A new measurement of the β-delayed α decay of N16 has been performed using a set of high efficiency ionization chambers. Sources were made by implantation of a N16 beam, yielding very clean α spectra down to energies as low as 400 keV. Our data are in good agreement with earlier results. For the S factor SE1, we obtain a value of 74±21keVb. In spite of improvements in the measurement, the error in SE1 remains relatively large because of the correlations among the fit parameters and the uncertainties inherent to the extrapolation

Ahmad, I.

Brune, C.R.

Champagne, A.

Greene, J.P.

Hecht, A.A.

Henderson, D.

Janssens, R.V.F.

Jiang, C.L.

Jisonna, L.

Kahl, D.

Moore, E.F.

Notani, M.

Pardo, R.C.

Patel, N.

Paul, M.

Rehm, K.E.

Savard, G.

Schiffer, J.P.

Segel, R.E.

Shumard, B.

Sinha, S.

Tang, X.D.

Wuosmaa, A.H.

Carolina Digital Repository

New Determination of the Astrophysical S Factor SE1 of the 12C;16O ReactionX. D. Tang,1,* K. E. Rehm,1 I. Ahmad,1 C. R. Brune,2 A. Champagne,3 J. P. Greene,1 A. A. Hecht,1,4 D. Henderson,1R. V. F. Janssens,1 C. L. Jiang,1 L. Jisonna,5 D. Kahl,1,† E. F. Moore,1 M. Notani,1,6 R. C. Pardo,1 N. Patel,1,7 M. Paul,8G. Savard,1 J. P. Schiffer,1 R. E. Segel,5 S. Sinha,1,‡ B. Shumard,1 and A. H. Wuosmaa91Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA2Ohio University, Athens, Ohio 45701, USA3University of North Carolina, Chapel Hill, North Carolina 27599, USA4University of Maryland, College Park, Maryland 20742, USA5Northwestern University, Evanston, Illinois 60208, USA6Michigan State University, East Lansing, Michigan 48824, USA7Colorado School of Mines, Golden, Colorado 80401, USA8Hebrew University, Jerusalem, Israel9Western Michigan University, Kalamazoo, Michigan 49008, USA(Received 16 March 2007; published 3 August 2007)A new measurement of the -delayed  decay of 16N has been performed using a set of high efficiencyionization chambers. Sources were made by implantation of a 16N beam, yielding very clean  spectradown to energies as low as 400 keV. Our data are in good agreement with earlier results. For the S factorSE1, we obtain a value of 74 21 keV b. In spite of improvements in the measurement, the error in SE1remains relatively large because of the correlations among the fit parameters and the uncertainties inherentto the extrapolation.DOI: 10.1103/PhysRevLett.99.052502 PACS numbers: 21.10.Pc, 23.60.+e, 25.55.e, 26.20.+fThe main isotopes of carbon and oxygen (12C and 16O),crucial to all living organisms, are produced by heliumburning in red giant stars. The ratio of their abundances isdetermined by the competition between the triple- pro-cess (  ! 12C) and the 12C; 16O reaction.This ratio also affects the future evolution of a star duringits carbon, neon, and oxygen burning phases [1]. While thecross section for the triple- process is experimentallyquite well determined [2], our knowledge of the12C;16O reaction under red giant conditions (E 300 keV) is considered to be the most serious uncertaintyin nucleosynthesis [1,3]. There are two main -capturemodes for the 12C;16O reaction. One is capture withmultipolarity E1 with contributions from the 1 resonanceat Ecm  2:418 MeV (Ex  9:585 MeV) and the subthres-hold 1 state at Ecm  45 keV (Ex  7:117 MeV), achannel that is isospin forbidden. The E2 capture modeincludes contributions from direct capture and the tail froma 2 (Ecm  245 keV, Ex  6:917 MeV) subthresholdstate. Contributions from cascade transitions are expectedto be small [4]. AtEcm  300 keV, the cross sections are ofthe order of 1017 b, i.e., beyond present experimentalcapabilities. One has, therefore, to rely on extrapolations[5] using measurements above 890 keV [6–11]. Thevalues quoted in the recent NACRE compilation [12] are79 21 keV b (SE1) and 120 60 keV b (SE2), respec-tively. In this Letter, SE1;E2 refer to the S factors at Ecm 300 keV.Measurements of the -delayed  decay of 16N providethe tightest constraints for determining SE1 [5]. In theseexperiments, SE1 is extracted from the height of a smallsatellite peak located at Ecm  1 MeV in the -energyspectrum. This peak originates from the interference be-tween the subthreshold 1 state (Ex  7:117 MeV) andthe higher-lying 1 state in 16O at Ex  9:585 MeV [13].The very small branching ratio of the 16N decay ( 105)complicates the -decay