19.Nuclear reactions by alpha particles.

The first transmutation reaction discovered by Rutherford was an alpha induced reaction, in which nitrogen nuclei were bombarded with alpha particles derived from a naturally occurring radioactive isotope polonium. Neutron was also discovered by Chadwick (1932) by bombarding beryllium with alpha particles. Before the discovery of neutron, the α-particle from natural radioactive sources were the only projectiles for nuclear bombardment. It is therefore, evident that alpha particle played an important role in the early development o nuclear chemistry and nuclear reactions. However, the high potential barrier of heavy nuclei is difficult to be penetrated by α – particle have greater kinetic energies than they generally posses. (α, p) and (α, n) are the most important transmutation by α-particles from natural sources.

By making use of high speed alpha particles some transuranium elements have also been synthesised

For alpha particles, the Coulomb barrier is still higher, reaching 25 MeV for heavy nuclei. At this energy of the incident alpha particles, the excitation energy of the nucleus is about 20 MeV, which is sufficient not only to compensate for the binding energy of the emitted nucleon but also to overcome the Coulomb barrier by the emitted proton. As a consequence, the (α, n) and (α, p) reactions are equally probable. Upon an increase in the energy of alpha particles, the (α, 2n) and (α, pn) reactions become most probable. A resonance structure of the energy dependence of the cross sections of these nuclear reactions is observed only in the case of light nuclei and at relatively low energies of alpha particles. The products of the (α, n) reaction are usually beta-active, and the products of the (α, p) reaction are stable nuclei.

28.Total cross sections

A cross section is the effective area that governs the probability of some scattering or absorption event. Together with particle density and path length, it can be used to predict the total scattering probability via the Beer–Lambert law.

In nuclearandparticle physics,the concept of a cross section is used to express the likelihood of interaction between particles.

When particles in a beam are thrown against a foil made of a certain substance, the cross section is a hypothetical areameasure around the target particles of the substance (usually its atoms) that represents a surface. If a particle of the beam crosses this surface, there will be some kind of interaction.

The term is derived from the purely classicalpicture of (a large number of)point-likeprojectiles directed to an area that includes a solid target. Assuming that an interaction will occur (with 100% probability) if the projectile hits the solid, and not at all (0% probability) if it misses, the total interaction probability for the single projectile will be the ratio of the area of the section of the solid (thecross section, represented by ) to the total targeted area.

The cross section σ_r is specifically for one type of reaction, and is called the partial cross section. The total cross section, and corresponding total rate of the reaction, can be found by summing over the cross sections and rates for each reaction:

4. Nuclear fission and fusion. Nuclear fission and nuclear fusion both are nuclear phenomena that release large amounts of energy, but they are different processes which yield different products. Learn what nuclear fission and nuclear fusion are and how you can tell them apart. Nuclear Fission Nuclear fission takes place when an atom's nucleus splits into two or more smaller nuclei. These smaller nuclei are called fission products. Particles (e.g., neutrons, photons, alpha particles) usually are released, too. This is an exothermic process releasing kinetic energy of the fission products and energy in the form of gamma radiation. Fission may be considered a form of element transmutation since changing the number of protons of an element essentially changes the element from one into another. Nuclear Fission Example ²³⁵₉₂U + ¹₀n → ⁹⁰₃₈Sr + ¹⁴³₅₄Xe + 3¹₀n Nuclear Fusion Nuclear fusion is a process in which atomic nuclei are fused together to form heavier nuclei. Extremely high temperatures (on the order of 1.5 x 10⁷°C) can force nuclei together. Large amounts of energy are released when fusion occurs. Nuclear Fusion Examples The reactions which take place in the sun provide an example of nuclear fusion:1) ¹₁H + ²₁H → ³₂He 2) ³₂He + ³₂He → ⁴₂He + 2¹₁H 3)¹₁H + ¹₁H → ²₁H + ⁰₊₁β. Distinguishing between Fission and Fusion. Both fission and fusion release enormous amounts of energy. Both fission and fusion reactions can occur in nuclear bombs. So, how can you tell fission and fusion apart? 1)Fission breaks atomic nuclei into smaller pieces. The starting elements have a higher atomic number than that of the fission products. For example, uranium can fission to yield strontium and krypton. 2)Fusion joins atomic nuclei together. The element formed has more neutrons or more protons than that of the starting material. For example, hydrogen and hydrogen can fuse to form helium.

31. Nuclear Chain Reactions are a simple, yet powerful method which to produce both constructive and destructive forces. Only understood to a significant degree within the last century, nuclear chain reactions have many practical uses in the modern era. Chain reactions can be addressed into two categories: first, controlled (like a nuclear power plant) and uncontrolled (an atomic bomb). Both are motivated by fission reactions, which are elaborated in this section.

13. Neutron Multiplication Factor The average lifetime of a single neutron in the reactor neutron cloud may be as small as one ten-millionth of a second. This means that in order for the cloud to remain in existence, each neutron must be responsible for producing another neutron in less than one ten millionth of a second. Thus, one second after a neutron is born, its ten-millionth generation descendent is born. (The term neutron generation will be used to refer to the "life" of a group of neutrons from birth to the time they cause fission and produce new neutrons). However, not all of the neutrons produced by fission will have the opportunity to cause new fissions because some will be absorbed by non-fissile material and others will leak out of the reactor. The number of neutrons absorbed or leaking out of the reactor will determine whether a new generation of neutrons is larger, smaller, or the same size as its predecessor. A measure of the increase or decrease in size of the neutron cloud is the ratio of the neutrons produced to the sum of the neutrons absorbed in fission or non-fission reactions, plus those lost in any one generation. This ratio is called the effective multiplication factor and may be expressed mathematicalIf the production of neutrons by one generation is greater than the sum of its absorption and the leakage, k_eff will be greater than 1.0, e.g., 1.1, and the neutron flux will increase with each generation. If, on the other hand, k_eff is less than 1.0, perhaps 0.9, the flux will decrease with each generation. If the size of each successive generation is the same then the production exactly equals the losses by absorption and leakage. k_eff is then exactly 1.0 and the reactor is said to be critical. The multiplication factor can, therefore, also be defined as:Changes in the neutron flux cause changes in the power level of the reactor. Since the change in power level is directly affected by the multiplication factor, it is necessary to know more about how this factor depends upon the contents and construction of the reactor.

22. Nuclear reactions induced by mesons Mesons may be emitted in nuclear reactions induced by nucleons, whose energy is greater than the threshold energy of meson production. The mesons may also initiate nuclear reactions and participate in the development of an intranuclear cascade. Nuclear reactions involving pions have been studied most extensively. Many nuclear reactions induced by pions are like the corresponding nu-cleon-induced reactions, for example, inelastic scattering (π, π^’), charge-exchange scattering (π⁺ π⁰), (π^– π⁰), and knock-out reactions [(π, πp), (π, πn), (π^–, πd)]. However, there are other nuclear reactions involving pions that do not have analogs in nucleon-nuclear interactions. Among these is the reaction of double charge exchange of pions (π^–, π⁺) and nuclear reactions of pion absorption (π^–, 2n). The study of these reactions makes possible the investigation of the correlation of nucleons in the nucleus.