There are a number of astrophysical environments in which the path of nucleosynthesis proceeds through proton-rich nuclei. Radioactive nuclei have traditionally not been available as beams, and thus proton-capture reactions on these nuclei could only be studied indirectly. At the Holifield Radioactive Ion Beam Facility (HRIBF), some of the first direct measurements of (p,γ) cross sections on radioactive beams have been made. The Daresbury Recoil Separator (DRS) has been used to separate the recoils of interest from the unreacted primary beam and identify them in an isobutane-filled ionization counter. Data from 17F(p,γ)18Ne and 7Be(p,γ)8B measurements are presented

Bardayan, D.W.

Blackmon, J.C.

Chae, K.Y.

Champagne, A.E.

Chipps, K.A.

Fitzgerald, R.P.

Greife, U.

Hatarik, R.

Kozub, R.L.

Matei, C.

Moazen, B.H.

Nesaraja, C.D.

Pain, S.D.

Peters, W.A.

Pittman, S.T.

Shrine Jr., J.F.

Smith, M.S.

Carolina Digital Repository

PoS(NIC XI)202Direct measurments of (p,γ) cross sections atastrophysical energies using radioactive beams andthe Daresbury Recoil SeparatorD. W. Bardayan∗a, K. A. Chippsb, R. P. Fitzgeraldc, J. C. Blackmona,d, K. Y. Chaee,A. E. Champagnec, U. Greifeb, R. Hatarik f , R. L. Kozubg, C. Mateih, B. H. Moazene,C. D. Nesarajaa, S. D. Paina, f , W. A. Peters f , S. T. Pittmane, J. F. Shriner, Jr.g, andM. S. SmithaaPhysics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831bDepartment of Physics, Colorado School of Mines, Golden, CO 80401cDepartment of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 37599dDepartment of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803eDepartment of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996f Department of Physics and Astronomy, Rutgers University, New Brunswick, NJ 08903gPhysics Department, Tennessee Technological University, Cookeville, TN 38505hOak Ridge Associated Universities, Oak Ridge, TN 37830E-mail: bardayandw@ornl.govThere are a number of astrophysical environments in which the path of nucleosynthesis proceedsthrough proton-rich nuclei. Radioactive nuclei have traditionally not been available as beams, andthus proton-capture reactions on these nuclei could only bestudied indirectly. At the HolifieldRadioactive Ion Beam Facility (HRIBF), some of the first direct measurements of (p,γ) crosssections on radioactive beams have been made. The DaresburyRecoil Separator (DRS) has beenused to separate the recoils of interest from the unreacted primary beam and identify them in anisobutane-filled ionization counter. Data from17F(p,γ)18Ne and7Be(p,γ)8B measurements arepresented.11th Symposium on Nuclei in the Cosmos, NIC XIJuly 19-23, 2010Heidelberg, Germany∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licen. http://pos.sissa.it/PoS(NIC XI)202Direct Measurements of (p,γ) cross sections D. W. Bardayan1. IntroductionIn novae and x-ray bursts, hydrogen fuel is burned through a series of proton-capture reactionson stable and radioactive nuclei followed by subsequentβ decays [1]. The nucleosynthesis can leadto proton-rich nuclei, many of which have scarcely been studied in the laboratory. Now, however,with the development of intense reaccelerated beams of radioactive nuclei, it is possible to makedirect measurements of proton-capture cross sections far from stability and thus calibrate this flowto heavier elements. At the HRIBF [2], some of the first measurements with radioactive beamshave been made.Proton-capture reactions are studied at the HRIBF by bombarding a windowless hydrogen gastarget [3] with intense radioactive beams. The recoil products from (p,γ) reactions are separatedfrom the primary beam with the DRS [4]. The DRS has two large 1.5-m-long Wien Filters withadjustable slits that are used to intercept the primary beamafter it has been steered off axis (see Fig.1). A final A/q focus is obtained at the focal plane of the DRS where a segmented, isobutane-filledgas ionization counter is located. Comparison of the differential energy loss with the total energyallows for the atomic number to be determined for the detected ions. Scattered primary beam that“leaks” through the DRS can therefore be distinguished fromthe (p,γ) reaction products, whichare counted to determine the cross sections for proton capture.Figure 1: The Daresbury Recoil Separator is used for studying proton-capture reactions on exotic nuclei.When tuned for capture recoils, the primary beam is intercepted by moveable slits after the first Wien filter.The recoils of interest are identified and counted by an ionization counter at the DRS focal plane.2. The 