Radioactivity

Prepared by: Timothy John D. Matoy

RADIOACTIVITYProcess of loosing energy to

reach a stable state.

Radioactivity and Radioactive Decay

The nuclei of some nuclides are not stable

They disintegrate or undergo nuclear transformation spontaneously and in random process called radioactivity

Henri Becquerel (1852-1908) Discover the radioactivity in

1896 He noted that a piece of

mineral containing uranium when placed over an exposed photographic plate just as if it has been exposed to light.

Marie Curie (1867-1924) Pierre and Marie

Curie discovered that polonium and radium also emit radiation.

Artificial Radioactivity Radioactivity produced by man

Irene Curie-Joliot produce the first radioactive product when they bombarded aluminum with alpha particles from polonium source to study the emitted neutrons and positrons.

Radioactive elements emitted into 3 types of radiation

Alpha Particles Beta Particles Gamma Rays

Alpha Particles Fast moving helium nuclei; positive

electrical charge

Beta Particles

Negative electrical charged electrons.

Electrons, Positrons Very light; in tissue do not travel in straight

lines but are deflected by coulombic repulsions from atomic orbital electrons

Lose an average 50% of their energy in interaction

Gamma Rays Electromagnetic waves of very short

wavelength and travelling within the speed of light.

No charge at all.

Protons Generated by cyclotron beams Because they are heavier than

electrons, travel mainly in straight line by boring a path through atomic clouds

Medium Z materials used for shielding

Half-Life(t½) the time in which a radioactive

substance will lose half of its activity through disintegration. Physical Half-life Biological Half-life Effective Half-Life

Physical Half-Life the average time required for the

decay of half the atoms in a given amount of a radioactive substance.

Biologic Half-Life the time in which a living tissue,

organ, or individual eliminates, through biologic processes, half of a given amount of a substance that has been introduced into it.

Effective Half-Life the half-life of a radioactive isotope in

a biologic organism, resulting from the combination of radioactive decay and biologic elimination.

Types of Decay Alpha Decay Beta Negative Decay Beta Positive Decay Gamma Ray Emission Electron Capture

ALPHA DECAY

ALPHA DECAY

BETA DECAY

BETA DECAY Beta minus

Electron antineutrino Interacts with neutron

Beta plus Electron Neutrino Interacts with protons

GAMMA RAY EMISSION

ELECTRON CAPTURE

ELECTRON CAPTURE

Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron (changing a nuclear proton to a neutron) and simultaneously emits a neutrino.

Radioactive Decay Law Elster and Geitel observed

that the strength of a pure radioactive substance decrease exponentially.

Radioactivity was found to be a property of the individual atoms, not of a substance as a whole.

Unit of Radioactivity Curie (Ci) Becquerel (Bq) 1 Bq = 1 disintegration per second 1 Ci = 3.7x1010 Bq

Exponential decay law expressed in the following equation:

N = N0e-λt or A = A0e-λt

Where: A = present activity

A0 = original activity

λ = disintegration constant/ decay constant t = elapsed time

The half life for radioactive radon gas is 3.83 days, what will be the present activity of the radon gas after 5 days if the initial activity is 30 mCi?

Sample Problem

Practice Problem A Cesium-138 radioactive source

has a half life of 30 years. If the initial activity is 10.25 Ci. What will be the present activity after 30 years?

Practice Problem A certain radioactive source has a

present activity of 2.1626 mCi after 47.5 days. If the initial activity is 15 mCi. Find the decay constant?

Activity Fraction the fraction of the activity that is remaining

after a given amount of time.

AF = 2 –n

Where n = time/ half life

The half life for radioactive radon gas is 3.83 days, what will be the its activity fraction after 5 day?

Sample Problem

Practice Problem A Cesium-138 radioactive source

has a half life of 30 years. If the initial activity is 10.25 Ci. What will be its activity fraction after 55 years?

With the activity fraction, we can write a single equation to solve for the final activity if we are given the initial activity

Nf = (AF) Ni

Where Nf = final activity,

Ni = initial activity

The half life for radioactive radon gas is 3.83 days, what will be the present activity of the radon gas after 5 days if the initial activity is 30 mCi?

Sample Problem

Practice Problem A Cesium-138 radioactive source

has a half life of 30 years. If the initial activity is 10.25 Ci. What will be the present activity?

Radioactive Decay LawActivity Remaining = original activity (0.5)n

Where n – number of half-lives

The half life for radioactive radon gas is 3.83 days, what will be the present activity of the radon gas after 5 days if the initial activity is 30 mCi?

Sample Problem

Practice Problem A Cesium-138 radioactive source

has a half life of 30 years. If the initial activity is 10.25 Ci. What will be the activity after 30 years?

PRACTICE PROBLEM

A certain radioactive source has an initial activity of 10.25 Ci. After 30 years only 5.125 Ci of activity remains. What is its half-life?

Practice problem

Technetium 99m with a half-life of 6 hours was left in the laboratory at 6 o’clock in the morning. If the original activity is 100 mCi, when do the radioactive substance will have a 6.25% of its original value? What will be its activity at that time?