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CHAPTER 18 - Radioactivity and Nuclear Energy

By Diane Brooks,2014-07-19 20:10
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CHAPTER 18 - Radioactivity and Nuclear Energy

CHAPTER 10 - Radioactivity and Nuclear Processes

Composition of atomic nucleus - neutrons and protons

    ANuclides are nucleus of different isotopes, which is represented by the symbol X - X Z

    being symbol of the element. A is the mass number, which is the sum of number of protons & neutrons; Z is the atomic number, the number of protons, which also represents the nuclear 14charge. Number of neutrons = (A Z). For example, the symbol of carbon-14 nuclide is C, 6

    which means a carbon-14 nuclide has 6 protons and 8 neutrons. The nuclide symbol of lead-206206 is Pb. 82

Radioactive Decay

    Nuclei with unsuitable compositions are unstable and they undergo spontaneous disintegration or nuclear decay. Nuclides that undergo spontaneous decay are said to be radioactive. Nuclear decay changes a radioactive nuclide into a stable one, which also change the identity of the nuclide. For example, when a carbon-14 nuclide decays by emitting a beta particle, it becomes nitrogen-14 (which is stable).

     14140 C ; N + e 671

    Therefore, unlike a chemical reaction, nuclear reactions alter the identity of the atom.

Types of Radioactive Decay

Alpha Decays

     There are several different types of radioactive decay. One frequently observed decay process involves production of an alpha () particle, which is a helium nucleus and assigned

    4the . This is the common mode of decay for heavy radioactive nuclides (those with atomic 2

    number, Z > 83). When a nucleus loses an alpha particle its mass number (A) decreases by 4 units and the atomic number (Z) decreases by 2 units. The product of an alpha decay is a nuclide with two less protons and neutrons, respectively, than the original nuclide. For 238226example, uranium-238 (U) and radium-226 (Ra) decay by alpha particle emission, which 9288

    change them to thorium-234 and radon-222, respectively.

     23823422622244 U ; Th + ; Ra ; Rn + 9290888622

Beta Decays

     Another common nuclear decay involves the production of a beta () particle, which is

    0a nuclear particle identical to an electron in mass and charge, and is assigned the symbol . 16014For example, radioactive nuclides such as cobalt-60 (Co) and carbon-14 (C), decay by 627

    emitting particle.

     6060141400 Co ; Ni + ; C ; N + ; 67112728

     110The lost of a beta particle convert a neutron to a proton: n ; H + . Thus, the product 011

    of beta decay is a nuclide with the same mass number (A), but with an atomic number one unit higher than the original nuclide. Both decay processes (as well as other types of

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    radioactive decay) are accompanied by the production of gamma () rays, which are high--13energy photons of electromagnetic radiation that has a very short wavelength (( ~ 10 m) 21-1and a very high frequency ( ~ 10 s). The new nuclide formed in the decay process goes to an excited nuclear energy state, which then relaxes to the ground state and releases the excess nuclear energy as radiation. For example, when uranium-238 decays, two types of

    4rays of different energy are produced in addition to alpha particle (): 2

     23823404 U ; Th + + 2 ; 929002

rays are photons, which have zero charge and mass. The production of ray does not

    change the mass number (A) nor the atomic number (Z).

Positron Emission

     A positron is identical to an electron in mass, but it is positively charged. The symbol 0used for positron is . For example, carbon-11 decays by a positron emission: 1

     11110 C ; B + ; 651

    110When a nuclide emits a positron, a proton is converted to neutron: p ; n + . Thus, the 011

    product of positron emission is a nuclide with the same mass number (A), but with one unit lower in atomic number relative to the original nuclide.

    Positron is not easily detected because as soon as it is released it will encounter an electron outside the nucleus and they annihilate each other, producing characteristic rays

    00that go in opposite directions: + ; 2. This annihilation process is an example 11

    whereby matter is converted to energy, and positron is often referred to as antimatter.

Electron Capture

     Some radioactive nuclides decay by the electron capture (EC) process; that is, an electron from the innermost shell (n = 1) is absorbed by the nucleus, where it combines with 110a proton to form a neutron: p + ; n. 110

    As in the positron emission, an electron capture produces a nuclide with the same mass number (A), but with one unit less in atomic number (Z). For example:

     1951950 Au + e ; Pt 79781

Radioactive Decay Series

    Heavy radioactive nuclides, such as radium-226 and uranium-238, cannot gain stability by emission of a single radioactive particle. They undergo a decay series, producing either an alpha or a beta particle and gamma radiation during each step, until a stable nuclide is formed. For example, uranium-238 undergoes a decay series to form a stable lead-206.

