Isotopes -A simple explanation.
a. Atoms of different elements have different numbers of protons in their nuclei. The term atomic number describes the number of protons in a nucleus. Although all the nuclei of a given element will have the same atomic number, they may have different atomic masses because they may contain different numbers of neutrons. Generally, this does not affect the chemical properties of the different atoms since the numbers of protons are not changed but does have profound effects upon nuclear stability of the different atoms. The total number of protons and neutrons in an atomic nucleus is referred to as the atomic mass number. Atomic species which have identical atomic numbers but different atomic mass numbers are called isotopes (Figure 2-III).
Figure 2-III. Isotopes of Uranium
b. The stable isotopes of elements have very definite ratios of neutrons to protons in their nuclei. As atomic mass numbers increase, the ratio of neutrons to protons increases according to a definite pattern (Figure 2-IV). If isotopes vary from this pattern, they are relatively unstable.
Figure 2-IV. Neutron to Proton Ratios
ATOMIC MASS UNIT
a. Common units of mass, such as grams, are much too large to conveniently describe the mass of an atomic nucleus or any of its constituent parts. To solve this problem a new unit was defined: the atomic mass unit (amu). The atomic mass unit is a relative unit defined arbitrarily by assigning a mass of 12 amu to the neutral atom carbon-12, the common isotope of carbon. One atomic mass unit equals 1.66 X 10-24 grams. Employing this value, the masses of the fundamental particles of an atom have been determined to be:
(1) Proton mass: 1.00727 amu.
(2) Neutron mass: 1.00867 amu.
(3) Electron mass: 0.00055 amu.
b. Logically, it should be possible, knowing the number of particles comprising a particular atom, to calculate the mass of that atom. However, experiments have shown that the total mass of an atom is less than the sum of the masses of the atom's electrons, protons, and neutrons. For example, the measured mass of the isotope fluorine-19 atom is 18.99840 amu, while the sum of the masses calculated for the individual particles of that atom is 19.15708 amu. The difference of 0.15868 amu between the measured and calculated mass of the fluorine-19 atom is defined as the mass defect (Figure 2-V).
c. Careful experimentation and study have shown that while the mass defect is real, the law of conservation of mass has not been violated. When basic particles combine to form an atom, a certain amount of mass is lost through conversion into energy in accordance with Einstein's equation E = mc2, where E is the energy, m is the mass, and c is the velocity of light in a vacuum. The converted energy is considered to be binding energy, i.e., energy necessary to hold the nucleus the nucleus together.
Figure 2-V. Illustration of a Mass Defect
SYMBOLS AND NOTATION
a. A standard notational form is used to identify the individual isotopes of a given element. The standard notation takes the following form:
A
X
Z
where X = chemical symbol of the element, Z = atomic number, and A = atomic mass number.
b. An example of the standard notation would be:
235
U
92
c. Reference to a chart of the nuclides would reveal that the element with an atomic number of 92 is uranium, the chemical symbol for which is U. The atomic mass number 235 identifies a uranium isotope having 92 protons and 143 neutrons (235 -92=143) in its nucleus. Thus the isotope identified by the example notation is the naturally occurring, readily fissionable isotope of uranium used in nuclear weapons. The atomic number is frequently left off, and such an isotope may then be represented only by its mass number and chemical symbol, i.e., 235U.
Fission
a. Fission is a nuclear process in which a heavier unstable nucleus divides or splits into two or more lighter nuclei, with the release of substantial amounts of energy. The materials used to produce nuclear explosions by fission are those isotopes of uranium or plutonium which undergo fission most readily. These are 235U and 239Pu. When as illustrated in Figure 2-VI, a free neutron of the proper energy is captured by the nucleus of a fissionable atom, the resulting unstable nucleus will "split" producing two or more fission products (atoms of different elements formed from the protons, neutrons, and electrons originally comprising the nucleus before its fission), two or three free neutrons and a tremendous amount of energy.
b. In terms of continued energy production, the most significant point about the fission process is the emission of free neutrons, which can in turn produce other fission events, which in turn produce still another generation of free neutrons. Each generation of fission produced neutrons can produce a large number of fissions; and so, within a few generations, the total number of fissions produced can be tremendous.
c. While in principle a single neutron could initiate a chain reaction of nuclear fissions which could ultimately result in the splitting of each fissionable atom in a given mass, not all of the neutrons produce more fissions. Some of the neutrons may escape from the fissionable mass. Others may be removed by nonfission reactions. To initiate a chain reaction, sustain that reaction for a period sufficiently long to permit a buildup of explosive energy, and confine the released energy for as long as possible to maximize the weapon's explosive effect requires that a variety of special conditions be met.
