experiment

The Coherent Elastic Neutrino-Nucleus Scattering process

Neutrinos, abbreviated $\nu$, are elementary particles, belonging to the category of fermions in the Standard Model of particle physics. They are very different from other know particles: first, they have an extremely low mass, so low that physicists have not been able to measure it yet! They also only interact through weak interaction, the weakest of all four forces known today, which makes them travel extremely long distance through matter without leaving a trace. On top of this, neutrinos do not have electric charge, and are only left-handed, making them one of the most mysterious elementary particle. Therefore, they are a topic of extreme interest for physicists, who study them to better understand nature at the subatomic scale.

At low energy, neutrinos interact mainly through coherent elastic neutrino-nucleus interactions, abbreviated as CEνNS. The interaction is coherent because the scattering wavelength is larger than the size of the nucleus : they interact coherently with the whole atomic nucleus. The process is elastic as neutrinos do not transfer enough energy to create new particle, they only scatter off the nucleus, hence conserving momentum and energy in the final state.

The CEνNS process has been predicted in 1974 by D. Z. Freedman, yet was only detected for the first time in 2017 by the COHERENT experiment at the Spallation Neutron Source (US) due to the challenging nature of detecting this faint signal. The Ricochet experiment aims at measuring the CEνNS interaction at lower energy by using a reactor as a source, pushing down the frontier of neutrino detection.

Testing the Standard Model with CEνNS

The study of the CEvNS process is a powerful probe for new physics searches. Indeed, recent theoretical and phenomelogical studies have found that any deviations from the Standard Model arising from a highly precise measurement of the CEvNS process at the lowest energies could lead to a myriad of exotic scenarios. These include for instance the existence of new mediators, that could be related to the long lasting Dark Matter problem, and the possibility of Non Standard Interactions that would dramatically affect our understanding of the electroweak sector. To that end, the Ricochet experiment aims at lowering the detection threshold of the CEνNS process detection with a large signal-to-background ratio, thanks to its unique particle identification capability, to test such extensions of the Standard Model.

The Laue-Langevin Institute

The Ricochet experiment is located at the Laue-Langevin Institute, holding the only nuclear reactor in France entirely dedicated to scientific research.

The H7 site, where the Ricochet experiment is located, starts at about 8 m from the ILL reactor core that provides a nominal nuclear power of 58.3 MW, leading to a neutrino flux at the Ricochet detectors 8.8 m from the reactor core of about 1.2×10¹² cm⁻²s⁻¹. The reactor is operated in cycles of typically 50 days’ duration with reactor-off periods sufficiently long to measure reactor-independent backgrounds, such as internal radioactivity or cosmogenic induced backgrounds, with high statistics. The available space is about 3 m wide, 6 m long and 3.5 m high. It is located below a water channel providing about 15 m water equivalent (m.w.e.) against cosmic radiations. It is not fed by a neutron beam and is well-shielded against irradiation from the reactor and neighboring instruments (IN20 and D19). The ILL is not new in the field of neutrino physics since a 1st experiment was started in 1976 (with the Nobel prize winner, Mössbauer R.L., being part of the team 1. More recently the experiment STEREO proved that the sterile neutrino, postulated to address the problem of a neutrino deficit in some circumstances, is highly unlikely to exist 2.

The Ricochet experimental setup

The Ricochet detectors are operated in a CryoConcept Hexa-Dry 200 dilution refrigerator equipped with ultra quiet technology (UQT) to minimize noise induced by vibrations from the pulse tube. Cryoconcept’s UQT is implemented by mounting the dilution refrigerator and the pulse-tube system on two different triangular frames. The thermal exchange between the cold head and the fridge plates is achieved by convection using the 3He/4He mixture within the dilution circuit. The significant reduction of mechanical contact between the pulse tube and the cryostat, which is limited to only a gas-sealing bellows with low natural vibration frequency, minimizes the propagation of vibrations.

