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The Majorana Search for Neutrinoless Double-Beta Decay
Nuclear Physics B (Proc. Suppl.) 143 (2005) 550 www.elsevierphysics.com
The Majorana Search for Neutrinoless Double-Beta Decay K. Kazkaza for the Majorana Collaboration a
CENPA, Box 354290, University of Washington Seattle, WA 98195-4290 USA The Majorana experiment[1] is a proposed 76 Ge-based search for neutrinoless double-beta (0νββ) decay and dark matter (DM). It will use segmented, enriched germanium detectors in a close-pack configuration. With 500 kg of germanium, Majorana would be expected to reach a sensitivity to the 0νββ decay half-life of 1027 years, corresponding to a neutrino mass near the atmospheric mass scale. Current development efforts are presented, along with performance expectations and background reduction methods. Additionally, the status of the related detectors SEGA (Segmented Enriched Germanium Assembly) and MEGA[2] (Multi-Element Gamma Assay) are presented. Current simulation efforts will be presented in a parallel poster by Reyco Henning.
With interactions as rare as 0νββ decay and DM recoils, background reduction is of greatest importance. All such experiments are situated deep underground to reduce the cosmic ray flux and have active and passive shielding to reduce or identify background radiation, but each individual experiment has specific and unique techniques to further reduce the background. For Majorana, the unique reductions fall into two categories: construction and data analysis. The hardware methods used to reduce background include using electroformed copper parts, incorporating recrystalization and baths with continuous microfiltration into the electroforming, along with glassware-free handling and the use of semiconductor-grade acids. Additionally, both the copper parts and germanium crystals themselves can be formed and machined deep underground to reduce cosmogenic activation. The crystal detectors will also be segmented to utilize the single-site nature of 0νββ and DM events. Particles that penetrate the innermost shield tend to interact multiple times throughout the detector array. Segmentation will help in separating the sought-after single-site events from the multi-site backgrounds. Events wholly contained within a single detector segment may still be multi-site. By digitizing the pulses with fine enough resolution, we may still be able to separate the desired signal from background. The efficacy of segmentation 0920-5632/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2005.01.215
and pulse-shape discrimination (PSD) have been tested and measured using a detector made up of four unenriched, segmented crystals. We compare the reduction of multi-site gamma events (the 1588 keV gamma from 228 Ac) to the retention of single-site events (the double-escape peak from 208 Tl at 1592 keV). Utilizing both segmentation and PSD, we reject 93% of multi-site events while retaining 74% of single-site events. The Majorana experiment will use expertise gained in other closely related experiments, SEGA and MEGA. SEGA is a single, segmented, enriched detector used in part to study the effects of segmentation on pulse shape and resolve issues of instrumentation. MEGA is a close-pack array of 18 detectors, and will be used in part to study structural engineering, heat load, and multi-element data analysis. MEGA will also be used as a low-background detector for screening of materials to be used in the construction of Majorana. REFERENCES 1. nucl-ex/0311013 2. Kazkaz, K., Aalseth, C.E., Hossbach, T.W., Gehman, V.M., Kephart, J.D., Miley, H.S., IEEE Trans. Nuc. Sci. 51 (2004) 1029.
The Majorana Search for Neutrinoless Double-Beta Decay K. Kazkaza for the Majorana Collaboration a
CENPA, Box 354290, University of Washington Seattle, WA 98195-4290 USA The Majorana experiment[1] is a proposed 76 Ge-based search for neutrinoless double-beta (0νββ) decay and dark matter (DM). It will use segmented, enriched germanium detectors in a close-pack configuration. With 500 kg of germanium, Majorana would be expected to reach a sensitivity to the 0νββ decay half-life of 1027 years, corresponding to a neutrino mass near the atmospheric mass scale. Current development efforts are presented, along with performance expectations and background reduction methods. Additionally, the status of the related detectors SEGA (Segmented Enriched Germanium Assembly) and MEGA[2] (Multi-Element Gamma Assay) are presented. Current simulation efforts will be presented in a parallel poster by Reyco Henning.
With interactions as rare as 0νββ decay and DM recoils, background reduction is of greatest importance. All such experiments are situated deep underground to reduce the cosmic ray flux and have active and passive shielding to reduce or identify background radiation, but each individual experiment has specific and unique techniques to further reduce the background. For Majorana, the unique reductions fall into two categories: construction and data analysis. The hardware methods used to reduce background include using electroformed copper parts, incorporating recrystalization and baths with continuous microfiltration into the electroforming, along with glassware-free handling and the use of semiconductor-grade acids. Additionally, both the copper parts and germanium crystals themselves can be formed and machined deep underground to reduce cosmogenic activation. The crystal detectors will also be segmented to utilize the single-site nature of 0νββ and DM events. Particles that penetrate the innermost shield tend to interact multiple times throughout the detector array. Segmentation will help in separating the sought-after single-site events from the multi-site backgrounds. Events wholly contained within a single detector segment may still be multi-site. By digitizing the pulses with fine enough resolution, we may still be able to separate the desired signal from background. The efficacy of segmentation 0920-5632/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2005.01.215
and pulse-shape discrimination (PSD) have been tested and measured using a detector made up of four unenriched, segmented crystals. We compare the reduction of multi-site gamma events (the 1588 keV gamma from 228 Ac) to the retention of single-site events (the double-escape peak from 208 Tl at 1592 keV). Utilizing both segmentation and PSD, we reject 93% of multi-site events while retaining 74% of single-site events. The Majorana experiment will use expertise gained in other closely related experiments, SEGA and MEGA. SEGA is a single, segmented, enriched detector used in part to study the effects of segmentation on pulse shape and resolve issues of instrumentation. MEGA is a close-pack array of 18 detectors, and will be used in part to study structural engineering, heat load, and multi-element data analysis. MEGA will also be used as a low-background detector for screening of materials to be used in the construction of Majorana. REFERENCES 1. nucl-ex/0311013 2. Kazkaz, K., Aalseth, C.E., Hossbach, T.W., Gehman, V.M., Kephart, J.D., Miley, H.S., IEEE Trans. Nuc. Sci. 51 (2004) 1029.