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Pediatric Terrorism and Disaster Preparedness

Public Health Emergency Preparedness

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Chapter 6. Radiological and Nuclear Terrorism

Radiological Threats: Scope and Implications

Nuclear and radiological weapons pose a significant terrorist threat. In the past, terrorists have attacked discrete locations with explosive materials that are not inherently toxic. However, the tactics and technological sophistication of terrorists are continually evolving. The September 2001 terrorist attacks demonstrated sophistication and planning not previously encountered. Future attacks with radiological devices are a real possibility that is outside the experience of most local emergency and health officials. Radiological terrorism could include detonation of one or more nuclear weapons, deployment of a radiation-producing device or other isotopic weapon (e.g., "dirty bomb"), or simply placement of a radioactive source (e.g., nuclear waste material) in a public location. The probability and nature of injuries depends on the type of disaster involved (Figure 6.1).

Such materials are sought by a variety of terrorist or criminal organizations and have been successfully seized by law enforcement personnel on many occasions. The increasing sophistication and inventiveness exhibited by transnational crime syndicates, drug cartels, anarchists, paramilitary warlords/insurgents and other groups challenge traditional law enforcement approaches. The danger posed by these various illicit organizations blurs the lines between international and domestic threats, as well as between criminal and military threats.

Incident Management

The National Council on Radiation Protection and Measurements (NCRP) suggests that attacks with radioactive materials can result in an area of contamination that is much larger than the immediate scene. Such "weaponized" hazardous materials are not readily amenable to local cleanup and require a paradigm shift in incident management. However, incident management will be difficult until the required and appropriate monitoring equipment is available, along with well-trained technical personnel. Incident response and the forensic investigation are also likely to be complicated by public fear and hysteria, need for personal protective equipment, potential contamination of evidence, and pressure for prompt cleanup and long-term site remediation. Consequently, police, fire, and emergency medical services (EMS) departments are training and equipping themselves to respond appropriately. State and Federal agencies are planning for how they will to coordinate transport and management of contaminated evidence for rapid diagnostic and forensic analysis, while still preserving safety and the evidentiary chain of custody.

There are several key public health and safety considerations in managing radiological incidents. These include the potential for both immediate and long-term health effects, depending on the specific radionuclide(s) and method of dispersal involved. Other concerns include protection of first responders (including forensic investigators) as well as the general public, casualty triage, decontamination, treatment, and management of emotional distress and fear associated with possible exposure to radiation. Key decision points include evacuation versus sheltering in place and potential restrictions on food and water consumption. Initial response capabilities will be limited unless appropriate expertise, specialized equipment, and supplies are readily available.

Nuclear Weapons

The so-called "dirty bomb" disperses radioactive material and is relatively simple to deploy. A nuclear weapon would be much more difficult to deploy. Nevertheless, the potential for detonation of a nuclear weapon in a major city cannot be dismissed. A crude nuclear weapon constructed outside of a national program would likely be limited to a yield <10 kilotons (10,000 tons) of TNT equivalent. Theft or provision of a stockpile weapon from a nuclear nation could provide a more efficient and higher yield device.

The destructive action of nuclear weapons is mainly due to blast and heat, as in conventional explosives. However, there are several important differences between nuclear and conventional explosions:

  • Nuclear explosions are hundreds to millions of times more powerful than conventional explosions.
  • The temperatures attained by nuclear weapons are much higher (tens of millions of degrees versus a few thousand), causing much more of the explosive energy to be emitted as light and heat (thermal radiation). This can result in skin burns and fires at considerable distances from the detonation.
  • Nuclear detonations release tremendous amounts of initial radiation.
  • The various substances remaining after a nuclear explosion are radioactive and may emit radiation for an extended period of time (i.e., residual radioactivity).

Medical providers should be prepared to adequately treat injuries complicated by ionizing radiation exposure and radioactive contamination. Medical facilities in the immediate area will be nearly unusable due to heavy physical damage. Medical facilities in the adjacent areas will be severely compromised by downed power and phone lines; probable loss of all city utilities; and damage to electronics, communications, and HVAC control induced by the electromagnetic pulse produced by a nuclear blast. Patients who need more than basic medical care will require transport to functioning medical facilities well outside the immediate area of destruction.

Radiation from a Nuclear Detonation

A nuclear detonation produces four kinds of ionizing radiation: neutron, gamma, beta, and alpha (go to: Radiation Physics). Neutron and gamma radiation are emitted immediately at detonation, in a ratio that decreases with increasing weapon yield and distance from ground zero. The residual radiation is composed largely of alpha particles, beta particles, and gamma rays. This residual radiation comes from fission products, unfissioned residual nuclear material, and ground-zero materials made radioactive by neutrons from the initial radiation (i.e., neutron-activation).

