Figure 1

Charge exchange cross sections due to ${\mathrm{He}}^0$, ${\mathrm{He}}^{1+}$, and ${\mathrm{He}}^{2+}$ interacting with ground state He atoms. The curves were fitted to experimental data by polynomial functions. (a) ${\sigma }^{01}$: solid black line (this work), red (light gray) filled circle (Ref. 87), black open circle (Ref. 88), blue (dark gray) filled upward triangle (Ref. 89); ${\sigma }^{02}$: dashed red line (this work), red (light gray) filled downward triangle (Ref. 89). (b) ${\sigma }^{10}$: solid black line (this work), red (light gray) filled upward triangle (Ref. 90), black open square (Ref. 91), red (light gray) open circle (Ref. 92), red (light gray) filled square (Ref. 93); ${\sigma }^{12}$: dashed red line (this work), black filled downward triangle (Ref. 94), blue (dark gray) crossed symbol (Ref. 92). (c) ${\sigma }^{20}$: solid black line (this work), black filled upward triangle (Ref. 95), red (light gray) open square (Ref. 96), blue (dark gray) filled downward triangle (Ref. 97), red asterisk symbol (Ref. 98); ${\sigma }^{21}$: dashed red line (this work), blue (dark gray) filled circle (Ref. 99), blue (dark gray) open diamond (Ref. 100), black filled diamond (Ref. 40), black open triangle (Ref. 101).Reuse & Permissions

Figure 2

Equilibrium fractions of the charge states of an energetic helium ion in liquid helium.Reuse & Permissions

Figure 3

Stopping power of a He ion in liquid helium. Data are drawn from the National Institute of Standards and Technology (NIST) database [41].Reuse & Permissions

Figure 4

Fractions of the charge states of an energetic helium ion as it slows down in liquid helium. The ion started as a ${\mathrm{He}}^{2+}$ with an initial energy of 50 keV, as indicated by the red (light gray) open circle. The arrows show how the fractions evolve as the particle loses its energy.Reuse & Permissions

Figure 5

Ionization cross sections due to ${\mathrm{He}}^0$, ${\mathrm{He}}^{1+}$, and ${\mathrm{He}}^{2+}$ interacting with ground state He atoms. The curves were fitted to experimental data by polynomial functions. (a) ${\sigma }_{\mathrm{ion}}^0$: solid black line (this work), red (light gray) filled circle (Ref. 102), blue (dark gray) filled triangle (Ref. 103), black open circle (Ref. 104). (b) ${\sigma }_{\mathrm{ion}}^{1+}$: solid black line (this work), black filled circle (Ref. 105), red (light gray) filled triangle (Ref. 106). (c) ${\sigma }_{\mathrm{ion}}^{2+}$: solid black line (this work), red (light gray) open circle (Ref. 105), blue (dark gray) open triangle (Ref. 107).Reuse & Permissions

Figure 6

Ionization and excitation yields of a recoil He ion in liquid helium as a function of the He ion energy.Reuse & Permissions

Figure 7

Calculated Lindhard factor for a recoil He ion in liquid helium as a function of the He ion energy.Reuse & Permissions

Figure 8

Electron extraction fraction $q$ as a function of the drift electric field. The blue solid curve represents the simulated electron extraction from beta tracks by Guo et al. [54]. The red dashed curve represents the simulated electron extraction from alpha tracks by Ito et al. [55]. Note that fields up to $40\mathrm{kV}/\mathrm{cm}$ have been readily applied in liquid helium [55].Reuse & Permissions

Figure 9

The ratio of the counts from different signal channels for 10 keV nuclear recoil and electronic recoil events as a function of the electric field.Reuse & Permissions

Figure 10

The ratio of the counts between different signal channels for nuclear recoil and electron recoil events as a function of the event energy. The electric field strength assumed in the simulation is $10\mathrm{kV}/\mathrm{cm}$.Reuse & Permissions

Figure 11

(a) The relative scintillation efficiency $L_{\mathrm{eff}}$ as a function of the recoil event energy. The $\times$ represents the calculated $L_{\mathrm{eff}}$ for helium under zero electric field. The measured data for liquid xenon by Plante et al. [71] [red (light gray) open triangle] and by Manzur et al. [38] (positive sign) are also shown. (b) The calculated light quenching factor $S_n$ for nuclear recoils as a function of event energy, for fields at $10\mathrm{kV}/\mathrm{cm}$, $20\mathrm{kV}/\mathrm{cm}$, and $40\mathrm{kV}/\mathrm{cm}$, respectively.Reuse & Permissions

Figure 12

(a) Monte Carlo simulation of the $\mathrm{S}1/\mathrm{S}2$ ratio for nuclear recoils (red dots) and electron recoils (black dots). The S1 scintillation light collection efficiency is assumed to be 20%. The electric field strength assumed in the simulation is $10\mathrm{kV}/\mathrm{cm}$. The black solid curve shows the calculated mean $\mathrm{S}2/\mathrm{S}1$ as a function of S1 counts for nuclear recoils. (b) Distribution of monoenergetic nuclear recoils and electron recoils on the $\mathrm{S}2/\mathrm{S}1$ versus S1 plot.Reuse & Permissions

Figure 13

Calculated rejection power for a helium detector as a function of event energy in keVnr scale.Reuse & Permissions

Figure 14

Spin-independent WIMP exclusion curves (solid lines), potential WIMP signals (solid regions), and projected liquid helium 90% sensitivity curves (dashed lines) in the region of 1–100 GeV WIMP mass. Exclusion curves include DAMIC in red [108], CDMS-II in green [10], XENON10 in magenta [12], and XENON100 in blue [14]. Potential WIMP signals include DAMA in red [109], CRESST in light blue [17], and CoGeNT in green [16]. Projected liquid helium sensitivities for different detector parameters are shown as dashed lines, including light blue ($10\mathrm{kV}/\mathrm{cm}$, 20% S1 collection, 4.8 keV threshold), green ($20\mathrm{kV}/\mathrm{cm}$, 20% S1 collection, 4.4 keV threshold), blue ($40\mathrm{kV}/\mathrm{cm}$, 20% S1 collection, 4.2 keV threshold), red ($20\mathrm{kV}/\mathrm{cm}$, 80% S1 collection, 2.8 keV threshold), and magenta ($40\mathrm{kV}/\mathrm{cm}$, 80% S1 collection, 2.6 keV threshold). Predicted limits assume an electron recoil background of $1\mathrm{event}/\mathrm{keVee}/\mathrm{kg}/\mathrm{day}$ and a 95% efficient gamma ray veto. Existing exclusion curves and potential WIMP signal regions are generated using DMTools [110].Reuse & Permissions