Session DE: Mini-Symposium: (alpha,n) Reactions for Astrophysics and Applications

Measuring ($\alpha $,n) reaction rates relevant for nuclear astrophysics

There are numerous reaction rates involving $\alpha $-particles that play a crucial role in nuclear astrophysics. For instance, some ($\alpha $,n) reactions have been found to be important for the nucleosynthesis of light nuclei in the rapid neutron-capture process (r-process) in neutrino-driven winds after a core collapse supernovae. Direct measurements of these reactions at relevant astrophysical energies are experimentally challenging. This is due to the typically small cross sections of these reactions and the experimental difficulties associated with low-intensity radioactive beams needed to study them. As a consequence, most of these reaction rates are still unknown. However, recent advances in the capabilities of radioactive ion beam facilities and experimental techniques have opened up new possibilities for the study of these astrophysically important reactions. In this talk I will review recent experimental efforts by different groups to measure such important reactions. Then, I will discuss recent advances and future possibilities that will enable better insights into the nuclear physics of these relevant reactions.   

HabaNERO: A new experimental tool for the study of ($\alpha,xn$) reaction rates in the weak r-process

Recent sensitivity studies have proposed $(\alpha,$x$n)$ reactions are the main production mechanism of $Z$=38-47 abundances in early nucleosynthesis, so called the weak r-process, with the condition of $(n,\gamma)-(\gamma,n)$ equilibrium and temperatures between 3.5 and 5.5~GK. While the uncertainty of $(\alpha,xn)$ reaction rates was critical to change the production yield of elements, they are relatively large due to little experimental data existed for $(\alpha,xn)$ cross sections involved in the nucleosynthesis calculation. We have developed the HabaNERO neutron detector to study $(\alpha,$x$n)$ compound reactions of neutron-rich nuclei near $Z$=26-40 region including $^{75}$Ga$(\alpha,$x$n)$. The HabaNERO is a neutron long counter system which consists of $^{3}$He and BF$_{3}$ gas-filled proportional tubes in the polyethylene matrix optimized to obtain a high average neutron detection efficiency as constant as possible in the wide neutron energy range ($E_n = 0.1$-$19.5$~MeV) that corresponds to the neutron energies of interest. Details of the detector design and its commissioning result will be presented.   

Measurements of (alpha,n) cross-sections relevant for the weak r-process.

The fast-expanding neutron-rich neutrino-driven winds in the Core-Collapse SNe is one of the most favorable scenarios for the nucleosynthesis of the Z=38-47 elements. Charge particle reactions, especially (α,n) on the A=80-90, create seeds for the weak r-process populating abundances of near stable isotopes for the Sr-Cd range. These abundances are significantly sensitive to the (α,n) reaction rates. Only very few of these (α,n) reactions had been measured in the energy range relevant for weak r-process astrophysical conditions. Sensitivity studies of such scenarios show that 85Br(α,n) is one of the most significant reactions to impact the abundances of the seeds to the weak r-process. Theoretical reaction rates calculations for reactions for such scenarios are very uncertain and model-dependent. To measure the (α,n) cross-sections of 85Br, 85Rb, and 75Ga, the HabaNERO detector was used which is a neutron counter system that includes either BF3 or 3He gas-filled proportional-counter tubes embedded in the matrix of polyethylene, designed to achieve constant and energy independent efficiency for neutrons in the range of 0.01-20 MeV. Preliminary results from these experiments along with brief details of the discrepancies with the Hauser-Fesbach model calculations will be presented.   

The Design, Validation, and Future Plans for a New Neutron Detector at Ohio University

Though ($\alpha$,n) reaction cross sections play a key role in nuclear astrophysics and applications, many are poorly constrained by nuclear experiments and have significant uncertainties in theoretical predictions. Improving this situation will be done in part using a newly developed neutron long counter, HeBGB, at the Ohio University Edwards Accelerator Lab. The detector was designed using the MCNP6 software to have near constant efficiency in the neutron energy range relevant for core-collapse supernovae and special nuclear materials. Efficiency validation measurements have been performed with HeBGB, which utilize well-characterized reactions with constrained cross sections and known neutron energies. The first measurement conducted with HeBGB is 27Al($\alpha$,n) near threshold, which dominates the astrophysical rate, has disagreement between theoretical predictions and has only one prior measurement in this energy regime. In preparation, various aluminum targets have been tested for purity using the Rutherford Backscattering (RBS) and Particle Induced X-ray Emission (PIXE) nuclear reaction analysis techniques. We find that store bought aluminum foils offer higher purity than traditional foil suppliers.   

Improving (a,n) cross sections on light elements at the University of Notre Dame

$(\alpha,n)$ cross sections on light elements are useful for a number of applications ranging from nuclear astrophysics and nuclear structure to reactor design and nuclear non-proliferation to neutrino and dark matter detection. In particular many of these applications require the partial cross sections from threshold up to about 9~MeV in order to accurately predict neutron energy spectra. For practical reasons, the majority of previous measurements are limited to total cross sections. Compilations attempt to give partial cross sections based on statistical model calculations, but there are large uncertainties associated with these calculations, which are not propagated on to simulation codes, giving end users a false sense that these cross sections are all ``well-known". In this talk I will describe our general efforts to improve these cross sections by building on previous experiments, using state-of-the-art deuterated liquid scintillators and HPGe detectors, and improving evaluations with comprehensive $R$-matrix analyses.   

Measurement of the 13C(α,n)16O reaction with implications for neutrino mixing, and geo-neutrino measurements

The $^{13}C(\alpha,n)^{16}O$ reaction has a broad range of implications ranging from nuclear astrophysics to nuclear non-proliferation and reactors to geoneutrino and neutrino mixing measurements. It was noted by W.Peters (PRC96, 029801) that the population of excited states and the effects of neutron detection efficiency could affect the interpretation of previous results. These comments were supported by statistical model calculations by P. Mohr (PRC97, 064613). In this talk, we will present results from a neutron spectroscopic measurement of the $^{13}C(\alpha,n)^{16}O$ reaction between $E_\alpha$ = 3.5 and 7.5 MeV which spans this important threshold region for population to the excited states. Neutron spectroscopy was performed using spectrum unfolding technique using deuterated liquid scintillators which provided clean separation of ground state and excited state contributions. Implications on uses of this reaction pertaining to neutrino physics measurements will be discussed.   

Measurement of the $^{18}O($\alpha$,n)$^{21}$Ne reaction from 2-8 MeV

$^{18}$O($\alpha$,n)$^{21}$Ne reaction has broad interest across a number of applications for nuclear physics. In nuclear astrophysics, this reaction acts as a competing channel for the production of $^{22}$Ne for the neutron source in the weak s-process. In neutrinoless double beta decay, this reaction poses a potential background in water shielded detectors. In addition, for nuclear nonproliferation, it acts as a neutron source in low burnup uranium oxide fuel. There is little existing data for this reaction, with large uncertainties dominating the cross section at higher energies. As such, a high-resolution measurement of the $^{18}$O($\alpha$,n)$^{21}$Ne reaction was performed at the University of Notre Dame over the range E$_{\alpha} = 2 - 8$ MeV to extract partial and total cross sections. 10 ODeSA detectors were used for neutron spectroscopy via the spectrum unfolding technique, and 2 HPGe detectors were also used to detect secondary gamma rays. Preliminary results from this measurement will be shown and compared to existing data.