Comment on "138La-138Ce-136Ce nuclear cosmochronometer of the supernova neutrino process"
aa r X i v : . [ a s t r o - ph . S R ] F e b Comment on ”
La-
Ce-
Ce nuclear cosmochronometer of thesupernova neutrino process”
P. von Neumann-Cosel, ∗ A. Richter,
1, 2 and A. Byelikov † Institut f¨ur Kernphysik, Technische Universit¨at Darmstadt, 64289 Darmstadt, Germany ECT*, Villa Tambosi, I-38050 Villazano (Trento), Italy (Dated: October 26, 2018)
Abstract
The nuclear chosmochronometer suggested by Hayakawa et al. [Phys. Rev. C , 065802 (2008)]based on the La-
Ce-
Ce abundance ratio in presolar grains would be affected by the existenceof a hitherto unknown low-energy 1 + state in La. Results of a recent high-resolution study of the
Ba( He, t ) reaction under kinematics selectively populating 1 + states in La through Gamow-Teller transitions provides strong evidence against the existence of such a hypothetical state.
PACS numbers: 26.30.jk, 25.55.Kr, 27.60.+j ∗ Electronic address: [email protected] † Present address: AREVA NP GmbH, 63067 Offenbach, Germany et al. [1] proposed a new cosmochronometer based on the
La-
Ce-
Ceabundance ratios in presolar grains. It utilizes the different dominant production mechanismsof
La and , Ce: while all result from explosive nucleosynthesis in type II supernovae,the former nuclide is a product of the ν process through charged-current reactions [2, 3, 4]while the latter nuclides are synthesized in the p process [5]. One possible pitfall for thescheme (and also for the ν process origin of La) would be the existence of a low-lying 1 + state in La allowing electron capture (EC) decay to the stable
Ba competing with thehighly hindered γ decay to the 5 + ground state. In fact, the final sentence of the paper reads ”We present that the energy of the lowest + state may affect the chronometer performanceand the ν process origin of La, and its measurement is desired.”
It is the purpose of thiscomment to report on recent experiments which allow to constrain the existence of such anEC branch.The problem discussed in [1] would be a 1 + state in La with an excitation energy belowthe presently known [6] lowest excited state ( E x = 72 keV, J π = 2 + ). Such a state could bepopulated in the ( ν e , e ) reaction either directly or by population of higher-lying 1 + statesand subsequent γ decay, since Gamow-Teller (GT) transitions are expected to dominate thereaction cross section [3]. For such a level EC decay to the Ba ground state (g.s.) couldbe competitive with γ decay because of the large transition multipolarity ( E
4) of the latter.In principle, also β − decay to the g.s. of Ce is possible. However, the smaller Q -valuelimits its branching ratio to a few percent of the EC decay.Recently, we have performed a study of the Ba( He, t ) La reaction at the ResearchCenter for Nuclear Physics, Osaka, Japan, at an incident energy of 140 MeV/nucleon andunder a scattering angle of 0 ◦ [7]. A brief account of the work was given in [4]. Suchexperiments can be performed with excellent energy resolution reaching values ∆ E/E ≃ − in heavy nuclei [8]. At angles close to 0 ◦ this reaction selectively excites GT transitions.It is thus a spectroscopic tool to investigate 1 + states in La. Furthermore, GT matrixelements can be extracted from the data utilizing the procedures discussed in [9] and [10],respectively. These permit to estimate the EC lifetimes of a possible back decay to
Ba.A spectrum of the reaction is shown in the upper part of Fig. 1, taken from Ref. [4].Since the
Ba target material was embedded in polyvinylalcohol (PVA), a background linefrom the O( He, t ) F reaction is visible close to the expected g.s. energy of
La. Thecontribution of the PVA was subtracted by a measurement on a pure PVA target under2 N ( . ) N ( g . s ) F ( . ) ( He,t) on PVA
Excitation Energy in
La (MeV) F ( g . s ) S n C oun t s ( He,t) on
BaCO +PVA FIG. 1: Top: Spectrum of the
Ba( He, t ) La reaction at E = 420 MeV and Θ = 0 ◦ − . ◦ ,taken from Ref. [4]. The target consisted of BaCO dissolved in PVA. Bottom: Spectrum of the( He, t ) reaction on PVA. identical kinematics (lower part of Fig. 1). No transition was observed in La for energiesbetween the g.s. and 72 keV. A conservative upper limit from the present experiment for thepopulation of a hypothetic low-energy state can be extracted varying between B (GT) = 0 . B (GT) = 0 .
02 around E x = 72keV. It is used to estimate an upper limit of the corresponding half life from the relationbetween f t and B (GT) values (see, e.g., Ref. [11]). For La EC decay one obtains upperlimits ranging from 3.48 h at E x = 72 keV to 1.90 h at E x = 0 keV.On the other hand, excitation of the previously known 1 + state in La at E x = 293 keVwas prominently observed (cf. Fig. 1) with B (GT) = 0 .
