Atomic nucleus bombardment

RBS is based on collisions between atomic nuclei and derives its name from Lord Ernest Rutherford who first presented the concept of atoms having nuclei. When a sample is bombarded with a beam of high-energy particles, the vast majority of particles are implanted into the material and do not escape. This is because the diameter of an atomic nucleus is on the order of 10 A while the spacing between nuclei is on the order of 1 A. A small fraction of the incident particles do undergo a direct collision with a nucleus of one of the atoms in the upper few pm of the sample. This collision actually is due to the Coulombic force present between two nuclei in close proximity to each other, but can be modeled as an elastic collision using classical physics. 

The number of protons in an element s atomic nucleus is called the atomic number, Z, of that element. For example, hydrogen has Z = 1 and so we know that the nucleus of a hydrogen atom has one proton helium has Z = 2, and so its nucleus contains two protons. Henry Moseley, a young British scientist, was the first to determine atomic numbers unambiguously, shortly before he was killed in action in World War I. Moseley knew that when elements are bombarded with rapidly moving electrons they emit x-rays. He found that the properties of the x-rays emitted by an element depend on its atomic number and, by studying the x-rays of many elements, he was able to determine the values of Z for them. Scientists have since determined the atomic numbers of all the known elements (see the list of elements inside the back cover). 

The most familiar fission reactions involve the splitting of uranium atoms. In these reactions, a uranium-235 atom is bombarded with neutrons. The uranium nucleus then splits apart into various product nuclei. Two examples of fission reactions that involve uranium-235 are shown in Figure 5.10. 

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. 

Spallation occurs in a particle accelerator when a high-energy proton bombards a heavy atomic nucleus, resulting in some neutrons being knocked out or spalled . 

To make a new element, scientists have to add protons to an element that already exists. (Remember that an element is defined by how many protons it has in its nucleus.) Scientists sometimes do this by bombarding an element with neutrons. These free neutrons can break tiny particles off of the neutrons in an atom s nucleus and turn them into protons. In some cases, researchers use light particles such as the nuclei of helium atoms to bombard a target element. Some of the heaviest elements (above atomic number 106) were created in a kind of gentle smashup of a heavy element such... 

Moreover, since the new atomic nucleus produced is in an excited nuclear state, it has an excess of energy to get rid of, whether by emission of neutrons and y-rays, or by fission (Figure 14.2) (If the energy of the bombarding particles is too low, they do not possess enough energy to overcome the Coulombic repulsion between them.)... 

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] ... 

Radiation sterilization includes the use of the ionizing radiation of x-rays and gamma-rays. The former are derived from bombardment of a heavy metal target with electrons. Gamma-rays are obtained from atomic nucleus decay from excited to ground state. 

At a spallation source a heavy-metal target, such as Pb, W, Ta or Hg, is bombarded with energetic particles, usually protons accelerated to energies of up to 1 GeV. Neutrons freshly released from an atomic nucleus have high energies, referred to as epithermal neutrons , and must be slowed down to be useful for powder diffraction experiments. This occurs by collisions between the neutrons and the moderator - such as liquid methane or water - placed in the path of the neutron beam, which cause the exchange of energy and a trend towards (partial) thermal equilibrium. 

Fission. The splitting of an atomic nucleus into two fragments that usually releases neutrons and y rays. Eission may occur spontaneously or may be induced by capture of bombarding particles. Primary fission products usually decay by particle emission to radioactive daughter products. The chain reaction that may result in controlled burning of nuclear fuel or in an uncontrolled nuclear weapons explosion results from the release of 2 or 3 neutrons/fission. Neutrons cause additional fissile nuclei in the vicinity to fission, producing still more neutrons, in turn producing still... 

In 1913, two years after Rutherford proposed the nuclear model of the atom (Section 2.2), English physicist Henry Moseley (1887-1915) developed the concept of atomic numbers. Bombarding different elements with high-energy electrons, Moseley found that each element produced X-rays of a unique frequency and that the frequency generally increased as the atomic mass increased. He arranged the X-ray frequencies in order by assigning a unique whole number, called an atomic number, to each element. Moseley correctly identified the atomic number as the number of protons in the nucleus of the atom. (Section 2.3)... 

The bombardment of compounds with neutrons or a particles causes major changes in the nature of the atomic nucleus and such effects are utilised in the preparation of unstable isotopes. 

Rutherford s experiments opened the door to nuclear transmutations of all kinds. Atoms were bombarded by alpha particles, neutrons, protons, deuterons (iH), electrons, and so forth. Massive instruments were developed for accelerating these particles to very high speeds and energies to aid their penetration of the nucleus. The famous cyclotron was developed by E. O. Lawrence (1901-1958) at the University of California later instruments include the Van de Graaf electrostatic generator, the betatron, and the electron and proton synchrotrons. With these instruments many nuclear transmutations became possible. Equations for a few of these are as follows ... 

Rutherford and others postulated that there must be another type of subatomic particle in the atomic nucleus the proof was provided by another English pltysicist, James Chadwick, in 1932. When Chadwick bombarded a thin sheet of beiylhum with a particles, a very high energy radiation similar to y rays was emitted by the metal. Later experiments showed that the rays actually consisted of electrically neutral particles having a mass slightly greater than that of protons. Chadwick named these particles neutrons. 

Rutherford found evidence for the existence of the atomic nucleus by bombarding gold foil with a beam of positively charged particles. 

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). 

---