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Open Access Publications from the University of California

Scholarly Works (11 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2021)

As part of the next generation of cancer treatment, targeted alpha therapy (TAT) has demonstrated the potential to improve patient outcomes by targeting cancer cells with highly cytotoxic $\alpha$-emitters. These are prized for their ability to efficiently kill cancer cells, while dealing minimal damage to nearby healthy tissue. When properly paired with a positron-emitting radionuclide, a rapid bio-assay of the therapeutic agent can be performed using positron emission tomography (PET), enabling micro-dosimetry and personalized treatment planning. One of the main challenges associated with these treatments is the production of the $\alpha$-emitting radionuclides in quantities which are relevant to widespread clinical use.

In this work, we seek to improve the understanding of the isotope production pathways for \ce{^{225}Ac}, one promising candidate for TAT, and \ce{^{134}Ce}, which is a positron-emitting analog of actinium. The main pathway which we explored was through the \ce{^{226}Ra}(n,2n) reaction, which produces the \ce{^{225}Ac} generator isotope \ce{^{225}Ra}. Thick target deuteron breakup was utilized as a high intensity, variable energy neutron source for inducing this reaction.

A parameterized hybrid model for the double differential neutron spectra from thick target deuteron breakup was developed, and the optimal parameters for the model were determined through a fit to selected literature data. Measurements of the spectrum were performed on a beryllium target at 33 and 40 MeV deuteron energies, using the activation and time-of-flight techniques, and these measurements were compared to the hybrid model and literature data. Additionally, production cross sections for several other emerging radionuclides were measured using the activation technique, with the deuteron breakup neutrons.

The \ce{^{226}Ra}(n,2n) reaction cross section was measured with the breakup spectra from 33 and 40 MeV deuterons on beryllium, using the activation method and $\alpha$ spectroscopy. These measurements were compared to the predictions of the TENDL-2015 and ENDF/B-VII.1 evaluations. A chemical separation of the produced \ce{^{225}Ac} was performed following both irradiations, to evaluate the separation capabilities of the DGA and AG50 cation exchange resins, as well as to search for any co-produced impurities such as \ce{^{227}Ac} or fission products.

Additionally, measurements of the \ce{^{139}La}(p,6n)\ce{^{134}Ce} cross sections from 35--60 MeV were performed using the stacked-foil activation technique. Thick target yield calculations showed this to be a promising pathway for \ce{^{134}Ce} production, which is a positron-emitting chemical analog to \ce{^{225}Ac}. Several other \ce{^{139}La}(p,x) reaction channels were also measured, which are useful for quantifying impurities and for evaluating nuclear reaction models. These measurements were compared to the predictions of the TALYS, EMPIRE and ALICE nuclear reaction codes, which showed that the default pre-equilibrium models were insufficient, providing motivation for further study of these processes.

Finally, a comparison study was performed between several of the most promising \ce{^{225}Ac} production routes. The \ce{^{232}Th}(p,x)\ce{^{225}Ac} pathway showed generally good yields, but was shown to contain significant \ce{^{227}Ac}, which may have negative long term side-effects, depending on the bio-chemistry of the complexed actinium molecule. The \ce{^{232}Th}(p,4n)\ce{^{229}Pa} pathway had a high purity, but poor yields. Pathways involving directly irradiating \ce{^{226}Ra} targets with charged particles or photons were shown to have engineering issues regarding target heating, that must be overcome if they are to be pursued further. And the \ce{^{226}Ra}(n,2n) pathway using secondary neutrons from deuteron breakup was shown to have high purity, and had yields comparable to \ce{^{232}Th}(p,x) if a radium target of 10--50 grams is used.

The primary goals of this thesis are to improve the understanding of deuteron breakup, both through measurements and modeling, and to highlight its potential applicability for isotope production purposes. It is the hope that this will inspire a more widespread adoption of technology utilizing deuteron breakup as a neutron source, and will lead to an improvement in the supply of \ce{^{225}Ac} available for research and clinical applications.

