The MAJORANA Demonstrator Project at the 4850 Level Davis Campus
The MAJORANA Collaboration began installing the ?MAJORANA Demonstrator (MJD)? detector at the 4850 Level Davis Campus underground in March 2012. This could be a significant step forward to answer one of the most intriguing physics questions in recent times: Are neutrinos Majorana particles?
A Majorana particle (named after the Italian physicist Ettore Majorana [1]) is a particle, which is also its own anti-particle. If neutrinos are indeed Majorana particles the consequences will be far reaching [2]: Majorana neutrinos could help to explain why the neutrino masses are so diminishingly small when compared to other known fundamental particles in the Standard Model (e.g. quarks and electrons), and possibly why there is only regular matter remaining in the universe today, while matter and anti-matter should have been created in equal parts in the Big Bang. Majorana neutrinos could also help to resolve the yet unknown ordering of the three neutrino masses among themselves, known as the neutrino mass hierarchy ambiguity.
Figure 1. Illustrations showing the difference between (a) the ??2? and (b) the ??0? processes. (c) shows the expected spectral shapes corresponding to (a) (blue) and (b) (red) in the summed beta energy spectrum (E??). Q?? is the Q-value for the decay and ROI is the region of interest centered at Q?? that the experiments will focus on.
It turns out that the only practical experimental approach known today to address this question is by studying the nuclear double beta decay processes in a few known isotopes such as 76Ge (decays to 76Se). From the Standard Model of physics, the two neutrino double beta decay mode (??2?, Figure 1a), where two anti-neutrinos are emitted together with two electrons, is allowed. However, only if the neutrino is a Majorana particle, the distinct neutrinoless double beta decay process (??0?, Figure 1b) could occur in nature. The experimental observation of the latter will provide very compelling evidence for neutrinos being Majorana particles. These two processes could in principle be distinguished experimentally by studying the summed-energy spectrum, E??, of the two electrons emerging in the final state of the decay. Contrast to the ??2? case where the kinematic sharing of the fixed decay energy by four particles resulting in a broad and continuous spectral shape (blue curve in Figure 1c), the summed energy for the ??0? process, by energy conservation, should show up as a discrete narrow peak in E?? (red line), corresponding to the known decay energy Q?? of the parent nucleus in question. The experimental challenge is huge however, as the extremely small signal will be concealed by known radioactive backgrounds typically many orders of magnitudes larger. For instance, the half-life for the observed ??2? decay in 76Ge is known to be ~1021 years [3], and the corresponding half-life for ??0? is expected to be orders of magnitudes larger than that. It is not surprising that even though there have been many experimental attempts to observe this important decay mode in the past, there is still no convincing and definitive conclusion [3].
The MAJORANA DEMONSTRATOR Project
The MAJORANA DEMONSTRATOR is a mixed array of enriched and natural germanium crystal detectors that will search for the neutrinoless double beta decay of the 76Ge isotope. Specific goals of the MAJORANA DEMONSTRATOR are to:
- Demonstrate a path forward to achieving a background rate at or below one count/tonne/year in the 4 keV region of interest (ROI) around the 2039-keV Q-value of the 76Ge 0???-decay. This is required for the next generation of tonne-scale germanium-based 0???-decay searches that will probe the neutrino mass scale in the inverted-hierarchy region (25 ~ 50 meV).
- Show technical and engineering scalability toward a tonne-scale instrument.
- Field an array that provides sufficient sensitivity to test the Klapdor-Kleingrothaus claim [3] and to be comparable with alternate approaches.
The MAJORANA Collaboration has designed a modular instrument composed of two cryostats built from ultra-pure electroformed copper, each containing 20 kg of HPGe detectors (Figure 2). These cryostats are capable of holding up to 35 p-type point-contact (PPC) HPGe detectors [4]. The baseline plan calls for 30 kg of the detectors to be built from 86% enriched material, resulting in a 76Ge mass of 30 kg enriched detectors and 10 kg of natural detectors. This amount of enriched material is sufficient to achieve the physics goals while still optimizing cost. The modular approach will allow the collaboration to assemble and optimize each cryostat independently, providing a fast deployment with minimum interference on already operational detectors. The inner passive shield will be constructed of electroformed and commercial high-purity copper, surrounded by high-purity lead, which itself is surrounded by an active muon veto and neutron moderator. The entire experiment will be located at the 4850 Level (1478 m) Davis Campus at the Sanford Lab Underground Research Facility (SURF).
The background goal for the MAJORANA DEMONSTRATOR requires reduction of backgrounds by a factor of ~100 relative to those demonstrated in previous Ge experiments. There are significant technical advances in the implementation of MJD in order to reach this goal. Some of those are discussed below.
