Physiea B 194-196 (1994) 515-516 North-Holland PHYSICA@ Quantum Evaporation of 4He: Angular Dependence and Efficiency C. Enss, S.R. Bandler, R.E. Lanou, H.J. Marls, T. More, F.S. Porter, and G.M. Seidel Department of Physics, Brown University, Providence, Rhode Island 02912, USA Investigations of quantum evaporation from the surface of superfhiid 4He at 20 mK have been carded out using both an electrical heater and a 5.5MeV ct source as generators of quasiparticles. The angular dependence of the evaporation signals for these two sources is found to be similar. The angular dependence and the total effi- ciency of the process are discussed in terms of models of quantum evaporation. The proposed design of a particle detector based on the use of liquid helium at low tempera- tures [1] makes use of quantum evaporation of he- lium atoms by elementary excitations in the liquid. In investigating the physical processes important in the functioning of such a detector, we have studied the dependence of quantum evaporation on the in- cident angle of the excitations on the surface using both o~ particles and a resistance heater as sources. We have also made an estimate of the efficiency of evaporation from the measurements using the ~x source. Our experiment consists of a 3 liter volume of isotopically pure 4He maintained at a temperature of 20inK using a specially designed 3He-4He dilution refrigerator. To detect the evaporation signal, both a 1.6cm diameter silicon and a 5cm diameter sap- phire wafer/calorimeter are suspended 1.5 cm above the helium bath in a region kept free of superfluid film [2]. The wafers are thermally linked to the mixing chamber at a base temperature of 20mK. Each wafer has an NTD Ge thermistor as the tem- perature sensing device for the calorimeter. The two heat sources are placed 1.5 cm under the liquid level and can be moved through a lateral distance of 12cm by a superconducting stepper motor. The quantum evaporation signal measured with the silicon calorimeter for a 1 p.J heat pulse into the heater is shown in Fig. 1. The horizontal scale rep- resents the lateral distance of the heater from the center of the calorimeter. As expected the maximum amplitude was observed for the source placed directly below the detector and the signal decreases rapidly at large angles. Wyatt [3] has shown, using collimated beams of rotons and phonons, that momentum parallel to the surface and energy are conserved in the evapo- ration of an atom by an excitation in the liquid. Based on this observation we constructed a Monte Carlo simulation to analyze the data more quantita- 0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All SSDI 0921-4526(93)E0837-7 tively. In this simulation excitations are generated with momenta in random directions and energies related to the momenta through the dispersion rela- tion for helium. Excitations striking the surface with sufficient energy are assumed to produce an evaporated helium atom with probability 1. Fur- thermore, our measurements indicate [4] that the reflection probability of an excitation at the walls of the container is small and is taken to be zero in the simulation. The heater is assumed to produce excitations predominately near the roton minimum since the temperature of the heater is below 1 K. The results of a simulation for excitations in the range 1.85A-l<k<l.95A "1 is shown in Fig. 1 (solid line) and is in good agreement with the data. For com- parison a simulation of excitations generated uni- formly in momentum space up to a cut-off of 2.3 A -1 is also shown (dashed line) and is considerably broader than the measurements. The different width of the two simulations is easy to understand. De- pending on the momentum, each excitation has a critical incident angle beyond which total internal reflection must occur. This follows directly from the conservation of parallel momentum at the liquid- vacuum interface. Since the critical angle is small- est for excitations in the region closest to the roton minimum [3] these excitations will lead to the nar- rowest position dependence. The positional dependence of the signal on the silicon wafer resulting from a 5.5 MeV c~ panicle stopped in the liquid is shown in Fig. 2. The two theoretical curves are, as before, simulations assum- ing rotons either with momenta restricted to the minimum in the dispersion curve or uniformly dis- tributed over the entire spectrum. For o~ panicles, as well as for a heater, the comparison between the simulation and the data indicates that the excita- tions leading to evaporation are primarily near the roton minimum. rights reserved