CRITICAL MASS
The first prerequisite to be met in producing a fission-type nuclear explosion is that there must be enough material present and in the right configuration so that successive generations of neutrons can cause equal or increased numbers of fissions. The amount capable of sustaining a continuous or chain reaction is termed a critical mass.
a. Although fission events release more than 2 million times more energy per event than do chemical reactions, there still must be a tremendous number of fissions to result in the release of a significant amount of energy. To meet this requirement, a mass of fissionable material having specific characteristics must be assembled. Depending on size, and other factors to be discussed, a given mass of fissionable material may support one of three types of chain reactions:
(1) Subcritical Chain Reaction. A reaction in which the number of neutrons decreases in succeeding generations, thus not continuing.
(2) Critical Chain Reaction. A reaction in which the number of neutrons remains constant in succeeding generations.
(3) Supercritical Chain Reaction. A reaction in which the number of neutrons increases in succeeding generations.
b. To produce a nuclear explosion, a weapon must contain an amount of uranium or plutonium that exceeds the mass necessary to support a critical chain reaction, i.e., a supercritical mass of fissionable material is required. Several methods can be used to make a mass of fissionable material supercritical.
(1) The active material can be purified to eliminate unwanted chemical impurities that might otherwise absorb neutrons.
(2) Fissionable material can be enriched, i.e., the amount of 235U as compared to 238U can be increased.
(3) The material can be machined into the most efficient shape. A spherical shape can be employed to provide the greatest volume with the least surface area, thereby reducing the probability of neutron loss.
(4) Moderators can be used to slow down fission neutrons, increasing the probability of their producing fissions.
(5) Finally, neutrons that have escaped the active material can be reflected back by using suitable materials as reflectors. Reflectors, used as tampers, can also physically delay the expansion of the exploding material allowing more fission to occur thereby resulting in an increase in explosive energy.
c. Because of the stray neutrons produced in the environment by spontaneous fission, those present in the atmosphere from cosmic ray interactions as well as others generated in various ways, a critical or supercritical mass would be likely to melt or possibly explode. It is necessary, therefore, that, before detonation, a nuclear weapon contain no piece of fissionable material as large as a critical mass. At the time of the detonation, some method must be employed to make the mass supercritical by changing its configuration. Two general methods have been developed for quickly converting a subcritical mass into a supercritical one.
(1) In the first, two pieces of fissionable material, each less than a critical mass, are brought together very rapidly to form a single supercritical one. This gun-type assembly may be achieved in a tubular device in which a high explosive is used to blow one subcritical piece of fissionable material from one end of the tube into another subcritical piece held at the opposite end of the tube (Figure 2-VII).
Figure 2-VII. Gun Assembly Principle
(2) In the second or implosion-type assembly method (see Figure 2-VIII), a subcritical mass of 235U or 239Pu is compressed to produce a mass capable of supporting a supercritical chain reaction. This compression is achieved by the detonation of specially designed high explosives surrounding a subcritical sphere of fissionable material. When the high explosive is detonated, an inwardly directed implosion wave is produced. This wave compresses the sphere of fissionable material. The decrease in surface to volume ratio of this compressed mass plus its increased density is then such as to make the mass supercritical. An enhanced radiation (ER) weapon, by special design techniques, has an output in which neutrons and x-rays are made to constitute a substantial portion of the total energy released. For example, a standard fission weapon's total energy output would be partitioned as follows: 50% as blast; 35% as thermal energy; and 15% as nuclear radiation. An ER weapon's total energy would be partitioned as follows: 30% as blast; 20% as thermal; and 50% as nuclear radiation. Thus, a 3-kiloton ER weapon will produce the nuclear radiation of a 10-kiloton fission weapon and the blast and thermal radiation of a 1-kiloton fission device (Figure 2-IX). However, the energy distribution percentages of nuclear weapons are a function of yield.
