Nuclear reactions artificial

Number OF Half- Lives OF Initial Quantity Remaining Quantity Remaining (MG) 

FIGURE 13.4 Radioactive decay of 20 mg of oxygen-15, which has a half-life of 2.0 min. The plotted data are given in the following table. After each half-life period, the quantity present at the beginning of the period is reduced by one-half. 

The first evidence for radioactivity occurred when photographic films were exposed when placed near samples of uranium, (a) True, 

Which of these forms of radiation is the most penetrating (a) Alpha particles, (b) Beta particles, (c) Gamma rays  

The mass number of a nucleus is unchanged after beta-particle emission, (a) True, (b) False. 

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A further group of elements, the transuranium elements, has been synthesized by artificial nuclear reactions in the period from 1940 onwards their relation to the periodic table is discussed fully in Chapter 31 and need not be repeated here. Perhaps even more striking today are the predictions, as yet unverified, for the properties of the currently non-existent superheavy elements.Elements up to lawrencium (Z = 103) are actinides (5f) and the 6d transition series starts with element 104. So far only elements 104-112 have been synthesized, ) and, because there is as yet no agreement on trivial names for some of these elements (see pp. 1280-1), they are here referred to by their atomic numbers. A systematic naming scheme was approved by lUPAC in 1977 but is not widely used by researchers in the field. It involves the use of three-letter symbols derived directly from the atomic number by using the... 

The discovery of neutron particles was based on artificial nuclear reactions. James Chadwick bombarded the nuclei of Be atoms with a-particles and obtained neutron particles in 1932. 

None of the reactions or processes stndied in previous chapters affected the nucleus of an atom. No atom changed from one element to another. This chapter considers the effects of nuclear change. In most cases, such changes cause a transformation from one element to another. They include the natural radioactivity of certain isotopes (Section 3.3), as well as the artificial nuclear reactions discovered during the twentieth century. Nuclear reactions differ from ordinary chemical reactions in the following ways ... 

These reactions are examples of artificial transmutation—the change of one element into another. Several small particles, in addition to those involved in natural radioactivity, are involved in artificial nuclear reactions. Some of these additional particles are listed in Table 21.4. They are used as projectiles to bombard nuclei or are produced along with other products of such reactions, or both. 

Bombardment of certain nuclei with small particles, such as alpha particles or neutrons, can lead to artificial nuclear reactions. The splitting of heavy atoms is one such process, called nuclear fission. Two fairly massive products plus some small particles are apt to result from splitting one large nucleus with a projectile particle. For example ... 

The hunt for new elements continues. To date, many elements between 93 and 118 have been synthesized artificially nuclear reactions. The heaviest elements are very unstable and cannot be of any practical use, but scientists predicted that an island of stability exists at around element 114 (one atom of this element was reported in January 1999, and it does seem to be more stable than other similarly heavy elements). Attempts have already been made to predict the properties of, as yet, undiscovered elements using Periodic trends. 

The law of conservation of matter Matter is neither lost nor gained in chemical reactions (Section 3.1). The only known exception to this law is in nuclear reactions, which occur only with radioactive isotopes or under the special conditions of artificial nuclear reactions (Section 13.5). The conservation law (so far) has always been reliable for chemical changes other than nuclear reactions. 

The neutron provided the nuclear physicist (or chemist) for the first time with a simple means to carry out artificial nuclear reactions. The production of neutrons did not require large and expensive accelerators. A mixture of beryllium powder with an a emitter in a little tube was sufficient to produce a neutron source of 10 neutrons per second. Neutrons possess no charge. Consequently, they do not have to overcome a Coulomb barrier and therefore do not have to be accelerated to enter an atomic nucleus. On the contrary, capture cross sections are usually larger for neutrons of smaller energy (thermal neutrons) because due to their lower velocity their residence time near a particular nucleus is larger. 

Many artificial (likely radioactive) isotopes can be created through nuclear reactions. Radioactive isotopes of iodine are used in medicine, while isotopes of plutonium are used in making atomic bombs. In many analytical applications, the ratio of occurrence of the isotopes is important. For example, it may be important to know the exact ratio of the abundances (relative amounts) of the isotopes 1, 2, and 3 in hydrogen. Such knowledge can be obtained through a mass spectrometric measurement of the isotope abundance ratio. 

Nucleosynthesis is the formation of elements. Hydrogen and helium were produced in the Big Bang all other elements are descended from these two, as a result of nuclear reactions taking place either in stars or in space. Some elements—among them technetium and promethium—are found in only trace amounts on Earth. Although these elements were made in stars, their short lifetimes did not allow them to survive long enough to contribute to the formation of our planet. However, nuclides that are too unstable to be found on Earth can be made by artificial techniques, and scientists have added about 2200 different nuclides to the 300 or so that occur naturally. 