experiments in two ways: It re-quires a strong and pure source of 16N particles, and thehigh  background severely masks the low-energy part ofthe  spectrum in the vicinity of the small satellite peakfrom which SE1 is obtained. This peak has been studied byseveral groups [5,14,15], all of them using thin Si-surfacebarrier detectors, which are sensitive to  particles, havedead layers, and are prone to deterioration during measure-ments lasting several weeks.Because of the importance of the 12C;16O reaction,we have remeasured the 16N decay using a different ap-proach. To reduce the sensitivity to electrons, we devel-oped an array of high-acceptance ionization chambers ofminimal thickness, to be used for the coincident detectionof 12C and  particles following the 16N decay. The maingoals of the experiment, described below, were (1) toremeasure the height of the small satellite  peak, usinga different experimental setup, subject to different system-atic uncertainties, (2) to extend the measurements towardslower energies and, (3) to resolve the discrepancies, ob-served in earlier measurements, in the shape of the low-energy part of the  spectrum [5,14–16].A schematic of the experimental setup is given in Fig. 1.A 61-MeV 16N (T1=2  7:1 sec ) beam is slowed down in agas-filled cell and stopped in a thin carbon foil (foil a)mounted on a rotating wheel (diameter 18.7 cm) located inthe main part of the detection chamber. The cell and themain chamber are filled with P10 counting gas at a pressurePRL 99, 052502 (2007) P H Y S I C A L R E V I E W L E T T E R Sweek ending3 AUGUST 20070031-9007=07=99(5)=052502(4) 052502-1 © 2007 The American Physical Societyof 150 Torr. After an irradiation of 15 s, the foil (a) isrotated (in 60 ms) by 120, placing it between a pair oftwin-ionization chambers (pair I) for the coincident detec-tion of 12C- pairs. The carbon foil is counted in this pairof ionization chambers (IC) for 15 sec. During this time, asecond foil (b), positioned on the wheel 120 relative to thefirst foil, is irradiated. At the end of the second irradiation,foil (b) is rotated to be between a second pair of ionizationchambers (pair II) and counted, while foil (a) is againirradiated with 16N. Detector pair I is meanwhile countinga nonirradiated foil to establish backgrounds. The samefoil, which is never irradiated, is also used for measuringthe background in detector pair II. In order to check thebackground in the gas system of the ionization chambers,some of the measurements were performed with a 15-s-on,15-s-off cycle.The ionization chambers are of the twin-ionization type,frequently used for the coincident detection of fission frag-ments [17,18]. In addition to the energy signal from theanode, an angle signal is obtained from the Frisch grid[18]. Details of the technique will be given in a forth-coming paper. The ionization chambers were calibratedwith  sources (228Th and 148Gd) as well as with 7Li and -triton coincidences obtained from the10Bn; 7Li and 6Lin; t reactions, respectively, usingmoderated neutrons from a Pu-Be neutron source. Thelatter two reactions provided  particles of 1.472, 1.776,and 2.056 MeV, energies that bracket the energy range ofinterest. The linearity of the electronics was tested with aprecision pulser. The energy resolution obtained from the1.472-MeV line is 40 keV. The accuracy of the calibrationis better than 5 keV. The long-term stability of thedetectors was monitored daily over a period of 10 dayswith ’s from the 10Bn; 7Li reaction and was found tobe better than 3	 103.To test the sensitivity of the ionization chambers to particles, a 22Na  source with an activity of about105 decays=s was mounted on the target wheel. Thestrength of this source is similar to the actual rate of particles from 16N in the experiment. The only changeobserved in the spectrum was a 25% increase in the singlescount rate at very low energies (E 100 keV), corre-sponding to an increase in count rate to about170 counts=hr above the threshold of 100 keV. For thecoincidence measurement, this background rate isnegligible.In the present experiment, the 16N beam was producedvia the In-Flight Technique [19], by bombarding adeuterium-filled cryogenic gas cell with an 82-MeVbeam of 15N from the ATLAS facility. No 17;18N ions canbe produced in this process. All beam