For most weapons, blast and thermal injuries will outnumber radiation injuries. The types of injuries associated with the initial detonation vary with yield, distance from ground zero, time of day, and other factors. For weapons <10 kilotons (KT), most injuries requiring medical attention are caused by blast and ionizing radiation within a km or so from ground zero. For weapons larger than 10 KT, thermal radiation is the primary cause of injury, as this extends over a greater distance than blast or radiation effects. Flash-blindness and retinal burns may occur out to 20 km during daytime and 50 km at night.

Fallout

Fallout is defined as radioactive material from a nuclear detonation that returns to the ground after the explosion. A detonation close to ground level (i.e., surface burst) will result in large quantities of earth and/or water being thermally vaporized and drawn up into a radioactive cloud. Much of this material may be blasted into the atmosphere and subsequently return to earth as fallout. This material becomes radioactive from either neutron activation or from condensing together with various radioactive isotopes. In highly contaminated areas, fallout can cause potentially lethal external radiation exposure, as well as provide a serious internal hazard from the inhalation or ingestion of radioactive materials, such as milk (Figure 6.2).

A surface burst over the ground produces particles that range in size from submicron to several millimeters in diameter. The finer particles rise into the stratosphere and may be dispersed as worldwide fallout. The larger particles settle to earth within 24 hours as local radioactive fallout. The heaviest fallout usually occurs in the first 4 hours. In contrast, bursts over water are characterized by lighter and smaller particles, producing a smaller volume of fallout that extends over a larger area. These particles consist mostly of sea salts and water, which may produce a "cloud seeding" effect that results in areas of high local fallout as radioactive materials are washed out of the air.

The primary method of fallout protection is initial sheltering, with rapid evacuation from the contaminated area until the risk has been eliminated through decay and/or remediation. In general, it is very difficult to accurately predict the rate of radioactive decay for fallout. Thus, relevant decisions should be based on actual radiological survey data. The 7:10 rule can be used to estimate residual radiation decay after all fallout is on the ground and the dose rate is beginning to measurably decrease.

Radiological Dispersal Devices (Dirty Bombs)

A radiological dispersal device (RDD) is designed to spread radioactive material through detonation of conventional explosives or other (non-nuclear) means. These "dirty bombs" blast radioactive material into the area around the explosion, exposing people and buildings. The purpose of a dirty bomb is to frighten people and make buildings or land unusable for a long period of time.

An RDD is a weapon of "mass disruption" more than a weapon of mass destruction. However, an RDD can produce external contamination, an exposure hazard, and a risk of internal contamination (via inhalation, ingestion, or wounds) if basic safety and hygiene precautions are not followed. An RDD also may pose a radiation injury risk in the event that a strong source (e.g., Cs-137 or Co-60) is kept relatively concentrated. In this situation, the RDD acts as a high-intensity sealed gamma source.

The principal use of an RDD is to cause fear and to disrupt infrastructure. Use of an RDD also tends to generate panic and social and economic disruption from the physical and psychological impacts of the attack. The severity of the psychological effects depends in part on the type of RDD material and the method of deployment. Mass psychosomatic symptoms resulting from an unrealistic fear of the effects of radioactive material could place unnecessary strain on the medical system, through medical screening of large numbers of anxious individuals. This would add to the chaos of identifying truly exposed cases, also mixed in among the existing cases of gastrointestinal, dermatologic, and respiratory illness that are prevalent in any population at baseline.

In the event an RDD is deployed, everything possible should be done to contain panic. It is very important to reassure the public that all necessary steps are being taken to safeguard their health and well being. This allows appropriate authorities to methodically contain and manage the incident in an atmosphere free of public interference. Public perception that appropriate treatment is not being provided, particularly for children and other vulnerable individuals, could markedly increase the psychological impact of the event. Medical authorities also need to manage the flow of casualties through proper layout and use of casualty reception stations, decontamination areas, and patient assessment and triage procedures.

Medical and Industrial Sources of Radiation

Radioactive materials used in medical or industrial settings can produce irradiation or contamination from accidental or intentional misuse. Irradiation and contamination are significantly different problems—a person can be irradiated from a distance without being contaminated by radioactive material. Radiological contamination is an issue because of the close proximity of the radionuclide to tissue, whether the contamination lies against the skin (external) or is inside the body (internal).

Powerful industrial radiography sealed sources used in the nondestructive testing of oil and water pipelines have caused severe exposures. Their bright stainless steel casings are eye-catching when the sources are accidentally left behind, and they have been stolen when positive control of the source was lost. They are a potential terrorist weapon and could present a serious localized radiation threat.

