Straw tube chambers work similarly to multiwire proportional chambers, but instead of high voltage being supplied along a plane, the high voltage is maintained on a metal coated tube (or straw) with a ground sense wire in the center. Because each sense wire has its own source of electric field, straw tube chambers can be operated reliably in higher rate environments. They also have the additional benefit of being more reliable since a single broken wire only impacts one channel.
It was originally feared that the FOCUS PWC system would not be able to handle the high rates present in the pair region and that the PWCs would have to be ``deadened'' in this region. To prepare for this possibility, three straw tube chambers were constructed to cover the pair region of each of the first three PWCs.
The three straw tube chambers have similar designs with the length and number of the straws being the primary difference between chambers. (P0 is smaller than P1 and P2 so the corresponding straw tube chamber is also smaller.) ST0 and ST1 are placed just in front of P0 and P1 respectively, ST2 is placed just behind P2.
There are three views per station, one vertical and two aligned at from vertical. Each view has three layers of straws. All the chambers use 5 mm diameter straws. The design of the straws is shown in spec:straws with their properties summarized in spec:straws.
In the end, deadening the PWC system was not necessary, so the straw tubes were not needed for tracking. However, because the straw tubes are read out with TDCs (Time to Digital Converters), they can provide useful information about the timing of events, rejecting tracks which occur in other accelerator buckets (see beam:extraction).