Theoretical investigation of possibilities for generating coherent helium-4 beams
O'Connor, Robert Barnard, Jr
Master of Arts
As part of an effort to find a suitable illuminant for the holographic microscopy of single biological molecules, a survey has been made of various possible methods for generating neutral He-4 beams. The required energies are in the range from 10 to the -2 to 10 to the -1 ev which correspond to particle velocities of about 10 to the 5th cm/sec. Coherence requirements for lensless holography set the maximum acceptable velocity bandwidth at about 10 -2 cm/sec. Continuous liquid beams, droplet beams, thermal effusion beams, supersonic nozzle beams, ionized beams, and excited beams have been considered. Of these the supersonic nozzle approach appears to be the most promising. However, filtering of the beam to achieve the necessary bandwidth appears to be a major obstacle. Droplet beams might also be possible if the liquid flux is not too great or if a suitable means for gasification can be found. Development of detection technology is required in order to produce a satisfactory recording of the scattered helium atoms. Fundamental questions still remain as to the ultimate suitability of helium as an illuminant for microscopy. The original idea and starting point was that since both He-4 atoms and photons were Bose,particles, it might be possible to construct the helium analog of an optical laser. After a review of laser theory and of the fundamental properties of He-4 below the A-point, the principal problem appears to be that the energies at which many of the interesting superfluid phenomena exist are two to three orders of magnitude below the required beam energies. Accelerating superfluid droplets seems to be the most practical way of bridging the gap, out there are the problems mentioned above. One possible analog model still appears to merit further work.