contaminants arestable and, therefore, do not bring radioactivities to thetarget. With 6	 1011=s 15N incident on the deuterium-filled production cell, the 16N beam intensity measuredbehind the catcher foil was about 3	 106=s.The thickness of the carbon foils used to stop the 16Nions after the absorption cell (nominally 10 g=cm2) wasmeasured after the experiments through the energy loss of228Th  particles using a Split Pole spectrograph and wasfound to be 17 2 g=cm2. The fraction of 16N stoppedin the foil was measured to be about 5%, in good agree-ment with estimates.In a study of the 16N -delayed  decay, three aspectsare important: (1)  particles with energies down to0.6 MeV have to be detected in coincidence with 12Cions of even lower energies ( 0:1 MeV). Any significantenergy loss of the outgoing particles in the catcher foil willdeform the shape of the spectrum. (2) If a particle, emittedfrom the foil, is stopped in the support frame, only a part ofthe energy is deposited in the gas. Such events must beclearly separated from the true coincidence events produc-ing the interference peak. (3) The detection efficiency mustbe constant over the energy range from 0.2 MeV to 2 MeV.Calibration measurements with pulser signals showed thatthe coincidence efficiency was constant down to 120 keV.The geometry of the target wheel and the detectors wasoptimized by Monte Carlo simulations. For a beam spot of5 mm, the foil was mounted on one side of a 1.75 mmthick target frame with a hole of 10 mm (see insert inFig. 1). Because of the upstream-downstream asymmetryof the target frame, the -12C coincidences exhibit slightdifferences depending on the direction in which the particle is emitted, as can be seen in Fig. 2, where coinci-dent events measured in the upstream and downstreamdetectors are shown. On the left, the results for a foilimplanted with 16N particles are given, while the rightpanel represents the background measurement mentionedabove. The two groups with the highest count rate seen inFig. 2(a) correspond to 12C- coincidences in the upstreamand downstream detectors, respectively. These two groupsexhibit a tail caused by energy-loss straggling of the 12Cparticles in the capture foil. The group observed very closeto the y-axis (around channel 2000) is caused by 16Ndecays where the 12C particle (emitted downstream) isstopped in the target frame, so that only part of the corre-sponding energy is detected (see the insert in Fig. 1). SuchFIG. 1 (color). Schematic of the experimental setup used for ameasurement of the -delayed  decay of 16N.PRL 99, 052502 (2007) P H Y S I C A L R E V I E W L E T T E R Sweek ending3 AUGUST 2007052502-2events can be eliminated by using the angle informationfrom the twin-ionization chambers. An  particle, emitteddownstream and hitting the target frame with only a frac-tion of the  energy detected in the ionization chamber,gives rise to the group seen near the origin.Here, we only discuss events where the  particle isemitted downstream and the full 12C energy is recorded,represented by the events in the lower part of Fig. 2(a). Adiscussion of all events will be given in the future.Selecting events below channel 800, and requiring thecorrect pulse height ratio of =12C events (as done in [5]),results in the  spectrum given in Fig. 3(a). The additionalrequirement that all particles are in the correct Frisch-gridanode region (see [18] for details) of the upstream (12C)and downstream () ionization chambers changes thenumber of counts in the region of the satellite peak byless than 1%. While the pressure of 150 Torr (equivalent toa Si detector thickness of 7 m) minimizes the interfer-ence from the low-energy  particles stopped in the targetframe (giving rise to the group next to the origin in Fig. 2),it also leads to a distortion of the  spectrum at channelnumbers above 2000 (corresponding to energies above1.9 MeV) due to incomplete stopping of higher energy particles. To verify this, some of the measurements wereperformed at a pressure of 195 Torr [Fig. 3(b)]. At thispressure, particles up to 2.4 MeV are stopped in the ion-ization chamber. The solid line in Fig. 3(b) represents thenormalized spectrum obtained at 150 Torr and illustratesthe effects of incomplete stopping. These effects were alsoreproduced in Monte Carlo simulations. Correcting the150 Torr data at energies above an  energy of 1.9 MeV,we obtain a summed spectrum shown in Fig. 4 containing2:2	 105 events. This number will be doubled in the finalanalysis. Due to the insensitivity of ionization chambers tothe copious ’s, the low-energy satellite peak in the-delayed  spectrum of 16N can now be followed, practi-cally background-free and without correcting for pileupevents from electrons or contributions from 17;18N decays,down to a c.m.