High-Precision Half-Life Measurements for the β Emitter C ......= constant Δ R = nucleus...

High-Precision Half-Life Measurements for the

Superallowed β+ Emitter 10C: Implications for Weak Scalar

Currents

Michelle R. DunlopUniversity of Guelph

Nuclear Structure, July 28 2016

Superallowed Fermi Beta Decays• In general, β decay ft values can be expressed as:

• 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+

T1/2, BR

ft =K

G2A|Mfi(GT )|2 +G2

V |Mfi(F )|2

Phase Space (Q-value) ConstantsMatrix

Element

Weak Coupling Strength

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:

• The superallowed ft values thus simplify:

{ {= 0 for Pure Fermi decays

= 2 for decays between T=1

IAS

{

constants (Conserved

Vector Current hypothesis)

ft =K

G2A|Mfi(GT )|2 +G2

V |Mfi(F )|2 ) K

2G2V

|Mfi(F )|2 = (T � TZ)(T + TZ + 1) = 2

A Selection of Beta Decay ft Values

Superallowed ft values

0 10 20 30 40Z of Daughter

3030

3040

3050

3060

3070

3080

3090

ft (s

)

10C14O

22Mg

26Alm

34Cl

34Ar

38Ca

38Km

42Sc

46V

50Mn

54Co

62Ga74Rb

ft ⇡ K

2G2V

ft(s)

{2%

= constant

ΔR = nucleus independent inner radiative correction: 2.361(38)%

δR = nucleus dependent radiative correction to order Z2α3: ≈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” ft value

Experiment ◦ Half-life ◦ Q-value ◦ Branching Ratio

Calculated corrections (≈1%) (nucleus dependent)

Inner radiative correction (≈2.4%) (nucleus independent)

CVC Hypothesis

Ft = ft (1+ δR )(1+ δNS - δC) = 2Gv (1+ΔR)2

K

Superallowed Fermi Beta Decay: Theoretical Corrections

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.

� =p

1� (↵Z)2

Ft = constant ! Ft(1 + bF �hW�1i) = constantFt = constant ) Ft(1 + bF �hW�1i) = constant

bF = Fierz Interference Term

W = Total Positron Energy

SM description With Scalar Interaction

{The ⟨W-1⟩ dependence

means that it is the lowest-Z decays that are most

sensitive to contributions from a scalar interaction

Ft = constant ! Ft(1 + bF �hW�1i) = constant

• The Fierz interference term (bF) can be extracted by a linear fit of 1/Ft vs Ɣ⟨W-1⟩

• Current limit from Hardy and Towner, Phys Rev. C 91 025501 (2015): bF = -0.0028(26)

0 0.1 0.2 0.3 0.4 0.5 0.6γ <W-1>

32.35

32.40

32.45

32.50

32.55

32.60

32.65

32.70

32.75

1/Ft

(1/s)

x 1

0-5

bF = -0.0028 ± 0.0025

Ft0 = 3070.1 ± 2.0 s

10C

14O

22Mg

26Alm

74Rb

62Ga54Co

50Mn

46V

38Ca

42Sc

34Ar

34Cl

38Km

χ2/ν = 0.40bF = -0.0028 ± 0.0026

Search for Physics Beyond the Standard Model

Ft = constant ! Ft(1 + bF �hW�1i) = constant

• The Fierz interference term (bF) can be extracted by a linear fit of 1/Ft vs Ɣ⟨W-1⟩

• Current limit from Hardy and Towner, Phys Rev. C 91 025501 (2015): bF = -0.0028(26)

• Limits are dominated by the lightest superallowed decays 14O and, in particular, 10C0 0.1 0.2 0.3 0.4 0.5 0.6

γ <W-1>

32.35

32.40

32.45

32.50

32.55

32.60

32.65

32.70

32.75

1/Ft

(1/s)

x 1

0-5

bF = -0.0028 ± 0.0025

Ft0 = 3070.1 ± 2.0 s

10C

14O

22Mg

26Alm

74Rb

62Ga54Co

50Mn

46V

38Ca

42Sc

34Ar

34Cl

38Km

χ2/ν = 0.40bF = -0.0028 ± 0.0026

Search for Physics Beyond the Standard Model

The 10C half-life• The adopted 10C half-life is evaluated using the 4 most precise

measurements.• The inconsistencies in the dataset result in a highly inflated uncertainty on

the adopted world average half-life.

