Baseline energy resolution of the first BULLKID prototype: 64 cubes of 5.4 × 5.4 × 5 mm³ carved in a 3-inch, 5 mm thick silicon wafer.
Research › Detectors and simulation
Cryogenic detectors
Silicon arrays for dark matter and neutrinos
A silicon wafer carved into dozens of cubes, each read by a superconducting resonator, listens for collisions of a few tens of eV.
The physics.
Light dark matter and coherent elastic neutrino–nucleus scattering (CEνNS) leave the same faint trace: a nucleus recoiling with tens to hundreds of electronvolts. Measuring it takes calorimeters cooled to millikelvin temperatures, where the heat capacity of a crystal is tiny and a small energy deposit gives a measurable signal.
In BULLKID the energy is read through phonons. The recoil creates phonons that spread through the silicon absorber and reach a thin aluminium film, a superconducting resonator. They break Cooper pairs, the kinetic inductance of the film rises and its resonance moves to lower frequency: a kinetic inductance detector (KID). Many resonators at different frequencies are read on a single line by microwave multiplexing.
The array is carved from one thick silicon wafer. Deep cuts divide it into cubes but stop short of the back, leaving a thin common disk that holds them and carries the KIDs: no glue and no individual holders, and the neighbours of each cube act as a veto against background from outside.
We dice the arrays in Ferrara. DANAE and BULLKID-DM scale the concept to sixteen wafers, over 2,000 elements and about 800 g at the Gran Sasso laboratories; for NUCLEUS, which will measure CEνNS at the Chooz reactor, we machine the germanium veto and the silicon holders.
What we have shown.
BULLKID carves a single silicon wafer into a monolithic array of absorbers, each read by a kinetic inductance detector, with no extra supports or wiring: the first 60-element prototype reached 26 eV baseline resolution. DANAE and BULLKID-DM scale this to sixteen wafers, over 2,000 elements and about 800 g at the Gran Sasso laboratories. For NUCLEUS, which will measure coherent neutrino–nucleus scattering at the Chooz reactor, we machine the germanium veto and silicon holders.
Energy threshold of BULLKID running continuously in a surface laboratory, with background from other cubes of the array rejected.
Resolution on bulk events from the 59.5 keV γ line of ²⁴¹Am, improved by a further factor of two by combining neighbouring cubes.
First observation of a nuclear-recoil peak induced by neutron capture, a new way to calibrate low-threshold detectors, in a NUCLEUS CaWO₄ detector.
Read first.
- BULLKID: Monolithic array of particle absorbers sensed by kinetic inductance detectorsCruciani A., Bandiera L., Calvo M. … Guidi V., Romagnoni M., Mazzolari A. et al. · Applied Physics Letters 121 (2022)
- Low-energy spectrum of the BULLKID detector array operated on surfaceDelicato D., Ahmad A., Bandiera L. … Guidi V., Romagnoni M., Mazzolari A. et al. · European Physical Journal C 84 (2024)
- Energy calibration of bulk events in the BULLKID detectorFolcarelli M., Delicato D., Acevedo-Rentería A. … Bandiera L., Guidi V., Malagutti L. et al. · European Physical Journal C 86 (2026)
- Germanium target sensed by phonon-mediated kinetic inductance detectorsDelicato D., Angelone D., Bandiera L. … Guidi V., Romagnoni M., Mazzolari A. et al. · Applied Physics Letters 126 (2025)
- Nucleus: Searching for Coherent Neutrino Nucleus Scattering at Lowest EnergiesKluck H., Angloher G., Bento A. … Guidi V., Mazzolari A. et al. · Journal of Low Temperature Physics 209 (2022)
- Observation of a Nuclear Recoil Peak at the 100 eV Scale Induced by Neutron CaptureAbele H., Angloher G., Bento A. … Mazzolari A., Romagnoni M. et al. · Physical Review Letters 130 (2023)