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

1Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, USA2Department of Nuclear Engineering, University of California, Berkeley, California 94720, USA

3Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA4Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada

5Department of Physics, University of Chicago, Chicago, Illinois 60637, USA6Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA7Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA

8Department 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 themany limitations associated with neutron detection by instead inferring the decay branching ratiosand energy spectra of the emitted neutrons by studying the nuclear recoil. Using the Beta-decayPaul Trap, fission-product ions were trapped and confined to within a 1-mm3 volume under vacuumusing only electric fields. Results from recent measurements of 137I+ and plans for developmentof a dedicated ion trap for future experiments using the intense fission fragment beams from theCalifornium Rare Isotope Breeder Upgrade (CARIBU) facility at Argonne National Laboratory aresummarized. The improved nuclear data that can be collected is needed in many fields of basic andapplied science such as nuclear energy, nuclear astrophysics, and stockpile stewardship.

I. INTRODUCTION

The properties of neutrons emitted following the β de-cay of fission fragments (known as delayed neutrons be-cause they are emitted on the timescales of the β-decayhalf-lives) play an important role in nuclear astrophysics,fission reactor performance and control, and stockpile-stewardship and radiochemistry applications.

Half of the isotopes of elements heavier than iron arebelieved to be produced through the rapid-neutron cap-ture process (r process) in which isotopes are producedthrough repeated neutron-capture reactions and β de-cays [1, 2]. Delayed-neutron branching ratios are neededfor determining how the neutron-rich isotopes synthesizedin r-process environments decay back to stability to formthe isotopic abundances observed today. The resultingdecay-chain shifts due to β-delayed neutron emission anda subsequent late capture of these neutrons during freezeout can be significant [3, 4] and needs to be well under-stood. To date, there have been few measurements ofdelayed-neutron properties near the proposed r-processpath and extremely sensitive techniques are needed to

∗ Corresponding author: scielzo1@llnl.gov

reach these exotic, short-lived isotopes.Reviews of delayed-neutron properties [5, 6] highlight

the need to obtain high-quality data to better understandthe time-dependence and energy spectrum of the neu-trons as these properties are essential for a detailed un-derstanding of reactor kinetics needed for reactor safetyand to predict the behavior of these reactors under vari-ous accident and component-failure scenarios. For fastbreeder reactors, criticality-calculation approximationsthat are used for light-water reactors (such as assum-ing the delayed-neutron and fission-neutron energy spec-tra are identical) are not acceptable [7] and improved β-delayed neutron data is needed [8]. With higher-qualitynuclear data, the delayed-neutrons flux and energy spec-trum could be calculated from the contributions from in-dividual isotopes and used to accurately model any fuel-cycle concept, actinide mix, or irradiation history. Addi-tional measurements are also critical to constrain mod-ern nuclear-structure calculations [9] and empirical mod-els [10] that predict the decay properties for nuclei forwhich no data exists.

In the interior of astrophysical bodies and nuclear re-actors, neutron-induced reactions can transmute the ra-dioactive isotopes present. For example, the importanceof neutron-capture rates for the non-equilibrium (“freeze-

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A Novel Approach to β-delayed . . . NUCLEAR DATA SHEETS N.D. Scielzo et al.

out”) phase of the r-process has been discussed [11–13]and recent works have shown that certain reaction ratescan have a strong influence on the dynamics [14, 15].Measuring cross sections on these neutron-rich species isa daunting technical problem and there is limited exper-imental 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 β-delayed neutron emission (which can be considered as theinverse process to neutron capture [16]) can provide im-portant constraints to reaction-theory calculations.

Neutron spectroscopy is challenging and the quality ofthe data available today for individual nuclei is limited.In some cases, discrepancies as large as factors of 2–4in β-delayed neutron branching ratios have been uncov-ered [17–20]. In addition, for the vast majority of neutronemitters, the energy spectrum has not been measured andeven for the few isotopes that have been studied in detail,large corrections for backgrounds and detector responsemust be applied to interpret the neutron energy spec-tra [5].

II. INNOVATIVE APPROACH TODELAYED-NEUTRON SPECTROSCOPY

The sensitive recoil-ion spectroscopy techniques devel-oped to test fundamental symmetries of the electroweakinteraction [22] can be applied to perform precision β-delayed neutron spectroscopy. When a radioactive iondecays in an ion trap, the recoil-daughter nucleus andemitted radiation emerge from the ∼1-mm3 trap volumewith negligible scattering and the recoil energy can there-fore be determined. This property of trapped samplesallows the momentum and energy of particles that wouldotherwise be difficult (or even impossible) to detect tobe precisely reconstructed. This approach has been usedto determine β-ν angular correlations by reconstructingthe neutrino momentum from measurements of the β andrecoil-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 recon-structed from measurements of the β and recoil-ion momenta.(b) In β-delayed neutron emission, the recoil from the leptonsis much smaller than the recoil from neutron emission. Theneutron energy can therefore be determined solely from thenuclear recoil as this can be approximated as a 2-body decay.

