An Inexpensive Positron Converter of High Reliability and High Yield

A new positron converter of the simplest construction is described, consisting of a multiple fixed tungsten target and a very small helical positron lens, both inside a small vacuum chamber. By utilizing the good vacuum insulating properties the lens may be pulsed with 6000 A at an operating voltage of 10 kV without insulating material, except for the vacuum feed throughs, for which the extremely radiation-resistant aluminiumoxide ceramic is used. The lens construction out of a copper pipe allows direct water cooling and gives sufficient mechanical strength without further reinforcement. The system has been in operation for 3 years and gives a positron conversion ratio I+/I- of 0.8 % as a routine value for an e--energy of 300 MeV, where the positron energy spread is within ± 1.5 MeV. Compared to a dc-lens saving factors of about 500 in weight, 50 in power consumption and 100 in cost have been achieved.