One possible source of neutrons for the astrophysical s process is the Ne22(n)25Mg reaction. However, the number of neutrons available to synthesize heavy elements may be limited by the rate of the Ne22(n,)23Ne reaction. To aid in the interpretation of recent (n,) measurements, we have used the Ne22(d,p)23Ne reaction to investigate the single-particle structure of states located near the Ne22+n threshold. No states that correspond to astrophysically significant resonances were found, and so the (n,) rate is effectively determined by direct capture

Champagne, A.E.

Howard, A.J.

Mao, Z.Q.

Carolina Digital Repository

PHYSICAL REVIEW C VOLUME 42, NUMBER 6Ne(d, p) Ne reaction and neutron balance in the s processDECEMBER 1990A. E. Champagne*Department ofPhysics, Princeton Uniuersity, Princeton, Pew Jersey 08544A. J. HowardDepartment of Physics, Trinity College, Hartford, Connecticut 06106Z. Q. MaoDepartment ofPhysics and Atmospheric Science, Drexe! Uniuersity, Philadelphia, Pennsylvania 19104(Received 19 July 1990)One possible source of neutrons for the astrophysical s process is the "Ne(a, n)"Mg reaction.However, the number of neutrons available to synthesize heavy elements may be limited by the rateof the "Ne(n, y)"Ne reaction. To aid in the interpretation of recent (n, y) measurements, we haveused the Ne(d, p) 'Ne reaction to investigate the single-particle structure of states located near the"Ne+n threshold. No states that correspond to astrophysically significant resonances were found,and so the (n, y) rate is effectively determined by direct capture.Neutron-capture nucleosynthesis via the s process isbelieved to occur during core He burning in massivestars' and/or in the He-burning shells of asymptotic-giant-branch (AGB) stars. ' In the core-burning andlow-mass AGB's (M(2Mo) scenarios, the ' C(ct, n)' 0reaction is thought to be the major source of neutrons,but in intermediate-mass AGB's (M=2 —10M&), theNe(a, n) Mg reaction, operating at kT=20 30 keV, —has been postulated as the primary neutron source (seeRef. 3 and references therein). A major unresolved ques-tion is the extent to which the Ne(n, y) Ne reactionserves to limit the amounts of neutrons and Ne avail-able to catalyze the synthesis of heavy elements.Whereas early calculations which included the effectof neutron capture by Ne used an estimate of o.~(k T=30 ke V ) =0.05 mb for the Maxwellian-averagedcapture cross section (evaluated at the canonical sprocess energy), a subsequent measurement indicated avalue in the range 0.2 mb~o~~1. 6 mb. Such a largeNe+n cross section would cause a substantial reductionin the s-process yield of heavy elements. However, arecent remeasurement' of this capture cross section indi-cates a reduced magnitude (by a factor =6) in the range0.04 mb(o. &0.27 mb and thus implies a reducedinhuence on the s-process yield. Using a neutron beamwith an approximately Maxwellian energy profile, Beeret al. " have made a direct measurement of the averagecapture cross section at kT =25 keV, yielding an extrapo-lated value of o (30 keV)=0. 060+0.005 rnb. The mag-nitude of this cross section is near what would be expect-ed for formation via a pure direct-capture (DC) process.The latter two results' '" are consistent with one anotherand with the previous theoretical estimate for energiesnear kT=30 keV.The (n, y) cross section presented in Ref. 10 was ob-tained by subtracting a substantial background from theraw data, and there may have been some systematiceffects associated with that procedure. In contrast, thetechnique employed in Ref. 11 (activation followed bymeasurement of residual nuclei) is comparatively free ofsystematic bias. However, in view of the importance ofthis cross section to models of the s process, an indepen-dent confirmation of these results is warranted. In thepresent work, we have used the Ne(d, p ) Ne neutron-transfer reaction to investigate the properties of states lo-cated near the Ne+ n threshold. Here neutron-captureresonances would manifest themselves as single-particlestates, and their neutron spectroscopic factors C S arelinearly related to the neutron width of the resonance, asdiscussed below. Note that the magnitude of the crosssection in Ref. 11 implies that there should be no astro-physically significant resonances located in the vicinity of25 keV above the neutron-capture threshold [Q~„=5.200(2) keV (Ref. 12)].The Ne(d, p) Ne reaction was induced using a 24. 