Nuclear Astrophysics

Relating Hubble observations to the inside of a novae explosion: Several years ago, fluorine was observed in the spectrum of a nova explosion for the first time by a joint exercise between the Hubble Space Telescope and the Nordic Optical Telescope. This provides a powerful tool to compare astronomical observations with theoretical stellar models, because only one stable isotope of fluorine exists, ¹⁹F, and its quantity is extremely sensitive to the nuclear reactions that create and destroy it as well as the temperature & density conditions in the explosion. One such reaction, p (¹⁹Ne,γ)²⁰Na, was measured for the first time at the DRAGON facility, using a beam of short-lived ¹⁹Ne produced at ISAC. This long sought-after reaction cross section was previously inaccessible to direct measurement. The results reduce the uncertainties resulting from nuclear physics inputs to negligible levels when comparing theoretical stellar models to the HST observations.    

Understanding gamma-ray emission from nova explosions: One of the first signals to emanate from a nova explosion is an intense burst of X- and gamma rays, long before the peak of the optical brightness is reached. At such a time, the 511 keV gamma-ray line is directly linked to the amount of radioactive ¹⁸F synthesized in the explosion. Thus, observing the 511 keV gamma ray intensity in a nova explosion would give astronomers a direct “thermometer” in the heart of the explosion. The problem is that the rates of nuclear reactions that create and destroy ¹⁸F in this environment are highly uncertain, including specifically the ¹⁸F(p,𝛼)¹⁵O and ¹⁸F(p,γ)¹⁹Ne reactions. Complementary to work on the ¹⁸F(p,𝛼)¹⁵O reaction performed at TUDA, DRAGON has measured a key resonance in ¹⁸F(p,γ)¹⁹Ne for the first time, finding it to be much weaker than previously thought, and reducing the uncertainties in the amount of ¹⁸F produced in these scenarios.  

TUDA Helps Lead Astrophysicists to Sources of Cosmic Aluminum: The rare isotope aluminum-26 (²⁶Al) with a lifetime of roughly a million years is one of the most important sources of information about galactic nucleosynthesis and has been carefully studied using the most advanced telescopes. However, what's highly uncertain in stellar models are certain nuclear reactions that produce and destroy ²⁶Al. As reported in Physical Review Letters (2015) scientists using TUDA achieved precise new measurements for one of these difficult to experimentally explore reactions, ²⁶Al(p,γ)²⁷Si. The results provide astrophysicists more precise estimates about the destruction of ²⁶Al in massive stars, and about the contributions of various cosmic sources to ²⁶Al production.

Studying the aluminium produced in massive stars: As reported in Physical Review Letters (2015), TUDA performed a direct measurement of the α (²³Na ,p)²⁶Mg reaction, whose strength influences the amount of aluminum-26 (²⁶Al) produced in convective burning in giant stars. Considerable disagreement existed in the literature as to its strength of this reaction, leading to large uncertainties in the predictions of the synthesis of ²⁶Al by massive stars. The TUDA measurement resolved the discrepancy, showcasing TUDA’s ability to directly measure a reaction of astrophysical interest at the energies inside the stars.

How old are the oldest stars? Using DSL, scientists have helped astrophysicists narrow-in on a key cosmic mystery: the age of the oldest stars in the Milky Way. To infer a star's age from its starlight, astrophysicists need to know the rates of nuclear reactions powering it, in particular the rate of the reaction when ¹⁴N captures a proton to become ¹⁵O. It's the slowest reaction, and thus determines the overall pace, of the carbon-nitrogen-oxygen cycle of hydrogen burning in stars. As reported in Physical Review C (2014), using the DSL facility, scientists measured the 6.79 MeV energy state and others in ¹⁵O and constrained its lifetime to be less than 1.8 femtoseconds, or quadrillionths of a second. The researchers believe that even more precise reaction rates could come from coupling the DSL facility to TRIUMF's new recoil spectrometer EMMA.

