High-Precision Half-Life Measurements for the Superallowed β+ Emitter 10C: Implications for Weak Scalar Currents
Michelle R. Dunlop University of Guelph Nuclear Structure, July 28 2016 Superallowed Fermi Beta Decays • In general, β decay ft values can be expressed as: Matrix Phase Space (Q-value) Constants Element K ft = 2 2 2 2 GA Mfi(GT ) + GV Mfi(F ) T1/2, BR | | | | Weak Coupling Strength
• For the special case of 0+ ⟶ 0+, we have a pure Fermi (Sβ = 0) allowed (Lβ = 0) transition. Transitions between isobaric analogue states are known as superallowed Fermi beta decays.
10C 0+ 10B 0+ Testing Fundamental Properties of the Weak Interaction with Superallowed Decays • The Fermi beta decay transition is the isospin ladder operator. For superallowed transitions between T=1 isobaric analogue states, the matrix element is simply:
M (F ) 2 =(T T )(T + T + 1) = 2 | fi | Z Z • The superallowed ft values thus simplify: K K ft = G2 M (GT ) 2 + G2 M (F ) 2 ) 2G2 A| fi | V | fi | V { { { = 0 for Pure = 2 for decays constants Fermi decays between T=1 (Conserved IAS Vector Current hypothesis) A Selection of Beta Decay ft Values
Superallowed ft values 3090 { K 3080 ft 62 ⇡ 2G2 Ga V 74 Rb 38 Ca 3070 2% 22 Mg 34 3060 Ar (s) ft (s) 54 ft 38 m 46 Co 34 10 K V C Cl 3050 14 O 50 42 Sc Mn 26 m 3040 Al
3030 0 10 20 30 40 Z of Daughter Superallowed Fermi Beta Decay: Theoretical Corrections δ δ δ K Ft = ft (1+ R )(1+ NS - C) = 2 = constant 2Gv (1+ΔR)
“Corrected” Experiment Calculated corrections (≈1%) CVC Hypothesis ft value ◦ Half-life (nucleus dependent) ◦ Q-value ◦ Branching Ratio Inner radiative correction (≈2.4%) (nucleus independent)
ΔR = nucleus independent inner radiative correction: 2.361(38)%
2 3 δR = nucleus dependent radiative correction to order Z α : ≈1.4% - depends on electron’s energy and Z of nucleus
δNS = nuclear structure dependent radiative correction: -0.3% to 0.03%
δC = nucleus dependent isospin-symmetry-breaking (ISB) correction: 0.2% to 1.5% - strong nuclear structure dependence (radial overlap) Corrected Superallowed Ft Values
• The superallowed data confirm the CVC hypothesis at the level of 1.2 x 10-4. Search for Physics Beyond the Standard Model
• The SM description of weak interaction is an equal mixture of vector and axial-vector which maximizes parity violation. • We can seek evidence that the weak interaction is not pure “V-A” by searching for contributions from scalar or tensor couplings in the weak interaction. • In particular, the superallowed data are sensitive to contributions from scalar interactions.
SM description With Scalar Interaction
1 1 t = constantt = constant t(1 +t(1b + bWF W) = constant) = constant F F ) F! F F h h i i { W = Total Positron Energy The ⟨W-1⟩ dependence means that it is the lowest-Z bF =FierzInterferenceTerm decays that are most = 1 (↵Z)2 sensitive to contributions from a scalar interaction p
Search for Physics Beyond the Standard Model 1 t = constant t(1 + b W ) = constant F ! F F h i
32.75 • The Fierz interference term 34 Ar 32.70 (bF) can be extracted by a b = -0.0028 ± 0.0025 2 -1 F bF = -0.0028 ± 0.0026χ /ν = 0.40 linear fit of 1/Ft vs Ɣ⟨W ⟩ 32.65 62 Ft = 3070.1 ± 2.0 s Ga 0
54 Co 34 Cl -5 32.60 42 Sc • Current limit from Hardy and
32.55 Towner, Phys Rev. C 91 50 Mn 38 m 26 m K 14 Al O 025501 (2015): 1/Ft (1/s) x 10 46 32.50 V bF = -0.0028(26)
32.45 10 38 C Ca 22 32.40 Mg 74 Rb 32.35 0 0.1 0.2 0.3 0.4 0.5 0.6 -1 γ
32.75 • The Fierz interference term 34 Ar 32.70 (bF) can be extracted by a b = -0.0028 ± 0.0025 2 -1 F bF = -0.0028 ± 0.0026χ /ν = 0.40 linear fit of 1/Ft vs Ɣ⟨W ⟩ 32.65 62 Ft = 3070.1 ± 2.0 s Ga 0
54 Co 34 Cl -5 32.60 42 Sc • Current limit from Hardy and
32.55 Towner, Phys Rev. C 91 50 Mn 38 m 26 m K 14 Al O 025501 (2015): 1/Ft (1/s) x 10 46 32.50 V bF = -0.0028(26)
32.45 10 38 C Ca 22 32.40 Mg • Limits are dominated by the 74 Rb lightest superallowed decays 32.35 0 0.1 0.2 0.3 0.4 0.5 0.6 14 10 -1 O and, in particular, C γ
T1/2 = 19.3052 ± 0.0071 s The 10C half-life • The 10C half-life is important because it has a significant impact on the limits set on scalar currents. • More than 200 individual superallowed measurements of comparable precision are currently used to set the limit on bF. • If the half-life from either of the two peaks in the ideograph is adopted, the central value of bF is shifted by more than 0.5σ.
