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A Novel Approach to β-delayed Neutron Spectroscopy Using the Beta-decay Paul Trap
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Nuclear Data Sheets 120 (2014) 70–73 www.elsevier.com/locate/nds
A Novel Approach to β-delayed Neutron Spectroscopy Using the Beta-decay Paul Trap N.D. Scielzo,1, ∗ R.M. Yee,1, 2 P.F. Bertone,3 F. Buchinger,4 S.A. Caldwell,3, 5 J.A. Clark,3 A. Czeszumska,1, 2 C.M. Deibel,6 J.P. Greene,3 S. Gulick,4 D. Lascar,3, 7 A.F. Levand,3 G. Li,3, 4 E.B. Norman,1, 2 S. Padgett,1 M. Pedretti,1 A. Perez Galvan,3 G. Savard,3, 5 R.E. Segel,7 K.S. Sharma,3, 8 M.G. Sternberg,3, 5 J. Van Schelt,3, 5 and B.J. Zabransky3 1
Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, USA 2 Department of Nuclear Engineering, University of California, Berkeley, California 94720, USA 3 Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA 4 Department of Physics, McGill University, Montr´eal, Qu´ebec H3A 2T8, Canada 5 Department of Physics, University of Chicago, Chicago, Illinois 60637, USA 6 Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA 7 Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA 8 Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada A new approach to β-delayed neutron spectroscopy has been demonstrated that circumvents the many limitations associated with neutron detection by instead inferring the decay branching ratios and energy spectra of the emitted neutrons by studying the nuclear recoil. Using the Beta-decay Paul Trap, fission-product ions were trapped and confined to within a 1-mm3 volume under vacuum using only electric fields. Results from recent measurements of 137 I+ and plans for development of a dedicated ion trap for future experiments using the intense fission fragment beams from the Californium Rare Isotope Breeder Upgrade (CARIBU) facility at Argonne National Laboratory are summarized. The improved nuclear data that can be collected is needed in many fields of basic and applied science such as nuclear energy, nuclear astrophysics, and stockpile stewardship. I.
INTRODUCTION
The properties of neutrons emitted following the β decay of fission fragments (known as delayed neutrons because they are emitted on the timescales of the β-decay half-lives) play an important role in nuclear astrophysics, fission reactor performance and control, and stockpilestewardship and radiochemistry applications. Half of the isotopes of elements heavier than iron are believed to be produced through the rapid-neutron capture process (r process) in which isotopes are produced through repeated neutron-capture reactions and β decays [1, 2]. Delayed-neutron branching ratios are needed for determining how the neutron-rich isotopes synthesized in r-process environments decay back to stability to form the isotopic abundances observed today. The resulting decay-chain shifts due to β-delayed neutron emission and a subsequent late capture of these neutrons during freeze out can be significant [3, 4] and needs to be well understood. To date, there have been few measurements of delayed-neutron properties near the proposed r-process path and extremely sensitive techniques are needed to
∗
Corresponding author: [email protected]
http://dx.doi.org/10.1016/j.nds.2014.07.009 0090-3752/© 2014 Elsevier Inc. All rights reserved.
reach these exotic, short-lived isotopes. Reviews of delayed-neutron properties [5, 6] highlight the need to obtain high-quality data to better understand the time-dependence and energy spectrum of the neutrons as these properties are essential for a detailed understanding of reactor kinetics needed for reactor safety and to predict the behavior of these reactors under various accident and component-failure scenarios. For fast breeder reactors, criticality-calculation approximations that are used for light-water reactors (such as assuming the delayed-neutron and fission-neutron energy spectra are identical) are not acceptable [7] and improved βdelayed neutron data is needed [8]. With higher-quality nuclear data, the delayed-neutrons flux and energy spectrum could be calculated from the contributions from individual isotopes and used to accurately model any fuelcycle concept, actinide mix, or irradiation history. Additional measurements are also critical to constrain modern nuclear-structure calculations [9] and empirical models [10] that predict the decay properties for nuclei for which no data exists. In the interior of astrophysical bodies and nuclear reactors, neutron-induced reactions can transmute the radioactive isotopes present. For example, the importance of neutron-capture rates for the non-equilibrium (“freeze-
A Novel Approach to β-delayed . . .
