Couder, M

Couture, A

Görres, J

Lee, H Y

Stech, E

Strandberg, E

Tan, W

Ugalde, C

Wiescher, M

CERN Document Server

PoS(NIC-IX)082Closing the Cold CNO Cycle: A New Measurementof 19F(p,γ)Aaron Couture∗Los Alamos National LaboratoryE-mail: acouture@lanl.govManoël CouderUniversity of Notre DameE-mail: mcouder@nd.eduJoachim GörresUniversity of Notre DameE-mail: jgoerres@nd.eduHye Young LeeUniversity of Notre DameE-mail: hlee2@nd.eduEdward StechUniversity of Notre DameE-mail: estech@nd.eduElizabeth StrandbergUniversity of Notre DameE-mail: emcnassa@nd.eduWanpeng TanUniversity of Notre DameE-mail: wtan@nd.eduClaudio UgaldeUniversity of North Carolina, Chapel HillE-mail: cugalde@unc.eduMichael WiescherUniversity of Notre DameE-mail: wiescher.1@nd.educ© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC-IX)082The cold CNO cycle is the primary energy source for main sequence hydrogen burning in inter-mediate to heavy mass stars (M >1.5 M). The 19F(p,γ) reaction provides the only breakout pathduring quiescent hydrogen burning, and the cyclic nature of the process enhances small break-out components. The sign of the interference components provided the largest uncertainty inthe reaction rate since no measurements had been sufficiently sensitive to measure the reactionoff-resonance. A new detection technique developed at the University of Notre Dame allowed ameasurement of the 19F(p,γ) and 19F(p,αiγ) reactions over an energy range of Ep=200-800 keV.This allowed a new measurement of the resonance parameters as well as a first measurement ofthe signs of the interference terms. Partial widths and resonance strengths are reported for theresonances in this region.International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IXJune 25-30 2006CERN∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.PoS(NIC-IX)082Closing the Cold CNO Cycle Aaron Couture1. IntroductionThe cold CNO cycle is a catalytic hydrogen burning mechanism, using carbon, nitrogen, andoxygen seed material in the hydrogen burning core of a main-sequence star. In order for the CNOcycle to compete with the primary p-p chain reactions, the temperatures must exceed 20 MK,corresponding to stars of 1.5 M or greater mass [1]. Since the reactions involved in the cold CNOcycle, illustrated in figure 1, are taking place at relatively low temperature, no nucleosynthesisbeyond the conversion of hydrogen to helium is possible. The CNO material comes solely fromthe proto-stellar material that condensed to form the star. The only reaction which can potentiallyremove the catalytic material from the cycle is 19F(p,γ)20Ne [2]. This reaction was expected to berather weak compared to the the 19F(p,α)16O reaction, so that the CNO material would be recycledback to 16O, with no material breaking out to 20Ne.Unfortunately, the strength of the gamma-producing 19F(p,αγ) reactions made measurementsof the 19F(p,γ) cross-section quite difficult. Previous measurements relied on detecting the >11 MeVprimary transition to the first excited state of 20Ne [3 – 7]. Most of these prior measurements reliedon low resolution and relatively small volume NaI(Tl) detectors. This made all of the measure-ments very susceptible to mis-identification of pile-up from the 6.1 MeV gamma-rays from thecompeting α−γ reaction as the energy resolution was insufficient to separate the two components.Subotic´ [7] was the only measurement which employed a high-resolution Ge(Li) detector, thoughdue to the low detection efficiency for such high energy gamma-rays, the measurement was lim-ited to observing on-resonance components [7]. Unsurprisingly, there was discrepancy betweenthe different measurements. One measurement observed an anomalously high partial width for thebroad resonance at Ecm=567 keV [6]. None of the measurements were sufficiently sensitive todetermine the signs of the interference