17F(p,γ)18Ne ReactionThe17F(p,γ)18Ne reaction is important in a number of astrophysical enviroments. In novae,the 17F(p,γ)18Ne reaction controls the flow through the sequence16O(p,γ)17F(p,γ)18Ne(β+)18Fleading to the production of the radioisotope18F, which is a target ofγ-ray astronomy [5]. If,however, theβ -decay of17F is faster than the proton-capture rate, the production of odd-massisotopes15N and17O is increased. These isotopes are thought to be primarily made in novae andthus their abundances are good indicators of the conditionspre ent in the explosion [6]. In x-ray bursts, it was found [7] that the energy generation rate during the preburst phase is stronglyaffected by the rate of this reaction. Heavy element production is later initiated by theα p chain14O(α , p)17F(p,γ)18Ne(α , p)21Na..., which, in turn, depends on the rates of reactions involved.2PoS(NIC XI)202Direct Measurements of (p,γ) cross sections D. W. BardayanFigure 2: The ion counter∆E vs. E spectrum obtained for the 3+ 17F(p,γ)18Ne resonance. Recoil18Neions are circled.The 17F(p,γ)18Ne reaction rate is thought to be dominated by direct capturebelow 0.3 GKand a single 3+ resonance atEc.m. = 600 keV at higher temperatures. The resonance energy andand proton width have been precisely measured in a1H(17F,p)17F study [8], but the strength of thisimportant resonance had never been directly measured. Since theγ width is much smaller than theproton width for this resonance, the resonance strength is directly proportional to theγ width. Asonly estimates existed for the strength of the 3+ resonance, the rate of the17F(p,γ)18Ne reactionhas been uncertain by orders of magnitude [9]. The goal of theHRIBF study was to make the firstdirect measurement of the strength of the 3+ 17F(p,γ)18Ne resonance.A mixed 17F/17O beam (17F was typically 35-70 % of the total beam) bombarded the window-less hydrogen gas target operated at 4 Torr. Beam currents upto 2×107 17F/s were obtained. Sincecharge state distributions of18Ne recoils were not known, the DRS was tuned in succession forthethree strongest charge states and the yields integrated to determine the total reaction rate. A17Fbombarding energy of 10.83 MeV was used which, after energy loss in the target, corresponded to acenter-of-mass energy range coverage from 590 - 606 keV. Data obtained on resonance are shownin Fig. 2. Data were also taken off resonance to estimate the background that was present in Fig.2. The resonance strength was extracted from the observed yield by integrating the Breit-Wignerresonance cross section weighted by the stopping cross section over the energy range covered bythe target [10]. The resonance strength obtained in this work is ωγ = 33±14(stat)±17(sys) meV[11]. The calculated17F(p,γ)18Ne reaction rate is larger than previous estimates by a factor of∼ 1.8 at 1.0 GK.3. The 7Be(p,γ)8B ReactionThe 7Be(p,γ)8B reaction is important for understanding the observed flux of solar neutrinosfrom the sun. There have been several studies of the7Be(p,γ)8B reaction by both direct andindirect techniques. Unfortunately, the required precision has not yet been reached as the directmeasurements and the indirect techniques (e.g., Coulomb dissociation) have converged at differentvalues for the astrophysicalS-factor at 0 keV,S17(0) [12]. As described in a review article [13], “Itis essential to have additional7Be(p,γ)8B measurements, to establish a secure basis for assessing3PoS(NIC XI)202Direct Measurements of (p,γ) cross sections D. W. BardayanFigure 3: The ion counter∆E vs. E spectrum obtained for the1H(7Be,γ)8B reaction. Twenty-two eventsfrom 8B recoils were observed.the best estimate and the systematic errors forS17(0).” “Experiments with7Be ion beams would bevaluable. Such experiments would avoid many of the systematic uncertainties that are important ininterpreting measurements of proton capture on a7Be target.”To address these issues, we are beginning a study of the7Be(p,γ)8B cross section at the HRIBF[14]. The goal of our initial measurement was to make the firststatistically-significant measurementof the cross section using a radioactive7Be beam. The measurement was made at a single energysince the energy dependence has been well characterized by previous studies but the absolute valueof the cross section is still uncertain. The radioactive7B material accelerated to make the7Bebeam was produced at the Triangle Universities Nuclear Laboratory (TUNL). A 10-MeV protonbeam bombarded a series of Li metal targets producing7Be via the7Li( p,n)7Be reaction. Thematerial was then loaded into sputter cathodes to be used in amulti-sample sputter ion source [15]at ORNL. Beam currents peaked at about 2×107 7Be/s with typical beam compositions of7Li/7Be∼ 7/1 [14].The 12-MeV beam was used to bombard the windowless hydrogen gas target run at a pressureof 5 Torr. The DRS was tuned for8B recoils of charge state q=+5 (> 90% charge state fraction)and data were accumulated for approximately 4 days. The resulting ion counter spectrum is shownin Fig. 3. Twenty-two8B events were observed during this time as shown encircled inFig. 