1. Triangle Universities Nuclear Laboratory
Precision Cross Section Measurement of Cu at 8 and 12 MeV for Background Reduction in the Next Generation of 0νßß Decay Experiments*
M.A. Antonacci 1, A. Chyzh2, J. Esterline3, S. Elliott4, B. Fallin3, A. Hime4, C.R. Howell3, A Hutcheson3, H.J. Karwowski5, J.H. Kelley2, M. Kidd3, D. Mei4, B. Spaun6, A.P. Tonchev3, W. Tornow3
1 Saint Vincent College, 2 North Carolina State University and TUNL, 3 Duke University and TUNL, 4 Los Alamos National Laboratory, 5 University of North Carolina at Chapel Hill, 6 Whitworth College
Abstract
Analysis and Results
A Proposed Majorana Setup
Renewed interest in observing 0nbb decay reactions has sparked efforts to experimentally verify the existence of such decays, and observe new physics beyond the Standard Model. These reactions, with half-lives around 1027 years, require an extensive understanding of background sources. The potential for neutron induced excitation of the shielding and detector materials is important for understanding and designing future 0nbb decay experiments. Gamma transitions at 2041, 2615, and 3062 keV directly obscure 0nbb decay detection of 76Ge at 2040 keV. Due to lack of experimental information in the nuclear database, high-resolution g-ray spectra from the interaction of mono-energetic and pulsed neutrons were measured at TUNL. The emitted g-rays were detected with 3 HPGe segmented clover detectors at 62°, 90°, and 135°. From these data, partial cross-sections for prominent g transitions in 63,65Cu were derived at En = 8 and 12 MeV. This experimental information will also help to understand the existing 0nbb data and serve as a benchmark for statistical model calculations.
Random Coincidences
Prompt Region
True Coincidences
Background Region
Gammas
Neutron Time of Flight (TOF)
Take advantage of 400 ns beam pulsing to cut out unwanted background coincidence events
Keep only events that happen in time with beam neutrons, exclude g’s from beam stop, subtract random coincidences
Results in significantly better peak to Compton ratio
Clusters of segmented HPGe detectors
Setup located deep underground to shield from muon-induced neutron production
Requires thorough understanding of background sources, especially at 2041 keV and 3062 keV
Segmented
Enriched
Germanium
Assembly
Region of Interest
2041 keV peak from 65Cu not evident
Suggests cross-section for this transition very low at 8 and 12 MeV
Good news for Majorana! Less obscuring of 76Ge 0nbb peak than originally expected
Motivation: The Majorana Project
In Search of 0νββ
(A,Z)  (A,Z+2) + 2e- + 2ν
(A,Z)  (A,Z+2) + 2e-
2νββ
0νββ
Experimental Setup
2nbb half life ~1020 yrs, 0νββ half life ~1027 yrs
Electrons and neutrinos share energy in 2nbb
Spike result of n annihilation, electrons carry away all decay energy in 0nbb
Demonstrates lepton number violation – physics beyond the Standard Model!
10MV FN Tandem
Accelerator
Shielded Neutron
Source Area
DENIS II negative
ion source
Observed Partial Cross-sections
Pulsed beam for precision coincidence measurements
Tunable energy range (4-18 MeV)
Easily collimated
*only 20% of
the TUNL
complex is
shown here
Answer standing questions about neutrinos:
Is the neutrino a Majorana particle?
What is the neutrino mass hierarchy?
What is the absolute neutrino mass scale?
n beam
d+ beam
4 atm
Beam pick-off
2H(d,n)3He
Beam Stop
8 MeV and 12 MeV neutrons produced at target area
3 HPGe Segmented Clover detectors at 62°, 90°, 135°
Efficiency and energy calibrations with 60Co, 22Na, and 226Ra sources
Target runs with Pb and Cu wrapped in Fe foil for cross-section normalization
Blank run to aid in background subtraction
0νββ Observed?
214Bi
0νββ?
Heidelberg-Moscow experiment1
Claimed to have observed 0nββ peak around 2040 keV in 76Ge
Other Possibilities:
Cosmic muons can produce underground neutron flux inducing reactions in Pb shielding
206Pb has (n,n′g) 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
Neutron Beam
References
Mary Kidd. Majorana and SEGA.
Dr. Henning Back (SEGA information)
Dongming Mei. Backgrounds in the Next Generation Double-Beta Decay Experiments. Los Alamos National lab.
J. Edward Ellis. PNNL Majorana Information Resource Page. < 7 July 2006.
Target Area
HPGe detector
*Supported in part by DOE grant DE-FG02-97ER41033 (Duke) and NSF grant NSF
1H.V. Klapdor-Kleingrothaus et al., Phys. Lett. B 586 (2004) 198