Figure 2-VIII. Implosion Assembly Principle
Figure 2-IX. Weapon Energy Distribution
FUSION
In general, fusion may be regarded as the opposite of fission. It is the combining of two light nuclei to form a heavier nucleus. For the fusion process to take place, two nuclei must be forced together by enough energy so that the strong, attractive, short-range, nuclear forces overcome the electrostatic forces of repulsion. The two conditions necessary for the fusion of appreciable numbers of nuclei are high temperatures to accelerate the nuclei and high pressure density to increase the probability of interaction. The only practical way to obtain the temperatures and pressures required is by means of a fission explosion. Consequently, weapons with fusion components must contain a basic fission component. The energy released in the explosion of a fission-fusion weapon originates in approximately equal amounts from the fission and fusion processes.
RADIOACTIVITY AND NUCLEAR RADIATION
General.
Above the isotope 235U was described as being "... the naturally occurring, readily fissionable isotope of uranium..." An expanded, but more complete, description would also have identified the isotope 235U as being radioactive. Similarly, in a fission reaction most, if not all, of the fission products produced are radioactive.
Radioactivity
The nuclei of certain naturally occurring isotopes, and of others produced artificially, contain excess energy, i.e., they are unstable. To attain stability, nuclei with excess energy emit that energy in the form of nuclear, ionizing radiation and, in that process, frequently change into different elements. (See paragraph 215e.) (Ionizing radiation is defined as radiation capable of removing an electron from a target atom or molecule, forming an ion pair.) Isotopes, the nuclei of which emit ionizing radiations to achieve stability, are termed radioactive. Radioactive isotopes are referred to as radioisotopes or radionuclides.
a. Radioactive Decay. The process wherein radionuclides emit ionizing radiation is also termed radioactive decay. Each radioisotope has its own characteristic decay scheme. A decay scheme identifies the type or types ionizing radiation emitted; the range of energies of the radiation emitted; and the decaying radioisotope's half-life.
b. Half-Life. Half-life is defined as the time required for half of the atoms of a given sample of radioisotope to decay. Half-life values range from fractions of a millionth of a second to billions of years. Theoretically, no matter how many half-lives have passed, some small number of nuclei would remain. However, since any given sample of radioactive material contains a finite number of atoms, it is possible for all of the atoms eventually to decay.
c. Data Plotting. Radioactive decay may be plotted in a linear form as shown in Figure 2-X or in a semilogarithmic form as in Figure 2-XI. The latter has the advantage of being a straight lineplot. The straight line form is used extensively in radiation physics, particularly when dealing with isotopes with short half-lives, since it allows direct determination by simple inspection of the activity at any given time with a precision adequate for most purposes.
Figure 2-X. Radioactive Decay Plotted in Linear Form
Figure 2-XI. Radioactive Decay Plotted in Semilogarithmic Form
MEASUREMENT OF RADIOACTIVITY
a. The international system of units is based on the meter, kilogram, and the second as units of length, mass, and time, and is known as Systems International (SI). The amount of radioactivity in a given sample of radioisotope is expressed by the new Systems International (SI) unit of the Becquerel (Bq). The old unit was the Curie (Ci). One Becquerel of a radioisotope is the exact quantity that produces one disintegration per second. The Curie is 3.7 x 1010 disintegrations per second. Thus 1 Bq = 2.7 x 10-11Ci and 1 Ci = 3.7 x 1010Bq. As the Becquerel is inconveniently small for many uses as was the Curie inconveniently large, prefixes such as micro (µ) (10-6), milli (m) (10-3), kilo (k) (103), mega (M) (106), and giga (G) (109) are routinely used. Following nuclear detonations, the amounts of radioactive material produced are very large and the terms peta-becquerel (PBq) (1015Bq) and exabecquerel (EBq) (1018Bq) may be used. The term megacurie (MCi) (106Ci) used to be used.
b. The amount of radioactive material available at any time can be calculated by using a specific mathematical formula:
At=A0e(-*t)
from which the following can be derived:
since
c. The terms in these formulae are as follows:
(1) At = activity remaining after a time interval, t.
(2) Ao = activity of sample at some original time.
(3) e = base of natural logarithms (2.718...).
(4) *= decay constant of the particular isotope, derived from the half-life.
(5) t = elapsed time.
(6) T1/2 = half-life of the particular isotope.
d. This formula can be used to calculate the activity (A) of an isotope after a specific time interval (t) if the half-life (T1/2) and the original activity (Ao) are known.