Radioactivity, Induced—Radioactivity produced in a substance after bombardment with neutrons or other particles. The resulting activity is "natural radioactivity" if formed by nuclear reactions occurring in nature and "artificial radioactivity" if the reactions are caused by man. 

Nuclear reactions involving technetium have been actively studied until today. Our interest in the nuclear chemistry of technetium is based on various reasons. Technetium was the first artificially produced element in the periodic table, a weighable amount of technetium ("Tc) is now available, and 99mTc is one of the most important radionuclides in nuclear medicine. In addition, technetium is an element of importance from a nuclear safety point of view. 

F is the remaining fraction of 38Ar at temperature t °C. t is derived from 37At outgassing data because this isotope is artificially produced by a nuclear reaction between fast neutrons and Ca. 

ISOTOPES There are 50 Isotopes of Yttrium. Only one Is stable (Y-89), and It constitutes 100% of the element s natural existence on Earth. The other Isotopes range from Y-77 to Y-108 and are all produced artificially In nuclear reactions. The radioactive Isotopes have half-lives ranging from 105 nanoseconds to 106.65 days. 

There is no natural curium on Earth. All of its isotopes are man-made and artificially produced through nuclear reactions with other elements. The curium isotope Cm-242 was first produced by bombarding plutonium-239 with helium nuclei (alpha particles), which contributed neutrons that changed Pu to g Cm. 

Californium is a synthetic radioactive transuranic element of the actinide series. The pure metal form is not found in nature and has not been artificially produced in particle accelerators. However, a few compounds consisting of cahfornium and nonmetals have been formed by nuclear reactions. The most important isotope of cahfornium is Cf-252, which fissions spontaneously while emitting free neutrons. This makes it of some use as a portable neutron source since there are few elements that produce neutrons all by themselves. Most transuranic elements must be placed in a nuclear reactor, must go through a series of decay processes, or must be mixed with other elements in order to give off neutrons. Cf-252 has a half-life of 2.65 years, and just one microgram (0.000001 grams) of the element produces over 170 mhhon neutrons per minute. 

Neither californium nor its compounds are found in nature. All of its isotopes are produced artificially in extremely small amounts, and all of them are extremely radioactive. All of its isotopes are produced by the transmutation from other elements such as berkelium and americium. Following is the nuclear reaction that transmutates californium-250 into cahfornium-252 Cf + (neutron and A, gamma rays) — Cf + (neutron and A, gamma rays) —> Cf. 

TRANSMUTATION. The natural or artificial transformation of atoms of one element into atoms of a different element as the result of a nuclear reaction. The reaction may be one in which two nuclei interact, as in the formation of oxygen from nitrogen and helium nuclei (/3-particles), or one in which a nucleus reacts widi an elementary particle such as a neutron or proton. Thus, a sodium atom and a proton form a magnesium atom. Radioactive decay, e.g., of uranium, can be regarded as a type of transmutation. The first transmutation was performed bv the English physicist Rutherford in 1919. 

Atoms with identical atomic numbers but different mass numbers are called isotopes, and the nucleus of a specific isotope is called a nuclide. There are 13 known isotopes of carbon, two of which occur commonly (12C and 13C) and one of which (14C) is produced in small amounts in the upper atmosphere by the action of neutrons from cosmic rays on 14N. The remaining 10 carbon isotopes have been produced artificially. Only the two commonly occurring ones are indefinitely stable the other 11 undergo spontaneous nuclear reactions, which change their nuclei. Carbon-14, for example, slowly decomposes to give nitrogen-14 plus an electron, a process we can write as... 

Following this theory, the transformation of one element into another one was realized. The nuclear equation of this artificial radioactive reaction is illustrated below ... 

One may rightfully raise the question as to why some products of nuclear reactions are radioactive while others are not. The answer concerns the stability of atomic nuclei. Essentially, any radioactive element, whether artificial or natural, can be considered abnormal. A nucleus that undergoes radioactive decay is in an unstable condition, and the process of decay always leads to stable isotopes. This tendency toward the achievement of stability is illustrated by the stepwise decay of naturally radioactive uranium to form a stable isotope of lead and the formation of stable carbon by the decay of artificial radioactive nitrogen. Although the conditions resulting in the instability of atomic nuclei are fairly well understood, further consideration of these factors is beyond the scope of this discussion. 

After 1933 Fermi turned increasingly to experimental physics. Inspired by recent work in which artificial radioactive substances were produced by a-particle bombardment, Fermi and several collaborators used neutron bombardment to create several transuranic elements heavier than uranium, including plutonium. This work, and his finding that slow neutrons produce nuclear reactions more efficiently than fast ones, earned Fermi wide acclaim and the 1938 Nobel Prize in physics. After accepting the prize in Sweden, Fermi and his Jewish wife immigrated to the United States to escape the Nazis. 

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