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Paul Garrett University of Guelph

Paul Garrett University of Guelph

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Paul Garrett University of Guelph

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  1. Pushing back the sensitivity frontier; high-statistics b-decay for nuclear structure studies Paul Garrett University of Guelph ISOL@MYRRHA Mol, Apr. 23 – 25, 2012

  2. The 8p spectrometer – a versatile tool for decay spectroscopy Auxiliary detectors as important for decay spectroscopy as in-beam spectroscopy – 8p uses auxiliaries for every experiment performed

  3. Moving tape collector for transport of activity – our most important auxiliary device • Beam implanted onto a moving tape at center of array • Allows for movement of long-lived activity out of focus of spectrometer • Tape speeds, dwell times, etc., variable • Control of beam kicker tightly coupled – beam collection time variable (down to ~ 100ms) • Tape transport very flexible: can be motionless, or operated in continuous motion • Disadvantage: study of long-lived activity in presence of intense, short-lived isobar Tape Cassette Lead Sheilding Built by E. Zganjar, LSU

  4. SCEPTAR – plastic scintillator array for tagging on b particles • SCEPTAR divided into two hemispheres. • Each half contains 10 thin (~1.6 mm) plastic scintillator panels. • 10 panels arranged in two pentagonal rings. SCEPTAR Plastic Scintillators Light Guides Photomultiplier Tubes 8p HPGe Detectors Photomultiplier Tubes Rectangular Plastic Scintillators • 1-to-1 correspondence between the SCEPTAR detectors and the Ge detectors allows to veto event that arises from high-energy b particles that reach Ge detector Light Guides Trapezoidal Plastic Scintillators

  5. PACES – 5 Si(Li) detectors inside target chamber for conversion electron detection E. Zganjar, LSU

  6. DANTE array for fast timing • 10 BaF2 detectors for fast timing measurements • In process of replacing with LaBr3

  7. LaBr3 detectors installed in 8p array with BGO shields • 5cm5cm LaBr3(Ce) detectors designed to fit within 8p BGO suppression shields • Currently have 6 detectors (7’th to be delivered in May) Timing with two LaBr3 detectors

  8. Nuclear structure studies with the 8p spectrometer • Three general themes • Studies related to fundamental symmetries, e.g. superallowed Fermi b decay, characterized by high-precision measurements • 14O, 18Ne, 19Ne, 26Alm, 38Km, 62Ga, 74Rb • Requires DAQ that has high degree of diagnostics, records pileup, deadtime, etc. • Studies related to nuclei far from stability, characterized by weak beams and low rates • 32Na, 50,52Ca, 102Rb • Requires high-efficiency detectors • Studies related to nuclei on or near stability, characterized by intense beams and high rates • 94Y, 110,112In/Ag, 124Cs, 156Ho, 158Tm • Studies on or near stability offer the huge advantage of large amounts of complementary data • Especially important are multi-step Coulex, (n,ng), (p,pg), etc., that offer a wide range of level lifetimes

  9. 124Xe – the fall of an “O(6)” nucleus Key 2+0+ transition never observed, crucial to establish band structure

  10. High-statistics 124Cs b-decay provides the solution 0.84% branch  59(18) W.u.! (preliminary) 1978, 24+ 289 = 58(18) W.u. 1690, 03+ 1335 354, 2+ 354 = 57.8(15) W.u. 0, 0+ 124Xe Intensity of 289-keV line is 10-5 of the 2+0+ Time-random background subtracted gg matrix 3.7108 events

  11. 124Cs isomer decay – ge- data This type of spectroscopy is especially important for the actinides

  12. 124Cs decay 1+, t½ =30.9s 124Cs 1548, 6+ 670 879, 4+ 525 354, 2+ 354 0, 0+ How does the 1+ parent populate the 6+ member of the daughter gsb? 124Xe It doesn’t!