-energy of 600 keV. The solid line is theresult of an R-matrix fit, which is discussed below.The insert in Fig. 4 shows a comparison of our spectrum with the results from earlier experiments. Thelines correspond to the spectra from [5] (dashed line) and[14] (solid line), normalized in height to the present ex-periment. While these previous experiments agree on thehigh-energy side of the spectrum, they show disagreementsfor the low-energy part of the main peak, with [14] exhib-iting a shallower slope, when compared to [5]. In thisenergy region, our spectrum is in better agreement withthe data of [14]. The height of the interference peak atFIG. 3. (a) The  spectrum measured in one of the twin-ionization chambers filled with 150 Torr of isobutane.(b) Same as (a), but at a pressure of 195 Torr. The solid line isthe spectrum obtained at 150 Torr, scaled in height.FIG. 4. Summed  spectrum obtained in this experiment incomparison with an R-matrix fit. The insert shows the low-energy part of spectrum together with the previous results (solid[14] and dashed lines [5]). See text for details.E_downstream (a.u.)0 500 1000 1500 2000E_upstream (a.u.)05001000150020002500E_downstream (a.u.)500 1000 1500 2000 25002500FIG. 2 (color). (Left) coincident energy spectrum, measuredwith one of the twin-ionization detectors for a foil with im-planted 16N particles. (Right) same as above, but for a non-implanted foil.PRL 99, 052502 (2007) P H Y S I C A L R E V I E W L E T T E R Sweek ending3 AUGUST 2007052502-3Ecm  1 MeV, which is sensitive to the S factor, showsgood agreement among the different experiments.Extracting the relevant S factor SE1 from the data isusually done using the R-matrix formalism through aleast-squares fit to data from the -delayed  decay of16N, in combination with experimental results from thecapture reaction 12C; 16O performed at higher ener-gies, as well as with phase shift parameters obtained fromelastic scattering of 12C;. For this, we have used theR-matrix program from [20]. The number of fitting pa-rameters is large (typically 14), and there are strong corre-lations among some of the parameters, even for high-statistics data. In a first step, we have employed the samephase shift and direct capture data as used in [5,14] for theR-matrix calculations. The result of such a fit (with achannel radius a  6:5 fm) is given by the solid line inFig. 4. We obtain SE1  74 21 keV b, including a sys-tematic uncertainty of 10 keV b, which is dominated by the5 keV uncertainty of the energy calibration.There are new data available for some of the other inputsused in the fits. With the phase shift data of [2,21], whichare not yet available in tabulated form, the SE1 valueobtained from our fits increases to 88 18 keV b, whichis closer to the value (SE1  101 17 keV b) obtained in[22]. A similar sensitivity of SE1 to the input parameters ofthe R-matrix calculation was observed when the-branching ratio of the subthreshold 1 state was in-creased by 10%, as suggested by a recent experiment[23]. Using the new set of capture data from [8–10],however, does not change SE1 appreciably. In the previousR-matrix fits, a distant 1 resonance was introduced, whichin [5] was found to be at Ex  19 MeV. No independentevidence for this resonance exists. Varying the location ofthis ‘‘background state‘‘ between 15 and 25 MeV changesSE1 by 10 keV b. We have also carried out fits thatinclude a constant plus linearly energy-dependent term,instead of the additional resonance, which gives SE1 77 23 keV b. The SE1 factor obtained by using all avail-able (but partially unpublished) data in the fit is 8518 keV b. These strong sensitivities to the input parametersindicate that improved measurements for all input parame-ters as well as a better theoretical description are needed inorder to reduce the uncertainties in the 12C;16O rate.We