42. The exact B (GT) values dependon the model used for conversion of experimental cross sections to transition strengths, butdifferences between the approaches of [9] and [10] are of the order of 10% in heavy nucleionly, which is of no relevance to the present discussion.The competing electromagnetic transition of a hypothetic low-energy 1 + state to the La g.s. would be of E /M M + state is unknown; for an estimate we assume an E E x = 72 keV of a hypothetical state would be 8.9 d (and even3arger for lower E x ), still significantly longer than the limit deduced for the EC decay.Nevertheless, the experimental results [7] provide an indirect argument against the ex-istence of another low-lying 1 + state besides the lowest known one at E x = 293 keV. Thestructure of the lowest states in La can be understood in the simplest approximation asproton-particle, neutron-hole states with respect to the N = 82 closed-shell nucleus Ba.The single-particle energies of shells near the Fermi level can be estimated from single-nucleon transfer reactions populating
Ba and
La, respectively. The lowest neutron-holestates observed in
Ba [13] are 2 d / (g.s.), 3 s / ( E x = 0 .
28 MeV), and 1 h / ( E x = 0 . La [14] one finds 1 g / (g.s.), 2 d / ( E x = 0 . s / ( E x = 1 .
21 MeV), and 1 h / ( E x = 1 .
44 MeV). A clear energy gap is ob-served in both cases suggesting the lowest states in
La to be formed by the configurations( πg / νd − / ) + , + , + , + , ( πg / νs − / ) + , + , ( πd / νd − / ) + , + , + , + , and ( πd / νs − / ) + , + . Thus,the low-energy spectrum of La should consist of the following number of states with agiven spin J π = 1 + (1), J π = 2 + (3), J π = 3 + (4), J π = 4 + (3), J π = 5 + (1). This is exactlywhat is found for the lowest states in La up to an excitation energy of E x = 642 keV[6]. The next-higher states all show negative parity (where known) indicating that theirstructure involves the 1 h / orbital.The experimental observation of only one 1 + state at 293 keV with a rather large GTstrength is qualitatively consistent with the above picture of a dominant ( πd / νd − / ) + structure. Any further hypothetical 1 + state in La at low excitation energies shouldmix with this one leading to a finite GT strength which can be largely excluded from thesensitivity limits of the ( He, t ) data. As an example, in a two-state model assuming aninteraction matrix element V = 10 keV and an energy spacing ∆ E = 250 keV between thetwo states one would obtain complete mixing with a corresponding share of the GT strength.As a final remark, evidence against the existence of an intruder 1 + state in La asthe first excited state is also provided by a study of the
Ba( p, nγ ) reaction [15]. In thisreaction low-spin states are preferentially excited but no γ transitions consistent with such apicture were found. We conclude that the existence of a low-energy 1 + state in La whichwould affect the cosmochronometer discussed in [1] and also the analysis of charged-currentreactions as a major nucleosynthesis source of
La [3, 4] is extremely unlikely.4 cknowledgments
This work was supported by the DFG under contracts SFB 634 and 446 JAP-113/267/0-2. [1] T. Hayakawa, T. Shizuma, T. Kajino, K. Ogawa, and H. Nakada, Phys. Rev. C , 065802(2008).[2] S. Goriely, M. Arnould, I. Barzov, and M. Rayet, Astron. Astrophys. , L35 (2001).[3] A. Heger, E. Kolbe, W. C. Haxton, K. Langanke, G. Mart´ınez-Pinedo, and S. E. Woosley,Phys. Lett. B , 258 (2005).[4] A. Byelikov et al. , Phys. Rev. Lett. , 082501 (2007).[5] M. Arnoud and S. Goriely, Phys. Rep. , 1 (2003).[6] A. A. Sonzogni, Nucl. Data Sheets , 515 (2003).[7] A. Byelikov, Doctoral thesis D17, Technische Universit¨at Darmstadt (2007).[8] Y. Kalmykov et al. , Phys. Rev. Lett. , 012502 (2006).[9] Y. Fujita, J. Phys. Conf. Series , 107 (2005).[10] R. G. T. Zegers et al. , Phys. Rev. Lett. , 202501 (2007).[11] Symmetries and Fundamental Interactions , Eds. W. C. Haxton, E. M. Henley (World Scien-tific, Singapore, 1995).[12] T. Kib´edi, T. W. Burrows, M. B. Trzhaskovskaya, P. M. Davidson, and C. W. Nestor, Jr.,Nucl. Instrum. Methods Phys. Res. A , 202 (2008).[13] A. Chaumeaux, G. Bruge, H. Faraggi, and J. Picard, Nucl. Phys.
A164 , 176 (1971).[14] B. H. Wildenthal, E. Newman, and R. L. Auble, Phys. Rev. C , 1199 (1971).[15] A. Islam, Thesis, McMaster University (1975); quoted in [6]., 1199 (1971).[15] A. Islam, Thesis, McMaster University (1975); quoted in [6].