Introduction (Atomic and Nuclear Structure-Radioactivity) Dr: Mohamed Bahaaeldin Afifi By Lecturer of Radiological Science

Matter &The Atom The term matter describes all of the physical substances around us Matter is anything that has mass and takes up space The Universe is made up of matter and energy

Matter is made up of atoms Matter is made up of atoms. An atom is the smallest whole particle of matter.

Early Models of the Atom Rutherford Mostly empty space Small, positive nucleus Contained protons Negative electrons scattered around the outside

Early Models of the Atom Bohr Electrons move in definite orbits around the nucleus

Modern Model of the Atom The electron cloud Sometimes called the wave model Spherical cloud of varying density

Which force holds the electrons in orbit? The Coulomb force

ATOMIC STRUCTURE

The nucleus of an atom is made up of tightly bound protons and neutrons, which are called nucleons. The nucleus contains most of the atomic mass.

Protons Neutrons Much larger and heavier than electrons Protons have a positive charge (+) Located in the nucleus of the atom Neutrons Large and heavy like protons Neutrons have no electrical charge Located in the nucleus of the atom 

Electrons Tiny, very light particles Have a negative electrical charge (-) Move around the outside of the nucleus in shells Fig1.2.

Each shell is assigned a principal quantum number (n), beginning with one for the K-shell, two for the L-shell, and so on. The number of electrons each shell can contain is 2n2.

Electrons are arranged in Energy Levels or Shells around the nucleus of an atom. first shell K a maximum of 2 electrons second shell L a maximum of 8 electrons third shell M a maximum of 18 electrons

The Atom The atom consists of two parts: 1. The nucleus which contains: protons neutrons 2. Orbiting electrons. Atoms in nature are electrically neutral so The number of electrons orbiting the nucleus equals the number of protons in the nucleus.

X Z A Mass number = number of protons + number of neutrons Element symbol Z Atomic number = number of protons

X A Z A = number of protons + number of neutrons Z = number of protons N = A – Z = number of neutrons Number of neutrons = Mass Number – Atomic Number

Electron binding energy The work that is required to remove an electron from an atom is called the electron binding energy. The binding energy of outer-shell electrons is smaller than the binding energy of the inner-shell.

Energetic particles can knock out inner-shell electrons only if their energy is equal to or greater than the electron binding energy.

A vacancy in the K-shell will be filled by an electron from a higher shell. Electrons moving from an outer shell to an inner shell may emit excess energy as electromagnetic radiation.

Electromagnetic radiation Radiation is the transport of energy through space. Electromagnetic radiation represents a transverse wave, in which the electric and magnetic fields oscillate perpendicular to the direction of the wave motion.

Wavelength (λ) is the distance between successive crests of waves. Amplitude is the intensity defined by the height of the wave.

Frequency (f) is the number of wave oscillations per unit of time expressed in cycles per second, or in hertz (Hz). The period is the time required for one wavelength to pass (1/f).

X-rays are an example of electromagnetic radiation. The product of the wavelength (λ) and frequency (f) of electromagnetic radiation is equal to the speed of light c (c = f .λ ).

Fig. 1.3 shows the electromagnetic spectrum from radio waves (long wavelength) to x-rays and gamma rays (short wavelength).

Photons Photons may behave as waves or particles but have no mass. Electromagnetic radiation is quantized, meaning that it exists in discrete quantities of energy called photons. Photons may behave as waves or particles but have no mass.

U U 235 92 238 92 There are many types of uranium: A Z Number of protons Number of neutrons A Z Number of protons Number of neutrons

U 235 92 U 238 92 A 235 Z 92 Number of protons Number of neutrons 143 A 238 Z 92 Number of protons Number of neutrons 146 Isotopes of any particular element contain the same number of protons, but different numbers of neutrons.

Most of the isotopes which occur naturally are stable. A few naturally occurring isotopes and all of the man-made isotopes are unstable. Unstable isotopes can become stable by releasing different types of particles. This process is called radioactive decay and the elements which undergo this process are called radioisotopes/radionuclides.

The word decay means to "break down." Radioactive Decay The word decay means to "break down." Radioactive decay results in the emission of either: an alpha particle (a), a beta particle (b), or a gamma ray(g).

Radiation Radiation: Energy in the form of particles or electromagnetic waves. Radiation is the transport of energy through space + -

Classification of radiation Radiation is classified into two main categories, non-ionizing and ionizing, depending on its ability to ionize matter. Ionizing Radiation: energy to remove an electron from an atom or molecule Radiation with sufficient . Non-ionizing radiation: Is radiation without enough energy to separate molecules or remove electrons from atoms. Examples are visible light, radio and television waves, ultra violet (UV), and microwaves with a large spectrum of energies.

Radioactivity Units of Activity Curie (Ci) Bequerel (Bq) The process by which unstable atoms spontaneously transform to new atoms* and in the process emit radiation. Units of Activity Curie (Ci) Bequerel (Bq) $-

Half-Life Half-life is the amount of time needed for the activity to reach one half of the original amount. f e l t l ln ( ) 2 T1/2 f 1 2 t T1/2 Days

Common Types of Radiation

Common Types of Radiation

Alpha Particle An alpha particle is identical to that of a helium nucleus. It contains two protons and two neutrons.

Alpha Particle

X Y + He Alpha Decay A Z A - 4 Z - 2 4 2 unstable atom alpha particle more stable atom

Alpha Decay Rn 222 86 He 4 2 Ra 226 88

Alpha Decay X A Z Y A - 4 Z - 2 + He 4 2 Ra 226 88 Rn 222 86 + He 4 2

Beta Particle A beta particle is a fast moving electron which is emitted from the nucleus of an atom undergoing radioactive decay. Beta decay occurs when a neutron changes into a proton and an electron.

Beta Decay As a result of beta decay, the nucleus has one less neutron, but one extra proton. The atomic number, Z, increases by 1 and the mass number, A, stays the same.

Beta Decay As a result of beta decay, the nucleus has one less neutron, but one extra proton. The atomic number, Z, increases by 1 and the mass number, A, stays the same.

Beta Decay b -1 At 218 85 Po 218 84

Beta Decay X A Z Y Z + 1 + b -1 Po 218 84 Rn 85 + b -1

Photons A photon is an x or gamma ray Has no weight Has no charge

Gamma Rays Gamma rays are not charged particles like a and b particles. Gamma rays are electromagnetic radiation with high frequency.

Photon Decay 99mTc 99Tc Stable Nucleus Excited Nucleus Gamma ray Gamma emissions occur during transitions such as fission, radioactive disintegration and electron-positron annihilation. Gamma rays are monoenergetic electromagnetic radiations that are emitted from nuclei of excited atoms following radioactive transformation; they provide a mechanism for ridding excited nuclei of their excitation energy. Source: Cember, H, Introduction to Health Physics, 1989 Stable Nucleus Excited Nucleus

Photon Decay When atoms decay by emitting a or b particles to form a new atom, the nuclei of the new atom formed may still have too much energy to be completely stable. This excess energy is emitted as gamma rays (gamma ray photons have energies of ~ 1 x 10-12 J).