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Determination of the 8B neutrino spectrum
Nuclear Physics A 746 (2004) 311c–315c
Determination of the 8 B neutrino spectrum W. T. Wintera , K. E. Rehmb , C. L. Jiangb , I. Ahmadb , S. J. Freedmana , J. Greeneb , A. Heinzb , D. Hendersonb , R. V. F. Janssensb , E. F. Mooreb , G. Mukherjeeb , R. C. Pardob , M. Paulc , T. Penningtonb , G. Savardb , J. P. Schifferb , D. Seweryniakb , G. Zinkannb a
Lawrence Berkeley National Laboratory, Berkeley, CA
b
Argonne National Laboratory, Argonne, IL
c
Hebrew University, Jerusalem
We have measured the total energy of the alpha particles emitted following the betadecay of 8 B. The alpha particle energy spectrum is used to infer the shape of the 8 B neutrino spectrum, which is an important input in the interpretation of experiments which detect energetic neutrinos from the Sun. The alpha-spectrum was measured with a new technique which involved the implantation of a beam of 8 B ions into the mid-plane of a 91 µm thick planar Si detector, allowing both of the decay alpha particles to be observed in the same detector. Calibration was performed by implanting a beam of 20 Na ions, which provide alpha lines in the energy region of interest. The experimental results are compared with a previous precision measurement of the alpha spectrum, and are found to disagree. 1. INTRODUCTION A significant fraction of the observed solar neutrino flux is due to neutrinos from the β-decay of 8 B. These 8 B neutrinos account for a majority of events in the Homestake 37 Cl ν-capture experiment [1], and nearly all of the events in water-Cerenkov experiments such as Super-Kamiokande [2] and Sudbury Neutrino Observatory (SNO) [3]. Three decades of observation of solar neutrinos have led to the conclusion that neutrinos are changing flavor, a result which may be interpreted by neutrino oscillations due to finite neutrino mass [4]. The Super-Kamiokande and SNO experiments are sensitive to the differential energy spectrum of observed neutrinos. An observed distortion of the 8 B ν-spectrum would provide further support for the oscillation interpretation, making it desirable that the shape of the (unmixed) neutrino spectrum be precisely determined. The neutrino spectrum of 8 B is complicated by the fact that the 8 B β-decay proceeds to the α-unstable 8 Be nucleus, which consists of a energetically broad region of overlapping and interfering nuclear levels. Thus, for example, the neutrino spectrum differs significantly from the allowed statistical shape. Measurements of the α-particle energy spectrum following the β-decay of 8 B determine the likelihood that the 8 B β-decay proceeds through a given excitation energy in 8 Be. The α-spectrum can then be used to construct the 8 B neutrino spectrum. 0375-9474/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysa.2004.09.029
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W.T. Winter et al. / Nuclear Physics A 746 (2004) 311c–315c
Measurements of the α-spectrum have been performed by several groups in the past [5]. While the data from these measurements agree in shape, they differ by energy offsets of ∼100 keV. This discrepancy has been attributed [5] to systematic effects originating from α-particle straggling corrections in insensitive regions external to the detector. Also, these measurements of the α-spectrum detected only a single α-particle from the decay and are subject to a significant recoil broadening effect due to the outgoing leptons from the β-decay [6]. A recent measurement [7] has detected both α-particles using two detectors in coincidence. This experiment was designed to reduce several of the systematic effects common to previous experiments, but was still subject to energy scale corrections due to α-particle energy straggling in insensitive regions. In order to eliminate α-particle energy loss in insensitive regions, we have performed a new measurement of the α-spectrum using a novel technique. A beam of 8 B ions was implanted directly into the mid-plane of a 91 µm thick planar Si detector, which was just sufficiently thick to stop the most energetic α-particles. This allows both α-particles to deposit their total energy in a single detector. The effect of energy deposition by the βparticle was minimized by the use of a coincidence β-detector which identified a subset of events where the β-particle exited the Si detector within ∼30◦ of normal, thus depositing a smaller amount of energy (∼25 keV). For calibration purposes, 20 Na ions were implanted into the same detector. The 20 Na nuclei, like 8 B, are β-delayed α-emitters, proceeding 20% of the time to α-unstable levels in 20 Ne which provide calibration lines in the energy range of interest. 