Qweak Main Detector Status Des Ramsay, Dave Mack, Michael Gericke
Main Detector Project Overview The Main Detector WBS has spent 85% of our $468.5K budget. $$ All custom PMT’s are at JLab and tested. $ All magnetic shields are at JLab (and actually fit!) $ All TRIUMF low-noise preamplifiers are at JLab and tested. $$$ Quartz bar shipments are complete. (some QA remaining) $$ Quartz lightguide shipments are complete. (lots of QA remaining) Remaining funds are for digital integrators, voltage dividers (bases), detector housing, and support structure. Several person-years of design, testing, and assembly remain.
Glue joints Glue Joints in the Optical Assembly There will be 5 glue joints per optical assembly which all need to be strong and UV transparent down to 250 nm. The central joint also has to be rad-hard: 100kRad nominal, (1 MRad for plan B) Two glue candidates. e-
Optical Transmission Apparatus Monochromator Due to lamp instabilities, we squat at one λ, normalize the beam with sample out, then measure with sample in, and repeat to estimate the random error. Sample on translation stage monochromator out in Integrating sphere with PMT Spectrophotometer and software by Carl Zorn of JLab Detector Group. Integrating sphere + PMT
Samples One of our original 100 cm x 2.5 cm x 12 cm prototype bars of Spectrosil 2000 was cut lengthwise into 15 slides of about 6 cm x 2.5 cm x 12 cm. Except for a few avoidable new scratches, the 6cm x 12 cm faces remained in excellent condition. Two glued samples and experimental controls.
Analysis T(λ) = Corr(λ) x (Iin(λ) – Idark)/(Iout(λ)– Idark) The correction for Fresnel reflection is typically 8%. Using precision n(λ) from Melles-Griot and a next higher order expression, the error on this correction is below 0.1%. Random errors are dominated by short-term lamp instability of about 0.2%. Corr(λ) Corr(λ) = 1/(T(2) + T(4)) where T(2) =TF2 T(4) = TF2(1-TF)2 TF = 4n1n2/(n1+n2)2
Pre-irradiation Baselines 2.5 cm Spectrosil 2000 (non-glued control) 5.0 cm Spectrosil 2000+ glue joint Below 300 nm, • SES403 glue joint reduces transmission 0.25%. • SES406 glue joint reduces transmission 0.75%. • With SES406, total light losses from all glue joints will be less than 3%. The control data are almost consistent with 100% transmission as expected for undamaged Spectrosil 2000. (The small dip near 275 nm is repeatable but not understood.)
Irradiation Irradiations with a 60Co source were done by Nuclear Services at North Carolina State University. Initial 100 KRad irradiation (nominal Qweak dose): controls SQ2, Air-Gap, and glued samples SES403, SES406. Final additional 1 MRad irradiation (plan B dose): control SQ2, and glued sample SES406 from Nuclear Services website:
Before and After 100 KRad 5.0 cm Spectrosil 2000 with glue joint 2.5 cm Spectrosil 2000 (non-glued control) Our central glue joint will not suffer any significant rad damage during the Qweak experiment. The 1 MRad data are still under analysis by Katie Kinsley (Ohio U.), but the preliminary results suggest no detectable damage.
PMT Nonlinearity Measurements Our goal is to keep the detector chain nonlinearity below 0.1%. (See D. Mack at http://qweak.jlab.org/doc-public/ShowDocument?docid=172) Measuring linearity to better than 1% requires special techniques: • We previously developed a 2-LED method with sensitivity at the few times 10-4 level. (See M. Geicke at http://qweak.jlab.org/doc-public/ShowDocument?docid=575) But Riad Suleiman wasn’t happy. If the nonlinearity is frequency dependent, this method may measure it only in the DC limit. So … 2. In the last few weeks, summer student M. Andersen (U. Manitoba) has demonstrated a new 3-LED technique which measures nonlinearity near the reversal frequency with sensitivity at the 10-6 level.
Testing PMT linearity quadratic nonlinearity VAC anode signal linear IAC cathode current IDC
Definitions The PMT transfer function from cathode to anode can be written Vanode = G Ik (1 + βIk) Vanode is the effective voltage at the anode G is the gain x 50 Ohms β is a small nonlinearity parameter Ik is the cathode current or signal Differentiating, ΔVanode = GΔIk (1 + 2βIk) The result depends on the load, so β alone isn’t very useful. We will call the dimensionless quantity 2βIk “the nonlinearity”: the relative error made when making a measurement with a bent ruler. It may help to think of nonlinearity as a load-dependent gain: G’ = G(1+βIk)
New 3-LED Technique • Nonlinearity is equivalent to self-multiplication. Multiplying two frequencies yields sum and differences. The appearance of mixing peaks at f1+-f2 therefore gives access to the nonlinearity. • Technique requires: • one DC LED to provide the load • one AC LED at f1 to mimic a small signal • one AC LED at f2 (near f1) to induce the mixing. • Inserting • Ik(f) = IDC + I1 + I2 • into • Vanode = GIk(1 +βIk) • the nonlinearity in terms of easily measurable quantities is • 2 IDC 50Ω |V(f1+-f2)| / (|V(f1)|| V(f2)| ) f1 f2 f2-f1 f1+f2 • Measures nonlinearity away from DC limit. • Self-normalizing: insensitive to drifts • Great sensitivity We have only demonstrated proof of principle. No reliable numbers yet.
