A method has been developed that can provide beams of many short-lived nuclei of interest in nucleosynthesis along the rp process path. With a 17F beam (T1/2=64s) the excitation function of the 17F(p, α)14O reaction was measured to determine properties of excited states in 18Ne. These states influence the rate of the 14O(α, p)17F reaction which is important for understanding energy generation and nucleosynthesis in x-ray bursts. The present direct measurements yield a pattern of resonances and cross sections which differ substantially from previous estimates

Blackmon, J.C.

Borasi, F.

Chen, A.

Greene, J.P.

Harss, B.

Henderson, D.

Janssens, R.V.F.

Jiang, C.L.

Nolen, J.

Pardo, R.C.

Parker, P.

Paul, M.

Rehm, K.E.

Schiffer, J.P.

Segel, R.E.

Siemssen, R.H.

Smith, M.S.

Sonzogni, A.A.

Uusitalo, J.

Wang, T.F.

Wiedenhöver, I.

Carolina Digital Repository

VOLUME 82, NUMBER 20 P H Y S I C A L R E V I E W L E T T E R S 17 MAY 1999Schiffer,Stellar Reactions with Short-Lived Nuclei: 17Fsp, ad14OB. Harss,* J. P. Greene, D. Henderson, R. V. F. Janssens, C. L. Jiang, J. Nolen, R. C. Pardo, K. E. Rehm, J. P.R. H. Siemssen, A. A. Sonzogni, J. Uusitalo, and I. WiedenhöverArgonne National Laboratory, Argonne, Illinois 60439M. PaulHebrew University, Jerusalem, IsraelT. F. WangLawrence Livermore National Laboratory, Livermore,California94550F. Borasi and R. E. SegelNorthwestern University, Evanston, Illinois 60439J. C. Blackmon and M. S. SmithPhysics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6354A. Chen and P. ParkerWright Nuclear Structure Laboratory, Yale University, New Haven, Connecticut 06520-8124(Received 11 December 1998)A method has been developed that can provide beams of many short-lived nuclei of interest innucleosynthesis along therp process path. With a17F beamsT1y2 ­ 64 s) the excitation function ofthe 17Fsp, ad14O reaction was measured to determine properties of excited states in18Ne. These statesinfluence the rate of the14Osa, pd17F reaction which is important for understanding energy generationand nucleosynthesis in x-ray bursts. The present direct measurements yield a pattern of resonances andcross sections which differ substantially from previous estimates. [S0031-9007(99)09166-8]PACS numbers: 25.60.Je, 25.40.Hs, 25.45.Hi, 26.30.+kn.theusedre-ntsted(upineh-InllReactions involving short-lived nuclei play an important role in nuclear astrophysics. In explosive astrophycal events, such as x-ray bursts, novas, or supernovas,reactions occur on a sufficiently fast time scale that threaction products on the proton-rich side of the valley ostability, that normally decay through the weak interaction, instead interact further with the protons and alphparticles which are present in the dense and hot stelenvironment. To study these reactions a method has bdeveloped for producing beams of such short-lived nucthat can be reached by eithersp, nd or sd, nd reactionsfrom stable nuclei. This report describes the productioof a 17F beam (T1y2 ­ 64 s) and the first measuremenof the 17Fsp, ad14O reaction.X-ray bursts [1–3] may be an important source for thproduction of proton-rich nuclei via the high temperaturrp process [4]. These events initially produce a largabundance of14O when the triple-alpha process is ignitedat a temperature of,2 3 108 K, but the peak energygeneration rate is not achieved until, at much higher temperatures, the14O bottleneck is broken. Measurementof waiting point reactions such as14Osa, pd17F are thusimportant for understanding the details of energy genertion and nucleosynthesis in these stellar environmentsPrior to the present measurements, estimates of thesolute value and energy dependence of the14Osa, pd17F3964 0031-9007y99y82(20)y3964(4)$15.00-si-theef-alareenleinteee-sa-.ab-cross