RadioactivityRadioactivityChapter 21Chapter 21

Natural occurring phenomena.Natural occurring phenomena. In the nucleus of an atom there are In the nucleus of an atom there are

protons and neutrons.protons and neutrons. Protons are positively charged so they Protons are positively charged so they

naturally want to repel each other.naturally want to repel each other. A strong nuclear force keeps them inside.A strong nuclear force keeps them inside. Neutrons have no charge – so they are Neutrons have no charge – so they are

neutral and help cushion the repulsive force neutral and help cushion the repulsive force the protons feel.the protons feel.• “ “the glue that holds the nucleus together”the glue that holds the nucleus together”

InstabilityInstability

Once the nucleus has 20 protons it needs Once the nucleus has 20 protons it needs more neutrons than protons to keep the more neutrons than protons to keep the nucleus together.nucleus together.

And once you get to a certain number of And once you get to a certain number of protons there is no number of neutrons protons there is no number of neutrons sufficient to keep the nucleus stable.sufficient to keep the nucleus stable.

Once the nucleus becomes unstable it Once the nucleus becomes unstable it wants to break down or decay.wants to break down or decay.

Band of StabilityBand of Stability The ratio between The ratio between

neutrons and neutrons and protons determine protons determine the stability of a the stability of a nucleus.nucleus.

If the number is too If the number is too large or too small the large or too small the nucleus is unstable.nucleus is unstable.

No atoms that have No atoms that have an atomic numbers an atomic numbers larger than 83 and a larger than 83 and a mass number larger mass number larger than 209 are stable.than 209 are stable.

Chemical SymbolChemical Symbol

U = UraniumU = Uranium Mass number = Mass number =

protons and neutronsprotons and neutrons Atomic number = Atomic number =

number of protonsnumber of protons

3 Main Types of Radiation3 Main Types of Radiation

Nuclear EquationsNuclear Equations

Alpha DecayAlpha Decay1.1. Alpha emission(greek symbol alpha - Alpha emission(greek symbol alpha - αα ))

• emission of a nucleus with a mass number emission of a nucleus with a mass number of 4 and atomic number of 2. of 4 and atomic number of 2.

• can be stopped by a few sheets of paper.can be stopped by a few sheets of paper. Example:Example:

Beta DecayBeta Decay

2.2. Beta emission (greek symbol Beta emission (greek symbol ββ ) ) emission of a high speed electron. emission of a high speed electron. can be stopped by wood or heavy protecting can be stopped by wood or heavy protecting

covering.covering. Example:Example:

Gamma DecayGamma Decay

3.3. Gamma emission(greek symbol Gamma emission(greek symbol γγ ) ) • emits a high energy non-particle radiation.emits a high energy non-particle radiation.• need several inches of lead to block need several inches of lead to block

Gamma.Gamma.

Example:Example:

Other Types of Particles…Other Types of Particles…

Particles in nuclear equations:Particles in nuclear equations: ProtonProton Positron Positron NeutronNeutron

Nuclear BombardmentNuclear Bombardment

When atoms are bombarded by other When atoms are bombarded by other particles.particles.

Usually take place in a particle Usually take place in a particle accelerator.accelerator.

Overview of Radioactive Overview of Radioactive EmissionEmission

Radiation ExposureRadiation Exposure

Radiation is measure by the non SI unit R Radiation is measure by the non SI unit R (roentgen equivalent for man).(roentgen equivalent for man).

Average dose received annually=120 RAverage dose received annually=120 R 70% of radiation comes from natural 70% of radiation comes from natural

resources such as minerals and cosmic resources such as minerals and cosmic rays. About 30% come from medical rays. About 30% come from medical procedures like x-rays.procedures like x-rays.

Fission vs. FusionFission vs. Fusion

FissionFission reactions - a reactions - a very heavy nucleus, very heavy nucleus, spontaneously or spontaneously or after absorbing after absorbing additional light additional light particles (usually particles (usually neutrons), splits into neutrons), splits into two or sometimes two or sometimes three pieces. (α decay three pieces. (α decay is not usually called is not usually called fission.) fission.) Occurs only with a Occurs only with a

critical mass.critical mass.

FusionFusion reactions - two reactions - two light nuclei join to light nuclei join to form a heavier one, form a heavier one, with additional with additional particles (usually particles (usually protons or neutrons) protons or neutrons) thrown off to conserve thrown off to conserve momentum. momentum.

