Nuclei, neutron bombardment

In 1938 Niels Bohr had brought the astounding news from Europe that the radiochemists Otto Hahn and Fritz Strassmann in Berlin had conclusively demonstrated that one of the products of the bom-bardmeiit of uranium by neutrons was barium, with atomic number 56, in the middle of the periodic table of elements. He also announced that in Stockholm Lise Meitner and her nephew Otto Frisch had proposed a theory to explain what they called nuclear fission, the splitting of a uranium nucleus under neutron bombardment into two pieces, each with a mass roughly equal to half the mass of the uranium nucleus. The products of Fermi s neutron bombardment of uranium back in Rome had therefore not been transuranic elements, but radioactive isotopes of known elements from the middle of the periodic table. 

Almost every nuclide undergoes neutron capture if a source of neutrons is available. Unstable nuclides used in radiochemical applications are manufactured by neutron bombardment. A sample containing a suitable target nucleus is exposed to neutrons coming from a nuclear reactor (see Section 22-1). When a target nucleus captures a... 

The element was discovered in the pitchblende ores by the German chemist M.S. Klaproth in 1789. He named this new element uranium after the planet Uranus which had just been discovered eight years earlier in 1781. The metal was isolated first in 1841 by Pehgot by reducing the anhydrous chloride with potassium. Its radioactivity was discovered by Henry Becquerel in 1896. Then in the 1930 s and 40 s there were several revolutionary discoveries of nuclear properties of uranium. In 1934, Enrico Fermi and co-workers observed the beta radioactivity of uranium, following neutron bombardment and in 1939, Lise Meitner, Otto Hahn, and Fritz Strassmann discovered fission of uranium nucleus when bombarded with thermal neutrons to produce radioactive iso-... 

This reaction is a fission reaction. It shows a heavy uranium nucleus being bombarded by a neutron and decaying into two lighter nuclei (barium and krypton). This is the very reaction that takes place in a nuclear reactor. 

The atom was once thought to be the smallest unit of matter, but was then found to be composed of electrons, protons, and neutrons. The question arises are electrons, protons, and neutrons made of still smaller particles In the same way that Rutherford was able to deduce the atomic nucleus by bombarding atoms with alpha particles (Chapter 3), evidence for the existence of many other subatomic particles has been obtained by bombarding the atom with highly energetic radiation.This research over the past centmy has evolved into what is known as the "standard model of fundamental particles, which places all constituents of matter within one of two categories quarks and leptons. 

When heavier nuclei are bombarded by slow neutrons, the nuclei of lighter elements are formed. Besides the energy released, several neutrons are emitted. The disintegration of a heavier nucleus into lighter nuclei by neutron bombardment is called nuclear fission (nuclear division). 

Disintegration of an 23fj nucleus into 92Kr and 141Ba as a result of neutron bombardment. 

Neutron Activation Analysis This technique permits the quantitative and qualitative identification of elements. It is based on the conversion of a stable atomic nucleus into a radioactive nucleus by bombarding it with neutrons. The radiation emitted by the radioactive nuclei is then measured [154]. The advantages of NAA include [16, 154] ... 

Hahn and Strassmann reported on their finding to Meitner. In discussion with Otto Frisch (1904-), her nephew and a physicist, Meitner recognized that the neutron bombardment had split the uranium nucleus. She called this process fission, and, in January 1939 with Frisch, she sent a letter to the journal Nature announcing the discovery of nuclear fission. 

Figure 23.13 Induced fission of A neutron bombarding a nucleus results in an extremely unstable nucleus, which becomes distorted in the act of splitting. In this case, which shows one of many possible splitting patterns, the products are Kr and Ba. Three neutrons and a great deal of energy are released also.

In nuclear fission, neutron bombardment causes a nucleus to split, releasing neutrons that split other nuclei to produce a chain reaction. A nuclear power plant controls the rate of the chain reaction to produce heat that creates steam, which is used to generate eiectricity. Potential hazards, such as radiation leaks, thermal pollution, and disposal of nuclear waste, remain current concerns. Nuclear fusion holds great promise as a source of clean abundant energy, but it requires extremely high temperatures and is not yet practical. 

The two young physicists had come to compare experiments with two of Rutherford s boys. An unanswered question hung over the neutron work, a question that called existing nuclear theory into doubt The Nature paper they brought with them discussed the difficulty frankly. It concerned what is called radiative capture, the typical reaction of the heavy elements to neutron bombardment a nucleus captures a neutron, emits a photon of gamma radiation to stabilize itself energetically and thus becomes an isotope one mass unit heavier. 

