The 17F(p, γ)18 reaction is important in stellar explosions, but its rate has been uncertain because of an expected 3+ state in 18Ne that has never been conclusively observed. This state would provide a strong l = 0 resonance and, depending on its excitation energy, could dominate the stellar reaction rate. We have observed this missing 3+ state by measuring the 1H(17F, p)17F excitation function with a radioactive 17F beam at the ORNL Holifield Radioactive Ion Beam Facility. We find that the state lies at a center-of-mass energy of Er = 599.8 ± 1.5stat ± 2.0sys keV (Ex = 4523.7 ± 2.9keV) and has a width of Γ = 18 ± 2stat ± 1sys keV

Bardayan, D.W.

Blackmon, J.C.

Brune, C.R.

Champagne, A.E.

Chen, A.A.

Cox, J.M.

Davinson, T.

Hansper, V.Y.

Hofstee, M.A.

Johnson, B.A.

Kozub, R.L.

Ma, Z.

Parker, P.D.

Pierce, D.E.

Rabban, M.T.

Shotter, A.C.

Smith, M.S.

Swartz, K.B.

Visser, D.W.

Woods, P.J.

Carolina Digital Repository

VOLUME 83, NUMBER 1 P H Y S I C A L R E V I E W L E T T E R S 5 JULY 1999Observation of the Astrophysically Important 31 State in 18Ne via Elastic Scatteringof a Radioactive 17F Beam from 1HD. W. Bardayan,1,2 J. C. Blackmon,1 C. R. Brune,3 A. E. Champagne,3 A. A. Chen,2 J. M. Cox,4 T. Davinson,5V. Y. Hansper,1,3 M. A. Hofstee,6 B. A. Johnson,4 R. L. Kozub,4 Z. Ma,1,7,8 P. D. Parker,2 D. E. Pierce,1M. T. Rabban,1 A. C. Shotter,5 M. S. Smith,1 K. B. Swartz,2 D. W. Visser,2 and P. J. Woods51Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 378312A. W. Wright Nuclear Structure Laboratory, Yale University, New Haven, Connecticut 065113Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina 275994Physics Department, Tennessee Technological University, Cookeville, Tennessee 385055Department of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom6Department of Physics, Colorado School of Mines, Golden, Colorado 804017Chinese Institute for Atomic Energy, Beijing, 102413, People’s Republic of China8Joint Institute for Heavy Ion Research, Oak Ridge, Tennessee 37831(Received 4 March 1999)The 17Fp, g18Ne reaction is important in stellar explosions, but its rate has been uncertain becauseof an expected 31 state in 18Ne that has never been conclusively observed. This state would providea strong   0 resonance and, depending on its excitation energy, could dominate the stellar reactionrate. We have observed this missing 31 state by measuring the 1H17F, p17F excitation function with aradioactive 17F beam at the ORNL Holifield Radioactive Ion Beam Facility. We find that the state liesat a center-of-mass energy of Er  599.8 6 1.5stat 6 2.0sys keV (Ex  4523.7 6 2.9 keV) and has awidth of G  18 6 2stat 6 1sys keV.PACS numbers: 27.20.+n, 25.40.Cm, 25.60.Bx, 26.30.+kThere are a number of extremely hot, dense astro-physical environments where hydrogen is expected toburn explosively. These include novae, supernovae, andx-ray bursts [1]. In a classical nova explosion, hy-drogen gas accretes onto a white dwarf star and burnsexplosively with the CNO nuclei present, creating a sub-stantial quantity of 13N, 14O, 15O, and 17F [2]. The fateof the 17F is uncertain and depends on the 17Fp, g18Nerate. If the proton-capture rate is slower than the17F-beta-decay rate at temperatures characteristic of novaexplosions [T9 # 0.2, where Tn  T10n K], thenthe reaction sequence 17Fe1ne17Op, a14Np, g15Ooccurs. This contributes to the 15O enrichment which isneeded to explain the large overabundance of nitrogen(originating from 15O beta decay) observed in novaejecta [3].If, on the other hand, the 17Fp, g18Ne rate is signifi-cant, there can be a substantial flow through the reactionsequence 17Fp, g18Nee1ne18F, and the 18F17F abun-dance ratio would be altered. Convection can bring 18Fand unburned 17F to the cooler surface regions where theycan only beta decay. This is important for two reasons.First, the release of the decay energy further increases theluminosity to a level in excess of 105LØ which can causerapid expansion and ejection of the envelope [4]. Second,the 511-keV gamma rays produced by the annihilation ofpositrons from the decay of 18F could be detectable be-cause the longer half-life