We have studied the b-delayed particle decay of 17Ne to test the feasibility of determining both the E1 and E2 components of the 12C(a,g )16O cross section at energies relevant to helium burning in stars. In this context we have observed the breakup of the isobaric analog state in 17F at 11.193 MeV into three particles via three channels: proton decay to the 9.59 MeV state in 16O; and a decay to the 2.365 and 3.502/3.547 MeV states in 13N. This is the first reported observation of the decay of the IAS to the 12 state in 16O at 9.59 MeV and the first reported b-delayed proton-a decay. With straightforward improvements to our detection apparatus to improve angular resolution, b suppression, and solid angle coverage, we should be able to proceed to the measurement of the effect of the tail of the subthreshold state at 7.117 MeV in 16O on thea spectrum from the breakup of the 9.59 MeV state

Azuma, R.E.

Bateman, N.

Boyd, R.N.

Buchmann, L.

Chow, J.C.

D'Auria, J.M.

Davinson, T.

Dombsky, M.

Galster, W.

Gete, E.

Giesen, U.

Iliadis, C.

Jackson, K.P.

King, J.D.

Morton, A.C.

Roy, G.

Shoppa, T.

Shotter, A.

Carolina Digital Repository

RAPID COMMUNICATIONSPHYSICAL REVIEW C FEBRUARY 1998VOLUME 57, NUMBER 2Three-particle breakup of the isobaric analog state in17FJ. C. Chow,1 A. C. Morton,1 R. E. Azuma,1 N. Bateman,1,2,4 R. N. Boyd,3 L. Buchmann,2 J. M. D’Auria,4 T. Davinson,5M. Dombsky,2 W. Galster,6 E. Gete,2 U. Giesen,2,4 C. Iliadis,1,2,* K. P. Jackson,2 J. D. King,1 G. Roy,7 T. Shoppa,2and A. Shotter51Physics Department, University of Toronto, Toronto, Ontario, Canada M5S 1A72TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 2A33Departments of Physics and Astronomy, Ohio State University, Columbus, Ohio 432104Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S65Department of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom EH9 3JZ6Département de Physique, Universite´ Catholique de Louvain, Louvain-la-Neuve, Belgium 13487Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2J1~Received 29 July 1997!We have studied theb-delayed particle decay of17Ne to test the feasibility of determining both theE1 andE2 components of the12C(a,g)16O cross section at energies relevant to helium burning in stars. In thiscontext we have observed the breakup of the isobaric analog state in17F at 11.193 MeV into three particles viathree channels: proton decay to the 9.59 MeV state in16O; anda decay to the 2.365 and 3.502/3.547 MeVstates in13N. This is the first reported observation of the decay of the IAS to the 12 state in16O at 9.59 MeVand the first reportedb-delayed proton-a decay. With straightforward improvements to our detection apparatusto improve angular resolution,b suppression, and solid angle coverage, we should be able to proceed to themeasurement of the effect of the tail of the subthreshold state at 7.117 MeV in16O on thea spectrum from thebreakup of the 9.59 MeV state.@S0556-2813~98!50602-4#PACS number~s!: 26.20.1f, 27.20.1n, 23.50.1z, 23.60.1etientoy-rlfue-tyeinotesr-gtto-forinyx-The4theT.Theb-delayed particle decay of17Ne has been the subjecof two major experimental studies@1,2#. However, there aresignificant differences between the results of these studand the energetically allowed proton decay into unboustates of16O was not observed in either work. The decay inunbound states in16O also offers, in principle, the possibilitof determining the reduceda width of the Jp521 boundstate atEx56.917 MeV in16O. This reduceda width isintimately related to the strength of theE2 component in theastrophysically important12C(a,g)16O radiative capture reaction @3–5#. This component has been shown to be pooconstrained by present data and difficult to determine byture