  13. 124Csm decay – unknown b branch b+/EC ~0.2% branch 124Xe 2+0+ 124Xe 6+,7+,8+ levels 124Cs 3+1+ 124Xe 6+4+

  14. b-decay of 124Csm – access to off-yrast high-spin states (7+), t½ =30.9s b+/EC ~0.2% 124Cs 4096, 6+–8+ 3741, 6+–8+ 2778, 6+–8+ 2193 1230 2548 1548, 6+ 670 879, 4+ 525 354, 2+ 354 0, 0+ 124Xe With high-statistics measurements, unexpected new decay branches can give access to states not anticipated beforehand

  15. 94Zr structure • Attempts at vibrational interpretation, albeit with problems, mixed-symmetry states • Characterization of 2+ level at 1671-keV is crucial – mixed-symmetry interpretation widely accepted based on strong M1, 0.31 mN2, to 21+ state Mixed-symmetry states Phonon states Elhami et al, PRC 78, 064303 (2008), based on (n,ng) study

  16. 94Zr structure – an alternative interpretation? • Characterization of 2+ level at 1671-keV is crucial – mixed-symmetry interpretation widely accepted based on strong M1, 0.31 mN2, to 21+ state A deformed coexisting band? Existence of 2+0+ transition is crucial

  17. Test alternative interpretation – shape coexistence band – with b decay 695 2366, 24+ 752 1671 1671, 23+ 371? 1300, 02+ 382 919, 21+ 919 = 4.9(3) W.u. 0, 0+ 94Zr No evidence for 2+0+ transition – might conclude no deformed coexisting band

  18. Test alternative interpretation – shape coexistence band – with b decay 695 2366, 24+ 752 1671 1671, 23+ 371 1300, 02+ 382 919, 21+ 919 = 4.9(3) W.u. 0, 0+ 94Zr Gate from below reveals 371-keV transition – a 0.15% branch.

  19. A single transition reveals the structure Deformed band The 1671-keV 2+ is not the mixed-symmetry state The conclusive proof of the band structure, the enhanced B(E2), is only available because of the extensive complementary measurements 94Zr A. Chakraborty, S.W. Yates, et al.,

  20. Why did the gate from above not reveal the structure? Coincidence efficiency Angular correlation • Peak area  feeding intensity  draining branching fraction • Gating from above is limited by the draining fraction of 0.15% • In case of the 371-keV 2+0+ transition, gating from below enhances peak by factor of 18 Normalization factor Intensity of feeding g Area of peak Efficiency of feeding g Efficiency of draining g Branching ratio of draining g

  21. Results from 112Ag b- and 112In b+/EC decay experiment • gg-matrix projection • 200106 prompt events, 100106 events random subtracted, 48106 events 180 suppressed 157-keV 112Inm internal transition Important to have sufficiently wide coincidence window to properly account for time-randoms

  22. Future of b-decay at TRIUMF – GRIFFIN • Phase I fully funded ($8.9M) • 16 large-volume unsegmented clover detectors (40% crystals – 220% with addback) • Support structure, new beam line, etc. • Digital DAQ – triggerless with aim to write 300 MB/s (~1 MHz event rate) • Commission 2014 • Gamma-Ray InfrastructureFor Fundamental Investigations ofNuclei

  23. GRIFFIN Efficiency • Operate in 2 modes • maximum efficiency mode (detectors fully forward, source-to-detector distance 11 cm) • full-suppression mode (allow for future BGO shields)

  24. Enabling b-delayed n measurements • DESCANT – 1.1psrdeuterated scintillator neutron detector array being assembled to be mounted to TIGRESS and GRIFFIN spectrometers • En from ~100 keV to 10 MeV

  25. Issues to consider for b-decay spectroscopy • Statistics, statistics, statistics,… • With the 8p at TRIUMF, we aim for 108 – 109 events in our gg matrices • Tape collector with variable beam-spill control is vital • Flexibility with deposit either internal or external of array is highly desirable • High-throughput DAQ • Auxiliary detectors to increase range of physics • Si for conversion e- for multipolarities, E0 transitions • Need LN2 cooling – restrictive geometry • Si count rates quickly become limiting factor – segmentation • Introduction of fast-timing detectors like LaBr3 • Neutron detectors for b-delayed n emission • Perhaps consider large hall, false floors, to minimize neutron scatter background • Measurement of angular correlations • Complements e- for multipolarities, spin sequences • Many ( perhaps most? ) previous g branching ratios wrong • Singles problematic, coincidences affected by angular correlations

  26. Issues to consider for ISOL@MYRRHA • Design of a collection station • For long-lived implanted sources • Spares target chamber from contamination • Enables e- measurements on long-lived activities, e.g. 152Eu • Perhaps after pre-separator – parasitic mode possible? • Production of exotic targets (at collection station) • Long lived species can be deposited on C (or other) foils for subsequent experiments • Can be used in “normal” kinematics experiments here or at other facilities • e.g. TRIUMF is producing a 26Al target • Harvesting of beam dump • e.g. 178Hfm target from LANL beam dump • Requires hot chemistry lab

