We have made two foam rings (outer diameter ca. 70 mm with a thickness of 10 mm each and placed them successively underneath PMT north. 

ring 1: ID = 55 mm 
ring 2: ID = 47 mm 

Without rings we had measured ca. 20 counts per channel in a time of about 7 minutes. (1st picture below) 

With ring 1 we have measured ca. 18 counts per channel in 7 mins. 

with ring 1 and ring 2 we have measured ca. 15 counts per channel in 7 mins. (2nd picture below)

It is clear that the rate of PMT north decreases with increasing distance from the beampipe. 

At this point it is not yet clear to us if the experiment will benefit from the 2 rings. 

If the rings should be implemented, we propose to have one proper ring (20 mm high) made by the mechanical workshop.