Research Highlights

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 magicity with 28 neutrons (N=28) was unknown. We measured the nuclear masses of 48Ar and 49Ar for the first time, demonstrating that the argon (Z=18) marks the lower-limit for N=28 magicity.

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

Nuclear Physics Uncertainties Substantial for Supernova Yields

Recent work has suggested that the possible site of origin for so-called 'light-element primary process' (LEPP) nuclides in metal-poor stars is the neutron-rich ν-driven wind of core-collapse supernovae. In this site a sequence of nuclear reactions, dubbed the α-process, ultimately produce the elements once attributed to the lowest nuclear-mass peak of the rapid neutron-capture process. However, nearly all of the (α,n) reactions rates, which dominate the α-process reaction network flow, are unmeasured to-date. We demonstrated that the present uncertainties in theoretical nuclear reaction rate calculations can have as large as an impact (and even larger) on astrophysics model calculation abundance yields as the astrophysics uncertainties themselves. This result demonstrates that measurements of (α,n) reaction cross sections for neutron-rich nuclides are sorely needed.

The full work, published in the Proceedings of Science, can be accessed here.

68Se Pumps the Breaks on the Rp-process

The rapid proton-capture (rp-)process is the nuclear reaction sequence that powers type-I x-ray bursts, thermonuclear explosions 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.