components which were expected to contribute almost 50%to the reaction rate at stellar temperatures [8, 9].Figure 1: The CNO cycle operating a temperatures below T6=1002PoS(NIC-IX)082Closing the Cold CNO Cycle Aaron Couture2. Experimental Procedure2.1 Beam and Target ProductionAll of the present measurements were made at the University of Notre Dame Nuclear StructureLaboratory. The JN model Van de Graaff accelerator was used to produce proton beams of energiesfrom 200 to 700 keV lab. The KN accelerator was used for the region from 600 to 800 keV, ensuringa broad overlap region to ensure the results were consistent. All of the experiments were carriedout on the same beamline, with only the accelerator providing beam to the target changing.The targets were prepared by evaporating CaF2 powder on to tantalum backings. Since beam-stop targets were used, the charge was collected on target and used to determine the total numberof protons. A biased cold finger was used both to prevent carbon build-up and ensure reliablecharge collection. Degradation of the fluorine was observed in the target and corrections weremade for this effect. Two different target thicknesses were used to confirm the results observed inthe inter-resonance regions. Reference [10] contains further details of the target preparation andthe corrections for target degradation.2.2 Detection ArraySignificant advances have been made in detector technologies since the previous measure-ments had been made, making available both large volume HPGe detectors and NaI(Tl) detectors.To take full advantage of this, a hybrid detection system was designed which could exploit the highefficiency of large volume NaI(Tl) detectors with the high resolution offered by HPGe detectors.Four 8” diameter and 6” tall cylindrical NaI(Tl) detectors were used together with a 150% HPGeclover detector. The detectors were arranged as is shown in figure 2. The HPGe detector was posi-tioned directly in front of the target holder, mounted at 0◦ in order to cover the largest possible solidangle. The segmentation of the HPGe clover allowed an instantaneous count rate in each crystal ofas much as 15 kHz. The four NaI(Tl) detectors were mounted at backward angles and rotated 45◦in φ and θ in order to offer an improved solid angle coverage and to maximize the detector depthas seen from the target.The idea of the setup was to observe the 1.63 MeV transition from the first excited state to theground state in the HPGe detector while observing the primary transition in the NaI(Tl) detectors.A coincidence requirement was placed between the NaI(Tl) detectors and the HPGe crystals. Inthe event of a coincidence, a sum energy gate was placed above 10 MeV. Since the Q-value was12.484 MeV, this gate was high enough to filter out most of the 6-7 MeV lines from the (p,αγ)reaction while keeping the coincidence efficiency fairly insensitive to the incident beam energy.If both conditions were met, then the 1.63 MeV gamma yield was measured in the HPGe clover.By requiring the coincidence in hardware, it was possible to run at rates of 10-12 kHz in theindividual HPGe detectors while still only requiring acquisition at a rate of 4 kHz, keeping the livetime well above 80%. The effectiveness of these gating conditions can be seen in figure 3. Atthe same time, one of every twenty events in the germanium detector was passed through to theacquisition regardless of coincidence so that the (p,αγ) yield could be monitored simultaneouslywith the (p,γ). These two features proved to be essential as it both gave us rapid feedback onthe energy and resonance structure as well as allowed running at high beam currents to see the(p,γ) yield in a reasonable amount of time. The coincidence requirement suppressed the detection3PoS(NIC-IX)082Closing the Cold CNO Cycle Aaron CoutureFigure 2: A schematic of the detector arrayused is