28/10/2015
Radioactivity
07.10.2015.
Nuclear stability
•
•
•
•
Up to atomic number 20, n=p is stable
Above atomic number 20, n>p stable
Above atomic number 84, all nuclei are unstable
Nuclei with magic numbers of protons and/or
neutrons are stable
• even number of protons and neutrons are more
stable.
1
28/10/2015
Radioactivity
• The emission of radiation by unstable atomic nuclei undergoing
radioactive decay.
• Types:
• Alpha-radiation
• Beta-radiation
• Gamma-radiation
• Ionization:
• energy transferred to an orbital electron to remove if from an atom
• an ion pair is produced (the newly freed electron(-) and the rest of the
atom(+))
• <-> excitation: energy transferred to an orbital electron to displace it
further away from the nucleus.
• LET (Linear Energy Transfer): the energy transferred to the
medium over a certain distance (J/m, ion pairs/cm of tissue).
Higher LET causes greater biological impact.
Discovery of radioactivity
• Antoine Henri Becquerel – 1896: radiation from uranium salts
exposed films (photopaper).
• Marie Currie – 1898: work with radioactive samples (discovery of
polonium, radium).
• The Nobel Prize in Physics 1903
•
•
•
•
Antoine Henri Becquerel 1/2
Pierre Curie 1/4
Marie Curie 1/4
"in recognition of the extraordinary services they have rendered by their joint
researches on the radiation phenomena discovered by Professor Henri
Becquerel".
• The Nobel Prize in Chemistry 1911
"in recognition of her services to the advancement of chemistry by the discovery
of the elements radium and polonium, by the isolation of radium and the study of
the nature and compounds of this remarkable element".
• Ernst Rutherford – 1903: discovered that alpha radiation contains
helium nuclei.
2
28/10/2015
Alpha-decay
• emission of a helium nucleus (alpha-particle) from a large
nucleus of an atom.
• He2+ ion is emitted
• A (mass number) decreased by 4
• Z (atomic number) decreased by 2
• Highly ionizing – LET: 4K-9K ion pairs/µm in tissue.
• range is few cm in air, < 0.5 cm in water, < 60 µm in tissue
• easily shielded (e.g., paper, skin).
• daughter nucleus left in excited state
• the energy spectrum of alpha decay is discrete
Beta radiation
•
•
•
•
•
•
•
•
•
High speed electron (e- = beta-particle) ejected from nucleus
A (mass number) does not change
Z (atomic number) decrease or increase by 1
range is a few tens of cm in air, < 2cm in water, a few mm in
tissue
Low LET causing light damage (6-8 ion pairs/µm in tissue)
Aluminium and other light materials can be used for shielding.
continuous energy spectrum
X-ray can be produced (Breaking or Characteristic Radiation)
e+: positron (antimatter) – annihilation (combination with an
electron) produce gamma radiation
3
28/10/2015
Generating beta radiation
1
0
n→11p + + 00e − + ν ( antineutrino )
1
1
p + →01n + 00e + + ν ( neutrino )
137
55
22
11
−
Cs→137
56 Ba + e +ν
22
Na →10
Ne + e + + ν
Radioactive sodium isotope
Gamma radiation
• Gamma rays are photons emitted from the nucleus
• Electromagnetic radiation (f>1019Hz, E>100keV
(1.6*10-14J), λ<10-12m)
• No mass; no charge, speed = Clight
• Long range (km in air, m in body)
• light damage
• usually shielded with lead or concrete
• radiation emitted by the atomic nucleus ↔ x-ray:
radiation emitted by the atom due to electron
transitions
4
28/10/2015
Radioactive Decay
λ
=
N
1/
λ:
=
2 0.693
=
λ
λ
!" # = half-life: the time for
50%
half of the atoms in a
radioactive
substance
to
disintegrate.
t
Radioactive Decay
λ
=
λ:
$
N
1/
%=
1
λ
& = average lifetime: the
length of time for 63.2% of
36.8%
the atoms in a radioactive
substance to disintegrate.
8225
t
5
28/10/2015
• The end!
6