section had been made from indirect informatioSince the reaction is mainly resonant, it depends onproperties of unbound states in the compound nucle18Ne. The properties of these levels had been inferr[5–9] by measuring the resonance energies with otheractions and by estimating widths and spin assignmefrom the mirror nucleus18O, utilizing isospin symmetrywith appropriate Coulomb shifts. The14Osa, pd17F reac-tion rate at lower temperatures is believed to be dominaby a resonance arising from a12 state that is the isobaricanalog of theEx ­ 6.20 MeV state in18O. This level hasbeen tentatively identified with a state atEx ­ 6.15 MeVin 18Ne [5]. Several states atEx ­ 7 7.5 MeV may alsocontribute to the reaction rate at higher temperaturesto ,3 3 109 K) relevant for x-ray bursts. The goal of thepresent work was to determine the properties of states18Ne that may contribute to the14Osa, pd17F reaction rateby measuring the excitation function for the time-invers17Fsp, ad14O reaction.The 17F beam was produced using an in-flight tecnique similar to the ones described in Refs. [10–12].this experiment, a primary beam of16O was used toproduce a17F secondary beam via theds16O, 17Fdn re-action. A liquid-nitrogen cooled, 3.5 cm long gas cetarget, filled with ø0.8 atm of D2 gas was bombardedwith an 16O beam of,100 pnA (6 3 1011ys) at energies© 1999 The American Physical SocietyVOLUME 82, NUMBER 20 P H Y S I C A L R E V I E W L E T T E R S 17 MAY 1999er-u-omicalfureri-ner-slye5].pre-nanoxi-othso-so-r-atex-ickmhisndtrgiesonldilensreds inverosss-nd-enbetween 70 and 90 MeV from the ATLAS acceleratorArgonne National Laboratory. The17F ions left the gascell in a 5± cone with respect to the primary beam drection, with a total energy spread of,20 MeV. A su-perconducting solenoid refocused the17F particles, andsuperconducting resonators before and after the gasreduced the energy spread of the beam. The17F19 ionswere separated from the16O beam in a bending mag-net and refocused onto a5 3 5 mm2 beam spot, withan energy spread less than 500 keV. Between2 3 105and2 3 106 particlesys were incident on the target. The(lower energy)16O contamination was between 10% an50% and did not interfere with the measurement. Dtails of the production, transport, and beam monitorinscheme are given elsewhere [12,13].Theps17F, ad14O reaction was measured at a numberincident energies, using several CH2 targets with thick-nesses of,100 or ,500 mgycm2. Each measurement re-presents an average over the energy loss in the target whcorresponds to 0.7(3) MeV in laboratory energy, an40(170) keV in the center-of-mass system. Two positiosensitive, double-sided, annular silicon strip detectobehind the target were used to measure the energyangle of the scattered particles. Coplanar kinemacoincidences were required between thea particle in thefirst detector, covering the angular range,13± 25±, andthe 14O residual nucleus in the second detector at ang,4± 8±.Figure 1a shows a plot of the energy of thea particles vsthe energy of the coincident14O partners. The backgroundseen below the line of constant sum energy is attributedreactions of the beam with12C in the CH2 target. Shown inFig. 1b are the14O-a coincidence events as a plot of ringnumber (scattering angleua) vs a energy, gated by the“good” events in theEa-E14O plot, with the requirement ofcoplanarity. The solid lines in Fig. 1 indicate the expectekinematic behavior for the reaction with the average enerloss of the real target.The measured total cross sections are presentedFig. 