CUORE: Creating the coldest cubic meter in the universe

Yale scientists working on the cryostat of the Cryogenic Underground Observatory for Rare Events (CUORE). 

The experiment is located in a clean room deep underneath the Gran Sasso mountain in Italy to shield the experiment from cosmic rays and other environmental backgrounds. 

Credit: CUORE collaboration

The drive to create the coldest cubic meter in the universe may be centered in Italy, but its ultimate success will depend on instruments developed at Yale University.

An international team of scientists recently set a world record by cooling a copper vessel with a volume of a cubic meter down to a temperature of 6 milliKelvins, or -273.144 degrees Celsius.

It was the first experiment to chill an object so large this close to absolute zero.

The collaboration, called CUORE (Cryogenic Underground Observatory for Rare Events), involves 130 scientists from the United States, Italy, China, Spain, France, and other countries.

It is based at the underground Gran Sasso National Laboratory (LNGS) of the Instituto Nazionale di Fisica Nucleare (INFN), in Italy.

"This is a major technological achievement," said Karsten Heeger, a professor of physics at Yale and director of Yale's Arthur W. Wright Laboratory.

CUORE is part of the new experimental program in neutrinos and dark matter pursued at the Wright Lab.

Yale physicists are building and testing instrumentation that will be used at temperatures of 10mK in the experiment's cryostat, which is the chilled chamber.

Reina Maruyama, an assistant professor of physics, is one of the original proponents for the US involvement in CUORE and is a coordinator of its data analysis

"In collaboration with the University of Wisconsin, we have developed a detector calibration system that will deploy radioactive sources into the coldest region of the cryostat and characterise our detectors," Heeger said.

Once the CUORE experiment is fully operational, it will study important properties of neutrinos, the fundamental, subatomic particles that are created by radioactive decay and do not carry an electrical charge.

Specifically, the experiment will look at a rare process called neutrino-less double-beta decay.

The detection of this process would let researchers demonstrate, for the first time, that neutrinos and anti-neutrinos are the same, thereby offering a possible explanation for the abundance of matter, rather than anti-matter, in the universe.

The experiment uses heat-sensitive detectors that operate in extremely cold temperatures. "It poses a unique challenge," Heeger said.

"We are trying to detect a minuscule amount of heat from nuclear decay, but need to know this very precisely. The detector calibration will tell us if we see the heat from double-beta decay or environmental backgrounds."

Now that the cryostat has reached base temperature, the commissioning and cryogenic testing of the calibration system will take place in the next few months, Heeger said.

More information: crio.mib.infn.it/wigmi/pages/cuore.php

Using antineutrinos to monitor nuclear reactors

Dr. Nils Haag developed an experimental setup that allowed him to determine the missing spectrum of uranium 238. 

Credit: Wenzel Schuermann / TU München 

When monitoring nuclear reactors, the International Atomic Energy Agency (IAEA) has to rely on input given by the operators. 

In the future, antineutrino detectors may provide an additional option for monitoring.

However, heretofore the cumulative antineutrino spectrum of uranium 238 fission products was missing.

Physicists at Technische Universität München have now closed this gap using fast neutrons from the Heinz Maier Leibnitz Neutron Research Facility.

In addition to neutrons, the fission reaction of nuclear fuels like plutonium or uranium releases antineutrinos.

These are also electrically neutral, but can pass matter very easily, which is why they can be discerned only in huge detectors.

Recently, however, detectors on the scale of only one cubic meter have been developed. They can measure antineutrinos from a reactor core, which has generated great interest at the IAEA.

Prototypes of these detectors already exist and collect data at distances of around 10 meters from a reactor core.

Changes in the composition of nuclear fuels in the reactor e.g., when weapons-grade U-239 is removed, can be determined by analyzing the energy and rate of antineutrinos.

This would free the IAEA from having to rely on representations of reactor operators.

Antineutrino spectrum of uranium 238 revealed
In the 1980s the antineutrino spectra of three main fuel isotopes, uranium 235, plutonium 239 and plutonium 241, were determined.

However, the antineutrino spectrum of the fourth main nuclear fuel, uranium 238, which accounts for approximately 10 percent of the total antineutrino flux, remained unclear.

It had only been estimated using inaccurate theoretical calculations and thus limited the accuracy of the antineutrino predictions.

Dr. Nils Haag

Dr. Nils Haag from the Chair of Experimental Astroparticle Physics at TU München recently developed an experimental setup at the FRM II that allowed him to determine the missing spectrum of uranium 238.

"I needed a high flux of fast neutrons to induce the fission of the U-238," says the physicist. This is why he located his experimental setup at the NECTAR radiography and tomography station of the FRM II, a source of fast neutrons.

Second detector allows background-free measurement
The neutrons induce nuclear fission in a film of U-238. The radioactive decay products then emit electrons and antineutrinos.

The electrons were investigated using a scintillator – a block of plastics that converts the kinetic energy of the electrons into light. A photomultiplier then translates this into electrical signals.

The nuclear decay also generates gamma radiation that produces unwanted events in the scintillator.

Therefore, Haag placed a second detector right in front of the scintillator: a so-called multi-wire proportional chamber.

Since only charged particles like electrons trigger a signal in the gas detector, the researcher was able to determine and subtract the proportion of gamma radiation.

Haag then inferred the antineutrino spectrum using this background-free measurement data.

Method allows better monitoring of reactor cores
The measurement of the antineutrino spectrum can be used to monitor the status, performance and even composition of reactor cores.

"Our results open the door to predict with significantly higher accuracy the expected antineutrino spectrum emitted by a reactor running on a fuel composition reported by the operator," explains Dr. Nils Haag.

"Deviations of antineutrino detector measurement data from expected reactor signals can thus be exposed."

The development of this methodology is embedded in basic research on the phenomenon of so-called "sterile" antineutrinos.

Comparing previously made measurements and predictions of reactor antineutrino spectra gave rise to the assumption that some of the antineutrinos turned "sterile" after being produced.

They were then no longer able to react with other matter. A better understanding of this effect would expand our knowledge of elementary physical processes.

More information: 
Experimental Determination of the Antineutrino Spectrum of the Fission Products of U238, N. Haag, A. Gütlein, M. Hofmann, L. Oberauer, W. Potzel, K. Schreckenbach, and F. M. Wagner, Phys. Rev. Lett. 112, 122501 (2014), DOI: 10.1103/PhysRevLett.112.122501