bF = -0.0016 ± 0.0026 bF = -0.0031 ± 0.0026
• An accurate and precise determination of the 10C half-life is thus critical to the limits set of bF set by the superallowed data. TRIUMF-ISAC
• Up to 100 μA, 500 MeV protons from TRIUMF’s main cyclotron are accelerated onto targets which produce high-intensity secondary radioactive ion beams by the ISOL technique • NiO target used to obtain CO+ and C+ ions • Radioactive ion beams of 10C16O (A=26) and 10C (A=10), with beam intensities of ~1.5x105 pps and ~2.5x104 pps, respectively, were delivered to the detectors The 10C decay scheme • 10C decays to the excited states in 10B giving us the opportunity to measure the half-life by both direct measurements of the emitted β particle or by measuring the ɣ-ray emitted during the decay to the ground state.
10 C 0+
T1/2 = 19.3 s Q = 3648 keV
β+ 0+ 1740 1.4645%
1022 keV
1+ 718 98.530%
718 keV 10 B 3+ g.s.
stable ɣ Counting — The 8π Spectrometer
• Spherical array of 20 BGO Compton suppressed HPGe detectors • ~1% photopeak efficiency at 1.3 MeV • Beam implanted onto a tape at the centre of the array. The decay activity was measured for 500 s (for ~25 half-lives). Tape moved into disposal box which is shielded by a lead wall (removes long lived contaminants out of view of the detector). ɣ Photopeak Counting • The 10C half-life was measured by gating on the characteristic 718-keV ɣ-ray which follows 100% of 10C β decays.
7 10 7 10
718 keV 6 10 6 5 10 10 Counts
4 10
5 3 10 10 705 710 715 720 725 E (keV) 1022 keV γ
4 10 Counts per 0.5 keV
3 10
2 10 0 200 400 600 800 1000 1200 1400 1600 1800 2000 E (keV) γ ɣ Photopeak Counting
• A total of 58 runs were taken, comprised of 562 cycles. • After a run, the shaping times and dead-times were varied. • The dead-time and pile-up corrected data are summed and a single fit to the data is performed.