A similar approach can be used to perform delayed-

neutron spectroscopy by inferring the neutron energyfrom the momentum it imparts to the nucleus (as il-lustrated in Fig. 1(b)). In β-delayed neutron emission,the nuclear recoil is dominated by the neutron emis-sion because of the massiveness of the neutron relativeto the leptons. The energy of the emitted neutron cantherefore be reconstructed using conservation of momen-tum. This novel way to perform delayed-neutron spec-troscopy allows neutron emission to be studied with alarge and energy-independent detection efficiency, fewbackgrounds, and good energy resolution. All the well-known challenges associated with direct neutron detec-tion are avoided.

The probability for the decay to occur via delayed-neutron emission can be determined from the ratio ofthe number of delayed-neutron recoil ions to the numberof (1) detected β particles, (2) β-delayed γ rays, and (3)recoil ions with longer time-of-flights characteristic of βand γ emission. These different measurements serve asan important check of many potential systematic effectsand will allow branching ratios to be reliably determined.

III. MEASUREMENTS OF IODINE-137

A proof-of-principle demonstration of this approach forstudying β-delayed neutron emission was recently com-pleted [27] using the Beta-decay Paul Trap (BPT) [30].This trap was originally designed for tests of the Stan-dard Model of particle physics such as precise studiesof β-decay angular correlations in the decay of 8Li [26].However, it was realized that this trap could be used fordelayed-neutron spectroscopy by outfitting it with the ap-propriate set of radiation detectors. A small plastic scin-tillator ΔE-E telescope and microchannel plate (MCP)detector (each subtending 3% of 4π) were used for β andrecoil-ion detection, respectively. These detectors, whichwere originally used in Refs. [23, 28], were installed.

The isotope 137I was selected for a proof-of-principledemonstration because its decay properties are well char-acterized and it has both a large independent yield fromthe spontaneous fission of 252Cf and a large delayed-neutron branching ratio, (7.33±0.38% [29]). A 1-mCi252Cf spontaneous fission source was placed in the gascatcher of the Canadian Penning Trap injection systemto produce low-energy, bunched beams of fission-fragmentions [31–33]. With this injection system, a 137I+ beamof 20–30 ions/s was delivered to the BPT. The time-of-flight (TOF) spectrum of recoil ions following detec-tion of a β particle in the plastic scintillator is shown inFig. 2(a). The structure observed at 0.4–2.0 μs is due torecoil ions that receive a large momentum kick followingneutron emission. The structure is consistent with theexpected TOF spectrum based on direct measurementsof the delayed-neutron energy spectrum.

Recently, additional data for a more detailed study of137I (shown in Fig. 2(b)) were collected after straightfor-ward upgrades to the trap electrode structure and de-

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A Novel Approach to β-delayed . . . NUCLEAR DATA SHEETS N.D. Scielzo et al.

100

keV

50 k

eV

(a)

(b)

FIG. 2. The TOF spectra of the recoil ions following 137I β de-cay. (a) The results of a proof-of-principle measurement usinga small plastic scintillator and a small MCP detector [27]. (b)The raw TOF spectrum for approximately half the data setcollected after the upgrades described in the text (shown with-out any physics cuts applied or corrections for backgrounds ordetector response). The higher energy recoils (peaked at 500–1000 ns) from delayed-neutron emission are clearly separatedfrom the other lower energy recoils (peaked at 3000–8000 ns).The statistics are nearly an order of magnitude higher thanthe earlier results of Ref. [27].

tector array were implemented. These upgrades resultedin greater statistics, lower β particle and neutron energythresholds, and improved neutron-energy resolution.

The solid angle for both β detection and recoil-ion de-tection were each increased by factors of roughly 3, lead-ing to an order of magnitude increase in coincident de-tection efficiency. Two large ΔE-E plastic scintillator de-tector telescopes were used for β spectroscopy. Each de-tector had a 1-mm-thick, 10.6-cm diameter ΔE detectorlocated in front of a 10.2-cm-thick, 13.3-cm diameter Edetector capable of stopping β particles with energies upto 15-20 MeV. The β particles were identified by energydeposition in the ΔE detector, as this thin detector hasonly a ∼1% intrinsic detection efficiency for γ rays andneutrons. The light from the ΔE scintillator is piped totwo 1.5-inch diameter photomultiplier tubes (PMTs) us-ing light-guide strips wrapped in thin specular reflectors.The E scintillator is coated in a layer of diffuse reflectorpaint and attached directly to a 5-inch diameter PMT.Each detector telescope was supported in its own vacuumchamber and was separated from the ultrahigh vacuumenvironment of the ion trap by a 10-μm-thick aluminized

kapton window. The vacuum in the detector region waskept below 10−3 torr. With minimal energy loss in thevacuum window and a low threshold for the ΔE detectorsignals, β particles with energies as low as 25 keV couldbe detected.

