Abstract
An ongoing experimental campaign at the National Ignition Facility (NIF) aims to measure neutron induced nuclear reaction cross–sections using radiochemically doped target capsules. Critical to this campaign is the ability to collect a representative sample of the reaction products using Solid Radiochemical Collectors (SRCs) fielded around the NIF chamber. The shot presented in this paper used a doped target capsule with a neopentane gas fill was to investigate the ratio of isotopes collected at three chamber angles. It was found that SRC samples of rare earth elements collected from NIF are representative of the ingoing dopant mix, thereby concluding that fractionation does not occur during a NIF implosion. This validates the doped capsule method for use in measuring neutron induced reaction cross-sections.
1 Introduction
Following the attainment of ignition at the National Ignition Facility (NIF) in 2021 [1], the high neutron yields provided by the Indirect Drive Inertial Confinement Fusion (ICF) platform [2] have become increasingly reliable, permitting the design of nuclear physics experiments that make use of the intense neutron output. The NIF Radiochemistry collaboration, formed of members from the Atomic Weapons Establishment (AWE) and Lawrence Livermore National Laboratory (LLNL), is engaged in one such effort to measure neutron induced reaction cross–sections on radioactive species, which have proved to be unobtainable using established methods.
The experiments in this campaign use the ratio method described by K. Moody et al. [3] applied to reaction products produced in a doped ICF capsule [4]. The violence of a NIF implosion scatters reaction products and other target debris around the chamber. The debris are collected on vanadium discs of 5 cm diameter and 0.5 mm thickness, called Solid Radiochemical Collectors (SRCs), which are fielded on Diagnostic Instrument Manipulators (DIMs) at various chamber angles [5]. Up to 16 SRCs can be fielded on each NIF shot, 4 each on DIMs (90,78), (90,124), (90,315) and the polar DIM (0,0). The solid angle covered by each SRC is roughly 0.1% of 4π at their typical stand-off distance of 30–50 cm from target chamber center [6]. To accurately calculate a reaction cross–section using the ratio method, it is imperative to ensure that the reaction products collected on the SRCs provide representative samples of the reaction products produced in the shot. The work presented here is an assessment of whether SRC samples retrieved from the NIF target chamber are representative of the true ratio of isotopes that were loaded into the target capsule.
The doped capsule cross–section experiments pursued by the NIF Radiochemistry collaboration are a relative measurement, requiring a subject, reference, and collection tracer isotope to be doped into the capsule, which is subsequently built into a complete target as seen in Figure 1. During a shot, all 192 beams of NIF are incident on the interior wall of the hohlraum, stimulating x–ray emissions which ablate the exterior capsule surface, causing rapid compression and achieving pressures of up to bar. At such high pressures, the capsule and its contents are vaporised into a hot plasma of ∼10 keV.
FIGURE 1
Simultaneously, laser–matter interactions in the target produce strong electromagnetic fields [8]. The different mass–to–charge ratios of the dopant species will determine how each behaves in the strong field environment, potentially resulting in preferential transport of certain species to different locations within the plasma. As space charge effects start to overcome the ablation pressure, the plasma expands outwards and the dopant atoms, along with any reaction products, condense on to the hohlraum surface, fixing their orientation with respect to the chamber axes. As the target proceeds to disassemble, the hohlraum material debris transports the dopant atoms radially outwards whereupon a small fraction is embedded into the SRCs. Further, during the flight of the debris, other interactions may occur with diagnostics or chamber components, which act to disrupt the isotropic distribution of the debris. In this work, the term fractionation applies to all interactions which may result in the ratio of isotopes detected from an individual SRC not being representative of the whole of the target contents.