Another important medical/industrial source is Cs-137, which is an important decay product resulting from the fission of uranium and plutonium fuels. This isotope is used in both industrial sealed-gamma sources as well as medical therapeutic sealed sources. Cesium (Cs) is an alkaline metal that is metabolized much like potassium and excreted through the kidneys. It has a radioactive half-life of 30 years but a biological half-time in adults ranging from 68 to 165 days (average 109 days). The biological half-time is shorter in children, ranging from 12 days in infants to 57 days in older children. It is also shorter in women (84 ± 27days) than in men.

An example of the potential danger from an uncontrolled Cs-137 source occurred in 1987. An abandoned radiotherapy sealed source containing 1400 Curies (Ci) of Cs-137 powder was opened by looters in Goiania, Brazil. About 250 people were subsequently exposed. The victims included children who rubbed the glowing powder on their bodies and, in at least one case, ingested it. Resulting radiation doses were as high as 10 Sv (1000 rem), resulting in four fatalities from acute radiation sickness (ARS). In this case, the Cs-137 had the exposures characteristic of both a sealed source (producing ARS) and an RDD causing both external and internal contamination. An estimated 120,000 concerned people had to be screened for possible contamination, with incident management costs in the millions of dollars.

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Nuclear Power Plants

The United States has 104 nuclear reactors licensed to provide electric power, as well as 36 reactors licensed for other uses. The U.S. Nuclear Regulatory Commission (NRC) has stringent physical protection requirements against sabotage, which cover both plant design and security protection features. Unlike the design of some foreign reactors, designs for power reactors operating in the United States incorporate a layered system of physical shields and walls, including a potentially pressurized containment vessel. Consequently, there have been relatively few mishaps involving American-designed power reactors.

The primary down-wind hazard from destruction or sabotage of a nuclear reactor is the venting of radioactive iodine gas. Power reactors cannot detonate like a nuclear bomb, because reactor fuel does not contain the highly enriched uranium needed for detonation.

The only significant U.S. catastrophic reactor failure occurred in March 1979, at the Three Mile Island (TMI) nuclear power facility in Pennsylvania. At that time, one of the TMI reactors experienced overheating and a meltdown of a portion of its fuel rods. This event resulted in a small release of radioactive gas to the environment, primarily Iodine-131. The radiation produced negligible doses to people residing near the plant (estimated maximum dose of 0.001 Sievert [Sv] [100 mrem]), which is equivalent to a routine chest radiograph). There has been no evidence to date of public injury resulting from the small amount of nuclear material that was vented as a result of this incident.

Pediatricians may be asked about the safety of consuming milk after a reactor accident. Iodine-131 fallout on vegetation has an effective half-life of about 5 days (combination of radioactive decay half-life [8 days] and vegetation half-life). An infant consuming 1 L of milk per day contaminated with 1 microCurie (µCi)/L would receive a total cumulative dose to the thyroid of about 16 rem. Therefore, locally produced milk, fruit, and vegetables should be declared fit for consumption only after clearance by appropriately-trained health inspectors. Emergency reference doses of 0.25 µCi/L peak level in milk and 1.5 µCi/m2 on pasture can be used to guide consumption (exposure) countermeasures. (For information on radioactive iodine and thyroid issues, go to the Medical Treatment section.)

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Historical Overview of Radiation Injury

Radiation in the form of "x-rays" was discovered in 1895 by the German physicist Wilhelm Roentgen. Radioactivity in ore was discovered by Antoine Henri Becquerel in 1898. Within a few years of discovery, ionizing radiation was used for a wide variety of medical diagnostic and treatment purposes. Radiation was found to be an extremely useful modality for medical imaging and an effective means for treating a variety of conditions from acne to malignancies. However, some of the side effects of radiation became known within a few years. Some early radiologists developed radiation skin injury and leukemia from their exposure to ionizing radiation.

Since 1940, there have been over 300 significant radiation accidents in the United States alone. These include both medical and industrial errors, as well as accidents involving the production and storage of nuclear weapons. Some of the most significant worldwide events associated with radiation injury are described below.

Hiroshima and Nagasaki, Japan—1945

The atomic bomb blasts over Hiroshima and Nagasaki in 1945 during World War II resulted in massive firestorms that obliterated much of the city. They also provided information on the medical effects of uncontrolled ionizing radiation over a wide dose range among those casualties who initially survived the blast and firestorm. The bomb detonated over Hiroshima was equivalent to 15,000 tons (15 KT) of TNT, while the Nagasaki weapon was equivalent to 22 KT of TNT.