T1/2 = 19.3052 ± 0.0071 s

• 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.

The 10C half-life

• An accurate and precise determination of the 10C half-life is thus critical to the limits set of bF set by the superallowed data.

bF = -0.0016 ± 0.0026 bF = -0.0031 ± 0.0026

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.

0+

1+

3+

β+

10C

10Bstable

T1/2 = 19.3 s Q = 3648 keV

0+

1740

718

g.s.

1.4645%

98.530%

1022 keV

718 keV

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).

• The 10C half-life was measured by gating on the characteristic 718-keV ɣ-ray which follows 100% of 10C β decays.

ɣ Photopeak Counting

0 200 400 600 800 1000 1200 1400 1600 1800 2000Eγ (keV)

102

103

104

105

106

107Co

unts

per 0

.5 k

eV

705 710 715 720 725Eγ (keV)

103

104

105

106

107

Coun

ts

1022 keV

718 keV

• 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.

ɣ Photopeak Counting

103

104

105

106

Coun

ts pe

r 1.2

s

0 100 200 300 400 500Time (s)

-4

-2

0

2

4

Resid

uals

[(yi-y

fit)/σ

i]

T1/2 (10C) = 19.2969 ± 0.0052 s

χ2/ν = 0.87

(a)

0 10 20 30 40 50 60Run Number

19.1

19.2

19.3

19.4

19.5

19.6

Hal

f-life

(s)

Half-life Vs. Run

T1/2(10C) = 19.2970 ± 0.0052 s

χ2/ν = 1.19

(b)

Symbols = Different Shaping Times Colours = Different Applied Dead Times

• The data are grouped according to their experimental running conditions. The grouping with the largest 𝞆2/ν > 1 is used to inflate the uncertainty on the measured half-life, as recommended by the Particle Data Group (PDG).

ɣ Photopeak Counting — Systematics

0 1 2 3 4 5 6 7 8 9 1019.25

19.26

19.27

19.28

19.29

19.3

19.31

19.32

19.33

19.34

19.35

10C

Hal

f-life

(s)

All DataDead Time (µs): Variable, 27, 40Shaping Time (µs): 0.5, 1.0, 2.0Beam: Pre-Switching; Switching

Systematics

χ2/ν = 1.01 χ2/ν = 2.01 χ2/ν = 0.72

T1/2,ɣ(10C) = 19.2969 ± 0.0052sys ± 0.0052stat = 19.2969 ± 0.0074 sRelative precision of 0.03%

β Counting — 4π 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.

Implantation SiteBeam

Gas Counter

Tape Disposal

Box

Tape Spool

~ 28 cm

Atmosphere

High vacuum (~10-6 T)

• Radioactive beams of both 10C (A=10) and 10C16O (A=26) were delivered to the gas counter

β Counting — In-beam Contaminant

• 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 as 13N2 (mass difference to 10C16O of 0.271 MeV) or HC13N (mass diff. of 1.673 MeV) in the A=26 beam.

• The literature T1/2 of 13N was included as a fixed parameter in the fit to the subset of data taken with the A=26 beam.

• 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.

β Counting

102

103

104

105

106

Coun

ts pe

r 1 s

10C decay13N contaminantBackgroundBest Fit Result

0 100 200 300 400 500Time (s)

-4

-2

0

2

4

Resid

uals

[(yi-y

fit)/σ

i] T1/2 (10C) = 19.2987 ± 0.0030 s χ

2/ν = 1.03

(a)

0 20 40 60 80Run Number

19.20

19.25

19.30

19.35

19.40

19.45

Hal

f-life

(s)

χ2/ν= 1.02

T1/2 (10C) = 19.3009 ± 0.0017 s

(b)

Open (Filled) Circles = 10C (10C16O) Radioactive Beam Colours = Different Gas Counters

