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## Radioactivity

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### Radioactivity

Prepared by: Timothy John D. Matoy

RADIOACTIVITY

- Process 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

BETA DECAY

- Beta minus
- Electron antineutrino
- Interacts with neutron
- Beta plus
- Electron Neutrino
- Interacts with protons

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 = 3.7x1010Bq

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

Sample Problem

- 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?

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

Sample Problem

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

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

Sample Problem

- 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?

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?

Sample Problem

- 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?

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?

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