NUCLEAR DATA SHEETS
out”) phase of the r-process has been discussed [11–13] and recent works have shown that certain reaction rates can have a strong influence on the dynamics [14, 15]. Measuring cross sections on these neutron-rich species is a daunting technical problem and there is limited experimental data available to guide theoretical predictions, which can vary by an order of magnitude or more [14]. However, detailed studies of decay processes such as βdelayed neutron emission (which can be considered as the inverse process to neutron capture [16]) can provide important constraints to reaction-theory calculations. Neutron spectroscopy is challenging and the quality of the data available today for individual nuclei is limited. In some cases, discrepancies as large as factors of 2–4 in β-delayed neutron branching ratios have been uncovered [17–20]. In addition, for the vast majority of neutron emitters, the energy spectrum has not been measured and even for the few isotopes that have been studied in detail, large corrections for backgrounds and detector response must be applied to interpret the neutron energy spectra [5].
N.D. Scielzo et al.
neutron spectroscopy by inferring the neutron energy from the momentum it imparts to the nucleus (as illustrated in Fig. 1(b)). In β-delayed neutron emission, the nuclear recoil is dominated by the neutron emission because of the massiveness of the neutron relative to the leptons. The energy of the emitted neutron can therefore be reconstructed using conservation of momentum. This novel way to perform delayed-neutron spectroscopy allows neutron emission to be studied with a large and energy-independent detection efficiency, few backgrounds, and good energy resolution. All the wellknown challenges associated with direct neutron detection are avoided. The probability for the decay to occur via delayedneutron emission can be determined from the ratio of the number of delayed-neutron recoil ions to the number of (1) detected β particles, (2) β-delayed γ rays, and (3) recoil ions with longer time-of-flights characteristic of β and γ emission. These different measurements serve as an important check of many potential systematic effects and will allow branching ratios to be reliably determined.
II. INNOVATIVE APPROACH TO DELAYED-NEUTRON SPECTROSCOPY
III.
MEASUREMENTS OF IODINE-137
A proof-of-principle demonstration of this approach for studying β-delayed neutron emission was recently completed [27] using the Beta-decay Paul Trap (BPT) [30]. This trap was originally designed for tests of the Standard Model of particle physics such as precise studies of β-decay angular correlations in the decay of 8 Li [26]. However, it was realized that this trap could be used for delayed-neutron spectroscopy by outfitting it with the appropriate set of radiation detectors. A small plastic scintillator ΔE-E telescope and microchannel plate (MCP) detector (each subtending 3% of 4π) were used for β and recoil-ion detection, respectively. These detectors, which were originally used in Refs. [23, 28], were installed. The isotope 137 I was selected for a proof-of-principle demonstration because its decay properties are well characterized and it has both a large independent yield from the spontaneous fission of 252 Cf and a large delayedneutron branching ratio, (7.33±0.38% [29]). A 1-mCi 252 Cf spontaneous fission source was placed in the gas catcher of the Canadian Penning Trap injection system to produce low-energy, bunched beams of fission-fragment ions [31–33]. With this injection system, a 137 I+ beam of 20–30 ions/s was delivered to the BPT. The timeof-flight (TOF) spectrum of recoil ions following detection of a β particle in the plastic scintillator is shown in Fig. 2(a). The structure observed at 0.4–2.0 μs is due to recoil ions that receive a large momentum kick following neutron emission. The structure is consistent with the expected TOF spectrum based on direct measurements of the delayed-neutron energy spectrum. Recently, additional data for a more detailed study of 137 I (shown in Fig. 2(b)) were collected after straightforward upgrades to the trap electrode structure and de-
The sensitive recoil-ion spectroscopy techniques developed to test fundamental symmetries of the electroweak interaction [22] can be applied to perform precision βdelayed neutron spectroscopy. When a radioactive ion decays in an ion trap, the recoil-daughter nucleus and emitted radiation emerge from the ∼1-mm3 trap volume with negligible scattering and the recoil energy can therefore be determined. This property of trapped samples allows the momentum and energy of particles that would otherwise be difficult (or even impossible) to detect to be precisely reconstructed. This approach has been used to determine β-ν angular correlations by reconstructing the neutrino momentum from measurements of the β and recoil-ion momenta [23–26] as indicated in Fig. 1(a).
FIG. 1. (a) In β decay, the neutrino momentum and energy (and therefore entire 3-body decay kinematics) can be reconstructed from measurements of the β and recoil-ion momenta. (b) In β-delayed neutron emission, the recoil from the leptons is much smaller than the recoil from neutron emission. The neutron energy can therefore be determined solely from the nuclear recoil as this can be approximated as a 2-body decay.
