Mark made a log entry 120017 for normalized test detector results with 2.3 + 1.6 mm lead shields added downstream of the right 5" photo tube as well as the quartz glass (right side) 2" photo tube.This is compared below with the last measurement I took before the lead sheets were added (07/31/06, 05:20). Note that the left 5" tube and left 2" tube (borosilicate) had no changes between taking these two data sets.
Rates are in kHz/uA of beam on a LH2 target. 07/31 08/06 08/06 no lead no lead with lead Left 5" tube 5.25 5.4 right 5" tube 2.38 --- 2.27 left 2" tube 0.625 0.645 --- right 2" tube 0.155 --- 0.136
Thus the 4 mm of lead has only a small impact on the count rate, somewhat more for the 2" than for the 5" tube. This means that the residual count rate in the detectors, after applying the Boroflex sheet + 4" of borated polyethylene ("no lead" column, right 5" tube), is - not due to low energy gamma flux (< 0.5 MeV) - not due to low energy charged particles
Note - that 4 mm lead stops 4.5 MeV e-, 10 cm polyethylene stops about 17 MeV e-; - that 4 mm lead is about 0.7 radiation lengths and 10 cm ployethylene is about 0.2 radiation lengths
Since we should have seen at least some effect from lead for any gammas (I guess), I suspect that the residual rate in the tubes is from high(er) energy neutrons - unless we get most of the residual counts indirectly from the sides/top/bottom and upstream. Certainly we get some rate from those directions as shown by the reduction in rate when borated polyethylene was stacked all around the right 5" tube. Note that the 2" tube is rather nicely shielded from upstream by the 4" polyethylene between the 2" and the 5" tube.
If my interpretation of higher energy neutrons is correct (I don't know how effectively 4" polyethylene thermalizes neutrons) then the ordered Quartz tubes should give us the benefits we hope for.
A copy of this log entry has been emailed to: pitt@vt.edu,dhbeck@uiuc.edu