Half-life Vs. Run
6 10 19.6 (a) (b) 10 T1/2( C) = 19.2970 ± 0.0052 s 5 2 10 19.5 χ /ν = 1.19 10 T1/2 ( C) = 19.2969 ± 0.0052 s
4 2 10 χ /ν = 0.87 19.4 Counts per 1.2 s 3 10
Half-life (s) 19.3 ] i
σ 4 )/
fit 19.2 2 -y i 0 -2 -4 19.1 Residuals [(y 0 100 200 300 400 500 0 10 20 30 40 50 60 Time (s) Run Number Symbols = Different Shaping Times Colours = Different Applied Dead Times ɣ Photopeak Counting — Systematics
• The data are grouped according to their experimental running conditions. The grouping with the largest �2/ν > 1 is used to inflate the uncertainty on the measured half-life, as recommended by the Particle Data Group (PDG). Systematics
19.35
19.34 All Data Dead Time (µs): Variable, 27, 40 19.33 Shaping Time (µs): 0.5, 1.0, 2.0 Beam: Pre-Switching; Switching 19.32
19.31
19.3
C Half-life (s) 19.29 10
19.28
19.27
2 2 2 19.26 χ /ν = 1.01 χ /ν = 2.01 χ /ν = 0.72
19.25 0 1 2 3 4 5 6 7 8 9 10
10 T1/2,ɣ( C) = 19.2969 ± 0.0052sys ± 0.0052stat = 19.2969 ± 0.0074 s Relative precision of 0.03% β Counting — 4π gas counter
Tape Spool Atmosphere High vacuum (~10-6 T)
Beam Implantation Site
~ 28 cm Tape Disposal Box
Gas Counter
• 4π continuous-flow proportional gas counter with tape transfer system • Directly detects β particles with ~100% efficiency • Cycles: Implant beam, move into gas counter and measured the decay activity for ~500 s, move tape into tape disposal box to remove long-lived contaminants from the gas counter. β Counting — In-beam Contaminant
• Radioactive beams of both 10C (A=10) and 10C16O (A=26) were delivered to the gas counter
• In the A=26 beam, a contaminant with T1/2 = 9.97(8) min was identified
• T1/2 consistent with the that of 13N, which could be delivered 13 as N2 (mass difference to 10C16O of 0.271 MeV) or HC13N (mass diff. of 1.673 MeV) in the A=26 beam. 13 • The literature T1/2 of N was included as a fixed parameter in the fit to the subset of data taken with the A=26 beam. β Counting
• A total of 82 runs were taken, comprised of 598 cycles. • After a run, parameters such as the dwell time, beam type, and gas counter, (etc..) were changed before a new run was started.
19.45 6 10 (a) (b) 10 C decay 10 5 T ( C) = 19.3009 ± 0.0017 s 13 19.40 1/2 10 N contaminant 2 Background χ /ν= 1.02 4 10 Best Fit Result 19.35
3 e (s)
10 lif Counts per 1 s - lf
2 Ha 19.30 10 10 2
] T ( C) = 19.2987 ± 0.0030 s χ /ν = 1.03 i 1/2 σ
)/ 4 fit
-y 19.25
i 2 0 -2 -4 0 100 200 300 400 500 19.20 Residuals [(y 0 20 40 60 80 Time (s) Run Number Open (Filled) Circles = 10C (10C16O) Radioactive Beam Colours = Different Gas Counters β Counting — Systematics • Extensive investigation of systematics was performed. • Each grouping of the experimental parameters yield a �2/ν < 1 and hence no inflation of the statistical uncertainty. • Vary the fixed parameters within their ±1σ limits. No statistically significant change in the half-life is observed. Systematics 19.36 All Data Bias Voltage (V): 2400, 2450, 2500, 2550, 2600, 2700, 2750, 2800 13 T ( N): +/- 1σ 19.35 Threshold (mV): 70, 95, 120 1/2 19.303 Deadtime: +/- 1 Dwell Time (s): 1.0, 1.2, 1.4 σ 10 16 10 19.34 Radioactive Beam: C O, C Gas Counter: 1, 2 19.302 10 19.33 Initial C Activity (kHz): <6, 6-8, 8-10, 10-12, >12
19.301 19.32 10 T ( C) = 19.3009 ± 0.0017 s 1/2 C Half-life (s) 10 Half-life (s) 19.31 19.3
19.30 19.299
19.29
2 19.298 χ /ν = 0.92 0.13 0.54 0.06 0.62 0.62 0 1 2 3 4 5 6 19.28 10 T1/2,β( C) = 19.3009 ± 0.0017 s
The most precise (0.009%) superallowed T1/2 reported to date! Results — 10C Half-life • The 6 most precise half-life measurements now yield 2 T1/2 = 19.3015 ± 0.0015 s with a � /ν = 1.90, which is an improvement in the uncertainty by a factor of nearly 3.
