A study of the level structure of Si26 using in-beam γ-ray spectroscopy is presented. A full level scheme is derived incorporating all states lying below the proton threshold energy. The results are in good agreement with shell model predictions and one-to-one correspondence is found with states in the mirror nucleus Mg26. Additionally, a γ-decay branch is observed from a state at 5677.0(17) keV, which is assigned to a 1+ resonance important in the astrophysical reaction Al25(p,γ)Si26. The newly derived resonance energy, Er=159.2(35) keV, has the effect of decreasing the reaction rate at the novae ignition temperature of 0.1 GK by a factor of 2 when compared with the previous most precise measurement of this state

Carpenter, M.P.

Davinson, T.

Janssens, R.V.F.

Jenkins, D.G.

Lauritsen, T.

Lister, C.J.

Seweryniak, D.

Shergur, J.

Sinha, S.

Woehr, A.

Woods, P.J.

Carolina Digital Repository

RAPID COMMUNICATIONSPHYSICAL REVIEW C 75, 062801(R) (2007)Level structure of 26Si and its implications for the astrophysical reaction rate of 25Al( p,γ )26SiD. Seweryniak,1 P. J. Woods,2 M. P. Carpenter,1 T. Davinson,2 R. V. F. Janssens,1 D. G. Jenkins,3 T. Lauritsen,1 C. J. Lister,1J. Shergur,1,4 S. Sinha,1 and A. Woehr51Argonne National Laboratory, Argonne, Illinois 60439, USA2University of Edinburgh, Edinburgh, EH9 3JZ, United Kingdom3University of York, Heslington, YO10 5DD, United Kingdom4University of Maryland, College Park, Maryland 20742, USA5University of Notre Dame, Notre Dame, YO10 5DD Indiana 46556, USA(Received 19 December 2006; published 8 June 2007)A study of the level structure of 26Si using in-beam γ -ray spectroscopy is presented. A full level schemeis derived incorporating all states lying below the proton threshold energy. The results are in good agreementwith shell model predictions and one-to-one correspondence is found with states in the mirror nucleus 26Mg.Additionally, a γ -decay branch is observed from a state at 5677.0(17) keV, which is assigned to a 1+resonance important in the astrophysical reaction 25Al( p,γ )26Si. The newly derived resonance energy, Er =159.2(35) keV, has the effect of decreasing the reaction rate at the novae ignition temperature of ≈0.1 GK by afactor of ≈2 when compared with the previous most precise measurement of this state.DOI: 10.1103/PhysRevC.75.062801 PACS number(s): 23.20.Lv, 27.30.+t, 25.40.Lw, 26.50.+xModern γ -ray spectroscopic techniques using large Gedetector arrays are providing a key alternative to studyingthe structure of astrophysically important proton-rich nuclei[1–4]. This approach gives very precise energies and relativelyunambiguous assignments in comparison with traditional lightion transfer methods. Surprisingly, the fusion productionmethod also proves to be relatively unselective for thesenuclei. This allows complete level structure determinationsbelow the particle threshold of interest for comparison withshell model calculations, and the observation of predominantlyγ -decaying states of astrophysical interest just above thisthreshold [1,3]. The present paper describes such a study ofthe nucleus 26Si. There are significant uncertainties in thenucleosynthesis of the important β-decaying cosmic γ -rayemitter, 26Al [5], that are associated with the 25Al( p,γ )26Sireaction rate at novae temperatures [6]. This reaction is thoughtto be dominated by resonant capture on low lying 1+ and3+ resonances above the proton threshold of 5517.8(31) keVin 26Si [7–10]. The effect of this reaction is to removeflux from the ground-state of 26Al, which is then bypassedby the 25Al( p,γ )26Si(βν)26mAl(βν)26Mg sequence. Presently,cosmic