M. Dombsky

{beta}-delayed {alpha} emission of {sup 18}N: Broad J{sup {pi}}=1{sup -} states in the {sup 14}C +{alpha} system

The {beta}-delayed {alpha}-spectrum of {sup 18}N has been measured at the TISOL facility at TRIUMF resolving the structure of a broad {alpha}-particle distribution at {alpha}-energies of 1.5-5.5 MeV into several states that show a distinct interference pattern. R-matrix fits assuming a solely J{sup {pi}}=1{sup -} partial wave confirm this finding. The extrapolated low energy part of this structure constitutes the natural parity background in a possible experiment to observe the parity forbidden {alpha}-decay of the E{sub x}=6.880 MeV, J{sup {pi}}=0{sup -} state in {sup 18}O that is estimated from the fit to be 2x10{sup -8} of the intensity of the E{sub {alpha}}=1081 keV peak. In addition, the tails of the broad 1{sup -} states observed likely contribute to the astrophysical {sup 14}C ({alpha},{gamma}){sup 18}O reaction rate though the experimental information available is too incomplete to allow precise predictions. The {beta}-delayed {alpha}-spectrum of {sup 18}N also excludes complex {beta}-feeding amplitudes at a significant level. The states seen in the {beta}-delayed {alpha}-decay produce a pattern of approximately equally spaced 1{sup -} states in {sup 18}O consistent with J{sup {pi}}=1{sup -} states seen at the same and higher excitation energies of {sup 18}O.

Constraints on the low-energy E1 cross section of 12C(α, γ) 16O from the β-delayed α-spectrum of 16N

The paper cited above omitted the explanation of our energy calibration of the 16 N ␤-delayed ␣ spectrum sent to us by Professor H. Wä ffler ͓1͔. Although this spectrum was not used in any way in our experiment or in its analysis, we showed a comparison of this spectrum ͑referred to below for brevity as the Mainz spectrum͒ with our 12 C-␣-coincidence ␣ spectrum in Fig. 15 of our paper. We present here a clarification of the calibration procedure. The Mainz spectrum consists of a quarter of the data on the basis of which the Mainz group first reported ͓2͔ the detection of the parity-violating group of ␣ particles from the 2 Ϫ excited state of 16 O, now known to be at E x ϭ8.8719Ϯ0.0005 MeV ͓3͔. The apparatus for this experiment was described in a paper published a year earlier, which also reported the observation of a narrow ␣ group resulting from the first-forbidden 16 N ␤ decay to the 2 ϩ 16 O state ͓4͔, now known to be at E x ϭ9.8445Ϯ0.0005 MeV ͓3͔. A third paper describes further work by the Mainz group, with improved apparatus, and ϳ4 times the number of ␣ particles detected for the 1970 letter, establishing the parity-violating ␣ width of the 2 Ϫ state more precisely ͓5͔. The location of the ␣ groups from the 2 Ϫ and 2 ϩ 16 O states, with energies of 1282.3Ϯ0.5 and 2011.5Ϯ0.6 keV, respectively , and the identification by Dr. Wä ffler of the position in the spectrum corresponding to the ␣ group from the 2 Ϫ 16 O state, made it possible for us to calibrate the true E ␣ energy scale for the Mainz spectrum. As noted in our paper, our coincidence ␣ spectrum was calibrated independently by the ␤-delayed ␣ particles from 18 N and 20 Na, in exactly the same experimental geometry as our measurement of the 16 N ␣ spectrum employed. It is clear from Fig. 15 of our paper that the two spectra agree on the high-energy side of the main peak well within the stated accuracy of either calibration, but the Mainz spectrum shows evidence of an enhancement on the low-energy side of the peak that is likely to be the result of the low-energy tail of the system response function. In the case of our experiment, it was possible to remove this tail of …