  27. Some final thoughts • The mad dash to the furthest reaches from stability will produce much speculation about structure, with few probes that provide conclusive evidence • Detailed systematic studies, anchored on nuclei on or near stability that can be studied with a wide variety of probes are a necessity • The detailed studies fit into the (refreshing) philosophy of ISOL@MYRRHA Special thanks • 94Zr – A. Chakraborty, S.W. Yates, and U. Kentucky group • 124Cs – A. Raddich (Guelph), 8p collaboration • LaBr3 – J. Michetti-Wilson (Guelph) • J. Wood

  28. ARIEL upgrade of TRIUMF facilities • TRIUMF plans include development of new 50 MeV 500 kW e-LINAC, ARIEL, as photo-fission driver • 25 MeV, 50kW by 2013, 100 kW by 2015 • 50 MeV, 100 kW by 2017, 500 kW by 2020 • Funded by Canadian Fund for Innovation (CFI) + BC provincial support for accelerator and civil infrastructure (~ $63M) Funding for target station, mass separator, beam transport, etc., must await next 5 year budget cycle beginning 2015

  29. Calculated in-target production 50 MeV, 100 kW N=50 Z=50 N=82 Z=28 GRIFFIN + ARIEL and p-induced spallation on actinide target will enable far-reaching science program for decades

  30. DESCANT detectors Detectors built by St. Gobain, filled with C6D6.

  31. DESCANT prototype performance

  32. Example of low beam-rate experiment – 32Na decay • 32Na decay investigated as a means to study the excited nuclear states of 32Mg (Z=12, N=20). • 32Na beam rate at ~2 ions/s • - coincidences measured with 8p and SCEPTAR. • Reduce background and allow weak 32Na decay spectrum to be measured. • Spectroscopy impossible without b tagging C.M. Mattoon et al., PRC 75, 017302 (2007)

  33. 112Cd – 2+ three-phonon candidate Unobserved transitions – upper limits placed Expected (HV) <2.2 <0.4 27(8) 2+ 42 31 17 0+ 4+ 2+ B(E20+)=17(5) B(E22+)<1.9 2+ B(E24+)<0.4 30 W.u. 0+ All values are B(E2)’s in W.u. unless indicated

  34. Analysis of 2+ states in 112Cd fed in b-decay 2+1 0+3 2+2 4+1 2674 keV 2+ 2765 keV 2+ 2945 keV 2+ 2121 keV 2+ 2506 keV 2+ 2231 keV 2+ 2723 keV 2+ 2853 keV 2+ 2156 keV 2+ 2980 keV 2+ Placed elsewhere in level scheme With high statistics and low backgrounds, can set restrictive limits on unobserved transitions – sometimes as important as what you see is what you don’t see!

  35. Summation of upper limits of B(E2) from excited 2+ states to 2-phonon states • Sum of B(E2) strength to 0+ 2-phonon level in agreement with harmonic expectations • But strength to 2+, and especially 4+, well below expectations • E2 strength to 2+ and 4+ 2-phonon levels does not appear to be fragmented, but missing • either non-existent or pushed beyond 3 MeV (5) Expected (HV) 2+ 42 31 17 0+ 4+ 2+

  36. Comparison of GRIFFIN efficiency vs 8p Single g detection

  37. 2+,3-,4+ ? Example: b decay of 32Na → 32Mg 1436 2+ 885 0+ eggGRIFFIN egg8p 32Mg ~ 300 3- 2+ 4+ Efficiency increase: 8p, 32Na @ 2 ions/s GRIFFIN gg: gate on 885 keV gg: gate on 885 keV C.M. Mattoon et al., PRC. 75, 017302 (2007)

  38. 124Cs isomer decay – gg data

  39. Improvement in g signal quality with b tagging with SCEPTAR – 62Ga decay bg coincidence, bremsstrahlung suppressed

  40. 11Li decay at ISAC with the 8p+SCEPTAR • b-gcoincidences: • Reduced background from room decays and induced reactions • Increased signal to background ratio, better quality lineshape 1’st experiment: 103 s-111Li 2’nd experiment 3104 s-111Li Mattoon et al., PRC, 80 034318 (2009)

  41. PACES with the 8p E. Zganjar, LSU

  42. Mass Separator (∆m/m ≈ 1000) removes neighboring isotopes. Variety of ion sources from surface ion to resonant ionization by laser Spallation reactions in primary target produce high-intensity secondary beams Up to 100 mA, 500 MeV proton beams from the TRIUMF main cyclotron impinge on the primary target.