Figure 2. Left: Sketch showing the overall configuration of the MAJORANA DEMONSTRATOR (MJD) experiment. The two main copper cryostats containing the germanium crystals are shown in the middle of a massive shield structure. Right: A 3D rendition of the MJD Monolith with a bearing floor. See text for details.
Ultra-Pure Materials
The selection and acquisition of ultra-low radioactivity materials for constructing the MJD detector are critical to the project. The production process for enriched germanium detectors themselves (enrichment, zone refining, and crystal growth) efficiently removes natural radioactive impurities from the bulk germanium. The cosmogenic activation isotopes 60Co and 68Ge are produced in the crystals while they are aboveground, but can be sufficiently reduced by the use of passive shielding during transport and storage.
Figure 3. Left: Photo showing the cave for storing the enriched germanium material in Oak Ridge, Tenn. Right: Photo for some of the zone refined enriched germanium ingots to be used by detector manufacturers.
The ~87% enriched 76GeO2 material for the MAJORANA DEMONSTRATOR was produced by the Electrochemical Plant (ECP) in Russia and transported to the United States in a customized steel-shield container, and stored in a shallow underground cave (Figure 3, left) in Oak Ridge, Tenn. The oxide material was then converted to metal and zone-refined in-house by the MAJORANA DEMONSTRATOR Collaboration. The photo in Figure 3 (right) shows some of the zone-refined enriched germanium ingots ready for use by detector manufacturers.
For the main structural material in the innermost region of the detector (such as the vacuum cryostat and the internal detector mounting and cooling hardware), the MAJORANA DEMONSTRATOR Collaboration chose copper for its lack of naturally occurring radioactive isotopes and its excellent physical properties. By starting with the cleanest copper stock available and electroforming it underground to eliminate primordial radioactivity and cosmogenically-produced 60Co, one expects to achieve several orders-of-magnitude background reduction over commercial alternatives.
Figure 4 shows the MAJORANA DEMONSTRATOR underground electroforming laboratory which has been operating since early 2011 near the 4850 Level Ross Shaft area at SURF. The MAJORANA DEMONSTRATOR electroformed material has been fabricated as cylinders that are up to 14 inches in diameter and 22 inches in length. The copper produced can range from a few thousandths of an inch to very thick plates exceeding 0.5 inches. Electroformed copper will also be employed for the inner passive, high-Z shield. Commercial copper stock is clean enough for use as the next 5 cm of shielding. Modern lead is available with sufficient purity for use as the bulk shielding material outside of the copper layers. Several clean plastics are available for electrical and thermal insulation. For the detector supports, the MJD collaboration selected pure PTFE, PEEK and Vespel material.
Figure 4. Left: Extracting an electroformed copper coated mandrel. Right: Interior view of the MJD electroforming laboratory in the 4850 Level Ross Shaft area.
P-type Point Contact HPGe Detectors
One important experimental signature of a double beta decay event in a HPGe detector is its localized (point-like single-site event, SE) energy deposition inside the detector due to the very short range (~mm) of ~ 2MeV beta particles in a germanium crystal. This is in great contrast to a typical background event, where the gamma ray will likely be scattered several times inside the crystal (multi-site event, MS). It is a great advantage to be able to identify the MS events efficiently for background rejection.
Figure 5. Scope charge (upper) and current (lower) traces of multi-site (MS) Compton scatter events in a standard coaxial p-type detector (left) and a point-contact detector PPC (right). This type of background event can be rejected efficiently by examining the shape of the rising edge of the waveform in a PPC detector.
HPGe detectors deployed in the MAJORANA DEMONSTRATOR are of the p-type point contact type [4]. While the PPC configuration is less common in typical gamma spectroscopic applications, their known characteristic long drift-time behavior [4] is particularly suitable for the present project. The relatively low electrical fields throughout most of the crystal results in slower carrier drift velocities, and hence larger time spreads between the arrivals of charge clouds liberated in spatially distinct energy depositions. In addition, the weighting potential increases sharply in the immediate vicinity of the point contact, resulting in an induced current spike just as the charge cloud is collected at the point contact. The current trace for a typical multi-site event in a PPC is contrasted with that of a standard coaxial detector in Figure 6. Discrimination of such events is essentially a matter of counting peaks, although the MAJORANA DEMONSTRATOR will employ more sophisticated techniques, such as ?2-based fits to ideal pulses.
Minimal Inactive Mass and Modular Engineering Design
To reduce radioactivity background contributions from all the inactive-mass components surrounding the germanium crystals, a minimalistic approach has been adopted. The engineering design effort also takes into account the serviceability and scalability of the detector array. Figure 5 shows an overview of the mechanical design for a Majorana Demonstrator detector cryostat module. An individual germanium detector is mounted in a stand-alone electroformed copper support mount, known as a single-detector unit. Several of the detector units are joined together to form a detector string assembly, and up to seven of these string assemblies could be mounted inside a common vacuum cryostat also fabricated from electroformed copper (Figure 5, left).
