The coulomb dissociation of 8B and the 8B solar neutrino flux

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Nuclear Physics B (Proc. Suppl.) 38 (1995) 77-80

The Coulomb Dissociation of gB and the gB Solar Neutrino Flux Moshe

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"A.W. Wright Nuclear Structure Laboratory, Yale University, New Haven, C T 06511 The rate of the 7Be(p,7)SB nuclear reaction, directly related to the production of a B in the sun, is most uncertain and yield one of the largest uncertainty in the standard solar model estimate of the SB solar neutrino flux. We discuss a first attempt to measure this reaction rate via the Coulomb dissociation method. Our initial results are consistent with the lower value of the cross section, as measured by Filippone et al., as well as allow for an estimate of the p-wave contribution that needs to be corrected for. Our preliminary result suggest a s B solar neutrino flux which is 25-30~0 smaller than predicted by the various standard solar model of BahcaU et al.

1. I n t r o d u c t i o n Light mass stars (with central temperature smaller then 17 MK) in the main sequence like our sun, spend most of their energy generating lifetime burning hydrogen. The 8B solar neutrino's generated in the PPIII chain, are composing: 75% of those detected by Ray Davis' chlorine detector, and 100% of the Kamiokande and the SNO detectors. The flux of SB solar neutrino's is very sensitive to details of the nuclear inputs and in particular to the 7Be(p,7)SB reaction rate at solar conditions, as well as details of the solar model including the central temperature (and opacity). The accepted value of the astrophysical S-factor ( = a x E x e ~ " , ~ = e~ZtZ2/hv) used by Bahcall and Ulrich for the rBe(p,7)SB reaction at zero energy is, Sty(0) = 24.3 eV-barn, and more recently the value adopted by Bahcall and Pinsonneault is 22.4 eV-b [1]. Turck-Chieze et al. [2] adopted the value measured by Filippone of 20.9 eV-b. This smaller value is one of the most significant differences between the various SSM of Turck-Chiez et al. and of Bahcall et al. The value of SIT(0) was studied in details by Barker and Spear [3] and Jhonson et al. [4], that point out to a considerable discrepancy between the data obtained by Filippone et al. [5] and the unpublished data of Kavanagh et al. [6]. The same discrepancy is however, observed in the data ofVaughn et al. [7] and Parker [8], see Fig. 1. *Permanent address: Dept. of Physics, University of Connectlcut, U-46, 2152 Hillside Dr., Storrs, CT 06269-3046. E-m~;l: [email protected], GAIQYaleVM.CIS.Yale.EDU 0920-5632/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0920 5632(94)00736-5

2. U n c e r t a i n t i e s in $17(0) In addition to the discrepancy in the absolute value of the cross section at low energies (25% f o r E < 450 keV, see Fig. 1), we also find considerable uncertainty (=t=7%) in the absolute value of the cross section of the 7Li(d,p)SLi measured on resonance (770keV). The last cross section was used by several authors to normalize the absolute value of the cross section of the 7Be(p,7)SB reaction. The last two uncertainties are experimental in nature and we now consider two major theoretical uncertainties. The value of &7(0) is derived from an extrapulation of laboratory measurements (from 117 keV [5], and 144 keV [6]) to its predicted value at solar energies (20 keV). This extrapulation is peformed in the SSM using the direct capture model. Bahcall et al. [9] show that the extrapulation of the direct capture model is higher by approximately 29% than that of a linear polinomial expansion. The last one is also used in other cases in nuclear astrophysics to extrapulate to stellar conditions. While the d a t a of Kavanagh et al. [6] appear to support the rise toward solar energies, as predicted in the direct capture model, the data of Filippone et al. [5] do not give credence to the predicted rise. Note that some authors [3] have criticised the validity of the direct capture model. Unfortunately an exprimental test of the direct capture model requires data at even lower energies (approximately 50 keV). In addition, the data measured at laboratory energies include contributions from partial

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is very optimistic, see discussion above. In addition the systematic nature of the uncertainty brings into question the meaning of the notion of theoretical uncertainties with N sigma standard deviations, as well the construction of 1,000 solar models where the values of the inputs parameters are randomiy varied over the range allowed by the uncertainties. The importance of the "tBe(p,v)SB reaction calls for a continued interest and additional accurate measurements of the ZBe(p,7)SB reaction, and in particular measurements that can distinguish between the two absolute values of the cross sections, see Fig. 1. We report here on an initial success, at resolving this problem with SB rad i o a c t i v e b e a m s and the use of a new technique involving the Coulomb dissociation of SB. 3. T h e C o u l o m b d i s s o c i a t i o n o f SB