γ -ray emission from 26Al is thought to be predomi-nantly from massive stars [11], but novae may also contributesignificantly to the flux.The basic experimental technique has already been outlinedin Ref. [3]. Here, an approximately 8 pnA beam of 58 MeV16O ions was used to bombard a stack of two 150 µg/cm2thick targets of 12C for about 56 h with the object of producing26Si residues via the 2n fusion evaporation channel. Promptγ rays were detected using the highly efficient Gammaspherearray of Compton suppressed Ge detectors [12] in coincidencewith A = 26, charge state 10+ recoils detected at the focalplane of the Argonne Fragment Mass Analyzer (FMA) [13].26Si, 26Al and 26Mg ions were cleanly resolved using E −Einformation from an ionization chamber situated behind theFMA focal plane. An energy spectrum for recoil-coincidentγ rays from 26Si is shown in Fig. 1. The inset in Fig. 1 containsthe high energy portion of the spectrum of γ rays observed incoincidence with the known 1797 keV, 2+1 → 0+1 transitionin 26Si [15]. A tabulation of the levels and γ -ray transitionsobserved for 26Si in the present study is given in Table I.152Eu and 56Co sources were used to calibrate the γ -ray de-tectors. Above 0.8 MeV, deviations from linearity were below0.1 keV up to the maximum energy of 3.251 MeV in 56Co.The intensities for strong transitions were fitted as a functionof the detection angle with respect to the beam axis using thefunction: W (θ ) = N (1 + a2 · P2(cos(θ )) + a4 · P4(cos(θ ))).In the high-spin limit and assuming perfect alignmentof an initial state, values of (a2, a4) = (0.357,−0.107),(− 0.25, 0), (0.5, 0) correspond to a pure I = 2 quadrupole,I = ±1 dipole, and I = 0 dipole transition, respectively.The proposed level structure and decay transitions of 26Siare presented in Fig. 2. For orientation, the 26Si levels arecompared with the levels in the mirror nucleus 26Mg, and shell-model calculations in Fig. 3. These results are discussed below.The recent overview of the then known structure of 26Siby Parpottas et al. [9] forms a very useful reference pointfor the present study, particularly Table I, which provides acomprehensive summary of level energies and assignments.The only γ -ray spectroscopic study of this nucleus wasreported by Bell et al. in 1969 [14]. We note that the first fourexcited levels observed and assigned by Bell et al. are alsodetected here, the γ -ray angular distributions support previousspin assignments, but the level energies do not all agreewithin errors. These energies, and those of two other higherlying states measured by Bell et al., were used by Parpottaset al. [9] to calibrate their (3He,n) study of higher lying excitedstates. Hence, those data may need to be recalibrated at the≈5 keV level. Bell et al. [14] reported also states at 3842 and4093 keV that have not been observed in any subsequentstudies of 26Si [9], and they were not observed here either.We propose that they do not exist, because, as we shall0556-2813/2007/75(6)/062801(5) 062801-1 ©2007 The American Physical SocietyRAPID COMMUNICATIONSD. SEWERYNIAK et al. PHYSICAL REVIEW C 75, 062801(R) (2007)FIG. 1. Gamma-ray spectrum measured incoincidence with 26Si residues. The inset con-tains the high-energy γ rays detected in coinci-dence with the 1797 keV, 2+1 → 0+1 transition.demonstrate, we observe all expected states below the protonthreshold based on the comparison with the mirror nucleus26Mg as well as shell-model calculations.The state at 4139 keV has been widely reported beforeand uniformly assigned to a 2+ level [9], and based on itsdecay pattern we agree with this assignment. The next