10 T1/2( C) = 19.3052 ± 0.0071 s Results — Scalar Current Limits • With the updated 10C half-life and newly updated 14O Q-value (A. A. Valverde et al., Phys. Rev. Lett. 114, 232502 (2015)) and 14O branching ratio (P. A. Voytas et al. Phys. Rev. C 92, 065502 (2015)) measurements, a linear fit to the superallowed Ft data yields an updated Fierz interference
term of: 32.75 34 Ar 32.70 bF = -0.0018(21) bF = -0.0018 ± 0.0021 Ft 32.65 62 0 = 3070.8 ± 1.8 s and Ga 2 54 χ /ν = 0.44
-5 Co 42 34 Cs/CV = -bF/2 = 32.60 Sc Cl 0.0009(11) 32.55 50 (1/s) x 10 Mn 26 m 14 for the ratio of weak 38 m K Al O Ft 46 32.50 V scalar to vector 1/
couplings assuming 32.45 10 38 C left handed neutrinos Ca 32.40 22 74 Mg Rb 32.35 0 0.1 0.2 0.3 0.4 0.5 0.6 -1 γ
• Measurements of the 10C half-life were performed in order to resolve inconsistencies in the data set. Conclusions
• Measurements of the 10C half-life were performed in order to resolve inconsistencies in the data set. • Resolving the discrepancy in the 10C half-life is important because it is the lowest-Z superallowed decay and is most sensitive to contributions from scalar currents in the weak interaction. Conclusions
• Measurements of the 10C half-life were performed in order to resolve inconsistencies in the data set. • Resolving the discrepancy in the 10C half-life is important because it is the lowest-Z superallowed decay and is most sensitive to contributions from scalar currents in the weak interaction. • We measured the half-life via both ɣ-ray photopeak counting and direct β counting, obtaining consistent results. Conclusions
• Measurements of the 10C half-life were performed in order to resolve inconsistencies in the data set. • Resolving the discrepancy in the 10C half-life is important because it is the lowest-Z superallowed decay and is most sensitive to contributions from scalar currents in the weak interaction. • We measured the half-life via both ɣ-ray photopeak counting and direct β counting, obtaining consistent results.
• The β measurement (T1/2 = 19.3009(17) s) represents the most precise superallowed half-life measurement reported to date and the first to -4 achieve a precision below 10 . Conclusions
• Measurements of the 10C half-life were performed in order to resolve inconsistencies in the data set. • Resolving the discrepancy in the 10C half-life is important because it is the lowest-Z superallowed decay and is most sensitive to contributions from scalar currents in the weak interaction. • We measured the half-life via both ɣ-ray photopeak counting and direct β counting, obtaining consistent results.
• The β measurement (T1/2 = 19.3009(17) s) represents the most precise superallowed half-life measurement reported to date and the first to -4 achieve a precision below 10 . • The updated Ft data, which includes the 10C half-life measurements presented here and recent measurements of the 14O Q-value and BR, are used to calculate the limits on scalar currents yielding bF = -0.0018(21) which remains fully consistent with the absence of weak scalar currents. Acknowledgements G. F. GRINYER
E. F. ZGANGAR V. BILDSTEIN G. C. BALL A. DIAZ VARELA T. BALLAST R. DUNLOP P. BENDER P. E. GARRETT A. B. GARNSWORTHY B. HADINIA D. MILLER C. ANDREOIU D. JAMIESON B. MILLS A. T. LAFFOLEY J. PARK A. D. MACLEAN M. M. RAJABALI A. J. RADICH C. UNSWORTH E. RAND Z. WANG J. R. LESLIE C. E. SVENSSON
R. A. E. AUSTIN A. VALENCIA Backup Slides Limits on Weak Scalar Currents
• Superallowed data gives us bF, which we can relate to a scalar coupling = as CS/CV = -bF/2 = +0.0009(11) given Cs Cʹs
• For general CS ≠ CʹS we have bF = -(Cs/CV + Cʹs/CV) and need to use β-ν angular correlation coefficient to provide an additional constraint
32.75 34 Ar 32.70 bF = -0.0018 ± 0.0021 Ft 32.65 62 0 = 3070.8 ± 1.8 s Ga 2 54 χ /ν = 0.44
-5 Co 42 34 32.60 Sc Cl
32.55 50 (1/s) x 10 Mn 26 m 14 38 m K Al O Ft 46 32.50 V 1/
