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Register a new userRevised Cross Section of the $^{13}$C($alpha$,n)$^{16}$O reaction between 5 and 8 MeV
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Peter Mohr
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As suggested in a Comment by Peters, Phys. Rev. C {bf 96}, 029801 (2017), a correction is applied to the $^{13}$C($alpha$,n)$^{16}$O data of Harissopulos {it et al.}, Phys. Rev. C {bf 72}, 062801(R) (2005). The correction refers to the energy-dependent efficiency of the neutron detector and appears only above the ($alpha$,n$_1$) threshold of the $^{13}$C($alpha$,n)$^{16}$O reaction at about $E_alpha approx 5$ MeV. The corrected data are lower than the original data by almost a factor of two. The correction method is verified using recent neutron spectroscopy data and data from the reverse $^{16}$O(n,$alpha$)$^{13}$C reaction.
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The $^{12}text{C}(alpha,gamma){}^{16}text{O}$ reaction plays a central role in astrophysics, but its cross section at energies relevant for astrophysical applications is only poorly constrained by laboratory data. The reduced $alpha$ width, $gamma_{11}$, of the bound $1^-$ level in $^{16}$O is particularly important to determine the cross section. The magnitude of $gamma_{11}$ is determined via sub-Coulomb $alpha$-transfer reactions or the $beta$-delayed $alpha$ decay of $^{16}$N, but the latter approach is presently hampered by the lack of sufficiently precise data on the $beta$-decay branching ratios. Here we report improved branching ratios for the bound $1^-$ level [$b_{beta,11} = (5.02pm 0.10)times 10^{-2}$] and for $beta$-delayed $alpha$ emission [$b_{betaalpha} = (1.59pm 0.06)times 10^{-5}$]. Our value for $b_{betaalpha}$ is 33% larger than previously held, leading to a substantial increase in $gamma_{11}$. Our revised value for $gamma_{11}$ is in good agreement with the value obtained in $alpha$-transfer studies and the weighted average of the two gives a robust and precise determination of $gamma_{11}$, which provides significantly improved constraints on the $^{12}$C$(alpha,gamma)$ cross section in the energy range relevant to hydrostatic He burning.
The $^{12}$C(n, 2n)$^{11}$C cross section was measured from just below threshold to 26.5 MeV using the Pelletron accelerator at Ohio University. Monoenergetic neutrons, produced via the $^3$H(d,n)$^4$He reaction, were allowed to strike targets of polyethylene and graphite. Activation of both targets was measured by counting positron annihilations resulting from the $beta^+$ decay of $^{11}$C. Annihilation gamma rays were detected, both in coincidence and singly, using back-to-back NaI detectors. The incident neutron flux was determined indirectly via $^{1}$H(n,p) protons elastically scattered from the polyethylene target. Previous measurements fall into upper and lower bands, the results of the present measurement are consistent with the upper band.
It has been suggested that hydrogen ingestion into the helium shell of massive stars could lead to high $^{13}$C and $^{15}$N excesses when the shock of a core-collapse supernova passes through its helium shell. This prediction questions the origin of extremely high $^{13}$C and $^{15}$N abundances observed in rare presolar SiC grains which is usually attributed to classical novae. In this context $^{13}$N($alpha$,p)$^{16}$O the reaction plays an important role since it is in competition with $^{13}$N $beta^+$-decay to $^{13}$C. The $^{13}$N($alpha$,p)$^{16}$O reaction rate used in stellar evolution calculations comes from the CF88 compilation with very scarce information on the origin of this rate. The goal of this work is to provide a recommended $^{13}$N($alpha$,p)$^{16}$O reaction rate, based on available experimental data. Unbound nuclear states in the $^{17}$F compound nucleus were studied using the spectroscopic information of the analog states in $^{17}$O nucleus that were measured at the Alto facility using the $^{13}$C($^7$Li,t)$^{17}$O alpha-transfer reaction, and spectroscopic factors were derived using a DWBA analysis. This spectroscopic information was used to calculate a recommended $^{13}$N($alpha$,p)$^{16}$O reaction rate with meaningful uncertainty using a Monte Carlo approach. The present $^{13}$N($alpha$,p)$^{16}$O reaction rate is found to be within a factor of two of the previous evaluation, with a typical uncertainty of a factor 2-3. The source of this uncertainty comes from the three resonances at $E_r^{c.m.} = 221$, 741 and 959 keV. This new error estimation translates to an overall uncertainty in the $^{13}$C production of a factor of 50. The main source of uncertainty on the re-evaluated $^{13}$N($alpha$,p)$^{16}$O reaction rate currently comes from the uncertain alpha-width of relevant $^{17}$F states.
We report on the construction and performance of a calibration source for KamLAND using the reaction C-13(alpha,n)O-16 with Po-210 as the alpha progenitor. The source provides a direct measurement of this background reaction in our detector, high energy calibration points for the detector energy scale, and data on quenching of the neutron visible energy in KamLAND scintillator. We also discuss the possibility of using the reaction C-13(alpha,n)O-16 as a source of tagged slow neutrons.
The neutron yield in $^{12}$C(d,n)$^{13}$N and the proton yield in $^{12}C(d,p)^{13}$C have been measured by deuteron beam from 0.6 MeV to 3 MeV which is delivered from a 4-MeV electro static accelerator bombarding on the thick carbon target. The neutrons are detected at $0degree$, $24degree$, $48degree$ and the protons at $135degree$ in the lab frame. The ratios of the neutron yield to the proton one have been calculated and can be used as an effective probe to pin down the resonances. The resonances are found at 1.4 MeV, 1.7 MeV, 2.5 MeV in $^{12}C(d,p)^{13}$C and at 1.6 MeV, 2.7 MeV in $^{12}$C(d,n)$^{13}$N. This method provides a way to reduce the systematic uncertainty and helps to confirm more resonances in compound nuclei.
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