Elsevier

Applied Radiation and Isotopes

Volume 143, January 2019, Pages 163-175
Applied Radiation and Isotopes

Fractionation of copper activation products in debris samples from the National Ignition Facility

https://doi.org/10.1016/j.apradiso.2018.10.014Get rights and content

Highlights

  • Debris from fusion experiments at the National Ignition Facility is inhomogeneous.

  • Fractionation techniques allow calculation of the average radionuclide inventory.

  • Corrected radionuclide concentrations support the calculation of nuclear cross sections.

Abstract

Nuclear fusion experiments performed at the National Ignition Facility produce radioactive debris, arising in reactions of fast neutrons with the target assembly. We have found that postshot debris collections are fractionated, such that isotope ratios in an individual debris sample may not be representative of the radionuclide inventory produced by the experiment. We discuss the potential sources of this fractionation and apply isotope-correlation techniques to calculate unfractionated isotope ratios that are used in measurements of nuclear reaction cross sections.

Introduction

In an inertial confinement fusion (ICF) experiment (Nuckolls et al., 1972, Nuckolls, 1982) at the National Ignition Facility (NIF) (Moses, 2003, Moses et al., 2009), ≥ 1 MJ of ultraviolet laser energy (Mead et al., 1981, Lindl et al., 2004) is delivered to the inner surface of a small cylindrical gold hohlraum (Kauffman et al., 1994) creating a bath of X-rays that ablates the outer surface of a 2-mm-diameter spherical capsule containing a mixture of deuterium and tritium (DT) (Boyd et al., 2009). The ablation creates a spherical rocket that compresses the hydrogen isotopes to densities beyond those found in normal terrestrial matter (Bodner, 1981, Lindl, 1995, Lindl et al., 2004, Nuckolls, 1982), leaving very little of the capsule material in contact with the DT fuel when it fuses (Spears et al., 2008, Landen et al., 2010). When the compressed fuel undergoes fusion reactions, it creates a 0.2-ns-long pulse of neutrons (Herrmann et al., 2010), produced mostly from the 3H(2H,n)4He reaction (Lindl, 1995, Lindl et al., 2004), with a peak energy of 14.1 MeV.

The target assembly, which contains the spherical capsule inside the hohlraum along with other associated parts (Alger et al., 2010, Haan et al., 2011, Hein et al., 2013, Hamza et al., 2016, Parham et al., 2016), is activated by the pulse of neutrons from the fusion reaction. Even though the deposited laser energy is enormous, the time scale is sufficiently short that the hohlraum material does not move significantly from its initial position until after the pulse of neutrons has passed (Eder et al., 2004, Eder et al., 2013, Hagmann et al., 2015, Moody et al., 2018), even though the inner surface of the hohlraum is heated to temperatures in excess of 0.2 keV (Haan et al., 1995, Lindl et al., 2004). Eventually, the hohlraum matrix disintegrates (Dombrowski and Johns, 1963) and the resulting debris moves at high speeds to passive collection plates fielded inside the NIF chamber at a distance 50 cm from the target assembly (Prussin et al., 1986, Nakaishi et al., 1990, Grim et al., 2010, Gostic et al., 2012). These collectors are retrieved from the NIF chamber and analyzed off-line for the activation products of gold originating from the hohlraum matrix (Gostic et al., 2012, Shaughnessy et al., 2014, Moody et al., 2018). Some neutrons produced in the fusion fuel scatter off the residual hydrogen isotopes, with a probability that is dependent on the degree of compression of the fuel (Shaughnessy et al., 2014, Hagmann et al., 2015). The lower-energy scattered neutrons are more likely to induce capture reactions in the hohlraum matrix than are the primary fusion neutrons, so the measurement of the relative concentrations of 197Au(n,γ)198Au and 197Au(n,2n)196Au in a debris sample is diagnostic of the capsule compression and ICF performance (Shaughnessy et al., 2014).

