National Superconducting Cyclotron Laboratory Research Articles

Point-cloud based machine learning for classifying rare events in the Active-Target Time Projection Chamber

In this work, we assess the use of machine learning to classify fission events in the Active Target Time Projection Chamber (AT-TPC) using data from an experiment performed at the National Superconducting Cyclotron Laboratory at Michigan State University. The experiment produces an extremely large quantity of data, less than 3% of which are fission events. Therefore, separating fission events from the background beam events is critical to an efficient analysis. A heuristic method was developed to classify events as Fission or Non-Fission based on hand-tuned parameters. However, this heuristic method places 5% of all events into an Unlabeled category, including 15% of all fission events. We present a PointNet model trained on the data labeled by the heuristic method. This model is then used to generate labels for the events in the Unlabeled category. Using the heuristic and machine learning methods together, we can successfully identify 99% of fission events.

Production and separation of 7Be for use in an ion source

Beryllium-7 (7Be) was created by proton irradiation of natural (natB) and enriched (10B) boron targets. The targets were dissolved in nitric acid, and the 7Be was separated from the bulk boron target material by cation-exchange chromatography. An average recovery of (99.4 ± 3.7)% was obtained for 6 separations. The purified 7Be sample was placed into a batch-mode ion source to create a 7Be beam that was delivered at an average rate of 5 × 105 pps to end users at the National Superconducting Cyclotron Laboratory.

Evolution of the nuclear spin-orbit splitting explored via the 32Si(d,p)33Si reaction using SOLARIS

The spin-orbit splitting between neutron 1p orbitals at 33Si has been deduced using the single-neutron-adding (d,p) reaction in inverse kinematics with a beam of 32Si, a long-lived radioisotope. Reaction products were analyzed by the newly implemented SOLARIS spectrometer at the reaccelerated-beam facility at the National Superconducting Cyclotron Laboratory. The measurements show reasonable agreement with shell-model calculations that incorporate modern cross-shell interactions, but they contradict the prediction of proton density depletion based on relativistic mean-field theory. The evolution of the neutron 1p-shell orbitals is systematically studied using the present and existing data in the isotonic chains of N=17, 19, and 21. In each case, a smooth decrease in the separation of the 1p3/2-1p1/2 orbitals is seen as the respective p-orbitals approach zero binding, suggesting that the finite nuclear potential strongly influences the evolution of nuclear structure in this region.

Parallel-plate avalanche counters for heavy-ion beam tracking: History and mysteries

Despite being an old detector concept, position-sensitive parallel-plate avalanche counters (PPACs) remain widely used today for heavy-ion position and timing measurements. In modern rare isotope beam facilities and large-acceptance fragment separators, PPACs are used to characterize beam properties (diagnostics), facilitate beam delivery (tuning), and provide event-by-event beam tracking for particle identification (PID). Most popular particle localization methods in PPAC detectors are based on strip electrodes electrically connected to resistive-chain circuits or delay lines. More exotic systems include optical readouts based on recording electroluminescence light with high-granularity photodetector arrays or high-resolution resistive anode electrodes. PPACs with conventional resistive-chain or delay-line readouts achieve typical position and time resolutions of below 1 mm (σ) and around 150 ps (σ), respectively. In addition, delay-line systems are capable of counting rates above several hundred kHz for beam areas of a few millimeters square, and around 1 MHz for larger beam sizes. Resistive-chain readouts have limited rates of a few tens of kHz. A review of the operation principles and performance of PPAC detectors is presented in this paper. We will discuss decades-long experience building and operating PPACs developed at the National Superconducting Cyclotron Laboratory, and then refined at the Facility for Rare Isotope Beams (FRIB), mostly focusing on the delay-line readout technique (DLPPAC). We will also discuss problems that arise due to electric discharges at high rates and briefly describe ongoing developments at FRIB. Published by the American Physical Society 2024

Deblurring decay-energy distribution from an invariant-mass measurement

Measured decay-energy spectra from invariant mass spectroscopy can give insights into the shell structure of particle-unbound systems. However, it is challenging to extract the underlying physics from a measured spectrum due to detector resolution and acceptance effects. In this work, we introduce a deblurring method used to restore the decay energy spectrum measured for the three-body decay of 26O. The method utilizes the Richardson-Lucy algorithm, which has proven to be successful in optics, and does not require any prior knowledge about the resonance states in the observed spectrum. It also circumvents the singularity issue by iteratively adjusting a positive definite distribution. The only inputs are the observed energy spectrum and the detector’s response matrix also referred to as the Transfer Matrix (TM). We test the method’s performance on a simulated spectrum using the MoNA-LISA-Sweeper setup and the associated TM. Finally, the approach is applied to the decay energy spectrum of the 26O system measured in an experiment conducted at the National Superconducting Cyclotron Laboratory (NSCL) by the Modular Neutron Array (MoNA) Collaboration. The deblurring approach suggests three peaks in the decay energy spectrum of 26O at 0.15, 1.5 MeV, and a broad peak between 4 and 6 MeV.