Solar fusion and Big Bang nucleosynthesis from ab initio theory: The ³He(α,γ)⁷Be and ³H(α,γ)⁷Li radiative-capture processes hold great astrophysical significance. Their reaction rates between ∼20 and 500 keV are essential to calculate the primordial ⁷Li abundance in the universe. In addition, standard solar model predictions for the fraction of pp-chain branches resulting in ⁷Be versus ⁸B neutrinos depend critically on the ³He(α,γ)⁷Be astrophysical S factor at about 20 keV. These capture cross sections are strongly suppressed at such low energies and thus hard to measure in a laboratory. As reported in Physics Letters B (2016), TRIUMF nuclear theorists investigated these reactions ab initio using a chiral nucleon–nucleon interaction. Our calculated ³He(α,γ)⁷Be cross sections agree reasonably with the higher-energy experimental data including those measured at TRIUMF while the ³H(α,γ)⁷Li ones are overestimated. Our low-energy predictions help reduce uncertainty of nuclear data evaluations for astrophysics.

Neutron-skin as a portal to neutron star properties: With the new era opened up by the LIGO and VIRGO observation of neutron-star mergers, multi-messenger astronomy will provide us with new ways to constrain the equation of states of neutron-rich matter. Stronger constraints will be obtained by combining such future data with observations on finite nuclei. Indeed, neutron-rich nuclei provide a portal to study neutron-rich matter, in that they form a neutron-skin around their surface which is directly related to the equation of state of nuclear matter. As reported in Nature (2016), TRIUMF theorists and international collaborators provided the first ab initio computation of calcium-48 (⁴⁸Ca), a nucleus with 20 protons and 28 neutrons. It was predicted that a neutron-skin is formed, though much smaller than previously thought, calling for new experimental investigations.  

Measurement of the most exotic neutron emitters at BRIKEN: The BRIKEN (Beta-delayed neutron measurements at RIKEN for nuclear structure, astrophysics, and applications) project started in 2016 at the RIKEN Nishina Center in Japan. As reported in the Journal of Instrumentation (2017) the ambitious goal of the collaboration is to design the most efficient neutron detector array for the measurement of the most exotic nuclei that can be produced today. With TRIUMF research collaboration, so far 268 nuclei have been measured, and for 180 of them the neutron branching ratio and for 60 the decay half-life has been measured for the first time. The neutron-branching ratio of the doubly-magic isotope nickel-78 has been measured for the first time, and will help to pinpoint theoretical predictions of neutron-magic nuclei.

Clarifying a key step in the origin of the elements: As reported in Physical Review C (2016) researchers using GRIFFIN have produced the highest-precision measurement ever of the half-life of cadmium-130  (¹³⁰Cd), a rare isotope that's a cornerstone for understanding cosmic element formation. Astrophysical observations are providing mounting evidence that heavy elements are forged in the merger of neutron stars. However, to understand and accurately simulate this element formation process (r-process), it's necessary to experimentally characterize the key rare isotopes involved. This is especially the case for the half-lives of isotopes with masses of about 130 since theoretical models have been tuned to reproduce the half-life of ¹³⁰Cd and then predict half-lives in the entire region. Thus, scientists used GRIFFIN to produce a ¹³⁰Cd half-life measurement three-times more precise than the previously adopted world average, a result which will help astrophysicists more clearly see our stardust origins.    

Explaining anomalies in the spectra of classical novae: The optical, ultraviolet and infrared spectra of the debris left over after nova explosions – thermonuclear detonations on the surface of accreting white dwarves in stellar binary systems – contain important fingerprints of the chemical elements synthesized and ejected during these cataclysmic events. However, for some elements, namely argon and calcium, much more than expected seems to be present. This flies in the face of theoretical models of nova explosions which say that nucleosynthesis in novae effectively stops at calcium, with around the same amount of calcium being present after the explosion as before the explosion. The volume of elements from Ar-Ca produced in these scenarios depends sensitively on the strengths of nuclear reactions around that region, in particular proton-induced radiative capture reactions. One of these,  p (³⁸K ,γ)³⁹Ca has now been experimentally measured for the first time using TRIUMF’s DRAGON facility, previously impossible because if the short lifetime of ³⁸K, but accessible to the inverse kinematics technique of DRAGON using an intense ³⁸K beam made at ISAC. This makes p (³⁸K,γ)³⁹Ca the highest mass reaction ever measured using this technique with radioactive beams.      