The size of a sample required for production of an observable radiochemical signature when irradiated at a distance of 50 cm precludes the fabrication of targets from most radioactive materials, including many of the actinide nuclides. The ability to collect debris samples from high-neutron-yield ICF experiments has led us to explore the possibility of irradiating materials loaded in the ICF capsule itself, i.e., a thin layer deposited on the inner surface of the capsule/ablator outside the thermonuclear fuel. The NIF capsule, which has an initial outside radius of 1 mm, is compressed to a radius of as little as 30 µm, a linear convergence ratio of more than 30 (Lindl, 1995). Assuming that the capsule remains spherical (Clark et al., 2015), a sample containing 1 ng of a heavy-element nuclide, spread uniformly over the inner surface, achieves an areal density of approximately 2.5 × 1016 atoms/cm2. An isotropic pulse of 1016 neutrons and a reaction cross section of 1 barn produces 2.5 × 108 product atoms. If we can collect at least 10−3 of the capsule debris, then we have produced a source of the product nuclide that is sufficiently strong to be measured by conventional radiochemical techniques. In this way, we plan to measure cross sections for neutron-induced reactions on radioactive species that would be dangerous to handle in bulk, as well as distributions of products arising from the thermonuclear-neutron-induced fission of exotic actinides.

Unfortunately, previous experiments involving collection of the hohlraum residuum and the measurement of the resultant gold isotopes demonstrated that debris produced during a NIF experiment is emitted and collected anisotropically. This is not surprising - while the goal of an ICF experiment is to create a spherical compression of the fusion fuel (Lindl, 1995, Amendt et al., 1995), the nuclear energy released from the capsule is small compared with the input laser energy, which drives the hohlraum debris axially rather than radially. Since the capsule residuum is likely to stagnate against the ablation front near the inner surface of the hohlraum after the pulse of fusion neutrons is produced (Dawson, 1964, Prussin et al., 1986, Anderson et al., 1996), we would also expect anisotropic behavior in the final distribution of initially isotropic capsule debris.

Individual collections of debris arising in the same NIF experiment accumulated on similar collectors mounted within a few centimeters of one another can display significantly different collection efficiencies, with sample-to-sample variations of as much as an order of magnitude (Gostic et al., 2012). Even though the inner surface of the hohlraum is heated beyond incandescence, some fraction of the activated matrix is not vaporized prior to being dispersed (Willmott and Huber, 2000, Eder et al., 2004), and arrives at the collection position as macroscopic particles whose anisotropic distribution is controlled by the statistics of small numbers. This has been verified by interrogation of collector plates with scanning-electron microscopy (Gostic et al., 2012). Our initial working assumption that NIF capsule debris arises from a homogeneous fireball (Eder et al., 2008) has proven to be inaccurate when extended to the rest of the target assembly.

Debris arising from a thin layer of target material deposited on the inner surface of an ICF capsule must have passed through a plasma state before condensing on the debris collectors. Therefore, it will not necessarily be distributed across a set of debris collectors as if it was intimately mixed with the debris from the hohlraum, a substantial fraction of which was never reduced to atomic vapor. This means that the debris from a NIF ICF experiment has the potential to be fractionated, and that any individual collection of debris from an ICF experiment will probably not be representative of the composition of the debris inventory as a whole. If it was possible to collect 100% of the debris produced in an ICF experiment, fractionation among the individual debris increments would be unimportant. Unfortunately, standard solid debris collectors subtend a total of less than 1% of the 4π solid angle, so we must be concerned that fractionation would result in an ensemble of debris collectors that would not be representative of the entire inventory of the debris field. Since radiochemical analysis uses measurements of nuclide ratios, the debris field need not be isotropic in its distribution; however, if the nuclide inventory of each debris increment is inhomogeneous, then we must be able to correct measurements of radionuclide concentrations to simulate the inventory of unfractionated debris in order to draw quantitative conclusions about fundamental nuclear properties or reaction cross sections.

Historically, the fractionation of debris samples was an issue in the analysis of the residues of the tests of nuclear explosives. Nuclear-test debris samples rarely contained more than 10−8 of the device residuum (Moody et al., 2015) and fractionation effects could cause significant deviations from homogeneity in collections of individual samples of radioactive glass. A large effort went into developing techniques for processing a collection of non-representative samples and extrapolating to the relative concentrations of a limited set of analytes in a hypothetical representative sample. We intend to build on this work and apply the techniques developed for the radiochemical diagnosis of fractionated nuclear-test debris to the NIF environment.

