Key reactions in the helium burning phase of stars

Progress in Particle and Nuclear Physics PERGAMON

Progress in Particle and Nuclear Physics 46 (2001) 43--44 http://www.elsevier.nl/loeateJnpe

Key Reactions in the Helium Burning Phase of Stars J. W. HAMMER Institutj~r Strahlenphysik (lfS), Universitat Stuttgart, D-70550 Stuttgart, Germany

Abstract From new experiments with considerably increased sensitivity cross sections and S-factors have been determined for the reactions 12C(v:,~,)lSO and 22Ne(a, n)2SMg.

The reaction 12C(a,7)160 The 12C(a, 7)1e0 reaction is considered as the key reaction in helium burning and caused in the past intensive experimental and theoretical investigations [1] and references herein. The cross section at burning temperatures is of the order of 10-lr b and can only be determined by extrapolations using informations on the interplay of several resonances below and above threshold. Using an array of four large actively shielded Ge detectors and high a - b e a m currents of up to 500/~A (DYNAMITRON) we have measured 15 angular distributions of 7 rays of 12C(a,7)leO in the energy range Ec.m. = 0.95 to 2.78 MeV. From the 7 angular distributions (9 reap. 12 positions) the S~1- and SE2-factors have been deduced, using all the necessary methods of data analysis as f.i. simulation of the geometry with the code GEANT,fit of the peak form of 7-lines and the proper background determination. Applying the Rmatrix method, our new data set has been described together with the data for elastic a-scattering [2, 3] for 1=1 and 3 and the/%delayed a-decay of :aN [4]. The result [5] is shown in Fig. 1. The E1 excitation

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Figure 1: Upper row : Experimental results for SE1 together with the R-matrix fit to the capture data, the elastic scattering data (l=1 and 3) [,$, 3] and the decay spectrum of 16N [4]. Lower row : Experimental results for SE2 together with the R-matrix fit to the capture data and the elastic scattering data (l=$). function has been described by a 3-level-R-matrix fit (ER =-45.1 and 2400keV + 'background'-level) with Ro = 6.5 fro. A 5-level-fit could not improve the description because of the lack of data at energies above 4MeV. The E2-data have been described with a 5-level-R-matrix fit (- 245, 2680, 4320, 5650 keV 0146-6410/01/$ - see front matter © 2001 Published by Elsevier Science BV. PII: S0146-6410(01)00106-5

44

J. W. Hammer / Prog. Part. Nucl. Phys. 46 (2001) 43-44

and the 'background'-level) using the elastic scattering data from literature for 1=2 {2, 3], 7-widths of Tilley-Weller [6] and the resonance parameters for the 2+ resonance from [6]. We obtained the following extrapolation values for S~i° Ss°° and ~qs0o E2 tot : this work [5]:

S~ ° = (76 + 20) keVb',

~2qs°°= (85 4- 30) keVb ;

Stao~ = (161 4- 50) keVb

NACRE

S s°°E]= (79 4- 21) keVb ,"

$3°°~2= (120 4- 60) keVb ;

~a°° = (199 4- 80) keVb

[1]:

New investigation of the reaction 22Ne(a, n)25Mg The reaction ~Ne(c~, n)2SMg is considered as one of the most important neutron sources in the astrophysical s-process. We investigated this reaction in the range from threshold at Ea= 570keV up to 1400 keV using a new actively shielded 41r neutron-detector and the gas target facility RHINOCEROS. The sensitivity was thus increased by more than one order of magnitude compared to the previous determination by Drotleff et al. [7]. The astrophysical reaction rate suffered from a large uncertainty of up to a factor of 70 at T9= 0.2 due to the uncertainty of the cross section near threshold.

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Figure 2: Ezcitation function of ~ Ne(a, n)2S Mg measured with the new experimental set-up. The resonance at Ea = 811~ke V is shown in the inset. For comparison some data of previous measurements have been plotted [7, 8, 9].

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Energy E~ [McV] Fig. 2 shows the new excitation curve from which parameters for all resonances and new upper limits for the low energy part have been obtained [10]. Further experimental details will be given elsewhere. Because of the extended geometry of the experiment very extensive Monte Carlo calculations (MCNP) were necessary to deduce the physical quantities for the astrophysical reaction rate. The resonance parameters are given in Tab. 1. For several resonances the width could be determined because of the better properties of the new detector. Considering the new results the uncertainty of the reaction rate could be reduced by more than a factor of 10 [10]. 832 4-2 r

[keV]

w7 [meV] Table 1:

975 4-2

998 4-2

10534-2 10774-2

12014-2 13604-2 1384 4-2

14344-2

0.424-0.10 3.24-1.0 10.24-3.0 18.64-3.0 2.54-1.0 15.24-2.0

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0.1~8(15) 0.040(6)0.056(12)

97(16)

75(13) 1092(112)

0.52(8) 0.92(14) 9.3(1.3)

1.14-0.2

Resonance parameters for the reaction 22Ne(a, n)2S Mg at low energies (with uncertainties).

References

[5]

[1] C. Angulo et al., Nucl. Phys. A 656 (1999) 3

[6] [4

[2] R. Plaga et al., Nucl. Phys. A465 (1987) 291 [3] M.D. Agostino et aL, Nuovo Cim. 27 (1975) 1

[8] [9]

[4] R.E. Azuma et al., Phys. Rev. C 50 (1994) 1194 [10]

R. Kunz, thesis, Stuttgart (2001) D.R. Tilley et al., Nucl. Phys. A564 (1993) 1 H.W. Drotleff et M., Ap. J. 414 (1993) 735 V. Harms et aL, Phys. Rev. C 43 (1991) 2849 U. Giesen et al., Nucl. Phys. A561 (1993) 95 M. Jaeger, thesis, Stuttgart (2001)