Transcript FRCRI-Atoms_and_Nuclei - hullrad Radiation Physics

Basic Physics
Manos Papadopoulos ([email protected])
Nuclear Medicine Department
Castle Hill Hospital
Hull & East Yorkshire Hospitals NHS Trust
FRCR LECTURES
 Lecture I – 24/09/09:
 Structure of Matter, the Atom and the Nucleus
 Lecture II – 01/10/2009:
 Radioactivity
 Lecture III – 08/10/2009:
 Interactions of Photons with Matter
 Lecture IV – 17/11/2009:
 Interactions of Charged Particles with Matter
BIBLIOGRAPHY
 Radiological Physics

P. Dendy, B. Heaton – Physics for Radiologists.
 Medical Imaging

S. Webb – The Physics of Medical Imaging.

J. Bushberg et al – The Essential Physics of Medical
Imaging.
 Nuclear Medicine

P. Sharp et al – Practical Nuclear Medicine.
Atoms and Nuclei
Manos Papadopoulos ([email protected])
Nuclear Medicine Department
Castle Hill Hospital
Hull & East Yorkshire Hospitals NHS Trust
STRUCTURE OF THE ATOM
 Definition of the atom:

Άτομο {átomo} – something that cannot
be divided any further

The smallest division of an element in
which the physical and chemical
properties of the element are maintained
STRUCTURE OF THE ATOM
 The atom is composed of:

an extremely dense, positively
charged NUCLEUS

an extra-nuclear cloud of light,
negatively charged
ELECTRONS
STRUCTURE OF THE ATOM
 The nucleus consists of NUCLEONS which are
of two types:

PROTONS – positively charged

NEUTRONS – no electric charge
 The number of electrons is equal to the number
of protons

the atom is electrically NEUTRAL
ATOMIC NOTATION
 ATOMIC NUMBER (Z): the number of protons in a
nucleus – defines the element of the atom
 MASS NUMBER (A): the total number of nucleons
(protons and neutrons) inside the nucleus
A=Z+N
238
92
U
uranium
197
79
Au
gold
NUCLEAR FORCES
 Two main forces in the nucleus:

Coulombic forces – between protons


Repulsive
Strong forces – between all nucleons

Attractive
 Strong forces operate over short nuclear distances
STRUCTURE OF THE ATOM
 Radius of an atom is:

10-10 m
 Radius of the nucleus is:

10-14 m
 Thus, size of atom is
10,000 times more than
that of nucleus
ELEMENTS
 Elements are groups of atoms with the same
 number of protons (Z)
 physical properties
 i.e. density, electrical conductivity, melting and boiling point
 chemical properties
 i.e. reactions with water, oxygen, acids
 There are more than 120 chemical ELEMENTS
 92 naturally occurring
ELEMENTS
BOHR’S ATOMIC MODEL
 Electrons orbit around the
nucleus at fixed distances
 Each shell is characterised
by its quantum number (n)
 Each shell contains a
maximum number of
electrons given by:

2n2
The Bohr model
ELECTRONIC STRUCTURE
ELECTRONIC STRUCTURE
 Each electron occupies

a discrete energy state

called binding energy
 It is the energy required
ionisation
excitation
to remove an electron
completely from the atom
 The atom is then ionised
Hydrogen Z=1
ELECTRONIC STRUCTURE
 Orbiting electrons

-
-
+
 Nucleus made up of:

Protons (+ charge)

Neutrons (0 charge)
-
- charge
-
EXCITATION & IONISATION
 Excitation of the atom

energy transferred to an orbiting electron

electron “jumps” from lower to higher energy levels

the atom is “excited”
 Ionisation of the atom

energy transferred to an orbiting electron

electron removed from the electric field of nucleus

the atom is ionised
CHARACTERISTIC X-RAYS
De-excitation of a tungsten atom
ELECTRON CASCADE
 Electron removed from its shell (i.e. ionisation) by

an X-Ray photon

a γ-Ray photon

a charged particle (e.g. alpha, beta)
 Vacancy created in shell

usually filled by an electron from outer shell
 Secondary vacancy in outer shell

filled by an electron transition from a more outer shell
 The phenomenon is called Electron Cascade
CHARACTERISTIC X-RAYS
 Electron transitions
emission of radiation
 Emissions from transitions exceeding 100eV

characteristic X-Rays
 Characteristic of the atom
Echaracteristic  Eb,vacant shell  Eb,transitionshell
X-RAYS TUBE
PRODUCTION OF X-RAYS
X-RAY SPECTRUM
AUGER ELECTRONS
 Predominant in low-Z elements
 Energy released  an orbital electron
 Ejected Auger electron has a kinetic energy equal to:

the difference between the transition energy and the
binding energy of the ejected electron
AUGER ELECTRONS
ISOTOPES
 Nuclides with the same number of protons but different
number of neutrons are called ISOTOPES
 they pertain to the same element
 they have the same chemical properties
 but different physical properties (different mass numbers)
NUCLEAR ENERGY LEVELS
 The nucleus has energy levels

analogous to orbital electron shells

of high energy
 The lowest energy state is called the ground state
 Nuclei in excess energy are in an excited state

average lifetimes from 10-16 to more than 100 years
 Excited states (t>10-12 sec) referred to as
 metastable or isomeric states (e.g. 99Tcm)
NUCLEAR STABILITY
 Nuclear line of Stability
 N=Z – for low-Z
 Nuclear line of Stability
 N≈1.5Z – for high-Z
 Higher n/p ratio

Heavy elements
 Nuclei with odd number

odd number of protons and
neutrons – unstable

even number of protons and
neutrons - stable
UNSTABLE NUCLEI

Combinations of unstable nuclei DO exist
 over time  decay to stable nuclei
 Two kinds of instability
 neutron excess
 neutron deficiency (proton excess)
 Such nuclei have excess internal energy
 Stability achieved through conversion of
 a neutron to a proton
 a proton to a neutron

Emission of energy
RADIOACTIVITY
 Nuclides decaying to more stable nuclei are

Radioactive
 The process is called

Radioactive Decay or Radioactivity
 A nucleus undergoes a series of radioactive decays

until it reaches a stable configuration
GAMMA RAYS
 Nucleus in excited state
 Nucleus decays to a lower (more stable) energy state
 Electromagnetic radiation emitted
 This electromagnetic radiation is called a
 gamma ray
 Analogous to the emission of characteristic X-Rays
 Gamma rays stem from the nucleus
SUMMARY I
 Atom – smallest division of an element
 Atom consists of:
 a nucleus (+ electric charge)



Protons – atomic number (Z)
Neutrons – neutronic number (N)

Mass number (A)
a cloud of orbiting electrons (– electric charge)

each electron  a discrete energy state  electron shell
 Ionisation
 electron removed from its shell  Ion Pair
 Excitation
 electron “moves” to a state of higher energy
SUMMARY II
 Ionisation
 Excitation

Radiation: Characteristic X-Rays / Auger electrons
 Nuclear Forces

Coulombic Force (repulsive) – between protons

Strong Forces (attractive) – between nucleons - protons and
neutrons
 Nuclear Energy Levels

Ground state – lowest energy state of an atom

Excited state – nuclei with energy in excess of the ground state
 metastable or isomeric state (10-12 sec)
THE END
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