2. EXPERIMENTAL DETAILS The experiment was performed at Argonne National Laboratory, using the Argonne Tandem-Linear-Accelerator-System (ATLAS). A beam of 8 B (t1/2 =778 msec) was produced with the In-Flight Technique [8] using the 3 He(6 Li,8 B)n reaction. A primary 6 Li beam of energy 36.4 MeV was incident on a 3.5 cm long 3 He gas cell held at 700 mbar pressure and cooled to 92 K. A 22◦ bending magnet was used to separate the 8 B reaction products from the primary 6 Li beam. The 8 B products were transported through the Enge Split Pole spectrograph where they were identified with respect to mass, nuclear charge, and energy in the gas-filled focal plane detector [9]. The magnetic field in the spectrograph was then adjusted so that 27.3 MeV 8 B ions (range ∼44 µm) were incident on the 91 µm thick planar Si detector located in the focal plane, adjacent to the gas-filled detector. A 11 mm diameter Ta collimator upstream of the Si detector ensured that the 8 B ions were implanted into the sensitive region of the Si detector. The coincidence β-particle detector consisted of a plastic scintillator, with 25.4 mm diameter and 2 mm thickness, mounted on a light guide optically coupled to a Hamamatsu R647 photo multiplier tube. The β-detector was located 12 mm downstream of the Si detector. The Si-scintillator system was cooled to -5◦ C. The 6 Li beam was cycled (1.5 sec on/1.5 sec off) and data were taken only during the beam-off cycle. An event was defined as a pulse above threshold in either the Si or the β-detector. Pulse height information from both detectors was recorded, as well as the relative timing between Si and β-detector, and the timing of the Si signal with
W.T. Winter et al. / Nuclear Physics A 746 (2004) 311c–315c
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Figure 1. Energy spectra obtained by observing the decay of 8 B (broad spectrum) or 20 Na (lines) which had been implanted into the Si detector. The vertical line at 1.2 MeV is the low energy cutoff, below which the data was not used.
respect to the start of the beam-off cycle. An average 6 Li beam current of 60 pnA led to about 3 8 B ions/sec implanted into the Si detector. Over three days, 4.4× 105 8 B events were implanted into the Si detector, 16% of which were in coincidence with events in the β-detector. The system was calibrated with a 20 Na (t1/2 =448 msec) beam immediately before the 8 B run. The energy released in the α-decays ranges from 2.66 to 6.06 MeV, covering a large fraction of the 8 B α-spectrum, including the spectral peak near 3 MeV. The 20 Na beam was produced using the same In-Flight Technique with a 199 MeV 19 F primary beam via the 3 He(19 F,20 Na)2n reaction. A Mylar foil was positioned upstream of the Si detector to slow the 20 Na beam to proper energies for implantation near the mid-plane of the detector. Data were taken in a fashion similar to that for 8 B. Because of the shorter half life, the on-off cycle time was reduced to 1.0 sec on and 1.0 sec off. With a 0.5 pnA 19 F beam, about 8 20 Na decays/min were detected in the Si detector, resulting in about 1×104 20 Na events in a one day run. In addition, calibrations with external sources of 228 Th and 148 Gd were performed before, during and after the experiment. These sources provide higher-energy alpha lines with energies from 3.183 to 8.784 MeV and required corrections for energy losses in the source as well as in the dead layer of the detector. The linearity of the electronics was checked with pulser calibrations at various times during the experiment. The leakage current in the detector was monitored and found to remain constant over the entire duration of the measurement. 3. EXPERIMENTAL RESULTS The raw α-particle energy spectra following the β-decays of 8 B and 20 Na are shown in Fig. 1. The dominant lines from the decay of 20 Na surround the 8 B spectrum peak, providing calibration in the energy range of interest. The 8 B spectrum was partially corrupted due to the acceleration of various ions by ATLAS during the beam-off cycle. This can be accounted for by requiring a coincidence count in the β-detector, which would indicate that the observed event was associated with
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Figure 2. R-matrix fits to the measured α-spectra presented in this work and from Ref. [7]. The spectrum lying higher in energy corresponds to the work reported here. The inset shows the location of the peaks of the spectra, on which the deduced neutrino spectrum is highly dependent. The width of the lines in the inset indicates the magnitudes of the systematic uncertainties of each measurement.
