Research Highlights

Nuclear Physics of Neutron Star Outer Layers in a Nutshell

Neutron stars, city-sized objects that weigh as much as the sun, host some of the most extreme conditions known to exist in nature. Studies of these objects ultimately aim to provide insight into the behavior of matter at densities near that of an atomic nucleus and temperatures low enough for quantum phenomena to manifest themselves. Neutron stars host a rich variety of nuclear processes, which shape their thermal and compositional structure and are especially prominent in the outer layers (often known as the atmosphere, ocean, and crust). In an invited review, we summarize the state of knowledge of nuclear physics of the outer layers of neutron stars, approaching the subject from the perspectives of observational astronomy, astrophysics theory, nuclear experiment, and nuclear theory. In this sense, this article is a one-stop-shop for practitioners looking to learn more about related research in other subfields and for students beginning to enter the exciting field of neutron star nuclear physics research.

The full work, published in Journal of Physics G, can be accessed here.

Meisel Recieves DOE Early Career Award

Research group leader Zach Meisel recieved one of the 2018 Department of Energy Office of Science Early Career Awards. This selective award "supports the development of individual research programs of outstanding scientists early in their careers" who "have the potential to develop new scientific ideas, promote them, and convince their peers to pursue them as new directions." This award will support innovative research employing nuclear physics experiments using stable and radioactive ion beams coupled with state-of-the-art astrophysics model calculations of phenomena occuring in the outer layers of accreting neutron stars. Ultimately this research will improve our understanding of how matter behaves at the highest densities observed in nature.

You can read the full story in the Ohio University Compass newsletter here or the Facility for Rare Isotope Beams newsletter here and find the Public Abstract for the funded research here.

Constraining Nuclear Physics with Astrophysics: Reconstruction of the Clock Burster

Type-I X-ray bursts are thermonuclear explosions on a neutron star surface powered by hydrogen and helium burning. The "textbook" example is the so-called clock burster GS 1826-24, which has displayed regular bursting behavior that resembles results obtained from astrophysics model calculations. We consistentely modeled the light curve of three bursting epochs of GS 1826-24 for the first time. This work resulted in improved astrophysical constraints on this source and demonstrated that such light curve reconstruction may be able to place limits on nuclear reaction rates which are presently out of reach from laboratory experiments.

The full work, published in the Astrophysical Journal, can be accessed here.
Model calculation details and results can be accessed here.

Brandenburg Recognized at SSAP Symposium

Research group member Kristyn Brandenburg took home a "best poster" prize at the National Nuclear Security Administration Stewardship Science Academic Program (NNSA SSAP) 2018 Symposium. Kristyn presented her work Developing a Long Counter for (α,n) Measurements Relevant to the α-process and Nuclear Reactor Diagnostics at Ohio University.

You can read more about this story from the NNSA Stockpile Stewardship Quarterly here, or from the Ohio University College of Arts and Sciences Forum here.

OU Expo 2017: Clean Sweep!

Research group members Kristyn Brandenburg and Doug Soltesz won poster presentation awards at the 2017 Ohio University Student Research Expo. Kristyn presented her work on the development of a neutron long-counter for measuring (α,n) cross sections. Doug presented his work on the development of a silicon detector array for detecting charged-particle emission from compound nuclear states populated via (3He,n) two-proton transfer measurements.

The full story, from the College of Arts and Sciences at Ohio University, can be accessed here.

Going Back in Time with Crust Coolers

Neutron stars often siphon hydrogen- and helium-rich material from a companion star, leading to a host of nuclear reactions near the neutron-star surface and associated observable phenomena. So-called quasi-persistent transients, neutron stars that cool after a long outburst of accretion ceases, can be used to infer the thermal and compositional structure of the neutron star's outer layers. However, Urca cooling, neutrino emission from e---decay cycles, has the potential to substantially impact the neutron star thermal structure. We included the impact of Urca cooling at realistic cooling strengths in models of quasi-persistent transients, shwoing it can impact the quasi-persistent transient light curve during quiesence. We use this effect to demonstrate that nuclear burning on the neutron star surface from centuries to millenia in the past can be constrained. We also identify nuclear physics uncertainties that must be reduced to improve the bygone surface nucleosynthesis constraints provided by our new technique.

The full work, published in the Astrophysical Journal, can be accessed here.

St. George Put through Its Paces

One of the most commonly occuring nuclear reaction types in stable and explosive stellar processes is the α-capture, γ-emission, (α,γ), reaction. These so-called radiative helium-burning reactions are notoriously diffuclt to measure directly with traditional techniques employing an α beam and γ-ray detector, since γ-rays of interest are often hidden due to prodigious background radiation. The St. George recoil separator was designed to work around this difficulty by instead measuring the nuclear recoil produced by (α,γ) reactions. We recently completed the crucial first test of St. George to make sure it could accept the full range of energies expected for nuclear recoils produced by (α,γ) reactions of interest for astrophysical helium-burning processes.

The full work, published in Nuclear Instruments and Methods A, can be accessed here.

Search Narrowed for Important Rp-process Reactions

Thermonuclear x-ray bursts, one of the most commonly occuring thermonuclear explosions in the universe, have been linked to hydrogen and helium burning reactions on neutron star surfaces for roughly four decades. Though it is understood that the rapid proton-capture (rp)-process powers the bursts, systematic studies identifying the most important nuclear reaction rates have been virtually absent. In fact, to-date only one study has examined the sensitivity of the rp-process reaction network to variations in thermonuclear reaction rates. To remedy this issue, we performed the first nuclear reaction sensitivity study of the rp-process which took into account the feedback from nuclear reactions on the environment temperature. We found several reactions which affect the light output and nuclear reaction products from x-ray bursts and quantified their impacts.

