radiochemistry dr nick evans [email protected]
TRANSCRIPT
Why Radiochemistry?
Radioisotopes are widely used in:1. Diagnostic and therapeutic nuclear medicine
• 500 000 per annum2. Mechanistic and kinetic studies of reactions 3. Analysis4. Agriculture5. Industry
Why?1. Ease and sensitivity of detection of radioisotopes2. Automation of counting of radioisotopes3. 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
0
PE
Distance from nucleus (r)
Range of attractive Nuclear Force
Coulombic Repulsion
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
to mass of nucleus, e.g. for 168O
Binding Energy of Nucleus (2)
• On 12C scale:– Mass of proton = 1.007825 amu– Mass of neutron = 1.008665 amu– Mass of electron = 0.0005485 amu
• Thus:– 8 protons = 8.0626– 8 neutrons = 8.06932– 8 electrons = 0.004388– Sum = 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
5
6
7
8
9
0 50 100 150 200 250
Mass Number
Bin
din
g E
ne
rgy
per
Nu
cle
on
(M
eV)
Fission releases energyFusion
releases energy
Fission releases energyFusion
releases energy
Binding Energy per Nucleon
• The most stable elements have mass numbers around 56, specifically
• 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
5626Fe
4 16 28 42 88 2082 8 14 20 38 82He, O, Si, Ca, Sr, 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
He-4
He-5
Li-6Li-7
Be-8
Be-9
B-10
B-11
C-12
C-13
N-14N-15
O-16
O-17
F-18F-19
Ne-20
Ne-21
Ne-22
Na-23
Mg-24
Mg-25
Al-26
Al-27
Si-28
Si-29
P-30P-31
S-32
-2
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32
Atomic Number
Neu
tron
Sep
arat
ion
Ener
gy (M
eV)
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
2He, 168O, 40
20Ca, 20882Pb
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:
• 21H, 6
3Li, 105B, 14
7N,
• 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
0
20
40
60
80
100
120
140
0 10 20 30 40 50 60 70 80 90
Atomic Number (Z)
Mas
s N
umbe
r (A)
Neutron-rich areaβ- decay favoured
Proton-rich areaβ+ or EC decay favoured
Nuclei Showing Ground State Energy (MeV)
61Zn56765
62Zn57692
63Zn58623
64Zn59550
65Zn60482
60Cu55832
61Cu56760
62Cu57690
63Cu58619
64Cu59551
59Ni54898
60Ni55826
61Ni56758
62Ni57686
63Ni58619
58Co53967
59Co54896
60Co55829
61Co56759
62Co57692
57Fe53036
58Fe53965
59Fe54898
60Fe55829
61Fe56763
Stable Isotopes
61Zn56765
62Zn57692
63Zn58623
64Zn59550
65Zn60482
60Cu55832
61Cu56760
62Cu57690
63Cu58619
64Cu59551
59Ni54898
60Ni55826
61Ni56758
62Ni57686
63Ni58619
58Co53967
59Co54896
60Co55829
61Co56759
62Co57692
57Fe53036
58Fe53965
59Fe54898
60Fe55829
61Fe56763
Mass = 61 Isobar
Fe
Co
Ni
Cu
Zn
-2
0
2
4
6
8
10
25 26 27 28 29 30 31
Atomic Number
Mas
s D
iffe
ren
ce (
MeV
)
Normally only 1 nucleus per mass number is stable