of 18F allows it to survive untilthe envelope becomes more transparent [5].The 17Fp, g18Ne reaction is also important forunderstanding other explosive events such as x-ray0031-90079983(1)45(4)$15.00bursts and supernovae. During the ignition phaseof x-ray bursts, the energy production is peaked bytwo reaction sequences: 12Cp, g13Np, g14O and16Op, g17Fp, g18Nee1ne18Fp, a15O [4]. Thesecond sequence and thus the x-ray burst energy produc-tion depend sensitively on the 17Fp, g18Ne reactionrate. For massive stars in the presupernova phase, thetemperature in the Ne-burning shell can rise to T9  1 2[6]. At these temperatures, 16O can burn to form 17Fwhich, depending on its proton-capture rate, may undergosubsequent burning. To understand the abundances pro-duced in these events, we must know the 17Fp, g18Nestellar reaction rate.Wiescher, Görres, and Thielemann [7] have proposedthat a low energy 31 state in 18Ne, the mirror to the31 state at Ex  5.378 MeV in 18O, dominates the17Fp, g18Ne stellar reaction rate for temperaturesgreater than T9  0.2, which is in the range of peaktemperatures produced in these explosive events. Onthe basis of a shell model calculation, they predictedthis state to have an excitation energy Ex  4.328 MeVand width G  Gp  5 keV. Subsequently, othershave also done analyses of the mass A  18 iso-bars and arrived at a wide variety of results. Garcı́aet al. calculated Ex  4.53 MeV and G  22 keV [8],while Sherr and Fortune predicted Ex  4.642 MeV andG  42 keV [9].Many experimental studies have also been conductedto examine states in 18Ne in this excitation energy range,none of which found conclusive evidence for the exis-tence of this 31 state [10]. Nero, Adelberger, and Dietrich© 1999 The American Physical Society 45VOLUME 83, NUMBER 1 P H Y S I C A L R E V I E W L E T T E R S 5 JULY 1999determined the properties of many 18Ne states using the16O3He, n18Ne and 20Nep, t18Ne reactions but foundno evidence for the 31 state [11]. Garcı́a et al. studiedthe 16O3He, n18Ne reaction and reported evidence at oneenergy and angle for a peak which has generally been in-terpreted as locating the missing 31 state at Ex  4.561 60.009 MeV [8]. This state was not seen in subsequent highresolution studies of 20Nep, t18Ne [12,13].All of the above studies were hindered from seeingthe 31 state because the reactions used suppress thepopulation of states with unnatural spin and parity.We, therefore, have performed a measurement of the1H17F, p17F excitation function using a radioactive 17Fbeam at the ORNL Holifield Radioactive Ion BeamFacility (HRIBF) [14]. Since the ground state of 17F hasJp 521, the 17F 1 p system populates 31 and 21 statesin 18Ne with   0 angular momentum transfers, and thusthis reaction is very sensitive to the missing 31 state.Furthermore, we were able to determine the resonanceenergy and width of the state being populated from theshape of the excitation function.A radioactive 17F beam was produced by an isotopeseparator online-type targetion source [14,15] via the16Od, n17F reaction using a fibrous refractory HfO2 targetbombarded with 8 mA of 44.5 MeV deuterons from theK  105 Oak Ridge Isochronous Cyclotron. Aluminumvapor was fed into the target to form Al17F moleculeswhich transported the highly reactive 17F atoms out ofthe target material and through a short (10 cm) transfertube to a modular ion source, where they were ionizedand extracted. After a first stage of mass analysis, theAl17F1 molecules entered a charge exchange cell wherethe molecules were dissociated. The resulting 17F2 ionswent through a second stage of mass analysis followingthe charge exchange cell and were then accelerated to theappropriate energy by the 25-MV tandem. After passingthrough an energy-analyzing magnet, the 17F beam wasdelivered to the experimental station. The average beamcurrent was 8 3 103 17F ions per second, and a total of2 3 109 17F ions were incident on the target over thecourse of the experiment.The 17F beam bombarded a 48-mgcm2 polypropyleneCH2n foil, and the scattered protons were detected in anannular array of single-sided silicon strip detectors 10.5 cmdownstream from the target