Integrating Detectors
Dave Mack
September 18, 2009
Moeller pre-Director’s Review Meeting
Radial Distribution at Focal Plane
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
Foci here are preliminary (proposal), but the detector clearly requires radial
binning to isolate e+e, estimate underlying e+pe+X backgrounds, and tune up
the spectrometer.
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Azimuthal Distribution at Focal Plane
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+p
Unwrapped Φ distribution.
e+e
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).
Foci here are preliminary (proposal), but the detector requires azimuthal binning
to measure parity-conserving cos(Φ) asymmetries, tune up the spectrometer, etc.
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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 other
noise 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 Estimate
Assuming 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 Requirements
for the Main Detector
•Full coverage of e+ee+e locus.
•Good coverage of e+pe+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
Front view
Side view
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First Simulation
courtesy of John Leacock (VPI)
John’s experience with Qweak lumis allowed him to quickly simulate
light collection in a 20cm (W) x 5 cm (H) x 2cm (T) piece of quartz with
no 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
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.
e+e
e+p
Inelastics
Direct electron backgrounds in the proposal are ~8%. I am hopeful for a factor
of 2 reduction in this by improving the focus.
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Direct Meson Backgrounds
Estimated in the proposal in 3.5.3.
Pion 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.
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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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Manpower
People 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-collimatorsshielding-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. Helicitycorrelated 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
19
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 Detectors
Tracking detector – event mode
detector which operates only at low
luminosity to measure the detector
response, search for backgrounds, etc.
Pion 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.
Luminosity
monitor –
integrating
detector at
small angles
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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
M. Gericke (U. Manitoba)
• Minimum position
resolution is a few mm
but with a Lorentzian
character
(consistent with rMoliere)
• Minimum resolution from
fused silica should be
similar.
D.J. Mack (TJNAF)
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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.
•
M. Gericke , E = 4.5 GeV
P Souder asked about soft
backgrounds.
still needs study
D.J. Mack (TJNAF)
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