Measurement of the reaction18O(α,n)21Ne

Abstract

Background: The reaction18 O(α, n) 21 Ne is a part of the reaction chains leading to the production of 19 F and 22 Ne during He burning in low-mass and massive AGB stars, respectively. Additionally, it it has been observed as a strong background source in the measurement of other (α, n) reactions. Purpose: Previously low-energy18 O(α, n) 21 Ne cross section data have only been available in a non peer-reviewed form. An improved measurement of this reaction has been done to both clarify its astrophysical influence as well as to provide background yield data for future (α, n) experiments. Method: The18 O(α, n (0+1) ) reaction has been measured with a moderating neutron detector. In addition the (α, n1γ) channel has been measured independently by observation of the characteristic 350.7 keV γ-transition in 21 Ne. The reaction cross section at energies above Eα = 1100 keV was determined by a simultaneous R-matrix fit to both channels. The strengths of the two lowest-energy resonances at Eα = 959 keV and Eα = 1066 keV were analyzed separately using individual Breit-Wigner fits. Results:The cross section of both reaction channels,18 O(α, n0) 21 Ne and 18 O(α, n1γ) 21 Ne, was determined from the threshold energies at 851 keV and 1280 keV, respectively to 2300 keV. A new reaction rate has been deduced for the temperature range of 0.1 GK to 10 GK. A previously reported resonance at Eα = 888 keV is explained as background from the contaminant reaction 17 O(α, n) 20 Ne. Conclusions:In general, our reaction rate is slightly lower than the reaction rates in recent compilations. At temperatures below 0.2 GK the present rate is significantly lower because it could be shown that the lowest reported resonance is background from the reaction 17 O(α, n) 20 Ne that has been wrongly assigned to 18 O(α, n) 21 Ne.

Uncertainty of the astrophysical O17,18(α,n)Ne20,21Mohr1 2017Phys. Rev. C9070View full textAdd to dashboardCite

“…1); obviously, the branching ratio of the individual resonances cannot be reproduced by the StM calculations. This leads to three conclusions for the 18 O(α,n) 21 Ne reaction: First, this cross section is well determined experimentally from several data sets which agree with each other [19,20,22,29,36]. Second, the statistical model is able to predict the average cross section for both open channels at low energies.…”

“…In particular, the (n 0 +n 1 ) group with cross sections of about 80 to 45 mb from 9.8 to 12.3 MeV are nicely reproduced within about 20%, giving some confidence in the calculated branching ratios to the lowest states in 21 Ne. The latest study by Best et al [29] at low energies improves the previous longcounter measurements by an ad-ditional determination of the 18 O(α,n 1 ) 21 Ne cross section by γ-ray spectroscopy. The information from the γ-ray data on the (α,n 1 ) cross section is used to calculate the contribution of the (α,n 1 ) channel to the total neutron yield which is measured as in the other studies by neutron counting.…”

“…The information from the γ-ray data on the (α,n 1 ) cross section is used to calculate the contribution of the (α,n 1 ) channel to the total neutron yield which is measured as in the other studies by neutron counting. After subtraction of the (α,n 1 ) yield, the remaining yield is assigned to the (α,n 0 ) channel (other channels are closed for the low energies under study in [29]), and this remaining yield is then converted to the (α,n 0 ) cross section with smaller uncertainties because the neutron energy in the (α,n 0 ) channel is now known from kinematics; thus, the neutron detection efficiency can be determined with improved accuracy.…”

“…The latest calculation of N A σv for the reaction 18 O(α,n) 21 Ne by Best et al [29] is based on an R-Matrix analysis of the experimental data which cover an energy range from the threshold up to about 2 MeV; i.e., the Gamow window is fully covered (including the width ∆) up to T 9 ≈ 1.5. Therefore, at higher temperatures the rate N A σv is calculated from the statistical model which has been scaled to N A σv from experiment at T 9 = 2.…”

“…Two potential explanations (or a combination of both) can be given for the deviation between the Best rate and the TALYS rate: (i) The number of resonances in the 18 O(α,n) 21 Ne reaction may be accidentially low in the Gamow window for T 9 = 2 around 2 MeV, leading to a scaling factor significantly below 1.0 in [29] for the adjustment of the statistical model calculations. (Unfortunately, this factor is not provided 21 Ne reaction from the NACRE compilation, from the experimental resonance properties by Best et al [29], and from the statistical model calculations using TALYS. The upper part (a) shows the rates NA σv ; the lower part (b) shows the rates normalized to the NACRE fit, including horizontal arrows which indicate the approximate validity of the different rates.…”