2 Experimental methods
This experiment was designed to provide a direct comparison between the isotope ratios measured from an SRC and those in the target. For this shot, which took place on 28 March 2023, a capsule was doped with a radiochemical cocktail of 152Eu ( atoms), 171Tm ( atoms) and 91Y ( atoms). The 91Y dopant was obtained by novel radiochemical methods described in Ref. [9]. The 152Eu was a high specific activity sample obtained from Eckert and Zeigler and the 171Tm was produced at the McClellan Nuclear Reactor by irradiation of 100 mg of natEr2O3. Following irradiation, the natEr2O3 was dissolved and separated from the 171Tm produced by a series of extraction chromatography columns packed with LN Resin (Eichrom Technologies, 50–100 µm). The capsule doping was performed using the Vaccuum Optimized Radionuclide–to–Capsule Administer for NIF (VORCAN) system detailed in Ref. [4]. High Purity Germanium (HPGe) detectors were then used to perform gamma–ray spectrometry on the doped capsule to quantify the number of dopant atoms. The capsule was built into an ICF style target as seen in Figure 1. The capsule was not filled with deuterium–tritium gas but was instead filled with neopentane to achieve a similar mass density at room temperature, resulting in no neutron yield but similar implosion properties to an ICF experiment [10].
A total of twelve SRCs were mounted within the chamber, with DIMs positioned at polar–azimuthal chamber angles 90–315, 90–124 and 00–00 holding four SRCs each. All SRCs were fielded at a stand–off distance of 0.5 m from TCC, constituting a total solid angle coverage of 0.768%. A DIM mounted with four SRCs may be seen in Figure 2.
FIGURE 2
The shot was executed as described above and debris was collected on the SRCs. Figure 3 is an optical image taken at shot time, in which the three DIMs fielded on the shot and the disassembling target may be seen. Upon completion of the shot, the SRCs were retrieved from the target chamber and placed in front of p–type coaxial HPGe detectors for counting. The background levels of the spectra were too great to observe the low–intensity, 66.7 keV gamma–ray from 171Tm, therefore, a low–background detector was used to observe this isotope. The low–background detector, which provides a higher efficiency in the low–energy region, was comprised of an n–type coaxial HPGe crystal with a thin front dead layer and a carbon fiber window. This detector is shielded by 6 inches of low–background Pb, a nitrogen inner shield radon purge, and a ∼2π active muon veto, drastically reducing the low–energy spectral background, when compared with the standard p–type detectors.
FIGURE 3
Both the p-type coaxial HPGe detectors, which were used to acquire the spectra showing 91Y and 152Eu, and the low–background detector, which was used to observe 171Tm, used the ORTEC DSPEC-50 data acquisition units. The DSPEC performs a zero dead–time correction as the data are acquired; therefore, a dead–time correction is not performed in the analysis detailed below. It should also be noted that the above detectors were compatible with different fitting software. This necessitated a slightly different approach to the analysis of the data from each detector, as detailed in Subsections 3.1 and 3.2.
Pre–shot capsule spectra were acquired on the p–type detectors detailed above for a live–time of 5 h. Post–shot SRC spectra taken on the p–type detectors were acquired for live–times of 4 h, 8 h and 12 h for SRCs fielded at DIM angles 90–315, 00–00 and 90–124, respectively. Post–shot SRC spectra taken on the n–type detectors were acquired for live–times of between 48–65 h. All spectral peaks used for the analysis presented in this paper were detected with a minimum confidence of 2σ. Dead–times for all spectra used were ≤0.19%. Gamma–ray spectra acquired from both detectors detailed above were then analyzed. Decay corrections were performed to the time of the shot, and isotope ratios were then calculated for the ingoing doped capsule and the SRCs, enabling a direct comparison of the ingoing and post-shot isotope ratios.
3 Analysis
Many experimental campaigns at NIF use solid debris collection as a diagnostic, providing a dataset for the debris collection efficiencies of different DIM angles. From this large shot history, it is well–known to the radiochemists who established solid debris collection at NIF, that DIM 90–315 is the optimal collection angle for target debris [12]. This informed the decision to use data from SRCs mounted on DIM 90–315 exclusively in this campaign. The present work confirms chemists’ observations, showing that SRCs fielded on DIM 90–315 account for 85.3% of all debris collected in the shot.
3.1 Gamma–ray spectrometry of 91Y and 152Eu
The p-type coaxial HPGe detectors in the Nuclear Counting Facility (NCF) at LLNL, make use of the in–house GAMANAL code [13] to fit, identify, and efficiency–and energy–calibrate the spectral peaks. In analyzing the 91Y and 152Eu data, the photon rate calculated by GAMANAL was used. The photon rate was multiplied by the live time to yield the efficiency–corrected number of counts in each spectral peak.