It has been estimated that 50% of those exposed to a 2.7-3.1 Gray (Gy) bone marrow dose died within 60 days from their radiation exposure, as little medical assistance was available. Radioactive fallout was very limited, because both of the detonations were airbursts, in which the actual fireball does not reach the ground. The estimates of maximum dose due to fallout are 1-3 centigray (cGy) in Hiroshima and 20-40 cGy in Nagasaki.

The principal effects of radiation in unborn infants in Hiroshima and Nagasaki were small head size (microcephaly) and mental retardation. Of the 1,600 children exposed before birth who were followed, 30 were found to have severe mental retardation. Those with severe mental retardation were noted to have received radiation exposure between 8 weeks and 25 weeks gestation. The most sensitive time of the gestational period was found to be 8-15 weeks after conception.

Ionizing radiation is a relatively weak carcinogen. Among 86,000 atomic-bomb survivors at Hiroshima and Nagasaki followed from 1950 to 1990, there were 7,578 deaths from solid cancer (versus 7,244 expected) and 249 deaths from leukemia (versus 162 expected), representing 421 additional deaths from cancer. The minimum elapsed time between radiation exposure and clinical disease was 2-3 years for leukemia, 3-4 years for bone cancer, 4-5 years for thyroid cancer, and 10 years for other solid tumors.

Mayak, Russia, Former Soviet Union—1948-1990

The Mayak Production Association (Mayak) is an industrial complex in the Southern Urals of Russia, where the former Soviet Union produced tons of plutonium for nuclear weapons. Between 1948 and 1956, radioactive waste was poured directly into the Techa River, which was the source of drinking water for many villages. It is reported that 124,000 people were exposed to medium and high levels of radiation. In 1957, one of the cooling systems at Mayak exploded, and more than half the amount of radioactive waste released at Chernobyl went into the atmosphere. Another incident occurred after a reservoir for waste storage evaporated during a dry hot summer. Windstorms then carried 600,000 curies of radioactive dust over 2,700 square kilometers. According to one source, the radiation accidents and radioactive discharge at Mayak killed thousands and made many more ill. The implications for children who were living in this area are enormous, although health data for this population are not readily available.

Marshall Islands—1954

Fallout played a large role in the Marshall Islands after a 1954 nuclear weapons test on Bikini Island caused fallout on nearby islands, resulting in significant health effects in children. Acute effects included skin injuries from beta radiation (so-called "beta burns"), especially of the feet. The most heavily exposed were the 64 people of Rongelap Island, who were exposed to 190 cGy of external radiation plus 1000-5000 cGy to the thyroid from radioactive iodine. Of 32 individuals who were younger than 20 years old when exposed to the radioactive fallout, 4 developed thyroid cancer and 1 developed leukemia. Two individuals who were younger than 1 year old at exposure developed myxedema and short stature.

Three Mile Island, PA—1979

On March 28, 1979, a nuclear power plant at Three Mile Island (TMI), PA, had a near meltdown (overheating of the fuel rods and a release of radioactive material). The accident produced negligible doses among people living nearby, with a maximum dose of 0.001 Sievert (Sv) (100 mrem) to the community. Immediate administration of potassium iodide (KI) was recommended for those living near TMI, but this drug was not available. There were no biological effects from the exposure, although significant psychological sequelae occurred.

Chernobyl, Ukraine, Former Soviet Union—1986

In April 1986, a power plant in Chernobyl (aka Chornobyl), Ukraine, had a mishap that produced a meltdown and hydrogen gas explosion within the reactor core. The area around the reactor was heavily contaminated with plutonium, cesium, and radioactive iodine because there was no containment vessel designed into the facility. An estimated 120 million curies (Ci) of radioactive material was released, contaminating more than 21,000 square kilometers of land. The greatest areas of fallout occurred in Ukraine, Belarus, and the Russian Federation. Approximately 135,000 people were permanently evacuated.

Almost 17 million people, including 2.5 million children younger than age 5, were exposed to excess radiation. The first delayed health effects were noted 4 years after exposure, with a sharp increase in the occurrence of thyroid cancers in children and adolescents, especially among those younger than 4 years at the time of the accident. As of 2002, there were more than 2,000 excess cases of childhood thyroid cancer in the exposed population. A large area remains uninhabitable because of environmental contamination that exceeds the legally allowable limits.

Goiania, Brazil—1987

An abandoned radiation therapy facility in Goiania, Brazil, was looted on September 13, 1987. The looters took a canister containing 1,400 Ci of radioactive Cs-137, which was subsequently opened and distributed to a junkyard owner and several families. Children played with the material, rubbing it on their bodies so that they glowed in the dark.

