Integrating Detectors

Dave MackSeptember 18, 2009

Moeller pre-Director’s Review Meeting

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Radial Distribution at Focal Plane

Foci here are preliminary (proposal), but the detector clearly requires radial binning to isolate e+e, estimate underlying e+pe+X backgrounds, and tune up the spectrometer.

Existing design produces a good radial e+e focus near z = 28.5 m. A reasonable e+p focus is found at the same z .

A better e+e focus would reduce backgrounds.

E158

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Azimuthal Distribution at Focal Plane

Foci here are preliminary (proposal), but the detector requires azimuthal binning to measure parity-conserving cos(Φ) asymmetries, tune up the spectrometer, etc.

Existing design produces a good azimuthal e+e focus near z = 28.5 m. An even better e+p focus is found at the same z .

e+ee+p

Unwrapped Φ distribution.

There are differences of opinion as to whether the remaining azimuthal defocusing at low E’ is a bug (higher cost and lower S/B) or a feature (systematic check).

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State of the Main Detector•For the sake of expedience, we agreed to use thin quartz radiators for the proposal.

•However, most of us would feel more comfortable using a shower-max pre-radiator design which would be less sensitive to soft backgrounds (even though these haven’t been simulated yet).

•Fused silica Cerenkov detectors as well as ion chambers have been discussed. The first is the devil we know. The latter would require more R&D, but potentially be much cheaper and simpler.

•Statistics are the lifeblood of the Moeller experiment. Key questions which require simulation if a pre-radiator is used are:

1. How much excess noise will there be due to shower fluctuations for E’ = 2-9 GeV?

2. What is the additional penalty for using a detector which weights events by energy?

I’m going to sometimes be vague about what the final detector will look like.

Our cost estimate is already based on the most expensive detector option (quartz detector with pre-radiator), but needs updating given the upheaval in the PMT industry given the departure of Phillips.

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Comments on Energy-Weighting Detectors The old issue that I’m now less worried about (aka, P. Souder was right again):

There must be a statistical penalty to weighting some events by 9 GeV of light, and other events by 2 GeV of light. No way the fluctuations are simply sqrt(N). I thought of doing a simple simulation to convince you, but it’s sufficient to show you our energy spectrum, which is equivalent to a “resolution” if the detector is linear in E’. The effective resolution is conservatively

σ = half-width/sqrt(12) = 1/5.5 = 18%

which sounds poor, but the excess noise scale factor is only

sqrt( 1 + (σ/S)2 ) = sqrt( 1 + 0.182) = 1.016

which is easily swept under the rug of othernoise sources.

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Comments on Energy-Weighting Detectors

The new issue that I’m worried about:

Shower-max light output presumably scales like aE + blnE.

The experimental asymmetry may differ significantly from the unweighted asymmetry in the proposal. That can affect our relative error (probably for the worse: higher energy, smaller angle, lower FOM = xsect*A2).

Of course, the detector response aE + blnE (or whatever) has to be well known.

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e+e Rings Dose EstimateAssuming a thin detector (no showering or absorption of bremsstrahlung), and a Moeller rate of 153 GHz distributed uniformly over radii of 88cm to 100cm:

Energy “Flux” [MeV/(g/cm2)] = 1.53E11 Hz x 2.4 MeV/(g/cm2) x 5040 hrs x 3600 sec/hr

= 6.7E18 MeV/(g/cm2)

Area [cm2] = π x [ (100cm)2 – (88cm)2 ] = 7100 cm2

Dose [MeV/g] = Energy Flux/Area = 6.7E18 MeV/(g/cm2)/7,100 cm2= 9.4E14 MeV/g

Dose [Rad] = 9.4E14 MeV/g x (100 Rad/6.24E9 MeV/g) = 1.5E7 Rad

or 15 Mrad. (documenting my assertion in C.4 of the proposal)

A more detailed examination, including focusing of distributions in r and Φ, may raise peak doses in the e+e rings to ~50 Mrad.

Doses in the e+p rings are similar (lower rate and smaller area compensate).

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Generic Requirementsfor the Main Detector

•Full coverage of e+ee+e locus.

•Good coverage of e+pe+p including radiative tail.

•Radiation-hard to 50 MRad for “thin” designs.

(at least one order of magnitude larger with a pre-radiator -> 1 GRad)

•Low excess noise (i.e., shower and electronic noise should be negligible).

•Radial and azimuthal binning for background measurement and spectrometer tune-up.

•Good linearity wrt charge sensitivity.

• Well-characterized dependence on electron energy (not necessarily linear).

•Insensitive to soft backgrounds .

•Event mode operation at low luminosities a big plus.

•Inexpensive and easy to build a plus.

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Integrating Detectors

Side viewFront view

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First Simulationcourtesy of John Leacock (VPI)

John’s experience with Qweak lumis allowed him to quickly simulatelight collection in a 20cm (W) x 5 cm (H) x 2cm (T) piece of quartz withno pre-radiator. Photo-cathode and coatings are lumi defaults.

Resolution is only ~50%. If this is not due to systematic position dependence, a pre-radiator will help.

So nothing to take too seriously, but John will help us look good in January.

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Direct Electron Backgrounds

Direct electron backgrounds in the proposal are ~8%. I am hopeful for a factor of 2 reduction in this by improving the focus.