shown above. The HPGe detector is onleft, centered and mounted at 0◦ with respectto the beam. The NaI(Tl) detectors are primar-ily at backward angles, rotated to provide maxi-mum solid angle coverage. The beam path wascollinear with the positive z-axis.0 2500 5000 7500 10000 12500Eγ (MeV)100101102103104105Raw Yield 1500 1600 1700100101102103104Figure 3: The suppression of the (p,αγ) compo-nents by the array is shown above. In red (light)is shown the yield seen by the HPGe clover withno cuts or conditions. In black (dark) is the yieldafter the multiplicity and Q-value cuts. The insetshows the region surrounding the 1.63 MeV firstexcited state decay of 20Ne in greater detail. Asuppression of close to 104 was achieved.of transitions directly to the ground state as these transitions only exhibit a single gamma ray.Previous measurements had not observed any significant yield from ground state transitions. Inorder to monitor any unexpected yield in this channel, signals in the HPGe detector with greaterthan 8 MeV of energy were also recorded. The acquisition was designed to allow the separationof events from the different trigger types (coincidence, singles, and high energy) when the datawas analyzed. No significant yield was seen in the HPGe detector above ≈11.5 MeV, an energycorresponding to a transition from the resonant state to the first excited state. Signals above 12 MeVwould have indicated that there was significant decay directly to the ground state which wouldwarrant closer inspection.3. Analysis and ResultsExperimental yield curves were measured for the (p,γ1), (p,α2), (p,α3), and (p,α4) channels.The (p,γ2) was also monitored, but no significant yield was observed outside of the Ecm=633keV resonance. The resonance energies, spins, and parities are well known [11], so no effort wasmade to measure them. The yield curves were fit with a set of Breit-Wigner resonance shapes.Interference terms were calculated for the resonances. The total yield was calculated byY = ∑i(∫ EE−χσi(E)ε(E)dE)+ ∑i< j(δ (Jpii ,Jpij )∫ EE−χ±2(σiσ j)12ε(E)(cos(ϕRi −ϕR j)dE). (3.1)The sum is over resonances i, the cross-sections were Breit-Wigner cross-sections [12], χ is thetarget thickness in terms of energy, ε(E) is the energy loss of the beam, Jpii is the spin and parity of4PoS(NIC-IX)082Closing the Cold CNO Cycle Aaron Couture10-710-610-510-410-310-210-1100Yield19F(p,γ1) DataFit to DataFit w/o InterferenceThin Target Yield10-710-610-510-410-310-210-1100YieldThick Target Yield200 300 400 500 600 700 800Ecm (keV)0.1110Ytheo/Yexp350 400 450 500Ecm (keV)0.1110Ytheo/YexpFigure 4: The fit of the (p,γ1) data is shown above.The agreement between the data and the fit is quitegood. On the panel on the right is shown the fit to thethick target data. Note the enhanced sensitivity to theinterference component. The ratio of the calculatedyield to the observed yield is shown at the bottom.10-310-210-1100101Yield19F(p,α2) DataFit to DataFit w/o InterferenceThin Target Yield10-210-1100101YieldThick Target Yield200 300 400 500 600 700 800Ecm (keV)0.1110Ytheo/Yexp350 400 450 500Ecm (keV)0.1110Ytheo/YexpFigure 5: The fit of the (p,α2) data is shown above.The acquisition method allowed the α channel datato be taken simultaneously with the γ channel datawhich was of significant advantage in the fitting ofthe data and improved systematics. Again, the thicktarget yield and fit are shown in the panel on the right.the resonant state, and ϕR is the resonance phase shift [13]. For total yield calculations, only termswith the same Jpi exhibit interference effects; the δ function performs this selection. It is worthnoting that since the phase shift is defined as an arctan function, care must be taken in choosingthe appropriate range so that the cosine function behaves properly. The numerical integration wascompleted with the dgauss routine from the cernlib