What I should like, Henry Tizard wrote Mark Oliphant after he had studied the Frisch-Peierls memoranda, would be to have quite a small committee to sit soon to advise what ought to be done, who should do it, and where it should be done, and I suggest that you, Thomson, and say Blackett, would form a sufficient nucleus for such a committee. Thomson was G. P. Thomson, J.J. s son, the Imperial College physicist who had ordered up a ton of uranium oxide the previous year to study and felt ashamed at the absurdity. He had concluded after neutron-bombardment experiments that a chain reaction in natural luranium was unlikely and a war project therefore impractical. Tizard, who had been skeptical to begin with and had taken Thomson s conclusions as support for his skepticism, appointed Thomson chairman of the small committee James Chadwick, now at Liverpool, his assistant P. B. Moon and Rutherford prot g John... 

Fig. 17. Contour diagrams showing the relative 3deld of radionuclides plotted in the proton-neutron plane. The circles represent nuclides whose yields were measured the star indicates the target nucleus (arsenic bombarded by 190 Mev deuterons in the upper, and copper bombarded by 340 Mev protons in the lower figure) the lines connect regions with approximately equal yields, each line representing a factor of ten in yield on an arbitrary scale. [Figure from Templeton Ann. Rev. Nucl. Sci. 2, 93 (1953).]...

Neutron bombardment produces transformation of the nucleus—a process which is invariably accompanied by the emission of electrons, y-rays or particles of some kind. These in turn will lead to chemical fragmentation. 

Nuclear transmutations are sometimes represent by listing, in order, the target nucleus, the bombarding particle, the ejected particle, and the product nucleus. Written in this fashion. Equation 21.8 is 7N(a, p) 80. The alpha particle, proton, and neutron are abbreviated as a, p, and n, respectively. 

In nuclear fission, neutron bombardment causes a nucleus to split into two smaller nuclei and release neutrons that split other nuclei, giving rise to a chain reaction. 

Graham s law has practical application in the preparation of fuel rods for nuclear fission reactors. Such reactors depend on the fact that the uranium-235 nucleus undergoes fission (splits) when bombarded with neutrons. When the nucleus splits, several neutrons are emitted and a large amount of energy is liberated. These neutrons bombard more uranium-235 nuclei, and the process continues with the evolution of more energy. However, natural uranium consists of 99.27% uranium-238 (which does not undergo fission) and only 0.72% uranium-235 (which does undergo fission). A uranium fuel rod must contain about 3% uranium-235 to sustain the nuclear reaction. 

EXERCISE 21.6 Carbon-14 is produced in the upper atmosphere when a particular nucleus is bombarded with neutrons. A proton is ejected for each nucleus that reacts. What is the identity of the nucleus that produces carbon-14 by this reaction ... 

From the onset, Meitner s team, as well as all other scientists at the time, operated under two false assumptions. The first involved the makeup of the bombarded nuclei. In every nuclear reaction that had been observed, the resulting nucleus had never differed from the original by more than a few protons or neutrons. Thus, scientists assumed that the products of neutron bombardment were radioisotopes of elements that were at most a few places in the periodic table before or beyond the atoms being bombarded (as Fermi had presumed in hypothesizing the transuranes). 

When a nucleus is bombarded by a neutron to produce a nucleus, what else is produced ... 

Today, many radioisotopes are prodneed in small amounts by converting stable, nonradioaclive isotopes into radioactive ones. In a process called transmutation, a stable nucleus is bombarded by high-speed particles such as alpha particles, protons, neutrons, and small nuclei. When one of these particles is absorbed, the stable nucleus is converted to a radioactive isotope and usually some type of radiation particle. 

Three researchers in Germany— Lise Meitner (1878-1968), Fritz Strassmann (1902-1980), and Otto Hahn (1879-1968)— repeated Fermi s experiments, and then performed careful chemical analysis of the products. What they found in the products— several elements lighter than uranium—would change the world forever. On January 6, 1939, Meitner, Strassmann, and Hahn reported that the neutron bombardment of uranium resulted in nuclear fission—the splitting of the uranium atom. The nucleus of the neutron-bombarded uranium atom had been split into barium, krypton, and other smaller products. They also determined that the process emits enormous amounts of energy. A nuclear equation for a fission reaction, showing how uranium breaks apart into the daughter nuclides, is shown here. 

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