The full work, published in the Astrophysical Journal, can be accessed here.

Thermostats Ubiquitous Near Neutron Star Surface

The possible existence of urca pairs, nuclides which cool dense environments via ν emission from e--capture/β--decay cycling, in the crust of accreting neutron stars was recently identified. We expanded upon that recent discovery by completing the census of all possible urca cooling nuclides throughout the neutron star ocean and crust, incorporating more realistic nuclear physics assumptions than had been made previously. We also assessed the impact of plausible urca cooling strenghts on x-ray superbursts and found a robust increase in the depth of carbon ignition (which is thought to trigger superbursts).

The full work, published in the Astrophysical Journal, can be accessed here.

Investigation of the Origin of Zn to Sn Heats Up with HABANERO

In the past decade, it has become apparent the elements from roughly zinc (Zn) to tin (Sn) are likely not made in the rapid neutron-capture process, as was previously thought. One favored creation site is the neutron-rich ν-driven winds of core-collapse supernovae, primarily via a sequence of (α,n) reactions. However, no (α,n) reaction cross sections on neutron-rich nuclides have been measured to-date due to experimental challenges. We recently developed the HABANERO neutron detector, which will enable us to measure many of the important (α,n) reactions at astrophysically relevant energies for the first time. We have now completed our first commissioning study of HABANERO at the Edwards Accelerator Laboratory at Ohio University, demonstrating its neutron-dection capabilities.

The full story, from the College of Arts and Sciences at Ohio University, can be accessed here.

Update: We completed the first (α,n) reaction cross section measurement on a neutron-rich nuclide July 2016 at the National Superconducting Cyclotron Laboratory. Analysis is ongoing.

Supersonic Gas-jet Target Optimized with CFD

Supersonic gas-jet targets play an important role in nuclear astrophysics studies by providing point-like nuclear reaction targets of gaseous species. The HIPPO gas-jet will provide the helium target for (α,γ) reaction cross section measurements with the St. George recoil separator at the University of Notre Dame's Nuclear Science Laboratory. The fluid dynamics properties of HIPPO were recently investigated with computational fluid dynamics (CFD) simulations and compared to data. This study marks the first step toward optimizing the performance of HIPPO and future gas-jets as nuclear reaction targets for nuclear astrophysics studies.

The full work, published in Nuclear Instruments and Methods A, can be accessed here.

Time-trial Expands Known Nuclear Mass Surface

Nuclear masses provide key insight into the evolution of nuclear structure for highly exotic nuclides, as well as crucial input into astrophysics model calculations. Mass determinations of the most exotic nuclides accessible in the laboratory are hindered due to their low-statistics and short half-lives. We recently overcame these challenges for neutron-rich isotopes of argon through iron via time-of-flight (TOF) mass measurements performed at the National Superconducting Cyclotron Laboratory at Michigan State University. By obtaining a calibrated mass-TOF relationship, we determined the masses of 18 nuclides, 7 of which were measured for the first time.

The full work, published in Physical Review C, can be accessed here.

Neutron Star Crusts Not So Hot (or Cold)

Recent calculations showed that urca cooling, ν-emission from e---decay cycling, may be present in the crusts of accreting neutron stars and that e--capture into 56Sc may be the strongest source of cooling. However, whether strong heating or cooling occurred depended on the unknown mass of 56Sc and the nuclear structure of its e--capture daughter 56Ca. We resolved this mystery by measuring the mass of 56Sc for the first time, using the time-of-flight mass measurement technique. When supplemented with new nuclear shell-model calculations of 56Ca's structure and implemented in a state-of-the-art neutron star crust composition evolution calculation, we found neither strong heating nor cooling. This result means that the abundance of other urca cooling nuclides, which were previously thought to have weaker cooling strengths, now must be determined to higher precision.

The full work, published in Physical Review Letters, can be accessed here.

Mind the (N=28) Gap

For roughly 70 years nuclides have been known to exhibit an enhanced stability to transmutation when they possess special, so called "magic", numbers of nucleons (neutrons and protons). However, it has been shown that magic numbers disappear (and new ones can take their place) for increasingly exotic nuclides. Until now, the lower limit for the number a protons (Z) a nucleus required for it to display N=28 magicity was not known. We determined this honor belongs to argon (Z=18) using time-of-flight mass measurement.

The full work, published in Physical Review Letters, can be accessed here.

68Se Pumps the Breaks on the rp-Process

Type-I X-ray bursts are thermonuclear explosions occuring on the surfaces of accreting neutron stars. Of the thousands of possible nuclear reactions, a handful are thought to play a dominant role in the rate at which the burning of nuclear fuel proceeds, and therefore on the burst light output and burning ashes. Waiting-point nuclides play such a role in the rp-process, where burning of fuel is slow and the process stalls until the waiting-point nuclide has decayed away. Recently it was an open question as to what extent 68Se stalled the rp-process. By determining the mass difference between 68Se and its proton-capture daughter 69Br in a β+-delayed proton-emission measurement at the National Superconducting Cyclotron Laboratory at Michigan State University, we found 68Se substantially stalls the rp-process, quenching the light output of x-ray bursts at late times.

The full work, published in Physics Letters B, can be accessed here.

The instrumentation paper corresponding to the new analysis technique developed for this work, published in Nuclear Instruments and Methods A, can be accessed here.