Two resistive-anode MCP detectors with nominal ac-tive areas of 50×50 mm2 were used in place of the 44-mm diameter metal-anode detector used for recoil-iondetection in the original work. These detectors have agrounded, 89% transmission grid located 4.5 mm fromthe front face of the MCP which is biased to approxi-mately −2.5 kV to accelerate the ions for detection. Theintrinsic detection efficiency of MCP detectors is nearlyindependent of energy for keV-energy heavy ions [34].The charge division at the resistive anode allowed therecoil-ion hit locations to be reconstructed with sub-mmprecision. This position sensitivity allowed the distancetravelled by each ion to be determined and together withthe TOF determined the recoil-ion velocity and energy.The MCP detector housings were specially designed tobe compact to fit between the electrodes of the BPT andto allow HPGe detectors to be brought within 10 cm ofthe trapped ion cloud.

In addition, new electrodes were installed with tips thatcame within 11 mm of the trap center. With these elec-trodes, the ions could be confined with peak-to-peak rfvoltages of around 200 V, a factor of 2 smaller than thefields used in Ref. [27]. The rf voltage was applied onlyto a region near the electrode tip while the back end ofthe electrode was held at ground. The amount of energyimparted to the low-energy recoil ions along their flightpath was therefore reduced, resulting in longer TOFs andbetter separation between the recoil ions from neutronemission and all the other β-decays. With this improve-ment, the neutron spectrum can be measured down to100 keV. The dashed lines in Fig. 2(b) illustrate the ap-proximate energy thresholds for measuring 100-keV and50-keV neutrons.

IV. ION TRAP FOR MEASUREMENTS ATCARIBU

A dedicated ion trap that is optimized for β-delayedneutron spectroscopy is currently being developed. Thedesign is being guided by the experience from the up-graded BPT setup and will implement several additionalimprovements.

The electrodes of this new system will have an opengeometry similar to the BPT but will incorporate morecomplex longitudinal segmentation to provide a highercapture efficiency and minimize the transverse losses thatcan contribute to the background. The electrodes willalso come even closer to the trap center to allow ion con-finement 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 spec-trum to energies where the neutron and lepton recoils arecomparable (typically 25–50 keV).

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The design of the new system will allow the trap to besurrounded by a larger array of plastic scintillator ΔE-E telescopes, position-sensitive microchannel plate detec-tors, and HPGe detectors for β, recoil-ion, and γ-rayspectroscopy, respectively. The anticipated β-recoil ioncoincidence efficiency will be over 2%.

Although initial demonstrations could be performedwith an offline 1-mCi 252Cf source, the ×103–104 in-crease in low-energy fission-fragment beam intensitiesavailable at the Californium Rare Ion Breeder Upgrade(CARIBU) [35] at Argonne National Laboratory will berequired for the experimental program. With CARIBUbeams and an optimized ion trap with a highly-efficientdetector array, high-quality delayed-neutron measure-ments can be performed on exotic neutron-rich isotopebeams as weak as ∼0.1 ion/s. This sensitivity will pro-vide new opportunities to study the nuclear structure anddecay properties of nuclei near and along the r-processpath. On the other hand, the highest intensity beamsare needed to precisely measure the decay properties ofthe isotopes at the fission-fragment mass peaks needed fornuclear-energy and stockpile-stewardship applications.

V. CONCLUSIONS

A new approach to β-delayed neutron spectroscopy hasbeen described that avoids all the difficulties associatedwith neutron detection by inferring the neutron energy

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 theneutron energy spectrum and branching ratio in 137I βdecay by measuring the recoil-ion TOF. Following thisinitial work, the BPT was upgraded with an array ofdetectors better suited to the approach while limita-tions (statistics, energy thresholds, energy resolutions)were addressed. Building off the experience gainedin this work, a dedicated ion trap is being developedto take advantage of CARIBU beams. With higherbeam intensities and an optimized apparatus, extremelyneutron-rich isotopes should be accessible in the nearfuture.

Acknowledgements: We thank P.A. Vetter for lendingthe β and MCP detectors used for the initial demonstra-tion, C.J. Lister and P. Wilt for assistance with HPGedetectors, and S. G. Prussin and P. Bedrossian for fruitfuldiscussions. This work was supported by U.S. DOE underContracts No. DE-AC02-06CH11357 (ANL), No. DE-AC52-07NA27344 (LLNL), No. DE-FG02-98ER41086(Northwestern U.); NSERC, Canada, under ApplicationNo. 216974; and the Department of Homeland Secu-rity. This material is based upon work supported bythe National Science Foundation under Grant No. DGE-0638477. R. M. Yee acknowledges support from theLawrence Scholar Program at LLNL and the BerkeleyNuclear Research Center.

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