PREX: Lead / Diamond Target Three bays (PREX-I) PREX-II to have 12 bays Lead (0.5 mm) sandwiched by diamond (0.25 mm) Liquid He cooling ( 50 Watts) P1/9

Experience with our Lead Targets 2 few-hour tests prior to PREX-I up to 100 uA 2 few-day runs of E06007 208Pb(e,e’p) with this lead target 3 targets used during PREX-I. 2 survived for >1 week 1 was performing to spec at 70 uA at the end. P2/9

How the Lead Targets Behave at 70 uA (data on later slides) After ~1 day the target becomes non-uniform, but stabilizes. Synching raster eliminated the additional noise. After ~1 week some targets melt. We believe the diamond loses its thermal conductivity. Support for this from Silviu Covrig’s CFD calcs. P3/9

Charge “Have proper measures to protect the 208Pb target from melting been taken ?” Answer: Yes, some targets melt. But the design “as is” has demonstrated a safety margin of a factor of ~2 to complete the experiment with 10 targets deployed. Have measures been taken and defined to prevent and monitor density fluctuations ? Answer: Yes, although the target becomes non-uniform, the fluctuations in rate are cancelled by synchronizing the raster to the helicity, as demonstrated in PREX-I. (next slide) P4/9

Synching Raster Eliminated Noise due to Target Non-uniformity this synch is now standard hardware Initial Running Flat Plot : Asy on the Left HRS vs Asy on the Right HRS. Should be uncorrelated – different electrons Not synched Synched After ~ 5 days NOT Flat PREX-I Data P5/9

Target Density Intact Melted Y X Y X with thin diamond backing Failure happens fairly suddenly after ~1 week PAVI 09 X P6/9

Thermal Analysis with ANSYS-Fluent16.0 work by Silviu Covrig If thermal conductivity of diamond is above 1000 W/mK then max ΔT ~ 40 K in Pb If the thermal conductivity of diamond drops below 100 W/mK then max ΔT > 450 K and Pb is likely to melt Here, kdiamond = 100 W/mK P7/9

Performance of Lead / Diamond Targets melted melted Targets with thin diamond backing (4.5 % background) degraded fastest. Thick diamond (8%) ran well and did not degrade at 70 uA. NOT melted Last 4 days at 70 uA Strategy: Run with 10 targets using the thicker (250 um) diamond foils P8/9

PREX-II Strategy Deploy 12 lead targets 2 not isotopically pure, used for commissioning 10 isotopically pure > 70 PAC days of beam time (factor of ~2 safety margin) Using “thicker” diamond with a higher starting thermal conductivity. (the thickness 250 um which equals the thickest in PREX-I ) Use graphite backing for 1 of the not isotopically pure and for 1 of the isotopically pure. P9/9

Backup

Safety Factor of 2 for Target Survivability Simplest calculation: Lifetime > 7 days, proposal 35 days need 5 targets have 10  Factor of 2 safety Alternative calculation : The total accumulated charge for the 3 targets was about 82 C (and the last target did not fail) 152 C is needed for PREX-II Safety factor ~ 82/152*10/3 = 1.8