2 as a function of the center-of-mass energies in bochannels. The uncertainties in the cross-section valushown are statistical. The detection efficiency was,65%,obtained from a Monte Carlo calculation assuming aisotropic angular distribution, and including the effects offinite beam spot and multiple scattering in the target. Tintegrated beam intensity was obtained from the17F ionsscattered by12C in the target. Only events withulab , 6±were used in this procedure, to assure the dominanceRutherford scattering.The energy intervals covered by the thin or thick targe(circles or crosses) are indicated in the figure by thhorizontal lines. The total systematic uncertainties aestimated to be,20%. The results of the thin- and thick-target measurements are consistent within statistical errPrevious estimates of the14Osa, pd17F reaction crosssection [5–9] utilized the known levels of18O to obtain as-signments to states in18Ne with Ex . 5 MeV. The ener-ati-cellde-gofichdnrsandticlestodgyinthesnaheoftsereors.gies of the unbound levels in18Ne were obtained from the20Nesp, td18Ne, 16Os3He, nd18Ne, and12Cs12C, 6Hed18Nereactions [5,6]. The cross section, in the energy intval covered here, for the14Osa, pd17F, and thus for theps17F, 14Oda, reaction is dominated by resonant contribtions from levels atEx ­ 6 8 MeV in 18Ne. In Ref. [5],the partial widths of resonances were taken either frthe known widths of the corresponding18O levels or wereassumed to have spectroscopic factors which are typin this region of nuclei. A comparison of the results othis estimate (see solid and dotted lines in Fig. 2) with odata shows distinct discrepancies. In the present expment three resonances were observed at excitation egies of7.60 6 0.05 MeV, 7.37 6 0.06 MeV, and7.16 60.15 MeV. These energies are consistent with previouobserved states in18Ne; however, the strengths of thesresonances differ substantially from earlier estimates [The current results are summarized and compared tovious results in Table I and in Fig. 2. Since the protowidths in this energy region are generally much larger ththe alpha widths, the resonance strength can be apprmated byvg ­ vGaGpyGtot ø vGa, where v is thestatistical factor from the spins involved.A strong resonance was observed at 7.60 MeV in bthe thin and the thick target measurements with a renance strength ofvg ­ s300 6 40d eV. In the energyrange Ex ­ 7.3 7.5 MeV, the thin-target data show apossible small structure that is attributed to a weak renance. Assuming a single level, a value ofvg ­ s23 612d eV is obtained for this resonance. Another factof-3 increase in the thick-target yield is observedEc.m.s17F 1 pd ­ 3.06 and 3.24 MeV. Assuming again asingle level, this suggests an additional resonance at ancitation energy of,7.16 MeV with vg ­ s75 6 20d eV.The yield measured at the lowest energy with the thtarget is about a factor of 2 larger than expected froan estimate of the direct capture cross section [9]. Tmight conceivably be yet another resonance arouEx ­ 7 MeV or alternatively a direct contribution aboutwice as large as the theoretical estimate.Cross sections have also been measured at c.m. enebetween 2.2 and 2.5 MeV, corresponding to excitatienergies of 6.1–6.3 MeV, where the12 state that isdominating the reaction rate is expected [5]. Some yie(,10 30 mb) was observed at the lower energies, whabove 2.3 MeV only upper limits for the cross sectiocould be set. Although differences between measuand predicted cross sections are observed, the yieldthis energy region are too small to allow a conclusiidentification of the important12 state.The difference between the observed and expected csections [5] is in part due to incorrect spin-parity asignments for particle-unbound states in18Ne. Some re-trictions on the spin-parity assignments in18Ne can bemade by comparing the present data with the correspoing well studied region of excitation energy in18O [14].Considering16O as the core, the energy difference betwe3965VOLUME 82, NUMBER 20 P H Y S I C A L R E V I E W L E T T E R S 17 