EMMA set to help reveal the hearts of stars and atoms: In 2018, the coupling of the EMMA and TIGRESS spectrometers will mark the beginning of EMMA’s scientific program to probe the deep nature of nuclear reactions in stars and the subtleties of extreme nuclear structure. EMMA's arrival is the culmination of a multi-year series of commissioning steps successfully demonstrating the spectrometer’s enormous potential for TRIUMF's rare isotope beam program. In December 2016, commissioning of the spectrometer as a whole began with a test beam of argon-36 (³⁶Ar) bombarding a very thick gold (Au) foil. The spectrometer was initially tuned for elastically scattered Ar ions and its first mass/charge spectrum was collected. As shown in figure A, both the 13⁺ and 14⁺ charge states were detected simultaneously, the dispersion–the key ability to distinguish between states–agreed exactly with calculations. In September 2017 an argon-40 (⁴⁰Ar) beam bombarded a very thin Au target and elastically back-scattered Au ions were detected in two charge states simultaneously and used to measure the energy, angular, and mass/charge acceptances; substantially improved resolving power was observed. In November 2017, EMMA accepted its first radioactive beam, sodium-24 (²⁴Na) which was used to induce fusion evaporation reactions on a copper target. Various fusion products with atomic masses from 80 to 85 were clearly detected and resolved in a single spectrometer setting. EMMA will use of both the light, neutron-deficient beams from ISAC and the future heavy, neutron-rich beams from ARIEL to explore radiative capture and transfer reactions that are central to astrophysical research, and fusion reactions important to the study of nuclear structure.

Rare isotope sheds light on how dead stars re-ignite: Novae are stellar element-making explosions caused when a white dwarf, the carbon-cinder of a once giant star, accretes hydrogen-rich material onto its surface from a companion star. A key to help astrophysicists better understand novae is through determining the rates of reactions that create and destroy the key rare isotopes they're observed to produce, including sodium-22 (²²Na). As reported in Physical Review C (2016) scientists used DSL to determine, for the first time using the Doppler-shift attenuation method, the lifetimes of several energetic states magnesium-23 (²³Mg) the rare isotope that dominates the destruction of ²²Na. The researchers' more precise lifetime measurements of related states help explain the structure and behaviour of these rare isotopes and thus the underlying nuclear physics driving novae.  

A new path to element formation: IRIS’ first results give astrophysicists and nuclear physicists a deeper understanding of the nuclear states and pathways involved in element formation. As reported in Physical Review Letters (2015), the results provide the experimental evidence for a new kind of nuclear excitation, soft dipole resonance. IRIS researchers identified this resonance in neutron-rich lithium-11 (¹¹Li), the heaviest bound lithium isotope, with three protons and eight neutrons, two of which orbit, barely held, in a halo, far from the nuclear core. Oscillation of the halo neutrons, the experiment revealed, leads to an extremely brief, quasi-bound nuclear resonance state, significantly changing the potential for neutron capture, as occurs in stars during element formation. The results confirm theoretical predictions and provide an important path for developing new nuclear models to predict the properties of neutron-rich isotopes.                                

TRIUMF helps provide IAEA with evaluation of beta-delayed neutron emitters In a coordinated research project under the auspices of the International Atomic Energy Agency (IAEA), Canadian researchers from TRIUMF and McMaster University have evaluated all existing beta-delayed neutron emitters and provided recommended values for their decay half-lives and neutron-branching ratios. These new recommendations, released as an IAEA report, together with the new data from ongoing experiments, will be integral part of a newly created database. Among a variety of applications, the data will be a key input in astrophysical studies for a better understanding of the heavy element production in explosive stellar events including core-collapse supernovae and binary neutron star mergers. Such a reliable and regularly updated database is essential for a better understanding of these important physical properties, especially for benchmarking theoretical predictions of yet unmeasured nuclei.