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LUNA and the neutrinos from the Sun
Nuclear Physics B (Proc. Suppl.) 188 (2009) 124–126 www.elsevierphysics.com
LUNA and the neutrinos from the Sun C. Brogginia (the LUNA Collaboration) a
Istituto Nazionale di Fisica Nucleare, Sezione di Padova, via Marzolo 8, 35131 Padova, Italy LUNA, Laboratory for Underground Nuclear Astrophysics, is studying thermonuclear reactions down to the energy of the stellar nucleosynthesis. The latest results obtained by LUNA on 3 He(α,γ)7 Be and 14 N(p,γ)15 O will be presented and their implications will be discussed.
2. The 7 Be and 8 B solar neutrino flux
1. Introduction Thermonuclear reactions that generate energy and synthesize elements take place inside the stars in a relatively narrow energy window: the Gamow peak. In this region, which is far below the Coulomb energy, the reaction cross-section σ(E) drops almost exponentially with decreasing energy E: σ(E) =
S(E) exp(−2 π η) E
(1)
where S(E) is the astrophysical factor and η is given by 2 π η = 31.29 Z1 Z2 (μ/E)1/2 . Z1 and Z2 are the charges of the interacting nuclei in the entrance channel, μ is the reduced mass (in amu), and E is the center of mass energy (in keV). The extremely low value of the cross-section has always prevented its measurement within the Gamow peak mainly because of the cosmic ray induced background. In order to explore this energy window two electrostatic accelerators have been installed underground in the Gran Sasso Laboratory by the LUNA Collaboration: a 50 kV accelerator and a 400 kV one. The initial underground activity has been focused on the search of a narrow resonance in the 3 He(3 He,2p)4 He cross section. As a matter of fact a resonance within the solar Gamow peak (16-28 keV) was suggested to explain the observed solar neutrino flux. We measured down to 16 keV, with a rate of about 2 events/month (σ =0.02 pbarn), and we could rule out [1] the astrophysical solution of the solar neutrino problem based on the existence of such a resonance. 0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.02.029
3
He(α, γ)7 Be (Q-value=1.586 MeV) is the key reaction for the production of 7 Be and 8 B neutrinos in the Sun since their flux depends almost linearly on the 3 He(α, γ)7 Be cross section. The cross section can be determined either from the detection of the prompt γ ray or from the counting of the produced 7 Be nuclei. The latter requires the detection of the 478 keV γ due to the excited 7 Li populated in the decay of 7 Be. Both methods have been used in the past to determine the cross section in the energy range Ec.m. ≥ 107 keV but the S3,4 extracted from the measurements of the induced 7 Be activity are 13% higher than the values obtained from the detection of the prompt γ-rays. The underground experiment has been performed with the 4 He+ beam from the 400 kV accelerator in conjunction with a windowless gas target filled with 3 He at 0.7 mbar. The beam enters the target chamber and it is stopped on a power calorimeter. The 7 Be nucleus produced by the reaction inside the 3 He gas target are implanted into the calorimeter cap which, after the irradiation, is removed and placed in front of a germanium detector for the measurement of the 7 Be activity. In the first phase of the experiment we obtained the 3 He(α, γ)7 Be cross section from the activation data [2] [3] alone with a total uncertainty of about 4%. In the second phase we performed a new high accuracy measurement using simultaneously prompt and activation methods. The prompt capture γ-ray is detected by a 135% ger-
C. Broggini / Nuclear Physics B (Proc. Suppl.) 188 (2009) 124–126
125
Figure 1. Astrophysical S-factor for 3 He(α, γ)7 Be. The results from the modern, high precision experiments are highlighted.
manium detector heavily shielded and placed in close geometry with the target. Data have been collected at Ec.m. = 170, 106, 93 keV. In this interval the cross section varies from 10.25 nbarn to 0.23 nbarn, with a total error of about 4%. The astrophysical factor obtained with the two methods [4] is the same within the quoted experimental error. Recently, similar conclusions have been reached in a new simultaneous activation and prompt experiment [5] which covers the Ec.m. energy range from 330 keV to 1230 keV. The energy dependence of the cross section seems to be theoretically well determined. The only free parameter is the normalization. If we rescale the fit of [6] in Figure 1 to our data we obtain S3,4 (0)=0.560±0.017 keV barn. Thanks to our small error, the total uncertainty on the 8 B solar neutrino flux goes from 12 to 10%, whereas the one on the 7 Be flux goes from 9.4 to 5.5% [4].
