Improved experimental determination of the branching ratio for β-delayed α decay of 16 N

We report on the preliminary results of an experimental study of the β decay of 16N, aiming to determine the branching ratio of the βα channel with a precision of ≤ 5%.


Introduction
During hydrostatic helium burning carbon is converted to oxygen via the reaction 12 C(α, γ) 16 O.The temperature at which this takes place is T ∼ 10 8 K, implying that reactions preferentially occur close to a center-of-mass energy of E cm ∼ 0.3 MeV.At such low energies the cross section is too small to be measured directly in laboratory experiments and must be determined by extrapolating the data obtained at higher energies (E cm 1.0 MeV).Typically, R-matrix theory [1] is used to parametrize the, a priori unknown, energy dependence of the cross section in terms of the properties of the levels in 16 O that are involved in the reaction.Since the same levels can be observed in other reactions many of the relevant parameters can be accurately determined or at least constrained by indirect techniques.Following this approach the cross section at 0.3 MeV has been determined with an estimated precision of ∼ 20% while a precision of at least ∼ 10% is desired [2].Among the indirect techniques used the β-delayed α decay of 16 N, shown in Fig. 1, has proven useful to determine how strongly the 7.12 MeV level in 16 O couples to the α + 12 C channel, and hence constrain the level's significant contribution (∼ 54%) to the capture cross section at 0.3 MeV.This requires precise measurements of the branching ratio to the 7.12 MeV level, the branching ratio for α emission, and the shape of the α spectrum.Here, we bring the preliminary results of a new experimental study of the β decay of 16 N recently performed at the ISOLDE facility [3] with the aim of obtaining precise values for the branching ratios.  1O are indexed 0-8.The main decay branches go to the ground state (28%) and the the 6.13 MeV level (66%) with smaller branches going to the levels at 7.12 MeV (4.8%) and 8.87 MeV (1.1%) while even smaller branches (< 10 −3 ) go to the other levels including an α branch of ∼ 10 −5 indicated by the two arrows.
2 Existing knowledge about the β decay of 16 N We proceed by giving a brief review of the existing empirical knowledge about the β decay of 16 N [4].
The main decay branches go to the ground state (28%) and the the 3 − level at 6.13 MeV (66%) with smaller branches going to the 1 − level at 7.12 MeV (4.8%) and the 2 − level at 8.87 MeV (1.1%) while even smaller branches (< 10 −3 ) go to the other levels, including a β-delayed α-decay branch of ∼ 10 −5 to the ground state of 12 C.It is remarkable that much of the existing data on the β decay of 16 N stems from γ-ray studies performed in the 1950s and 1960s using NaI detectors with poor energy resolution.For example, the branching ratio to the 7.12 MeV level is known with a precision of ∼ 8% based on three independent measurements of the intensity ratio of the 6.13 and 7.12 MeV γ-rays performed in the 1950s [5][6][7].With modern HPGe detectors it should be possible to obtain significantly improved values, not only for the branching ratio to the 7.12 MeV level, but also for the branching ratios to some of the other levels. 1 On the other hand, the ratio of the two main branches, which account for 94% of the total intensity, has been determined very precisely (∼ 1%) by careful measurements of the shape of the β spectrum.The α-decay branching ratio was first determined to have a value of 1.20(5) × 10 −5 by Kaufmann et al.

New experiment and preliminary results
In order to close some of the gaps in our knowledge about the β decay of 16 N, we have studied the decay at ISOLDE [3].In this study, performed in May 2016, a 30 keV beam of 14 N 16 N + molecular ions was delivered to the ISOLDE Decay Station (IDS) [12] at an average rate of ∼ 2 × 10 4 s −1 for a total of 32 hours.The ions were stopped in a thin (30 µg/cm 2 ) carbon foil surrounded by five doublesided silicon strip detectors (DSSD) and four high-purity germanium (HPGe) clovers, allowing for the simultaneous detection of charged particles and γ rays, while auxiliary detectors were used to check that the beam was being fully transmitted to the center of the setup and fully stopped in the foil.Three of the DSSDs were sufficiently thin (40-60 µm) to allow the α spectrum to be clearly separated from the β background, as shown in Fig. 2. The other two DSSDs were much thicker (300 µm and 1 mm)  16 N have energies between 0.8 MeV and 2.2 MeV and are seen to be well separated from the β particles and the 12 C recoils.The data shown here represents 1/3 of the total acquired data.The remaining data have yet to be analyzed.and served primarily to detect the β particles.The γ-ray spectrum measured in the HPGe clovers (Fig. 3) contains several γ-rays from the decay of 16 N with no evidence of other radioactive isotopes.In order to convert the observed γ-ray yields to intensity ratios it is necessary to correct for the energy dependent detection efficiency of the HPGe array.An absolutely calibrated 152 Eu source was used to determine the detection efficiency at low energies (E γ < 1.5 MeV), while βγ and γγ coincidences will be used to extend the calibration up to 7 MeV.

Summary and outlook
The β decay of 16 N has been studied in an experiment at the ISOLDE Decay Station in which both charged particles and γ rays were detected.It is expected that the data will constrain the branching ratio for β-delayed α emission and the branching ratio to the 7.12 MeV level with a precision of 5% or better.The data analysis is nearing completion and the results will soon be published, including an assessment of the impact on the inferred cross section of the 12 C(α, γ) 16 O reaction at E cm = 0.3 MeV.
[9].More recently,Zhao et  al. have obtained the value 1.3(3) × 10 −5 [10], while Refsgaard et al. find 1.49(5) × 10 −5 with a possible systematic uncertainty of −0.10 × 10 −5 [11].There is evidently a significant discrepancy between the values of Kaufmann et al. and Refsgaard et al., while the value of Zhao et al. has sufficiently large error bars to be consistent with either of the two other values.A new measurement of α-decay branching ratio is needed to resolve the discrepancy.

Figure 2 .
Figure2.Energy spectrum measured in the 40-µm thick DSSD.The α particles emitted in the decay of16 N have energies between 0.8 MeV and 2.2 MeV and are seen to be well separated from the β particles and the12 C recoils.The data shown here represents 1/3 of the total acquired data.The remaining data have yet to be analyzed.

3 EPJFigure 3 .
Figure 3. Add-back γ-ray spectrum obtained in the present experiment. 16N: solid (black) line.Background: dashed (red) line.Photopeaks due to transitions i → j in 16 O are indicated by the symbol γ i j while first-and second-escape peaks are indicated by a single asterisk ( * ) and a double asterisk ( * * ), respectively.