state, at4187 keV, has been variously assigned as either 3+ or 4+ [9].The angular distributions for the γ -ray transitions from thislevel are consistent with a stretched, dipole character withquadrupole admixture, and, as the decay pattern of this stateis similar to that of the 3+ level at 4350 keV in the mirrornucleus 26Mg, we assign 3+ in agreement with the analysis ofTABLE I. The energies, intensities, angular distribution coefficients, and proposed assignments for theγ -ray transitions detected in coincidence with 26Si nuclei produced in the 16O+12C reaction at 58 MeV. Thededuced level energies included in the table were corrected for the recoil energy of 26Si nuclei.J π Level γ energy γ intensity a2/a4 Assignmentenergy (keV) (keV)2+1 1797.3(1) 1797.2(1) 100.0(15) 0.18(4)/−0.08(4) 2+1 → 0+12+2 2786.4(2) 988.8(1) 26.4(7) 0.14(5)/0.01(7) 2+2 → 2+12787.5(3) 12.9(7) 0.36(12)/−0.22(14) 2+2 → 0+10+2 3336.4(6) 1539.1(5) 2.6(5) 0+1 → 2+13+1 3756.9(2) 970.4(1) 8.1(4) −0.30(9)/0.07(11) 3+1 → 2+21960.4(2) 10.6(6) −0.15(11)/0.01(15) 3+1 → 2+12+3 4139.3(7) 1355(2) 2+3 → 2+22341.9(6) 6.8(6) 2+3 → 2+14141(3) 0.8(4) 2+3 → 0+13+2 4187.1(3) 1400.7(2) 10.1(6) 0.41(13)/0.09(15) 3+2 → 2+22391.4(5) 5.8(6) 0.52(18)/0.20(20) 3+2 → 2+14+1 4446.2(4) 1657(2) 4+1 → 2+22648.8(3) 17.3(8) 0.32(11)/−0.15(13) 4+1 → 2+14+2 4798.5(5) 3001.0(4) 12.4(8) 0.12(11)/−0.37(14) 4+2 → 2+1(2+4 ) 4810.7(6) 2024.2(5) 4.3(5) 2+4 → 2+2(0+2 ) 4831.4(10) 2044.9(9) 1.2(4) 0+2 → 2+22+5 5146.7(9) 2360.2(8) 3.6(5) 0.42(24)/0.17(26) 2+5 → 2+23351(2) 2+5 → 2+14+3 5288.2(5) 842.1(3) 3.6(4) 4+3 → 4+11531.1(5) 4.8(5) 4+3 → 3+12503(2) 4+3 → 2+24+4 5517.2(5) 1071.8(4) 2.9(4) 4+4 → 4+11329.4(3) 3.9(4) 4+4 → 3+21764.4(8) 4.0(7) 4+4 → 3+12733(3) 4+4 → 2+21+1 5677.0(17) 3879.4(17) 1.4(4) 1+1 → 2+1062801-2RAPID COMMUNICATIONSLEVEL STRUCTURE OF 26Si AND ITS . . . PHYSICAL REVIEW C 75, 062801(R) (2007)FIG. 2. The γ -decay scheme of 26Si obtained in this work.Ref. [9]. The 4446 keV level has been widely observed before,but with varying 2+, 3+, 4+ assignments [9]. Here, the angulardistribution of the 2649 keV transition is consistent with thatof a stretched quadrupole, thus suggesting a 4+ assignment.Moreover, the γ -ray transition strengths to the 2+1 and 2+2 levelsare consistent with those observed for the 4+ mirror state at4318 keV [15]. Therefore, we assign the 4446 keV level to a4+ state, again in agreement with the analysis of Ref. [9]. Itshould be noted that the energy of the mirror 4+ state is lower,in contrast to all other analog pairs observed in 26Si and 26Mg.In the shell model analysis of Illiadis et al. [7] the spins for the4187 keV and 4446 keV states were interchanged, which ledto a large positive energy shift for the 3+ state.Nearly degenerate 0+, 2+, 4+ states are expected around≈ 4.8 MeV in 26Si [7]. However, these levels have not beenresolved in previous studies [9]. The 3001 keV transitiondeexciting the state at 4798 keV is in strong coincidencewith the 2+1 → 0+1 transition and has an angular distributionconsistent with a stretched quadrupole character. In all respectsit is analogous to the state 4+2 at 4901 keV in26Mg, alsoproduced in this experiment. Therefore, we assign this levelto the 4+2 state in26Si. Weaker transitions are observed at2024 and 2045 keV feeding the 2+2 state at 2786 keV. Theseare analogous to the decays of the 0+2 and 2+4 levels in the26Mg mirror. In the present experiment, the 2+ state in 26Mgis significantly more strongly populated than the 0+ level.Hence, we tentatively assign the relatively stronger 2024 keVtransition in 26Si to the decay of the 2+4 state at 4810 keV, and,correspondingly, the 2045 keV