A spectral acquisition takes a non–zero, finite amount of time. Therefore, the nuclei being measured will decay over the length of the acquisition. To extract a true value for the number of atoms present at the start of the acquisition, this decay during counting must be corrected for. Using the efficiency–corrected peak counts, this correction was performed using Equation 1,where λ is the decay constant for a given isotope and t is the real time of the acquisition.
The activity at the start of the acquisition was calculated using Equation 2,where is the gamma–ray intensity associated with a given spectral peak.
The number of atoms present at the start of the acquisition, calculated from a given spectral peak was determined as,
The number of atoms was subsequently decay corrected back to the time of the shot, t = 0, by,where Δt is the time difference between the start of the spectral acquisition and the shot.
The statistical error associated with peak fitting, as reported by GAMANAL, was propagated through each step in the analysis detailed above. Using this propagated error, an inverse variance weighted mean of the atom numbers calculated from each spectral peak across the p-type coaxial detectors was calculated, yielding a single atom number for each isotope on each SRC. The statistical errors, once propagated through the mean calculation were combined in quadrature with the 4% systematic error associated with the GAMANAL processing. The systematic error is dominated by the total uncertainty on the NIST-traceable Eckert and Ziegler point sources which are used as inputs in the characterization of the GAMANAL efficiency model.
3.2 Gamma–ray spectrometry of 171Tm
The low–background detector detailed in Section 2 was used to observe the low–intensity 66.7 keV peak of 171Tm. Spectra acquired on this detector were processed using the Genie2K spectroscopy software from Mirion Technologies [14], to fit the spectral peaks and perform an energy calibration. Ten Eckert and Ziegler sources, which include 152Eu, were used to produce the energy calibration spectra. The peak area and associated statistical uncertainty provided in the Genie2K output file were then used in the analysis.
The efficiency of the detector and source geometry was modelled using Monte Carlo methods and benchmarked against GAMANAL using the 152Eu lines measured on the p-type coaxial HPGe detectors used in the section above. Dividing the peak area by the efficiency calibration for its energy provided the number of decays which took place within the spectral acquisition. Applying a decay during counting correction using Equation 5 then yielded the activity at the start of the acquisition,
The number of atoms at the start of the acquisition and at shot time were then calculated using Equations 3, 4, respectively. A single atom number for each isotope on each SRC was then calculated as in Section 3.1, using the statistical uncertainty reported by the Genie2K output file. Combined errors were handled as in Section 3.1. The atom numbers calculated as described above, for SRCs fielded on DIM 90–315 may be seen in Table 1.
TABLE 1
| SRC name | 171Tm (atoms) | Δ171Tm (atoms) | 91Y (atoms) | Δ91Y (atoms) | 152Eu (atoms) | Δ152Eu (atoms) |
|---|---|---|---|---|---|---|
| 315–V22 | ||||||
| 315–V26 | ||||||
| 315–V28 | ||||||
| 315–V46 |
Atom numbers at shot time calculated for each isotope on SRCs fielded at DIM position 90–315. Quoted errors include systematic contributions.
4 Results
Using the 152Eu as a collection efficiency tracer, when summed over all SRCs, 0.148% of the loaded 152Eu was collected. When normalized to this value, the contribution of each SRC to the total amount of material collected can be seen in Table 2. This clearly demonstrates the facility knowledge that DIM 90–315, the DIM opposite to the port on which the target is fielded, yields the best collection efficiency.
TABLE 2
| Chamber angle | Relative percentage of collected152Eu |
|---|---|
| 00–00 | 5.76 |
| 7.33 | |
| 1.26 | |
| 0.163 | |
| 90–124 | 0.296 |
| 0.123 | |
| 0 | |
| 0 | |
| 90–315 | 16.7 |
| 21.5 | |
| 21.2 | |
| 25.7 |
Percentage contribution from each SRC to the total amount of152Eu collected in the shot. The first three numbers in the SRC sample name equate to the DIM that the samples were fielded on. Two 90–124 SRCs are noted to have collected zero Eu atoms, in both cases the number of counts in the152Eu peaks were below the critical limit, and therefore no activity must be declared. The upper limit for the number of152Eu atoms in each case is and .
