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Register a new userStudy of the astrophysically important $boldsymbol{^{23}mathrm{Na}(alpha,p)^{26}mathrm{Mg}}$ and $boldsymbol{^{23}mathrm{Na}(alpha,n)^{26}mathrm{Al}}$ reactions
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The $^{23}$Na$(alpha,p)^{26}$Mg and $^{23}$Na$(alpha,n)^{26}$Al reactions are important for our understanding of the $^{26}$Al abundance in massive stars. The aim of this work is to report on a direct and simultaneous measurement of these astrophysically important reactions using an active target system. The reactions were investigated in inverse kinematics using $^{4}$He as the active target gas in the detector. We measured the excitation functions in the energy range of about 2 to 6 MeV in the center of mass. We have found that the cross sections of the $^{23}$Na$(alpha,p)^{26}$Mg and the $^{23}$Na$(alpha,n)^{26}$Al reactions are in good agreement with previous experiments, and with statistical model calculations.
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According to sensitivity studies, the $^{38}mathrm{K}left( p, gamma right){}^{39}mathrm{Ca}$ reaction has a significant influence on $mathrm{Ar}$, $mathrm{K}$, and $mathrm{Ca}$ production in classical novae. In order to constrain the rate of this reaction, we have performed a direct measurement of the strengths of three candidate $ell = 0$ resonances within the Gamow window, at $386 pm 10~mathrm{keV}$, $515 pm 10~mathrm{keV}$, and $689 pm 10~mathrm{keV}$. The experiment was performed in inverse kinematics using a beam of unstable $^{38}mathrm{K}$ impinged on a windowless $mathrm{H}_2$ target. The $^{39}mathrm{Ca}$ recoils and prompt $gamma$ rays from $^{38}mathrm{K}left( p, gamma right){}^{39}mathrm{Ca}$ reactions were detected in coincidence using a recoil mass separator and a BGO array, respectively. For the $689$ keV resonance, we observed a clear recoil-$gamma$ coincidence signal and extracted resonance strength and energy values of $120^{+50}_{-30}~mathrm{(stat.)}^{+20}_{-60}~mathrm{(sys.)}~mathrm{meV}$ and $679^{+2}_{-1}~mathrm{(stat.)} pm 1~mathrm{(sys.)}~mathrm{keV}$, respectively. We also performed a singles analysis, extracting a resonance strength of $120 pm 20~mathrm{(stat.)} pm 15~mathrm{(sys.)}~mathrm{meV}$, consistent with the coincidence result. For the $386$ keV and $515$ keV resonances, we extract $90%$ confidence level upper limits of $2.54$ meV and $18.4$ meV, respectively. We have established a new recommended $^{38}mathrm{K}(p, gamma){}^{39}mathrm{Ca}$ rate based on experimental information, which reduces overall uncertainties near the peak temperatures of nova burning by a factor of ${sim} 250$. Using the rate obtained in this work in model calculations of the hottest oxygen-neon novae reduces overall uncertainties on $mathrm{Ar}$, $mathrm{K}$, and $mathrm{Ca}$ synthesis to factors of $15$ or less in all cases.
The $^{23}$Na($alpha,p$)$^{26}$Mg reaction has been identified as having a significant impact on the nucleosynthesis of several nuclei between Ne and Ti in type-Ia supernovae, and of $^{23}$Na and $^{26}$Al in massive stars. The reaction has been subjected to renewed experimental interest recently, motivated by high uncertainties in early experimental data and in the statistical Hauser-Feshbach models used in reaction rate compilations. Early experiments were affected by target deterioration issues and unquantifiable uncertainties. Three new independent measurements instead are utilizing inverse kinematics and Rutherford scattering monitoring to resolve this. In this work we present directly measured angular distributions of the emitted protons to eliminate a discrepancy in the assumptions made in the recent reaction rate measurements, which results in cross sections differing by a factor of 3. We derive a new combined experimental reaction rate for the $^{23}$Na($alpha,p$)$^{26}$Mg reaction with a total uncertainty of 30% at relevant temperatures. Using our new $^{23}$Na($alpha,p$)$^{26}$Mg rate, the $^{26}$Al and $^{23}$Na production uncertainty is reduced to within 8%. In comparison, using the factor of 10 uncertainty previously recommended by the rate compilation STARLIB, $^{26}$Al and $^{23}$Na production was changing by more than a factor of 2. In type-Ia supernova conditions, the impact on production of $^{23}$Na is constrained to within 15%.