a 8 B β-decay. The exception is protons accelerated by ATLAS, which were able to pass through the Si detector and trigger the β-detector, producing a peak near 800 keV. As a result, only coincidence data below 1.2 MeV was used in the final analysis. Extrapolation to low energies was performed using the R-matrix approach, as described for example in [6]. The dominant systematic uncertainties in this experiment appear as multiplicative uncertainties in the energy scale arising from the calibration and from an observed gain shift. The calibration allowed the energy scale to be determined with a 5 keV precision at the peak of the α-spectrum. The gain shift, which was monitored by observing the centroids of the α-spectrum from run to run, contributes an additional 7 keV uncertainty at the spectral peak. Besides these two dominant uncertainties, we considered uncertainties associated with β-particle energy deposition, uncertainties in implantation depth of the 8 B and 20 Na, and extrapolation to low α-particle energies. The neutrino spectrum deduced from the measured α-spectrum is far more dependent on systematic uncertainties than on statistical uncertainties, so that the use of a smooth R-matrix fit provides a convenient way of comparing α-spectrum measurements. Fits to the data set presented here and the other precision measurement [7], along with the quoted systematic uncertainties, are shown in Fig. 2. The α-spectrum reported here lies 50 keV higher in energy than the previous measurement. This discrepancy is beyond the uncertainties in the energy scale of the two experiments, both of which were near ±10 keV. The 8 B neutrino spectrum was deduced from the measured α energy spectrum, taking into account contributions from recoil order matrix elements [10] and radiative corrections [11]. The neutrino spectrum deduced here disagrees with the spectrum recommended in [7], particularly for neutrinos at the high energy end of the spectrum. A graphical comparison of the neutrino spectra based on the this measurement and the previous precision measurement [7] is shown in Fig. 3. This work was supported by the United States Department of Energy Nuclear Physics
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W.T. Winter et al. / Nuclear Physics A 746 (2004) 311c–315c
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Figure 3. (a) The normalized neutrino energy spectrum deduced from this measurement. (b) The line in the shaded region is the ratio between the neutrino spectrum recommended by Ortiz et al. [7] to the spectrum deduced in this work. The line at unity represents the spectrum deduced here. The bands indicate the ±1σ experimental uncertainties, which are the result of propagating the uncertainties in the alpha spectrum, shown in Fig. 2, to the neutrino spectrum. The Ortiz et al. spectrum was smoothed to account for binning effects. Division, under Contract Nos. W-31-109-ENG-38 and DE-AC03-76SF00098. REFERENCES 1. R. Davis, Jr., D. S. Harmer, and K. C. Hoffman, Phys. Rev. Lett. 20 (1968) 1205. 2. The Super-Kamiokande Collaboration, Phys. Lett. B539 (2002)179, contains recent solar neutrino results. 3. SNO collaboration, Phys. Rev. Lett. 89 (2002) 011301, contains the most recent published results. a-Garay, J. High Energy Phys. 7 4. J. N. Bahcall and M. C. Gonzalez-Garcia and C. Pen (2002) 54, is one recent review of available solar neutrino data and its interpretation. 5. J. Bahcall et al., Phys. Rev. C54 (1996) 411, gives a comprehensive description of the 8 B alpha and beta spectrum measurements performed before 1996. 6. M. Bhattacharya and E. G. Adelberger, Phys. Rev. C 65 (2002) 055502, describes the lepton recoil broadening effect and contains references to earlier work on the application of the R-matrix technique to 8 B β-decay. 7. C. Ortiz et al., Phys. Rev. Lett. 85 (2000) 2909. 8. B. Harss et al., Rev. Sci. Instrum. 71, (2000) 380. 9. K. E. Rehm and F. L. H. Wolfs, Nucl. Inst. Meth. A 273 (1988) 262. 10. J. N. Bahcall and B. R. Holstein, Phys. Rev. C 33 (1986) 2121. 11. I. S. Batkin and M. K. Sundaresan, Phys. Rev. D 52 (1995) 5362.
