Background Subtraction in Next Generation 0 Experiments

Double-Beta Decay Challenges in 0 Decay Detection• Small 0νββ decay half-life leads to low statistics in decay spectrum.

• Cosmic muons produce underground neutron flux inducing reactions in Pb shielding and Cu in cryo-system:

206Pb has (n,n’γ) transition at 2041 keV.

65Cu has (n,n’γ) transition at 2041 keV.

207Pb has (n,n’γ) transition at 3062 keV, which can produce a double escape peak at 2040 keV.

• Must know partial cross sections of these neutron-induced (n,n’γ) transitions in order to subtract these events out of final 0νββ decay spectrum.

Cross Section Measurement Setup

Experimental Results(A,Z) (A,Z+2) + 2e- + 2ν (t1/2 ≈ 1020 yrs)

(standard ββ decay)

(A,Z) (A,Z+2) + 2e- (t1/2 1027 yrs)

(theorized neutrinoless ββ decay)

If detected, 0νββ decay would indicate:

• Lepton number violation, contrary to present formulation of the Standard Model.

• The neutrino is its own antiparticle (Majorana particle).

• An absolute scale for neutrino mass (from half-life).

• Mass hierarchy for 3 neutrino flavors (also from half-life).

Detection of 0 Decay

Setup(MAJORANA PROJECT)

• Very long half-life, so huge quantities of 76Ge required as well as long measurement times.

• Underground for background reduction

• Enriched 76Ge used as source and detector

• Segmented to distinguish between background (in the form of random coincidences) and physical events.

• Continuum: a result of many-body kinematics – electrons and neutrinos share energy.

• Spike at 2040 keV: a result of neutrino annihilation – electrons carry all of the decay energy.

• Heidelberg-Moscow experiment claimed to have observed 0νββ peak around 2040 keV in 76Ge.

• 8 MeV and 12 MeV neutrons produced at target area

• 3 HPGe segmented clover detectors at 62°, 90°, and 135°

• Efficiency and energy calibrations with 60Co, 22Na, and 226Ra sources of known intensity.

• Target runs with Pb and Cu wrapped in Fe foil for cross-section normalization.

• 400 ns pulsed neutron beam allows for TOF background subtraction.

• Distinguish between beam-related and random coincidence events.

• Shield veto reduces Compton background.

Region of Interest: 2040 keV

• 2041 keV peak from 206Pb only showed up in clover 3.

• 2041 keV peak from 65Cu was not evident above background at either energy.

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Region of Interest: 3062 keV

Measured σ for 207Pb at 3062 keV

Det. angle 8 MeV 12 MeV

62° 388±51 mb –

90° 300±42 mb 59±13 mb

135° 133±79 mb –

Measured σ for 206Pb at 2041 keV

62° 225±49 mb 107±19 mb

Partial Cross Sections: Peaks of interest

• Angular and energy dependence of 3062 keV partial cross-section.

• Low statistics causes high uncertainty in cross-section measurement.

• May obscure 0νββ decay 2040 keV region of interest.

i. M. Kidd. Majorana and SEGA. (private communication)

ii. H. Back. (private communication)

iii. D. Mei. Backgrounds in the Next Generation Double-Beta Decay Experiments. Los Alamos National Lab. (prepatred for publication)

iv. J.E. Ellis. PNNL Majorana Information Resource Page. <http://majorana.pnl.gov/>. 7 July 2006.

v. H.V. Klapdor-Kleingrothaus et al., Phys. Lett. B 586 (2004) 198.

References

• 3062 keV peak from 207Pb evident in most clovers at both energies

SegmentedEnrichedGermaniumAssembly

1460.8 keV 40K (background)

Beam related events

Energy (keV)

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Time (ns)

Benjamin Spaun1, Anton Tonchev2, and Werner Tornow2

1Whitworth College, Spokane, WA 2Duke University & TUNL, Durham, NC (advisors)

Work funded by the Department of Energy (grant number DE-FG02-97ER41033) and the National Science Foundation (grant number NSF-PHY-05-52723).