     238234234234230226222218 U ; Th ; Pa ; U ; Th ; Ra ; Rn ; Po ; 9290919290888684

    214214214210210210206Pb ; Bi ; Po ; Pb ; Bi ; Po ; Pb 82838482838482

    (How many and particles, respectively, are produced in this decay series?)

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Balancing Nuclear Reactions

    In nuclear reactions, equations are balanced by matching the atomic number (that is, the charges) and the mass number on both side of the equation. Once the atomic number of the product nuclide is determined, it must be assigned proper symbol. For example, polonium-212 decays by an alpha emission. Identify the new nuclide formed.

     2124 Po ; + ? 842

    To identify the second product, its mass number and atomic number must be determined. That is, the new nuclide has an atomic number = 84 - 2 = 82 (which is lead) and a mass number = 212 - 4 = 208. The nuclide symbol of the new nuclide is ;S(208, 82)Pb. The nuclear equation is written as:

     2122084 Po ; + Pb 84822

Nuclear Transformation or Transmutation

    Transmutation is an induced nuclear reaction that results when a target nucleus is bombarded with a fast moving particle or nuclear particle called projectile. The first transmutation process was observed by Rutherford in 1919, who noticed that when nitrogen-14 was bombarded with an alpha particle an oxygen-17 is formed:

     141741 N + ; O + p 7821

    About 14 years later, Irene Curie and her husband, Frederick Joliot, observed a similar transformation of aluminum to phosphorus when bombarded by alpha particles.

     273014 Al + ; P + n 201315

    Transmutation can also be carried out by bombarding the target with other positive ions. Since the bombarding particles (the projectile) are positively charged and strongly repelled by the target nuclei, they must be accelerated to a very high speed in order to acquire sufficient kinetic energy that would overcome the repulsion force. This is accomplished in various types of particle accelerators, such as the Stanford Linear Accelerator, the cyclotron at UC Berkeley Lawrence Livermore Lab, and other particle accelerators.

    All transuranium elements (those with Z > 93) are produced by transmutation processes. For example, element-106 (Seaborgium) was formed by bombarding californium-249 with oxygen-18 nuclei, the projectiles are accelerated in a linear accelerator, called Superhilac.

     249182631 Cf + O ; Sg + 4 n 9881060

    Element 110, which is not yet named, was produced by bombarding lead-208 with nickel-64 ions: 2086427119+ Pb + Ni ; E + n; (*E is not actual symbol) 82110028

    Neutrons are also used as projectiles in many transmutation processes. Since neutrons are neutral particles, they are not strongly repelled by target nuclei and do not to be accelerated as fast as positively charged particles. Instead, neutrons are accelerated by heat, thus called

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    thermal neutrons, which gives them enough energy to enter target nuclei. The reaction is called neutron capture:

     591600 Co + n ; Co + 002727

Exercise-1:

    1. Write a balanced nuclear equation for the decay of the following nuclides. The mode of

    decay is indicated in parentheses).

     131 (a) I (beta emission) 53

    235 (b) U (alpha emission) 92

    55 (c) Fe (electron capture) 26

    50 (d) Mn (positron emission) 25

    2. Identify the missing species (give complete isotope symbols) in the following nuclear

    reactions:

     2394061400 (a) Pu ; ? + ; (b) K ; ? + ; (c) Cu ; ? + ; 942111929

     110912844 (d) Si + ; ? + n; (e) Ag + ; ? + 2n; 20472014

    __________________________________________________________________________

The Detection of Radioactivity

Radioactivity or the rate of nuclear decay is measured in units called becquerel (Bq), where 1

    Bq = 1 event (disintegration) per second. Another commonly used unit of radioactivity is the 10Curie (Ci),where 1 Ci = 3.70 x 10 Bq.

    Radioactivity may be measured using a Geiger Counter or scintillation detector. A Geiger counter counts activities by measuring the pulses of electric current produced by ionized gas particles in the probe. The probe in a Geiger counter is filled with argon gas. When the high-energy radioactive particles, such alpha or beta particles, enter the probe, they collide with argon atoms and ionize the latter. The argon ions create an electrical potential and produce electrical pulses which are detected and counted by the detector.