2H + 3H → 4He + n

FissionFission

FusionFusion

Uses of RadiationUses of Radiation

Nuclear PowerplantsNuclear Powerplants Age DatingAge Dating Radioactive TracersRadioactive Tracers Cancer TreatmentCancer Treatment SterilizingSterilizing Smoke DetectorsSmoke Detectors Genetic StudiesGenetic Studies

Radioactive DatingRadioactive Dating

Animals and plants have a known proportion of Animals and plants have a known proportion of Carbon-14 (a radioisotope of Carbon) in their Carbon-14 (a radioisotope of Carbon) in their tissues.tissues.

When they die they stop taking Carbon in, then When they die they stop taking Carbon in, then the amount of Carbon-14 goes down at a known the amount of Carbon-14 goes down at a known rate rate (Carbon-14 has a half-life of 5700 years). (Carbon-14 has a half-life of 5700 years).

The age of the ancient organic materials can be The age of the ancient organic materials can be found by measuring the amount of Carbon-14 found by measuring the amount of Carbon-14 that is left. that is left.

Radioactive TracersRadioactive Tracers

The most common tracer is called Technetium-99 and is The most common tracer is called Technetium-99 and is very safe because it only emits gamma rays and doesn't very safe because it only emits gamma rays and doesn't cause much ionisation.cause much ionisation.

Radioisotopes can be used for medical purposes, such Radioisotopes can be used for medical purposes, such as checking for a blocked kidney. as checking for a blocked kidney. To do this a small amount of Iodine-123 is injected into To do this a small amount of Iodine-123 is injected into the patient, after 5 minutes 2 Geiger counters are placed the patient, after 5 minutes 2 Geiger counters are placed over the kidneys. over the kidneys.

Also radioisotopes are used in industry, to detect leaking Also radioisotopes are used in industry, to detect leaking pipes. To do this, a small amount is injected into the pipes. To do this, a small amount is injected into the pipe. It is then detected with a GM counter above pipe. It is then detected with a GM counter above ground. ground.

Cancer TreatmentCancer Treatment Because Gamma rays can kill living cells, they are used Because Gamma rays can kill living cells, they are used

to kill cancer cells without having to resort to difficult to kill cancer cells without having to resort to difficult surgery. This is called "surgery. This is called "RadiotherapyRadiotherapy", and works ", and works because cancer cells can't repair themselves when because cancer cells can't repair themselves when damaged by gamma rays, as healthy cells can.damaged by gamma rays, as healthy cells can.

It's vital to get the dose correct - too much and you'll It's vital to get the dose correct - too much and you'll damage too many healthy cells, too little and you won't damage too many healthy cells, too little and you won't stop the cancer from spreading in time. stop the cancer from spreading in time.

Some cancers are easier to treat with radiotherapy than Some cancers are easier to treat with radiotherapy than others - it's not too difficult to aim gamma rays at a others - it's not too difficult to aim gamma rays at a breast tumor, but for lung cancer it's much harder to breast tumor, but for lung cancer it's much harder to avoid damaging healthy cells. Also, lungs are more avoid damaging healthy cells. Also, lungs are more easily damaged by gamma rays, therefore other easily damaged by gamma rays, therefore other treatments may be used.treatments may be used.

SterilizingSterilizing

Even after it has been packaged, gamma Even after it has been packaged, gamma rays can be used to kill bacteria, mould rays can be used to kill bacteria, mould and insects in food. This process prolongs and insects in food. This process prolongs the shelf-life of the food, but sometimes the shelf-life of the food, but sometimes changes the taste. changes the taste.

Gamma rays are also used to sterilize Gamma rays are also used to sterilize hospital equipment, especially plastic hospital equipment, especially plastic syringes that would be damaged if heated. syringes that would be damaged if heated.

Smoke DetectorsSmoke Detectors

Smoke alarms contain a weak source Smoke alarms contain a weak source made of Americium-241.made of Americium-241.Alpha particles are emitted from here, Alpha particles are emitted from here, which ionize the air, so that the air which ionize the air, so that the air conducts electricity and a small current conducts electricity and a small current flows. flows. If smoke enters the alarm, this absorbs the If smoke enters the alarm, this absorbs the a particles, the current reduces, and the a particles, the current reduces, and the alarm sounds. alarm sounds. Am-241 has a half-life of 460 years. Am-241 has a half-life of 460 years.

Half-lifeHalf-life

The half-life of a substance is the time it The half-life of a substance is the time it takes for a radioactive sample to decay takes for a radioactive sample to decay into half.into half.