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waves (p and d waves) which are assumed to be hindered at solar energies, where the e-wave contribution is predicted to be dominant. The extrapulation to solar energies requires an estimate of the contributions of the various partial waves at given energies. And we note that the failure of theory to predict the unexpectedly large p-wave

component (at least 40%) in the zLi(p,q,)SBe reaction [10] is of great concern, as it may suggest that our understanding of nuclear structure at very low energies is incomplete. W e emphasize that all four uncertainties discussed above are essentially systematical in nature, and thus it is hard to estimate the overall uncertainty in Siz(0). However it appears that the uncertainty adopted by Johnson et al. [4] (4-11%) and the S S M of Bahcall and Pinsonneault

The Coulomb dissociation [Ii] Primakoff [12] process, is the time reverse of the radiative capture process. The reaction is characterized by the absorption of a virtual photon from the field of a high Z nucleus such as 2°SPb. In this case since ~ for a photon is approximately 1,000 times larger than that of a particle beam, the small cross section is enhanced. The large virtual photon flux (typically 100-1,000 photons per collision) also gives rise to enhancement of the cross section. Our understanding of the Coulomb excitation mechanism and the virtual photon flux allow us (as in the case of electron scattering) to deduce the inverse nuclear process. However in Coulomb dissociation since c~Z approaches unity (unlike the case in electron scattering), higher order Coulomb effects (Coulomb post acceleration) may be non-negligible and they need to be understood [13]. The success of such an experiment hinges on understanding such effects and the choice for kinematical conditions so as to minimize these effects; i.e. distance of closest approach considerably larger then 20 fm (hence very small forward angles scattering) [13]. Measurements must also be carried out at high energies (many tens of MeV/u), so as to maximize the virtual photon flux and reduce the Coulomb post acceleration effect.

M. Gai/Nuclear Physics B (Proc. Suppl.) 38 (1995) 77-80

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the .qzz-factor extracted from the radiative capture work, but not the Sir-factor extracted from the Coulomb dissociation. Hence our data on the Coulomb dissociation of SB [14] is expected to be slightly below direct measurements of the capture rate, as observed in Fig. 2. In fact this allows us to estimate the amount of p-waves contribution to the ZBe(p, 7)SB reaction by comparing for example to the data of Filippone et al.

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An experiment to study the Coulomb dissociation of SB was performed at the Riken radioactive beam facility. Details on the results and procedures of this study of the Rikkyo-RIKEN-YaleTokyo-Tsukuba-LLN collaboration can be found in Ref. [14]. Indeed we demonstrated that the Coulomb disscoiation allowed us to measure the radiative capture Z B e ( p , ~,)SB cross section, and preliminary results are consistent with the absolute value of the cross section measured by Filippone et al. [5] and by Vaughn et al. [7], with a preliminary extracted value of Stz(0) = 16.7 -4- 3.2 e V - b, implying a considerable reduction (25-30%) in the predicted S B solar neutrino flux, as compared to the SSM of Bahcal et al., see above. We note that the Coulomb dissociation process is insensitive to the M1 component of the cross section, since the M1 virtual photon flux is smaller by approximately/~. In this case the 1+ state in SB at E~,= = 632 keV is not expected to be observed, see Fig. 2. Such an M 1 contribution is predicted to be approximately 10% of the zBe(p,'y)SBcross section measured just below 1 MeV, which yield to a 10% correction of

