Measurement of the Coulomb Dissociation of8Bat 254 MeV/nucleon and the

Abstract

We have measured the Coulomb dissociation of 8 B into 7 Be and proton at 254 MeV/nucleon using a large-acceptance focusing spectrometer. The astrophysical S17 factor for the 7 Be (p,γ) 8 B reaction at Ec.m. = 0.25 − 2.78 MeV is deduced yielding S17(0) = 20.6±1.2 (exp.) ± 1.0 (theo.) eV-b. This result agrees with the presently adopted zero-energy S17 factor obtained in direct-reaction measurements and with the results of other Coulomb-dissociation studies performed at 46.5 and 51.2 MeV/nucleon. PACs: 25.40.Lw, 25.70.De, 26.65.+t The precise knowledge of the solar thermonuclear fusion of 8 B (from 7 Be plus proton) is crucial for estimating the 8 B solar neutrino flux and the predicted neutrino rates in terrestrial neutrino measurements. The relevant 7 Be(p,γ) 8 B cross section σ(E) is parameterized in terms of the astrophysical factor S 17 (E) which is defined by2 /hv is the Sommerfeld parameter. The flux of 8 B solar neutrinos is particularly important for the results of the Homestake, Super Kamiokande, and SNO experiments [1] which measure high-energy solar neutrinos mainly or solely from the 8 B decay. Unfortunately, this cross section has not been known with sufficient accuracy for a long time, despite the fact that several comprehensive direct measurements were reported for the 7 Be(p,γ) 8 B reaction [2][3][4][5][6]. The main difficulty in such experiments is the determination of the effective target thickness of the radioactive 7 Be target. This difficulty is reflected in the fact that the results of these measurements can be grouped into two distinct data sets which agree in their energy dependence but disagree in their absolute normalization by about 30%. In view of this discrepancy, experimental studies with different methods are highly desirable.As an alternative approach one can measure the inverse process, the Coulomb dissociation (CD) of 8 B into 7 Be and proton [7]. The CD yields are enhanced because thicker targets can be used and a larger phase space is available for CD. This method uses stable targets and thus is free from the difficulty of determining the effective target thickness. On the other hand, direct (p,γ) and Coulomb dissociation measurements have different sensitivities to the multipole composition of the photon fields. The E2 amplitude is enhanced in CD due to the large flux of E2 virtual photons, whereas it can be neglected in the (p,γ) reaction.Recently, Motobayashi et al. have performed a CD experiment at E( 8 B) = 46.5 MeV/nucleon, yielding values for S 17 in the energy range 0.6−1.7 MeV [8]. The extracted (p,γ) cross section is consistent with the results from the lower group of direct-reaction data points [4][5][6]. Another measurement at 51.9 MeV/nucleon by the same group with improved accuracy led essentially to the same conclusion [9].In this article, we report on an experiment of the CD of 8 B at a higher energy of 254 MeV/nucleon performed at the SIS facility at GSI, Darmstadt, Germany. The present incident energy has several advantages compared to those used in Refs. ...

S17(0)Determined from the Coulomb Breakup of 83 MeV/Nucleon<

“…The present result is in good agreement with three of the capture measurements [2,11,27], and with the RIKEN (18.9 ± 1.8 eV b) and GSI (20.6 ± 1.2 ± 1.0 eV b) Coulomb breakup measurements [8,9]. It is also in excellent agreement with the results of asymptotic normalization coefficient determinations (17.3 ± 1.8 eV b) [3,28].…”

“…It is also in excellent agreement with the results of asymptotic normalization coefficient determinations (17.3 ± 1.8 eV b) [3,28]. Although the results agree within the errors, the E1 strength found here is about 15% smaller than reported in the GSI Coulomb breakup measurement [9]. This might be ascribed to the neglect of E2 transitions in the analysis of the GSI measurement.…”

“…As a consequence, the reported [12] ratio of E2 and E1 S factors at E rel = 0.6 MeV should be replaced by 4.7 +2.0 −1.3 × 10 −4 . The E2 strength extracted from the inclusive breakup measurement [12] is a factor of 10 to 100 larger than the upper limits reported in other experimental studies [7,9]. However, it is only slightly smaller than or in good agreement with recent theoretical calculations of E2 strength [13,[16][17][18][19], and is consistent with the measurement of [20].…”

“…As illustrated in [9], there is a disagreement in the energy dependence of the S factors extracted from the Coulomb dissociation experiments at RIKEN and GSI. Furthermore, the radiative capture reaction proceeds almost exclusively by E1 transitions at solar energies (≈ 20 keV), but E2 and M 1 transitions also play a role in Coulomb breakup for relative energies less than 1 MeV.…”

“…The disagreements among the direct measurements make an independent approach desirable. Peripheral transfer reactions that yield asymptotic normalization coefficients [3] and Coulomb breakup [4][5][6][7][8][9][10] permit the extraction of S factors with different systematic uncertainties. In the Coulomb breakup of 8 B, a virtual photon emitted by a heavy target nucleus such as Pb dissociates an incident 8 B projectile into 7 Be + p. This is the inverse of the radiative capture reaction.…”