Alpha Particle Interactions

Neutron interaction with atomic nuclei is directly linked to the production of several noble gas isotope species in the crust. Before considering the production of crustal noble gases it is essential to first understand the factors controlling the subsurface neutron flux. There are three main types of reaction that produce neutrons in the crust Cosmic ray interactions spontaneous fission and alpha particle interaction with light nuclei. Cosmic ray interaction is only important within the top few meters of the crust (Niedermann 2002, this volume) and we neglect this source of neutrons here. 

Alpha particles cause extoisive ionization in matter. If the particles are allowed to pass into a gas, the electrons released by the ionization can be collected on a positive electrode to produce a pulse or curr t. Ionization chambers and proportional counters are instruments of this kind, which permit the individual counting of each a-particle emitted by a sanq)le. Alpha particles interacting with matter may also cause molecular excitation, which can result in fluoresc ce. This fluorescence — or scintillation — allowed the first observation of individual nuclear particles. The ionization in semiconductors caused by a-particles is now the most common means of detection, see Ch. 8. 

Silver-activated zinc sulfide [ZnS(Ag)] has been used since radioactivity was first measured to detect alpha particles. It is relatively insensitive to electrons and gamma rays because it is not transparent to its own radiation. Radiation interactions within the detector are not recorded only its surface, where alpha particles interact, emits scintillations. The ZnS is doped with silver to shift its scintillations to a longer wavelength for better PMT response. 

A new detector unit has been designed to add an internal check of radiation detection sensitivity by including a Th source in the detector LiF foil assembly. This provides continuous alpha-particle emission to test the ability of the foil detector to sense the neutron alpha-particle interaction in the LiF foil. An internal clock circuit forces the SCR to trigger if the detector amplifier fails for longer than 1 min. The unit produces a radiation alarm with any detected pulse rate from the LiF foil >4 pulse/s. Mechanical and electrical compatibility of the new detector design with the existing detection system has been a basic requirement not only to maintain the existing radiation sensitivity but also to minimize installation costs. 

Alpha particles interact with matter by pulling off orbital electrons from atoms as they are deflected by the positively charged nuclei of these atoms. They also can cause excitation of the atoms by pulling orbital electrons into higher orbits. The energy thus deposited in the atoms can break chemical bonds, and secondary ionizations can be caused by production of free electrons and by secondary x-rays produced when the excited atom drops back into the ground state. Alpha particles have very high specific ionizations. 

During the red giant phase of stellar evolution, free neutrons are generated by reactions such as C(a,n) and Ne(a,n) Mg. (The (ot,n) notation signifies a nuclear reaction where an alpha particle combines with the first nucleus and a neutron is ejected to form the second nucleus.) The neutrons, having no charge, can interact with nuclei of any mass at the existing temperatures and can in principle build up the elements to Bi, the heaviest stable element. The steady source of neutrons in the interiors of stable, evolved stars produces what is known as the "s process," the buildup of heavy elements by the slow interaction with a low flux of neutrons. The more rapid "r process" occurs in... 

Proportional counters can also count neutrons by introducing boron into the chamber. The most common means of introducing boron is by combining it with tri-fluoride gas to form Boron Tri-Fluoride (BF3). When a neutron interacts with a boron atom, an alpha particle is emitted. The BF3 counter can be made sensitive to neutrons and not to gamma rays. 

Blackett discovered that the process was not one of disintegration, but one of integration only two tracks were seen after the interaction occurred, meaning that the alpha particle was absorbed as the proton was ejected. The resulting nucleus was a heavy isotope of oxygen. 

Since all Rutherford could know from his scintillation experiments was that alpha particles infrequently caused nitrogen nuclei to emit protons—he could not see the actual interaction—he had assumed it was a disintegration process. Only the cloud chamber could provide a visual representation of the transmutation process itself and give physicists the chance to discover the intricacies of the exchange. 

Theoretical studies [25,42] have shown that significant amounts of a number of radionuclides usually assumed to be derived only from the atmosphere may actually be produced in the subsurface, largely through interactions with secondary neutrons produced by alpha capture reactions. The alpha particles are derived mostly from normal decay of natural U and Th. Whether or not subsurface production of radionuclides can indeed influence dating has yet to be demonstrated by field and laboratory tests. The matter needs further study, particularly in relation to 14C dating of water which is more than 40,000 years old. 

In their studies with cathode rays, researchers observed different rays traveling in the opposite direction of cathode rays. In 1907, Thomson confirmed the rays carried a positive charge and had variable mass depending on the gas present in the cathode-ray tube. Thomson and others found the positive rays were as heavy or heavier than hydrogen atoms. In 1914, Ernest Rutherford (1871-1937) proposed that the positive rays were composed of a particle of positive charge as massive as the hydrogen atom. Subsequent studies on the interaction of alpha particles with matter demonstrated that the fundamental positive particle was the proton. By 1919, Rutherford was credited with identifying the proton as the second fundamental particle. 

Second, lithium, beryllium, and boron have very low abundances. These elements are, for the most part, not made in stars and were not made efficiently in the Big Bang. They are produced via cosmic ray interactions. Nuclei of heavier atoms, when hit by fast moving protons or other nuclei, can break into pieces, including protons, neutrons, alpha particles, and heavier fragments. Some of these fragments are lithium, beryllium, and boron nuclei. 

Pressure measurements involve the interaction of alpha radiations with a gas, which results in the formation of positive and negative ions. The latter can be collected and measured as electric current The number of ions produced in a gas by alpha particles depends upon the density and composition of the gas. Where either of these factors is known the other can be inferred from these measurements. Several vacuum gages employ this principle. 

The absorption of thermal neutrons in B produces alpha particles according to the following interaction ... 

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