(1) Example: If 3.7 x 1010Bq (= 1.0 Ci) of 60Co (cobalt) is the original amount of radioactive material at time to, what will be the activity of the 60Co remaining 1 month later?
A1 month = activity remaining after 1 month (t)
Ao = 3.7 x 1010Bq (original activity)
T1/2 = 5.27 years (half-life of 60Co is 5.27 years)
t = 1 month (time elapsed since the original time).
(2) Substituting in the formula gives the following:
(3) All values have to be converted to the same time units, in this case, years. Therefore:
(4) In other words, the activity of 60Co after 1 month is 0.99 of its original activity, a reduction of only 1%. This could not be determined with precision from a graphic plot of activity versus time.
Nuclear Radiation.
Radioisotopes of heavy elements such as radium or uranium characteristically decay by emission if ionizing radiation in the form of alpha particles. Some heavy elements also decay by spontaneous fission which results in neutron releases. For the lighter elements, emission of beta particles is common. In addition, emissions of gamma or x-ray photons almost invariably accompany both alpha and beta particle radiation. This is important since gamma or x radiation constitutes the principal casualty producing form of ionizing electromagnetic radiation associated with nuclear explosions. X-ray and gamma photons are essentially identical, differing only in their points of origin. Gamma photons originate in the nuclei of decaying atoms while x-rays originate in the electron shells surrounding nuclei. Refer to paragraphs 503-506 for detail of penetration capabilities of the types of radiation.
a. Even though they possess no net electrical charge, gamma and x-ray photons interact with atoms to produce ionization. Gamma photons have discrete energies over a very wide range, but are considerably less ionizing than alpha or beta particles but much more penetrating.
b. An alpha particle is a helium nucleus consisting of two protons and two neutrons all strongly bound together by nuclear forces. Alpha particles have a mass about 7000 times that of electrons and are ejected from the nuclei of radioactive atoms with one, or at the most several, characteristic and discrete energies. Although highly ionizing, alpha particles are only slightly penetrating.
c. Beta particle decay involves the conversion of a neutron into a proton and electron within the nucleus. While the proton is retained in the nucleus, the beta particle (electron) is ejected with a velocity dependent upon its kinetic energy. Opposed to alpha particles, beta particles show a continuous energy spectrum. Because of its smaller mass and relatively higher emission energies, a beta particle is less ionizing than an alpha particle but more penetrating.
d. In a fission process, neutrons are also released and consequently, make up a significant portion of the total radiation output.
e. From the discussion in paragraphs 215b and 215c, it can be seen that, depending upon the type of particulate radiation emitted in decay, decaying nuclei can, in addition to changing their energy states, be transformed into new elements. Examples of the transformations resulting from alpha and beta particle decay are shown in Table 2-I.
INTERACTION WITH MATTER
a. Ionizing radiation interacts with matter in one of two ways. It is either scattered or absorbed. Both result in deposition of energy in the target system. The mechanisms of absorption are of particular interest because:
(1) Absorption in body tissue may result in physiological injury.
(2) Absorption is a phenomenon upon which the detection of ionizing radiation is based.
(3) The degree of absorption or type of interaction is a primary factor in determining shielding requirements.
b. Transfer of energy from an incident photon or particle to the atoms of an absorbing target material may occur by several mechanisms.
(1) Excitation. This process involves the addition of energy to an atomic or molecular system, thereby transferring it from its ground or stable state to an excited or unstable state. Depending upon the type of interaction, either the atomic nucleus or one of its orbital electrons may absorb the excitation energy.
(a) Electron excitation occurs when relatively small amounts of energy are transferred. Here the electrons may only be moved to a higher energy level in the atom (Figure 2-XII).
(b) An excited electron will not retain its energy but will tend to return to its original energy level either by emitting the excess energy in the form of a photon of electromagnetic radiation (x-ray) or by transferring its energy to the electrons of other atoms or molecules.
(2) Ionization. As indicated previously, ionization is any process which results in the removal of an electron (negative charge) from an atom or molecule thereby leaving the atom or molecule with a net positive charge. Ionization occurs if alpha or beta particles, or gamma photons transfer sufficient energy to dislodge one of the electrons from the outer orbital shells of the target atom. Each ionization event produces an ion pair consisting of a free electron and the positively charged remainder of the atom (Figure 2-XIII).
Figure 2-XII. Excitation of an Electron
Figure 2-XIII. Electron Removal by Ionization
(TO BE CONTINUED... :)