transition to the decay of the0+2 level at 4831 keV.All previous studies observing the 5147 keV state havereported a 2+ assignment [9] and our results are consistent withthis conclusion as well, since its decay pattern is similar to thatof the mirror 2+5 state in26Mg. Similarly, a level is observed at5288 keV and is assigned to a 4+ state in agreement with allprevious work [9]. We observe a state at 5517 keV very close tothe proton threshold. Parpottas et al. [9] and Caggiano et al. [8]proposed a 4+ assignment, whereas Illiadis et al. [7] suggested1+ based on a comparison with the shell model. As seen fromthe decay scheme, a transition is observed to a 4+ level, whichrules out the 1+ assignment. The decay characteristics arealso found to be analogous to those of the known 4+4 statein the 26Mg mirror, also observed here. Consequently, the5517 keV level is given a 4+ assignment, which wouldcorrespond to the calculated shell-model 4+ state at 6009 keV[7]. On the other hand, we associated the calculated 1+ state at5833 keV [7] with the 5677 keV level above the protonemission threshold (see below). This exhausts all the predictedshell model states near/below the threshold [7] and all theobserved mirror states [15] (see Fig. 3).In Fig. 1 one can see a γ -ray line at 3879 keV, which isin coincidence with the 2+1 → 0+1 transition (see inset). Thiscorresponds to a level excitation energy of 5677.0(17) keV,consistent with the energy reported by Caggiano et al. of062801-3RAPID COMMUNICATIONSD. SEWERYNIAK et al. PHYSICAL REVIEW C 75, 062801(R) (2007)FIG. 3. Comparison between the 26Si levels,the states in 26Mg, and the results of shell-modelcalculations [7].5678(8) keV [8], where possible 1+ or 3+ assignmentswere considered, and the former was proposed, based onthe comparison with the calculated Coulomb displacementenergies. More recently, the (3He,n) differential cross sectionmeasurements by Parpottas et al. [9] led to a specific1+ assignment to this state, with corresponding energy of5670(4) keV. Based on mirror symmetry, from the known levelscheme and γ decays of 26Mg [15], the dominant γ decay ofthe 1+ level is expected to be to the 2+1 , consistent with what isfound here, supporting the previous assignments. Conversely,a 3+ assignment is clearly ruled out as a dominant 1490 keVtransition to the 4187 keV 3+2 level is expected [15], whichis not observed here. The transition to the 2+1 state in thiscase is expected to be about 20 times weaker. The 3+ levelis in fact observed at 5912(4) keV by Parpottas et al. [9],and this assignment has been supported by a recent (p,t)transfer experiment, which is strongly indicative of a 3+state at 5914(2) keV [16]. Such a level would predominantlyproton decay, and would thus not be observed in the presentexperiment. Parpottas et al. [9] report the only other stateexpected around this energy, a 0+ level, to be at 5946(4) keV.Based on these results, Bardayan et al. [16] predicted adominating proton decay branch, although the γ -decay tothe 2+1 state is expected to be of comparable strength. Nosuch γ -ray transition was observed. This is not surprisingsince the 0+ analog state in the much more strongly produced26Mg mirror nucleus is only weakly populated in the presentexperiment. Combined with the competing proton decaybranch, we would not expect to observe this transition withthe present experimental detection sensitivity.The present results suggest that the complete detailedstructure of 26Si below, and in the region of the protonthreshold, can be well reproduced by comparisons with theshell model [7], although some states were reassigned in062801-4RAPID