Nuclear Radiation Interactions

Sidney Yip

• Publisher: WSPC
• Year: 2014
• ISBN:
  9789814644570
• Topic: Physical Sciences
• Subtopic: Atomic & Molecular Physics

This book is a treatment on the foundational knowledge of Nuclear Science and Engineering. It is an outgrowth of a first-year graduate-level course which the author has taught over the years in the Department of Nuclear Science and Engineering at MIT. The emphasis of the book is on concepts in nuclear science and engineering in contrast to the traditional nuclear physics in a nuclear engineering curriculum. The essential difference lies in the importance we give to the understanding of nuclear radiation and their interactions with matter. We see our students as nuclear engineers who work with all kinds of nuclear devices, from fission and fusion reactors to accelerators and detection systems. In all these complex systems nuclear radiation play a central role. In generating nuclear radiation and using them for beneficial purposes, scientists and engineers must understand the properties of the radiation and how they interact with their surroundings. It is through the control of radiation interactions that we can develop new devices or optimize existing ones to make them more safe, powerful, durable, or economical. This is why radiation interaction is the essence of this book. Contents:

• • Context and Perspective
  • Organization
• Part 1: Nuclear Physics Background:
  • Nuclear Properties and Data
  • Stability of Nuclei
  • Energy-Level Models
  • Nuclear Disintegrations and Decays
  • Collision Cross Sections
  • Nuclear Reactions Fundamentals
• Part 2: Unit Processes of Nuclear Radiation Interactions:
  • Neutron Scattering
  • Gamma Scattering and Absorption
  • Charged Particle Stopping
  • Neutron Reactions
• Part 3. Cumulative Effects of Nuclear Radiation Interactions:
  • Neutron Transport

Advanced undergraduate or graduate students and researchers in nuclear physics. Key Features:

• A textbook on the basic study of nuclear interactions that are foundational to the field of nuclear science and engineering
• 40 pages of problems to test basic understanding and physical insight
• A special section — neutron transport — to connect with further studies of neutron interactions

1

Context and Perspective

Nuclear Engineering is that branch of science and engineering concerned with the various beneficial uses of nuclear radiations. In this discipline, for which we will use the broader designation of Nuclear Science and Engineering (NSE), two areas of study are foundational. One deals with the basic understanding of individual interactions of nuclear radiations with the atoms in a materials medium. The other deals with the collective evolution of nuclear radiations in systems that are engineered for specific technological applications. These two areas are intimately connected, and it is the fundamental connection between the two that we feel is noteworthy in the present overview. We will use the term Nuclear Interactions to denote a collection of single reaction events, each being an interaction (collision) between a particular type of nuclear radiation (particle) with an atom (nucleus and electrons) in a specified medium, while the term Radiation Transport will denote the distribution of nuclear radiations after many sequences of collision-induced reactions and spatial migrations have occurred. With this distinction it should be clear that Interactions are the elementary (unit) processes that provide the essential information needed in the study of Transport phenomena at the system level.

This book is intended to be primarily a treatment of the fundamentals of Nuclear Interactions for students in (NSE). Additionally the connections between Nuclear Interactions and Radiation Transport are also addressed, albeit to a limited extent. These connections are important for the students’ appreciation of the broader context of this work. To see this we briefly consider how the discipline of NSE has evolved during its approximately 60-year-old history (Sec. 1.1). In the first ∼30 years, NSE experienced rapid growth as many countries embraced the peaceful uses of nuclear energy. A period of uncertainty then followed as the acceptance of nuclear fission power became a subject of intense public debate. Over the last decade interests in nuclear energy gradually returned until another incident occurred in 2011 (Sec. 1.2). Now the overall picture for nuclear science and technology appears likely to evolve further. Against this background, we give an outlook on the relevance of Nuclear Interactions and Radiation Transport in a world of increasing technological complexity (Sec. 1.3).

1.1EVOLUTION OF NUCLEAR SCIENCE AND ENGINEERING

The historical development of NSE follows closely the development of nuclear fission power. As the use of this technology rises and falls worldwide, the discipline either thrived or struggled. Following World War II, the peaceful use of nuclear energy quickly gained attention in many countries. A number of nuclear engineering departments were established across the U.S., offering a multidisciplinary curriculum drawing heavily on nuclear physics (theoretical and experimental, particle transport), chemical engineering (fuel processing, isotope separation, chemical thermodynamics), and mechanical engineering (heat transfer and power engineering). Nuclear Interactions and Radiation Transport (equivalently nuclear reactor physics) emerged as two fundamental subjects in all the nuclear engineering curricula. This is the period of Early Promise extending from ~1950 to ~1980. The overlap between NSE and the discipline of nuclear physics was strong because of opportunities for research and applied technology.

