11 GeV PV Møller Detector Considerations BRAINSTORMING JLab Workshop August 2008

1. 11 GeV PV MøllerDetector Considerations BRAINSTORMING JLab Workshop August 2008 Michael Gericke and Dave Mack

2. This is a current mode experiment: From the point of view of detector development: Every undesirable effect that we don’t “design away” to begin with will increase our RMS width in the signal. Some of them will introduce the potential for false asymmetries. There are no cuts (short of beam properties) once the data is taken. A custom tailored detector set is paramount ! Simple is better !!

3. What do(n’t) we (I) know ? • several proposed spectrometer designs (but no collimators) • we (I) have some idea of the focal plane profile (FPP)shape • but no information about rate or q2 variation (do we care ?) • we don’t yet know where the background is hitting the FP • These are important factors in determining the detector geometry, materialand type! • Nonetheless, what one CANsay about possible detectors: • the experiment should be statistics limited: we want to suppress excess noise (electronic and detector geometry), i.e. as close to counting statistics as possible • ideally, we want to be insensitive to anything but electrons • we want something that works (realistically) and can be • funded • These already constrain to a large extend what technology we should use …

4. Detector Cause and Effect - Driving Issues Given by exp. Detector physical Signal properties design choices FPP Type (Technology) Yield (light, etc…) Rate Geometry (Shape) Yield uniformity Q2 Active Material Q2 uniformity Rad. Dose Shower Material E, Q2, spatial resolution Background Readout Background rejection Linearity Noise

5. Basic Detector Technology • We can get some things out of the way immediately: • Čerenkov (bare quartz): Rad hard, largely insensitive to soft photon background, hard to shape, can have low signal (light) yield, good noise performance, expensive, … • Čerenkov Shower Calorimeter: Rad hard, insensitive to photon background, can accommodate quartz fibers/rods for odd shapes (as in E158), larger excess noise, can have much larger light yields, expensive, … • PSICs Rad hard, must have radiator shields to remove background sensitivity and increase signal yield, inexpensive, handles weird FFP shapes, larger excess noise, … • Scintillator Not rad hard enough … Am I missing something …?

6. Focal Plane Profile Shape • We have 4 (?) spectrometer designs with slightly different FPPs. • The profile shape dictates the minimum detector geometry constraints which in turn affects all other detector properties: • yield weird detector geometries produce less light at the photo-cathode (Čerenkov) (PSICs presumably less sensitive to this unless you have to do really weird things …) • Y unif. complicated detector geometries produce light yield non uniformities across the detector … • q2 unif. if the focus is not uniform and the rate or light yield is not flat over the FPP then extended detector sizes give rise to q2 bias … • (need better spatial resolution) • backgr. larger geometries invite more background … • All of the above then in turn influence excess noise and the yield and q2 non-uniformities produce systematic false asymmetries with helicity correlated beam effects.

7. Willie Falk; 3 toroid design Put a thin rectangular (?) quartz bar there (a la Qweak) (20 cm in x) Maybe encase in tungsten ? x [m] z [m] Calculation and plot by Kent Paschke

8. How important is this region? Same rate ? Same q2 ? There are obvious problems with interference between neighboring sections. Keeping these away is a collimation problem; but at what cost in statistics? Calculation and plot by Willie Falk

9. But is there an e-p radiative tail in here ? Calculation and plot by Kent Paschke

10. Annulus sections of a PSIC. Or a quartz shower calorimeter a la E158. This would allow binning in q2 if the focus is not so good. 2 Toroid Calculation and plot by Willie Falk

11. Kent Paschke: Nested Toroids Candidate for a ring shaped detector again.

12. Krishna Kumar and Luis Mercado: quads Focal plane profile is a ring. Use a set of ring detectors. (out of what ?)

13. The End

14. Q2 Bias Average momentum transfer calculated from collimator apertures and detector geometry. The photoelectron yield varies with hit location along the detector ! The Q2 distribution is not uniform across the bar ! How big is mean Q2 bias introduced by PE weighing ?

15. No NPE Weighting A detector asymmetry will be calculated by averaging left and right PMT asymmetries. Q2 bias is troubling in combination with radiation damage and PMT aging ! Non uniform Q2 bias across the detector is troubling in combination with helicity correlated beam motion ! Left PMT NPE Weighting Right PMT NPE Weighting Sum PMT NPE Weighting Back

16. Detector Thickness and Excess Noise Optimal quartz thickness based on excess noise simulations at 0 degree tilt-angle. QWeak Statistical Error + Excess Noise: Modeled as a contribution from photoelectron noise and shower noise: Shower activity inside the detector increases with detector thickness. The number of PEs will decrease as the detector is made thinner to suppress shower activity. The two competing processes lead to an optimal detector thickness which minimizes the total excess noise.

17. 4% Excess Noise Bialkali Cathode S20 Cathode Detector thickness was selected at 1.25 cm Back

18. Soft photon background The 10 keV to 1 MeV photon rate is as high as the elastic electron rate ! Photons with E < 10 keV mostly stopped in detector housing or wrapping. Photons with 10 keV ≤ E < 1MeV potentially stopped in the detector. Photons with E ≥ 1 MeV deposit ~10%. Photons with E ≤ 10 MeV produce ≤ 30% of electron Cherenkov light (photon rate is down by 2 orders of magnitude for E ≥ 10 MeV). electrons Back

19. Lead Pre-Radiator Study Can we cut soft photon background using a pre-radiator? Questions: How thick does this radiator have to be? Can we live with the excess noise ?

20. Simulate various radiator thicknesses and establish an ideal thickness that minimizes the excess noise while attenuating the soft photons: Excess noise – a function of photoelectron yield and shower size Overall asymmetry error with excess detector noise

21. Simulations were run for 8 different setups with the lead radiator thickness varied between 1 and 4 cm. Lead radiation length = 0.5 cm Shower max is reached at ~ 4 radiation lengths --- on these grounds it is expected that the minimum in excess noise is reached at about 2 cm A 2 cm lead radiator would produce about 12% excess noise requiring about 370 hours of additional running time – but keep it in our back pocket if we end up seeing too much background with beam. Back

22. Position Sensitive Ion Chambers (PSIC’s) • Fused silica-based Cerenkov detectors are expensive/difficult to sculpt to match the shape of a crude hardware focus. • An ion chamber with an optimized pre-radiator is very promising: a clever E158 implementation had good time response, good linearity, low susceptibility to dielectric breakdown. • Ion chambers are intrinsically rad-hard with the signal size determined by geometry and pressure. • By partitioning the anode into strips, it is possible to make detectors with radial resolutions of < 1 cm. • Cost will be dominated by the electronics.

23. PSIC’s: Minimum Position Resolution • Simulation: Ee = 4.5 GeV 1.9 cm W (5.4 X0) (shower max!) 10 cm, 1 atm He gas • Minimum position resolution is a few mm (= rMoliere) • Need to control point to point variations in the gas column M. Gericke (U. Manitoba)

24. Simulations of the energy resolution and the corresponding excess noise for a PSIC detector with various pre-radiator strengths. The interplay between the number of shower particles and the corresponding energy deposition yields an optimal radiator tickness. Back