Determination of the 8 B neutrino spectrum W. T. Wintera , K. E. Rehmb , C. L. Jiangb , I. Ahmadb , S. J. Freedmana , J. Greeneb , A. Heinzb , D. Hendersonb , R. V. F. Janssensb , E. F. Mooreb , G. Mukherjeeb , R. C. Pardob , M. Paulc , T. Penningtonb , G. Savardb , J. P. Schifferb , D. Seweryniakb , G. Zinkannb a
Lawrence Berkeley National Laboratory, Berkeley, CA
b
Argonne National Laboratory, Argonne, IL
c
Hebrew University, Jerusalem
We have measured the total energy of the alpha particles emitted following the betadecay of 8 B. The alpha particle energy spectrum is used to infer the shape of the 8 B neutrino spectrum, which is an important input in the interpretation of experiments which detect energetic neutrinos from the Sun. The alpha-spectrum was measured with a new technique which involved the implantation of a beam of 8 B ions into the mid-plane of a 91 µm thick planar Si detector, allowing both of the decay alpha particles to be observed in the same detector. Calibration was performed by implanting a beam of 20 Na ions, which provide alpha lines in the energy region of interest. The experimental results are compared with a previous precision measurement of the alpha spectrum, and are found to disagree. 1. INTRODUCTION A significant fraction of the observed solar neutrino flux is due to neutrinos from the β-decay of 8 B. These 8 B neutrinos account for a majority of events in the Homestake 37 Cl ν-capture experiment [1], and nearly all of the events in water-Cerenkov experiments such as Super-Kamiokande [2] and Sudbury Neutrino Observatory (SNO) [3]. Three decades of observation of solar neutrinos have led to the conclusion that neutrinos are changing flavor, a result which may be interpreted by neutrino oscillations due to finite neutrino mass [4]. The Super-Kamiokande and SNO experiments are sensitive to the differential energy spectrum of observed neutrinos. An observed distortion of the 8 B ν-spectrum would provide further support for the oscillation interpretation, making it desirable that the shape of the (unmixed) neutrino spectrum be precisely determined. The neutrino spectrum of 8 B is complicated by the fact that the 8 B β-decay proceeds to the α-unstable 8 Be nucleus, which consists of a energetically broad region of overlapping and interfering nuclear levels. Thus, for example, the neutrino spectrum differs significantly from the allowed statistical shape. Measurements of the α-particle energy spectrum following the β-decay of 8 B determine the likelihood that the 8 B β-decay proceeds through a given excitation energy in 8 Be. The α-spectrum can then be used to construct the 8 B neutrino spectrum. 0375-9474/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysa.2004.09.029
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W.T. Winter et al. / Nuclear Physics A 746 (2004) 311c–315c
Measurements of the α-spectrum have been performed by several groups in the past [5]. While the data from these measurements agree in shape, they differ by energy offsets of ∼100 keV. This discrepancy has been attributed [5] to systematic effects originating from α-particle straggling corrections in insensitive regions external to the detector. Also, these measurements of the α-spectrum detected only a single α-particle from the decay and are subject to a significant recoil broadening effect due to the outgoing leptons from the β-decay [6]. A recent measurement [7] has detected both α-particles using two detectors in coincidence. This experiment was designed to reduce several of the systematic effects common to previous experiments, but was still subject to energy scale corrections due to α-particle energy straggling in insensitive regions. In order to eliminate α-particle energy loss in insensitive regions, we have performed a new measurement of the α-spectrum using a novel technique. A beam of 8 B ions was implanted directly into the mid-plane of a 91 µm thick planar Si detector, which was just sufficiently thick to stop the most energetic α-particles. This allows both α-particles to deposit their total energy in a single detector. The effect of energy deposition by the βparticle was minimized by the use of a coincidence β-detector which identified a subset of events where the β-particle exited the Si detector within ∼30◦ of normal, thus depositing a smaller amount of energy (∼25 keV). For calibration purposes, 20 Na ions were implanted into the same detector. The 20 Na nuclei, like 8 B, are β-delayed α-emitters, proceeding 20% of the time to α-unstable levels in 20 Ne which provide calibration lines in the energy range of interest. 