HALF-LIFE AND MODES OF DECAY

Physical, Biological, Effective

Types of half-life

Physical half-life

The time required for a quantity of radioactivity to be reduced to one-half its original value

Every radioactive material has its own unique half life value

All radioactivity never disappears

Quantity decreases but never reaches zero

T1/2 = 0.693/ lambda

formula for physical half-life

3.3 half-lives

1 tenth life

Biological Half-life

The time required for the body to eliminate one-half of the dose of any substances by biological processes

Determined by the clearance of the radionuclides from the organ, tissue or body

Perspiration, urine, feces, exhalation

sample of biological half-life

Biological half-life

Most radiopharmaceuticals are also cleared from organs by various physiologic processes

Effective half-life

A combination of both T1/2 and Tb

The time required for half of initial radioactivity to disappear from an organ or body by combination of excretion and physical decay

Must always shorter than T1/2 or T

1/Te = 1/T1/2 + 1/Tb

formula for effective half-life

Alpha Decay, Beta Decay, Electron Capture, Isomeric Transition, Internal Conversion, Isobaric Transition

Decay Modes

Alpha Decay

Spontaneous decay emission of an alpha particle

Not used in medical imaging

Most common in A > 150 and Z > 82

Z - 2, A - 4, N - 2

Result of Alpha Decay

Alpha Particle

Identical to a helium nucleus

Consists of 2 protons and 2 neutrons

Heaviest and least penetrating form of radiation

Emitted from the atomic nucleus

Carries an electronic charge twice that of the proton

High LET

a

Symbol of Alpha particle

4

A of Alpha particle

+2

Charge of alpha particle

Little risk

external source of alpha particle

High risk

internal source

Ingestion, inhalation, wound

types of internal source

Paper

Alpha particles can be absorbed by?

Beta Decay

Occurs in allradionuclides

Occurs much more frequently than alpha particle

Emits beta particles

Beta minus, beta plus

Types of beta decay

Beta minus

Results:

Z + 1

A = no change

N – 1

Emits negatron and antineutrino (v)

Beta plus

Results:

Z - 1

A = no change

N + 1

Emits positron and neutrino (v)

Positron

Positively charge

Polyenergetic

Causes ionization

React violently with electron causing annihilation

Negatron

Negatively charge

Causes ionization

Antiparticle of positron

Antineutrino, Neutrino

Subatomic particle

Neutral charge

Lighter than electron

Electron Capture

K-capture

Method of radioactive decay that involves the capture of an orbital electron most likely in the K or L-shell by its nucleus

Similar to positron decay

Occurs in proton rich (neutron deficient) nuclei

Electron Capture

Most common in heavy proton rich nuclei

1 e- + 1 p+ ⟶ 1 n + neutrino

<1.02 MeV

common threshold for e- capture

≥1.02 MeV

both e- capture or beta plus decay may occur

Same as beta plus decay, characteristic xrays, gamma rays, auger electron

results of electron capture

Auger Process

The process of removing electron by a characteristic x-rays within an atom

Isomeric Transition

A decay process occurs when a nuclei in the excited state goes to ground state by emission of gamma radiation to attain stability

Isomeric State

Metastable state

Exists for >10^-9 s

Always unstable

No change in Z,A,N, Emission of gamma radiation, characteristic radiation, Auger electron

results of isomeric transition

Internal Conversion

Multipole electric fields of the nucleus interact with the orbital e

Causes ionization of electron

Not the same as gamma decay

Excess Electromagnetic Transition Energy

The surplus energy stored within a nucleus when its protons and neutrons are in an excited, unstable arrangement

Unstable configuration

After a nucleus undergoes alpha or beta decay, its protons and neutrons (nucleons) are often left rearranged in a high-energy "excited state."

EM Force (279keV), Ionized atom, Inner-shell vacancy, Internal conversion electron, stable nucleus, characterostic xrays, auger electrons

Results of internal conversion

Isobaric Transition

Modes of decay in which the mass number remains constant

Negatron decay, Positron decay, Electron Capture

isobaric transition occur in?