Science 10 Chapter 7 - Radiation Chapter 7, Radiation and Nuclear Energy 7.1 ELECTROMAGNETIC RADIATION Radiation -Energy in the form of electromagnetic waves Electromagnetic wave -Composed of both a varying electric field and a varying magnetic field. The two fields run at a right angle to each other Electromagnetic Spectrum -Is a name that scientists give a bunch of types of radiation when they want to talk about them as a group. Lists the types of radiation from low frequency to high frequency. 1 Science 10 Chapter 7 - Radiation Wave length -The length of 1 wave, the higher the frequency the shorter the wavelength A radio’s wavelength is approx. 1m in length A gamma ray’s wavelength is 0.000 000 000 000 001 m in length! Electromagnetic radiation Can be described in terms of a stream of photons, which are particles each traveling in a wavelike pattern and moving at the speed of light = 299 792 458 m / s. Each photon contains a certain amount of energy, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and ... the most energetic of all ... gamma-rays. 2 Science 10 Chapter 7 - Radiation 7.2 RADIATION IN ATOMS Radioactive -Any atom that emits radiation from their nuclei -Three types of radiation are emitted from a nuclei - alpha, beta and gamma Alpha: composed of 2 protons and 2 neutrons and have a charge of +2 - Are helium nuclei Beta: high energy electrons and have a charge of -1 Gamma: a form of electromagnetic radiation and are not made of particles - can penetrate very deep into objects Isotopes Review – atomic number = protons and electrons - atomic mass minus atomic number = neutrons -Atoms of the same element that have different mass numbers, due to the number of neutrons in the nucleus. Example : Carbon 12 has 6 protons and 6 neutrons Carbon 13 has 6 protons and 7 neutrons Carbon 14 has 6 protons and 8 neutrons Radioactive Decay The unstable nuclei emit radiation and decompose over time Alpha Decay Emit alpha particles Alpha particles are helium nuclei, therefore it emits 2 protons and 2 neutrons Example Uranium Helium and Thorium 238 92 U 4 2 He + 234 90 Th 3 Science 10 Chapter 7 - Radiation Beta Decay Emit high energy electrons Example Carbon electron and Nitrogen 14 6 C 01 e + 14 7 N Carbon will lose 1 electron (e-1) therefore 6 – (-1) = 7 Gamma Decay Does not give off a particle, there is no change to the nucleus Emits excess energy from the nucleus Half-life The time required for half of the nuclei to decay Therefore, after 1 half life, exactly half of the original starting material is left. Example Carbon 14 - half-life = 5730 years Radon 222 – half life = 3.8 days Radon 216 – half life = 0.000045 seconds Half-life The time required for half of the nuclei to decay Therefore, after 1 half life, exactly half of the original starting material is left. Example Carbon 14 - half-life = 5730 years Radon 222 – half life = 3.8 days Radon 216 – half life = 0.000045 seconds 4 Science 10 Chapter 7 - Radiation If you point a Geiger counter at a radioactive substance for a period of time, you'll notice that the reading on the meter decreases as you watch. This is shown on the graph. The radioactivity from some substances dies away very fast - perhaps in a few microseconds. Others take thousands of years before you'll notice that the radioactivity had decreased at all. In theory, every radioactive substance should stay slightly radioactive for ever - the graph should never actually fall to zero. This means that we can't usefully talk about the "life" of a radioactive source. Instead, we use the idea of "half-life". This is the time it takes for the radioactivity to fall by half. This graph shows what would happen with an imaginary radioactive substance. Notice that the radioactivity falls by half every 2 hours. Thus we say that this imaginary substance has a half-life of 2 hours. The count rate coming from a radioactive source depends on how many unstable atoms it contains. That's the number of un-decayed atoms. If the count rate has fallen by half, then the number of unstable atoms has fallen by half. A typical exam question may be "A radioactive substance has a half-life of 2 hours. How much of the substance will remain after 6 hours have passed?" 