CUORE Experiment

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The Cryogenic Underground Observatory for Rare Events (CUORE) [1] is an upcoming experiment at LNGS whose primary purpose is to search for neutrinoless double beta decay in 130Te, a relatively abundant isotope of the element tellurium.  Neutrinoless double beta decay (0νββ) is a process hypothesized to occur very rarely in some elements, if at all, whereby the neutrino (ν) emitted by the beta decay of one neutron (n → p + e- + ν) is absorbed in the simultaneous beta decay of another neutron (ν + n → p + e-).  This can occur only if the neutrino is its own anti-particle and thus a so-called "Majorana" particle, unlike all other known constitutents of matter (for example, electrons and quarks) which are so-called "Dirac" particles.  0νββ is the only feasible way to discover if the neutrino is a Majorana particle; if observed, it would have enormous consequences for nuclear and particle physics, astrophysics, and cosmology.  An observation of 0νββ also has the potential to determine the absolute mass of neutrinos, which is still unknown.  0νββ therefore offers a unique opportunity to probe the fundamental nature of neutrinos in several ways.

The CUORE detector will consist of an array of 988 tellurium dioxide (TeO2) crystals (Figure 1 and 2), which contain 27% 130Te by mass and therefore also serve as the source of 0νββ.  The crystals will be cooled inside a specially built dilution refrigerator --one of the world's largest-- to a temperature of ~ 10 mK, at which point they have such a small heat capacity that the energy deposited by individual particles or gamma rays in a crystal produces a temporary, measurable rise its temperature.  The measured temperature pulses will be used to construct an energy spectrum of the interactions occurring inside the crystals, and the spectrum is then inspected for a small peak at 2527 keV, the amount of energy that would be released if 0νββ occurs in 130Te

 
Figure 1.  Prototype with four TeO2 detectors assembled in a copper frame.   Figure 2. Drawing of the full detector inside the future CUORE cryostat and shields.

 

If 0νββ does exist, it will occur so rarely that its peak in the energy spectrum will be very small.  In order to make the peak visible, it is necessary to suppress the underlying background in the energy spectrum as much as possible.  This is accomplished in several ways.  The first and most essential step is to conduct the experiment at an underground facility like LNGS, whose 1.5-km-thick overhead rock barrier absorbs most of the cosmic rays that continually bombard the surface of the earth.  The second step is to manufacture and assemble the crystal detectors under the cleanest possible conditions so that they don't collect common, naturally occurring radioactive contaminants like radon.  The third step is to surround the detectors with a lead shield to block any remaining ambient radiation.  In fact, the CUORE cryostat will contain several tons of ancient Roman lead, salvaged from a shipwreck off the Sardinian coast (Figure 3), which possesses extremely low levels of radioactivity due to the fact that the lead was smelted thousands of years ago [2].

 

Figure 3. A scuba diver recovering lead ingots from an ancient Roman shipwreck off the eastern coast of Sardinia.  Some of the lead ingots will be melted down and used for shielding inside the CUORE cryostat.

 

CUORICINO, a demonstrator experiment for CUORE, operated from 2003-2008 at LNGS using the same basic techniques as CUORE, but with only 62 TeO2 crystals and higher backgrounds (Figure 4).  0νββ was not observed in CUORICINO, and the experiment has been able to set the world's most stringent lower limit for the half-life for 0νββ in 130Te, T1/2 ≥ 2.8×1024 y [3].

 

Figure 4. The CUORICINO Tower during the construction, before the installation of the copper shields. The 13 layers of TeO2 detectors are visible.

 

 

The next project goal for CUORE will be the construction and operation of CUORE-0, the first 52-crystal tower produced by the CUORE detector assembly line.  The CUORE-0 tower will be installed in the existing CUORICINO cryostat in Spring 2011, and it will take data for the next two years while the 19 CUORE towers are assembled.  CUORE-0 is primarily intended to serve as a test of the CUORE detector assembly protocols and to verify the functionality of the experimental components, but it will nevertheless represent a significant measurement in its own right: it will be comparable in size to CUORICINO, yet its energy spectrum will have a lower background due to improvements in materials and assembly procedures.

At present, CUORE-0 and CUORE are in a construction phase: the hut that will house the CUORE apparatus is being built in Hall A at LNGS; TeO2 crystals are being manufactured in China; and the cryostat and refrigeration equipment are being built in Holland.  In addition, each institution in the CUORE collaboration is working on its own particular responsibility, such
as building the data-acquisition electronics, acquiring and machining low-activity copper for the detector scaffolding, etc.  CUORE is expected to begin taking data in 2013.

As indicated in its name, CUORE is a low-background observatory for rare events, so in principle it also has the potential to detect processes involving exotic particles such as dark matter or axions.

 

[1] C. Arnaboldi et al. [CUORE Collaboration], Nucl. Instrum. Meth. A518 (2004) 775-798.

[2] E. Fiorini, Il Nuovo Saggiatore 7 (1991) 29.

[3] C. Arnaboldi et al. [CUORICINO Collaboration], "Results from a search
for the 0νββ of 130Te," Phys. Rev. C 78, 035502 (2008).  A final result was announced at the Neutrinos 2010 Conference, July 14, 2010, and a paper is now in preparation.

 

 

 


 


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