3. The CNO cycle and the metallicity of the Sun In our Sun the CNO cycle accounts for just a small fraction of the nuclear energy production, whereas the main part is supplied by the protonproton chain. 14 N(p,γ)15 O (Q-value of the reaction: 7.297 MeV) is the slowest reaction of the cycle and it rules its energy production rate. In particular, it is the key reaction to know the 13 N and 15 O solar neutrino flux, which depends almost linearly on its cross section. In the first phase of the LUNA study, data have been obtained down to 119 keV energy with solid targets of TiN and a 126% High Purity Germanium detector. This way we could measure the five different radiative capture transitions which contribute to the 14 N(p,γ)15 O cross section at low energy. The total cross section was then measured down to very low energy in the second phase
126
C. Broggini / Nuclear Physics B (Proc. Suppl.) 188 (2009) 124–126
of the experiment by using a 4π BGO summing detector placed around a windowless gas target (1 mbar pressure). At the lowest energy of 70 keV we measured a cross section of 0.24 pbarn, with an event rate of 11 counts/day from the reaction. The results we obtained first with the germanium detector data [7][8] and then with the BGO set-up [9] are about a factor two lower than the existing extrapolation at very low energy. As a consequence the CNO neutrino yield in the Sun is decreased by about a factor two, with respect to the estimates. The lower cross section is affecting also stars which are more evolved than our Sun. In particular, the age of the Globular Clusters is increased by 0.7-1 Gyr and the dredge-up of carbon to the surface of AGB stars is much more efficient. The main conclusion from the LUNA data has been confirmed by an independent study at higher energy [10]. However, there is a 15% difference between the total S-factor extrapolated by the two experiments at the Gamow peak of the Sun. In particular, this difference arises from the extrapolation of the capture to the ground state in 15 O, a transition strongly affected by interference effects between several resonances and the direct capture mechanism. In order to provide precise data for the ground state capture we performed a third phase of the 14 N(p,γ)15 O study with a segmented germanium detector to reduce the summing correction and, in order to obtain sufficient statistics, we concentrate on the beam energy region immediately above the 259 keV resonance, where precise data effectively constrain the R-matrix fit for the ground state transition. With these improvements we could finally reduce to 8% the total error on the S-factor: S1,14 (0)=1.57±0.13 keV barn [11]. Thanks to this relatively small error it will soon be possible to measure the metallicity of the central region of the Sun by comparing the detected CNO neutrino flux with the predicted one. As a matter of fact, the CNO neutrino flux is decreased by about 35% in going from the high to the low metallicity scenario.
4. Conclusions Underground nuclear astrophysics started almost 18 years ago with the goal of exploring the fascinating domain of nuclear astrophysics at very low energy. During these years LUNA has proved that, by going underground and by using the typical techniques of low background physics, it is possible to measure nuclear cross sections down to the energy of the nucleosynthesis inside stars. In particular, the measurements of 3 He(3 He,2p)4 He has shown that nuclear physics was not the origin of the solar neutrino puzzle. The cross section of 3 He(α, γ)7 Be has been measured with two different experimental approaches and with a 4% total error. Thanks to this small error, the total uncertainty on the 7 Be solar neutrino flux has been reduced to 5.5%. Finally, the study of 14 N(p,γ)15 O has shown that the expected CNO solar neutrino flux has to be decreased by about a factor two, with an error small enough to pave the way for a measurement of the central metallicity of the Sun. REFERENCES 1. R. Bonetti et al., Phys. Rev. Lett. 82, 5205 (1999). 2. D. Bemmerer et al., Phys. Rev. Lett. 97, 122502 (2006). 3. G. Gyurky et al., Phys. Rev. C 75, 035805 (2007). 4. F. Confortola et al., Phys. Rev. C 75, 065803 (2007). 5. T.A.D. Brown et al., Phys. Rev. C 76, 055801 (2007). 6. P. Descouvement et al., Data Nucl. Data Tables 88, 203 (2004). 7. A. Formicola et al., Phys. Lett. B 591, 61 (2004). 8. G. Imbriani et al., Eur. Phys. J.A 25, 455 (2005). 9. A. Lemut et al., Phys. Lett. B 634, 483 (2006). 10. R.C. Runkle et al., Phys. Rev. Lett. 94 082503 (2005). 11. M. Marta et al., Phys. Rev. C 78, 022802 (2008).
