Google is your friend. Andersson and Braun designed a neutron rem meter with a cylindrical BF3 or 3HE tube surrounded by a PE moderator. These slides give you a nice overview and explain the main feature of AB detectors:
To reduce the instrument’s over-response to low energy neutrons, they incorporated a borated plastic sleeve (7.6 cm diameter) around the detector tube and inner moderator.
You can also find a good introduction in this paper, explaining what an "rem counter" is and how Andersson-Braun detectors work:
It is well known that the solution usually adopted by health physicists for the determination of the old concept of maximum dose equivalent (MADE) is that of using a so-called rem counter. This is an instrument in which a detector with a high efficiency to thermal neutrons is placed inside a moderator- attenuator of such a nature and geometry that the response function of the rem counter reproduces the curve of the conversion coefficient from neutron fluence to dose equivalent on an energy range as wide as possible. The best instrument of this type is the Andersson-Braun (A-B) rem counter [...], whose response is considered acceptable, for radiation protection purposes, from thermal to 14 MeV neutrons.