1-MeV deuteron beam provided by the Princeton AVF cy-clotron. The target was prepared' by ion implantationinto a 40-ling/cm carbon foil. This produced a stable tar-get with a Ne areal density of -6 pg/cm . Outgoingprotons were detected in the focal plane of a QDDDspectrometer at angles 10'(8&,b ~ 20' in 2. 5' steps and at8&,b=25', 30', and 35'. A spectrum collected at 8&,b=15'is shown in Fig. 1. Excitation energies were determinedfrom a calibration based upon the energies of well-knownstates (tabulated in Ref. 14) populated in the ' O(d, p)' 0reaction. The resulting Ne energies are in good agree-ment with prior results. ' ' The angular-distributiondata were analyzed using the distorted-wave Born ap-proxitnation (DWBA) code DwUcK4 (Ref. 16) withoptical-model parameters appropriate for this mass andenergy region. ' ' An absolute cross-section scale wasestablished by normalizing the relative spectroscopic fac-tors measured for several low-lying bound states to tabu-lated' values. The overall uncertainty in the absolute2730 1990 The American Physical Society42 BRIEF REPORTS 2731)0422Ne(d p)23Ne Ed=24. 1 MeVlab=155220- keV STATE= 3=2103-00102 .I400+) +,LAI600LA+IQlOJC)LA+Vi ~LO +I800Z'.OOCt10—01-CHANNELFIG. 1. Spectrum from the 'Ne(d, p)"Ne reaction. The ex-citation energies (in keV) are from the present work. The stateslabeled with parentheses are at the limit of detectability, but areobserved at more than one angle and shift kinematically aswould be expected for Ne. The neutron-capture threshold islocated at E„=5200keV.I10I I I I I15 20 25 30 35FIG. 2. Angular distribution and associated DWBA fits for1„=2and 3 for the 5220-keV state. The intercomparisons estab-lish 1„=3for this case.cross sections obtained by this procedure is +30%.In this mass and energy region, the (d,p) reactionpredominantly populates states formed by angularmomentum transfers l„~3 (in a first-order direct reac-tion) and is therefore less selective than the (n, y) reac-tion which, because of barrier-penetrability considera-tions, is dominated by any l„=0 and 1 resonances at lowenergy. Thus in regions of high level density, it can bedifficult to make a direct comparison between (d,p) and(n, y) data. ' However, in the present case, the leveldensity is low enough so that a direct correspondence canbe made with existing ( n, y ) data: The lowest tabulated'(n, y) resonance is a p-wave resonance at E, =262keV, which corresponds to the state observed atE„=5462 keV. Its (d,p) angular distribution clearly in-dicates 1„=1transfer. Only one state below this level isstrongly populated: that at an excitation energyE„=5220 keV. Its angular distribution (Fig. 2) is charac-teristic of /„=3 angular momentum transfer. The 1„=2DWBA calculation, also shown in Fig. 2, clearly does notfit the data. Therefore, this state corresponds to an fwave resonance at E, =20+6 keV. However, thestrong centrifugal barrier implies that this resonance isquite weak, as is demonstrated below. No other states inthis energy region show discernible single-particle struc-ture in their angular distributions.The Maxwellian-averaged capture cross section for anarrow resonance can be written as4 3~~ (2J+ 1) I „I" E Eykz. —k~(E ) 2 I (kT )where k(E, ) is the neutron wave number evaluated atthe resonance energy E, , J„ is the spin of the reso-nance, and I „,I, and I are, respectively, the neutronand gamma-ray partial widths and the total width of theresonance. The neutron width can be calculated from theusual prescriptionI „=2y, C SPI(E, ),where y, is a single-particle reduced width, C S is theneutron spectroscopic factor, and P, (E, ) is the neu-tron penetrability. The spectroscopic factor can be ob-tained from the relationship' between the measured(d,p) cross section o,„„and the DWBA prediction crow:o,„=1.53C So.Dw .For the 20-keV resonance, y, was calculated by findingthe wave function for a neutron scattered from a realistic,diffuse nuclear potential, and C S=0.04 was obtainedfrom the angular-distribution data (assuming J"=—', ).The resulting neutron width is I „=6X 10 eV. The un-certainty in the resonance energy results in an uncertain-ty in the neutron width of about a factor of 2.The magnitude of the measured thermal cross sectioncr,h =45 5+6.0 mb .