A similar approach can be used to perform delayed71
A Novel Approach to β-delayed . . .
NUCLEAR DATA SHEETS
N.D. Scielzo et al.
kapton window. The vacuum in the detector region was kept below 10−3 torr. With minimal energy loss in the vacuum window and a low threshold for the ΔE detector signals, β particles with energies as low as 25 keV could be detected. Two resistive-anode MCP detectors with nominal active areas of 50×50 mm2 were used in place of the 44mm diameter metal-anode detector used for recoil-ion detection in the original work. These detectors have a grounded, 89% transmission grid located 4.5 mm from the front face of the MCP which is biased to approximately −2.5 kV to accelerate the ions for detection. The intrinsic detection efficiency of MCP detectors is nearly independent of energy for keV-energy heavy ions [34]. The charge division at the resistive anode allowed the recoil-ion hit locations to be reconstructed with sub-mm precision. This position sensitivity allowed the distance travelled by each ion to be determined and together with the TOF determined the recoil-ion velocity and energy. The MCP detector housings were specially designed to be compact to fit between the electrodes of the BPT and to allow HPGe detectors to be brought within 10 cm of the trapped ion cloud. In addition, new electrodes were installed with tips that came within 11 mm of the trap center. With these electrodes, the ions could be confined with peak-to-peak rf voltages of around 200 V, a factor of 2 smaller than the fields used in Ref. [27]. The rf voltage was applied only to a region near the electrode tip while the back end of the electrode was held at ground. The amount of energy imparted to the low-energy recoil ions along their flight path was therefore reduced, resulting in longer TOFs and better separation between the recoil ions from neutron emission and all the other β-decays. With this improvement, the neutron spectrum can be measured down to 100 keV. The dashed lines in Fig. 2(b) illustrate the approximate energy thresholds for measuring 100-keV and 50-keV neutrons.
(a)
50 keV
100 keV
(b)
FIG. 2. The TOF spectra of the recoil ions following 137 I β decay. (a) The results of a proof-of-principle measurement using a small plastic scintillator and a small MCP detector [27]. (b) The raw TOF spectrum for approximately half the data set collected after the upgrades described in the text (shown without any physics cuts applied or corrections for backgrounds or detector response). The higher energy recoils (peaked at 500– 1000 ns) from delayed-neutron emission are clearly separated from the other lower energy recoils (peaked at 3000–8000 ns). The statistics are nearly an order of magnitude higher than the earlier results of Ref. [27].
tector array were implemented. These upgrades resulted in greater statistics, lower β particle and neutron energy thresholds, and improved neutron-energy resolution. The solid angle for both β detection and recoil-ion detection were each increased by factors of roughly 3, leading to an order of magnitude increase in coincident detection efficiency. Two large ΔE-E plastic scintillator detector telescopes were used for β spectroscopy. Each detector had a 1-mm-thick, 10.6-cm diameter ΔE detector located in front of a 10.2-cm-thick, 13.3-cm diameter E detector capable of stopping β particles with energies up to 15-20 MeV. The β particles were identified by energy deposition in the ΔE detector, as this thin detector has only a ∼1% intrinsic detection efficiency for γ rays and neutrons. The light from the ΔE scintillator is piped to two 1.5-inch diameter photomultiplier tubes (PMTs) using light-guide strips wrapped in thin specular reflectors. The E scintillator is coated in a layer of diffuse reflector paint and attached directly to a 5-inch diameter PMT. Each detector telescope was supported in its own vacuum chamber and was separated from the ultrahigh vacuum environment of the ion trap by a 10-μm-thick aluminized
IV.
ION TRAP FOR MEASUREMENTS AT CARIBU
A dedicated ion trap that is optimized for β-delayed neutron spectroscopy is currently being developed. The design is being guided by the experience from the upgraded BPT setup and will implement several additional improvements. The electrodes of this new system will have an open geometry similar to the BPT but will incorporate more complex longitudinal segmentation to provide a higher capture efficiency and minimize the transverse losses that can contribute to the background. The electrodes will also come even closer to the trap center to allow ion confinement with peak-to-peak rf voltages of less than 100 V. By further reducing the perturbation from the rf fields, it may be possible to measure the neutron-energy spectrum to energies where the neutron and lepton recoils are comparable (typically 25–50 keV). 72
A Novel Approach to β-delayed . . .