5 Science 10 Chapter 7 - Radiation Thus, for a substance with a half-life of 2 hours, 1/8 of the original atoms will remain after 6 hours. Note that this also means that 7/8 of the atoms will have decayed in that time. Take another look at the graph above, you'll see that after 6 hours the activity has fallen from 8,000 to 1,000; i.e. it's fallen to 1/8 of the starting value. Question 1: A radioactive substance has a half-life of 30 minutes. What fraction of the atoms will not have decayed after 1 hour? Answer: The half-life of the substance is 30 minutes. That means that 1 hour is 2 half-lives for the substance. After one half-life the activity will fall to 1/2 of the starting value, after a second half-life it will have fallen to half of that - i.e. a quarter. So one quarter of the original atoms remain. 6 Science 10 Question 2: Another substance has a half-life of 3 hours. decayed after 6 hours? Answer: Chapter 7 - Radiation What fraction of the atoms will have The half-life is 3 hours, so 6 hours is 2 halflives. After 2 half-lives only one quarter of the original atoms remain, which means that three-quarters of them have decayed. Question 3: A researcher measures 200 counts per minute coming from a radioactive source at midday. At 3 o'clock, she finds that this has dropped to 25 counts per minute. What is the half-life of the radioactive source? Answer: After 3 hours, the count rate has fallen from 200 to 25 counts per minute Calculate the number of half lives there is from 200 to 25 200 – 100 -50 - 25 so this means that three half lives have passed. If 3 half-lives are 3 hours, then the half-life is one hour. 7 Science 10 Chapter 7 - Radiation Chapter 7, Radiation and Nuclear Energy 7.1 ELECTROMAGNETIC RADIATION Radiation -Energy in the form of electromagnetic waves Electromagnetic wave -Composed of both a varying electric field and a varying magnetic field. The two fields run at a right angle to each other Electromagnetic Spectrum -Is a name that scientists give a bunch of types of radiation when they want to talk about them as a group. Lists the types of radiation from low frequency to high frequency. 8 Science 10 Chapter 7 - Radiation Wave length -The length of 1 wave, the higher the frequency the shorter the wavelength A radio’s wavelength is approx. 1m in length A gamma ray’s wavelength is 0.000 000 000 000 001 m in length! 9 Science 10 Chapter 7 - Radiation Electromagnetic radiation Can be described in terms of a stream of photons, which are particles each traveling in a wave-like pattern and moving at the speed of light = 299 792 458 m / s. Each photon contains a certain amount of energy, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and ... the most energetic of all ... gamma-rays. Electromagnetic radiation (including visible light) travels 149 million kilometers (93 million miles) from the sun to Earth in about 8 minutes. In contrast, an automobile speeding at 100 kilometers per hour (60 miles per hour) would require 177 years to make the same one-way trip. In only one second, light can circumnavigate the Earth seven times. 10 Science 10 Chapter 7 - Radiation 7.2 RADIATION IN ATOMS Radioactive -Any atom that emits radiation from their nuclei -Three types of radiation are emitted from a nuclei - alpha, beta and gamma Alpha: composed of 2 protons and 2 neutrons and have a charge of +2 - Are helium nuclei Beta: high energy electrons and have a charge of -1 Gamma: a form of electromagnetic radiation and are not made of particles - can penetrate very deep into objects Isotopes Review – atomic number = protons and electrons - atomic mass - atomic number = neutrons -Atoms of the same element that have different mass numbers, due to the number of neutrons in the nucleus. Example : Carbon 12 has 6 protons and 6 neutrons Carbon 13 has 6 protons and 7 neutrons Carbon 14 has 6 protons and 8 neutrons 11 Science 10 Chapter 7 - Radiation Radioactive Decay The unstable nuclei emit radiation and decompose over time Alpha Decay - Emit alpha particles - Alpha particles are helium nuclei, therefore it emits 2 protons and 2 neutrons Example; Uranium Helium and Thorium 238 4 2 U 92 He + 234 90 Th Beta Decay Emits high energy electrons Example; Carbon electron and Nitrogen 14 6 C 0 1e + 14 7 N Carbon will lose 1 electron (e-1) therefore 6 – (-1) = 7 Gamma Decay Does not give off a particle, there is no change to the nucleus Emits excess energy from the nucleus 12 Science 10 Chapter 7 - Radiation Half-life - The time required for half of the nuclei to decay - Therefore, after 1 half life, exactly