11 GeV PV Møller11 GeV PV Møller Detector ConsiderationsDetector Considerations

BRAINSTORMINGBRAINSTORMING

JLab WorkshopJLab Workshop

August 2008August 2008

Michael Gericke and Dave MackMichael Gericke and Dave Mack

This is a current mode experiment:This is a current mode experiment:

From the point of view of detector development:From the point of view of detector development:

Every undesirable effect that we don’t “design away” Every undesirable effect that we don’t “design away” to to begin with begin with will increase our RMSwill increase our RMS width in the signal. width in the signal.

Some of them Some of them will introduce the potential for false will introduce the potential for false asymmetries.asymmetries.

There are no cuts (short of beam properties) once the There are no cuts (short of beam properties) once the data data is taken. is taken.

A custom tailored detector set is paramount !A custom tailored detector set is paramount !

Simple is better !!Simple is better !!

What do(n’t) we (I) know ?What do(n’t) we (I) know ?

• several proposed spectrometer designs (but no several proposed spectrometer designs (but no collimators)collimators)• we (I) have some idea of the focal plane profile (FPP)we (I) have some idea of the focal plane profile (FPP) shape shape • but no information about rate or but no information about rate or qq22 variation (do we variation (do we care ?)care ?)• we don’t yet know where the background is hitting we don’t yet know where the background is hitting the FPthe FP

These are important factors in determining the These are important factors in determining the detector detector geometry, materialgeometry, material and type!and type!

Nonetheless, what one CANNonetheless, what one CAN say about possible detectors:say about possible detectors:

• the experiment should be statistics limited: we want the experiment should be statistics limited: we want to to suppress excess noise (electronic and detector suppress excess noise (electronic and detector geometry), geometry), i.e. as close to counting statistics as i.e. as close to counting statistics as possiblepossible• ideally, we want to be insensitive to anything but ideally, we want to be insensitive to anything but electronselectrons• we want something that works (realistically) and can we want something that works (realistically) and can be be fundedfunded

These already constrain to a large extend what These already constrain to a large extend what technology technology we should use …we should use …

Detector Cause and Effect - Driving IssuesDetector Cause and Effect - Driving Issues

Given by exp.Given by exp. Detector physical Detector physical Signal Signal propertiespropertiesdesigndesign choiceschoices

FPPFPP Type (Technology)Type (Technology) Yield (light, etc…)Yield (light, etc…)

RateRate Geometry (Shape)Geometry (Shape) Yield uniformityYield uniformity

QQ22 Active MaterialActive Material QQ22 uniformity uniformity

Rad. DoseRad. Dose Shower MaterialShower Material E, QE, Q22, spatial , spatial resolutionresolution

BackgroundBackground ReadoutReadout Background rejectionBackground rejection

LinearityLinearity

NoiseNoise

Basic Detector TechnologyBasic Detector TechnologyWe can get some things out of the way immediately:We can get some things out of the way immediately:

• Čerenkov (bare quartz):Čerenkov (bare quartz): Rad hard, largely insensitive to soft Rad hard, largely insensitive to soft photon photon background, hard to shape, background, hard to shape, can have low can have low signal (light) yield, good noise signal (light) yield, good noise performance, performance, expensive, …expensive, …

• Čerenkov Shower Calorimeter:Čerenkov Shower Calorimeter: Rad hard, insensitive to Rad hard, insensitive to photon photon background, can background, can accommodate quartz accommodate quartz fibers/rods for odd shapes (as in E158), fibers/rods for odd shapes (as in E158), larger excess noise, can have much larger larger excess noise, can have much larger

light yields, expensive, …light yields, expensive, …

• PSICsPSICs Rad hard, must have radiator Rad hard, must have radiator shields to shields to remove background remove background sensitivity and increase sensitivity and increase signal yield, signal yield, inexpensive, handles weird FFP inexpensive, handles weird FFP shapes, shapes, larger excess noise, …larger excess noise, …

• ScintillatorScintillator Not rad hard enough … Not rad hard enough … Am I missing something …?Am I missing something …?

Focal Plane Profile ShapeFocal Plane Profile ShapeWe have 4 (?) spectrometer designs with slightly We have 4 (?) spectrometer designs with slightly different FPPs.different FPPs.