direct measurements@5#.Recently, we have used theb-delayeda-particle spec-trum from 16N (t1/257.13 s! to constrain theE1 cross sec-tion at low energies. Simultaneous fits were made to the16Nb-delayeda spectrum, to the four sets of12C(a,g)16O E1cross-section data, and to the12C(a,a)12C phase shifts.From the fits we were able to determine thea width of thesubthreshold 12 state at 7.117 MeV, and thereby much rduce the uncertainty in the12C(a,g)16O E1 cross section a300 keV @6,7#. However, 21 states are not significantlpopulated in the decay of16N and our knowledge of theE2component was not improved by that experiment. Howevin the b-delayed proton decay of17Ne, both 12 and 21states in16O are populated. Therefore, we are investigatthe decay of17Ne to see if we can determine the strengths*Present address: Department of Physics and Astronomy,University of North Carolina at Chapel Hill, Chapel Hill, N.C27599-3255.570556-2813/98/57~2!/475~4!/$15.00s,dy-r,gfthe tails of both the 6.917 and 7.117 MeV subthreshold stain the 12C(a,g)16O reaction. Here we report the first obsevation of the decay of the isobaric analog state~IAS! in 17Fto the 12 state of16O at 9.59 MeV. A detailed study of thisdecay mode should provide an independent check on theE1strength in12C(a,g)16O. We also give corrected branchinratios for the decay of the IAS.The energy available for theb1 decay of 17Ne is 13.51MeV @8#. Since 17F is bound by only 0.6005 MeV againsproton decay to16O, most states populated in theb decay of17Ne will decay by proton emission@1,2,8#. States in17Fwith energy greater than 7.762 MeV may decay ina-unbound states in16O, including the tails of the subthreshold 6.917 and 7.117 MeV states. The partial level scheme17Ne decay@2,8# shown in Fig. 1 indicates that all states17F above 5.819 MeV may also decay into13N plus anaparticle.The TISOL facility @9# at the TRIUMF laboratory hasbeen used to investigate theb-delayed proton decay of17Neto excited states of16O. A 17Ne beam was produced bbombarding a MgO target with 500 MeV protons and etracting a mass 17 beam from an on-line ECR source.branching ratios for the decay to bound excited states in16Owere determined by measuring proton–g-ray coincidences.Transitions were observed to the 21 state at 6.917 MeV in16O from states in17F at 11.193, 10.0, 9.45, 8.83, and 8.4MeV. Many new transitions to the 12 state at 7.117 MeVand to the 32 state at 6.130 MeV were also observed, anddecay of the IAS to the 22 state in 16O at 8.872 MeV wasseen for the first time@10,11#.To collect a low-backgrounda-particle spectrum with acomponent from the tail of the subthreshold state in16O atheR475 © 1998 The American Physical SocietyRAPID COMMUNICATIONSR476 57J. C. CHOWet al.FIG. 1. Partial decay scheme of17Ne @2,8#. States below 8 MeV in17F are omitted since they cannot populate the 21 state at 6.917 MeVin 16O via proton decay.e--nttemTh5hefwadthth-6.917 or 7.117 MeV will require a triple coincidence btween thea particle, the recoiling12C nucleus, and the proton emitted from the parent17F state. The IAS in17F at11.193 MeV has a width of only 0.18 keV@8# and is popu-lated in '0.7% of the 17Ne decays. It decays by protoemission to all states in16O up to the 9.59 MeV state~firstreported here! and by a emission to the ground and firsexcited states of13N. Thus, the IAS seems a good candidastate in 17F for the observation of p-a-12C triple coinci-dences.A Monte Carlo simulation was used to define an optimudetector arrangement for observing triple coincidences.experimental arrangement~see Fig. 2! consisted of two 900mm2 ion-implanted Si detectors at right angles, and two 4mm2 ion-implanted Si detectors placed at 110o to one of thelarger detectors. The detector plane was