half of the original starting material is left. Example Carbon 14 - half-life = 5730 years Radon 222 – half life = 3.8 days Radon 216 – half life = 0.000045 seconds Half-life The time required for half of the nuclei to decay Therefore, after 1 half life, exactly half of the original starting material is left. Example Carbon 14 - half-life = 5730 years Radon 222 – half life = 3.8 days Radon 216 – half life = 0.000045 seconds If you point a Geiger counter at a radioactive substance for a period of time, you'll notice that the reading on the meter decreases as you watch. This is shown on the graph. The radioactivity from some substances dies away very fast - perhaps in a few microseconds. Others take thousands of years before you'll notice that the radioactivity had decreased at all. In theory, every radioactive substance should stay slightly radioactive for ever - the graph should never actually fall to zero. This means that we can't usefully talk about the "life" of a radioactive source. Instead, we use the idea of "half-life". This is the time it takes for the radioactivity to fall by half. 13 Science 10 Chapter 7 - Radiation This graph shows what would happen with an imaginary radioactive substance. Notice that the radioactivity falls by half every 2 hours. Thus we say that this imaginary substance has a half-life of 2 hours. The count rate coming from a radioactive source depends on how many unstable atoms it contains. That's the number of un-decayed atoms. If the count rate has fallen by half, then the number of unstable atoms has fallen by half. A typical exam question may be "A radioactive substance has a half-life of 2 hours. How much of the substance will remain after 6 hours have passed?" Thus, for a substance with a half-life of 2 hours, 1/8 of the original atoms will remain after 6 hours. Note that this also means that 7/8 of the atoms will have decayed in that time. Take another look at the graph above, you'll see that after 6 hours the activity has fallen from 8,000 to 1,000; i.e. it's fallen to 1/8 of the starting value. 14 Science 10 Chapter 7 - Radiation Question 1: A radioactive substance has a half-life of 30 minutes. What fraction of the atoms will not have decayed after 1 hour? Answer: The half-life of the substance is 30 minutes. That means that 1 hour is 2 half-lives for the substance. After one half-life the activity will fall to 1/2 of the starting value, after a second half-life it will have fallen to half of that - i.e. a quarter. So one quarter of the original atoms remain. 15 Science 10 Question 2: Another substance has a half-life of 3 hours. decayed after 6 hours? Answer: Chapter 7 - Radiation What fraction of the atoms will have The half-life is 3 hours, so 6 hours is 2 halflives. After 2 half-lives only one quarter of the original atoms remain, which means that three-quarters of them have decayed. Question 3: A researcher measures 200 counts per minute coming from a radioactive source at midday. At 3 o'clock, she finds that this has dropped to 25 counts per minute. What is the half-life of the radioactive source? Answer: After 3 hours, the count rate has fallen from 200 to 25 counts per minute Calculate the number of half lives there is from 200 to 25 200 – 100 -50 - 25 so this means that three half lives have passed. If 3 half-lives are 3 hours, then the half-life is one hour. 16 Science 10 Chapter 7 - Radiation Chapter 7, Radiation and Nuclear Energy (student handout) 7.1 ELECTROMAGNETIC RADIATION Radiation - ___________________________________________________________________ _____________________________________________________________________________ Electromagnetic wave - ___________________________________________________________________ ___________________________________________________________________________ Electromagnetic Spectrum - ________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ 17 Science 10 Chapter 7 - Radiation Wave length _________________________________________________________________________________ A radio’s wavelength is approx. 