As word of this event spread, thousands of people became anxious about their possible exposure. A stadium was used to screen self-referred potential casualties. Approximately 112,000 people were evaluated, and 249 individuals were found to have either external or internal exposure. Radiation doses were determined to be as high as 10 Sievert (1,000 rem) using cytogenetic techniques (go to: Radiation Biology and Dosimetry section in this chapter). Twenty individuals needed hospitalization, and four died of acute radiation sickness. Victims developed radiation-associated illnesses that ranged from significant skin irradiation injury to acute radiation sickness and long-term health problems.

Treatment for victims with significant exposure included Prussian Blue, which binds cesium and helps eliminate it from the body, and granulocyte macrophage colony stimulating factor (GM-CSF), which stimulates white blood cell production for victims with radiation-induced neutropenia. Mitigation efforts required the removal of 6,000 tons of dirt, furniture, clothing, and other materials.

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Radiological Dispersal Devices (Dirty Bombs)

Terrorists are clearly interested in obtaining RDDs, although a dirty bomb has not yet been exploded. Some incidents involving RDDs are described below.

  • November 1995: Chechen rebels in Moscow, Russia, informed a Russian television station that they had buried a cache of radiological materials in a park. Authorities found a partially buried source of cesium.
  • March 1998, Greensboro, NC: 19 tubes of Cs-137 were stolen from a hospital.
  • December 1998, Argun, Chechnya: A container filled with radioactive material was found attached to an explosive mine near a railway line.
  • June 2002, Chicago, IL: An American citizen was arrested on suspicion of planning to build and detonate a dirty bomb in an American city.
  • January 2003, Herat, Afghanistan: British intelligence agents reported that Al Qaeda had succeeded in constructing a small dirty bomb.

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Other Radiation Uses and Injuries

Prior to the 1960s, ionizing radiation was deemed nearly innocuous and was often believed to be beneficial. Individual exposures to low-level radiation commonly occurred from cosmetics, luminous paints, medical and dental x-ray machines, and gadgets for fitting shoes in retail stores. Fluoroscopy was widely used, as in the routine monthly well-baby visits of at least one large pediatric practice, and to shrink the thymus gland in other pediatric practices. This put infants at risk of thyroid cancer years later. Radium-224 was given intravenously to treat tuberculosis of the bone, resulting in an excess incidence of bone malignancies. Thorotrast® was also used as an intravenous radiographic contrast medium in Europe, Japan, and the United States, producing excess malignancies of the liver, bile ducts, spleen, brain, and bone in all age groups. Children and adults were also subjected to radiation exposure as part of experimental research in people that would not have met today's ethical standards.

In 1956, the U.S. National Academy of Sciences—National Research Council and the U.K. Medical Research Council published similar conclusions about the potential hazards and the late effects of radiation. Since that time, the uses of ionizing radiation have come under increasing scrutiny and regulation.

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Physical Principles of Ionizing Radiation

Atomic Structure

Atoms have two distinct parts that are made up of three main particles. Protons and neutrons are found in the nucleus, which is the tiny, dense center that contains almost the entire mass of the atom. Electrons surround the nucleus, moving in paths called orbitals.

Protons

Protons have a mass of about 1.7 x 10-27 kg, which is assigned the value of 1 atomic mass unit (AMU). Protons carry an electrical charge, which is assigned a value of +1. The density of an atom is determined by the number of protons in the nucleus. For example, uranium has 92 protons, making it much denser than hydrogen, which has only one proton.

Neutrons

Neutrons also have a mass of about 1 AMU, differing from the proton by <0.14%. Unlike the proton, the neutron has no charge. While the number of protons determines the density/identity of an atom, the number of neutrons can vary. Atoms with the same number of protons but different numbers or neutrons are called isotopes of each other. For example, 235U and 237U are two different isotopes of uranium.

Electrons

The mass of an electron is about 1/1800th of an AMU, or about 9 x 10-31 kg. The electron charge is -1. In a neutral atom, the number of electrons and protons match, with their charges canceling each other out. An ion is an atom that has either gained or lost electrons and hence carries a net negative or positive charge, respectively.

Stability and Radiation

Several factors can influence the stability of an element, leading to the potential for radioactivity. An electron or nucleus can become excited by radiation, causing it to become unstable and to give up energy in the form of photons. For example, technetium-99m (99mTc) is an unstable (i.e., metastable) form of technetium that is used in medical settings. When 99mTc goes from an excited state to a non-excited state, it gives off a photon and becomes 99Tc.