location. The silicon detec-tor array (SIDAR) is comprised of 128 segments with16 radial (from 5 to 13 cm) and 8 azimuthal divisions,similar to the Louvain-Edinburgh Detector Array arrayused at Louvain-la-Neuve [16]. The array covered an-gles 25± # ulab # 51± allowing detection of the forward-focused scattered protons while passing the 17F beam outof the target chamber. The experimental configuration isshown in Fig. 1. The recoil 17F ions were detected in co-incidence with the protons in an isobutane-filled ioniza-tion counter. This detector provides DE-E informationfor particle identification and is described in Ref. [17].This experimental configuration was tested previously with46a 1H17O, p17O measurement, and the coincidence effi-ciency was found to be greater than 90%. The unscatteredprimary beam was prevented from entering the ionizationcounter by a 1.5-cm-diameter disk which was inserted infront of the ionization counter entrance window duringeach run. The size of the disk was chosen so that for theproton angles covered by the SIDAR, the correspondingrecoil 17F ions were not blocked by the disk. In betweenruns, this disk was removed and a 4-mm aperture was in-serted for beam tuning and beam purity measurements viaparticle identification in the ion counter. Typical beam pu-rities were found to be 17F17O  1000.Proton yields were measured at 12 beam energiesbetween 10 and 12 MeV. A spectrum from the SIDARis shown in Fig. 2. The proton peak was clearly distin-guishable by its energy, angular dependence, and narrowwidth. Only the proton peak remained when coincidencewith 17F ions in the ionization counter was required. Theyield at each energy was determined by summing thecoincident proton yields, Ycoin, in all strips of the SIDARand normalizing to the incident beam current. Thisnormalization was achieved by monitoring the amountof 17F, YF , that was scattered from carbon in the targetand detected by the ionization counter. The normalizedproton yields, YcoinYFEinEout 3 const where Ein Eout is theenergy the beam has before (after) it transverses the tar-get, are displayed in Fig. 3 along with a fit to the data andclearly show the presence of a resonance. It was prefer-ential to use the coincidence spectrum to extract protonyields, because at some energies and angles, beta particlesand scattered 17F ions overlap with the proton peak in thesingles spectrum. There was no appreciable targetdegradation or dead time during the experiment. Themeasurement at 10.5 MeV was repeated to test thereproducibility of the system and found to lie withinthe uncertainty of the measurements. The beam energycalibration [18] was checked with a precision of 64 keVby measuring the 1H19F, a16O excitation function in theregion of the 20Ne resonance at Er  828 keV [19]. Thisintroduces a negligible uncertainty in the center-of-massenergy of 0.2 keV.A fit to the data was performed using a Breit-Wignerformalism [20] with three fit parameters: the normalization,resonance energy, and width. The theoretical cross sectionassuming a Jp  31 resonance was integrated over theangles covered by the SIDAR and then averaged over theenergy loss in the target. The average energy loss wasmeasured with a 19F beam, corrected for the mass of 17F,and found to be 690 6 50 keV. The energy loss in thetarget changes only by 20 keV as the bombarding energychanges from 10 to 12 MeV. The best fit (x2n  1.20) wasobtained for a center-of-mass resonance energy of Er 599.8 6 1.5 keV and a total resonance width of G 18 6 2 keV. The statistical uncertainties were determinedin the standard way from the least-squares fit to the data[21]. The fitting procedure was varied to estimate system-atic uncertainties. The singles data set was used instead ofVOLUME 83, NUMBER 1 P H Y S I C A L R E V I E W L E T T E R S 5 JULY 1999FIG. 1. Our experimental configuration is shown with the 17F ions impinging on a polypropylene target. The scattered protonswere detected in the SIDAR, while recoil 17F ions were detected in coincidence in a gas-filled ionization counter.the coincidence data, and an R-matrix fit was performedusing the code MULTI [22] instead of the