1

Atomic StructureAtomic StructureProtons 1

1p(1.007276 amu)

Neutrons 10n

(1.008665 amu)Electrons(0.0005486 amu)

Neon-20 2010Ne

(19.992434 amu)

2

Nucleus of the AtomNucleus of the Atom

The nucleus contains protons and neutrons

All atoms of the same element contain the same number of protons

Isotopes of an element can have differing numbers of neutrons

Isotopes having a nearly equal number of protons and neutrons are most likely to be stable

3

Nuclear Shorthand and Nuclear Shorthand and DefinitionsDefinitions

Mass Number: (A) # Protons + #Neutrons Atomic Number: (Z) # Protons

(the atomic number defines the element)

Isotope: Nuclides of an element with the same atomic number (number of protons) but differing numbers of neutrons

THE ATOM The nucleus structure

– protons and neutrons = nucleons– Z protons with a positive electric charge

(1.6 10-19 C) – neutrons with no charge (neutral)– number of nucleons = mass number A

The extranucleus structure – Z electrons (light particles with electric charge)

equal to proton charge but negative

Particle Symbol Mass Energy Charge (kg) (MeV)----------------------------------------------------------Proton p 1.672*10-27 938.2 +Neutron n 1.675*10 -27 939.2 0Electron e 0.911*10 -30 0.511 -

Identification of an Isotope

6

Question #1Question #1

A P32 atom has 15 protons in the nucleus. How many neutrons does it have?

a. 15

b. 16

c. 17

d. 18

IONIZATION-EXCITATION

Energy

characteristicradiation

Auger-electron

DEEXCITATION

THE NUCLEUSENERGY LEVELS

The nucleons can occupy different energy levels and the nucleus can be present in a ground state or in an excited state. An excited state can be reached by adding energy to the nucleus. At deexcitation the nucleus will emit the excess of energy by particle emission or by electromagnetic radiation. In this case the electromagnetic radiation is called a gamma ray. The energy of the gamma ray will be the difference in energies between the different energy levels in the nucleus.

Occupied levels

~8 MeV

0 MeV

ENERGY

Particle emission

Gamma ray

DeexcitationExcitation

EnergyEnergy particlesphotons

NUCLEAR EXCITATION

alpha-particlebeta-particle

Gamma radiation

NUCLEARDEEXCITATION

INTERNAL CONVERSION

characteristicradiation

conversionelectron

Radioactive decay

IAEA Training Material on Radiation Protection in Nuclear Medicine

STABLE NUCLIDES

long rangedelectrostatic

forces

short rangednuclear forces

p

p

n

Line of stability

Stable and unstable nuclides

Too manyneutronsfor stability

Too manyprotonsfor stability

RADIOACTIVE DECAYFissionThe nucleus is divided into two parts, fission fragments. and3-4 neutrons. Examples: Cf-252 (spontaneous), U-235 (induced)

-decayThe nucleus emits an -particle (He-4). Examples: Ra-226, Rn-222

-decayToo many neutrons results in -decay. n=>p++e-+. Example:H-3, C-14, I-131.Too many protons results in -decayp+=>n+ e++ Examples: O-16, F-18 or electron capture (EC). p+ + e-=>n+ Examples: I-125, Tl-201

86226

84222

24Ra Rn+

Multiple & prefixes (Activity)Multiple Prefix Abbreviation

1 - Bq

1 000 000 Mega (M) MBq

1 000 000 000 Giga (G) GBq

1 000 000 000 000 Tera (T) TBq

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Types of Ionizing Radiation: Alpha, Beta, & Types of Ionizing Radiation: Alpha, Beta, & GammaGamma

Alpha ParticleAlpha Particle

Beta ParticleBeta Particle

Large Mass (nuclei) – HeliumLarge Mass (nuclei) – HeliumAtom with a +2 chargeAtom with a +2 charge

Small Mass - Small Mass - ElectronElectron(subatomic (subatomic particleparticle))

Gamma PhotonGamma Photonand X-Raysand X-Rays

No MassNo Mass(Electromagnetic(Electromagnetic Radiation)Radiation)

+

+

19

Penetrating Power of Radiations

Penetrating Power of Radiations

Alpha

Beta

Gamma and X-rays

Paper Plastic Lead

20

Radioactivity UnitsRadioactivity UnitsActivity = Rate of decay of a radioactive sample or the

number of atoms that decay per unit time

1 Curie (Ci) = 3.7x1010 dps

or 2.22 x 1012 dpm

1 millicurie (mCi) = 3.7 x 107 dps or 2.22 x 109 dpm

1 microcurie (Ci) = 3.7 x 104 dps or 2.22x106 dpm

1 Becquerel (Bq) = 1 disintegration per second

Specific Activity: amount of radioactivity in a given mass or volume, e.g. μCi/ml

or mCi/mg

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Half-LifeHalf-Life

The time required for theamount of radioactive materialto decrease by one-half

22

Half-Life: I-131 Half-Life: I-131

8 day half-life

0 8 16 24 321.0 ½ ¼ 1/8 1/16

20 mCi 10 mCi 5 mCi 2.5 mCi 1.25 mCi

It is impossible to know at what time a certain radioactive nucleuswill decay. It is, however possible to determine the probability l of decay in a certain time. In a sample of N nuclei the number of decays per unit time is then:

2lnT

eN=N(t)

Ndt

dN

2/1

t-0

RADIOACTIVE DECAY

24

Activity CalculationsActivity Calculations

A = Aoe-t

(λ = ln 2 ÷ T½ = 0.693/ T½)

Phosphorus-32 has a half-life of 14.3 days. How much is left of a one millicurie shipment after 30 days?

A = 1 mCi × e-(0.693/14.3 d) x 30 d

A = 0.234 mCi