0 1 2 3 4 5 619.298

19.299

19.3

19.301

19.302

19.303

10C

Hal

f-life

(s)

T1/2(13N): +/- 1σDeadtime: +/- 1σ

Systematics

19.28

19.29

19.30

19.31

19.32

19.33

19.34

19.35

19.36

Hal

f-life

(s)

All DataBias Voltage (V): 2400, 2450, 2500, 2550, 2600, 2700, 2750, 2800Threshold (mV): 70, 95, 120Dwell Time (s): 1.0, 1.2, 1.4Radioactive Beam: 10C16O, 10CGas Counter: 1, 2Initial 10C Activity (kHz): <6, 6-8, 8-10, 10-12, >12

χ2/ν = 0.92 0.13 0.54 0.06 0.620.62

T1/2(10C) = 19.3009 ± 0.0017 s

• Extensive investigation of systematics was performed.• Each grouping of the experimental parameters yield a 𝞆2/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.

β Counting — Systematics

T1/2,β(10C) = 19.3009 ± 0.0017 sThe most precise (0.009%) superallowed T1/2 reported to date!

• The 6 most precise half-life measurements now yield T1/2 = 19.3015 ± 0.0015 s with a 𝞆2/ν = 1.90, which is an improvement in the uncertainty by a factor of nearly 3.

Results — 10C Half-life

T1/2(10C) = 19.3052 ± 0.0071 s

• 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:

Results — Scalar Current Limits

0 0.1 0.2 0.3 0.4 0.5 0.6

γ <W-1

>

32.35

32.40

32.45

32.50

32.55

32.60

32.65

32.70

32.75

1/F

t (1

/s)

x 1

0-5

bF = -0.0018 ± 0.0021

Ft0= 3070.8 ± 1.8 s

10C

14O

22Mg

26Al

m

74Rb

62Ga

54Co

50Mn

46V

38Ca

42Sc

34Ar

34Cl

38K

m

χ2/ν = 0.44

bF = -0.0018(21)and

Cs/CV = -bF/2 = 0.0009(11)

for the ratio of weak scalar to vector

couplings assuming left handed neutrinos

M. R. Dunlop et al., Phys. Rev. Lett. 116, 172501 (2016)

• Measurements of the 10C half-life were performed in order to resolve inconsistencies in the data set.

Conclusions


• 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



• We measured the half-life via both ɣ-ray photopeak counting and direct β counting, obtaining consistent results.

Conclusions




• The β measurement (T1/2 = 19.3009(17) s) represents the most precise superallowed half-life measurement reported to date and the first to achieve a precision below 10-4.

Conclusions




• The β measurement (T1/2 = 19.3009(17) s) represents the most precise superallowed half-life measurement reported to date and the first to achieve a precision below 10-4.

• 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.

Conclusions

Acknowledgements

G . C . B A L LT. B A L L A S TP. B E N D E RA . B . G A R N S W O R T H YD . M I L L E RB . M I L L SJ . PA R KM . M . R A J A B A L IC . U N S W O R T HZ . WA N G

G . F. G R I N Y E R

V. B I L D S T E I NA . D I A Z VA R E L AR . D U N L O PP. E . G A R R E T TB . H A D I N I AD . J A M I E S O NA . T. L A F F O L E YA . D . M A C L E A NA . J . R A D I C HE . R A N DC . E . S V E N S S O N

E . F. Z G A N G A R

C . A N D R E O I U

J . R . L E S L I E

R . A . E . A U S T I NA . VA L E N C I A

Backup Slides

• 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

Limits on Weak Scalar Currents

0 0.1 0.2 0.3 0.4 0.5 0.6

γ <W-1

>

32.35

32.40

32.45

32.50

32.55

32.60

32.65

32.70

32.75

1/F

t (1

/s)

x 1

0-5

bF = -0.0018 ± 0.0021

Ft0= 3070.8 ± 1.8 s

10C

14O

22Mg

26Al

m

74Rb

62Ga

54Co

50Mn

46V

38Ca

42Sc

34Ar

34Cl

38K

m

χ2/ν = 0.44

Testing Fundamental Properties of the Weak Interaction

0

@d0

s0

b0

1

A =

0

@Vud Vus Vub

Vcd Vcs Vcb

Vtd Vts Vtb

1

A

0

@dsb

1

A

Vud = GV /GF

• 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.