The ratio of protons to neutrons in the nucleus is also important to stability. The lighter elements are usually stable, with a ratio of about 1:1. As elements get heavier, the ratio reaches about 2:3, increasing the chances of instability. Whenever the ratio shifts from a stable configuration to an unstable one (for whatever reason), the nucleus attempts to regain stability by emitting radiation.

When an unstable atom emits radiation, it is said to decay. The half-life of a radionuclide is the average amount of time that it takes for half of the atoms in a sample to decay. Half-lives can vary from fractions of a second to millions of years. When an atom decays, it does not necessarily become stable. Rather, it is quite common for decay to be a series process, in which a given atom might decay dozens of times in different ways before becoming stable. Radon gas is a common example of this, with a very long decay chain. Radioactive decay can be associated with several different types of radiation.

Photons (x-rays and gamma rays)

Photons are (nearly) massless "bundles" of energy. The energy determines the wavelength, which can be classified into various categories for convenience. Radio waves, visible light, ultraviolet light, x-rays, and gamma rays are all photons of various energies. X-rays and gamma rays differ only in their origin. An electron losing energy yields an x-ray photon, while loss of energy from a nucleus exciting produces a gamma photon. X-rays and gamma rays with the same energy are otherwise identical.

Gamma or x-rays are often released to remove residual energy that remains after other forms of decay (e.g., alpha or beta particles). For example, 60Co, a common industrial and medical radionuclide, decays by beta emission, while simultaneously giving off two powerful gamma rays during its transformation into the stable element 60Ni.

Beta Particles (electrons)

One of the ways that an unstable nucleus can adjust its proton:neutron ratio is to emit an electron, effectively transforming a neutron into a proton and an electron. This electron is then ejected from the nucleus as a "beta particle," which in all respects is identical to any other electron. Beta particles can cause damage to tissue directly or through secondary processes.

Bremstrahlung. When an electron decelerates (or turns), it loses energy in the form of photons, which is a process called "bremstrahlung." The quicker the deceleration, the greater the energy imparted to the photons. Electron paths tend to curve near heavy nuclei, so that electrons lose energy when interacting with dense matter. This is one of the reasons that lightweight materials are preferred as protection against beta radiation. For example, in old televisions, leaded glass was used for the picture tube. When the electrons from the electron guns hit the back of the screen, they produced bremstrahlung that were roughly equivalent to weak x-rays. Newer televisions are designed to produce minimal bremstrahlung.

Positrons. Some nuclei (e.g., 12N or 124I) emit an anti-electron or positron, converting a proton to a neutron. The ejected positron is identical to an electron but has a +1 charge rather than a -1 charge. Because it is an anti-particle to an electron, the two will annihilate when they interact. The annihilation completely converts both particles to energy, releasing two photons of exactly 0.511 MeV. Positron emission and the detection of the annihilation photons form the basis of positron emission tomography (PET), a fairly common medical imaging procedure.

Alpha Particles

Alpha particles are stable helium nuclei (two protons and two neutrons) ejected mainly (but not exclusively) by heavy nuclei during adjustment of improper proton:neutron ratios. The two protons and two neutrons are emitted, often with considerable energy, and the particle has a +2 charge. Alpha particles are the most massive and highly charged of the common types of radiation. Both of these properties make them potentially very damaging but also very easy to shield.

Neutrons

Neutrons are rarely emitted as decay products. Rather, they are more commonly emitted as by-products of nuclear fission or fusion. As neutrons have no charge, they are able to interact readily with the nucleus of an atom, possibly being absorbed into it. This affects the proton:neutron ratio and can lead to the atom becoming radioactive or otherwise unstable.

Radiation Interactions

Radioactive particles can interact in many ways, leading to potentially hazardous processes and by-products.

Electron Excitation

The simplest way for a radiation particle (including a photon) to interact with a target atom is for it to impart some energy to the electrons of the atom. If enough energy is imparted, the electron will be knocked out, and an ion will be formed. If the energy is not sufficient to form an ion, the electron will merely be excited, giving back this extra energy in the form of a photon. This photon can be in the form of visible light (e.g., the phosphors in a television) or as a more hazardous x-ray.

Ionization

The primary radiation effect of medical concern is ionization, in which an electron is completely removed from an atom. Ionization can directly or indirectly break chemical bonds, leading to tissue damage. Furthermore, if an electron is removed from the inner shell of an atom, the cascade of electrons dropping to fill the gap may produce a series of x-rays, which can carry the damage beyond the original track of the incident particle. Similarly, if the electron that was removed has sufficient energy, it may induce its own ionization events, again carrying the effects away from the original track. These follow-on events are called secondary radiation.

