During a visit yesterday with Alexander M at Cornell, I came to realize that the design of the E-166 positron transport was less than optimal -- in that there would still be considerable loss of particles that passed through the momentum slit.

I eventually recalled that what we should be building is a "nondispersive, translating spectrometer".

So old lore on this can be seen in the book of Steffen (DESY): steffen_nondispersive%20spectrometers.pdf

One way to fix things up would be to put an inverting lens between the two dipole magnets. Since space is limited, a rather strong lens would be required. Maybe we could do this with a solenoid....

A sketch of the desirable patterns of rays is e166_positron_spectrometer_rays.jpg

Steffen shows another design, based on 3 60 deg bend magnets (and no quad or solenoid). This might also work for us. e166_positron_spectrometer_3_magnets.jpg
In this scheme, the central ray is bent by 180 deg....

The present state of my drawing is: e166_positron_spectrometer.dwg

--Kirk