The new Batch Mode Ion Source for stand-alone operation at the Facility for Rare Isotope Beams (FRIB)

Gas stoppers have been used for a long-standing successful science program at Michigan State University with stopped and rare-isotope beams produced by projectile fragmentation. The National Superconducting Cyclotron Laboratory’s Coupled Cyclotron Facility has recently transitioned into the Facility for Rare Isotope Beams (FRIB) laboratory to provide rare isotopes using a high-power superconducting linear accelerator and new production facilities. To allow the science program with stopped and reaccelerated beams to continue during the transition period, a stand-alone capability was added. The Batch Mode Ion Source (BMIS) was built and has been providing beams of long-lived and stable isotopes of a variety of elements for successful user experiments. The BMIS system is described and results from the production of various beams are presented.

The cyclotron gas stopper at FRIB getting ready for operations

Linear gas stopping cells have been used for nearly two decades to slow down projectile fragments at the National Superconducting Cyclotron Laboratory, now the Facility for Rare Isotope Beams, for experiments with low-energy and reaccelerated beams. In order to efficiently stop and rapidly extract light to medium-mass fast ions, a cyclotron gas-stopper has been constructed. It uses a cyclotron-type magnet and a helium-gas filled stopping chamber to slow down the injected beam. RF ion guides transport the stopped ions to the center of the magnet and axially through the bore before acceleration to <60 keV.Following successful offline tests, the cyclotron stopper was moved to an experimental vault and connected to a new momentum–compression beam line. Beam transport to and into the cyclotron stopper was tested with stable beams. Using 46K fragments, the first successful stopping and extraction of a high-energy beam with the cyclotron stopper was demonstrated.

Harvesting 88Zr from heavy-ion beam irradiated tungsten at the National Superconducting Cyclotron Laboratory

Tungsten is a commonly used material at many heavy-ion beam facilities, and it often becomes activated due to interactions with a beam. Many of the activation products are useful in basic and applied sciences if they can be recovered efficiently. In order to develop the radiochemistry for harvesting group (IV) elements from irradiated tungsten, a heavy-ion beam containing 88Zr was embedded into a stack of tungsten foils at the National Superconducting Cyclotron Laboratory and a separation methodology was devised to recover the 88Zr. The foils were dissolved in 30% hydrogen peroxide, and the 88Zr was chemically purified from the tungsten matrix and from other co-implanted radionuclides (such as 85Sr and 88Y) using strong cation-exchange (AG MP-50) chromatographic resin in sulfuric acid media. The procedure provided 88Zr in approximately 60 mL 0.5 M sulfuric acid with no detectable radio-impurities. The overall recovery yield for 88Zr was (92.3 ± 1.2)%. This proof-of-concept experiment has facilitated the development of methodologies to harvest from tungsten and tungsten-alloy parts that are regularly irradiated at heavy-ion beam facilities.

4DCT is long overdue for improvement.

4DCT is long overdue for improvement.

