Temperature dependence of alkali-antimonide photocathodes: Evaluation at cryogenic temperatures

${\mathrm{Cs}}_x{\mathrm{K}}_y\mathrm{Sb}$ photocathodes were manufactured on a niobium substrate and evaluated over a range of temperatures from 300 to 77 K. Vacuum conditions were identified that minimize surface contamination due to gas adsorption when samples were cooled below room temperature. Measurements of the photocathode spectral response provided a means to evaluate the photocathode band gap dependence on the temperature and to predict the photocathode quantum efficiency at 4 K, a typical temperature at which superconducting radio frequency photoguns operate.

Figure 1

Photocathode QE at 532 nm following successive iterations of sample cooling and heating inside the unbaked vacuum chamber. Note that QE did not restore following a return to room temperature, indicating that the vacuum conditions promoted surface contamination via gas adsorption. The blue text boxes indicate the temperature at which the sample was heated to rejuvenate the QE and the percentage increase in ${\mathrm{H}}_2\mathrm{O}$ partial pressure relative to the system total pressure during the brief heating period, as reported by the RGA.

Figure 2

Photocathode QE at 532 nm following successive iterations of sample cooling and heating inside the baked vacuum chamber. Photocathode QE at 77 K was consistently $\sim 6\%$ to 7%, and when the sample was returned to room temperature, photocathode QE actually increased from $\sim 7\%$ to 13%.

Figure 3

QE spectral response from the ${\mathrm{Cs}}_x{\mathrm{K}}_y\mathrm{Sb}$ photocathode grown on the Nb substrate measured at the indicated temperatures using a wavelength tunable laser. The axes units were chosen based on Fowler’s model and Eq. (1).

Figure 4

Exponential fit of the photoemission threshold $V_o$ (square) and parameter $B$ (circle), each obtained from the QE spectral fit for each temperature indicated in Fig. 3. The open markers indicate extrapolation of $V_o$ and $B$ values to 4 K.

Figure 5

Measured QE spectral response for the ${\mathrm{Cs}}_x{\mathrm{K}}_y\mathrm{Sb}$ photocathode grown on the Nb substrate for the indicated temperatures: circles, ice water; rectangle, dry ice; triangles, liquid nitrogen. The dashed lines through QE data points correspond to fits based on Fowler’s model (see the text). The solid black line is a simulated spectral response using the photoemission threshold $V_o$ and fit parameter $B$ extrapolated to 4 K. The vertical dashed line at 532 nm indicates a QE of 1.4% at 4 K.