NUCLEAR DATA SHEETS
and momentum from the nuclear recoil. The BPT, instrumented with a β detector, a recoil-ion detector, and two HPGe detectors was used to determine the neutron energy spectrum and branching ratio in 137 I β decay by measuring the recoil-ion TOF. Following this initial work, the BPT was upgraded with an array of detectors better suited to the approach while limitations (statistics, energy thresholds, energy resolutions) were addressed. Building off the experience gained in this work, a dedicated ion trap is being developed to take advantage of CARIBU beams. With higher beam intensities and an optimized apparatus, extremely neutron-rich isotopes should be accessible in the near future.
The design of the new system will allow the trap to be surrounded by a larger array of plastic scintillator ΔEE telescopes, position-sensitive microchannel plate detectors, and HPGe detectors for β, recoil-ion, and γ-ray spectroscopy, respectively. The anticipated β-recoil ion coincidence efficiency will be over 2%. Although initial demonstrations could be performed with an offline 1-mCi 252 Cf source, the ×103 –104 increase in low-energy fission-fragment beam intensities available at the Californium Rare Ion Breeder Upgrade (CARIBU) [35] at Argonne National Laboratory will be required for the experimental program. With CARIBU beams and an optimized ion trap with a highly-efficient detector array, high-quality delayed-neutron measurements can be performed on exotic neutron-rich isotope beams as weak as ∼0.1 ion/s. This sensitivity will provide new opportunities to study the nuclear structure and decay properties of nuclei near and along the r-process path. On the other hand, the highest intensity beams are needed to precisely measure the decay properties of the isotopes at the fission-fragment mass peaks needed for nuclear-energy and stockpile-stewardship applications. V.
N.D. Scielzo et al.
Acknowledgements: We thank P.A. Vetter for lending the β and MCP detectors used for the initial demonstration, C.J. Lister and P. Wilt for assistance with HPGe detectors, and S. G. Prussin and P. Bedrossian for fruitful discussions. This work was supported by U.S. DOE under Contracts No. DE-AC02-06CH11357 (ANL), No. DEAC52-07NA27344 (LLNL), No. DE-FG02-98ER41086 (Northwestern U.); NSERC, Canada, under Application No. 216974; and the Department of Homeland Security. This material is based upon work supported by the National Science Foundation under Grant No. DGE0638477. R. M. Yee acknowledges support from the Lawrence Scholar Program at LLNL and the Berkeley Nuclear Research Center.
CONCLUSIONS
A new approach to β-delayed neutron spectroscopy has been described that avoids all the difficulties associated with neutron detection by inferring the neutron energy
[1] E.M. Burbidge et al., Rev. Mod. Phys. 29, 547 (1957). [2] K.-L. Kratz et al., Prog. Part. Nucl. Phys. 59, 147 (2007). [3] B. Pfeiffer et al., Nucl. Phys. A 693, 282 (2001). [4] K. Farouqi et al., Astrophys. J. 712, 1359 (2010). [5] S. Das, Prog. Nucl. Energy 28, 209 (1994). [6] A. D’Angelo, Prog. Nucl. Energy 41, 5 (2002). [7] E. Kiefhaber, Nucl. Sci. Eng. 111, 197 (1992). [8] R.J. Onega, R.J. Florian, Ann. Nucl. Energy 10, 477 (1983). [9] T. Kawano, P. Moller, W.B. Wilson, Phys. Rev. C 78, 054601 (2008). [10] E.A. McCutchan et al., Phys. Rev. C 86, 041305 (2012). [11] R. Surman, J. Engel, Phys. Rev. C 64, 035801 (2001). [12] T. Rauscher, Nucl. Phys. A 758, 655 (2005). [13] S. Goriely, Phys. Lett. B 436, 10 (1998). [14] R. Surman et al., Phys. Rev. C 79, 045809 (2009). [15] J. Beun et al., J. Phys. G 36, 025201 (2009). [16] K.-L. Kratz, W. Ziegert, W. Hillebrandt, F.-K. Thielemann, Astron. Astrophys. 125, 381 (1983). [17] U.C. Bergmann et al., Nucl. Phys. A 714, 21 (2003). [18] J.A. Winger et al., Phys. Rev. Lett. 102, 142502 (2009). [19] P. Hosmer et al., Phys. Rev. C 82, 025806 (2010).