LUNA and the neutrinos from the Sun C. Brogginia (the LUNA Collaboration) a
Istituto Nazionale di Fisica Nucleare, Sezione di Padova, via Marzolo 8, 35131 Padova, Italy LUNA, Laboratory for Underground Nuclear Astrophysics, is studying thermonuclear reactions down to the energy of the stellar nucleosynthesis. The latest results obtained by LUNA on 3 He(α,γ)7 Be and 14 N(p,γ)15 O will be presented and their implications will be discussed.
2. The 7 Be and 8 B solar neutrino flux
1. Introduction Thermonuclear reactions that generate energy and synthesize elements take place inside the stars in a relatively narrow energy window: the Gamow peak. In this region, which is far below the Coulomb energy, the reaction cross-section σ(E) drops almost exponentially with decreasing energy E: σ(E) =
S(E) exp(−2 π η) E
(1)
where S(E) is the astrophysical factor and η is given by 2 π η = 31.29 Z1 Z2 (μ/E)1/2 . Z1 and Z2 are the charges of the interacting nuclei in the entrance channel, μ is the reduced mass (in amu), and E is the center of mass energy (in keV). The extremely low value of the cross-section has always prevented its measurement within the Gamow peak mainly because of the cosmic ray induced background. In order to explore this energy window two electrostatic accelerators have been installed underground in the Gran Sasso Laboratory by the LUNA Collaboration: a 50 kV accelerator and a 400 kV one. The initial underground activity has been focused on the search of a narrow resonance in the 3 He(3 He,2p)4 He cross section. As a matter of fact a resonance within the solar Gamow peak (16-28 keV) was suggested to explain the observed solar neutrino flux. We measured down to 16 keV, with a rate of about 2 events/month (σ =0.02 pbarn), and we could rule out [1] the astrophysical solution of the solar neutrino problem based on the existence of such a resonance. 0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.02.029
3
He(α, γ)7 Be (Q-value=1.586 MeV) is the key reaction for the production of 7 Be and 8 B neutrinos in the Sun since their flux depends almost linearly on the 3 He(α, γ)7 Be cross section. The cross section can be determined either from the detection of the prompt γ ray or from the counting of the produced 7 Be nuclei. The latter requires the detection of the 478 keV γ due to the excited 7 Li populated in the decay of 7 Be. Both methods have been used in the past to determine the cross section in the energy range Ec.m. ≥ 107 keV but the S3,4 extracted from the measurements of the induced 7 Be activity are 13% higher than the values obtained from the detection of the prompt γ-rays. The underground experiment has been performed with the 4 He+ beam from the 400 kV accelerator in conjunction with a windowless gas target filled with 3 He at 0.7 mbar. The beam enters the target chamber and it is stopped on a power calorimeter. The 7 Be nucleus produced by the reaction inside the 3 He gas target are implanted into the calorimeter cap which, after the irradiation, is removed and placed in front of a germanium detector for the measurement of the 7 Be activity. In the first phase of the experiment we obtained the 3 He(α, γ)7 Be cross section from the activation data [2] [3] alone with a total uncertainty of about 4%. In the second phase we performed a new high accuracy measurement using simultaneously prompt and activation methods. The prompt capture γ-ray is detected by a 135% ger-
C. Broggini / Nuclear Physics B (Proc. Suppl.) 188 (2009) 124–126
125
Figure 1. Astrophysical S-factor for 3 He(α, γ)7 Be. The results from the modern, high precision experiments are highlighted.
manium detector heavily shielded and placed in close geometry with the target. Data have been collected at Ec.m. = 170, 106, 93 keV. In this interval the cross section varies from 10.25 nbarn to 0.23 nbarn, with a total error of about 4%. The astrophysical factor obtained with the two methods [4] is the same within the quoted experimental error. Recently, similar conclusions have been reached in a new simultaneous activation and prompt experiment [5] which covers the Ec.m. energy range from 330 keV to 1230 keV. The energy dependence of the cross section seems to be theoretically well determined. The only free parameter is the normalization. If we rescale the fit of [6] in Figure 1 to our data we obtain S3,4 (0)=0.560±0.017 keV barn. Thanks to our small error, the total uncertainty on the 8 B solar neutrino flux goes from 12 to 10%, whereas the one on the 7 Be flux goes from 9.4 to 5.5% [4].
