Radioactivity Prepared by: Timothy John D. Matoy

RADIOACTIVITY Process of loosing energy to reach a stable state. -This excited state can last for a few moments to billion of years and is measured in terms of half-lives. -This lose of energy can be in form of pure energy, particles, or both. -To maintain nuclear stability as the atomic number of an atom goes up to the number of neutrons must rise faster than the number of neutrons. -radiocnuclide -The highest stable atomic number is bismuth at 83, all higher elements are radioactive (any element with more than 83 protons will be radioactive.)

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 Radioactivity is the emission of particles and energy in order to become stable. Radioactivity decay results in emission of Alpha particles, beta particles and gamma rays. Radioisotopes. – beta and alpha emission are important Half-life – the time required for a quantity of radioactivity to be reduced to one-half its original value. 1Bq – 1 per second – the number of decaying nuclei per second 1 Cu = 3.7 x 10 10 process of losing energy to reach a stable state This excited state can last for a few moments to billion of years and is measured in terms of half-lives

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. - The blackening of the photographic plate was due not to light but to a radiation being emitted by the uranium mineral.

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 10 MV produce neutrons 4 MV – 23 MV therapeutic Slow electrons- High Z materials must be avoided because they produce secondary radiation

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 http://medical-dictionary.thefreedictionary.com/physical+half-life The amount of time that is required to reduce the radioactivity to ½ of its present value. Radon gas – 3.83 days Associated dose rate is also reduce to ½ Gold-198 - 2.7 days

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 Americium 241 – smoke detectors If substances emitting alpha particles are ingested, inhaled, injected or introduced through the skin, then it could result in a measurable dose. accompanied by gamma photon emission. Russian dissident Alexander Litvinenko's 2006 murder by radiation poisoning is thought to have been carried out with polonium-210, an alpha emitter.

ALPHA DECAY

BETA DECAY

Beta minus Beta plus BETA DECAY 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. http://en.wikipedia.org/wiki/Electron_capture Emission of 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. Statistical nature of disintegration was established.

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

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? N = 12.14 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? 2.877 Ci

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? 0.04

Where n = time/ half life 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? N = 12.14 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 its activity fraction after 55 years? 2.877 Ci

Nf = (AF) Ni Where Nf = final activity, Ni = initial activity 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? N = 12.14 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? 2.877 Ci

Radioactive Decay Law Activity Remaining = original activity (0.5)n Where n – number of half-lives

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? N = 12.14 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? 2.877 Ci

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?