Nuclear Excitation

If an incident particle interacts with the nucleus of an atom, several outcomes are possible. Nuclear fission or fusion is possible, although the simplest outcome is excitation. As noted above, when a nucleus goes from an excited to an unexcited state, it will usually emit a photon in the form of a gamma ray.

Nuclear Fission

When certain heavy nuclei become unstable with respect to their proton:neutron ratio, they can undergo a splitting process termed fission. Typically, this process is induced by the absorption of a neutron into the nucleus of the target atom. The nucleus then breaks into two or more fission fragments, including lighter elements, subatomic particles, and energy.

The subatomic particles that can be released during fission include more neutrons, which can in turn induce new fissions if the conditions are appropriate. This latter process is called a chain reaction and is the heart of nuclear reactors and weapons. Uranium, thorium, and plutonium have isotopes that can undergo fission. A large number of different possible fission fragments can be created, and many of them are themselves unstable and hence radioactive. Some of the most common and hazardous byproducts of fission include 131I, 137Cs, and 90Sr.

Activation

Neutrons lack a charge and can therefore readily reach the nucleus of an atom. If the neutron is absorbed by the nucleus, the new nucleus may have an unstable proton:neutron ratio as well as excess energy. This new configuration may or may not be stable. If it is not stable, it will eventually decay. In other words, the new nucleus may be radioactive. This process of creating a new radioactive radionuclide is called activation and is mainly associated with nuclear reactors and nuclear detonations.

Secondary Radiation

The radiation "particle" that initially enters a volume/space is considered primary radiation. Any electrons or photons (or anything else) liberated with enough energy to produce new ionizations are considered secondary radiation. Secondary radiation does not necessarily travel in the same direction as the primary energy, leading to secondary damage in new spaces. Furthermore, the secondary radiation is not necessarily of the same type as the primary radiation, leading to different types of effects. For example, a neutron will liberate electrons and numerous gamma and x-rays as it decelerates. Very high energy radiation (e.g., cosmic radiation) travels quickly through a given space (e.g., human tissue), producing only a small number of interactions. However, each of these interactions can transfer large amounts of energy as secondary radiation, which can then distribute throughout the given space.

Radiation Damage and Protection

Linear Energy Transfer (LET)

As a radiation "particle" passes through matter, it loses energy through the various mechanisms discussed above. The rate at which it loses that energy is the LET, which is measured in energy per unit distance (e.g., MeV per micron). A particle with high LET transfers energy very quickly, slowing down rapidly and depositing energy on a dense "track" in the target volume. A particle with low LET interacts much less often and slows down only gradually. For example, photons are low LET radiation because they have no mass or charge, while alpha particles are high-LET radiation due their large mass and +2 charge. Beta particles and neutrons are intermediate, with neutrons having the higher LET of the two.

Penetration and Radiation Shielding

The more rapidly a given radiation particle loses energy (high LET), the more quickly and easily it is stopped. Therefore, the LET and the degree to which a particle is considered penetrating are inversely related—i.e., the higher the LET, the less penetrating the particle. For example, an alpha particle deposits very large amounts of energy in a very short distance, which can result in considerable damage to living cells. However, alpha particles cannot penetrate even the thickness of a piece of paper or the dead layer of skin on your body. Therefore, alpha particles generally do not need to be shielded, although the alpha-emitting materials should be prevented from entering the airways (e.g., with filtering masks) to protect delicate lung tissue. On the other hand, high-energy gamma rays need to be shielded with very thick lead or concrete because they interact so little and penetrate so deeply. As noted earlier, lightweight materials such as Plexiglas® or aluminum are preferred as shielding for beta particles to reduce bremstrahlung.

Radiation Units

The measurement of radiation required the creation of new units to describe various aspects that were new to science. The major units used to describe radiation and its effects are discussed below. This discussion will include both the older American nomenclature and the newer nomenclature under the System International (SI), because both are still widely used.

Radioactivity; curie/becquerel

The units used to describe radioactivity are the curie and the becquerel (SI). Activity represents the number of radioactive decays that take place in 1 second. One curie (Ci) is the number of decays that occur in 1 gram of pure radium-226 in 1 second (i.e., 3.7 1010 disintegrations/second). The becquerel (Bq) is one disintegration per second, so that there are 3.7 1010 Bq per Ci.

The specific activity (activity per unit of mass) varies from radionuclide to radionuclide, so that the same weight of different materials does not necessary have the same level of radioactivity. For example, 1 curie of 226Ra weighs about 1 gram, as noted above, while 1 Ci of 232Th weighs just under 10 million grams (ten metric tons).

Exposure; roentgen

Formally, the roentgen (R) is a unit of electrical charge liberated by photons per kilogram of air. This unit is specific to x- and gamma rays and cannot be applied to any other form of radiation or to any other medium. It does not have an SI equivalent.