inclined at 45o tothe incoming17Ne beam in order to provide access for tbeam to a 10mg cm22 carbon collector foil at the center othe array. The beam intensity was typically 1–23104 s21.In order to reduce the event rate, the master triggerset to require coincidences between any adjacent pair oftectors. Triple coincidence spectra were combined intotriple-energy-sum spectrum of Fig. 3. The breakup ofrecoiling 16O and 13N nuclei produces a back-to-backa-12Ce0se-eeor proton-12C pair, respectively, in the center-of-mass system. Since the decay of excited states in17F is predomi-nantly by proton emission@1,2,10,11#, and each proton decayis accompanied by an16O recoil, an accidental event in theFIG. 2. Detector arrangement for the detection of p-a-12C triplecoincidences from the decay of the IAS in17F.fmofedf9RAPID COMMUNICATIONS57 R477THREE-PARTICLE BREAKUP OF THE ISOBARIC . . .third counter~at 90o) will provide a triple coincidence. If alltriple coincidences are assumed to be p-a-12C events then,from the measured energies and conservation of mometuthe angles between the particles can be calculated. Rejectof events with angles outside the ranges covered by the dtectors eliminated most of the background in the spectrudue to two-body decays.The energy difference between the IAS in17F and thethreshold for breakup intoa 1 12C in 16O is 3.43 MeV@8#.A narrow peak at about 3.4 MeV is apparent in Fig. 3. Thpeak near 2.4 MeV is due primarily to the stronga decay ofthe 8.08 MeV state to the ground state of13N, which has notFIG. 4. Two-dimensional plot of protons versus12C recoil nu-clei emitted during the breakup of17F in its IAS. The ellipses showthe gates used to obtain the particle spectra of Fig. 5.FIG. 3. Triple sum spectrum for theb-delayed particle decay of17Ne. See text for details.m,ione-mebeen removed completely by the kinematic cut. The width othe peak associated with the IAS decay is compounded frothe inherent resolution of the system and the inaccuracyenergy calibration of lower-energya particles and12C re-coils, plus a possible contribution from misidentified low-energy particles.Single and double coincidence spectra were then obtainwith a gate set on the 3.4 MeV peak of Fig. 3. A plot of12CTABLE I. Branching ratios for the decay of the IAS of17F.Particle Ex Jp;T B.R. B.R. Present~MeV! ~%! @1# ~%! @2# ~%!(16O!0 01;0 1062 10.760.6 7.860.4p 6.049 01 ,3 1163 9.660.86.130 32 2262 2562 17.660.66.917 21 2466 ,4 0.560.17.117 12 4464 1863 13.460.38.872 22 – – 8.661.29.59 12 – – 22.7611.3(13N!a 0 122; 12 – 1.160.5 0.860.22.365 121 – 2969 14.661.13.50/3.55 322/ 521 – – 1.060.5(17F!g 0.495 121; 12 3.461.5aaBranching ratio forg decay taken as given in Ref.@2#.FIG. 5. Spectrum ofa particles and12C recoil nuclei in triplecoincidence with protons resulting from the breakup of the IAS o17F. The full curve is the spectrum for proton decay to the 9.5MeV state of 16O followed by breakup intoa112C. The dashedcurve is the spectrum fora decay to the 2.365 MeV state in13Nfollowed by breakup into p112C. The spectra were obtained bysetting conditions on proton-12C coincidences as indicated in Fig. 4.2efVeci-caone. 4nte.sti-be-veolddeomea-oldeoldhle,hisol-ci-theRAPID COMMUNICATIONSR478 57J. C. CHOWet al.recoil energy versus proton energy summed over the F1Fand F1F2F4 detector configurations~see Fig. 2! is shown inFig. 