3. The CNO cycle and the metallicity of the Sun In our Sun the CNO cycle accounts for just a small fraction of the nuclear energy production, whereas the main part is supplied by the protonproton chain. 14 N(p,γ)15 O (Q-value of the reaction: 7.297 MeV) is the slowest reaction of the cycle and it rules its energy production rate. In particular, it is the key reaction to know the 13 N and 15 O solar neutrino flux, which depends almost linearly on its cross section. In the first phase of the LUNA study, data have been obtained down to 119 keV energy with solid targets of TiN and a 126% High Purity Germanium detector. This way we could measure the five different radiative capture transitions which contribute to the 14 N(p,γ)15 O cross section at low energy. The total cross section was then measured down to very low energy in the second phase
126
C. Broggini / Nuclear Physics B (Proc. Suppl.) 188 (2009) 124–126
of the experiment by using a 4π BGO summing detector placed around a windowless gas target (1 mbar pressure). At the lowest energy of 70 keV we measured a cross section of 0.24 pbarn, with an event rate of 11 counts/day from the reaction. The results we obtained first with the germanium detector data [7][8] and then with the BGO set-up [9] are about a factor two lower than the existing extrapolation at very low energy. As a consequence the CNO neutrino yield in the Sun is decreased by about a factor two, with respect to the estimates. The lower cross section is affecting also stars which are more evolved than our Sun. In particular, the age of the Globular Clusters is increased by 0.7-1 Gyr and the dredge-up of carbon to the surface of AGB stars is much more efficient. The main conclusion from the LUNA data has been confirmed by an independent study at higher energy [10]. However, there is a 15% difference between the total S-factor extrapolated by the two experiments at the Gamow peak of the Sun. In particular, this difference arises from the extrapolation of the capture to the ground state in 15 O, a transition strongly affected by interference effects between several resonances and the direct capture mechanism. In order to provide precise data for the ground state capture we performed a third phase of the 14 N(p,γ)15 O study with a segmented germanium detector to reduce the summing correction and, in order to obtain sufficient statistics, we concentrate on the beam energy region immediately above the 259 keV resonance, where precise data effectively constrain the R-matrix fit for the ground state transition. With these improvements we could finally reduce to 8% the total error on the S-factor: S1,14 (0)=1.57±0.13 keV barn [11]. Thanks to this relatively small error it will soon be possible to measure the metallicity of the central region of the Sun by comparing the detected CNO neutrino flux with the predicted one. As a matter of fact, the CNO neutrino flux is decreased by about 35% in going from the high to the low metallicity scenario.
4. Conclusions Underground nuclear astrophysics started almost 18 years ago with the goal of exploring the fascinating domain of nuclear astrophysics at very low energy. During these years LUNA has proved that, by going underground and by using the typical techniques of low background physics, it is possible to measure nuclear cross sections down to the energy of the nucleosynthesis inside stars. In particular, the measurements of 3 He(3 He,2p)4 He has shown that nuclear physics was not the origin of the solar neutrino puzzle. The cross section of 3 He(α, γ)7 Be has been measured with two different experimental approaches and with a 4% total error. Thanks to this small error, the total uncertainty on the 7 Be solar neutrino flux has been reduced to 5.5%. Finally, the study of 14 N(p,γ)15 O has shown that the expected CNO solar neutrino flux has to be decreased by about a factor two, with an error small enough to pave the way for a measurement of the central metallicity of the Sun. REFERENCES 1. R. Bonetti et al., Phys. Rev. Lett. 82, 5205 (1999). 2. D. Bemmerer et al., Phys. Rev. Lett. 97, 122502 (2006). 3. G. Gyurky et al., Phys. Rev. C 75, 035805 (2007). 4. F. Confortola et al., Phys. Rev. C 75, 065803 (2007). 5. T.A.D. Brown et al., Phys. Rev. C 76, 055801 (2007). 6. P. Descouvement et al., Data Nucl. Data Tables 88, 203 (2004). 7. A. Formicola et al., Phys. Lett. B 591, 61 (2004). 8. G. Imbriani et al., Eur. Phys. J.A 25, 455 (2005). 9. A. Lemut et al., Phys. Lett. B 634, 483 (2006). 10. R.C. Runkle et al., Phys. Rev. Lett. 94 082503 (2005). 11. M. Marta et al., Phys. Rev. C 78, 022802 (2008).