[20] S. Padgett et al., Phys. Rev. C 82, 064314 (2010). [21] G. Savard, G. Werth, Ann. Rev. Nucl. Part. Sci. 50, 119 (2000). [22] J.A. Behr, G. Gwinner, J. Phys. G 36, 033101 (2009). [23] N.D. Scielzo, S.J. Freedman, B.K. Fujikawa, P.A. Vetter, Phys. Rev. Lett. 93, 102501 (2004). [24] A. Gorelov et al., Phys. Rev. Lett. 94, 142501 (2005). [25] X. Flechard et al., Phys. Rev. Lett. 101, 212504 (2008). [26] G. Li et al., Phys. Rev. Lett. 110, 092502 (2013). [27] R.M. Yee et al., Phys. Rev. Lett. 110, 092501 (2013). [28] N.D. Scielzo et al., Phys. Rev. A 68, 022716 (2003). [29] Proceeding of the Beta-Delayed Neutron Emission Evaluation INDC(NDS)-0599 (IAEA, Vienna, Austria, 2011). [30] N.D. Scielzo et al., Nucl. Instrum. Methods Phys. Res. A 681, 94 (2012). [31] J. Fallis et al., Phys. Rev. C 78, 022801(R) (2008). [32] N.D. Scielzo et al., Phys. Rev. C 80, 025501 (2009). [33] J. Fallis et al., Phys. Rev. C 84, 045807 (2011). [34] G.W. Fraser, Int. J. Mass Spectrom. 215, 13 (2002). [35] G. Savard et al., Nucl. Instrum. Methods Phys. Res. B 266, 4086 (2008).
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Nuclear Data Sheets 120 (2014) 70–73 www.elsevier.com/locate/nds
A Novel Approach to β-delayed Neutron Spectroscopy Using the Beta-decay Paul Trap N.D. Scielzo,1, ∗ R.M. Yee,1, 2 P.F. Bertone,3 F. Buchinger,4 S.A. Caldwell,3, 5 J.A. Clark,3 A. Czeszumska,1, 2 C.M. Deibel,6 J.P. Greene,3 S. Gulick,4 D. Lascar,3, 7 A.F. Levand,3 G. Li,3, 4 E.B. Norman,1, 2 S. Padgett,1 M. Pedretti,1 A. Perez Galvan,3 G. Savard,3, 5 R.E. Segel,7 K.S. Sharma,3, 8 M.G. Sternberg,3, 5 J. Van Schelt,3, 5 and B.J. Zabransky3 1
Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, USA 2 Department of Nuclear Engineering, University of California, Berkeley, California 94720, USA 3 Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA 4 Department of Physics, McGill University, Montr´eal, Qu´ebec H3A 2T8, Canada 5 Department of Physics, University of Chicago, Chicago, Illinois 60637, USA 6 Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA 7 Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA 8 Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada A new approach to β-delayed neutron spectroscopy has been demonstrated that circumvents the many limitations associated with neutron detection by instead inferring the decay branching ratios and energy spectra of the emitted neutrons by studying the nuclear recoil. Using the Beta-decay Paul Trap, fission-product ions were trapped and confined to within a 1-mm3 volume under vacuum using only electric fields. Results from recent measurements of 137 I+ and plans for development of a dedicated ion trap for future experiments using the intense fission fragment beams from the Californium Rare Isotope Breeder Upgrade (CARIBU) facility at Argonne National Laboratory are summarized. The improved nuclear data that can be collected is needed in many fields of basic and applied science such as nuclear energy, nuclear astrophysics, and stockpile stewardship. I.
INTRODUCTION
The properties of neutrons emitted following the β decay of fission fragments (known as delayed neutrons because they are emitted on the timescales of the β-decay half-lives) play an important role in nuclear astrophysics, fission reactor performance and control, and stockpilestewardship and radiochemistry applications. Half of the isotopes of elements heavier than iron are believed to be produced through the rapid-neutron capture process (r process) in which isotopes are produced through repeated neutron-capture reactions and β decays [1, 2]. Delayed-neutron branching ratios are needed for determining how the neutron-rich isotopes synthesized in r-process environments decay back to stability to form the isotopic abundances observed today. The resulting decay-chain shifts due to β-delayed neutron emission and a subsequent late capture of these neutrons during freeze out can be significant [3, 4] and needs to be well understood. To date, there have been few measurements of delayed-neutron properties near the proposed r-process path and extremely sensitive techniques are needed to
∗
Corresponding author: [email protected]
http://dx.doi.org/10.1016/j.nds.2014.07.009 0090-3752/© 2014 Elsevier Inc. All rights reserved.
