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| Fig. 1: The Sandia labs miniature pulsed neutron generator. (Source: Wikimedia Commons) |
James Chadwick won the Nobel Prize in Physics for the discovery of the neutron in 1932; this final piece in understanding the atomic puzzle sparked a revolution leading to the nuclear age. However, this piece will focus neutron generation for metrology experiments. One of the key features in the discovery of neutron radiation in the disintegration of atoms was that it has incredibly strong penetrating capabilities. This capability allows neutrons to penetrate atomic electron clouds and probe the inner structure of nuclei. At the time, gamma radiation was considered the most powerful form of radiation, so the discovery of a radiation beam with much stronger penetrating powers was quite puzzling. Additionally, the radiation proved to be completely neutral to electric fields. These experiments, along with specific modelling of the new radiation led him to deduce the existence of a new subatomic particle, the neutron. This discovery solved the problem of the perceived imbalance of charge for a given atom: the mass and charge of the proton was known, along with the number of electrons per atom, but the atoms could not be made electrically neutral while having the correct mass. [1]
In the first neutron generation experiments, beryllium was bombarded with helium nuclei. The discovery of neutrons led to the discovery of unstable isotopes of chemical elements that would, by way of their thermal energy, decay through emission of a neutron. These, sources proved to be much stronger generators of neutron radiation and hence, they found their way into practical applications. [2] One such practical application is in well logging. Here, the extremely penetrating nature of the radiation was taken advantage of for the purposes of identifying elemental composition of deep bore holes in the earth. The neutron radiation would bombard the earth formations and scatter both elastically and inelastically; in the process, they would generate gamma radiation, whose energy could be studied in order to uncover the elemental composition of the formations. In a more modern use, neutron-based metrology is considered for detection of contraband. [3]
However, these sources had severe limitations in that their neutron flux contains a wide spectrum of energies. Furthermore, fast pulsed operation turned out to be very useful for elemental identification through the ability to measure decay rates after neutron interaction events. Additionally, such sources cannot be "turned off." [2] Breakthroughs for better, monoenergetic sources of neutrons came through developments in the understanding of fusion reactions. This developments led to neutron generators operating off the principle of fusing two hydrogen atoms together, releasing a helium atom and a neutron. The two reactions are the so-called "DT" reaction and the "DD" reaction. In the DT reaction, two isotopes of hydrogen: deuterium and tritium, are fused together to form helium-4 and in the process release a neutron. In the DD reaction, two deuterium atoms fuse together to form helium-3 and also release a neutron. The challenge in creating these reactions is that their energy of formation is extremely high - this challenge is solved with the application of an extremely high voltage that ionizes and accelerates the isotopes of helium toward a hydride target, producing favorable conditions for fusion to occur.
The first such machines actually derived their high-voltage pulse from a Van de Graaff electrostatic generator. Here, a belt drive shuffles electrons until extremely high voltages are generated. Not surprisingly, such a system was quite unstable. Such machines were nevertheless capable of generating pulses a few microseconds in length and at kilohertz rates. [2] Further advances in electronics technology, however, allowed all of the high-voltage equipment to be packaged into a solid state form. [4] Somewhat surprisingly, these rates turned out to essentially be limited by the physical timescales of the occurring reactions. The main improvements primarily occurred exclusively in neutron generation rate and source stability.
Improvements in miniaturization technologies allowed Sandia labs to develop an extremely miniaturized version of the neutron source in 2012 that can fit in the palm of a hand (Fig. 1). [5] Their device is capable of producing approximately half-second neutron pulses with around 1000 neutrons. Beyond metrology applications, this development may bring neutron-based cancer therapies to the home.
© Kevin Fischer. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] H. Pleijel, Nobel Award Ceremony Speech 1935, Nobel Lectures, Physics 1922-1941 (World Scientific, 1998).
[2] A. H. Youmans and E. C. Hopkinson, "Pulsed Neutron Generator," US Patent 3,309,522, 14 Mar 67.
[3] A. Buffler, "Contraband Detection with Fast Neutrons," Radiat. Phys. Chem. 71, 853 (2004).
[4] J. Reijonen et al., "D-D Neutron Generator Development at LBNL," Appl. Radiat. Isotopes 63, 757 (2005).
[5] J. M. Elizondo-Decanini et al., "Novel Surface-Mounted Neutron Generator," IEEE Trans. Plasma Sci. 40, 2145 (2012).
