This work should be preceded by two things. First,
students should have an opportunity to explore electric forces on a
macroscopic scale by using pieces of plastic that obtain an electric
charge when rubbed. Second, students should be introduced to the main
concepts in the text below in a lecture-style or class discussion format.
Most importantly, they should be shown a full table of the isotopes: one
is available from Wikipedia.
Here are some links with visuals and charts for various concepts that need
to be covered in lecture/discussion: Nuclear Notation and Forces Fundamental Forces Neutron Excess Table of Isotopes Diagram
Chapter Twelve, The Nuclear Age: Energy, Medicine, Weapons, and Terrorism
Case Study: Elimination by Irradiation
Who was Alexander Litvinenko? He was a former colonel in the Soviet Union’s secret police (the KGB). While still living in Russia he made accusations against his superiors by saying that they had ordered agents to kill a prominent Russian citizen. He fled to the UK, where he received political asylum and continued to make allegations against the Russian secret police.
What did Mr. Litvinenko accuse his government of doing? He accused the government of staging bombings in which Russian civilians were killed in order to justify a war in Chechnya. Also he accused the government of colluding with organized crime, smuggling illegal drugs, and killing critics and enemies of President Putin.
How was Mr. Litvinenko killed? Apparently, he was killed by radiation poisoning which was traced back to a Nov. 1, 2006 meeting at a London hotel with two men. He died only 22 days later.
Section 12.1 The Discovery of Natural Radioactivity
What was Henri Becquerel actually trying to investigate when he accidentally discovered natural radioactivity? He was trying to establish that phosphorescence (spontaneous emission of light after exposure to light) involved not just the emission of visible light but also the newly discovered x-rays.
How did he go about his experiments? He placed a phosphorescent mineral on a photographic plate, which was wrapped in black paper. The black paper prevented exposure to ordinary light but allowed x-rays to pass through it.
How did he accidentally discover radioactivity? He placed the mineral on a photographic plate in a dark drawer because there was no sunlight to activate the phosphorescence for a few days due to cloudy weather. When he developed the plate anyway, he found that it still showed signs of exposure to x-rays, even though—as far as he knew—this was’t possible. Eventually, he traced this activity to the uranium in the phosphorescent mineral.
Who was Marie Curie, née Sklodowska and what did she do? She was an early researcher into radioactive phenomena and in fact was responsible for coining the word radioactivity. She also discovered the radioactive elements polonium (named for her native Poland) and radium. She won two Nobel prizes: one for physics and one for chemistry. Element 96 is named in her honor: curium (Cm).
Section 12.2 Radiation Types and Hazards
What is the danger of radioactivity? Radioactivity is the emission of rays of highly energetic light or charged particles. When these rays or particles strike molecules in the body they can cause damage. This damage is caused by breaking molecules into non-functional pieces and—more importantly—by ionizing them. Ionization means that the molecules become positively charged due to the loss of electrons. Charged and broken molecules do not function normally and can actually spread the damage to other molecules as they come into contact with them. High levels of exposure to ionizing radiation is linked to severe illness and death.
What is background radiation? Background radiation refers to the fact that even in the absence of human activity we are all exposed to some level of ionizing radiation at all times. Most of this exposure comes as a result of the noble gas radon (Rn) which seeps up from bedrock due to the radioactive decay of trace amounts of radioactive uranium. Other sources include trace amounts of radioactive elements in minerals, medical scans (x-rays), cosmic radiation (from the sun, especially), and even radioactive elements naturally present in human bodies.
What is it that determines whether or not a given atomic nucleus is stable or radioactive? Atomic nuclei have extremely tiny volumes and all the positive charges in an atom are packed into it. The positively charged protons repel each other powerfully. Neutrons, on the other hand, are not subject to this repulsion and instead act to hold the nucleus together. In order for a nucleus to be stable it has to have the right ratio of neutrons to protons.
How does the right ratio of neutrons to protons change as larger and larger nuclei are considered? For the first twenty or so elements (up to Ca with 20 protons) the ratio is about 1:1 (n0/p+). After that the ratio slowly increases to about a 1.5:1 ratio. Radioactive isotopes have a n0/p+ ratio outside of the belt of stability which represents those elements that are stable. (It is extremely helpful to look at and carefully consider the illustration at the bottom of pg. 363 (fig. 2.12) which shows the belt of stability and demonstrates the change in the n0/p+ ratio with increasing atomic number. Another version of the same diagram, which also shows natural and man-made radioactive isotopes, is available on the Hyperphysics site: Neutron Excess Table of Isotopes Diagram.