With hindsight, it can be said the initial utilization of the emerging nuclear technology apparently did not adequately appreciate the full complexity of the technology, or the critical role of human error in reactor plant operation and control. Thus when an accident such as Three-Mile Island occurred (see Sec. 1.2), public opposition quickly gained momentum. Coupled to this fear of nuclear accident are concerns over safe disposals of the long-term waste. The public debates are factors that made a sophisticated technology more costly to introduce. In this period of Lean Years, the survival of nuclear engineering departments in the US became problematic for a number of universities. Correspondingly, there was little incentive for research in Nucleat Interactions, and efforts in developing more powerful computational methods in Radiation Transport were limited. It was during this period that NSE started to decouple from nuclear physics, as research opportunities moved to higher energies with less relevance to nuclear power. Around the turn of the last century interests in nuclear power started to increase as a result of two developments. One was the significant improvement in plant performance sustained over several years. The other, even more important, was the growing environmental concern of CO₂ emission. Additionally new demands for nuclear power have emerged in countries undergoing strong industrial growth. This period of Partial Renaissance continued for about a decade until the Fukushima incident (Sec. 1.2). The incident reminded the world of the vulnerabilities of nuclear power technology to unexpected environmental conditions, in this case a combination of earthquake and tsunami of unprecedented proportions.

1.2HISTORICAL EVENTS

A survey of the historical events relevant to the study of Nuclear Interactions would entail a section that would be more lengthy and detailed than what we feel would be appropriate for this work. The decision was made at the outset to focus on only a few events to illustrate the broad impact that nuclear interactions can have in the evolution of scientific and technological enterprises in our society. All the chapters in the book, except for this and Chap. 2, deals with technical contents. Here we take the opportunity to connect with the history of nuclear science and engineering, in a highly selective (but not arbitrary) manner. In aiming to be brief and coherent, we find it necessary to leave many interesting details to further reading of the references cited.

We propose to draw a connection among just five events, the discovery of a new nuclear radiation (the particle neutron), the discovery of a new nuclear reaction (neutron fission), and a group of three incidents each involving a nuclear power plant accident. This is admittedly an unusual set of events being considered together in a book of this type. In the last section of the chapter we will suggest what implications the present discussion may have in the context of motivation and historical perspective. We hope to show the thematic foundation for this book may be characterized as the linking of science to systems to society in the study of nuclear phenomena.

We begin with the discovery of the neutron as a form of nuclear radiation in 1932 by James Chadwick. This will be followed by a second discovery, that of neutron-induced fission six years later by Otto Hahn and Fritz Strassmann. Both are bona fide major events in the history of science. Chadwick was awarded the Nobel Physics Prize in 1936 and Hahn the Nobel Chemistry Prize in 1944. Historically the period surrounding the fission discovery, when many other significant events also took place, is perhaps the most fascinating era of nuclear science development, and much of the events can be related to nuclear radiation interactions in one way or another [Rhodes, 1986]. We then skipped forward to recent times and the issue of nuclear safety facing our society. By nuclear safety we mean the safety of nuclear power reactor technology [Seghal, 2012] rather than anything pertaining to nuclear weapons technology and proliferation [Garwin and Charpak, 2002; Glasstone and Dolan, 1977; Satori, 1983; Murray, 2009].

Discovery of neutron

Among the nuclear particles which we will study, neutron is the most special and important. Neutron interactions are the fundamental processes underpinning nuclear science and technology. Without neutrons there would be no nuclear power as we know it, and the discipline of nuclear science and engineering very likely would not exist. While there is a great deal to be said about the neutron [Schofield 1982], for setting the context for this book we focus exclusively on the discovery of this nuclear particle. The scientific announcement of the discovery itself was anything but spectacular, certainly not by the current standards of how scientific breakthroughs are presented to the public. This contrast should not go unnoticed by the reader, keeping in mind how the neutron’s discovery soon led to the discovery of nuclear fission, and from that event the development of nuclear technology in two major forms, energy production and weapons. In a one-page Letter to the Editor in the journal Nature James Chadwick presented a summary of experiments and analysis under the heading, “Possible Existence of a Neutron” [Chadwick 1932a]. The Letter was dated February 17, 1931. On May 10, 1932 he communicated to the Royal Society a full discussion of his findings in a paper now entitled “The Existence of a Neutron” [Chadwick 1932b]. With hindsight it is rather remarkable that a discovery with such great subsequent impact could be demonstrated in just a single page. A lesson for the students here is that nuclear science at that time was in its infancy, and much of what we now regard as common knowledge in fact was not known. If one follows Chadwick’s arguments in deducing the existence of a particle with no charge and yet great ionizing power, one can see that he relied basically on the conservation laws and kinematic relations between energy and momentum. While Chadwick made use of the work of others to arrive at his own conclusions, he was, in effect, “forced” to break with traditional thinking to postulate the existence of the neutron. Both the Letter to the Editor and the follow-up paper are worthwhile reading for insights into early-day research (and incisive deductions) in nuclear radiation interactions, especially after the reader has studied Chap. 8.