1m in length A gamma ray’s wavelength is 0.000 000 000 000 001 m in length! Electromagnetic radiation Can be described in terms of a stream of photons, which are particles each traveling in a wave-like pattern and moving at the speed of light = 299 792 458 m / s. Each photon contains a certain amount of energy, and all electromagnetic radiation consists of these photons. Electromagnetic radiation (including visible light) travels 149 million kilometers (93 million miles) from the sun to Earth in about 8 minutes. In contrast, an automobile speeding at 100 kilometers per hour (60 miles per hour) would require 177 years to make the same one-way trip. In only one second, light can circumnavigate the Earth seven times. The only difference between the various types of electromagnetic radiation is _____________ _________________________________________________________________________________. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and ... the most energetic of all ... gamma-rays. 18 Science 10 Chapter 7 - Radiation 7.2 RADIATION IN ATOMS Radioactive -__________________________________________________________________________ -Three types of radiation are emitted from a nuclei - alpha, beta and gamma Alpha: composed of _________________________________ and have a charge of +2 - Are _______________ nuclei Beta: _________________________________________________________________ Gamma: a form of electromagnetic radiation and are not made of particles - can penetrate very deep into objects Isotopes Review : atomic number = protons and electrons - atomic mass minus atomic number = neutrons -Atoms of the same element that have different mass numbers, due to the number of neutrons in the nucleus. Example : Carbon 12 has 6 protons and 6 neutrons Carbon 13 has 6 protons and 7 neutrons Carbon 14 has 6 protons and 8 neutrons Radioactive Decay The unstable nuclei emit radiation and decompose over time Alpha Decay Emit alpha particles Alpha particles are helium nuclei, therefore it emits 2 protons and 2 neutrons Example 19 Science 10 Chapter 7 - Radiation Beta Decay Emit high energy electrons Example Gamma Decay Does not give off a particle, there is no change to the nucleus _______________________________________________________________ Half-life The time required for half of the nuclei to decay Therefore, after 1 half life, exactly half of the original starting material is left. Example Carbon 14 - half-life = 5730 years Radon 222 – half life = 3.8 days Radon 216 – half life = 0.000045 seconds If you point a Geiger counter at a radioactive substance for a period of time, you'll notice that the reading on the meter decreases as you watch. This is shown on the graph. The radioactivity from some substances dies away very fast - perhaps in a few microseconds. Others take thousands of years before you'll notice that the radioactivity had decreased at all. In theory, every radioactive substance should stay slightly radioactive for ever - the graph should never actually fall to zero. This means that we can't usefully talk about the "life" of a radioactive source. Instead, we use the idea of "half-life". This is the time it takes for the radioactivity to fall by half. 20 Science 10 Chapter 7 - Radiation This graph shows what would happen with an imaginary radioactive substance. Notice that the radioactivity falls by half every 2 hours. Thus we say that this imaginary substance has a half-life of 2 hours. The count rate coming from a radioactive source depends on how many unstable atoms it contains. That's the number of un-decayed atoms. If the count rate has fallen by half, then the number of unstable atoms has fallen by half. A typical exam question may be "A radioactive substance has a half-life of 2 hours. How much of the substance will remain after 6 hours have passed?" Thus, for a substance with a half-life of 2 hours, 1/8 of the original atoms will remain after 6 hours. Note that this also means that 7/8 of the atoms will have decayed in that time. Take another look at the graph above, you'll see that after 6 hours the activity has fallen from 8,000 to 1,000; i.e. it's fallen to 1/8 of the starting value. 21 Science 10 Chapter 7 - Radiation Question 1: A radioactive substance has a half-life of 30 minutes. What fraction of the atoms will not have decayed after 1 hour? Answer: 22 Science 10 Question 2: Another substance has a half-life of 3 hours. decayed after 6 hours? Answer: Chapter 7 - Radiation What fraction of the atoms will have Question 3: A researcher measures 200 counts per minute coming from a radioactive source at midday. At 3 o'clock, she finds that this has dropped to 25 counts per minute. What is the half-life of the radioactive source? Answer: 23