The β-decay branching ratio of 12B to the Hoyle state in 12C was measured by detection of γ rays. 12B nuclei were produced via the 11B(d,p)12B reaction in inverse kinematics on a TiD2 target. The present results corroborate those obtained recently for the β branch by implantation. The value from both experiments is inconsistent with that accepted in the literature

Albers, M.

Alcorta, M.

Almaraz-Calderon, S.

Bertone, P.F.

Carnelli, P.F.F.

Carpenter, M.P.

Chiara, C.J.

Digiovine, B.

Fynbo, H.O.U.

Greene, C.J.P.

Hoffman, R.

Janssens, R.V.F.

Lauritsen, T.

Laursen, K.L.

Marley, S.T.

Nair, C.

Nusair, K.O.

Rehm, E.

Seweryniak, D.

Ugalde, C.

Zhu, S.

Carolina Digital Repository

β-decay measurements of 12B with GammasphereM. Alcorta1,a, H. O. U. Fynbo2, M. Albers1, S. Almaraz-Calderon1, P. F. Bertone1,b,P. F. F. Carnelli3, M. P. Carpenter1, C. J. Chiara1,4, B. DiGiovine1, J. P. Greene1, C. R. Hoffman1,R. V. F. Janssens1, T. Lauritsen1, K. L. Laursen2, S. T. Marley1,5,c, C. Nair1, O. Nusair1,K. E. Rehm1, D. Seweryniak1, C. Ugalde1,6,7, and S. Zhu11Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA2Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark3Laboratorio Tandar, Comisión Nacional de Energía Atómica, B1650KNA Buenos Aires, Argentina4Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA5Physics Department, Western Michigan University, Kalamazoo, MI 49008, USA6Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637, USA7Joint Institute for Nuclear Astrophysics, Chicago, IL 60637, USAAbstract.The β−decay branching ratio of 12B to the Hoyle state in 12C was measured by detectionof γ rays. 12B nuclei were produced via the 11B(d,p)12B reaction in inverse kinematics ona TiD2 target. The present results corroborate those obtained recently for the β branch byimplantation. The value from both experiments is inconsistent with that accepted in theliterature.1 IntroductionRecent measurements of the β branch of 12B to the Hoyle state in 12C give a value of 0.58(2)% [1],about half that found in the literature (although in ref. [2] the value given is 1.5(3)%, in ref. [3] anargument is given for updating the literature value to 1.2(3)%). This branching ratio is important forunderstanding the R-Matrix fits of excitation energies in 12C between 9 and 13 MeV. Accurate fits tothe data are necessary in order to fully disentangle the different states of natural spin and parity in theregion, which is key in the search for the 2+ excitation built on the Hoyle state [4].The method used to determine the β branch of 12B to the Hoyle state in ref. [1] consisted ofdirectly implanting the 12B nuclei into a highly-segmented Si detector, thus measuring the sum ofthe three α energies emitted in the subsequent decay. It is important to verify this result using anindependent method. This manuscript reports on a coincidence measurement of the γ branch of theHoyle state (a cascade going through the 4.44 MeV, 2+ state) using Gammasphere (GS) at ATLAS.By accurately measuring the γ cascade, the β branch to the Hoyle state can be determined using thefollowing relation:ae-mail: malcorta@gmail.combPresent address: Marshall Space Flight Center, Huntsville, Alabama 35811, USAcPresent address: Physics Department, University of Notre Dame, Notre Dame, Illinois 46556, USADOI: 10.1051/C© Owned by the authors, published by EDP Sciences, 2014,/0 70 01 (2014)20166epjconfEPJ Web of Conferences4 6 6070 01This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20146607001BR(7.65) = BR(4.44) ·NγγN4.44 · ε3.211Cθ · Γγ/Γ(1)where BR(4.44) is the branching ratio of 1.28(4)% [2] to the 4.44 MeV state in 12C. The efficiencyof the 3.21-MeV γ ray, ε3.21, was determined for this experiment. Cθ, a correction factor allowing forthe angular correlation in the 0+ → 2+ → 0+ cascade, is assumed to be 1 due to the 4π coverage ofGS. The gamma width of the Hoyle state, Γγ/Γ, is well known with a value Γγ/Γ = 4.12(11) × 10−4[6]. The major inhibiting factor in obtaining a precise measurement of the branching ratio is the largeamount of bremsstrahlung radiation from the high-energy beta particles (up to 13 MeV), as describedin ref. [5].2 ExperimentThe experiment was carried out at the ATLAS facility at Argonne National Laboratory, using the GSarray [7]. A pulsed 11B beam (40 ms on, 40 ms off) with an intensity of 1 pnA and an energy of40 MeV impinged on a 22 mg/cm2 TiD2 target (1.5 mg/cm2 D2 on a thick Ti foil). The GS arraywas operated in singles mode, and the γ rays from excited 12C states populated in the β decay of 12B(T1/2=20.2 ms) were measured during the beam-off period.As mentioned in the previous section, the difficulty in measuring the weak 0+ → 2+ → 0+cascade is due to the large background induced by bremsstrahlung radiation of the high-energy betaparticles. In order to reduce this radiation, a low-Z chamber was designed from a Bonner sphere (Fig.1) that surrounded the target and greatly reduced the bremsstrahlung yield in GS.Figure 1. Schematic drawing of a low-Z chamber made from a Bonner sphere. This sphere