We investigate an implication of the most recent observation of a second $J^{\pi }=2^{+}$ state in ${}^{12}\mathrm{C}$, which was measured using the ${}^{12}\mathrm{C}(\gamma ,\alpha ){}^8\mathrm{Be}_{(\mathrm{g}.\mathrm{s}.)}$ reaction. In addition to the dissociation of ${}^{12}\mathrm{C}$ to an $\alpha$-particle and ${}^8\mathrm{Be}$ in its ground state, a small fraction of events (2%) were identified as direct decays and decays to excited states in ${}^8\mathrm{Be}$. This allowed a limit on the direct $3\alpha$ partial decay width to be determined as ${\mathrm{\Gamma }}_{3\alpha }<32\left(4\right)$ keV. Since this $2^{+}$ state is predicted by all theoretical models to be a collective excitation of the Hoyle state, the $3\alpha$ partial width of the Hoyle state is calculable from the ratio of $3\alpha$ decay penetrabilities of the Hoyle and $2^{+}$ states. This was calculated by using the semiclassical Wenzel-Kramers-Brillouin approach and we deduce a stringent upper limit for the direct decay branching ratio of the Hoyle state of $\frac{{\mathrm{\Gamma }}_{3\alpha }}{\mathrm{\Gamma }}<5.7\times {10}^{-6}$, over an order of magnitude lower than previously reported. This result places the direct measurement of this rare decay mode beyond current experimental capabilities.

The carbon/oxygen (C/O) ratio at the end of stellar helium burning is the single most important nuclear input to stellar evolution theory. However, it is not known with sufficient accuracy, due to large uncertainties in the cross-section for the fusion of helium with ¹²C to form ¹⁶O, denoted as ¹²C(α, γ)¹⁶O. Here we present results based on a method that is significantly different from the experimental efforts of the past four decades. With data measured inside one detector and with vanishingly small background, angular distributions of the ¹²C(α, γ)¹⁶O reaction were obtained by measuring the inverse ¹⁶O(γ, α)¹²C reaction with gamma-beams and a Time Projection Chamber (TPC) detector. We agree with current world data for the total reaction cross-section and further evidence the strength of our method with accurate angular distributions measured over the 1⁻ resonance at E_cm ~ 2.4 MeV. Our technique promises to yield results that will surpass the quality of the currently available data.

The neutron beams produced by the Soreq Applied Research Accelerator Facility (SARAF) operating with the liquid lithium target (LiLiT) present scientific opportunities in nuclear astrophysics. Of interest are measurements (in our setup) of 1.5 to 3.0 MeV alpha-particles and 1.5 MeV protons, emanating from the interaction of neutrons with ${}^7\mathrm{Be}$. The deduced cross sections, are important for understanding the cosmological “Primordial ${}^7\mathrm{Li}$ Problem.” However, major experimental challenges arise when measuring such low-energy charged particles due to the high flux neutron beams produced by the LiLiT (up to $\sim 5\times {10}^{10}$ $\mathrm{n}/\sec /{\mathrm{cm}}^2$). We present a study of the operation of CR39 nuclear track detectors (NTD), in such a high neutron flux. The CR39 plates were energy calibrated with 3.18 MeV alpha-particles from a ${}^{148}\mathrm{Gd}$ standard radioactive source, and by using Rutherford backscattering of 1.5 MeV alpha-particles as well as 1.5 MeV protons from a thin (100 $\mu \mathrm{g}/{\mathrm{cm}}^2$) gold foil. The plates were etched in a 6.25 N NaOH solution for 30 minutes at ${90}^{\circ }\mathrm{C}$ to produce micron-sized circular pits. The alpha (3.18 MeV) and proton (1.5 MeV) etch efficiencies were measured to be $93.3\pm 6.1$% and $8.7\pm 1.3$%, respectively. After etching, the plates were scanned with a fully automated microscope. A segmentation algorithm that addressed the challenges posed by the intense neutron beam was developed. A measurement of the interaction of cold neutrons with CR39 showed that the ${}^{17}\mathrm{O}(n,\alpha ){}^{14}\mathrm{C}$ reaction inside the CR39 was a major source of background. Since the tracks were formed inside the CR39, this background extended up to the full energy deposit of $\sim 2.1$ MeV. This neutron-beam-induced background inside the CR39 plates, observed here for the first time, will most likely be the limiting factor for observing $\sim 2$ MeV alpha-particles with CR39, in measurements of small cross sections (below a few mb) of ($n,\alpha$) reactions. However, in measurements of larger cross sections, this ${}^{17}\mathrm{O}(n,\alpha )$ “background line” can be used as an internal self-calibration of alpha-particles with energies up to $\sim 2.1\mathrm{MeV}$, and thus allow to correct for piece-to-piece variations in the energy-to-track size calibration of CR39 plates.

We report the first measurement of alpha-particles from the interaction of neutrons with 7Be at “temperatures” of Big Bang Nucleosynthesis (BBN). We measured the Maxwellian averaged cross sections (MACS), with neutron beams produced by the LiLiT at the SARAF in Israel (with kT = 49.5 keV hence 0.57 GK). In addition, we measured the cross section of the 7Be(n,p) reaction, which is in excellent agreement with the recent measurement of the n_TOF collaboration, further substantiating our method as a demonstration of “proof of principle”. The cross section for the 7Be(n,ga) and the 7Be(n,a) reaction measured in the “BBN window” is considerably smaller than compiled by Wagoner in 1969 and used today in Big Bang Nucleosynthesis (BBN). We also rule out a hitherto unknown resonance in 8Be at the BBN window, that was conjectured as a possible standard nuclear physics solution to the “Primordial 7Li Problem”. Together with previous results, we deduce a new Wagoner-like Rate for the destruction of 7Be by neutrons which is based on all current measured data. We conclude the lack of a standard nuclear solution to the “Primordial 7Li Problem”. Our upper limit on the cross sections for the high energy alpha-particles is in agreement with recent measurement of the n_TOF collaboration, but it is considerably smaller than the p-wave extrapolation of the Kyoto collaboration. We measured the alpha-particles from the 7Be(n,gi)8Be*(3.03 MeV) reaction, which is considerably larger than a previous s-wave estimate. Hence, in contrast, we conclude s-wave dominance at BBN energies, as would be expected due to the broad (122 keV) low lying 2” state at En = 10 keV.

Gamma-Beams now available at the HIγS facility of the TUNL at Duke University and soon to be available at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) in Magurele, near Bucharest, Romania, present scientific opportunities for progress in Nuclear Physics and Nuclear Astrophysics. In particular the use of Time Projection Chamber (TPC) detectors in these facilities will address the large confusion of conflicting data on the ¹²C(α, γ) reaction and promise to yield progress on this four decades old problem.