The internal crystal mounting hardware front-end electronics are designed to be low-mass and ultra-low background as well because they must be located in the interior of the array adjacent to the detectors in order to maintain signal fidelity. The circuit board is fabricated by sputtering thin traces of pure gold and titanium on a silica wafer, upon which a bare FET is mounted using silver epoxy (Figure 5, upper right). The proximity of the traces provides sufficient feedback capacitance without the need for additional material. A ~G?-level feedback resistance is provided by depositing intrinsically pure amorphous Ge in an appropriate geometry. The detector signals are read out by custom designed low-noise amplifiers (Figure 5, lower right).
Figure 6. Left: Sketch showing the arrangement of detectors (detector units and string assemblies) inside a Majorana Demonstrator cryostat module. The low-mass front-end board with a sputtered amorphous germanium feedback resistor on a fused silica substrate is shown in the upper right, with four of the low-noise preamps modules mounted in a daughterboard shown in the lower right. The preamps are connected to a computer configurable controller-pulser board for performance optimization and calibration.
The detector cryostat and supporting sub-systems are integrated into a stand-alone, scalable mechanical module, known as a monolith unit (Figure 2, right). A MAJORANA DEMONSTRATOR monolith consists of a cryostat of Ge detectors, the materials of the inner and outer shields, a panel of the Rn exclusion box, and the cryostat?s cooling, electrical, and vacuum hardware. It is equipped with a bearing floor and could be repositioned.
The MAJORANA DEMONSTRATOR Laboratory at the Davis Campus
The MAJORANA DEMONSTRATOR will be assembled and operated in the ?Transition Area? of the SURF Davis Campus.
Figure 7. Left: Floor plan layout for the Majorana Demonstrator experiment in the transition area of the Davis Campus 4850 Level. Right: A recent photo of the assembly room where the detector will be installed.
The MAJORANA DEMONSTRATOR Davis campus has three main areas (Figure 6, left). The first is the machine shop which contains several machine tools. The tools will be dedicated to the underground machining of the copper part of the MAJORANA DEMONSTRATOR. The second main area is called the electroforming room. It will be used for clean storage, detector string testing in string test cryostats, and clean bagging.
Figure 8. A 3D rendition of the clean assembly line for the Majorana Demonstrator cryostat with multiple and coupled glove boxes.
The third main area is the assembly room. This is where the MAJORANA DEMONSTRATOR will be assembled and operated. It also houses the glove box in which the cryostat internals will be assembled and a soft-walled clean room with a fume hood for cleaning and etching of small parts (Figure 7).
Summary
It is going to be an extremely exciting time for members of the MAJORANA Collaboration. All the efforts in the various tasks such as Project Management, Simulations and Analysis, Host Lab Infrastructure, Materials and Assay, Electroforming, Enriched Ge Production, Detectors, Detector Modules, Mechanical Systems and Integration, Data Acquisition and Testing and Commissioning etc. will come together for a full integration [5]. A detailed construction schedule and EH&S plan have been established with SURF, and heavy machinery and hardware sub-systems are ready to go underground. It is a very challenging experiment but if the design goals are reached, together with results coming from other research programs all over the world, the science community will soon know more about properties of the neutrinos.
References:
[1] E. Majorana, ?Teoria Symmetrica del l?Electrone e del Positrone,? Nuovo Cimento 14, 171, (1937) ?Symmetric theory of the electron and positron?.
[2] See, for example, ?Neutrino Properties? talk by Boris Kayser, Neutrino 2008.
www.slac.stanford.edu/econf/C0805263/Slides/Kayser.pdf
[3] H. V. Klapdor-Kleingrothaus and I. V. Krivosheina, "The Evidence for the Observation of 0??? Decay: the Identification of 0??? Events from the Full Spectra," Mod. Phys. Lett. A 21 (2006) p. 1547; Steven R. Elliott and Petr Vogel, Annu. Rev. Nucl. Part. Sci. 115, (2002).
[4] Luke, P.N. et al., ?Low capacitance large volume shaped-field germanium detector,? LBL-25694, Luke P., et al. "Low capacitance large volume shaped-field germanium detector". IEEE Transactions on Nuclear Science 36 (1): 926?930. (1989). Barbeau P., et al., "Large-mass ultra low noise germanium detectors: performance and applications in neutrino and astroparticle physics". Journal of Cosmology and Astroparticle Physics (2007).
[5] Majorana Demonstrator Final Design Report, The MAJORANA Collaboration (2012).