Breit-Wignerformalism. Additionally, the dependence of the fit resultson the target thickness was examined. All of these resultedin variations in the resonance energy of less than 2 keVand in the width of less than 1 keV. From this andfrom a previous study of the 1H17O, p17O excitationfunction [23], we estimate the systematic uncertainty forthe resonance energy to be 2 keV and for the width to be1 keV.Combining the resonance energy with the measuredmass excess of 18Ne [24] and the well-known massexcesses of 1H and 17F [25] yields an excitation energyin 18Ne of 4523.7 6 2.9 keV. This is very close to theknown 12 state in 18Ne at 4519 6 8 keV and explainswhy this 31 state was not observed during measurementsof reactions which more readily populate states of naturalspin and parity. The resonance we have observed isnot the known 12 state because this spin and paritywould require an   1 transfer and, as demonstrated0204060800 2 4 6 8 10Energy (MeV)02040601H17F12Cβcounts/channel(a) Singles(b) Coincidence1HFIG. 2. (a) The raw particle spectrum from a ring of SIDARstrips at u  27.7± 29.9± is shown. A 10-MeV 17F beamimpinged on a 48-mgcm2 polypropylene target. (b) Sameas (a) when coincidence with recoil 17F ions in the ionizationcounter was required.in Fig. 3, would not be observable above the Rutherfordscattering background. Furthermore, the expected width(0.1 keV) of the 12 state [8] is inconsistent with ourresults. We conclude that we are populating either a 31 or21 state in 18Ne. From examination of the nuclear leveldiagrams, we see there are no 21 states in 18O whoseanalogs have not been identified in 18Ne. While it ispossible that the mirror assignments are not well known,and that we are observing the mirror to the 21 state atEx  5.255 MeV in 18O, this seems highly unlikely basedupon an examination of the expected widths of the states.If we take the spectroscopic factors for the 21 and 31states in 18O from Li et al. [26] and an appropriate singleparticle width [27], we estimate a proton width for the21 31 state in 18Ne of 7 (19) keV. If we fit our dataassuming that the resonance is a 21 state, the best fit(x2n  1.72 compared to 1.20 for the 31 case) is obtained9 10 11 12 13Average 17F Energy (MeV)4005006007008009001000Proton Yield (Arbitrary Units)Er = 599.8 ± 1.4 keVΓ = 18 ± 2 keVFIG. 3. The normalized proton yields are plotted as a functionof the average 17F beam energy in the target. The heavysolid line is a fit to the data with three fit parameters: thenormalization, the resonance energy, and the width of the 31state. The thin solid line shows the excitation function expectedif the only resonances in this region were the previouslyobserved 12 and 01 states in 18Ne. The dotted line shows theexcitation function if the width of the 12 state were 20 keVinstead of the expected 0.1 keV. This curve demonstratesthat the scattering anomaly could not be caused by an   1resonance.47VOLUME 83, NUMBER 1 P H Y S I C A L R E V I E W L E T T E R S 5 JULY 19990.0 0.2 0.4 0.6 0.8 1.0T (109 K)10-1010-810-610-410-2100102NA<σv> (cm3  mole-1 s-1)This Work- TotalThis Work(Jπ=3+) Garcia et al.(Jπ=3+)Wiescher et al.(Jπ=3+)Sherr and Fortune(Jπ=3+)FIG. 4. The contribution to the 17Fp, g18Ne reaction ratefrom the 31 state is plotted as a function of stellar temperature.This is compared to estimates of the rate from previouslypublished predictions of the resonance parameters from Garcı́aet al. [8], Wiescher et al. [7], and Sherr and Fortune [9]. Thetotal reaction rate, which includes contributions from nearbyresonances as well as direct capture, is also shown.for a width of 30 keV. This exceeds the total possible1s12 single particle strength and is a factor of 4 greaterthan the estimated width. On the other hand, the 31 fitresult of 18 6 2 keV for the width agrees very well withthe estimate of 19 keV. We, therefore, conclude that weare observing the long-sought 31 state in 18Ne.Using these new resonance parameters, we have recalcu-lated the 17Fp, g18Ne stellar reaction rate as a functionof temperature. The new rate is plotted in Fig. 4 alongwith rates using previous predictions of the resonance pa-rameters. The