Nuclear-test diagnostics postulated two groups of processes resulting in fractionation, those occurring at early times during debris formation (geometrical fractionation) and those occurring at late times (chemical fractionation) (Moody et al., 2015). We propose the same demarcation for fractionated NIF debris. In geometrical fractionation, the debris originating from the ICF target assembly never becomes well mixed (Hicks, 1982); the inhomogeneous distribution of capsule and hohlraum debris due to the physical scale of the debris particles falls into this category. Materials that are intimately mixed with one another in the initial assembly should not be geometrically fractionated in collections of debris. Chemical fractionation entails the segregation of chemical species during recombination of atomic-scale debris (Irons and Peacock, 1974), nucleation and condensation at high temperatures and low concentrations (Edvarson et al., 1959, Freiling, 1961), and the affinity of the material for a particular collection matrix. Chemical fractionation is most important for debris arising from the capsule and the innermost portions of the hohlraum, which pass through a state where matter has been reduced to the atomic scale.

The effects of fractionation on the collection of debris samples must be understood before we can consider making quantitative radiochemical measurements of nuclear properties at NIF. For instance, the chemical fractionation of a reaction product from the target nuclide from which it derives would lead to an incorrect determination of a reaction cross section from an isotope ratio. Chemical effects in the collection of fission products would lead to incorrectly measured cumulative fission yields. In the diagnostic measurements of 198Au/196Au, mentioned above, chemical effects do not apply since both nuclides in the ratio are isotopes of the same element, thereby chemically indistinguishable. We do consider the potential for geometric fractionation; the gold hohlraum is not spherically symmetric, making the flux of neutrons at the waist higher than that at the ends of the cylinder. Providing the shape of the neutron energy spectrum does not change as a function of direction, the diagnostic isotope ratio is invariant to any effects of geometrical fractionation. However, the ratio of a reaction product concentration to that of the residual target species (e.g., 196Au/197Au), changes according to the location along the length of the hohlraum, and we do observe these differences between debris samples, thus making the determination of an absolute collection efficiency problematic.

In this work, we report the results from initial experiments aimed at quantifying the degree of fractionation observed in NIF debris collections. We apply the mixing techniques developed for nuclear-test diagnostics to obtain representative nuclide concentration ratios among the components of the debris and identify radionuclides arising from the same piece of the hohlraum assembly. We focus on the activation products of copper to demonstrate that the elements Co, Ni, and Cu are neither chemically nor geometrically fractionated to any significant degree under the conditions of these experiments.

Section snippets

Experimental

Experiments were performed onsite at the Lawrence Livermore National Laboratory. Debris samples were collected from several different experiments at NIF. A diagram of a capsule in a hohlraum under laser irradiation is shown in Fig. 1. In each experiment, a capsule with an outer diameter of approximately 2 mm containing an approximately equimolar mixture of deuterium and tritium was suspended (Kucheyev and Hamza, 2010) at the geometrical center of the hohlraum, a thin-walled cylindrical tube

Results

Between 5 and 7 gamma-ray spectra were acquired from each counting sample over the span of 7–10 days. These spectra were processed with GAMANAL to give a table of radionuclide concentrations at irradiation time. As many as 30 radionuclide concentrations were determined for each sample.

We selected seven radionuclides, the production of which was often observed in the gamma-ray spectra, arising in different parts of the target assembly. These radionuclides are representative of seven different

Discussion

In Fig. 5 we show the energy spectrum of neutrons incident on the hohlraum specific to experiment N141106. The other experiments listed in Table 2 produced neutron spectra that were similar in form, i.e., strongly peaked at 14.1 MeV but with significant contributions at lower energies from neutrons scattering off hydrogen atoms in the thermonuclear fuel (Cerjan et al., 2013, Frenje et al., 2013, Clark et al., 2015). The Fig. 5 spectrum is the result of a simulation (Cerjan et al., 2013, Cerjan

Conclusions

When debris from ICF experiments fielded at NIF is collected, the radionuclides induced in the various components of the target assembly are determined to be fractionated from one another in their spatial distribution. The ICF capsule and the innermost portions of the hohlraum are ionized and reduced to a mono-atomic/ionic state by the deposited laser energy, and are subject to fractionation arising from both geometrical and chemical effects. However, the outer portions of the target assembly

Acknowledgements

The authors thank the NIF Target Diagnostics Engineering Group and the NIF Operations staff. We also thank the staff of the LLNL Nuclear Counting Facility for their help with gamma-ray spectrometry. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract no. DE-AC52-07NA27344. This work was funded by the Laboratory Directed Research and Development Program at LLNL under project tracking code 16-SI-001.

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