    In the scintillation detector, the probe uses a substance such as sodium iodide that produces flashes of light when struck by radioactive particles. These flashes produce photoelectric current, which are detected and measured by the detector. Unlike a Geiger counter which only measures the rate of nuclear decay, a scintillation detector also measures radiation energy.

The biological units for radiation dose are roentgen (R), rad (D), gray (Gy), and rem

    (roentgen equivalent man). One roentgen is the quantity of radiation that will produce 2.1 x 9310 ion pairs in 1 cm of dry air at normal temperature and pressure, or generates about 1.8 x 1210 ion pairs per gram of tissue.

The rad and gray are biological units to describe the effects of radiation dose in terms of the -2-3amount of radiation energy absorbed by the affected tissue. 1 rad = 1.0 x 10 J (or 2.4 x 10

    cal) of radiation energy per kilogram of tissue. 1 gray = 1 J per kg tissue. The rem is a

    biological radiation unit that takes into account the different effects of various types of

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    radiation. A 1-rad dose of gamma radiation does not produce the same health effects as a 1-rad of alpha radiation. Since alpha particles are more massive, at the same energy, alpha radiation would cause a greater damage to tissue than either beta or gamma radiation. Therefore, the unit rem is created as a standard comparative unit, such that one rem of any

    type of radiation has the same health effect as 1 roentgen of gamma rays or X-rays.

Half-Lives of Radioactivity

    The activity of a radioactive substance is normally given in the form of its half-live, which is the time taken for 50% of the original amount of the radioactive substance to decay. For example, the half-life of carbon-14 is 5730 years, which means that, if a sample originally contains 100 millions carbon-14 nuclides, the first 50 millions will decay in 5730 years, the next 25 millions nuclei decay in another 5730 years, the next 12.5 millions nuclides decay in another 5730 years, and so on.

    The half-life of iodine-131, which is used in radiotherapy for thyroid cancer, is 8.0 days. If a sample contains 100 millions I-131 nuclides, it will take only 8.0 days for the first 50 millions nuclides to decay, and the next 25 millions will take the same number of days to decay, and so on. On the other hand, a sample containing the same number of uranium-238 nuclides, 99which has a half-life 4.5 x 10 years, will take 4.5 x 10 years for the first 50% of the sample

    to decay.

     5730 y 5730 y 141414100 millions C ; 50 millions C ; 25 millions C 666

     8.0 d 8.0 d 131131131100 millions I ; 50 millions I ; 25 millions I 535353

     99 4.5 x 10 y 4.5 x 10 y 238238238100 millions U ; 50 millions U ; 25 millions U 929292

    Thus, radioactive elements with short half-lives have higher activity (decay at faster rates) than those with longer half-lives. It also means that, radioactive materials with longer half-lives remains active for longer periods than those with short half-lives. Nuclides with very long half-lives also means that they are relatively more stable, and nuclides with half-lives longer 11than 10 years are considered as stable.

Exercise-2:

    1. Technetium-99 has a half-life of 6 hours. If a sample of technetium initially has an activity

    of 50.0 mCi (millicurie), what is its activity 24 hours later?

    2. Iodine-131 is a beta emitter used to treat thyroid cancer and has a half-life of 8.0 days.

    The dosage is 1.0 mCi/kg body weight. How long would it take for a 70-mCi dose to decay

    to an activity of 2.2 mCi?

    3. Gold-198 is a beta emitter and has a half-life of 2.7 days. It is used as an implant for

    cancer therapy. For an implant containing 50 g of gold-198, approximately how much

    gold-198 remains after 14 days?

__________________________________________________________________________

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Dating Using Radioactivity

    Archeologists and geologists use radioactive substances to determine the ages and accurate dates of artifacts and rocks. Artifacts that were made from woods or plant materials can be dated using carbon-14 dating. Rocks can be dated using uranium or other radioactive substances present in them.

     Since the rate of radioactive decay is constant and the half-life of a given radioactive substance is fixed, it can serve as a dating clock. For example, carbon-14 is continuously being formed in the upper atmosphere by the bombardment of nitrogen-14 with neutrons from cosmic rays.