    Cover page: Next-Generation Isotope Production via Deuteron Breakup
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2023)
    Thick target deuteron breakup is a variable-energy accelerator-based source of high-energy neutrons, with applications in fundamental and applied nuclear science and engineering. However, the breakup mechanism remains poorly understood, and data on neutron yields from thick target breakup remain relatively scarce. In this work, the double-differential neutron yields from deuteron breakup have been measured on a thick beryllium target at ϵd=33 and 40 MeV, using both time-of-flight and activation techniques. We have also introduced a simple hybrid model for the double-differential deuteron breakup cross section, applicable in the ϵd=10-100 MeV energy range on light (Z≤6) targets. This model features four empirical parameters that have been fit to reproduce experimental breakup measurements on beryllium targets, using the method of least squares. It was shown that these parameters extrapolate well to higher energies, and to other low-Z target materials. We also include optimization of the parameters that modify the Kalbach systematics for compound and preequilibrium reactions, in order to better reproduce the experimental data for beryllium targets at large angles.
      Cover page: Secondary neutron production from thick target deuteron breakupCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2020)
      A stacked-target of natural lanthanum foils (99.9119% 139La) was irradiated using a 60 MeV proton beam at the LBNL 88-Inch Cyclotron. 139La(p,x) cross sections are reported between 35–60 MeV for nine product radionuclides. The primary motivation for this measurement was the need to quantify the production of 134Ce. As a positron-emitting analogue of the promising medical radionuclide 225Ac, 134Ce is desirable for in vivo applications of bio-distribution assays for this emerging radio-pharmaceutical. The results of this measurement were compared to the nuclear model codes TALYS, EMPIRE and ALICE (using default parameters), which showed significant deviation from the measured values.
        Cover page: Measurement of 139La(p,x) cross sections from 35–60 MeV by stacked-target activation
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2020)
        We report here the results of a measurement of the scattered versus unscattered neutron fluence on polyethylene determined via neutron activation of multiple natural indium foils from a deuterium-deuterium (D-D) neutron generator. The neutrons were produced by the High Flux Neutron Generator (HFNG) at the University of California, Berkeley, a specially designed source to maximize neutron flux on a sample while minimizing the total neutron yield. During the experiment, approximately 108 n/s were produced with the energies at the indium foils ranging from 2.2 to 2.8 MeV. Both the angle-integrated and the partial angle differential results are consistent with the predictions of the Monte Carlo N-Particle Transport (MCNP) code, using ENDF/B-VII.1. This supports shielding calculations in the fast energy region with high-density polyethylene.
          Cover page: An Integral Experiment on Polyethylene Using Radiative Capture in Indium Foils in a High Flux D-D Neutron Generator
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2021)
          Theoretical models often differ significantly from measured data in their predictions of the magnitude of nuclear reactions that produce radionuclides for medical, research, and national security applications. In this paper, we compare a priori predictions from several state-of-the-art reaction modeling packages (CoH, EMPIRE, TALYS, and ALICE) to cross sections measured using the stacked-target activation method. The experiment was performed using the Lawrence Berkeley National Laboratory 88-Inch Cyclotron with beams of 25 and 55 MeV protons on a stack of iron, copper, and titanium foils. Thirty-four excitation functions were measured from 4–55 MeV, including the first measurement of the independent cross sections for natFe (p,x)49 , 51Cr , 51 , 52 m , 52 g , 56Mn , and 58 m , 58 gCo. All of the models, using default input parameters to assess their predictive capabilities, failed to reproduce the isomer-to-ground state ratio for reaction channels at compound and pre-compound energies, suggesting issues in modeling the deposition or distribution of angular momentum in these residual nuclei.
            Cover page: Proton-induced reactions on Fe, Cu, and Ti from threshold to 55 MeV
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2020)
            In view of several significant discrepancies in the excitation function of the 86Sr(p,n)86g+xmY reaction which is the method of choice for the production of the non-standard positron emitter 86Y for theranostic application, we carried out a careful measurement of the cross sections of this reaction from its threshold up to 16.2 MeV at Forschungszentrum Jülich (FZJ) and from 14.3 to 24.5 MeV at LBNL. Thin samples of 96.4% enriched 86SrCO3 were prepared by sedimentation and, after irradiation with protons in a stacked-form, the induced radioactivity was measured by high-resolution γ-ray spectrometry. The projectile flux was determined by using the monitor reactions natCu(p,xn)62,63,65Zn and natTi(p,x)48V, and the calculated proton energy for each sample was verified by considering the ratios of two reaction products of different thresholds. The experimental cross section data obtained agreed well with the results of a nuclear model calculation based on the code TALYS. From the cross section data, the integral yield of 86Y was calculated. Over the optimum production energy range Ep = 14 → 7 MeV the yield of 86Y amounts to 291 MBq/μA for 1 h irradiation time. This value is appreciably lower than the previous literature values calculated from measured and evaluated excitation functions. It is, however, more compatible with the experimental yields of 86Y obtained in clinical scale production runs. The levels of the isotopic impurities 87mY, 87gY, and 88Y were also estimated and found to be <2% in sum.
              Cover page: Accurate determination of production data of the non-standard positron emitter 86Y via the 86Sr(p,n)-reaction
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2021)