When a radioactive isotope decays does it move closer to or farther away from the belt of stability on the n0/p+ graph? It moves closer to the belt of stability. If it is still not yet stable then it decays again, moving still closer until it is finally a stable isotope.
Are there any elements that have no stable isotopes? Where are they found on the periodic table? Yes. The elements technetium (43Tc) and Promethium (61Pm) have no stable isotopes. Also, no elements with an atomic number greater than 83 have any stable isotopes. Most of these elements have no well-defined average atomic mass and the mass shown for each of them on the periodic table is the mass of the longest-lived isotope. However, Thorium (90Th), Protactinium (91Pa) and Uranium (92U) all have true average atomic masses because a number of isotopes of each of them have extremely long half-lives and so have relatively stable natural abundances in the Earth’s crust.
What are the three basic kinds of radiation? They are alpha particles (α or 42He), beta particles (β or 0 -1e-), and gamma rays (γ or 0 0γ).
What does ionizing power mean? Ionizing power means the strength of a given type of radiation to cause molecules and atoms it strikes to become ionized—that is, to lose one or more electrons.
What is penetrating power? Penetrating power describes the ability of radiation to penetrate into materials. The more penetrating power the radiation has, the more difficult it is to create physical shielding against it to block it completely.
What are alpha particles? Alpha particles are atomic nuclei with 2 protons and 2 neutrons; in fact, they are identical with helium-4 nuclei. It seems likely that most of the helium found on Earth was produced by the alpha decay of radioactive elements in the Earth’s crust. When an atom emits an alpha particle it reduces its atomic mass number by 4 units and its atomic number by 2 units. This happens because the nucleus undergoing this type of decay was too large for the Strong Nuclear Force’s limited range and needed to be smaller in order to be stable. For this reason, alpha decay is most common for radioactive isotopes with atomic numbers greater than 83. Picture the isotope moving down two units and to the left two units on the n0/p+ graph.
What are the penetrating and ionizing powers of alpha particles? The penetrating power of alpha particles is low and they can be blocked by a piece of paper or even by clothing. The ionizing power of alpha particles is, on the other hand, very large. They are relatively massive particles moving at high speeds and each one has a +2 ionic charge (no electrons accompany the helium-4 nucleus on its trip out of the parent radioactive isotope). For this reason, although alpha-emitting isotopes are of relatively low danger outside the body they can be particularly deadly within the body.
What are beta particles? Beta particles come in two varieties: beta-minus and beta-plus. The textbook only describes beta-minus particles, which are actually electrons. In a beta-minus decay the nucleus has a n0/p+ ratio that is too high (there are too many neutrons relative to the number of protons for the isotope to be stable). The nucleus emits an electron and in the process a neutron in the nucleus becomes a proton. The atomic mass number stays the same but the atomic number increases by one. In this way the isotope moves down one unit and to the right one unit on the n0/p+ graph, closer to the belt of stability.
Beta-plus decay involves nuclei which have too many protons relative to the number of neutrons (the n0/p+ ratio is too low). The nucleus emits a positron—the antimatter partner of an electron, which has the same mass but a positive charge. The emission of a positron (symbol: 0 +1β+ or 0 +1e+) causes a proton in the nucleus to become a neutron. The atomic mass number stays the same but the atomic number decreases by one. In this way the isotope moves down one unit and to the left one unit on the n0/p+ graph, closer to the belt of stability. This same transformation can occur, often accompanied by gamma rays (see below) when a nucleus absorbs an electron, changing a proton into a neutron. This alternate pathway to the same transformation is called electron capture. Aside: all of these processes are governed by the Weak Nuclear Force.
What are the penetrating and ionizing powers of beta particles? The penetrating power of beta particles is higher than that of alpha particles and they require thicker shielding to block. Several millimeters’ thickness of metal is required, although larger thicknesses of other materials will do as well. The ionizing power of beta particles is less than that of alpha particles. They are less massive particles and their -1 (beta-minus) or +1 (beta-plus) is not as great. Whether inside or outside the body, beta particles pose a serious hazard.