2. EXPERIMENTAL DETAILS The experiment was performed at Argonne National Laboratory, using the Argonne Tandem-Linear-Accelerator-System (ATLAS). A beam of 8 B (t1/2 =778 msec) was produced with the In-Flight Technique [8] using the 3 He(6 Li,8 B)n reaction. A primary 6 Li beam of energy 36.4 MeV was incident on a 3.5 cm long 3 He gas cell held at 700 mbar pressure and cooled to 92 K. A 22◦ bending magnet was used to separate the 8 B reaction products from the primary 6 Li beam. The 8 B products were transported through the Enge Split Pole spectrograph where they were identified with respect to mass, nuclear charge, and energy in the gas-filled focal plane detector [9]. The magnetic field in the spectrograph was then adjusted so that 27.3 MeV 8 B ions (range ∼44 µm) were incident on the 91 µm thick planar Si detector located in the focal plane, adjacent to the gas-filled detector. A 11 mm diameter Ta collimator upstream of the Si detector ensured that the 8 B ions were implanted into the sensitive region of the Si detector. The coincidence β-particle detector consisted of a plastic scintillator, with 25.4 mm diameter and 2 mm thickness, mounted on a light guide optically coupled to a Hamamatsu R647 photo multiplier tube. The β-detector was located 12 mm downstream of the Si detector. The Si-scintillator system was cooled to -5◦ C. The 6 Li beam was cycled (1.5 sec on/1.5 sec off) and data were taken only during the beam-off cycle. An event was defined as a pulse above threshold in either the Si or the β-detector. Pulse height information from both detectors was recorded, as well as the relative timing between Si and β-detector, and the timing of the Si signal with
W.T. Winter et al. / Nuclear Physics A 746 (2004) 311c–315c
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Counts per 40 keV
1000
100
10
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2
4
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10 8 Energy (MeV)
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Figure 1. Energy spectra obtained by observing the decay of 8 B (broad spectrum) or 20 Na (lines) which had been implanted into the Si detector. The vertical line at 1.2 MeV is the low energy cutoff, below which the data was not used.
respect to the start of the beam-off cycle. An average 6 Li beam current of 60 pnA led to about 3 8 B ions/sec implanted into the Si detector. Over three days, 4.4× 105 8 B events were implanted into the Si detector, 16% of which were in coincidence with events in the β-detector. The system was calibrated with a 20 Na (t1/2 =448 msec) beam immediately before the 8 B run. The energy released in the α-decays ranges from 2.66 to 6.06 MeV, covering a large fraction of the 8 B α-spectrum, including the spectral peak near 3 MeV. The 20 Na beam was produced using the same In-Flight Technique with a 199 MeV 19 F primary beam via the 3 He(19 F,20 Na)2n reaction. A Mylar foil was positioned upstream of the Si detector to slow the 20 Na beam to proper energies for implantation near the mid-plane of the detector. Data were taken in a fashion similar to that for 8 B. Because of the shorter half life, the on-off cycle time was reduced to 1.0 sec on and 1.0 sec off. With a 0.5 pnA 19 F beam, about 8 20 Na decays/min were detected in the Si detector, resulting in about 1×104 20 Na events in a one day run. In addition, calibrations with external sources of 228 Th and 148 Gd were performed before, during and after the experiment. These sources provide higher-energy alpha lines with energies from 3.183 to 8.784 MeV and required corrections for energy losses in the source as well as in the dead layer of the detector. The linearity of the electronics was checked with pulser calibrations at various times during the experiment. The leakage current in the detector was monitored and found to remain constant over the entire duration of the measurement. 3. EXPERIMENTAL RESULTS The raw α-particle energy spectra following the β-decays of 8 B and 20 Na are shown in Fig. 1. The dominant lines from the decay of 20 Na surround the 8 B spectrum peak, providing calibration in the energy range of interest. The 8 B spectrum was partially corrupted due to the acceleration of various ions by ATLAS during the beam-off cycle. This can be accounted for by requiring a coincidence count in the β-detector, which would indicate that the observed event was associated with
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W.T. Winter et al. / Nuclear Physics A 746 (2004) 311c–315c
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Figure 2. R-matrix fits to the measured α-spectra presented in this work and from Ref. [7]. The spectrum lying higher in energy corresponds to the work reported here. The inset shows the location of the peaks of the spectra, on which the deduced neutrino spectrum is highly dependent. The width of the lines in the inset indicates the magnitudes of the systematic uncertainties of each measurement.