              As72 is a promising positron emitter for diagnostic imaging that can be employed locally using a Se72 generator. However, current reaction pathways to Se72 have insufficient nuclear data for efficient production using regional 100-200 MeV high-intensity proton accelerators. In order to address this deficiency, stacked-target irradiations were performed at LBNL, LANL, and BNL to measure the production of the Se72/As72 positron emission tomography (PET) generator system via As75(p,x) between 35 and 200 MeV. This work provides the most well-characterized excitation function for As75(p,4n)Se72 starting from threshold. Additional focus was given to report the first measurements of As75(p,x)Ge68 and bolster an already robust production capability for the highly valuable Ge68/Ga68 PET generator. Thick target yield comparisons with prior established formation routes to both generators are made. In total, high-energy proton-induced cross sections are reported for 55 measured residual products from As75, Cunat, and Tinat targets, where the latter two materials were present as monitor foils. These results were compared with literature data as well as the default theoretical calculations of the nuclear model codes talys, coh, empire, and alice. Reaction modeling at these energies is typically unsatisfactory due to few prior published data and many interacting physics models. Therefore, a detailed assessment of the talys code was performed with simultaneous parameter adjustments applied according to a standardized procedure. Particular attention was paid to the formulation of the two-component exciton model in the transition between the compound and preequilibrium regions, with a linked investigation of level density models for nuclei off of stability and their impact on modeling predictive power. This paper merges experimental work and evaluation techniques for high-energy charged-particle isotope production in an extension to an earlier study of this kind.
                Cover page: Measurement and modeling of proton-induced reactions on arsenic from 35 to 200 MeVCreative Commons 'BY-NC' version 4.0 license
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2021)
                Background: A number of accelerator-based isotope production facilities utilize 100- to 200-MeV proton beams due to the high production rates enabled by high-intensity beam capabilities and the greater diversity of isotope production brought on by the long range of high-energy protons. However, nuclear reaction modeling at these energies can be challenging because of the interplay between different reaction modes and a lack of existing guiding cross-section data. Purpose: A Tri-lab collaboration has been formed among the Lawrence Berkeley, Los Alamos, and Brookhaven National Laboratories to address these complexities by characterizing charged-particle nuclear reactions relevant to the production of established and novel radioisotopes. Method: In the inaugural collaboration experiments, stacked-targets of niobium foils were irradiated at the Brookhaven Linac Isotope Producer (Ep=200MeV) and the Los Alamos Isotope Production Facility (Ep=100MeV) to measure Nb93(p,x) cross sections between 50 and 200 MeV. First measurements of the Nb93(p,4n)Mo90 beam monitor reaction beyond 100 MeV are reported in this work, as part of the broadest energy-spanning dataset for the reaction to date. Nb93(p,x) production cross sections are additionally reported for 22 other measured residual products. The measured cross-section results were compared with literature data as well as the default calculations of the nuclear model codes TALYS, CoH, EMPIRE, and ALICE. Results: The default code predictions largely failed to reproduce the measurements, with consistent underestimation of the preequilibrium emission. Therefore, we developed a standardized procedure that determines the reaction model parameters that best reproduce the most prominent reaction channels in a physically justifiable manner. The primary focus of the procedure was to determine the best parametrization for the preequilibrium two-component exciton model via a comparison to the energy-dependent Nb93(p,x) data, as well as previously published La139(p,x) cross sections. Conclusions: This modeling study revealed a trend toward a relative decrease for internal transition rates at intermediate proton energies (Ep=20-60 MeV) in the current exciton model as compared to the default values. The results of this work are instrumental for the planning, execution, and analysis essential to isotope production.
                  Cover page: Investigating high-energy proton-induced reactions on spherical nuclei: Implications for the preequilibrium exciton model
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2024)
                  Excitation functions of the 86Sr(d,n)87m,87gY, 86Sr(d,2n)86m,86gY and 86Sr(d,3n)85m,85gY reactions on enriched 86Sr target were measured by the activation technique up to deuteron energies of 49 MeV. The isomeric cross-section ratios as a function of projectile energy were deduced from the measured data for 87mY, 87gY(cum), 86mY, 86gY(cum), 85mY and 85gY pairs for the same energy range. All measurements are reported for the first time. The experimental data were compared with the data from the TENDL library which is based on TALYS calculation with default parameters. No satisfactory agreement was observed. Nuclear model calculations were then performed using the codes TALYS and EMPIRE with some parameter adjustments, and compared with the experimental data. The quality of the agreement between experimental data and model calculations was numerically quantified. In general, the data as well as the isomeric cross-section ratios are partially reproduced by the model calculations, provided the input model parameters are properly chosen and the level structure of the product nucleus is thoughtfully considered.
                    Cover page: Excitation functions and isomeric cross-section ratios of (d,xn) reactions on 86SrCreative Commons 'BY' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2019)
                    We designed and tested a compact deuteron-deuteron fusion neutron generator for application to 40Ar/39Ar geochronology. The nearly monoenergetic neutrons produced for sample irradiation are anticipated to provide several advantages compared with conventional fission spectrum neutrons: Reduction of collateral nuclear reactions increases age accuracy and precision. Irradiation parameters within the neutron generator are more controllable compared with fission reactors. Confidence in the prediction of recoil energies is improved, and their likely reduction potentially broadens applicability of the dating method to fine-grained materials without vacuum encapsulation. Resolution of variation in the 39K(n,p)39Ar neutron capture cross section at 1.3 to 3.2 MeV and discovery of a strong resonance at ~2.4 MeV illuminate future pathways to improve the technique for 40Ar/39Ar dating.
                      Cover page: Boutique neutrons advance <sup>40</sup>Ar/<sup>39</sup>Ar geochronology.
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