A large number of $(alpha,p)$ and $(alpha,n)$ reactions are known to play a fundamental role in nuclear astrophysics. This work presents a novel technique to study these reactions with the active target system MUSIC whose segmented anode allows the investigation of a large energy range of the excitation function with a single beam energy. In order to verify the method, we performed a direct measurements of the previously measured reactions $^{17}$O$(alpha,n)^{20}$Ne, $^{23}$Na$(alpha,p)^{26}$Mg, and $^{23}$Na$(alpha,n)^{26}$Al. These reactions were investigated in inverse kinematics using $^{4}$He gas in the detector to study the excitation function in the range of about 2 to 6 MeV in the center of mass. We found good agreement between the cross sections of the $^{17}$O$(alpha,n)^{20}$Ne reaction measured in this work and previous measurements. Furthermore we have successfully performed a simultaneous measurement of the $^{23}$Na$(alpha,p)^{26}$Mg and $^{23}$Na$(alpha,n)^{26}$Al reactions.
The ${}^{12}mathrm{C}(alpha,gamma){}^{16}mathrm{O}$ reaction plays a key role in the evolution of stars with masses of $M >$ 0.55 $M_odot$. The cross-section of the ${}^{12}mathrm{C}(alpha,gamma){}^{16}mathrm{O}$ reaction within the Gamow window ($E_textrm{c.m.}$ = 300 keV, $T_textrm9$ = 0.2) is extremely small (about $10^{-17}$ barn), which makes the direct measurement in a ground-based laboratory with existing techniques unfeasible. Up until now, the cross-sections at lower energies can only be extrapolated from the data at higher energies. However, two subthreshold resonances, located at $E_x$ = 7.117 MeV and $E_x$ = 6.917 MeV, make this extrapolation more complicated. In this work, the 6.917 MeV subthreshold resonance in the ${}^{12}mathrm{C}(alpha,gamma){}^{16}mathrm{O}$ reaction was investigated via the ${}^{12}mathrm{C}({}^{11}mathrm{B},{}^{7}mathrm{Li}){}^{16}mathrm{O}$ reaction. The experiment was performed using the Q3D magnetic spectrograph at the HI-13 tandem accelerator. We measured the angular distribution of the ${}^{12}mathrm{C}({}^{11}mathrm{B},{}^{7}mathrm{Li}){}^{16}mathrm{O}$ transfer reaction leading to the 6.917 MeV state. Based on the FRDWBA analysis, we derived the asymptotic normalization coefficient (ANC) of the 6.917 MeV level in $^{16}$O to be (1.10 $pm$ 0.29) $times 10^{10}$ fm$^{-1}$, with which the reduced $alpha$ width was computed to be $18.0pm4.7$ keV at the channel radius of 6.5 fm. Finally, we calculated the astrophysical $S_{E2}(300)$ factor of the ground-state transitions to be 46.2 $pm$ 7.7 keV b. The result for the astrophysical $S_{E2}(300)$ factor confirms the values obtained in various direct and indirect measurements and presents an independent examination of the most important data in nuclear astrophysics.
We report on a precise measurement of double-polarization asymmetries in electron-induced breakup of $^3mathrm{He}$ proceeding to $mathrm{pd}$ and $mathrm{ppn}$ final states, performed in quasi-elastic kinematics at $Q^2 = 0.25,(mathrm{GeV}/c)^2$ for missing momenta up to $250,mathrm{MeV}/c$. These observables represent highly sensitive tools to investigate the electromagnetic and spin structure of $^3mathrm{He}$ and the relative importance of two- and three-body effects involved in the breakup reaction dynamics. The measured asymmetries cannot be satisfactorily reproduced by state-of-the-art calculations of $^3mathrm{He}$ unless their three-body segment is adjusted, indicating that the spin-dependent part of the nuclear interaction governing the three-body breakup process is much smaller than previously thought.
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