MAY 1999nalolidet.FIG. 1. (a) Energies of coincidenta and 14O particles. The coplanarity condition has been applied. (b) A two-dimensioscatter plot ofa-particle angle (ring number) vsa energy for the events from (a), requiring also the correct sum energy. The slines represent the expectedEa E14O andua Ea correlations, respectively, for events originating from the midpoint of the targhinretrgeteectonaresingle neutron and proton states can be estimated fromA ­ 17 mirror levels that are well known [14]. Whilethe total binding energies of the5y21 ground states of17Fand 17O differ by 3.54 MeV, those of the1y21 excitedstates differ only by 3.17 MeV, a 376 keV differenceThis arises from the diffuse wave function of the looselbounds-wave proton, and its lower Coulomb energy. Thdifferences in excitation energies between correspondistates in18O and 18Ne should then arise primarily fromthe amount ofs1y2 admixtures in their wave functions.Since the difference in isospin is two units, the shiftcould be as high as,800 keV. Since the ground stateof 18O has approximately a 15%s1y2 admixture [14], thefluctuations of excitation energies of mirror states in18Oand 18Ne should be limited to2250 keV , Exs18Od 2Exs18Ned , 800 keV.The parameters for the resonances found in the preswork are summarized in Table I, which also includethe parameters used in Refs. [5,6]. The possible spparity assignments have been restricted by the limitsenergy differences given above. The remaining optionand constraints from the known widths of some of thlevels in 18O are also shown in Table I. By isospinsymmetry, the reduceda-particle widths for the decay ofexcited levels in18O should be the same as the widths foalpha emission from the corresponding levels in18Ne.With these criteria, the 3.24 MeV (Ex ­ 7.16 MeV)resonance in18Ne is consistent only with a12 assignment,and corresponds to the 7.62 MeV,12 level in 18O. The3.48 MeV (Ex ­ 7.37 MeV) level can be assigned eithera 12 or a 41 value. With the first assignment, this statewould be the partner of the 8.04 MeV12 level in18O, witha Coulomb energy difference near the limit. The41 as-signment would make the state the partner of the 7.13 Me41 level in 18O with the widths in rather good agree-ment. Finally, the strong 3.69 MeV (Ex ­ 7.60 MeV)resonance could haveJp ­ 12, 21, or 32. A Jp ­ 32assignment, however, agrees poorly with thea width ofthe corresponding state in18O. Since there are only two3966the.yengsentsin-onserVFIG. 2. Measured cross sections for theps17F, 14Oda re-action (top). The circles represent measurements with t(100 mgycm2) targets, while the crosses are for thicke(500 mgycm2) ones. The horizontal bars,hown for one pointonly, indicate the energy interval corresponding to the targthickness. The dotted line represents the expected thick-tayield and the solid line that for the thinner targets using thresonance parameters of [5]. The dashed line is the dircomponent estimated in [9]. The arrow indicates the locatiof a state in18Ne that was listed in Ref. [5], but not includedin the estimates. The energy scales at the top and bottomfor the entrance and exit channels involved in the reaction.VOLUME 82, NUMBER 20 P H Y S I C A L R E V I E W L E T T E R S 17 MAY 1999astTABLE I. Resonance parameters for various states in18Ne. The first three rows are from the present experiment, while the lsix lines were taken from Ref. [5].Ex Ec.m. vg Ex Exs18Od2s18Ned sp, ad sp, ad Gexpt.a Q2a s18Od Q2a Exs18NedfMeVg fMeVg feVg Jp fkeVg s18Ned fMeVg s18Od fkeVg7.16 6 0.15 3.24 75 6 20 12 0.3 0.016 7.62 0.012a 4607.37 6 0.06 3.48 23 6 12 12 0.08 0.002 8.04 ,0.1b 67041 0.03 0.09 7.11 0.11a 22507.60 6 0.05 3.69 300 6 40 12 1.2 0.015 8.04 ,0.03b 44021 0.7 0.026 8.21 0.03 6 0.02c 61032 0.5 0.09 8.28 0.72 6 0.04c 6806.15 2.23 