Getting to know dark matter’s traces

The ionization efficiency for silicon is plotted as a function of nuclear recoil energy. The black line and dots with error bars show the best measurements to date. The solid red line shows our fit to preliminary new data, from 2 to 20 keV. The dashed lines display the 1 sigma error bands of a single parameter χ2 fit to the model (developed by Lindhard, et al, in 1963). In our next run we expect these errors, for points every 1 keV, to shrink to the yellow band. The recoil energy range will cover from 1 to 30 keV.

When an energetic particle — perhaps a dark matter particle — interacts with the nucleus of an atom, the nucleus can recoil. Some fraction of the energy transferred to the recoiling nucleus disturbs electrons in adjacent atoms, producing free electric charge. This fraction is called ionization efficiency. The bigger this number, the larger the signal in the detector and the easier it is to detect nuclear recoils.

Ionization efficiency measurements at low energies are important to calibrate the energy measurement of silicon detectors used in dark matter direct-detection experiments. The calibration will also help experiments trying to observe coherent neutrino scattering, such as CONNIE, which is at a nuclear power plant in Angra dos Reis, Brazil.

At low energies, the current best measurements of the ionization efficiency in silicon have considerable uncertainty.

Scientific charge-coupled devices (better known as CCDs) made of silicon are now able to detect a few electronvolts of ionization energy. These detectors can detect low-energy nuclear recoils where the ionization efficiency has never been measured. A test beam apparatus, shown in the picture below, will provide a measurement of the ionization efficiency in silicon for low recoil energies, in the range of 1 to 30 kiloelectronvolts, or keV.

The test beam, located at the Institute for Structure and Nuclear Astrophysics at the University of Notre Dame, provides 30- to 600-keV neutrons. The neutrons scatter off a silicon detector and are measured by an array of plastic scintillators and devices called photomultipliers. Scientists will use this apparatus to determine how the ionization efficiency changes with the lower nuclear recoil energy.

A preliminary, proof-of-concept run of seven hours using only two scintillator bars generated the result shown in red in the upper plot. A total of 69 scattering neutron events were used in the measurement. Scientists compared the data with simulations using a theoretical model developed by Lindhard, et al, in 1963. The measurement produced the preliminary result shown by the red solid line in the plot above.

The team will soon run for two weeks, with a full setup of 21 scintillator bars. Calculations and simulations predict a collection of about 1,000 neutron events. With these statistics the errors bars will be reduced from the red dashed lines to the yellow band shown in the plot.

Federico Izraelevitch, Fermilab, and Marco A. Reyes, University of Guanajuato

Scientists working on today’s result stand next to the experiment’s scintillator bar array, located at SiDet at Fermilab. From left: Andrew Lathrop (Fermilab), Federico Izraelevitch (Fermilab), Marco Reyes (University of Guanajuato), Gustavo Cancelo (Fermilab), Gaston Gutierrez (Fermilab), Junhui Liao (University of Zurich) and Juan Estrada (Fermilab). Inset, from left: Javier Tiffenberg (Fermilab), Vic Scarpine (Fermilab), Leonel Villanueva-Rios (University of Guanajuato), Jorge Molina (National University of Asuncion), Alex Kavner (University of Michigan) and Dante Amidei (University of Michigan).