package [14]. The resonance parameterswere allowed to vary freely and minimized with the minuit minimization package [15]. The sign ofthe interference term, which has to be determined experimentally, was changed manually in orderto find the optimal fit. All four reaction channels were fit simultaneously. The total width wascalculated from the partial widths and thus, varied, though not independently from the physicalconstraints. A background state to simulate the contributions of all higher lying resonances wasincluded, though it only contributed significantly in the (p,γ) yield. Fits to two of the yield curvescan be seen in figures 4 and 5. As can be seen, the yield is well reproduced. The resonanceparameters are given in table 1. While previous measurements were generally normalized to thestrength of 669 keV resonance, in this measurement the absolute efficiency of the HPGe clover wasdetermined both by use of a 60Co source as well as looking at decays from the 12.239 MeV state of28Si populated via the 27Al(p,γ) reaction. The resonance strength was taken from ref. [16], whilethe branching ratios were taken from ref. [17]. The state was chosen because the energies of thegamma-decays were similar to the gamma-ray energies from the decay of the states in 20Ne∗ and16O∗. Summing corrections were accounted for in the determination of the absolute efficiency [10].The determination of the coincidence efficiency was made by comparing the singles yield to thecoincidence yield for the 669 keV resonance in 19F(p,γ). GEANT [18] simulations were done todetermine any change in coincidence efficiency due to the incident proton energy. The correctiondue to proton energy was less than 5% at lowest proton energies.5PoS(NIC-IX)082Closing the Cold CNO Cycle Aaron CoutureAuthor Energy, cm (keV)Quantity 212 323 460 567 636Jpi [11] 2− 1+ 1+ 2− 1+ThisMeasurementΓp (eV) 0.011 35.9 12.2 44.5 6540Γγ1 (eV) n.o. 0.096 0.248 n.o. 1.28Γα2 (eV) 861 1960 753 35800 71.2Γ (keV) 870 2060 1060 36100 6680(ωγ)γ1 (meV) n.o. 1.25×10−3 2.14×10−3 n.o. 940σR (µb) n.o. 0.51 1.2 n.o. 610Subotic´ [7]Γγ1 (eV) — 0.80 0.59 3.93 —(ωγ)γ1 (meV) — 0.01 0.0025 0.0053 —Berkes [6]Γp (eV) ≈ 0.43 — 4.8 — —Γγ1 (eV) < 0.06 0.27 0.40 11.4 —Γα2 (eV) 950 — 670 — —Sinclair [3]σR (µb) — — — — 480Table 1: Note that all widths and strengths are in center of mass frame. For those entries where “—” islisted, the relevant parameter was not measured by that experiment. Those resonances with a value of “n.o.”for this measurement were not observed. The strength was sufficiently small as to be below the limits ofdetection. It is worth noting that the values observed by Subotic´ and Berkes for the 567 keV resonance werewell within the experimental sensitivity.4. ConclusionsThe new measurements of the resonance parameters and most importantly, the interferenceterms, is quite promising. In order to properly fit the high energy tail of the 633 keV resonance, itwas necessary to include a broad, high energy 1+ state to account for the contribution of all higherlying resonances. Unfortunately, the properties of this background state were not well constrainedby the present data, and while these uncertainties had little effect on the resonance properties inthe region studied, they are sufficient to cause large uncertainties on the S-factor at astrophysicallyinteresting regions (30-120 keV). Measurements at higher energies, particularly of the possible T=1resonance at Ecm=1.35 MeV, [11] are needed to constrain this background term.The measurement technique has shown to be very effective at isolating the 19F(p,γ) reac-tion and is sufficiently sensitive to determine interference terms, even in regions of resonancesin the competing channels. There was no observed strength for the lowest-lying resonance at6PoS(NIC-IX)082Closing the Cold CNO Cycle Aaron CoutureEcm=212 keV. The broad resonance observed