First Look at FLASH Thick Target Test Beam (Jan 2004) Results The Alumina Underground Jan. 13, 2004

Target Setup Alumnia bricks are 6”x4”x2” Assume 3” of alumina ~ 1 r.l. Picture shows ~14 r.l. of target

Target Setup Target thickness reduced by removing bricks from the updtream end of pile Picture shows ~10 r.l. target

Background Measurement Background measured using 6 “blind” PMT These are located behind at least 30 r.l. of lead shielding from target 2” and 4” optical channels alternately tried Signals seen in time with beam, but many late hits. ADC results analyzed using standard calibration of PMT from Stan Thomas

Background at 2000V, 2” optical channel

Background at 2000V, 4” optical channel

Target variation Worst case of background appears to be at 10 r.l. and only marginally (~10-15%) different from 14 r.l. Average background level equivalent to ~30-40 p.e. at 10 7 e - per pulse and 10 r.l. Decrease by ~30-40% from 10 r.l. to 6 r.l. Minimal background at 2 r.l.

Target variation (cont’d) The variation of the background vs. target thickness is inconsistent with GEANT prediction for gamma background + large number of late hits suggests we are probably dominated by neutrons PMT4 calibration is probably off; the other 5 are quite consistent

2” and 4” optical channel comaprison

2” vs. 4” Difference in background between 2” and 4” optical channel configuration is small (less than ~10-20%)

Varying HV: 2000V vs. 1600V

Varying HV: 2000V vs. 1200V

HV variation No Discernible HV variation in background level estimate…suggests that background is dominated by electrons from photo-cathode

Conclusion Background probably dominated by neutrons Maximum background level is equivalent to ~30-40 p.e. at 10 7 electrons per pulse. 2” and 4” optical channels give similar background levels Equivalent background level does not change with HV between V.