3.The amount of beam that bombarded the gas target was determined from the number of scatteredprotons observed in the±45◦ silicon detectors. The ratio of scattered protons to impinged beamparticles was measured by implanting the beam into metal plates while counting scattered protonswith the detectors. The plate was then removed and the amountof 7Be determined from aγ-rayspectrum taken with a calibrated Ge detector. These samplings were performed at regular intervalsto account for long-term changes in the beam purity, none of which were observed.Putting this information together, the cross section of the7B (p,γ)8B reaction was determinedto be(1.12±0.27)µb at Ec.m.=1.5 MeV [14]. This agrees with the values measured in Ref. [16]with a7Be target. From Ref. [16], the extrapolated value of the cross section from the 1.4 - 1.6 MeVdata points is(0.91±0.03)µb at 1.5 MeV. Using the extrapolation from [16] of our cross section tostellar energies yields anS-factor ofS17(0) = (26.8±6.5) eV-b [14]. The uncertainty in our result4PoS(NIC XI)202Direct Measurements of (p,γ) cross sections D. W. Bardayanis dominated by statistics. To address this, future runs will make use of higher amounts of activityinitially loaded into the sputter cathode to produce higherintensity7Be beams. Arrangements havebeen made for future7Be production to occur at the Institute of Nuclear Research of t e HungarianAcademy of Sciences (ATOMKI) in Debrecen, Hungary. It is hoped that with increased beamintensity, measurements at several energies can be made.4. ConclusionsThe availability of intense radioactive beams is providingopportunities to directly measureproton-capture rates on astrophysically-important nuclei. The Daresbury Recoil Separator has beeninstalled at the HRIBF and complemented with a windowless hydrogen gas target to perform suchmeasurements. The first direct measurement of the17F(p,γ)18Ne reaction has been made, alongwith the first statistically-significant measurement with a7Be beam of the7Be(p,γ)8B cross section.This measurement, however, is not yet statistically competitiv with 7Be target experiments andthus work is ongoing to improve the intensity of the7Be beam at HRIBF.This research was sponsored in part by the Office of Nuclear Physics, U.S. Department ofEnergy (DOE) and by the National Nuclear Security Administration under the Stewardship ScienceAcademic Alliances program through DOE Cooperative Agreemnt DE-FG52-08NA28552.References[1] M. S. Smith and K. E. Rehm, Annu. Rev. Nucl. Part. Sci.51, (2001) 91.[2] D. W. Stracener, G. D. Alton, R. L. Auble, J. R. Beene, P. E.Mueller, and J. C. Bilheux, Nucl.Instrum. Methods Phys. Res. A521, (2004) 126.[3] R. Fitzgeraldet al., Nucl. Phys. A748, (2005) 351.[4] A. N. Jameset al., Nucl. Instrum. Methods Phys. Res. A267, (1985) 144.[5] M. Hernanz, J. José, A. Coc, J. Gómez-Gomar, and J. Isern,Astrophys. J. Lett.526, (1999) L97.[6] Y. Chin, C. Henkel, N. Langer, and R. Mauersberger, Astrophys. J. Lett.512, (1999) L143.[7] M. Wiescher, J. Görres, and H. Schatz, J. Phys. G: Nucl. Part hys.25, (1999) R133.[8] D. W. Bardayanet al., Phys. Rev. Lett.83, (1999) 45.[9] C. Iliadis et al., Astrophys. J. Suppl. Ser.142, (2002) 105.[10] C. Rolfs and W. S. Rodney,Cauldrons in the Cosmos, University of Chicago Press (1988).[11] K. A. Chippset al., Phys. Rev. Lett.102, (2009) 152502.[12] R. H. Cyburt, B. Davids, and B. K. Jennings, Phys. Rev. C70, (2004) 045801.[13] E. G. Adelbergeret al., Rev. Mod. Phys.70, (1998) 1265.[14] R. Fitzgerald, Ph. D. Thesis, University of North Carolina at Chapel Hill, (2005).[15] G. D. Alton, G. D. Mills, and J. Dellwo, Rev. Sci. Inst.65, (1994) 2006.[16] A. Junghanset al., Phys. Rev. C68, (2003) 065803.5

Direct measurments of (p,γ) cross sections at astrophysical energies using radioactive beams and the Daresbury Recoil Separator

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Direct measurments of (p,γ) cross sections at astrophysical energies using radioactive beams and the Daresbury Recoil Separator

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