Absorbed Dose; Rad/Gray

"Absorbed dose" is the amount of energy absorbed per unit of mass, which applies to any type of radiation in any type of matter. The Rad (radiation absorbed dose) is defined as 100 erg per gram. The SI equivalent is the Gray (Gy), defined as 1 joule per kilogram. One Gy is equivalent to approximately 100 Rad. Often the unit of centiGray (cGy) is used because it is equal to 1 Rad.

Effective Dose

Absorbed dose is a very useful concept, but it does not account for the different biological effects of different types of radiation on different types of tissues. Two artificial weighting factors are used to represent the impact of these differences in "effective dose."

Relative biological effectiveness. This weighting factor accounts for how strongly a given radiation interacts with the particular mix of elements that makes up the human body. This factor does not account for shielding and is partially dependent on energy. Note that this weighting factor seems to match up reasonably well with the discussion of LET. For example, alpha particles and certain neutrons have the greatest effectiveness, while photons and beta particles have the lowest effectiveness (Table 6.1).

Tissue weighting factors. Different tissues in the human body have differing sensitivities to radiation. Nerve tissue, for example, is fairly insensitive to radiation, while the lining of the gastrointestinal (GI) tract is very sensitive. These weighting factors apply only to cancer risk and cannot be correctly applied to other risks. The sum of all the weighting factors is one. A whole body exposure receives a weighting factor of one, while a partial body exposure would receive the appropriate fraction.

Dose equivalent. The dose equivalent is the product of absorbed dose and the various weighting factors in units of energy per unit mass. The traditional American unit is the REM, which stands for roentgen equivalent man. The REM is defined as 100 ergs per gram, just like the Rad. The SI equivalent is the Sievert (Sv), which is defined as 1 joule per kilogram and is equal to 100 REM. The absorbed dose and the dose equivalent are not the same. The former is a measurable physical quantity, while the latter is the product of consensus weighting factors with a special focus on biological effects.

Radiation Background Exposure

Americans typically receive about 360 millirem of background radiation per year. Of this, approximately 295 millirem is considered natural (cosmic rays, radon, etc.), and the other 65 millirem is considered manmade (x-rays, etc). The main sources of radiation exposure are discussed below.

Natural Exposures

There are four sources of natural exposure: radon, cosmic radiation, internal radiation, and terrestrial exposure.

Radon. Radon gas is a natural by-product of the decay of uranium and thorium in rocks and soil. Remember that decay can occur in chains of radionuclides. Radon is part of a long chain. Because it is a gas, it "percolates" up through the soil and into the atmosphere. If it happens to come up under a building, it can become trapped and accumulate in the basement or first floor, leading to sometimes appreciable radiation doses to the occupants. Radon decays eight times before becoming stable, and many of the decays produce very high-energy alpha particles. Because radon is a gas, it can enter the lungs where there is no dead layer of skin to block the alpha particles. Radon and its daughter products account for about 200 millirem of exposure per person per year.

Cosmic radiation. Cosmic radiation, as the name implies, originates from space, either from the sun or from extra-solar sources like supernovae and quasars. Much cosmic radiation is of exceedingly high energy and does not interact at all with the human body. Much more is blocked either by the solar atmosphere or by the earth's atmosphere. The annual cosmic radiation dose at sea level is about 27 millirem, while people living or working at higher elevations (e.g., airline pilots) receive more.

Internal radiation. A number of materials are present in food, water, etc., that expose us to small doses of radiation. Most commonly considered is 40K, a common, naturally radioactive isotope of potassium. Bananas and some nuts contain elevated levels of radioactivity from this radionuclide. The total dose due to naturally occurring radioactive material incorporated into the body is about 39 millirem per year.

Terrestrial exposure. The earth itself contains a number of radioactive elements. Uranium and potassium have already been mentioned. The annual dose from external radiation by rocks and soils is about 28 millirem.

Manmade Exposures

Manmade exposures may occur through medical procedures, consumer products, and/or fallout/nuclear power fallout and waste.

Medical. By far the largest manmade exposure is from medical procedures, such as radiology, radiation therapy, and so forth. For example, the ubiquitous chest x-ray exposes a patient to about 8 millirem. The average annual dose from all medical procedures is about 53 millirem.

Consumer products. A variety of small radiation doses come from consumer products. Cathode ray tubes, radium dials, airport x-rays, etc., all contribute to the 10 millirem annual dose.

Fallout and nuclear power. Combined, fallout from weapons testing and waste from the nuclear power industry produce an annual dose of <1 millirem.

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