     141141 N + n ; C + p 7061

     14140Carbon-14 then undergoes beta-decay to nitrogen-14: C ; N + . 671

    After millions of years, the rate of formation and decay of carbon-14 reach a steady state level, and the natural abundance of carbon-14 becomes constant. The steady state level for carbon-14 in a fresh plant material is about 15.0 dis. per min. per gram of total carbon (dpm/g C).

    This level continues to decrease and after several hundred to a few thousand years, only a fraction of the activity remains. From the ratio of the residual activity to the steady state activity of carbon-14, and the half-life of carbon-14, the age of the artifact can be estimated. The following equation is used to calculate the age of artifacts or rocks:

     ln(N/N) = - 0.693t t0

     t 1/2

where, N = radioactivity at the present time or at the time the artifact was discovered t

    N = the initial or steady state activity; 0

    t = half-life of the radioisotope; 1/2

    t = the age of the artifact.

Example:

    A piece of charcoal from a tree killed by volcanic eruption that formed the Crater Lake in Oregon showed a radioactivity of 7.0 dpm/g C. If the steady state activity of carbon-14 in the living tree is 15.0 dpm/g C, how long ago did the volcanic eruption occur?

We calculate the age of the charcoal using the following equation:

     ln(N/N) = - 0.693t t0

     t 1/2

     ln(7.0 dpm/g C) = - 0.693t;

     (15.0 dpm/g C) t 1/2

     -0.76 = - 0.693 t t = (0.76 x 5730 y,0.693) = 6300 y

     5730 y 0.693

    where t represent the age of the charcoal and also of the volcanic eruption occurred.

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Exercise-3:

    1. A wooden artifact showed a carbon-14 activity of 5.0 dpm/g-C. If the activity of carbon-14

    in a fresh wood is 15.0 dpm/g-C, how old is the artifact? Carbon-14 has a half-life of 5730

    years.

    2. A rock sample is found to contain 1.24 mmol of lead-206 and 1.37 mmol uranium-238.

    Assuming that all of lead-206 is the product of uraniuim-238 decay and the half-life of 9uranium-238 is 4.5 x 10 yrs, determine the age of the rock.

    _________________________________________________________________________

Medical Applications of Radioisotopes

    In the medical fields, radioisotopes have been used as radiotracers for diagnosis and in cancer therapy. Radio tracing involves the use of radionuclides that can be introduced into the patient in food or drugs and subsequently traced by monitoring their radioactivity. This technique is very useful if the radionuclide has a tendency to accumulate in specific organ or tissues. For example, iodine-123, which decay by gamma emission and has a half-life of 13.3 hours, is used in the diagnosis of thyroid gland function. The patient is given a small amount of NaI that includes I-123 isotope, which will be taken up by the thyroid gland. An overactive gland will absorb an excess amount of the radioactive iodine and forms a hot spot, whereas a

    damaged thyroid gland will absorb less iodine and produces a cold spot. Iodine-131, which is a

    beta emitter and has a half-life of 8.0 days, is also used in the diagnosis and treatment of illnesses of the thyroid gland.

    Technetium-99 and thalium-201 are used as radiotracers to assess the damage to the heart muscle in a person who has suffered a heart attack. Both are useful as tracers in the diagnosis of heart illnesses because of their tendency to accumulate in healthy muscle tissues that received sufficient blood supply, but not in damaged heart muscle tissues that do not receive sufficient blood (and oxygen). Thus, a “cold spot” will be observed in the image of a damaged heart.

    Radioisotopes used as diagnostic tracers should have the following characteristics:

    1. Radioisotope tracers should have short half-lives so they will decay while the diagnosis

    is being done but will quickly disappear after the diagnosis procedure is completed.

    However, the half-life should be long enough to allow it to be prepared, transported,

    and administered conveniently.

    2. The decay product of the tracer should be nontoxic and nonradioactive.

    3. The radioisotope tracer decays by emitting penetrating gamma-radiation that can be

    easily detected from outside the patient’s body.

    4. The radioisotope should have chemical properties that allow the tissue being studied to

    either concentrate it in diseased areas and form a hot spot or essentially reject it from

    diseased areas to form a cold spot.

    Radioisotopes administered internally for therapeutic use should ideally have the following characteristics:

    1. The radioisotope should emit less penetrating alpha or beta radiation to restrict the

    extent of damage to the desired tissue.