most clearly by [6] at Ecm=567 keV is excluded sothat it will not contribute in any significant way to the astrophysical reaction rate. The resonancestrengths for the other resonances were generally smaller than that previously observed, and thenet interference effect at low energies was seen to be destructive. All of these effects suggest thatthe reaction rate will be lower than previously predicted, effectively closing the cold CNO cycle.A final determination of the stellar reaction rate and S-factor will require further measurements athigher energies to constrain the role of the background state needed to properly fit the data.5. AcknowledgmentsThe authors would like to thank Dr. Larry Lamm, Mr. Bradley Mulder, Mr. James Kaiser,and Mr. Jerry Lingle for their technical assistance with the maintenance and operation of theaccelerators at the Nuclear Structure Laboratory at the University of Notre Dame.This work was supported by the National Science Foundation under Grant No.PHY01-40324, the Joint Institute for Nuclear Astrophysics, NSF-PFC, under Grant No.PHY02-16783 and the US Department of Energy under the auspices of Los Alamos NationalSecurity, LLC, DOE contract number DE-AC52-06NA25396.References[1] C. Rolfs, W. Rodney. Cauldrons in the Cosmos. University of Chicago Press, Chicago (1988).[2] M. Wiescher, J. Görres, H. Schatz. Break-out reactions from the CNO cycles. J. Phys. G: Nucl. Part.Phys. 25 (1999) R133.[3] R. M. Sinclair. Gamma Radiation from Certain Nuclear Reactions. Phys. Rev. 93 (1954) 1082.[4] G. K. Farney, H. H. Given, B. D. Kern, H. T. M. High Energy Gamma Rays from the ProtonBombardment of Fluorine. Physical Review 97 (1955) 720.[5] L. Keszthelyi, I. Berkes, I. Demeter, I. Fodor. Resonances in 19F+p Reactions at 224 and 340 keVProton Energies. Nucl. Phys. 29 (1962) 241.[6] I. Berkes, I. Dézsi, I. Fodor, L. Keszthelyi. The Resonances at 483 and 597 keV Proton Energies inF19 + p Reactions. Nucl. Phys. 43 (1963) 103.[7] K. M. Subotic´, R. Ostojic´, B. Z. Stepancˇic´. Study of the 19F(p,γ)20Ne Radiative Capture Reactionfrom 0.2-1.2 MeV. Nuclear Physics A 331 (1979) 491.[8] G. R. Caughlan, W. A. Fowler. Thermonuclear Reaction Rates V. Atomic Data and Nuclear DataTables 40 (1988) 283.[9] C. Angulo, M. Arnould, M. Rayet, P. Descouvemont, C. Leclercq-Willain, et al. A compilation ofcharged-particle induced thermonuclear reaction rates. Nuclear Physics A 656 (1999) 3.[10] A. Couture. 19F(p,γ)20Ne and the Stellar CNO Burning Cycle. Ph.D. thesis, University of NotreDame (2005).[11] F. Ajzenberg-Selove. Energy levels of light nuclei A = 18-20. Nucl. Phys. A 475 (1987) 1.[12] H. E. Gove. Resonance Reactions, Experimental. In P. M. Endt, M. Demeur, eds., Nuclear Reactions,vol. 1, chap. 6. Interscience (1959), 259–317.[13] A. J. Ferguson. Angular Correlation Methods in Gamma-Ray Spectroscopy. North-Holland (1965).7PoS(NIC-IX)082Closing the Cold CNO Cycle Aaron Couture[14] CERN Development Team (2003).[15] Application Software Group. MINUIT–Function Minimization and Error Analysis. CERN, Geneva,Switzerland (1998). CERN Program Library Long Writeup D506.[16] P. B. Lyons, J. W. Toevs, D. G. Sargood. Total Yield Measurements in 27Al(p,γ)28Si. Nucl. Phys. A130 (1969) 1.[17] M. A. Meyer, I. Venter, D. Reitmanm. Energy Levels of 28Si. Nucl. Phys. A 250 (1975) 235.[18] Application Software Group. GEANT – Detector Description and Simulation Tool. CERN, Geneva,Switzerland (1993). CERN Program Library Long Writeup W5013.8

Closing the Cold CNO Cycle: A new measurement of $^{19}F(p,\gamma)$

Closing the Cold CNO Cycle: A new measurement of $^{19}F(p,\gamma)$

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Closing the Cold CNO Cycle: A new measurement of 19F(p,γ)^{19}F(p,\gamma)19F(p,γ)

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Closing the Cold CNO Cycle: A new measurement of $^{19}F(p,\gamma)$