Branching Ratio of 18Ne(7.06 MeV, 4+)

The recently reported branching ratio (BR) for the 4+ state in 18Ne at Ex = 7.06 MeV strongly disagrees with the BR computed using the known properties of this state. Disciplines Physical Sciences and Mathematics | Physics Comments Fortune, H. T. (2012). Branching Ratio of 18Ne(7.06 MeV, 4+). Physical Review C, 86(6), 068802. doi: 10.1103/PhysRevC.86.068802 © 2012 American Physical Society This journal article is available at ScholarlyCommons: http://repository.upenn.edu/physics_papers/269 PHYSICAL REVIEW C 86, 068802 (2012) Branching ratio of 18Ne(7.06 MeV, 4+) H. T. Fortune Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA (Received 19 August 2012; published 13 December 2012) The recently reported branching ratio (BR) for the 4+ state in 18Ne at Ex = 7.06 MeV strongly disagrees with the BR computed using the known properties of this state. DOI: 10.1103/PhysRevC.86.068802 PACS number(s): 26.20.−f, 26.50.+x, 21.10.Jx, 27.20.+n There appears to be a serious problem with at least one of the proton branching ratios (BR’s) recently reported [1] for astrophysically interesting states near 5–8 MeV in 18Ne. Almaraz-Calderon et al. [1] populated these states with the 16O(3He,n) reaction and detected the decay protons. Their reported BR’s for the 4+ state at 7.06(10) MeV are listed in Table I. At temperatures above about T9 ∼ 2, this resonance is the most important for the reaction 14O(α,p). Yet the proton branching ratios are in some considerable disagreement. Sometimes the cross section for the reaction 14O(α,p) is obtained by applying detailed balance to a measured cross section for the time-reversed reaction 17F(p, α). The presence of p1 decays invalidates that procedure. Harss et al. [2] initially assigned 1− to a state at 7.16(15). We proved it was 4+ [3]. They later agreed [4] and gave Ex = 7.05(10). Our calculated energy and alpha width were 7.086(40) MeV [5] and 22.6(3.2) eV [6]. This state should not have a measureable p1 decay for reasons I now discuss. The largest component in the structure of this state [7] (see Table II) is a collective excitation that is primarily of a four-particle two-hole (4p-2h) configuration, i.e., (sd)4(1p)−2, where the (sd)4 part is basically the first 4+ state of 20Ne. By use of mirror correspondence, we had earlier calculated the expected energy and proton and alpha widths [3,5,6]. They are listed in Table III. The problem with the new BR is the reported branch to the 1/2+ excited state of 17F. In order for a 4+ state to decay to 1/2+, the value must be 4. This 4+ state is very unlikely to have any appreciable g9/2 strength. Furthermore, because of the large centrifugal barrier the maximum = 4 width is very small. With standard parameters r0 = 1.26, a = 0.60, r0c = 1.40 (all in fm), I get sp( = 4) = 0.68 keV for 4+ to 1/2+. But, the actual situation is even worse. The g9/2 spectroscopic factor is almost certainly no larger than about 0.01–0.02, so the expected width for p1 decay is calc = S sp < 14 eV. The 1/2+/g.s. BR, with my calculated ground-state width, is thus less than about 2 × 10−4, to be compared with the recently reported value [1] of 0.19 = 0.16(7)/0.83(3) for this state. The present value is compared with others in Table IV. I can only conclude that the p1 decays must be from a nearby state—perhaps the one TABLE I. Branching ratios from Ref. [1] for the 42 state of 18Ne. Ex (MeV) J π p0 p1 7.06(4) 4+ 0.83(3) 0.16(7) TABLE II. Wave functions from Ref. [7] for 18O/18Ne(42 ). Configuration Wave-function amplitude d2 0.120 dd’ − 0.392 Coll. 0.912 at 7.37 MeV, about which little is known. The recent paper states that the authors did not observe this state, but it was seen in an earlier (3He,n) study [8] with a cross section of about 3% of that for the 18Ne(g.s.). Perhaps it is strong enough in the present experiment to account for the p1 decays. Or, they might be from a previously unknown state in this region of excitation. Hahn et al. [8] reported two states near here—at 7.05 and 7.12 MeV. If the peak attributed [1] to the decay 18Ne(7.06 MeV) → 17F(1/2+) arises instead from the decay of some other state to 17F(g.s.), Almaraz-Calderon et al. [10] indicate that the excitation energy of this other state would be about 6.7 MeV— an energy corresponding to no known state in 18Ne. As they state, this would “indicate the possibility of a new, previously unobserved state in 18Ne.” Clearly, more work is needed in this important region of 18Ne. I note that the new paper states that Harss et al. [4] assigned 2+ to the 7.37-MeV state. But that was a suggestion, not an assignment. Harss et al. stated that their data are consistent with any natural-parity J , up to some high J . They suggested 2+ simply because the lowest state of 18O without an identified mirror was the 2+ state at 8.21 MeV. I will not repeat the argument here, but we proved [6] that the 7.37-MeV state in 18Ne is not the mirror of the 8.21-MeV state in 18O. Mirrors of both states remain to be identified. I note that, with our calculated alpha width of 22.6(3.2) eV for the 7.06-MeV state, our value of the relevant astrophysical strength parameter ωγ is only 0.56 of the one in common use. TABLE III. Properties of the 42 state. Quantity Exp. [1, 4] Calc. Ex (MeV) 7.06(4) 7.086(40) [5] α (eV) 39(13) 22.6(3.2) [6] p (keV) 90(40) 64(13) [6] 068802-1 0556-2813/2012/86(6)/068802(2) ©2012 American Physical Society BRIEF REPORTS PHYSICAL REVIEW C 86, 068802 (2012) TABLE IV. Reported branching ratios p1/p0 for 18Ne(7.06 MeV, 4+). Source Branching ratio Harss et al. [4] 1/90 Notani et al. [9] Large Almaraz-Calderon et al. [1] 0.19 Present 2 × 10−4 In summary, my calculated p1/p0 BR for the 7.06-MeV 4+ state of 18Ne is less than about 2 × 10−4, in agreement with an earlier limit of 1/90 from Harss et al. [4], but not with the value of 0.19 in a recent report [1]. The value from Notani et al. [9] is even larger. Finally, the “best” ωγ for this resonance is only 0.56 of the value in common use. I am grateful to S. Almaraz-Calderon for informative correspondence. [1] S. Almaraz-Calderon et al., Phys. Rev. C 86, 025801 (2012). [2] B. Harss et al., Phys. Rev. Lett. 82, 3964 (1999). [3] H. T. Fortune and R. Sherr, Phys. Rev. Lett. 84, 1635 (2000). [4] B. Harss et al., Phys. Rev. 65, 035804 (2002). [5] R. Sherr and H. T. Fortune, Phys. Rev. C 58, 3292 (1998). [6] H. T. Fortune and R. Sherr, Phys. Rev. C 68, 034307 (2003). [7] R. L. Lawson, F. J. D. Serduke, and H. T. Fortune, Phys. Rev. C 14, 1245 (1976). [8] K. I. Hahn et al., Phys. Rev. C 54, 1999 (1996). [9] M. Notani et al., Nucl. Phys. A 746, 113 (2004). [10] S. Almaraz-Calderon et al. (private communication).