CUORE Experiment
CUORE uses bolometers to search for neutrinoless double beta (0νββ) decay and other rare processes. Our bolometers are ultra-cold tellurium dioxide (TeO2) crystals containing the candidate 0νββ isotope Te-130. Every time a tellurium nucleus decays or a particle interacts in the crystal, it releases a minute amount of energy (less than a few MeV), causing the temperature of the crystal to rise slightly. This rise in temperature is then converted into an electrical signal using temperature-dependent resistors (thermistors). For this temperature rise to be measurable, the baseline temperature of the crystals must be very low. We use ultra-cold cryogenic temperatures: a few thousandths of a degree above absolute zero.
One major advantage of crystal bolometers for a 0νββ search is that the crystals can be grown from the double-beta-decaying isotope. When the source is fully contained inside the detector, as is the case in our bolometers, the emitted electrons are nearly always observed. In 1984, Ettore Fiorini proposed using bolometers to search for 0νββ decay. In particular, TeO2 crystals were chosen to investigate the decay of Te-130, as bolometers made from TeO2 can be made extremely radiopure and with an excellent energy resolution (0.2% at the 0νββ energy region of interest).
Ten years later, Fiorini’s research group demonstrated the potential of this idea by successfully operating a single 340 g TeO2 crystal. In 2003, MiDBD, an array of 20 crystals (with a total mass of 6.8 kg of TeO2), published results that paved the way to develop large-mass bolometric detectors. Cuoricino (2003–2008) and its successor, CUORE-0 (2013–2015) have published compelling results. These experiments did not observe 0νββ decay in 130Te, but allowed us to set the most stringent limit to date on its half-life: 4.0 × 1024 years at 90% C.L. CUORE aims to improve the sensitivity of this half-life up to 1026 years by using a mass 19 times larger than that of CUORE-0, and decreasing the background rate in the region of interest.
Working Principle
In 1930, Wolfgang Pauli wrote a letter addressed to the “dear radioactive ladies and gentlemen” at a physics conference in Tübingen, Germany. In the letter, he predicted the existence of a new, neutral particle to solve a problem in beta decay experiments. These experiments had found that, counter to expectations, the electrons emitted in beta decays did not carry the full energy of the decays. Pauli, as a “desperate attempt” to explain this apparent violation of energy conservation, postulated that some of the energy of the decay was being carried away by an undetected new particle. Enrico Fermi named this new particle the neutrino, and proposed a full theory of beta decay incorporating this new particle.
The neutrino was observed for the first time 25 years later by Frederick Reines and Clyde Cowan, when they detected antineutrinos from a nuclear reactor. Since then, neutrinos have been the subject of experiments with ever-increasing sensitivity. Experiments have shown that more than one family of neutrinos exist, that neutrinos from different families can change into other another, and that neutrinos have a very small—but nonzero—mass.
The Detector
To view a larger, more detailed version of the detector simply click the detector image below.
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The Cryostat
The CUORE detectors and their shielding must be cooled to approximately 10 mK before they can take data. We cool them using one of the largest and most powerful helium dilution refrigerators ever constructed. Our cryostat produces the coldest cubic meter in the known universe.
To reach such a low temperature, the cryostat is cooled in different stages. The first, which allows the cryostat reach a few degrees above absolute zero, can be performed using cryogenic liquids (liquid helium and liquid nitrogen) or pulse tube cryocoolers. For CUORE, we chose the latter, as it ensures that the cryostat will be stable over several years without needing to replace boiled off liquid cryogens, which disturbs the detector operating conditions.
The second stage, that brings the temperature down to a few mK, exploits the unique properties of two isotopes of helium, 3He and 4He, to cool matter to a fraction of a degree above absolute zero. At very cold temperatures, 3He and 4He cannot be mixed in an arbitrary ratio; the mixture will spontaneously separate into two phases, much like oil and water. These phases are known as the concentrated phase, containing mostly 3He, and the dilute phase, containing mostly 4He. A pump outside the cryostat pulls 3He across the boundary from the concentrated phase to the dilute phase, an endothermic process that absorbs energy and is the key to the cooling power of the refrigerator. This 3He is then returned to the dilute phase, completing the circuit. We pump on the dilute phase to encourage faster phase change from concentrated to dilute, just like blowing across the gas above a cup of tea encourages a phase change from liquid to gas, cooling your tea.