4. Protons from the IAS to the 9.59 MeV state in16Oshould peak at about 1 MeV in the laboratory frame of rerence, whilea particles from the breakup of the 9.59 Mestate should appear at about 1.8 MeV, with the12C recoils atabout 0.6 MeV. If the IAS decays by emission of anpar-ticle to the excited state in13N at 2.365 MeV, the proton,a,and 12C energies are near 0.4, 2.3, and 0.7 MeV, resptively, in our choice of detector geometry. The two coindence peaks outlined in Fig. 4 show that these two demodes are fairly well separated; this separation arisesbecause thea energies of the two modes are themselves wseparated. Figure 5 shows thea and 12C spectra obtained bysetting gates on the two ellipsoidal areas outlined in FigThis observation ofbp-delayed a particles from 17Nethrough the IAS of17F adds another branch to the protodecay of the IAS reported in Ref.@11# and represents the firsreportedb-delayed pa decay.In Table I we summarize the branching ratios for the dcay of the IAS in 17F @1,2,10,11#. New proton branches tothe 8.872 and 9.59 MeV states in16O have been observedThe proton branch to the 9.59 MeV state in16O and thea-particle branch to the 2.365 MeV state in13N have bothbeen observed~in true triple coincidence! to lead to breakupof the recoiling unbound residual nucleus.The detection of 14,000 triple-coincidence decay eventeKF3-c-ylyll.-at0.15% coincidence efficiency into the 9.59 MeV state of16Ocorresponds to approximately 231026 of the total observed17Ne decays. This is about a factor of 104 more than thenumber ofb-delayed proton decays through the IAS esmated to populate the tail of the 7.117 MeV state in16O andto have been detected in this experiment. While it wouldstraightforward with the17Ne yields recently achieved withTISOL (>23105 s21mA21), with improved angular resolution, and with much larger solid angle coverage, to achiethe count rate required to detect the tail of this subthreshstate in16O, further detector improvements have to be mato suppress false triple coincidences resulting primarily frcoincidences withb particles.In the next phase of our study, with improvedb suppres-sion and much better statistical accuracy, it should be fsible to determine the influence of the tail of the subthresh7.117 MeV state on thea spectrum from the decay of thIAS into the 9.59 MeV state of16O. This will provide anindependent determination of the effect of this subthreshstate on the12C(a,g)16O cross section for comparison witthe result obtained from16N decay@6,7#.We wish to thank H. Biegenzein, D. Jones, P. Machuand H. Sprenger for help with the technical aspects of texperiment and M. Trinczek for assistance with the data clection. The work was supported in part by the Natural Sences and Engineering Research Council of Canada, byNational Science Foundation, and by TRIUMF..dslospto,@1# J.C. Hardy, J.E. Esterl, R.G. Sextro, and J. Cerny, Phys. RC 3, 700 ~1971!.@2# M.J.G. Borgeet al., Nucl. Phys.A490, 287 ~1988!.@3# W.A. Fowler, Rev. Mod. Phys.56, 149 ~1984!.@4# T.A. Weaver and S.E. Woosley, Phys. Rep.227, 65 ~1993!.@5# L. Buchmann, R.E. Azuma, C.A. Barnes, J. Humblet, andLanganke, Phys. Rev. C54, 393 ~1996!.@6# L. Buchmannet al., Phys. Rev. Lett.70, 726 ~1993!.@7# R.E. Azumaet al., Phys. Rev. C50, 1194~1994!.@8# D.R. Tilley, H.R. Weller, and C.M. Ceves, Nucl. Phys.A564,1 ~1993!.v..@9# J.M. D’Auria, L. Buchmann, M. Dombsky, P. McNeely, GRoy, H. Sprenger, and J. Vincent, Nucl. Instrum. MethoPhys. Res. B70, 75 ~1992!.@10# J.D. King et al., in Proceedings of the Fourth InternationaSymposium on Nuclear Astrophysics: Nuclei in the Cosm,Notre Dame, IN, 1996@Nucl. Phys.A621, 169c~1997!#.@11# J.D. Kinget al., in Proceedings of the International Workshoon the Physics of Unstable Nuclear Beams, Serra Negra, Sa˜oPaulo, Brazil, 1996, edited by C.A. Bertulani, L. Felipe Canand M.S. Hussein~World Scientific, New Jersey, 1997!, p. 15.

Three-particle breakup of the isobaric analog state in 17F

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Three-particle breakup of the isobaric analog state in 17F

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