949148
• Therefore, mass defect = 0.141394 amu
Binding Energy of Nucleus (3)
• Decrease in mass is due to energy release
when atom is formed, i.e.:
• E = mc2
= 0.141394 x 10-3 kg x (3 x 108 ms-1)2/6.023 x 1023
= 2.1128 x 10-11 J
• But 1 eV = 1.6021 x 10-19 J
• Thus E = 131.9 MeV
or
= 8.24 MeV per nucleon
• Sun loses 4.2 million tonnes per second as it
builds heavier nuclei
• Plot binding energy per nucleon vs. mass number
Binding Energy per Nucleon vs. Mass Number
Binding Energy per Nucleon (MeV)
9
8
7
Fusion
releases
energy
6
Fission releases energy
5
0
50
100
150
Mass Number
200
250
Binding Energy per Nucleon
• The most stable elements have mass numbers
around 56, specifically 56 Fe
26
• 8 MeV is high energy compared with
electromagnetic radiation
– UV is a few electron volts (eV) to ~100 eV
– X-ray photons have energies ~100 eV to ~100 keV
– Gamma-ray energies > 100 keV
• Small peaks represent particularly stable nuclei
– high binding energy per nucleon
4
2
He,
16
8
O,
28
14
Si,
42
20
Ca,
88
38
Sr,
208
82
Pb etc.
Separation Energy
• Energy required to remove a single neutron from
the nucleus
• Shows the stability of nuclei built from α-particles
• Mass increasing in jumps of 4
Neutron Separation Energies
28
Si-28
26
24
Neutron Separation Energy (MeV)
22
20
He-4
Be-8
18
C-12
Ne-20
16
Mg-24
O-16
S-32
14
12
B-11
N-14N-15
10
Al-26
Ne-22
Mg-25
Ne-21
Li-6
C-13
4
2
P-31
P-30
Si-29
Li-7
6
O-17
Be-9
0
-2 0
F-19
F-18
B-10
8
Al-27
Na-23
2
He-5
4
6
8
10
12
14
16
18
Atomic Number
20
22
24
26
28
30
32
Magic Numbers
• Leads to concept of ‘magic numbers’ for certain numbers
of neutrons and protons
• Suggests there are energy levels in the nucleus
• Equivalent to idea of full outer shell of electrons in noble
gases
• Magic Nos.: 2, 8, 20, 28, 50, 82, 126
• Nuclei with this number of protons, or neutrons or sum of
the 2 which is a magic number are especially stable, e.g.
4 He, 16 O, 40 Ca, 208 Pb
2
8
20
82
Nuclear Energy Levels
Two Theories of Nuclear Structure:
• Liquid drop model
• Assumes nucleons behave like molecules in a liquid
• random movement and exchange of position
• Scattering experiments suggest that nuclei have
approximately constant density (2.4 x 1014 g cm-3)
• Takes into account that the forces on nucleons on surface
are different from those in interior where the nucleons are
completely surrounded by others
• Like taking into account surface tension of liquid drop
Nuclear Energy Levels (2)
• Shell Model
• Accounts for energies of particles emitted
• Dense-gas type models of nuclei with multiple
collisions between particles didn't fit data
• Patterns like magic numbers suggest shell
structure
Nuclear Energy Levels
• Analogous to filled electron shells
• No principal quantum number
• Levels are determined by angular momentum
quantum number
• Jumps between levels caused by absorption or
emission of energy
• Often gamma
Neutron : Proton Ratio
• Approximately 275 nuclei have shown no evidence
of radioactive decay
• ~60% of these have:
• even numbers of protons and
• even numbers of neutrons
• In general the most abundant on earth
• Remaining ~40% are about equally divided
between:
• even number of protons and odd number of neutrons
• odd number of protons and even number of neutrons
Neutron : Proton Ratio (2)
• There are only 4 ‘stable’ nuclei with an odd
number of protons and neutrons:
•
2
6 Li, 10 B, 14 N,
H,
1
3
5
7
• Relative abundances of 0.015, 7.42, 19.6, 99.63%
• Very light nuclei
• Elements of even atomic number have more
stable isotopes than those of odd atomic number
• Occurs due to energy stabilisation of pairs of
protons and/or neutrons
The Stable Region
• Stability is favoured by even numbers of protons
and neutrons
• Not usually equal numbers
• Plotting neutron number (A) against proton
number (Z) for all known nuclei, shows area of
stability
• For very light elements N ≈ Z gives stable
elements
• 1:1 up to 4020Ca
• Ratio gradually rises (A>Z) until by element 83
(Bi, the last one with a stable isotope) it is ~1.5
The Stable Region (2)
• If the N/P ratio is too high for stability
then isotope is neutron rich
• likely to decay by β- emission
• If the N/P ratio is too low for stability then
isotope is proton rich
• likely to decay by β+ emission or electron
capture
Stable Isotopes
140
Mass Number (A)
120
Neutron-rich area
β- decay favoured
100
80
60
40
Proton-rich area
β+ or EC decay favoured
20
0
0
10
20
30
40
50
Atomic Number (Z)
60
70
80
90
Nuclei Showing Ground State Energy (MeV)
61Zn
62Zn
63Zn
64Zn
65Zn
56765
57692
58623
59550
60482
60Cu
61Cu
62Cu
63Cu
64Cu
55832
56760
57690
58619
59551
59Ni
60Ni
61Ni
62Ni
63Ni
54898
55826
56758
57686
58619
58Co
59Co
60Co
61Co
62Co
53967
54896
55829
56759
57692
57Fe
58Fe
59Fe
60Fe
61Fe
53036
53965
54898
55829
56763
Stable Isotopes
61Zn
62Zn
63Zn
64Zn
65Zn
56765
57692
58623
59550
60482
60Cu
61Cu
62Cu
63Cu
64Cu
55832
56760
57690
58619
59551
59Ni
60Ni
61Ni
62Ni
63Ni
54898
55826
56758
57686
58619
58Co
59Co
60Co
61Co
62Co
53967
54896
55829
56759
57692
57Fe
58Fe
59Fe
60Fe
61Fe
53036
53965
54898
55829
56763
Mass = 61 Isobar
Mass Difference (MeV)
10
8
Fe
6
Zn
4
Co
2
Cu
0
Ni
-2
25
26
27
28
29
30
31
Atomic Number
Normally only 1 nucleus per mass number is stable