Dark Matter Detection Using Helium Evaporation and Field Ionization

We describe a method for dark matter detection based on the evaporation of helium atoms from a cold surface and their subsequent detection using field ionization. When a dark matter particle scatters off a nucleus of the target material, elementary excitations (phonons or rotons) are produced. Excitations which have an energy greater than the binding energy of helium to the surface can result in the evaporation of helium atoms. We propose to detect these atoms by ionizing them in a strong electric field. Because the binding energy of helium to surfaces can be below 1 meV, this detection scheme opens up new possibilities for the detection of dark matter particles in a mass range down to $1\mathrm{MeV}/c^2$.

Figure 1

Schematic diagram of (a) the original HERON experiment and (b) a dark matter direct detection experiment based on the detection of evaporated helium atoms by field ionization. In both cases, the recoil of a helium nucleus in the superfluid produces rotons and phonons which, upon arriving at the free surface, cause helium atoms to be released by quantum evaporation.

Figure 2

Field ionization. (a) Illustration of a helium atom a distance $x_c$ away from a positively charged metal tip. (b) Diagram of the electron potential energy $U$ showing the field-distorted barrier of a bound electron (black line), the influence of the applied field on a free electron (dashed line), the work function of the tip $W$, and the helium ionization energy $E_{\mathrm{ion}}$.

Figure 3

Schematic diagram of a field ionization tip array indicating $D$, $L$, $s$, and $d$.