The profile shape dictates the minimum detector The profile shape dictates the minimum detector geometry constraints which in turn affects all other geometry constraints which in turn affects all other detector properties:detector properties:

• yield yield weird detector geometries produce less weird detector geometries produce less light at light at the photo-cathode the photo-cathode (Čerenkov) (PSICs presumably (Čerenkov) (PSICs presumably less less sensitive to this unless you have to do really sensitive to this unless you have to do really

weird things …)weird things …)• Y unif.Y unif. complicated detector geometries complicated detector geometries produce light produce light yield non yield non uniformities across the detector … uniformities across the detector … • qq2 2 unif.unif. if the focus is not uniform and the rate if the focus is not uniform and the rate or light or light yield is not flat over the yield is not flat over the FPP then extended FPP then extended detector sizes detector sizes give rise to give rise to q2 bias … …

(need better spatial resolution)(need better spatial resolution)• backgr.backgr. larger geometries invite more larger geometries invite more background …background …

All of the above then in turn influence excess noise All of the above then in turn influence excess noise and the yield and qand the yield and q22 non-uniformities produce non-uniformities produce systematic false asymmetries with helicity systematic false asymmetries with helicity correlated beam effects.correlated beam effects.

Willie Falk; 3 toroid designWillie Falk; 3 toroid design

Calculation and plot by Kent PaschkeCalculation and plot by Kent Paschke

Put a thin Put a thin rectangular rectangular (?) quartz (?) quartz bar there bar there (a la Qweak)(a la Qweak)

(20 cm in x)(20 cm in x)

Maybe Maybe encase in encase in tungsten ?tungsten ?

x [m

]x

[m]

z [m]z [m]

Calculation and plot by Willie FalkCalculation and plot by Willie Falk

How How important is important is this region?this region?

Same rate ?Same rate ?Same qSame q22 ? ?

There are There are obvious obvious problems problems with with interference interference between between neighboring neighboring sections.sections.

Keeping Keeping thesetheseaway is a away is a collimation collimation problem; but problem; but at what cost at what cost in statistics?in statistics?

Calculation and plot by Kent PaschkeCalculation and plot by Kent Paschke

But is there But is there an e-p an e-p radiative tail radiative tail in here ?in here ?

Annulus sections Annulus sections of a PSIC. Or a of a PSIC. Or a quartz shower quartz shower calorimeter a la calorimeter a la E158.E158.

This would allow This would allow binning in qbinning in q22 if the if the focus is not so focus is not so good.good.2 Toroid Calculation and plot by Willie Falk2 Toroid Calculation and plot by Willie Falk

Kent Paschke: Nested ToroidsKent Paschke: Nested Toroids

Candidate for a Candidate for a ring shaped ring shaped detector again.detector again.

Krishna Kumar and Luis Mercado: quadsKrishna Kumar and Luis Mercado: quads

Focal plane profile is a ring. Use a set of ring detectors. Focal plane profile is a ring. Use a set of ring detectors. (out of what ?)(out of what ?)

The End

Q2 BiasQ2 Bias Average momentum transfer calculated Average momentum transfer calculated fromfromcollimator apertures and detector collimator apertures and detector geometry. geometry.

The photoelectron yield varies with hit The photoelectron yield varies with hit location along the detector !location along the detector !

The Q2 distribution is not uniform The Q2 distribution is not uniform across the across the bar !bar !How big is mean Q2 bias introduced by How big is mean Q2 bias introduced by PE weighing ?PE weighing ?

A detector asymmetry will be A detector asymmetry will be calculated by averaging left and calculated by averaging left and right PMT asymmetries.right PMT asymmetries.

Q2 bias is troubling in Q2 bias is troubling in combination with combination with radiation damage and PMT aging !radiation damage and PMT aging !

Non uniform Q2 bias across the Non uniform Q2 bias across the detector is troubling in detector is troubling in combination with helicity combination with helicity correlated beam motion !correlated beam motion !

No NPE Weighting

Left PMT NPE Weighting

Right PMT NPE Weighting

Sum PMT NPE Weighting

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Detector Thickness and Excess NoiseDetector Thickness and Excess NoiseOptimal quartz thickness based on excess noise simulations at 0 degree Optimal quartz thickness based on excess noise simulations at 0 degree tilt-angle. tilt-angle. QWeak Statistical Error + Excess Noise:QWeak Statistical Error + Excess Noise:

Modeled as a contribution from photoelectronModeled as a contribution from photoelectronnoise and shower noise:noise and shower noise:

Shower activity inside the detector increases with detector thickness.Shower activity inside the detector increases with detector thickness.The number of PEs will decrease as the detector is made thinner to The number of PEs will decrease as the detector is made thinner to suppress shower activity. The two competing processes lead to an optimal suppress shower activity. The two competing processes lead to an optimal detector thickness which minimizes the total excess noise.detector thickness which minimizes the total excess noise.

