Double - beta decay of 96zr and double - electron capture of 156dy to excited final states

Outline
Introduction to second-order weak decays
Nature of the neutrino
Double-β decay of 96Zr
Two-coaxial HPGe apparatus
Analysis and results
Resonantly enhanced double-electron capture of 156Dy
Two-clover HPGe apparatus
Analysis and results
Concluding remarks

Outline
Introduction to second-order weak decays
Nature of the neutrino
Double-β decay of 96Zr
Two-coaxial HPGe apparatus
Analysis and results
Resonantly enhanced double-electron capture of 156Dy
Two-clover HPGe apparatus
Analysis and results
Concluding remarks

Weak Nuclear Decays
β decay: n → p + e- + νe
Double-β decay: 2n → 2p + 2e- + 2νe
35 nuclides capable of ββ; Observed in 11
n
n
p
p
Spectator Nucleons
e-
νe
e-
νe

Neutrinoless-ββ Decay
n1 → p1 + e-1 + “νe”
“νe” + n2 → p2 + e-2
Never observed (one questionable claim)
n
n
p
p
Spectator Nucleons
e-
νe
e-
νe
n
n
p
p
Spectator Nucleons
e-
νe
e-
νe

Majorana Neutrinos
0νββ requires:
Helicity flip
Solved by massive neutrinos
Neutrinos are their own antiparticle
Solved by Majorana neutrinos

Outline
Introduction to second-order weak decays
Nature of the neutrino
Double-β decay of 96Zr
Two-coaxial HPGe apparatus
Analysis and results
Resonantly enhanced double-electron capture of 156Dy
Two-clover HPGe apparatus
Analysis and results
Concluding remarks

2νββ to Excited Final States
Q value

Nuclear Matrix Elements

Experimental Technique
Sample between two coaxial-HPGe detectors
All energies in keV
An excited state decay with two coincident γs

Two-Coaxial HPGe Apparatus
Two-coaxial HPGe detectors sandwich sample
Active veto
NaI annulus for Compton suppression
Plastic end caps
Passive shielding
¾” OFHC copper
6” lead

HPGe Spectrum

Kimballton Underground Research Facility (KURF)
Active limestone mine
Ripplemead, Va
30 minutes from VT
4 hours from Duke
1700 feet limestone overburden
1450 m.w.e. shielding from cosmic rays
Internet access allows experiments to be controlled remotely

KURF

KURF Lab
micoLENS (VT)
Neutron Spectroscopy (Maryland)
Present experiment (Duke)
Low background radioassays (UNC)
MALBEK (UNC)
Low activity Ar (Princeton)
Watchboy (LLNL)

96Zr Sample
150Nd and 100Mo are the only two nuclides where ββ decay to an excited state has been observed
96Zr as a ββ-decay candidate
High Q Value (3347 keV)
Ground state decay measured by NEMO collaboration
T1/2 = [2.35 ± 0.14 (stat) ± 0.16 (syst)] x 1019 yr
2.8% natural abundance
ZrO2 sample from ORNL:
7.283 g enriched to 91.39%
26.968 g enriched to 64.18%
Total of 17.914 g 96Zr

96Zr 2νββ Data
96Zr source in place
623.8 days (1.92 yr) of data
4 events in ± 3σ ROI
2 events in ± 2σ ROI
Consistent with background
Backgrounds
212Bi (232Th decay chain)
Compton scattering
Discriminate with energy resolution

Limit Setting
Limit versus sensitivity
Limit: Nd(Nobs, Nbkgd)
Sensitivity: Ns(Nbkgd)
The mean limit Nd reported by an experiment with background Nbkgd and no signal

New limits
Limit at 90% C.L.
T1/2 > 3.2 x 1020 yr
Sensitivity: T1/2 > 2.8 x 1020 yr
Previous limit
T1/2 > 6.8 x 1019 yr
Used single well-type HPGe; no coincidence
Limited by high background and uncertainty from statistical fits

To Higher Excited States
96Mo
0+
0+g.s.
0+1
2+
96Zr
3351
1148.1
778.2
0.0
2+2
0+2
70.3%
1497.8
1330.0
0+3
2+3
90.3%
2622.5
1625.9
All energies in keV
ββ

Limits on NME
1 J. Barea, J. Kotila, and F. Iachello, Phys. Rev. C 87, 014315 (2013)

Single-β decay of 96Zr
T1/2(96Nb) = 23.35 h
All energies in keV

Experimental Motivation
Help understand the nuclear structure of 96Zr
Possible background for experiments searching for 0νββ in 96Zr
Previous experiments used a single HPGe detector
β decay forms an irreducible background for excited state decays
Using the coincidence technique in present work can unambiguously distinguish β decay and ββ decay

Limits on Single-β decay of 96Zr
Three coincident γs
Present Experiment
Sum three most intense decay modes
Branching ratio 81.6%
13 events in ROI , expect 11.1 events background
T1/2 > 2.4 x 1019 yr
Previous measurement
T1/2 > 3.8 x 1019 yr
Theory2 (QRPA)
T1/2 = 2.4 x 1020 yr
2 H. Heiskanen, M.T. Mustonen, and J. Suhonen, J. Phys. Rev. G 34, 837 (2007)

Outline
Introduction to second-order weak decays
Nature of the neutrino
Double-β decay of 96Zr
Two-coaxial HPGe apparatus
Analysis and results
Resonantly enhanced double-electron capture of 156Dy
Two-clover HPGe apparatus
Analysis and results
Concluding remarks