Measurements and computational analysis of the natural decay of Lu176

Background: Mainly because of its long half-life and despite its scientific relevance, spectroscopic measurements of $^{176}\mathrm{Lu}$ forbidden $\ensuremath{\beta}$ decays are very limited and lack formulation of shape factors. A direct precise measurement of its Q value is also presently unreported. In addition, the description of forbidden decays provides interesting challenges for nuclear theory. The comparison of precise experimental results with theoretical calculations for these decays can help to test underlying models and can aid the interpretation of data from other experiments.Purpose: Perform the first precision measurements of $^{176}\mathrm{Lu}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$-decay spectra and attempt the observation of its electron capture decays, as well as perform the first precision direct measurement of the $^{176}\mathrm{Lu}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$-decay Q value. Compare the shape of the precisely determined experimental $\ensuremath{\beta}$ spectra to theoretical calculations, and compare the end point energy to that obtained from an independent Q value measurement.Method: The $^{176}\mathrm{Lu}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$-decay spectra measurements and the search for electron capture decays were performed with an experimental setup that employed lutetium-containing scintillator crystals and a NaI(Tl) spectrometer for coincidence counting. The $\ensuremath{\beta}$ decay Q value was determined via high-precision Penning trap mass spectrometry (PTMS) with the LEBIT facility at the National Superconducting Cyclotron Laboratory. The $\ensuremath{\beta}$-spectrum calculations were performed within the Fermi theory formalism with nuclear structure effects calculated using a shell model approach.Results: Both $\ensuremath{\beta}$ transitions of $^{176}\mathrm{Lu}$ were experimentally observed and corresponding shape factors formulated in their entire energy ranges. The search for electron capture decay branches led to an experimental upper limit of $6.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ relative to its $\ensuremath{\beta}$ decays. The $^{176}\mathrm{Lu}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$-decay and electron capture Q values were measured using PTMS to be 1193.0(6) and 108.9(8) keV, respectively. This enabled precise $\ensuremath{\beta}$ end point energies of 596.2(6) and 195.3(6) keV to be determined for the primary and secondary $\ensuremath{\beta}$ decays, respectively. The conserved vector current hypothesis was applied to calculate the relativistic vector matrix elements. The $\ensuremath{\beta}$-spectrum shape was shown to significantly depend on the Coulomb displacement energy and on the value of the axial vector coupling constant ${g}_{A}$, which was extracted according to different assumptions.Conclusion: The implemented self-scintillation method has provided unmatched observations of $^{176}\mathrm{Lu}$, independently validated by the first direct measurements of its $\ensuremath{\beta}$-decay Q value by Penning trap mass spectrometry. Theoretical study of the main $\ensuremath{\beta}$ transition led to the extraction of very different effective ${g}_{A}$ and ${log}_{10}\phantom{\rule{0.16em}{0ex}}f$ values, showing that a high-precision description of this transition would require a realistic nuclear structure with nucleus deformation.

Relative population of states in Mg21 from few-nucleon removal reactions

Nuclear reactions with intermediate-energy beams in which three, four, and five nucleons are removed are expected to proceed through a combination of nondissipative and statistical processes. In an experiment at the National Superconducting Cyclotron Laboratory, in-beam $\ensuremath{\gamma}$-ray spectroscopy was utilized to study few-nucleon removal reactions from incoming beams of $^{24}\mathrm{Mg}, ^{25}\mathrm{Al}$, and $^{26}\mathrm{Si}$ projectiles to the same reaction product, $^{21}\mathrm{Mg}$. New $\ensuremath{\gamma}$-ray transitions and $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences in $^{21}\mathrm{Mg}$ were established using the CsI(Na) array CAESAR and the inclusive cross section for three-neutron removal from $^{24}\mathrm{Mg}$ was measured. Significant differences in the relative population of states in $^{21}\mathrm{Mg}$ from $^{25}\mathrm{Al}$ compared to $^{26}\mathrm{Si}$ were observed and found to be correlated with the spins of the $^{21}\mathrm{Mg}$ states. As a result, this intermediate regime between direct and statistical nucleon removal may have potential as a tool to deliver unique patterns of level populations in fast-beam experiments and alter isomer to ground state ratios in the production of exotic beams.

Evolution of collectivity in 126,128Xe studied in Coulomb excitation measurements

The characteristics of 126,128Xe were investigated in Coulomb excitation measurements performed at the National Superconducting Cyclotron Laboratory (NSCL) Re-accelerator facility, ReA3, Michigan State University (MSU). The Xe nuclei were accelerated to sub-barrier energies and were impinged on 196Pt and 208Pb targets in separate experimental runs. The scattered nuclei and the de-excitation γ-rays were detected using the JANUS setup. Electromagnetic matrix elements were extracted from the experimental data with the help of the GOSIA/GOSIA2 codes. The results were compared to schematic Davydov-Filippov γ-rigid rotor theoretical calculations and large-scale calculations within a newly-established microscopic shell model (called PMMU model). The experimental results agree well with the theoretical predictions, except for the quadrupole moments of the second 2+ states in both nuclei, therefore challenging the interpretation of the γ-bands structure.