3.1. Is T h e r e E v i d e n c e for an E2 C o m p o nent? The much publicized [15] paper of Langanke and Shoppa (LS) [16], claims that the data analysis performed by the RIKEN collaboration is invalid due to the model dependent prediction of LS of a large E2 component in the CD of SB, which was ignored in our paper [14]. In the following we show that their assertion arise from a misunderstanding of the experimental procedures of the RIKEN experiment. W e first note that I have pointed to LS that they have used and published (before the R I K E N collaboration!) incorrect data, including wrong error bar. Langanke and Shoppa now attempt to correct it in a form of (a revised) Erratum. A m o n g several mistakes, LS appear to have ignored the angular resolution of the Riken experiment. The finite angular resolution implies that the acceptance (or response) of the Riken detector is not the same for E1 and E2. This invalidates the basic assumption of LS that "Assumes the detector efficiency is the same for El and E2 contributions" [16]. As it turns out the angular averaging tends to push the predicted E1 cross section to large angles, where the E2 dominates, and the large E2 predicted by LS appears to be a compensation for their neglect of the angular resolution of the Rikcn experiment. In that sense the entire analysis of LS is misleading and in fact wrong. A search for E2 component in the R I K E N data was performed by Gai and Bertulani [17]. When the experimental resolutions are correctly taken into account, together with the correct RIKEN data (l) the best fit of the angular distributions is obtained with E1 amplitude alone, as shown in

M. Gai/Nuclear Physics" B (Proc. Suppl.) 38 (1995) 7~80

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Fig. 3. Our analysis invalidates the claims of LS and support the analysis presented in Re£ [14].

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4. A c k n o w l e d g e m e n t s I would like to acknowledge the work of Dr. Naohita Iwasa and Professor Tohru Motobayashi that performed the data analysis of the SB experiment at Rikkyo University; the Ph.D. thesis of Dr. lwasa. I would also like to acknowledge discussions and encouragements from Professor Carlos Bertulani, Professor Gerhard Baur, and m y collaborator Professor Thierry Delbar.

REFERENCES . J.N. Bahcall, and R.K. Ulrich, Rev. Mod. Phys. 60(1988)297; and J.N. Bahcall, and M.H. Pinsonneault; Rev. Mod. Phys. 64(1992)885.

2. S. Turck-Chieze et al.; ApJ 335(1988)415; and Sylvaine Turck-Chieze, and Ilidio Lopes; ApJ. 408(1993)347; and S. Turck-Chieze et al.; Phys. Rep. 230(1993)57. 3. F.C. Barker and H.R. Spear; ApJ 307(1986)847. 4. C.W. Johnson, E. Kolbe, S.E. Koonin, and K. Langanke, ApJ 392(1992)320. 5. B.W. Filippone et al.; Phys. Rev. Lett. 50 (1983)412, ibid Phys. Rev. C28(1983)2222. 6. R.W. Kavanagh et al.; Bull. Amer. Phys. Soc. 14(1969)1209. 7. F.J. Vaughn, R.A. Chalmers, D. Kohler, and L.F. Chase Jr; Phys. Rev. C2(1970)1657. 8. P.D. Parker; Phys. Rev. 150(1966)851, ApJ 153(1968)L85. 9. J.N. Bahcall et al; Rev. Mod. Phys. 54(1982)767. I0. R.M. Chastler, H.R. Weller, D.R. Tilley,R.M. Prior; Phys. Rev. Lett. 72(1994)3949. 11. G. Baur, C.A. Bertulani, and H. Rebel; Nucl. Phys. A458(1986)188. 12. H. Primakoff; Phys. Rev. 81(1951)899. 13. C.A. Bertulani; Phys. Rev. C49(1994)2688. 14. T. Motobayashi, N. lwasa, Y. Ando, M. Kurokawa, H. Murakami, 3. Ruan (Gen), S. Shimoura, S. Shirato, N. Inabe, M. Ishihara, T. Kubo, Y. Watanabe, M. Gai, R.H. France III, K.I. Hahn, Z. Zhao, T. Nakamura, T. Tcranishi, Y. Futami, K. Furataka, and T. Delbar; preprint Rikkyo R U P 94-2, Yale-40609-1141, Physical Review Letters, in press, October, 1994. 15. J.N. Bahcall, C.A,. Barnes, J. ChristensenDalsgaard, B.T. Cleveland, S. Degl'innocenti, B.W. Filippone, A. Glasner, R.W. Kavanagh, S.E. Koonin, K. Lande, K. Langanke, P.D. Parker, M.H. Pinsonneault, C.R. Pro~tt, and T. Shoppa; placed on the World Wide W e b Electronic Bulletin, 3 April, 1994, and preprint L A S S N S - A S T 94/13, Institute of advanced study, 1994. 16. K. Langanke and T.D.Shoppa; Phys. Rev. 49, R1771(1994). 17. Moshe Gai, and Carlos A. Bcrtulani; comment submitted to Phys. Rev. C, May 22, 1994, preprint UConn-40870-0005.