Dr. Mohammed Alnafea [email protected] Methods of Radioactive Decay.

Dr. Mohammed AlnafeaDr. Mohammed [email protected]@ksu.edu.sa

Methods of Radioactive Decay

ATOMIC STRUCTUREATOMIC STRUCTUREAtomic number (Z):Atomic number (Z):number of protons in nucleusnumber of protons in nucleusMass number (A):Mass number (A):Number of protons + neutronsNumber of protons + neutronsNeutron number (N):Neutron number (N):

Nuclear forces:Nuclear forces:• "Strong" "Strong" attractive attractive forceforce• electrostatic electrostatic repulsive repulsive force force

Radioactive decay caused by Radioactive decay caused by nuclear instabilitynuclear instability

Due to p-p electrostatic repulsionDue to p-p electrostatic repulsion

2 21/04/234th lecture RAD 311

Nuclear Transformation

When the atomic nucleus undergoes spontaneous transformation, called radioactive decay, radiation is emittedIf the daughter nucleus is stable, this

spontaneous transformation endsIf the daughter is unstable, the process

continues until a stable nuclide is reachedMost radionuclides decay in one or more

of the following ways: (a) alpha decay, (b) beta-minus emission, (c) beta-plus (positron) emission, (d) electron capture, or (e) isomeric transition.

21/04/233 4th lecture RAD 311

Methods of Radioactive Decay

4

Consider a hypothetical nucleus that can undergo many of the major forms of radioactive decay. This hypothetical nucleus is shown below:

21/04/234th lecture RAD 311

RADIONUCLIDE DECAY MODESRADIONUCLIDE DECAY MODES

NNo stable nuclei wo stable nuclei when hen Z > Z > 83 or 83 or N > N > 126126

Stable nucleiUnstable – radioactive : half-life < 1msUnstable – radioactive : half-life > 1000 years

Number of protons (Z)

Nu

mb

er

of

neu

tron

s (A

-Z)


Alpha Decay

Alpha () decay is the spontaneous emission of an alpha particle (identical to a helium nucleus) from the nucleus.

Typically occurs with heavy nuclides (A > 150) and is often followed by gamma and characteristic x-ray emission.

energyn transitioHe YX 242

4A2Z

AZ


NNo stable nuclei wo stable nuclei when hen Z > Z > 83 or 83 or N > N > 126126

RADIONUCLIDE DECAY MODESRADIONUCLIDE DECAY MODES


NNuclei with Zuclei with Z >> 8383

RADIONUCLIDE DECAY MODESRADIONUCLIDE DECAY MODES decaydecay


Beta-Minus (Negatron) DecayBeta-minus (-) decay characteristically

occurs with radionuclides that have an excess number of neutrons compared with the number of protons (i.e., high N/Z ratio)

Any excess energy in the nucleus after beta decay is emitted as gamma rays, internal conversion electrons or other associated radiations

energy β YX -A1Z

AZ


Occurs in Occurs in nuclei with nuclei with high neutron:proton high neutron:proton ratioratio

RADIONUCLIDE DECAY MODESRADIONUCLIDE DECAY MODES-- decay decay



Beta-Plus Decay (Positron Emission)

Beta-plus (+) decay characteristically occurs with radionuclides that are “neutron poor” (i.e., low N/Z ratio).

Eventual fate of positron is to annihilate with its antiparticle (an electron), yielding two 511-keV photons emitted in opposite directions.

energy β YX A1-Z

AZ


Occurs in Occurs in nuclei with a nuclei with a low neutron:proton low neutron:proton ratioratio

RADIONUCLIDE DECAY MODESRADIONUCLIDE DECAY MODES++ decay decay



Electron Capture Decay

Alternative to positron decay for neutron-deficient radionuclides

Nucleus captures an orbital (usually K- or L-shell) electron

Electron capture radionuclides used in medical imaging decay to atoms in excited states that subsequently emit detectable gamma rays

energy Y e X A1-Z

-AZ


RADIONUCLIDE DECAY MODESRADIONUCLIDE DECAY MODESElectron captureElectron capture

Occurs in Occurs in nuclei with a nuclei with a low neutron:proton low neutron:proton ratioratio


Isomeric Transition

During radioactive decay, a daughter may be formed in an excited state

Gamma rays are emitted as the daughter nucleus transitions from the excited state to a lower-energy state

Some excited states may have a half-lives ranging up to more than 600 years

energy X X AZ

AmZ


Generally accompanies other radioactive decayGenerally accompanies other radioactive decayassociated with associated with energy losenergy loss from changes in nuclear s from changes in nuclear

energy statesenergy states

RADIONUCLIDE DECAY MODESRADIONUCLIDE DECAY MODES emissionemission


Used by high Z nucleiUsed by high Z nuclei

2 nuclei of approximately 2 nuclei of approximately equal mass produced equal mass produced

Accompanied by releaseAccompanied by release of energy and neutronsof energy and neutrons

RADIONUCLIDE DECAY MODESRADIONUCLIDE DECAY MODESSpontaneous fissionSpontaneous fission


Decay SchemesEach radionuclide’s decay process is a

unique characteristic of that radionuclide.Majority of pertinent information about

the decay process and its associated radiation can be summarized in a line diagram called a decay scheme

Decay schemes identify the parent, daughter, mode of decay, intermediate excited states, energy levels, radiation emissions, and sometimes physical half-life.


Generalized Decay Scheme







Summary: Radioactive Decay

27

Fission: Some heavy nuclei decay by splitting into 2 or 3 fragments plus some neutrons. These fragments form new nuclei which are usually radioactive;

Alpha Decay: Two protons and two neutrons leave the nucleus together in an assembly known as an alpha-particle;

An alpha-particle is a He-4 nucleus; Beta Decay - Electron Emission: Certain nuclei with

an excess of neutrons may reach stability by converting a neutron into a proton with the emission of a beta-minus particle;

A beta-minus particle is an electron; 21/04/234th lecture RAD 311


28

Beta Decay - Positron Emission: When the number of protons in a nucleus is in excess, the nucleus may reach stability by converting a proton into a neutron with the emission of a beta-plus particle;

A beta-plus particle is a positron; Positrons annihilate with electrons to produce

two back-to-back gamma-rays; Beta Decay - Electron Capture: An inner

orbital electron is attracted into the nucleus where it combines with a proton to form a neutron;



29

Electron capture is also known as K-capture; Following electron capture, the excited nucleus

may give off some gamma-rays. In addition, as the vacant electron site is filled, an X-ray is emitted;

Gamma Decay - Isomeric Transition: A nucleus in an excited state may reach its ground state by the emission of a gamma-ray;

A gamma-ray is an electromagnetic photon of high energy;

Gamma Decay - Internal Conversion: the excitation energy of an excited nucleus is given to an atomic electron.


Dr. Mohammed Alnafea [email protected] Methods of Radioactive Decay.

Documents

lecture rad

beta decay

positron decay

radionuclide decay modes

methods of radioactive

alpha decayalpha decay

medical imaging decay

daughter nucleus