What are gamma rays? Gamma rays are photons of enormous energy. Gamma rays are only produced by processes within atomic nuclei and they are the most energetic form of electromagnetic radiation (light). As such, they have no mass and no charge. They can be produced alongside other radiation in nuclei that decay but continue to have an excess amount of energy. This excess energy is released in the form of gamma rays much as excess energy is released as longer wavelengths of light when an electron moves from a higher energy level to a lower energy level.
What are the penetrating and ionizing powers of gamma rays? The penetrating power of gamma rays is very high because they are small and moving at the speed of light. To shield an area completely from gamma rays requires several centimeters of lead or thick concrete walls. The ionizing power of gamma rays is relatively low due to their small size and lack of charge. Whether inside or outside the body, gamma rays pose a serious hazard.
Section 12.3 Balancing Nuclear Equations
In equations written to describe chemical reactions the number of atoms of each element must be the same on both sides of the equation due to the Law of Conservation of Matter. How are nuclear equation different and do they involve a violation of the Law of Conservation of Matter? Nuclear equations involve atoms changing their identity from one element to another so they do not have the same number of atoms of each element on both sides of the equation. Instead, the total of all mass numbers must be the same on both sides and the total of all atomic numbers must be the same on both sides. Only in nuclear equations are we allowed to change the element! The Law of Conservation of Matter is not violated because although the atoms have changed their identity, the amount of mass is still the same before and after the change.
What values must be balanced on both sides of a nuclear equation? The mass numbers on both sides must sum to the same amount. Also, the atomic numbers on both sides must sum to the same amount.
Section 12.4 Half-lives and Risk Assessment
What are the symptoms of severe radiation sickness? A person afflicted with radiation sickness as a result of exposure to a large dose of radiation will suffer nausea, diarrhea, skin burns, hair loss, and bleeding from the mouth, nose, and gums. Higher doses lead to more severe symptoms.
What is does half-life mean? The half-life of a substance is the time it takes for the concentration of that substance to decrease to half of its original value. Radioactive isotopes each have their own characteristic half-life which measures how long it takes for half of a sample of that isotope to undergo radioactive decay.
How does the half-life of a radioactive isotope affect the level of hazard that it poses? For example, all other things being equal, which type of radioisotope is more dangerous: one with a short half-life or one with a long half-life? The variables that need to be kept equal between the two are the time someone is exposed, the type of radiation, whether the source is inside or outside the body, and the dose. All other things being equal, the radioisotope with the longest half-life is the less dangerous. This is because, for a given amount of time, the radioisotope with the shorter half-life will undergo more radioactive decays than the one with the longer half-life.
How does the half-life of a radioisotope determine the best method for disposing of it? For very short-lived isotopes a carefully monitored landfill is acceptable since they will quickly become harmless. For long-lived radioisotopes disposal is more problematic since they can have half-lives from 710 million years (uranium-235) to 4.5 billion years (uranium-238).
Section 12.5 Medical Applications of Nuclear Isotopes
What two roles do radioisotopes play in medicine? They can be used to diagnose and they can be used to treat cancers by exposing tumors to radiation.
What type of radiation is best suited to diagnosing problems? Gamma rays are particularly useful in diagnosis because the isotope can be taken internally and images can be made of how they are absorbed and manipulated by the body in the same way x-rays are made. The gamma rays travel easily out of the body and can strike detectors in order to make an image. Different isotopes are used depending on the body system to be investigated. Iodine-131 is used to image the activity of the thyroid and cobalt-60 is used (as part of specially-made vitamin B12 molecules) to investigate the ability of the intestines to absorb vitamin B12.
How is radiation used to destroy tumors? Gamma rays can be applied from outside the body. Alpha- and beta-emitters must be implanted within the body in order for the ionizing radiation to kill tumor cells.
Why do radioisotopes used for imaging have short half-lives? Radioisotopes used for imaging need only be present in the body for short periods of time. Also, for the patient’s health a shorter exposure to ionizing radiation is desirable.
Section 12.6 Forensic Applications of Carbon-14
How does radioactive carbon-14 come to be present in living organisms? Cosmic radiation dislodges neutrons from atoms in the upper atmosphere. These neutrons strike nuclei of nitrogen-14 and convert them to carbon-14 nuclei. (147N + 10n0 --> 146C + 11p+) These react with oxygen to form carbon dioxide, which is incorporated into plant tissues via ordinary photosynthesis. Animals eat plants and so also have carbon-14 in their tissues.