4% Excess Noise

Bialkali Cathode S20 Cathode

Detector thickness was selected at 1.25 cmDetector thickness was selected at 1.25 cm

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Soft photon backgroundSoft photon backgroundThe 10 keV to 1 MeV photon rate is as The 10 keV to 1 MeV photon rate is as high as the elastic electron rate !high as the elastic electron rate !

Photons with E < 10 keV mostly Photons with E < 10 keV mostly stopped in detector housing or stopped in detector housing or wrapping.wrapping.Photons with 10 keV Photons with 10 keV ≤ E < 1MeV ≤ E < 1MeV potentially stopped in the detector.potentially stopped in the detector.Photons with E ≥ 1 MeV deposit ~10%.Photons with E ≥ 1 MeV deposit ~10%.Photons with E ≤ 10 MeV produce ≤ Photons with E ≤ 10 MeV produce ≤ 30% of electron Cherenkov light 30% of electron Cherenkov light (photon rate is down by 2 orders of (photon rate is down by 2 orders of magnitude for E ≥ 10 MeV).magnitude for E ≥ 10 MeV).

electrons

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Lead Pre-Radiator StudyLead Pre-Radiator StudyCan we cut soft photon background using a pre-radiator?Can we cut soft photon background using a pre-radiator?

Questions: How thick does this radiator have to be? Can we live with Questions: How thick does this radiator have to be? Can we live with the excess noise ?the excess noise ?

Excess noise – a function of photoelectron yield and shower size

Overall asymmetry error with excess detector noise

Simulate various radiator thicknesses and establish an ideal thicknessSimulate various radiator thicknesses and establish an ideal thicknessthat minimizes the excess noise while attenuating the soft photons:that minimizes the excess noise while attenuating the soft photons:

Simulations were run for 8 different setupsSimulations were run for 8 different setupswith the lead radiator thickness varied with the lead radiator thickness varied

between 1 between 1 and 4 cm.and 4 cm.

Lead radiation length = 0.5 cmLead radiation length = 0.5 cm

Shower max is reached at ~ 4 radiation Shower max is reached at ~ 4 radiation lengthslengths

--- --- on these grounds it is expected that the on these grounds it is expected that the minimum in excess noise is reached at minimum in excess noise is reached at

about about 2 cm2 cm

A 2 cm lead radiator would produce about 12% excess noise requiring 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 about 370 hours of additional running time – but keep it in our back pocket if we end up seeing too much background with beam.if we end up seeing too much background with beam. Back

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Position Sensitive Ion Chambers (PSIC’s)Position Sensitive Ion Chambers (PSIC’s)

• Fused silica-based Cerenkov detectors Fused silica-based Cerenkov detectors are expensive/difficult to sculpt to match are expensive/difficult to sculpt to match the shape of a crude hardware focus. the shape of a crude hardware focus.

• An ion chamber with an optimized pre-An ion chamber with an optimized pre-radiator is very promising: radiator is very promising:

a clever E158 implementation had a clever E158 implementation had good time response, good linearity, good time response, good linearity, low susceptibility to dielectric low susceptibility to dielectric breakdown.breakdown.

• Ion chambers are intrinsically rad-hard Ion chambers are intrinsically rad-hard with the signal size determined by with the signal size determined by geometry and pressure. geometry and pressure.

• By partitioning the anode into strips, it By partitioning the anode into strips, it is possible to make detectors with radial is possible to make detectors with radial resolutions of < 1 cm. resolutions of < 1 cm.

• Cost will be dominated by the Cost will be dominated by the electronics.electronics.

23 M. Gericke (U. Manitoba)

– Simulation:Simulation:EEee = 4.5 GeV = 4.5 GeV

1.9 cm W (5.4 X1.9 cm W (5.4 X00))(shower max!)(shower max!)

10 cm, 1 atm He gas10 cm, 1 atm He gas

– Minimum position Minimum position resolution is a few mm resolution is a few mm (= r(= rMoliereMoliere) )

– Need to control point to Need to control point to point variations in the point variations in the gas columngas column

PSIC’s: Minimum Position ResolutionPSIC’s: Minimum Position Resolution

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Simulations of the energy Simulations of the energy resolution and the resolution and the corresponding excess noise corresponding excess noise for a PSIC detector with for a PSIC detector with various pre-radiator various pre-radiator strengths.strengths.

The interplay between the The interplay between the number of shower particles number of shower particles and the corresponding energy and the corresponding energy deposition yields an optimal deposition yields an optimal radiator tickness.radiator tickness.

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