Smaller dilution refrigerators are commonly used in modern low-temperature experimental physics; they typically have working spaces of tens of cubic centimeters. The CUORE detectors, on the other hand, weigh approximately a ton and are approximately a cubic meter in size, and are accompanied by several tons of copper and lead shielding that need to be cooled as well. Also, the CUORE detectors are extremely sensitive to any radioactive background, so the cryostat had to be constructed from extremely radio-pure materials. For these reasons, the design of a cryogenic facility for CUORE was a real challenge.
The cryostat is surrounded by an octagonal lead shield and borated polyethylene neutron shield. Some of the lead used for shielding close to the detector was recovered from an Ancient Roman shipwreck off the coast of Sardinia. For more information, see Roman ingots to shield particle detector in Nature News.
Performances
Physics with CUORE
[ref]| Background goal in 0νββ region of interest | 0.01 counts/keV/kg/yr |
| Expected energy resolution | 5 keV at 2615 keV |
| Expected sensitivity to 0νββ | T1/2 = 9.5 × 1025 yr in 5 years (90% C.L.) |
CUORE Detectors [ref]
| Number of bolometers | 988, arranged into 19 towers with 13 floors of 4 crystals each |
| Detector composition | TeO2 crystals, with natural isotopic abundance of tellurium |
| Crystal dimensions | 5 × 5 × 5 cm3 |
| Detector mass | 742 kg, with 206 kg of 130Te |
CUORE Cryostat [ref]
| Configuration | Dilution refrigerator with pulse tube cryocoolers |
| Internal shielding | Roman lead shield on sides and below detector, and modern low-radioactivity lead shield above detector |
| External shielding | Modern low-radioactivity lead shield and borated polyethylene neutron shield |
Physics with CUORE-0
| 0νββ half-life limit | T1/2 > 2.7 × 1024 yr (90% C.L.) |
| 0νββ half-life limit (with Cuoricino) | T1/2 > 4.0 × 1024 yr (90% C.L.) |
| Effective Majorana neutrino mass limit (with Cuoricino) | mββ < 270–760 meV (90% C.L.) |
| Energy resolution in 0νββ region of interest | 5.1 ± 0.3 keV FWHM |
| Background level in 0νββ region of interest | 0.058 ± 0.004 (stat.) ± 0.002 (syst.) counts/keV/kg/yr |
| Final isotopic (130Te) exposure | 9.8 kg yr |
CUORE-0 Detectors [ref]
| Number of bolometers | 52, arranged into 1 tower with 13 floors of 4 crystals each |
| Detector composition | TeO2 crystals, with natural isotopic abundance of tellurium |
| Crystal dimensions | 5 × 5 × 5 cm3 |
| Detector mass | 39.1 kg, with 10.9 kg of 130Te |
CUORE Cryostat [ref]
| Configuration | Dilution refrigerator |
| Internal shielding | Roman lead shield cylindrical shield around detector and disks above and below the detector |
| External shielding | Modern low-radioactivity lead shield, borated polyethylene neutron shield, and acrylic anti-radon box flushed with nitrogen gas |
Physics with Cuoricino
| 0νββ half-life limit | T1/2 > 2.8 × 1024 yr (90% C.L.) |
| Effective Majorana neutrino mass limit (with Cuoricino) | mββ < 300–710 meV (90% C.L.) |
| Energy resolution in 0νββ region of interest | 6.3 ± 2.5 keV FWHM for 5 × 5 × 5 cm3 crystals |
| Background level in 0νββ region of interest | 0.169 ± 0.006 counts/keV/kg/yr |
| Final isotopic (130Te) exposure | 19.75 kg yr |
Cuoricino Detectors
| Number of bolometers | 62, arranged into 11 floors of four 5 × 5 × 5 cm3 crystals each and 2 floors of nine 3 × 3 × 6 cm3 crystals each. |
| Detector composition | All the TeO2 crystals feature natural isotopic abundance of tellurium, with the exception of two 3 × 3 × 6 cm3 crystals enriched in 130Te (82%) and two 3 × 3 × 6 cm3 crystals enriched in 128Te (75%) |
Bolometers
| Detector working temperature | T ≈ 10 mK |
| Crystal heat capacity | C ≈ 2 × 10-9 J/K at 10 mK |
| Thermal coupling of crystal to heat bath | G ≈ 2 × 10-9 W/K at 10 mK |
| Bolometer time constant | τ = C/G ≈ 1 s at 10 mK |
| Thermistor resistance | R ≈ 100 MΩ at 10 mK |
| Typical pulse decay time | t ≈ 0.2 s |
| Thermistor voltage signal | ΔV ≈ 0.3 mV/MeV |
| Crystal temperature rise | ΔTcrystal ≈ 0.1 mK/MeV |
Inquiries:
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