a 8 B β-decay. The exception is protons accelerated by ATLAS, which were able to pass through the Si detector and trigger the β-detector, producing a peak near 800 keV. As a result, only coincidence data below 1.2 MeV was used in the final analysis. Extrapolation to low energies was performed using the R-matrix approach, as described for example in [6]. The dominant systematic uncertainties in this experiment appear as multiplicative uncertainties in the energy scale arising from the calibration and from an observed gain shift. The calibration allowed the energy scale to be determined with a 5 keV precision at the peak of the α-spectrum. The gain shift, which was monitored by observing the centroids of the α-spectrum from run to run, contributes an additional 7 keV uncertainty at the spectral peak. Besides these two dominant uncertainties, we considered uncertainties associated with β-particle energy deposition, uncertainties in implantation depth of the 8 B and 20 Na, and extrapolation to low α-particle energies. The neutrino spectrum deduced from the measured α-spectrum is far more dependent on systematic uncertainties than on statistical uncertainties, so that the use of a smooth R-matrix fit provides a convenient way of comparing α-spectrum measurements. Fits to the data set presented here and the other precision measurement [7], along with the quoted systematic uncertainties, are shown in Fig. 2. The α-spectrum reported here lies 50 keV higher in energy than the previous measurement. This discrepancy is beyond the uncertainties in the energy scale of the two experiments, both of which were near ±10 keV. The 8 B neutrino spectrum was deduced from the measured α energy spectrum, taking into account contributions from recoil order matrix elements [10] and radiative corrections [11]. The neutrino spectrum deduced here disagrees with the spectrum recommended in [7], particularly for neutrinos at the high energy end of the spectrum. A graphical comparison of the neutrino spectra based on the this measurement and the previous precision measurement [7] is shown in Fig. 3. This work was supported by the United States Department of Energy Nuclear Physics
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Figure 3. (a) The normalized neutrino energy spectrum deduced from this measurement. (b) The line in the shaded region is the ratio between the neutrino spectrum recommended by Ortiz et al. [7] to the spectrum deduced in this work. The line at unity represents the spectrum deduced here. The bands indicate the ±1σ experimental uncertainties, which are the result of propagating the uncertainties in the alpha spectrum, shown in Fig. 2, to the neutrino spectrum. The Ortiz et al. spectrum was smoothed to account for binning effects. Division, under Contract Nos. W-31-109-ENG-38 and DE-AC03-76SF00098. REFERENCES 1. R. Davis, Jr., D. S. Harmer, and K. C. Hoffman, Phys. Rev. Lett. 20 (1968) 1205. 2. The Super-Kamiokande Collaboration, Phys. Lett. B539 (2002)179, contains recent solar neutrino results. 3. SNO collaboration, Phys. Rev. Lett. 89 (2002) 011301, contains the most recent published results. a-Garay, J. High Energy Phys. 7 4. J. N. Bahcall and M. C. Gonzalez-Garcia and C. Pen (2002) 54, is one recent review of available solar neutrino data and its interpretation. 5. J. Bahcall et al., Phys. Rev. C54 (1996) 411, gives a comprehensive description of the 8 B alpha and beta spectrum measurements performed before 1996. 6. M. Bhattacharya and E. G. Adelberger, Phys. Rev. C 65 (2002) 055502, describes the lepton recoil broadening effect and contains references to earlier work on the application of the R-matrix technique to 8 B β-decay. 7. C. Ortiz et al., Phys. Rev. Lett. 85 (2000) 2909. 8. B. Harss et al., Rev. Sci. Instrum. 71, (2000) 380. 9. K. E. Rehm and F. L. H. Wolfs, Nucl. Inst. Meth. A 273 (1988) 262. 10. J. N. Bahcall and B. R. Holstein, Phys. Rev. C 33 (1986) 2121. 11. I. S. Batkin and M. K. Sundaresan, Phys. Rev. D 52 (1995) 5362.