(Ref. 21) is consistent with pure directcapture (DC). Thus the DC component of the capturecross section at kT =30 keV can be obtained from a 1/vextrapolation of cr,h with the result that o ~ (30keV)DC=4. 2X10 mb. The total cross section is thesum of DC and resonant parts, but the resonance contri-bution appears to be negligible: Assuming that I ~ && I „,2732 BRIEF REPORTS 42the capture cross section for the 20-keV resonace is5X10 mb, a factor of 84 smaller than the DC crosssection. Similarly, the tail of the next known resonance(at 262 keV) contributes 7X10 mb to the total capturecross section. Note that this resonance is slightlystronger at kT=30 keV than the 20-keV resonance eventhough it is located well above threshold. This situationis a rather dramatic consequence of penetration throughthe centrifugal barrier, which inhibits formation of thelow-energy f-wave resonance (20 keV} as compared tothe higher-energy p-wave resonance (262 keV}. It is pos-sible that intermediate states, corresponding to astro-physically significant s- or p-wave resonance, could es-cape our detection if weakly populated in the (d,p) reac-tion (in this case, if C S ~ 3 X 10 ). However, no statist-ically significant evidence for any such state exists in the(n, y) data of Winters and Macklin. 'The lack of any threshold states which are character-ized by low angular momentum transfer and measurablesingle-particle strength implies that the dominant contri-bution to the Ne(n, y) Ne reaction is most probablyDC at stellar energies. Therefore, the low-energy crosssection should display a 1/u velocity dependence. Thisconclusion is consistent with the allowed range for a r (30keV) in Ref. 10 and is in qualitative agreement with themeasurement of o (25 keV} by Beer et al. " In conse-quence, it does not appear that the Ne(n, y)2 Ne reac-tion will cause a significant reduction in the number ofneutrons available for the s process.This work was supported in part by the National Sci-ence Foundation and in part by the U.S. Department ofEnergy.'Present address: Department of Physics and Astronomy, Uni-versity of North Carolina at Chapel Hill, Chapel Hill, NC27599.W. D. Arnett and F.-K. Thielemann, Astrophys. J. 295, 589(1985).N. Prantzos, M. Arnould, and J.-P. Arcoragi, Astrophys. J.315, 209 (1987).3I. Iben, Jr. and A. Renzini, Phys. Rep. 105, 331 (1984).4W. M. Howard, G. J. Mathews, K. Takahashi, and R. A.Ward, Astrophys. J. 309, 633 (1986).5S. A. Lamb, M. W. Howard, J. W. Truran, and I. Iben, Jr., As-trophys. J. 217, 213 (1977).6J. Almeida and F. Kappeler, Astrophys. J. 265, 417 (1983).7M. Busso and R. Gallino, Astron. Astrophys. 151,205 (1985).M. Busso, G. Picchio, R. Gallino, and A. Chic%, Astrophys. J.326, 196 (1988).N. Langer, J.-P. Arcoragi, and M. Arnould, Astron. Astro-phys. 210, 187 (1989).' R. R. Winters and R. L. Macklin, Astrophys. J. 329, 943(1988).H. Beer, G. Rupp, F. Voss, and F. Kappeler, in Proceedingsof the 5th Ringberg Workshop on Nuclear Astrophysics, edit-ed by W. Hillebrandt and E. Muller [Max-Planck Institut Re-port No. MPA/PI, 1989 (unpublished}].' P. M. Endt and C. van der Leun, Nucl. Phys. A310, 1 (1978).' M. S. Smith, Ph.D. thesis, Yale University, 1990 (unpub-lished).F. Ajzenberg-Selove, Nucl. Phys. A475, 1 (1987).' P. Desgrolard, B.Chambon, D. Drain, C. Pastor, and A. Dau-chy, Phys. Rev. C 12, 1711 (1975).' P. D. Kunz, University of Colorado report, 1974 (unpub-lished).' A. J. Howard, J. G. Pronko, and C. A. Whitten, Nucl. Phys.A152, 317 (1970).' E. Fabrici, S. Micheletti, M. Pignanelli, F. G. Resmini, R.DeLeo, G. D'Erasmo, A. Pantaleo, J. L. Escudie', and A.Tarrats, Phys. Rev. C 21, 830 (1980).' R. Liljestrand, J. McIntyre, G. Blanpied, J. Lynch, L. Ray, W.R. Coker, and G. W. Hoffmann, Phys. Rev. C 11, 1570 (1975).G. Blanpied, R. Liljestrand, J. Lynch, J. McIntyre, L. Ray,and G. W. Hoffmann, Phys. Rev. C 12, 1726 (1975).S. F. Mughabghab, M. Divadeenam, and N. E. Holden, Neu-tron Cross Sections (Academic, New York, 1981),Vol. 1.J. M. Blatt and V. F. Weisskopf, Theoretical Nuclear Physics(Wiley, New York, 1962).z3D. D. Clayton, Principles of Stellar Evolution and Nucleosynthesis (University of Chicago Press, Chicago, 1983).

Ne22(d,p)23Ne reaction and neutron balance in the s process

https://cdr.lib.unc.edu/downloads/6682xf34w

Ne22(d,p)23Ne reaction and neutron balance in the s process

Abstract

Similar works

Full text

Available Versions

Carolina Digital Repository