was designed tomaximize the coverage with a low-Z material while minimizing the attenuation of γ rays. The beam enters forthe left side, and impinges on the target, located in the middle. The target is removable and is easily slid in andout. The sources used for efficiency calibrations were placed in the target position before and after the run wascompleted.EPJ Web of Conferences07001-p.23 AnalysisThe branching ratio given in Eq. 1 is determined from two values given by the literature, with thefollowing obtained from experiment: Nγγ, N4.44, ε3.21, and Cθ. As stated above, the correction factoris assumed to be 1 given the symmetric 4π coverage of GS. Nγγ and N4.44 were determined by fittingthe peaks displayed in Fig. 2. The difficulty in the analysis arises from the determination of ε3.21, thedetection efficiency for a γ ray of 3.21 MeV.Figure 2. Coincidence spectrum accumulated from the TiD2 target displayed together with the projection fromthe gated 4.44-MeV peak on top; the energies are binned in 0.666 keV/channel. The 4.44 MeV (ch. 2961) gatedspectrum clearly shows a peak at 3.21 MeV (ch. 2142).The determination of ε3.21 was carried out in two steps. The first was to determine the relativeefficiency of GS. This was done by using 152Eu and 56Co calibration sources, placed in the center ofthe GS chamber. The absolute efficiency can then be determined from the following. In the case of aγ − γ cascade where all of the flux in the intermediate level proceeds via the second γ ray, the ratioof the areas of the peaks in a γ − γ coincidence matrix will give the efficiency of a detector at theenergy of the second transition. By summing over all detectors, one can then determine the absoluteefficiency of GS. For this reaction, it was found that 24Mg was an ideal candidate since it was producedthroughout the irradiation and so the decay was measured in the same conditions as the decay of the12B nuclei. The 24Mg was likely produced from background reactions with the 13C in the Bonnersphere. The 4+ → 2+ → 0+ cascade was used, so that the absolute efficiency was determined forEγ=1.369 MeV. The absolute efficiency for a 3.21 MeV γ was subsequently deduced to be 2.94(9)%.4 ConclusionsThe preliminary value for the β-decay branching ratio of 12B to the Hoyle state isBR(7.65)=0.68(11)%, in agreement with the 0.58(2)% branch determined using β decay [1]. Thesefindings support the view that the literature value of 1.2(3)% should be revised. In this experiment,the error is dominated by statistics (± 0.09%) rather than by systematics (± 0.02%). The large amountINPC 201307001-p.3of background reactions on the Ti backing greatly limited the usable beam current. Changing to ahigher-Z backing, such as Hf, would enable the use of higher beam intensities and, therefore, result ina significant increase in statistics.With the verification of the β-decay result presented here, it is possible to use the same method tothen determine the radiative branching ratio, Γγ/Γ from Eq. 1:Γγ/Γ =BR(4.44)BR(7.65)·NγγN4.44 · ε3.211Cθ(2)Reducing the error estimate on the radiative branching ratio is important for lowering the error onthe radiative width, which dominates the uncertainty of the triple-α reaction rate, R(3α):R(3α) ∝ T−3/2Γ(8Be)ΓradΓtotexp(−Q3αkT),Γrad =ΓradΓtot·ΓtotΓπ· Γπ (3)Currently, the literature gives the following values and uncertainties for those quantities:Quantity Value Error(%) ReferenceΓγ + ΓπΓtot4.12(11)×10−4 2.7 [6]ΓπΓtot6.7(6)×10−6 9.2 [8]Γπ 59.6(1.5) µeV 2.5 [9]While the main source of error for R(3α) is the pair decay partial width, the current value forthe radiative branching ratio is determined from a weighted average of various measurements, mostlycomprised of inelastic scattering data. Using the method described in this manuscript would give avalue of ΓradΓtotwith different systematics. With an increase in statistics, it may be possible in a futureexperiment to obtain a value with a similar error to the literature value for Γγ+ΓπΓtot.5 AcknowledgementsThis work is supported by the U.S. Department of Energy, Office of Nuclear Physics, under ContractsNo. DE-AC02-06CH11357 and No. DE-FG02-04ER41320 and Grant No. DE-FG02-94ER40834.We also acknowledge financial support from the European Research Council under ERC starting grant“LOBENA”, No. 307447.References[1] S. Hyldegaard et al., Phys. Lett. B 678, 459 (2009).[2] F. Ajzenberg-Selove, Nucl. Phys. A506, 1 (1990).[3] S. Hyldegaard et al., Phys. Rev. C 80, 044304 (2009).[4] H. O. U. Fynbo and M. Freer, Physics 4, 94 (2011).[5] D. E. Alburger, Phys. Rev. 131, 1624 (1963).[6] R. G. Markham, S. M. Austin, and M. A. M. Shahabuddin, Nucl. Phys. A270, 489 (1976).[7] I. Y. Lee, Nucl. Phys. A520, 641c (1990).[8] D. E. Alburger, Phys. Rev. C 16, 2394 (1977).[9] M. Chernykh et al., Phys. Rev. Lett. 105, 022501 (2010).EPJ Web of Conferences07001-p.4

β -decay measurements of 12B with Gammasphere

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β -decay measurements of 12B with Gammasphere

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