reach these exotic, short-lived isotopes. Reviews of delayed-neutron properties [5, 6] highlight the need to obtain high-quality data to better understand the time-dependence and energy spectrum of the neutrons as these properties are essential for a detailed understanding of reactor kinetics needed for reactor safety and to predict the behavior of these reactors under various accident and component-failure scenarios. For fast breeder reactors, criticality-calculation approximations that are used for light-water reactors (such as assuming the delayed-neutron and fission-neutron energy spectra are identical) are not acceptable [7] and improved βdelayed neutron data is needed [8]. With higher-quality nuclear data, the delayed-neutrons flux and energy spectrum could be calculated from the contributions from individual isotopes and used to accurately model any fuelcycle concept, actinide mix, or irradiation history. Additional measurements are also critical to constrain modern nuclear-structure calculations [9] and empirical models [10] that predict the decay properties for nuclei for which no data exists. In the interior of astrophysical bodies and nuclear reactors, neutron-induced reactions can transmute the radioactive isotopes present. For example, the importance of neutron-capture rates for the non-equilibrium (“freeze-
A Novel Approach to β-delayed . . .
NUCLEAR DATA SHEETS
out”) phase of the r-process has been discussed [11–13] and recent works have shown that certain reaction rates can have a strong influence on the dynamics [14, 15]. Measuring cross sections on these neutron-rich species is a daunting technical problem and there is limited experimental data available to guide theoretical predictions, which can vary by an order of magnitude or more [14]. However, detailed studies of decay processes such as βdelayed neutron emission (which can be considered as the inverse process to neutron capture [16]) can provide important constraints to reaction-theory calculations. Neutron spectroscopy is challenging and the quality of the data available today for individual nuclei is limited. In some cases, discrepancies as large as factors of 2–4 in β-delayed neutron branching ratios have been uncovered [17–20]. In addition, for the vast majority of neutron emitters, the energy spectrum has not been measured and even for the few isotopes that have been studied in detail, large corrections for backgrounds and detector response must be applied to interpret the neutron energy spectra [5].
N.D. Scielzo et al.
neutron spectroscopy by inferring the neutron energy from the momentum it imparts to the nucleus (as illustrated in Fig. 1(b)). In β-delayed neutron emission, the nuclear recoil is dominated by the neutron emission because of the massiveness of the neutron relative to the leptons. The energy of the emitted neutron can therefore be reconstructed using conservation of momentum. This novel way to perform delayed-neutron spectroscopy allows neutron emission to be studied with a large and energy-independent detection efficiency, few backgrounds, and good energy resolution. All the wellknown challenges associated with direct neutron detection are avoided. The probability for the decay to occur via delayedneutron emission can be determined from the ratio of the number of delayed-neutron recoil ions to the number of (1) detected β particles, (2) β-delayed γ rays, and (3) recoil ions with longer time-of-flights characteristic of β and γ emission. These different measurements serve as an important check of many potential systematic effects and will allow branching ratios to be reliably determined.
II. INNOVATIVE APPROACH TO DELAYED-NEUTRON SPECTROSCOPY
III.
