Alpha-particle beams, bombardment

The experiment conducted by Rutherford and his co-workers involved bombarding gold foil with alpha particles, which are doubly charged helium atoms. The apparatus used in their experiment is shown in Figure 14-9. The alpha particles are produced by the radioactive decay of radium, and a narrow beam of these particles emerges from a deep hole in a block of lead. The beam of particles is directed at a thin metal foil, approximately 10,000 atoms thick. The alpha particles are delected by the light they produce when they collide with scintilltaion screens, which are zinc sulfide-covered plates much like the front of the picture tube in a television set. The screen... 

Neutrons bear no charge, so they are not repelled by nuclei as positively charged projectiles are. They do not need to be accelerated to produce bombardment reactions. Neutron beams can be generated in several ways. A frequently used method involves bombardment of beryllium-9 with alpha particles. 

Indeed, there were no practicable methods for accumulation of berklium and californium in sufficient quantities to prepare targets to be bombarded by alpha particles as a means of synthesizing elements 99 and 100. This was due to short half-lives of berklium and californium measured in hours and minutes (long-lived isotopes were unknown at the time). There was only one more or less feasible method, namely, to bombard plutonium with a high-intensity neutron beam but then the results would be obtained only many years later. 

RBS can provide absolute quantitative analysis of elemental composition with an accuracy of about 5%. It can provide depth-profile information from surface layers and thin films to a thickness of about 1 pm. In some cases, however, the high-energy beam can damage the surface. This is particularly a problem with insulating materials, such as polymers, alkali halides, and oxides. The Mars Pathfinder mission in 1997 contained an alpha proton X-ray spectrometer (APXS). In its RBS mode, the spectrometer bombarded samples with alpha particles and determined elemental composition via energy analysis of the backscattered particles. In addition to RBS, the APXS instrument was designed to carry out proton emission and particle-induced X-ray emission (PIXE) experiments. Soil and rock compositions were measured and compared to those from the earlier Viking mission. 

In 1919 Ernest Rutherford answered Yes to that question. He found that he could bombard the nucleus of a nitrogen atom with a beam of alpha particles from a radioactive source, producing an atom of oxygen-17 and a hydrogen atom ... 

Nuclear reactions resulting in particle emission Some nuclear reactions result in the emission of a particle (proton or alpha) with an energy higher than the primary beam energy so that they can be detected unambiguously in the detector used for RBS analysis. Relatively few reactions are useful for analytical purposes. One example is Li (p,a) He, which occurs when Li is bombarded with 2-3 MeV protons. Two alpha particles are emitted which have energies of 7-8 MeV and can be detected with no interference from the spectrum of backscattered protons. 

Radioactive isotopes for tracer studies may be prepared artificially from nonradioactive elements by bombarding them with suitable nuclear particles produced in a cyclotron or a nuclear reactor. The discovery of this effect was made in 1934 by the French physicists Irene Joliot-Curie (1897-1956) and her husband Frederic Joliot-Curie (1900-1958). They were studying the effect of bombarding light elements such as aluminum with alpha (a) particles, which are beams of helium nuclei, fHe. They noticed that, after the bombardment had ceased, a new form of radiation continued to be emitted, and they concluded that a new isotope had been formed. In the case of the bombardment of ordinary aluminum, HAl, with a particles, the product is an isotopic form of phosphorus, ifP, the most abundant isotope of phosphorus being f P. The process is... 

---