COMMUNICATIONSLEVEL STRUCTURE OF 26Si AND ITS . . . PHYSICAL REVIEW C 75, 062801(R) (2007)the present work when compared to Ref. [7]. In addition, aone to one correspondence is found with all known levelsin the mirror nucleus 26Mg [15]. This would reinforce themost recent assumptions about the location and nature ofthe key astrophysical resonances in 26Si [9,16], and wouldfurther make unlikely the replacement of the important 3+level at 5914 keV with a 2+ assignment, a possibility exploredby Bardayan et al. when analyzing uncertainties in the25Al( p,γ )26Si reaction rate [16]. Our results suggest that theprecise calibration of excitation energies in the key study ofParpottas et al. [9], including that of the 3+ at 5914 keV, mayneed be reconsidered. This study used γ -ray energies froma single work in 1969 [14] with which we report discrepantvalues at the ≈5 keV level. The 1+ excitation energy measuredhere with improved precision gives a resonance energy in thecenter of mass of 159.2(35) keV, limited by the precision of theproton threshold energy of 5517.8(3) keV [17]. Compared tothe most recent precise measurement, by Parpottas et al. [9],this would have the effect of increasing the 25Al( p,γ )26Sireaction rate at the novae ignition temperature of ≈ 0.1 GKby a factor of ≈2. This will result in a slight increasein the predicted yield of the cosmic γ -ray emitter 26Al innovae modeling calculations [16]. Ideally, the key resonancestrengths influencing the 25Al( p,γ )26Si reaction rate shouldultimately be determined directly. In such approaches usingradioactive beams, it is important to have accurate and preciseresonance energies to facilitate measurements (see for exampleRef. [18]).In summary, we have completed a detailed study of thestructure of levels below and in the region of the protonthreshold. The results agree well with shell model calculationsand the mirror nucleus 26Mg, and further constrain likelyuncertainties in the 25Al( p,γ )26Si reaction rate.This work was supported by the U. S. Department ofEnergy, Office of Nuclear Physics under contracts No. DE-AC02-06CH11357 and DE-FG02-94-ER49834. PJW, TD,acknowledge support from EPSRC. AW acknowledges sup-port from the National Science Foundation Grant PHY0140324.[1] D. Seweryniak et al., Phys. Rev. Lett. 94, 032501 (2005).[2] D. G. Jenkins et al., Phys. Rev. Lett. 92, 031101 (2004).[3] D. Seweryniak et al., Phys. Lett. B590, 170 (2004).[4] D. G. Jenkins et al., Phys. Rev. C 73, 065802 (2006).[5] R. Diehl et al., Astron. Astrophys. 97, 181 (1993).[6] J. Jose et al., Astrophys. J. 520, 347 (1999).[7] C. Iliadis, L. Buchmann, P. M. Endt, H. Herndl, and M. Wiescher,Phys. Rev. C 53, 475 (1996).[8] J. Caggiano et al., Phys. Rev. C 65, 055801 (2002).[9] Y. Parpottas et al., Phys. Rev. C 70, 065805 (2004).[10] D. W. Bardayan et al., Phys. Rev. C 65, 032801(R) (2002).[11] R. Diehl et al., New Astron. Rev. 48, 81 (2004).[12] R. V. F. Janssens and F. S. Stephens et al., Nucl. Phys. News 6,9 (1996).[13] C. N. Davids et al., Nucl. Instrum. Methods. Phys. Res. B 70,358 (1992).[14] R. A. Bell et al., Nucl. Phys. A133, 337 (1969).[15] P. Endt et al., Nucl. Phys. A633, 1 (1998).[16] D. W. Bardayan et al., Phys. Rev. C 74, 045804(2006).[17] G. Audi et al., Nucl. Phys. A729, 3 (2003).[18] S. Bishop et al., Phys. Rev. Lett. 90, 162501 (2003).062801-5

Level structure of Si26 and its implications for the astrophysical reaction rate of Al25(p,γ)Si26

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Level structure of Si26 and its implications for the astrophysical reaction rate of Al25(p,γ)Si26

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