have performed a new measurement of the-delayed  decay of 16N using a setup that is insensitiveto the high electron background. The low-energy part ofthe spectrum was found to be in better agreement withearlier data measured at Mainz [16] and Yale [14] than withthe data from the TRIUMF [5] group. The satellite peak,which originates from the interference between the sub-threshold 1 state and the higher-lying 1 level in 16O, wasclearly resolved. Its height is in agreement with earliermeasurements. The extrapolation of SE1 to regions whereit cannot be measured directly involves input from manyparameters, such as phase shifts, capture cross sections,branching ratios and contributions from nonresonant am-plitudes. Our analysis shows that improvements in theaccuracies of all these values are needed in order to reducethe uncertainty of SE1.We would like to thank the operations group of theATLAS accelerator for providing the 16N beam for theexperiment. This work was supported by the USDepartment of Energy, Office of Nuclear Physics, underContract No. DE-AC02-06CH11357 and by the NSF GrantNo. PHY-02-16783 (Joint Institute for NuclearAstrophysics).*Currently at University of Notre Dame, Notre Dame, IN,USA†Currently at McMaster University, Hamilton, Canada‡Currently at University of CA, Davis, CA, USA[1] T. A. Weaver and S. Woosley, Phys. Rep. 227, 65 (1993).[2] L. R. Buchmann and C. A. Barnes, Nucl. Phys. 777, 254(2006).[3] W. Fowler, Rev. Mod. Phys. 56, 149 (1984).[4] C. Matei et al., Phys. Rev. Lett. 97, 242503 (2006).[5] R. Azuma et al., Phys. Rev. C 50, 1194 (1994); L.Buchmann et al., Phys. Rev. Lett. 70, 726 (1993).[6] A. Redder et al., Nucl. Phys. A 462, 385 (1987).[7] J. M. L. Ouellet et al., Phys. Rev. Lett. 69, 1896 (1992).[8] R. Kunz et al., Phys. Rev. Lett. 86, 3244 (2001).[9] R. Kunz et al., Astrophys. J. 567, 643 (2002).[10] M. Assunção et al., Phys. Rev. C 73, 055801 (2006).[11] R. Plag, Ph.D. thesis, FZ Karlsruhe [FZKA ReportNo. 7099, 2005].[12] C. Angulo et al., Nucl. Phys. A 656, 3 (1999).[13] J. Humblet, Phys. Rev. C 42, 1582 (1990).[14] Z. Zhao et al., Phys. Rev. Lett. 70, 2066 (1993).[15] R. H. France et al., Nucl. Phys. A 621, 165c (1997).[16] K. Neubeck, H. Schober, and H. Wäffler, Phys. Rev. C 10,320 (1974).[17] C. Budtz-Jørgensen et al., Nucl. Instrum. Methods Phys.Res., Sect. A 258, 209 (1987).[18] A. Göpfert, F.-J. Hambsch, and H. Bax, Nucl. Instrum.Methods Phys. Res., Sect. A 441, 438 (2000).[19] B. Harss et al., Rev. Sci. Instrum. 71, 380 (2000).[20] J. Powell, Ph.D. thesis, University of Toronto, 1994.[21] P. Tischhauser et al., Phys. Rev. Lett. 88, 072501 (2002).[22] C. Brune et al., Phys. Rev. Lett. 83, 4025 (1999).[23] K. E. Rehm et al. (to be published).TABLE I. SE1 values obtained from R-matrix fits with differ-ent input parameters. See text for details.source of16N data SE1 keV b 2 other data used in fitsRef. 5 79 21 1.6this work 74 21 2 standard input88 18 2.9 phase shifts from [21]70 20 2 new -branching ratio [23]73 21 1.7 with (; ) data from [8]77 23 2 constant linear term85 18 2.7 Ref. [8,21,23]PRL 99, 052502 (2007) P H Y S I C A L R E V I E W L E T T E R Sweek ending3 AUGUST 2007052502-4

New determination of the astrophysical S factor SE1 of the C12(α,γ)O16 reaction

A new measurement of the β-delayed α decay of N16 has been performed using a set of high efficiency ionization chambers. Sources were made by implantation of a N16 beam, yielding very clean α spectra down to energies as low as 400 keV. Our data are in good agreement with earlier results. For the S factor SE1, we obtain a value of 74±21 keV b. In spite of improvements in the measurement, the error in SE1 remains relatively large because of the correlations among the fit parameters and the uncertainties inherent to the extrapolation

Tang, X. D.

Rehm, K. E.

Brune, C. R.

Greene, J. P.

Hecht, A. A.

Janssens, R. V. F.

Jiang, C. L.

Moore, E. F.

Pardo, R. C.

Schiffer, J. P.

Segel, R. E.

Wuosmaa, A. H.

NEUROSURGERY ENTHUSIASTIC WOMEN SOCIETY

New Determination of the Astrophysical S Factor S E 1 of the C 12 ( α , γ ) O 16 Reaction

https://cdr.lib.unc.edu/downloads/cz30q364d

New determination of the astrophysical S factor SE1 of the C12(α,γ)O16 reaction

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NEUROSURGERY ENTHUSIASTIC WOMEN SOCIETY