MEASUREMENTS OF IODINE-137
A proof-of-principle demonstration of this approach for studying β-delayed neutron emission was recently completed [27] using the Beta-decay Paul Trap (BPT) [30]. This trap was originally designed for tests of the Standard Model of particle physics such as precise studies of β-decay angular correlations in the decay of 8 Li [26]. However, it was realized that this trap could be used for delayed-neutron spectroscopy by outfitting it with the appropriate set of radiation detectors. A small plastic scintillator ΔE-E telescope and microchannel plate (MCP) detector (each subtending 3% of 4π) were used for β and recoil-ion detection, respectively. These detectors, which were originally used in Refs. [23, 28], were installed. The isotope 137 I was selected for a proof-of-principle demonstration because its decay properties are well characterized and it has both a large independent yield from the spontaneous fission of 252 Cf and a large delayedneutron branching ratio, (7.33±0.38% [29]). A 1-mCi 252 Cf spontaneous fission source was placed in the gas catcher of the Canadian Penning Trap injection system to produce low-energy, bunched beams of fission-fragment ions [31–33]. With this injection system, a 137 I+ beam of 20–30 ions/s was delivered to the BPT. The timeof-flight (TOF) spectrum of recoil ions following detection of a β particle in the plastic scintillator is shown in Fig. 2(a). The structure observed at 0.4–2.0 μs is due to recoil ions that receive a large momentum kick following neutron emission. The structure is consistent with the expected TOF spectrum based on direct measurements of the delayed-neutron energy spectrum. Recently, additional data for a more detailed study of 137 I (shown in Fig. 2(b)) were collected after straightforward upgrades to the trap electrode structure and de-
The sensitive recoil-ion spectroscopy techniques developed to test fundamental symmetries of the electroweak interaction [22] can be applied to perform precision βdelayed neutron spectroscopy. When a radioactive ion decays in an ion trap, the recoil-daughter nucleus and emitted radiation emerge from the ∼1-mm3 trap volume with negligible scattering and the recoil energy can therefore be determined. This property of trapped samples allows the momentum and energy of particles that would otherwise be difficult (or even impossible) to detect to be precisely reconstructed. This approach has been used to determine β-ν angular correlations by reconstructing the neutrino momentum from measurements of the β and recoil-ion momenta [23–26] as indicated in Fig. 1(a).
FIG. 1. (a) In β decay, the neutrino momentum and energy (and therefore entire 3-body decay kinematics) can be reconstructed from measurements of the β and recoil-ion momenta. (b) In β-delayed neutron emission, the recoil from the leptons is much smaller than the recoil from neutron emission. The neutron energy can therefore be determined solely from the nuclear recoil as this can be approximated as a 2-body decay.
A similar approach can be used to perform delayed71
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NUCLEAR DATA SHEETS
N.D. Scielzo et al.
kapton window. The vacuum in the detector region was kept below 10−3 torr. With minimal energy loss in the vacuum window and a low threshold for the ΔE detector signals, β particles with energies as low as 25 keV could be detected. Two resistive-anode MCP detectors with nominal active areas of 50×50 mm2 were used in place of the 44mm diameter metal-anode detector used for recoil-ion detection in the original work. These detectors have a grounded, 89% transmission grid located 4.5 mm from the front face of the MCP which is biased to approximately −2.5 kV to accelerate the ions for detection. The intrinsic detection efficiency of MCP detectors is nearly independent of energy for keV-energy heavy ions [34]. The charge division at the resistive anode allowed the recoil-ion hit locations to be reconstructed with sub-mm precision. This position sensitivity allowed the distance travelled by each ion to be determined and together with the TOF determined the recoil-ion velocity and energy. The MCP detector housings were specially designed to be compact to fit between the electrodes of the BPT and to allow HPGe detectors to be brought within 10 cm of the trapped ion cloud. In addition, new electrodes were installed with tips that came within 11 mm of the trap center. With these electrodes, the ions could be confined with peak-to-peak rf voltages of around 200 V, a factor of 2 smaller than the fields used in Ref. [27]. The rf voltage was applied only to a region near the electrode tip while the back end of the electrode was held at ground. The amount of energy imparted to the low-energy recoil ions along their flight path was therefore reduced, resulting in longer TOFs and better separation between the recoil ions from neutron emission and all the other β-decays. With this improvement, the neutron spectrum can be measured down to 100 keV. The dashed lines in Fig. 2(b) illustrate the approximate energy thresholds for measuring 100-keV and 50-keV neutrons.
(a)
50 keV
100 keV
(b)
FIG. 2. The TOF spectra of the recoil ions following 137 I β decay. (a) The results of a proof-of-principle measurement using a small plastic scintillator and a small MCP detector [27]. (b) The raw TOF spectrum for approximately half the data set collected after the upgrades described in the text (shown without any physics cuts applied or corrections for backgrounds or detector response). The higher energy recoils (peaked at 500– 1000 ns) from delayed-neutron emission are clearly separated from the other lower energy recoils (peaked at 3000–8000 ns). The statistics are nearly an order of magnitude higher than the earlier results of Ref. [27].