32.45 10 38 C Ca 32.40 22 74 Mg Rb 32.35 0 0.1 0.2 0.3 0.4 0.5 0.6 -1 γ
• Using GV as determined from the superallowed ft data can be used to calculate Vud and test CKM unitarity. • The most demanding test of the CKM unitarity comes from the sum of the squares of the top-row elements • The superallowed data provide the most precise experimental measure for Vud. 2 2 2 Vud = GV /GF Vud + Vus + Vub =0.99978(55)
d0 Vud Vus Vub d s0 = V V V s 0 1 0 cd cs cb 1 0 1 b0 Vtd Vts Vtb b @ A @ A @ A Pile-Up Correction 0.5 µs Shaping Time Pileup Correction 0.04 Pile-Up Correction 2.0 µs Shaping Time 0.03 0.025
Data Best-fit Curve Pile-up of 2 or more events 0.02 Cosmic Pile-up 0.02
0.015 Probability of Pile-Up
0.01 Data Best Fit 0.01 Cosmic pile-up
Probability of Pile-Up Pile-up of 2 or more events
0.005 0 0 50 100 150 200 250 300 350 400 ChopTime (1.2 s/bin)Plot
0 Summed Data 0 50 100 150 200 250 300 350 400 19.33 Time (1.2 s/bin)
Pile-Up Correction 1.0 µs Shaping Time 19.32 0.03
19.31
0.02 19.3 Half-life (s)
19.29
Probability of Pile-Up 0.01 Data Best Fit Cosmic pile-up 19.28 No pile-up correction applied Pile-up of 2 or more events Pile-up correction applied
0 0 50 100 150 200 250 300 350 400 Time (1.2 s/bin) 19.27 0 20 40 Number of Leading Channels Removed Gas CounterHigh Rate Data Removal Rates
19.31
19.305
19.3 Half-life (s)
19.295 Systematics 10 C Activity at t=0 19.35 19.29 4 5 6 7 8 9 10 11 12 13 14 15 16 Initial Count Rate Upper Limit (kHz) 19.34 2-4 kHz 2 χ /ν = 0.69
19.33
19.32
12-14 kHz
Half-life (s) 19.31 4-6 kHz 8-10 kHz 6-8 kHz 10-12 kHz 19.30 14-16 kHz 19.29
19.28 0 2 4 6 8 10 12 14 10 t=0 C Activity (kHz) ɣ Photopeak Counting - Contaminants
• Searched for possible contribution within the 718 keV photopeak from possible contaminants in the A = 26 beam.
γ−β Coincidence Spectrum Gated on the first 7.2 seconds of the implation 10 26 (1) Na (T1/2 = 1.07128(25) s) 10000
1 Counts • Absence of the 1000 characteristic 1809 keV ɣ- 0.1 26 1800 1805 1810 1815 1820 ray from the decay of Na 100 E (keV) γ
ɣ-β coincidence spectrum Counts during the initial beam on 10 period. 1
0.1 0 500 1000 1500 2000 E (keV) γ ɣ Photopeak Counting - Contaminants
• Searched for possible contribution within the 718 keV photopeak from possible contaminants in the A = 26 beam.
26m Al Run 10 Cycles 1000 Values from fit for 10 cycles 10 I( C) = 733(11) c/s 26m 26m I( Al) = 46(21) c/s 78 (2) Al (T1/2 = 6.34602(54) s) I( Br) = 77.5(29) c/s BG = 6.4(17) c/s
• No ɣ radiation. Per cycle, 23.9 implant, 2.3 s cool time 10 • R ( C, in beam) = 138.2(21) c/s Dedicated β counting 26m
Counts R( Al, in beam) = 6.4(29) c/s 78 measurement with the mass 100 R( Br, in beam) = 186(70) c/s separator was tuned to the mass of 26mAl. 2 • No significant contribution χ /ν = 0.96 from 26mAl measured. 0 100 200 300 400 500 Time (s) ɣ Photopeak Counting - Contaminants
• Searched for possible contribution within the 718 keV photopeak from possible contaminants in the A = 26 beam.
13 (3) N (T1/2 = 9.9670(37) min)
• Observed in β counting expt • No ɣ radiation. • GEANT4 simulation of the Bremsstrahlung production and annihilation radiation 718 keV photopeak 5 • Less than 2 counts expected 10 10C Counts 13 13 10 N within the 718 keV peak in the N/ C: 0.03% 104 entire experiment.
103 • The upper limits obtained for each contaminant was included in the fit 102 to the data. No change in the half- 10 710 712 714 716 718 720 722 724 life observed. Energy (keV) Contaminants
Mass difference between 26mAl and 78Br is 12.274 MeV
The 10C beam intensity is suppressed by a factor of ~700 when the beam is tuned to 26mAl relative to the optimized tune to 10C16O. No such suppression factor is assumed when including the 26mAl component deduced from the 26mAl run. We expect the true amount of 26mAl to be even smaller than included in the above numbers.