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Chapter 7
Characterization of Energetic Particles Emitted During Pd/D Co-Deposition for Use in a Radioisotope Thermoelectric Generator (RTG) Pamela A. Mosier-Boss1, Frank E. Gordon1, and Lawrence P.G. Forsley2 1
SPAWAR Systems Center Pacific, Code 71730, San Diego, CA 92152 2 JWK International Corp., Annandale, VA 22003
CR-39 is a solid-state nuclear-track etch detector. Using these detectors in Pd/D co-deposition experiments, researchers have detected energetic particles and neutrons. The source of these particles and neutrons is the cathode. In this communication, spacer experiments and track modeling are done to characterize the energetic particles emitted. The potential use of these energetic particles to power a RTG is discussed.
Introduction A radioisotope thermoelectric generator (RTG) is a simple electrical generator which obtains its power from radioactive decay by the Seebeck effect using an array of thermocouples. Figure 1 shows a schematic of an RTG that has been used on the Cassini probe. The main component of the RTG is a container that holds the radioactive material (fuel). An array of thermocouples is placed in the walls of the container. The outer end of each thermocouple is connected to a heat sink. As the fuel undergoes radioactive decay, heat is produced and flows through the thermocouples to the heat sink, generating electricity in the process.
© 2009 American Chemical Society
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In Low-Energy Nuclear Reactions and New Energy Technologies Sourcebook Volume 2; Marwan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
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Figure 1. Schematic of an RTG used in the Cassini probe (1). In order to be used to power an RTG, the fuel needs to have a half-life long enough to produce energy at a relatively continuous rate for a reasonable time. At the same time, the half-life needs to be short enough so that it decays sufficiently quickly to generate a usable amount of heat. It would be desirable for the fuel to produce high energy radiation that has low penetration. If there are size considerations, the fuel must produce a large amount of energy per mass and volume. Based on these criteria, alpha radiation is the preferred fuel because it is easier to shield against than other forms of radioactive decay. The properties of sources commonly used in RTGs are summarized in Table 1. Radioactive sources typically used in these devices include 238Pu, 90Sr, and 210Po. Although 238 Pu has a long half-life, reasonable energy density, and exceptionally low gamma and neutron radiation levels, it is not naturally occurring. 238Pu is usually prepared by the irradiation of 237Np. Consequently, 238Pu is expensive. 90 Sr has a shorter half-life and much lower energy density and produces gamma radiation, but it is cheaper. Although 210Po has a high energy density, it has limited use because of its very short half-life and significant gamma ray production. The Pd/D system is another source of energetic particles that potentially could be used in RTGs. Using CR-39, researchers have detected the emission of particles in experiments using bulk palladium in both gas permeation (2) and electrolysis (3, 4, 5). Recently, particle emission has been reported using electrodes prepared by the Pd/D co-deposition process (6, 7). The Pd/D system has the following advantages over traditional radioactive alpha sources: (i) the rate of energetic particle production can be controlled by manipulating the experimental parameters and (ii) the rate of gamma emissions is low (7, 8). However, in order to use the particles generated as the result of Pd/D codeposition, researchers need to know the particles’ energies. To determine the energies of these particles, researchers conducted the following experiments: placement of spacers between the CR-39 detector and cathode, sequential etching of the CR-39 detector, and track modeling. The results of these experiments are discussed in this communication.
In Low-Energy Nuclear Reactions and New Energy Technologies Sourcebook Volume 2; Marwan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
121 Table 1. Properties of sources commonly used in RTGs. Isotope 238
Pu
242
Cm
90
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Sr
144
Cs
210
Po
Emission Half-Life 5.5 MeV α 86.8 y
W/g (thermal) $/W (thermal) 0.55 3000
6.1 MeV α 0.445 y 0.2 MeV β 28 y
120 495 0.93 250
46 keV β 0.781 y 5.3 MeV α 0.378 y
25 15 141 570
Comments Exceptionally low γ and neutron radiation levels,