Probing spin-isospin excitations in proton-rich nuclei via theC11(p,n)N11reaction

Tracking the evolution of nuclear properties away from stability serves as a valuable test for nuclear models. In the present work, the $(p,n)$ charge-exchange reaction was used to test the extraction of ${\ensuremath{\beta}}^{\ensuremath{-}}$ Gamow-Teller transition strengths, $B(\mathrm{GT})$, from proton-rich unstable isotopes, and the resulting $B(\mathrm{GT})$ values were compared to shell-model and ab initio calculations. The $^{11}\mathrm{C}(p,n)^{11}\mathrm{N}$ reaction was measured in inverse kinematics at 95 MeV/u at the National Superconducting Cyclotron Laboratory. The $B(\mathrm{GT})$ values to the ${\frac{1}{2}}^{\ensuremath{-}}$ state at 0.73 MeV and the ${\frac{3}{2}}^{\ensuremath{-}}$ state at 2.86 MeV in $^{11}\mathrm{N}$ were determined to be $0.18{(1)}^{\mathrm{stat}}{(3)}^{\mathrm{sys}}$ and $0.18{(1)}^{\mathrm{stat}}{(4)}^{\mathrm{sys}}$, respectively. These results are consistent with shell-model calculations using the wbp interaction after introducing a phenomenological quenching factor and with ab initio variational Monte Carlo calculations using the $NV2+3I{a}^{*}$ $NN$ and $3N$ interactions without any scaling. Additionally, this result is consistent with the $B(\mathrm{GT})$ values extracted from mirror $^{11}\mathrm{B}(n,p)$ and $^{11}\mathrm{B}(t,^{3}\mathrm{He})$ reactions. This experiment demonstrates the feasibility of using the $(p,n)$ probe in inverse kinematics to extract $B(\mathrm{GT})$ from proton-rich nuclei, although improved background suppression will be important in future experiments.

Single-particle properties of the near-threshold proton-emitting resonance in B11

The excitation function of proton elastic scattering from $^{10}\mathrm{Be}$ at keV energy is calculated using the method of self-consistent Skyrme Hartree-Fock in the continuum. The calculation successfully reproduces the narrow near-threshold proton-emitting resonance (${E}_{x}=11.4$ MeV, $\mathrm{\ensuremath{\Gamma}}=6$ keV, and quantum number ${J}^{\ensuremath{\pi}}=1/{2}^{+}$) in $^{11}\mathrm{B}$ relevant to the $\ensuremath{\beta}$-delayed proton emission of $^{11}\mathrm{Be}$. This supports the recent experimental result of Ayyad et al. at the ReA3 reaccelerator facility of the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. The resonance is interpreted as the ${s}_{1/2}$ single-proton resonance state in the Skyrme Hartree-Fock mean-field theory.

Structure of Xe126,128 studied in Coulomb excitation measurements

The electromagnetic properties of $^{126,128}\mathrm{Xe}$ were studied in subbarrier Coulomb excitation measurements performed at the National Superconducting Cyclotron Laboratory Re-accelerator facility, ReA3, at Michigan State University (MSU). $^{126}\mathrm{Xe}$ and $^{128}\mathrm{Xe}$ nuclei were accelerated to 3.74 and 3.81 MeV/nucleon, respectively, and were impinged on $^{196}\mathrm{Pt}$ and $^{208}\mathrm{Pb}$ targets. The $\ensuremath{\gamma}$ rays deexciting the populated low-lying states were detected in coincidence with the scattered nuclei using the JANUS setup. Transition and diagonal matrix elements for low-lying states and transitions in $^{126,128}\mathrm{Xe}$ were extracted from the experimental data using the gosia and gosia2 codes. The experimental results were compared with the theoretical calculations by the microscopic shell model and the Davydov-Filippov $\ensuremath{\gamma}$-rigid rotor model. The calculated results from the newly established shell model (called the PMMU model), which is based on the advanced Hartree-Fock Bogoliubov plus generator coordinate method (HFB $+$ gcm) for a large model space, agree well with the measurements in both nuclei, except for the second ${2}^{+}$ state. Interpretation for the experimentally determined nearly vanishing electric-quadrupole moment of this state remains a challenge for theory.

Solid-phase isotope harvesting of 88Zr from a radioactive ion beam facility

During routine operation of the Facility for Rare Isotope Beams (FRIB), radionuclides will accumulate in both the aqueous beam dump and along the beamline in the process of beam purification. These byproduct radionuclides, many of which are far from stability, can be collected and purified for use in other scientific applications in a process called isotope harvesting. In this work, the viability of 88Zr harvesting from solid components was investigated at the National Superconducting Cyclotron Laboratory. A secondary 88Zr beam was stopped in a series of collectors comprised of Al, Cu, W, and Au foils. This work details irradiation of the collector foils and the subsequent radiochemical processing to isolate the deposited 88Zr (and its daughter 88Y) from them. Total average recovery from the Al, Cu, and Au collector foils was (91.3 ± 8.9) % for 88Zr and (95.0 ± 5.8) % for 88Y, respectively, which is over three times higher recovery than in a previous aqueous-phase harvesting experiment. The utility of solid-phase isotope harvesting to access elements such as Zr that readily hydrolyze in near-neutral pH aqueous conditions has been demonstrated for application to harvesting from solid components at FRIB.