Discovery of nuclear fission

Following the neutron discovery experiments using neutrons to bombard uranium soon began at various laboratories in Rome (E. Fermi), Paris (I. Curie and F. Joliot), and Berlin (O. Hahn, L. Meitner, later F. Strassmann) [Segrè, 1989]. The thinking was that the bombardment would lead to the absorption of neutrons, and therefore elements heavier than uranium would be produced. What was not expected was that the neutrons could actually cause the uranium nucleus to be unstable and undergo a “fission” reaction. The events leading up to the discovery of fission, and the process of discovery, now involving more than a single individual, are considerably more complicated compared to the neutron discovery. Moreover, with each fission event producing more than 2 neutrons on the average, a self-sustained chain reaction could be achieved to produce a large amount of energy. Because the fission discovery occurred during the war time in Europe, it soon became apparent that this process can have two applications, peaceful power generation and weapons.

The immediate events prior to the discovery could be summarized as follows. With the expectation of producing transuranic element 93 with chemical properties resembling rhenium (element 75), the Rome group reported finding products (actually fission products like ₄₃Tc) which were interpreted to be element 93. Similar conclusions were made in Berlin and Paris. On the other hand, the possibility of fission had been pointed out as early as 1934 [Noddack, 1934]; however, no experiments were performed to verify the hypothesis. In 1937–1938 I. Curie and P. Savitch reported finding bombardment product chemically resembling lanthanum, but did not realize or prove that it was indeed ₅₇La¹⁴¹[Curie and Savitch, 1938(a)]. In 1938, L. Meitner, an Austrian citizen, fled to Sweden (when Hitler annexed Austria) and continued work with nephew Otto Frisch in Copenhagen. In two papers, dated July 12, 1938 [Curie and Savitch, 1938(b)] and November 8, 1938 [Hahn et al., 1938], the participating scientists still believed that neutron bombardment of uranium would lead to only transuranic products. The dramatic change, the realization that fission was being observed, was announced by Hahn and Strassmann, in a paper dated December 22, 1938 [Hahn and Strassmann, 1939]. In this publication four reactions were observed and reported. They were written out as:

“RaI”? → AcI → “Th”?

“RaII”? → AcII → “Th”?

“RaIII”? → AcIII → “Th”?

“RaIV”? → AcIV → “Th”?

The elements were identified as Ra and Th in quotation mark because the authors, while thinking they should be interpreted as Radium and Thorium, were no longer sure. In fact in the paper they argued to the contrary [Segrè, 1989]:

“As chemists, in consequence of the experiments just described, we should change the schema given above and introduce the symbols Ba, La, Ce in place of Ra, Ac, Th. As ‘nuclear chemists’ working very close to the field of physics, we cannot yet bring ourselves to take such a drastic step, which goes against all previous experiences of nuclear physics.”

as quoted in Segrè (1989)

It is interesting that in those early days of nuclear research, the chemists seemed to defer to the physicists for the last word on fundamental scientific knowledge. In 1944, Otto Hahn was awarded the Nobel Prize in chemistry for the discovery of nuclear fission. One might ask whether the discovery was worthy of a Physics Prize. Also, to this date, discussions continue on whether Meitner also deserved recognition. She and Frisch came to the same conclusion regarding the possibility of fission, which they published in a paper received just two weeks later than the receipt date of the paper of Hahn and Strassmann [Meitner 1939]. Emilio Segrè, a collaborator of Fermi who himself was awarded the Nobel Prize in Physics (1959) for the discovery of antiproton, later observed [Segrè, 1989]:

“The discovery of fission has an uncommonly complicated history; many errors beset it. Nature had, however, truly complicated the problem. One had to contend with the radioactivity of natural uranium and the presence of two long-lived isotopes – U235 and U238. The heavier isotope, as is well-known, does not undergo fission when bombarded by slow neutrons. The lighter isotope, which makes up 0.7% of natural uranium, is responsible for all the slow-neutron fission. This is a tricky set-up. Above all, it seems to me that the human mind sees only what it expects.”

As a follow-up to the above narrative, one can consider two major applications of the nuclear fission reaction (neutron interaction with uranium), nuclear weapons and power generation. On the former we make a few observations below and then refer the reader to the literature. On the latter we go a bit further in the next section.

In 1930’s many scientists, including Rutherford, Milliken, Einstein, did not believe that one can get more energy out of nuclear reacti...