    2. The target tissue has the ability to concentrate the radioisotope to restrict the radiation

    damage to the target tissue.

    3. The half-life should be long enough to allow sufficient time for the desired therapy to

    be accomplished.

    4. The decay products should be nontoxic and nonradioactive.

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    Radiotherapy is widely used in the treatment of various types of cancers and blood

    disorder such as leukemia. Cancer cells are usually more sensitive to radiation than normal cells. Radiation kills more of the cancerous cells per dose than healthy cells. The radioactive substance can be enclosed in needles or small capsules and implanted to deliver a slow but continuous dose of radiation to small tumors that can be reached easily. The slow

    administration of radiation will also allow affected normal cells to repair damages caused by radiation. Iodine-131 is used therapeutically to treat thyroid cancer and hyperthyroidism. Its primary advantage is the ability of the target organ to concentrate this radioisotope and localize its effect. Cobalt-60 is used as an external source of gamma radiation for cancer treatment. The damage to normal tissues is greater and causes side effects such as nausea and loss of hair.

Food Radiation to Increase Shelf-Life

     Bacteria and fungi that cause food spoilage can be killed by radiation. Preservation of food by radiation uses much less energy than preservation by heating and freezing. Food

    irradiation is commonly done using -rays from sources such as cobalt-60 and cesium-137.

    Medical supplies can also be sterilized by radiation.

Nuclear Fission Process

    The most important nuclear fission is the fission of uranium-235 when bombarded with

    thermal neutrons:

     2351236141921 U + n ; U* ; Ba + Kr + 3n; 9209256036

    Bombarding a uranium-235 nuclides with a slow (thermal) neutron produces an excited

    uranium-236, called a compound nucleus. The unstable uranium-236 then undergoes fission to form two "daughter nuclei" and more neutrons. The fission is a random process, that is all types of daughter nuclei will be formed. The neutrons produced in one fission reaction can be used to cause further fission on other nuclei, which causes the number of neutrons and fission reactions to increase very rapidly, producing branching chain reactions.

    The extent of chain reactions depends on the size of the target sample. A critical sample size is needed to maintain the chain reactions in a controllable fashion. If the size is too large (supercritical size), the number of neutrons produced increases very rapidly, and it will cause a chain reactions in an uncontrolled manner. An enormous amount of energy is released in a very short time and a tremendous explosion may occur.

    Since slow or thermal neutrons are needed to bring about nuclear fission on uranium-235, a moderator, such as liquid sodium, is used to slow down neutrons produced by the fission process. These are then fed back into the reactor core to bring about more fissions. The core in a nuclear reactor contains the fuel and the control rods. The control rods, usually contain cadmium or boron carbide, absorbs slow neutrons effectively. If the number of neutrons produced from fission increases, the control rods are lowered in between the fuel rods to absorb excess neutrons, and this will slow down the fission reactions. When the number of neutrons becomes too low, the control rods are raised and the fission reactions increase.

Problem Associated with Nuclear Power

     The supply of uranium-235 is very limited. The relative natural abundance of uranium-235 is only about 0.7%; naturally occurring uranium is mostly uranium-238, which does not undergo fission reaction. Therefore, a uranium sample has to go through an extensive enrichment process, to

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increase the concentration of uranium-235 to about 3%, before it can be used as fuel in a nuclear

    reactor. The spent fuel, which is still highly radioactive must be properly stored.

Breeder Reactors and The Production of Plutonium-239 for Nuclear Energy

     In the fuel rod, uranium-238 which is not a fissile isotope can capture fast neutrons and undergoes a two-step beta-decays to form plutonium-239, another fissile isotope:

    238123923923923900 U + n ; U* ; Np + !?;Np ; Pu + ; 9209293939411

    Plutonium-239 can undergo nuclear fission process when bombarded with fast neutrons, therefore a moderator is not needed. Plutonium-239 is also a by-product in power reactors using uranium-235, where a lot of uranium-238 is also present. The plutonium-239 formed is harvested from spent fuel rods and then purified and used as fuel in nuclear reactors or for weaponry.

    A special type of reactor, called breeder reactor, is designed to produce plutonium-239 from uranium-238. Some of the neutrons produced from plutonium-239 fission are used to sustain the chain reaction, while others are used to transmute uranium-238 to plutonium-239.