tector array were implemented. These upgrades resulted in greater statistics, lower β particle and neutron energy thresholds, and improved neutron-energy resolution. The solid angle for both β detection and recoil-ion detection were each increased by factors of roughly 3, leading to an order of magnitude increase in coincident detection efficiency. Two large ΔE-E plastic scintillator detector telescopes were used for β spectroscopy. Each detector had a 1-mm-thick, 10.6-cm diameter ΔE detector located in front of a 10.2-cm-thick, 13.3-cm diameter E detector capable of stopping β particles with energies up to 15-20 MeV. The β particles were identified by energy deposition in the ΔE detector, as this thin detector has only a ∼1% intrinsic detection efficiency for γ rays and neutrons. The light from the ΔE scintillator is piped to two 1.5-inch diameter photomultiplier tubes (PMTs) using light-guide strips wrapped in thin specular reflectors. The E scintillator is coated in a layer of diffuse reflector paint and attached directly to a 5-inch diameter PMT. Each detector telescope was supported in its own vacuum chamber and was separated from the ultrahigh vacuum environment of the ion trap by a 10-μm-thick aluminized
IV.
ION TRAP FOR MEASUREMENTS AT CARIBU
A dedicated ion trap that is optimized for β-delayed neutron spectroscopy is currently being developed. The design is being guided by the experience from the upgraded BPT setup and will implement several additional improvements. The electrodes of this new system will have an open geometry similar to the BPT but will incorporate more complex longitudinal segmentation to provide a higher capture efficiency and minimize the transverse losses that can contribute to the background. The electrodes will also come even closer to the trap center to allow ion confinement with peak-to-peak rf voltages of less than 100 V. By further reducing the perturbation from the rf fields, it may be possible to measure the neutron-energy spectrum to energies where the neutron and lepton recoils are comparable (typically 25–50 keV). 72
A Novel Approach to β-delayed . . .
NUCLEAR DATA SHEETS
and momentum from the nuclear recoil. The BPT, instrumented with a β detector, a recoil-ion detector, and two HPGe detectors was used to determine the neutron energy spectrum and branching ratio in 137 I β decay by measuring the recoil-ion TOF. Following this initial work, the BPT was upgraded with an array of detectors better suited to the approach while limitations (statistics, energy thresholds, energy resolutions) were addressed. Building off the experience gained in this work, a dedicated ion trap is being developed to take advantage of CARIBU beams. With higher beam intensities and an optimized apparatus, extremely neutron-rich isotopes should be accessible in the near future.
The design of the new system will allow the trap to be surrounded by a larger array of plastic scintillator ΔEE telescopes, position-sensitive microchannel plate detectors, and HPGe detectors for β, recoil-ion, and γ-ray spectroscopy, respectively. The anticipated β-recoil ion coincidence efficiency will be over 2%. Although initial demonstrations could be performed with an offline 1-mCi 252 Cf source, the ×103 –104 increase in low-energy fission-fragment beam intensities available at the Californium Rare Ion Breeder Upgrade (CARIBU) [35] at Argonne National Laboratory will be required for the experimental program. With CARIBU beams and an optimized ion trap with a highly-efficient detector array, high-quality delayed-neutron measurements can be performed on exotic neutron-rich isotope beams as weak as ∼0.1 ion/s. This sensitivity will provide new opportunities to study the nuclear structure and decay properties of nuclei near and along the r-process path. On the other hand, the highest intensity beams are needed to precisely measure the decay properties of the isotopes at the fission-fragment mass peaks needed for nuclear-energy and stockpile-stewardship applications. V.
N.D. Scielzo et al.
Acknowledgements: We thank P.A. Vetter for lending the β and MCP detectors used for the initial demonstration, C.J. Lister and P. Wilt for assistance with HPGe detectors, and S. G. Prussin and P. Bedrossian for fruitful discussions. This work was supported by U.S. DOE under Contracts No. DE-AC02-06CH11357 (ANL), No. DEAC52-07NA27344 (LLNL), No. DE-FG02-98ER41086 (Northwestern U.); NSERC, Canada, under Application No. 216974; and the Department of Homeland Security. This material is based upon work supported by the National Science Foundation under Grant No. DGE0638477. R. M. Yee acknowledges support from the Lawrence Scholar Program at LLNL and the Berkeley Nuclear Research Center.
CONCLUSIONS
A new approach to β-delayed neutron spectroscopy has been described that avoids all the difficulties associated with neutron detection by inferring the neutron energy
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