0.55 s12d 2.2 3 1023 s6.20d6.29 2.36 0.2 s32d 3.4 3 1024 s6.40d7.05 3.13 36 s41d 4.8 3 1022 s7.11d7.12 3.20 · · · · · · · · ·7.35 3.43 375 s12d 1.7 s7.62d7.62 3.70 · · · · · · · · ·a [15]. b [14]. c [16].yctätett.s.s...12 states known in18O in this energy region, assign-ments ofJp for the observed resonances at 7.16, 7.3and 7.60 MeV can bes12; 41; 12d, s12; 41; 21d, ors12; 12; 21d.The contribution of these resonances to the14Osa, pd17Fastrophysical reaction rate in the temperature rangeT9 ­1 3 is found to be about a factor of 2 less than previoupredictions. However, these measurements still leave opthe question of the partial widths for proton decay to th1y21 excited state in17F in the 14Osa, pd17F reaction,since the decay to that state would also contributethe astrophysical yield. Future experiments which aoptimized for measuring these proton widths will haveaddress this question.The present experiment has shown that beams of shlived, radioactive nuclei can be produced with the in-flightechnique with energy resolutions of about 30 keV in thc.m. system. With a17F beam, a direct measuremenof an excitation function of the17Fsp, ad14O reactionhas been performed, which is dominated by resonastates in18Ne. The measured cross section was fouto differ substantially from predictions based on indiremeasurements. The results of this experiment illustrathat the cross section estimates for astrophysical procesbased on indirect methods are likely to be uncertain.was found both here and in the18Fsp, ad reaction [17,18],direct measurements of these cross sections are necesto obtain good quantitative knowledge of the reaction crosection, even for nuclei as near to stability as in the prsent case.This work was supported by the U.S. D.O.E., NuclePhysics Division, under Contracts No. W-31-109-ENG38 (ANL), No. W-7405-Eng-48 (LLNL), No. DE-FG02-98ER-41086(NU), and No. DE-FG02-91ER-40609 (YaUniversity), and by the University of Chicago ArgonnNational Laboratory Collaborative Grant. Oak Ridge Na7,senetoretoort-tetntndcttesesAssarysse-ar-lee-tional Laboratory is managed by Lockheed Martin EnergResearch Corp. for the U.S. D.O.E. under ContraNo. DE-AC05-96OR22464. One of us (B. H.) wants tothank Professor Hans Körner (Technische UniversitMünchen) for support of his Ph.D. thesis at ArgonnNational Laboratory.*Also Physics Department E12, Technische UniversitäMünchen, Munich, Germany.[1] A. E. Champagne and M. Wiescher, Annu. Rev. Nucl. ParSci. 42, 39 (1992).[2] H. Schatzet al., Phys. Rep.294, 168 (1997).[3] M. Wiescher, H. Schatz, and A. E. Champagne, PhiloTrans. R. Soc. London A356, 2105 (1998).[4] R. K. Wallace and S. E. Woosley, Astrophys. J. Suppl.45,389 (1981).[5] K. I. Hahn et al., Phys. Rev. C54, 1999 (1996).[6] S. H. Parket al., Phys. Rev. C59, 1182 (1999).[7] M. Wiescheret al., Astrophys. J.316, 162 (1988).[8] M. Wiescher, J. Görres, and F.-K. Thielemann, AstrophyJ. 316, 384 (1988).[9] C. Funck, B. Grund, and K. Langanke, Z. Phys. A332,109 (1989).[10] J. J. Kolataet al.,Nucl. Instrum. Methods Phys. Res., SectB 40y41, 503 (1989).[11] T. Yamayaet al.,Nucl. Instrum. Methods Phys. Res., SectB 70, 374 (1992).[12] B. Harss et al., in Proceedings of the Particle Accel-erator Conference, Vancouver, 1997(IEEE, New York,1998).[13] B. Harsset al. (to be published).[14] D. R. Tilley et al., Nucl. Phys.A595, 1 (1995).[15] A. Cunsoloet al., Phys. Rev. C24, 476 (1981).[16] J. A. Weinman and E. Silverstein, Phys. Rev.111, 277(1958).[17] K. E. Rehmet al., Phys. Rev. C52, R460 (1995).[18] R. Coszachet al., Phys. Lett. B353, 184 (1995).3967

Stellar reactions with short-lived nuclei: 17F(p,α)14O

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Stellar reactions with short-lived nuclei: 17F(p,α)14O

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