Nuclear Fusion Process

    Fusion is the combination of nuclides of low mass numbers. A simple example is the fusion of hydrogen-2 to form hydrogen-3 or helium-3:

     12231223 H + H ; H + p; H + H ; He + n; 01111112

    For fusion to occur, the nuclei must be accelerated to a very high speed so that they have enough kinetic energy to overcome the nuclear repulsion and to bind the nuclei together. The 67process requires a temperature in the order of 10 - 10 K.

The following fusion reactions are believed to occur in the interior of the sun, which produces 19in the order of 10 kJ/day of solar energy.

     1120 2 p + 2 p ; 2 H + 2 ; 1111123 2 p + 2 H ; 2 He + 2 1123341 He + He ; He + 2 p; 2221

     ———————————————

    140 Net: 4 p ; He + 2 + 2 121

     ———————————————

    Biological Effects of Radiation

    Radiation produced by radioactive decay contains very high energy particles and gamma-photons can cause the ionization of biological molecules. Ionization cause bonds to break and cells get damaged. The radiation of water molecules, both inside and outside the cells, produces free radicals which can damage cells by causing oxidation-reduction reactions. A person exposed to ionizing radiation may suffer either short term or long term injury. The offspring of women exposed to radiation may also be affected. Cancer is the main delayed effect of ionizing radiation. The extent of damage that a person will suffer from radiation depends on the combination of exposure dose and the duration of exposure.

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    Radiation damage to organisms can be classified as somatic or genetic damage. Somatic

    damage is damage to the organism itself, resulting in sickness or death. The effects may appear almost immediately if a massive dose of radiation is received; for smaller doses, a delayed effect, usually in the form of cancer, may only appear years later. Genetic damage is

    damage to the genetic machinery of reproductive cells, creating problems that often afflict the offspring. The biological effects of a particular source of radiation depend on several factors:

    1. The energy of radiation the higher the energy content of the radiation, the more damage

    it can cause.

    2. The penetrating ability of the radiation. Different types of radiation vary in their ability to

    penetrate human tissues: -rays and X-rays are highly penetrating, particles can

    penetrate approximately 1 cm of the skin tissue, and particles will not penetrate the

    upper dermal layer of the skin.

3. The ionizing ability of the radiation. The ionizing ability of radiation also varies a lot. -rays

    are very penetrating but cause little ionization. On the other hand, particles, though not

    very penetrating, will cause serious damage inside the body because of its powerful

    ionizing effect. Therefore, ingestion of particles emitters, such as plutonium or radium, is

    particularly damaging.

4. The chemical properties of the radiation source. When a radioactive nuclide is ingested, its

    capacity to cause damage depends on how long it remains in the body. For example, both

    krypton-85 and strontium-90 are -particles emitters. Because krypton (a noble gas) is

    chemically inert, it passes through the body quickly and does not have much time to cause

    damage. Strontium, on the other hand, is chemically similar to calcium and can

    accumulate in the bones, where it can cause leukemia and bone cancer.

    5. The susceptibility of the affected tissues. Different tissues also have different sensitivity to

    radiation. Some are more sensitive than others when exposed to the same type radiation.

    Specialized cells and cells undergoing active cell divisions are more susceptible. For

    example, radiation is 100 times more dangerous to the fetus during the third to seventh

    weeks of pregnancy than it is to pregnant mothers.

6. The duration of exposure to radiation. The dose required to produce a certain amount of

    damage depends on the period over which the dose is received. A dose that is deadly

    within a short period of time may lead to very few symptoms if the same dose is spread

    over the normal life-time of the individual. Some effects of radiation exposure may not be

    evident for many years.

Effects of Instantaneous Whole-body Radiation Doses on Human

    ——————————————————————————————————————————

    Dose, (in rem) Effects

    ——————————————————————————————————————————

    > 1000 Death within 24 h from destruction of the neurological system

    750 - 1000 Death within 4 - 30 days from gastrointestinal bleeding

    150 - 750 Intensive hospital care are required for survival. At the higher end of range,

    death occurs due to infection resulting from the destruction of white blood cell-

    forming organs, which usually takes place within 4-8 weeks after the accident. 50 decrease in white blood cell count.

    ——————————————————————————————————————————

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