High-precision mass measurement of Si24 and a refined determination of the rp process at the A=22 waiting point

We report a high-precision mass measurement of $^{24}\mathrm{Si}$, performed with the Low Energy Beam and Ion Trap (LEBIT) facility at the National Superconducting Cyclotron Laboratory. The atomic mass excess, $10\phantom{\rule{0.16em}{0ex}}753.8$(37) keV, is a factor of 5 more precise than previous results. This substantially reduces the uncertainty of the $^{23}\mathrm{Al}(p,\ensuremath{\gamma})^{24}\mathrm{Si}$ reaction rate, which is a key part of the rapid proton capture ($rp$) process powering type I x-ray bursts. The updated rate constrains the onset temperature of the $(\ensuremath{\alpha},p)$ process at the $^{22}\mathrm{Mg}$ waiting point to a precision of 9%.

Constraining the astrophysical p process: Cross section measurement of the Kr84(p,γ)Rb85 reaction in inverse kinematics

One of the biggest questions in nuclear astrophysics is understanding where the elements come from and how they are made. This work focuses on the p process, a nucleosynthesis process that consists of a series of photodisintegration reactions responsible for producing stable isotopes on the proton-rich side of stability. These nuclei, known as the p nuclei, cannot be made through the well-known neutron-capture processes. Currently p-process models rely heavily on theory to provide the relevant reaction rates to predict the final p-nuclei abundances and more experimental data is needed. The present work reports on an experiment performed with the SuN detector at the National Superconducting Cyclotron Laboratory, NSCL, at Michigan State University using the ReA facility to measure the $^{84}$Kr(p,$\gamma$)$^{85}$Rb reaction cross section in inverse kinematics. The reverse $^{85}$Rb($\gamma$,p)$^{84}$Kr reaction is a branching point in the p-process reaction network that was highlighted as an important reaction in sensitivity studies in the production of the $^{78}$Kr p nucleus. A new hydrogen gas target was designed and fabricated and a new analysis technique for background subtraction and efficiency calculations of the detector were developed. The experimental cross section is compared to standard statistical model calculations using the NON-SMOKER and TALYS codes.

Development of Chemical Procedures for Isotope Harvesting: Separation of Trace Hafnium from Tungsten

ABSTRACT The separation of trace hafnium from bulk tungsten alloys is of interest for isotope harvesting at the National Superconducting Cyclotron Laboratory and the Facility for Rare Ion Beams because 172Hf, the parent of 172Lu, accumulates in tungsten alloy beam blockers at these facilities. In this work, a procedure for the separation of trace hafnium from a bulk tungsten alloy (454 g) was established using tracer isotopes (175Hf, 88Zr, 173Lu, and 88Y). The procedure employed dissolution in an HF–HNO3 solution followed by a calcium fluoride precipitation, and then extraction chromatography was used for more selective separation steps. Two stages of column separations using LN resin (HDEHP based) and TRU resin (CMPO based) were performed. Gamma-ray spectroscopy and mass spectrometry were used to analyze the final hafnium sample and follow the steps in the chemical processing. The final recovery of hafnium was high (90 ± 8)%, and the mass of tungsten and other transition metals was reduced to near background levels (as determined by ICP-MS of the blank acid solutions). Zirconium follows hafnium quantitatively in this procedure; there was no detectable 173Lu in the final hafnium sample.

57Zn β-delayed proton emission establishes the 56Ni rp-process waiting point bypass

We measured the Zn57β-delayed proton (βp) and γ emission at the National Superconducting Cyclotron Laboratory. We find a Zn57 half-life of 43.6±0.2 ms, βp branching ratio of (84.7±1.4)%, and identify four transitions corresponding to the exotic β-γ-p decay mode, the second such identification in the fp-shell. The p/γ ratio was used to correct for isospin mixing while determining the Zn57 mass via the isobaric multiplet mass equation. Previously, it was uncertain as to whether the rp-process flow could bypass the textbook waiting point Ni56 for astrophysical conditions relevant to Type-I X-ray bursts. Our results definitively establish the existence of the Ni56 bypass, with 14-17% of the rp-process flow taking this route.