V 2ud + V 2

us + V 2ub = 0.99978(55)

0 50 100 150 200 250 300 350 400Time (1.2 s/bin)

0

0.01

0.02

0.03

0.04

Prob

abili

ty o

f Pile

-Up

DataBest FitCosmic pile-upPile-up of 2 or more events

Pile-Up Correction0.5 µs Shaping Time

Pileup Correction

0 50 100 150 200 250 300 350 400Time (1.2 s/bin)

0

0.005

0.01

0.015

0.02

0.025

Prob

abili

ty o

f Pile

-Up

DataBest-fit CurvePile-up of 2 or more eventsCosmic Pile-up


0 50 100 150 200 250 300 350 400Time (1.2 s/bin)

0

0.01

0.02

0.03

Prob

abili

ty o

f Pile

-Up

DataBest FitCosmic pile-upPile-up of 2 or more events


0 20 40Number of Leading Channels Removed

19.27

19.28

19.29

19.3

19.31

19.32

19.33

Hal

f-life

(s)

No pile-up correction appliedPile-up correction applied

Chop PlotSummed Data

Gas Counter Rates

4 5 6 7 8 9 10 11 12 13 14 15 16Initial Count Rate Upper Limit (kHz)

19.29

19.295

19.3

19.305

19.31

Hal

f-life

(s)

High Rate Data Removal

0 2 4 6 8 10 12 14t=0 10C Activity (kHz)

19.28

19.29

19.30

19.31

19.32

19.33

19.34

19.35

Hal

f-life

(s)

Systematics10C Activity at t=0

χ2/ν = 0.692-4 kHz

4-6 kHz 6-8 kHz 8-10 kHz10-12 kHz

12-14 kHz

14-16 kHz

• Searched for possible contribution within the 718 keV photopeak from possible contaminants in the A = 26 beam.

ɣ Photopeak Counting - Contaminants

0 500 1000 1500 2000Eγ (keV)

0.1

1

10

100

1000

10000

Coun

ts

γ−β Coincidence Spectrum Gated on the first 7.2 seconds of the implation

1800 1805 1810 1815 1820Eγ (keV)

0.1

1

10

Coun

ts

(1) 26Na (T1/2 = 1.07128(25) s) • Absence of the

characteristic 1809 keV ɣ-ray from the decay of 26Na ɣ-β coincidence spectrum during the initial beam on period.



(2) 26mAl (T1/2 = 6.34602(54) s)

• No ɣ radiation. • Dedicated β counting

measurement with the mass separator was tuned to the mass of 26mAl.

• No significant contribution from 26mAl measured.

0 100 200 300 400 500Time (s)

100

1000

Coun

ts

26mAl Run10 Cycles

χ2/ν = 0.96

I(10C) = 733(11) c/sI(26mAl) = 46(21) c/s I(78Br) = 77.5(29) c/sBG = 6.4(17) c/s

R (10C, in beam) = 138.2(21) c/sR(26mAl, in beam) = 6.4(29) c/s

Values from fit for 10 cycles

Per cycle, 23.9 implant, 2.3 s cool time

R(78Br, in beam) = 186(70) c/s



(3) 13N (T1/2 = 9.9670(37) min)

• Observed in β counting expt • No ɣ radiation. • GEANT4 simulation of the

Bremsstrahlung production and annihilation radiation

• Less than 2 counts expected within the 718 keV peak in the entire experiment.

• The upper limits obtained for each contaminant was included in the fit to the data. No change in the half-life observed. Energy (keV)

710 712 714 716 718 720 722 724

Cou

nts

10

210

310

410

510 C10

N13C: 0.03%10N/13

718 keV photopeak

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.

High-Precision Half-Life Measurements for the β Emitter C ......= constant Δ R = nucleus...

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Transcript of High-Precision Half-Life Measurements for the β Emitter C ......= constant Δ R = nucleus...