1. 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)
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2. Experience with our Lead Targets
2 few-hour tests prior to PREX-I up to 100 uA
2 few-day runs of E Pb(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.
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3. 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.
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4. 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)
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5. 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
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6. Target Density Intact Melted Y X Y X
with thin diamond backing
Failure happens fairly suddenly after ~1 week
PAVI 09 X
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7. 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
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8. 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
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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.
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10. Backup

11. 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 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