Inelastics

e+ee+p

The dominant background in terms of yield is from the radiative tail of e+p elastics. (red curves below)The inelastic yield is much smaller (green curves below), but the asymmetry is much larger resulting in an error contribution similar to elastic e+p. Resolve any discrepancies between simulation and back of the envelope estimates.

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Direct Meson BackgroundsPion detector – integrating detector which operates at high luminosity to measure the PV asymmetry of muons and charged pions after EM shower products are ranged out by lead shielding.

The pi/mu detector needs a mother. It will need a lot of simulation, and could potentially be large and heavy.

Estimated in the proposal in 3.5.3.

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Multi-Bounce Backgrounds I Not yet simulated.

Start by looking at photons from the defining collimators.

The intense neutral beam shining through the defining collimator may produce significant backgrounds if it strikes air or beam-pipes near the detectors. Can we dump it upstream, or will we have to transport it in vacuum and dump it downstream of the detectors? What about the lumi light-guides? Need to study this.

Neutral beam

Primary beam

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Multi-Bounce Backgrounds II A “shower-max” detector would improve S/B, but signal will become proportional to E. Need to study whether that really leads to excess noise given our large energy bite. If PMT’s are used, the proposal implies a shutter scheme to measure interaction of soft backgrounds in the tubes. Very difficult to implement for 300+ channels, can only be used invasively hence unlikely to permit a soft background asymmetry to be measured, and ignores Compton scattering or neutron capture in the quartz.

Need to develop a comprehensive soft background measurement strategy which includes a few shuttered detectors, and shadow-shielded, nearby PMT’s with and without quartz radiators.

In general, for diffuse backgrounds, improving the focus would allow us to build fewer, smaller detectors with a corresponding improvement in S/B at reduced cost.

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ManpowerPeople who have expressed interest in the main detector working group are:

Paul Souder (Syracuse)

Dave Mack (Jlab)

Michael Gericke (U Manitoba)

Pete Markowitz (FIU)

VPI and Umass will undoubtedly play an important role as well.

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How This Presentation Contributes to Addressing the Charge

1. Review the relevance and potential risk to the physics case. This should include:

a. The completeness and credibility of the proposed error estimate.

The main detector could potentially negatively impact the counting statistical error by having excess noise or low uptime/reliability. This is unlikely for the following reasons: The detector will have adequate resolution such that excess noise will not be more than a few percent. The measured noise of the TRIUMF electronics will be negligible. Detector local components will either be rad-hard or shielded, and digitizing electronics will be located outside Hall A. This is the same planning which has gone into the high-luminosity Qweak experiment which has similar low noise requirements.

The main detector could potentially negatively impact the systematic error by being sensitive to backgrounds which are difficult to measure. The largest backgrounds, and methods for measuring them, were established in E158. Our new spectrometer and detector designs, our tracking system, and are strategy for measuring soft backgrounds are expected to reduce backgrounds and/or their uncertainties.

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How This Presentation Contributes to Addressing the Charge

2. Review the viability of the approach used in the project with respect to the general experimental technique proposed to measure the weak mixing angle. This should include the evaluation of credible plans for:

a. R&D required to meet the technical challenges of the experiment.

The main detector requires a great deal of simulation to optimize the design (mainly to minimize backgrounds). Our experience from Qweak showed this is a time-consuming process because the spectrometer-beamline-collimators-shielding-main detector function as a system, and many iterations are required. However, none of the technology is new.

b. Proposed detector concept and associated calibration/background measurements, including helicity-correlated and beam-target generated backgrounds.

We have addressed the major beam-target generated backgrounds. Helicity-correlated backgrounds are covered in the talk by ???.

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Suggestions for Director’s Review

1. Need a separate low-noise electronics talk.I thought I could do it because Des Ramsay had organized everything nicely in the proposal and various Qweak technical notes. But in practice there aren’t enough hours in the day.

2. At the end of the Director’s Review, summarize for the committee how we have explicitly tried to address each item in the charge. Shelley Page did this for the Qweak Readiness Review and it seemed very helpful (i.e., to draft the committee’s report for them).

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Extras

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JLab Moeller Experiment Parameters

APV = 35.6 ppb

E = 11 GeV

E’ = 1.8-8.8 GeV

Θlab = 0.230-1.10

150 cm LH2 target

153 GHz rate

5040 hours

∆sin2θW = ± 0.00026(stat) ± 0.00013(sys)

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Apparatus Overview

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Additional DetectorsPion detector – integrating detector which operates at high luminosity to measure the PV asymmetry of muons and charged pions after EM shower products are ranged out by lead shielding.

Tracking detector – event mode detector which operates only at low luminosity to measure the detector response, search for backgrounds, etc.

Luminosity monitor – integrating detector at small angles

D.J. Mack (TJNAF) 23

Minimum Position Resolution with Preshower

• 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 but with a Lorentzian character

(consistent with rMoliere)

• Minimum resolution from fused silica should be similar.

M. Gericke (U. Manitoba)

D.J. Mack (TJNAF) 24

Position Sensitive Ion Chambers (PSIC’s)• Ion chambers are promising: good time response, good linearity,

rad-hard, no fast gain changes, easy to match octants, cheap

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

• M. Gericke modeled 10cm of 1atm He gas with 2 cm Pb preshower

• Excess noise is 1.055, or 11% additional running time.

• P Souder asked about soft backgrounds.

still needs study

M. Gericke , E = 4.5 GeV