Of the three DIMs on which SRCs were fielded, only the spectra from 90 to 315 displayed gamma signatures above 2σ confidence for all doped isotopes. For each SRC at position 90–315 and the pre–shot capsule, three ratios were taken between the atom numbers calculated for the doped isotopes, 91Y/152Eu, 91Y/171Tm and 171Tm/152Eu. Plots of these ratios may be seen in Figure 4, confirming that the isotope ratios calculated from each SRC match those of the ingoing capsule to within a 3σ confidence interval, and that therefore, fractionation of these elements, relative to one another was not observed.
FIGURE 4
5 Summary
A capsule was doped with a radiochemical cocktail of 171Tm, 91Y, and 152Eu. Gamma–ray spectrometry was performed on the pre–shot capsule to establish the atom ratio between the dopants. Post–shot, debris–containing SRCs were retrieved from the chamber and gamma–ray spectrometry was performed to quantify the dopant atoms collected. The collection efficiency from varied chamber angles agreed with the results of observations from many previous NIF experiments fielding SRC collectors. The ratio between the number of dopant atoms collected on SRCs from the optimal collection angle, 90–315, were calculated. Dopant ratios for all 90–315 SRCs agreed with those of the pre–shot capsule to within a 3σ confidence interval demonstrating that fractionation of 171Tm, 91Y, and 152Eu does not occur. This finding demonstrates that subsequent experiments, which aim to collect reaction products on the ingoing capsule dopants 171Tm, 91Y, and 152Eu, may consider data collected by SRCs on DIM 90–315 to be representative of the ratio of all reaction products produced in the shot. It should be noted that for other elements which may be used as capsule dopants, the occurrence of fractionation would have to be investigated further due to different transport dynamics as described in Section 1.
Statements
Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Author contributions
DP-W: Writing – review and editing, Investigation, Writing – original draft, Formal Analysis. JD: Investigation, Writing – review and editing, Formal Analysis, Methodology, Conceptualization. KK: Methodology, Investigation, Writing – review and editing, Conceptualization, Formal Analysis. JJ: Investigation, Formal Analysis, Writing – review and editing. KT: Writing – review and editing, Formal Analysis, Investigation, Methodology. DS: Writing – review and editing, Investigation, Supervision, Conceptualization, Funding acquisition. JB: Investigation, Writing – review and editing. CY: Methodology, Writing – review and editing, Investigation. TB: Investigation, Writing – review and editing. WK: Investigation, Writing – review and editing. RR: Methodology, Writing – review and editing, Investigation. BS: Investigation, Writing – review and editing. EM: Writing – review and editing. TW: Investigation, Writing – review and editing.
Funding
The author(s) declared that financial support was received for this work and/or its publication. This work was performed under the auspices of U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees make any warranty, expressed or implied, or assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represent that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.
Acknowledgments
The author would like to thank the NIF Radiochemistry team for facilitating the shot, and Paula Rosen for introductions which resulted in this collaboration.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
capsule doping, fractionation, national ignition facility, nuclear reaction, solid radiochemistry collectors
Citation
Pitman-Weymouth D, Despotopulos JD, Kmak KN, Jeet J, Thomas K, Shaughnessy DA, Benstead J, Yeamans CB, Braun T, Kerlin WM, Reynolds R, Sammis B, Monzon E and Wooddy T (2026) Assessment of rare earth element fractionation in NIF implosions with radiochemically doped capsules. Front. Phys. 14:1815424. doi: 10.3389/fphy.2026.1815424
Received
22 February 2026
Revised
30 March 2026
Accepted
31 March 2026
Published
13 May 2026
Volume
14 - 2026
Edited by
Chong Qi, Royal Institute of Technology, Sweden
Reviewed by
Heshani Jayatissa, Los Alamos National Laboratory (DOE), United States
Mauricio Portillo, Michigan State University, United States
Updates
Copyright
Crown Copyright © 2026 AWE. Authors: Pitman-Weymouth, Despotopulos, Kmak, Jeet, Thomas, Shaughnessy, Benstead, Yeamans, Braun, Kerlin, Reynolds, Sammis, Monzon and Wooddy.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Daniel Pitman-Weymouth, d.pitman-weymouth@awe.co.uk
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.