calculation of the total reaction rate usesresonance properties for the nearby 01 and 12 states aswell as the direct capture rate from Garcı́a et al. [8]. Ourcalculation follows the prescription in Ref. [28] but usesour new resonance parameters for the 31 state.In conclusion, by measuring the 1H17F, p17F excita-tion function with a radioactive 17F beam, we have foundthe missing 31 state in 18Ne. We measure its resonanceenergy to be 599.8 6 1.5stat 6 2.0sys keV and width tobe 18 6 2stat 6 1sys keV. Because its resonance energyis 37 keV lower than was found in Ref. [8], its con-tribution to the 17Fp, g18Ne stellar reaction rate is afactor of 2 larger at T9  0.5 than the prediction inRef. [28]. It is different, however, by orders of magni-tude from the predictions of Wiescher et al. [7] and Sherrand Fortune [9]. Because of its excitation energy, the 31state contributes strongly to the rate at temperatures aboveT9  0.5 and is thus very important for explosive eventssuch as x-ray bursts and supernovae. In the lower tem-perature environments of novae, the rate is dominated bythe (unmeasured) direct capture contribution. While dis-covery of the 31 state resolves the greatest uncertainty inthe rate, a direct measurement of the 17Fp, g18Ne crosssection is needed to address the remaining uncertainties.48We thank the staff of the HRIBF. Without theirhard work and dedication to this project, this experimentwould not have been possible. We also thank J. F.Shriner for his help with the MULTI code. Oak RidgeNational Laboratory is managed by Lockheed MartinEnergy Research Corp. under Contract No. DE-AC05-96OR22464 with the U.S. Department of Energy. Thework was supported in part by the DOE under ContractNo. DE-FG02-91ER-40609.[1] A. E. Champagne and M. Wiescher, Annu. Rev. Nucl. Part.Sci. 42, 39 (1992).[2] S. A. Glasner, E. Livne, and J. W. Truran, Astrophys. J.475, 754 (1997).[3] Y. Chin, C. Henkel, N. Langer, and R. Mauersberger,Astrophys. J. Lett. 512, L143 (1999).[4] M. Wiescher, H. Schatz, and A. E. Champagne, Philos.Trans. R. Soc. London A 356, 2105 (1998).[5] J. Gómez-Gomar, M. Hernanz, J. José, and J. Isern, Mon.Not. R. Astron. Soc. 296, 913 (1998).[6] M. El Eid (private communication).[7] M. Wiescher, J. Görres, and F.-K. Thielemann, Astrophys.J. 326, 384 (1988).[8] A. Garcı́a et al., Phys. Rev. C 43, 2012 (1991).[9] R. Sherr and H. T. Fortune, Phys. Rev. C 58, 3292 (1998).[10] D. R. Tilley, H. R. Weller, C. M. Cheves, and R. M.Chasteler, Nucl. Phys. A595, 1 (1995).[11] A. V. Nero, E. G. Adelberger, and F. S. Dietrich, Phys.Rev. C 24, 1864 (1981).[12] K. I. Hahn et al., Phys. Rev. C 54, 1999 (1996).[13] S. H. Park et al., Phys. Rev. C 59, 1182 (1999).[14] G. D. Alton and J. R. Beene, J. Phys. G 24, 1347 (1998).[15] R. F. Welton et al. (to be published).[16] R. Coszach et al., Phys. Lett. B 353, 184 (1995).[17] A. N. James et al., Nucl. Instrum. Methods Phys. Res.,Sect. A 267, 144 (1988).[18] D. K. Olsen et al., Nucl. Instrum. Methods Phys. Res.,Sect. A 254, 1 (1987).[19] D. R. Tilley et al., Nucl. Phys. A636, 247 (1998).[20] J. M. Blatt and L. C. Biedenharn, Rev. Mod. Phys. 24, 258(1952).[21] P. R. Bevington, Data Reduction and Error Analysis forthe Physical Sciences (McGraw-Hill, New York, 1969).[22] R. O. Nelson, E. G. Bilpuch, and G. E. Mitchell, Nucl.Instrum. Methods Phys. Res., Sect. A 236, 128 (1985).[23] D. W. Bardayan, Ph.D. thesis, Yale University, 1999.[24] P. V. Magnus, E. G. Adelberger, and A. Garcı́a, Phys.Rev. C 49, 1755 (1994).[25] G. Audi, O. Bersillon, J. Blachot, and A. H. Wapstra,Nucl. Phys. A624, 1 (1997).[26] T. K. Li, D. Dehnhard, Ronald E. Brown, and P. J. Ellis,Phys. Rev. C 13, 55 (1976).[27] C. Iliadis, Nucl. Phys. A618, 166 (1997).[28] D. W. Bardayan and M. S. Smith, Phys. Rev. C 56, 1647(1997).

Observation of the astrophysically important 3+ state in 18Ne via elastic scattering of a radioactive 17F beam from 1H

https://cdr.lib.unc.edu/downloads/d217r035h

Observation of the astrophysically important 3+ state in 18Ne via elastic scattering of a radioactive 17F beam from 1H

Abstract

Similar works

Full text

Available Versions

Carolina Digital Repository