How is carbon-14 used to find out the age of things? The amount of carbon-14 stays roughly constant while an organism lives but when it dies it is no longer renewed as the carbon-14 decays by beta-minus decay. Comparing the amount present to the amount that would be present in a living organism allows a carbon-14 date to be calculated. The half-life of carbon-14 is 5,730 years.
What are the security risks with respect to nuclear power plants? First, a direct attack on a nuclear power plant could disrupt the power grid and lead to the release of radioactive materials into the environment. These materials would be dispersed both by the air and by water since nuclear power plants must be sited next to a natural source of water such as a river or lake. Second, nuclear power plants always store fuel and reactor waste on site. If terrorists obtained these materials they could potentially use them to harm others.
What is nuclear fission? Nuclear fission is a nuclear reaction in which a large nucleus splits into smaller pieces. An example is the fission of uranium-235 after it is struck by a neutron. One set of particles that results from the subsequent fission of the (now) uranium-236 atom includes 3 neutrons, 1 nucleus of barium-142 and 1 nucleus of krypton-91.
See the text book pp. 376 - 377 for a discussion of how fission-reactor nuclear power plants generate electricity.
Is fusion power likely to soon eliminate our energy worries? No, fusion requires temperatures above 10 million degrees Celsius. No physical container can hold materials at this temperature and magnetic ‘bottles’ are still not good enough to maintain a fusion reaction for long periods of time.
Section 12.8 Military Uses of Nuclear Isotopes
What are the abundances of the naturally-occurring isotopes of uranium? 238U—99.2754%, 235U—0.720%, and 234U—0.0055%.
Only 235U will undergo nuclear fission so natural uranium metal must be enriched in this isotope to be useful for fueling nuclear power plants or to be used to build bombs. What levels must 235U be enriched to for the various purposes to which it is put? For nuclear power generation the 235U has to have an abundance of 3 - 5 %. For a crude nuclear weapon, a 20% enrichment is sufficient. To make modern, high-yield nuclear weapons requires uranium enriched to 85% 235U. Equipment used to enrich uranium for use in nuclear power plants could also be used to enrich uranium for weapons, hence the diplomatic troubles over Iran’s uranium enrichment program.
See the discussion in the text on pp 379 - 380 about how nuclear weapons work and the controversy regarding the use of depleted uranium to make shells for use in anti-tank weapons.
See also: PhET Simulation for Nuclear Fission, which has a tab allowing scenarios to be created that mimic the critical and super-critical conditions in a nuclear reactor or nuclear bomb.
Section 12.9 Nuclear Transmutations
Is natural radioactive decay the only way that atoms can be changed from one element to another? No, nuclear transmutation reaction can be carried out which can produce isotopes and elements different from the starting material.
How are elements beyond uranium made? They are made in nuclear reactors in which atoms are bombarded by other atomic nuclei or by neutrons. (Worked Example 6 on pg 381 has some worthwhile example equations).
Section 12.10 Nuclear Terrorism
Does a dirty bomb involve the detonation of a nuclear device? No, a dirty bomb is a conventional explosive which contains within it some radioactive material. The bomb disperses the radioactive material and exposes nearby people to the radiation produced by those materials.
Why is the security of nuclear power stations so important for preventing the building of dirty bombs? It is important because if persons intending to do harm have access to the materials at nuclear power stations (or other sites which house radioactive materials) then they could take them and use them to build a dirty bomb.
Other interesting issues and topics for discussion are brought up on pg 382 regarding how to respond to a dirty bomb detonation and how to prepare for one.
Section 12.11 Case Study Finale: Elimination by Irradiation
How was polonium-210, an alpha-emitter—and therefore a radiological source of low penetrating power—able to kill Alexander Litvinenko? The poison was inside his body and so the high ionizing power of alpha particles were brought into direct contact with Mr. Litvinenko’s cells.
Is it possible that Mr. Litvinenko was poisoned accidentally by polonium-210 from the environment? No, it is too rare. Only 100 grams of it are made each year, mostly in Russia nuclear reactors. Also, traces of it were found in British Airways planes that travel between London and Moscow.
Was the prime suspect, former KGB officer Andrei Lugovoi, ever held responsible? No, the Russian government protected him from foreign prosecution by refusing to extradite him. The Russian Prosecutor General never pursued the case, even though the evidence provided was sufficient for an indictment.