Resonant ECEC to Excited Final States
Nucleus captures atomic electron
p + e- → n + νe
Second-order nuclear decay
156Dy + 2e- → 156Gd + 2νe
For Majorana neutrinos
156Dy + 2e- → 156Gd*
Possible experimental alternative to 0νββ
No outgoing particles
No method to dissipate excess energy
Requires “monumental coincidence” that an excited nuclear state in the daughter nucleus is degenerate with the Q value

Resonant ECEC to Excited Final States
Two neutrino mode strongly disfavored by phase space
Observation would be evidence for Majorana neutrinos
Rate enhancement if the Q value is degenerate with an energy level
ECEC
Q value

ECEC in 156Dy
3 S. Eliseev et al., Phys. Rev. C 84, 012501(R) (2011)

Resonant ECEC in 156Dy
156Dy is currently one of the most promising candidates
Extremely low natural abundance: 0.056%
Enriched sample from ORNL
1.15 grams
Enriched to 21%
Total = 213 mg
Detectable only by γ-ray transitions in daughter
ECEC
Q value

New Two-Clover HPGe Apparatus
Use two clover HPGe detectors
Segmented
Larger volume
NaI annulus
6-8” Lead shielding
Timeline
Built in 2010
Characterization and efficiency measurements 2010-2012
Moved to KURF Fall 2012
156Dy sample 2013-2014

External coincidences
Internal coincidences
Addback coincidences
Coincidence and Addback

Previous Searches
Belli et al.4
322 g natural Dy2O3
157 mg 156Dy
104.7 days
Single HPGe at LNGS
This work
Enriched sample
Reduce γ-ray attenuation by sample
Cover a larger solid angle
Ability to look at coincidence γ-rays from cascades
Reduce background
4 P. Belli et al., J. Phys. Conf. Ser. 375, 042024 (2012)

156Dy Data Acquisition
Run #1
151.95 mg
99.13 days
Run #2
213.57 mg
132.82 days
Presented results are the sum of both runs
0.119 g·yr of exposure

Capture to the 1946 keV State
Branching ratio 57.8%
Gate on 88.9 keV γ ray in one clover segment
Allow singles and addback on 1857 keV
All energies in keV

Capture to the 1946 keV State
Branching ratio 57.8%
Gate on 88.9 keV γ ray in one clover segment
Allow singles and addback on 1857 keV
1 event in ROI, expected 4.12 events background
Nd = 1.275
Ns = 4.86

Capture to the 1988 keV State
No nuclear data available on state
Assume a strong transition to the 2+ state
Branching ratio = 100%
Gate on 88.9 keV γ ray in one clover segment
All energies in keV

Capture to the 1988 keV State
No nuclear data available on state
Assume a strong transition to the 2+ state
Branching ratio = 100%
Gate on 88.9 keV γ ray in one clover segment
2 events in ROI, expected 3.76 events background

Capture to the 1952 keV State
Two γ-ray decay mode
Branching ratio 44%
Allow addback on both γs
Three γ-ray decay mode may be reconstructed in addback
Additional 26.3% increase to sensitivity

Capture to the 1952 keV State
Two γ-ray decay mode
Branching ratio 44%
Allow addback on both γs
Three γ-ray decay mode may be reconstructed in addback
Additional 26.3% increase to sensitivity
Given 709.9 keV event
2 events in ROI, expected 2.38 events background

Capture to the 2003.7 keV State
All energies in keV

Capture to the 2003.7 keV State
Two γ-ray decays
Branching ratio 25.8%
Including contribution from ternary γ-ray decays
Increase sensitivity by 18%
Top: in coincidence with 1319.7
Bottom: in coincidence with 1242.5
Sum all three decay modes
6 events in ROI, expected 5.64 events background

Summary of Results
4 P. Belli et al., J. Phys. Conf. Ser. 375, 042024 (2012)

Outline
Introduction to second-order weak decays
Nature of the neutrino
Double-β decay of 96Zr
Two-coaxial HPGe apparatus
Analysis and results
Resonantly enhanced double-electron capture of 156Dy
Two-clover HPGe apparatus
Analysis and results
Concluding remarks

The Future
Many 0νββ experiments are currently underway with more about to begin
Underway: EXO-200, GERDA, KamLAND-Zen
Near future: CUORE, Majorana, SNO+
Collaborations are progressing towards 1-ton scale experiment
Goals:
Illuminate the Dirac or Majorana nature of the neutrino
Set limits on the neutrino mass

Summary
ββ of 96Zr to excited states
Improve theoretical understanding of NMEs
Reduce uncertainty
Excited state decay modes could help verify discovery
Decreased background by coincidence technique
T1/2(ββ) > [1.0-3.2] x 1020 yr
T1/2(β) > 2.4 x 1019 yr
0νECEC to excited states
Experimental alternative to 0νββ
Resonant enhancement could greatly increase experimental sensitivity
T1/2 > [0.67-10] x 1017 yr

Thank you

How to Observe 0νββ
How to observe
Neutrinos escape
Electrons deposit energy in detector
Majorana Experiment
76Ge

HPGe Detectors
Photoelectric effect
Low energies (<140 keV)
Full energy deposition
Compton scattering
Median energies
Partial energy deposition
Pair production
High energy
Requires 1.022 MeV
Dominates > 8 MeV

Previous Data: 100Mo
5 M.F.Kidd, J.H. Esterline, and W. Tornow, Nuc. Phys. A 821, 251 (2009)

Previous Data: 150Nd
6 M.F. Kidd, J.H. Esterline, S.W. Finch, and W. Tornow, Phys. Rev. C 90, 055501 (2014)

Two-Coaxial Apparatus Efficiency

Coaxial HPGe efficiency
HPGe 1
HPGe 2

Angular Distribution

Two-Clover Apparatus Efficiency

Two-Clover Apparatus Efficiency

Clover Efficiency

Method of Feldman-Cousins