The cryostat is surrounded by a 22-ton external shield to limit reactogenic and radiogenic backgrounds. It is composed of two parts: (1) a cup-shaped shield around the detector area which consists of a 35-cm-thick inner layer of borated high-density polyethylene (HDPE) and a 20-cm-thick outer layer of lead; (2) two top layers of 20-cm-high lead (bottom) and 35-cm-high HDPE (top) that further attenuate neutrons and backgrounds coming from above. The external shield is enclosed in a soft iron shell, which attenuates the magnetic fields generated by the neighboring experiments, and is mounted on a rail system to ease access to the cryostat.

The external shield does not cover the top of the cryostat. For this reason, an internal shielding was designed to fill the cylindrical space above the detectors. The internal shield consists of two parts: (1) a cylindrical plug mounted at the 1-K stage, composed of two 4.25-cm-high lead disks inserted between three 1.5-cm-high copper disks, followed by 16 layers of alternating ∼3.5-cm-high HDPE disks and 1-cm-high copper disks; (2) 8-mm-thick HDPE sheets installed around the copper cans of the 1-K, 4-K and 50-K stages at the height of the internal plug.

The outer muon veto consists of 34 plastic scintillator panels (200×50×3 cm³) repurposed from the GERDA experiment. The panels are arranged in 17 pairs to form an inner and outer layer. Each muon veto panel is read out via a single photomultiplier tube (Hamamatsu R960) located at one extremity of the panel, the light collection is ensured by optical fibers glued on the panel’s lateral faces. Lastly, a cryogenic muon veto system, made of two scintillating panels readout with a total of eight optical fibers and 2 SIPM, as been installed in the cryostat below the 4K stage.

The Ricochet cryogenic detectors

Searching for Coherent Elastic Neutrino-Nucleus Scattering (CEνNS) interactions involves detecting antineutrino-induced sub-keV nuclear recoils, which are overwhelmed by environmental backgrounds. To achieve this ambitious goal, the Ricochet collaboration has developed an array of 18 ultra-sensitive 42 g germanium crystals, known as the CryoCube, cooled to 15 mK. When an antineutrino scatters off a nucleus in the crystal, the subsequent nuclear recoil disturbs the crystal lattice, generating phonons that propagate through the detector, and ionizes the surrounding material, additionaly creating electron-hole pairs. Each crystal is equipped with both a phonon sensor, namely a Neutron Transmutation-Doped (NTD) germanium thermistor, and aluminum electrodes. This setup enables the simultaneous readout of the heat and ionization energies deposited in the crystal following any particle interaction. The collaboration has designed two types of electrodes: planar electrodes (PL), which cover the top and bottom surfaces, and fully interdigitated electrodes (FID), which allow for surface background rejection at the cost of a reduced fiducial volume. The dual readout of heat and ionization energies is the key technological feature of the Ricochet experiment. It uniquely enables the identification of each recoiling particle, down to the sub-keV recoil energy range, thus providing highly efficient background suppression which is critical for an accurate CEνNS measurement. Although the first science phase has already begun, the collaboration continues an intensive R&D program aimed at improving CEνNS sensitivity and expanding the experiment’s physics reach. These improvements include: 1) the development of the Q-array detector technology using superconducting cryogenic crystals, and 2) the addition of a second semiconductor target material, such as silicon, which has already shown promising results.

The Ricochet collaboration

Ricochet is an international collaboration formed by the following institutes:

• Centre for Nanoscience and Nanotechnology [C2N] (France)
• Colorado School of Mines [Mines] (USA)
• Institut Néel (France)
• Institute of Physics of the 2 Infinities of Lyon [IP2I-Lyon] (France)
• Joint Institute for Nuclear Research [JINR] (Russia)
• Laboratory of Subatomic Physics and Cosmology of Grenoble [LPSC] (France)
• Laboratory of the Physics of the two Infinities Irène Joliot-Curie [IJCLab] (France)
• Laue-Langevin Institute [ILL] (France)
• Massachusetts Institute of Technology [MIT] (USA)
• Northwestern University (USA)
• University of Massachusetts Amherst [UMass Amherst] (USA)
• University of Toronto [UoT] (Canada)