Low Gain Base Tests Michael Gericke
PMT Low Gain Base Testing Status We went through several generations of low gain dividers since summer 2006 We need a nominal gain of 2000. We want to be able to go up to a “contingency” gain of 16000. Last year we had 3 base generations with 4 and 5 active stages respectively: Voltage required to get to a gain of 16000 was still too high for 5 stages. Go to more stages : 7 active stages with 6 141 kOhm Resistors and 2 Zeners
Small setup at University of Manitoba: As before: Used reference PMT 128 and 2 280 nm UV LEDs Assembly and measurements done by summer student Charles Koop Currently at JLab
Dark current increased too much with voltage with 5 stage. 7 stage is good. Several tests indicate the dark current is mostly coming from the PMT (not leakage in the base) Thedark currentis now only a0.05% dilution for a 6 A nominal signal. Measured gain vs voltage for 5 and 7stage Contingency gain range can be obtained with 1000 to 1250 Volts of bias. Everything looks good so far, but more stages means less stability and linearity measurements are in progress
Dave Mack Miscellaneous • High gain divider tests • Panel stiffness measurement • Magnetic field sensitivity
High Gain Divider Tests • Mitchell Andersen also built our first high gain divider to look at pulses. (gain = few x 106) • Large pulses OK, but found unacceptable baseline noise for spe at 1-2 mV. • Problem tracked down, with great difficulty, to noisy zeners. • For now, we are using all-resistive dividers which give acceptable pulses and quiet baselines for upcoming cosmic tests. • Lower noise zeners and external amplifiers were ordered and have arrived. • Tests continuing.
Panel Stiffness Measurement • The glued quartz bars will be supported in front by a low radiation length (1.7%) composite panel. • The prototype panel from Composiflex has a core of IG-71 Rohacell (0.075 g/cm3) wrapped in 7 layers of epoxy-impregnated Carbon fiber. The deflection was measured with a Mitutoyo Dial Caliper BS-74 under a load representing the 10 kG weight of a 200 cm long quartz bar. Measurements by Mkrtchyan et al. Application of weights Blue = more realistic loading Maximum deflection is 0.5 mm. We’re pretty happy with this, but still need to check that the glue joint won’t pop during transport.
Magnetic Field Sensitivity Stray fields from QTOR will be < 0.1 Gauss. (W. Falk) Earth’s field dominates. Mkrtchyan et al. < 1% variation 10% variation Sensitivity of our 5” PMT’s with Vk-d1 = 280 V is negligible with shields.
Current Mode Electronics Update Des Ramsay
ITEM current signals voltage signals I-V channels I-V modules + spares type VME integrator channels Main Detectors: 8 bars x 2 tubes 16 16 8 + 4 main 16 Lumi Monitors: 2 monitors x 8 tubes 16 16 8 + 4 Lumi 16 Real-time isolation detector 2 2 1 + 1 main 2 Soft background detector 2 2 1 + 1 Lumi 2 “Fake BCM” isolation monitor 1 1 “Fake BPM” isolation monitor 2 2 Target BPM (in scattering chamber) 4 4 Essential Beamline Monitors 20 20 TOTALS 36 27 36 63 MINIMUM MODULES 18 8 SPARE MODULES 10 6 TOTAL MODULES 28 Dual preamps 14 main 14 Lumi 14 Octal integrators Overall Tally of Current Mode Electronics
Current Mode Electronics Summary • 10 dual preamplifiers (20 channels) with transimpedance selection 0.5, 1, 2, 4 M are already at JLab for the main detectors. • 4 More main detector style preamps are finished at TRIUMF. 14 Lumi-style preamps with gain selection 0.5, 1, 25, 50 M are also complete. • The testing is almost finished at TRIUMF. • Paul king is testing the prototype VME octal integrator. We have made a couple of firmware upgrades and the module appears to be working properly. We now need more detailed tests. • Preliminary designs are ready for a TRIUMF test source that will give us a realistic current of ~5 mA, upon which a small simulated parity violating signal can be superimposed.
Modulated Current source • Reference current of 5 mA DC • Choice of 16 modulations from ~10-6 to ~10-9 • Unmodulated reference channel available • Responds to external spin state signals, or can run in stand-alone mode.
Modulated Current Source Block Diagram Voltage ramp on small capacitor ~10-15 A
switching function -- 18 ms quartet 4 ms 4 ms 0.5 ms 0.5 ms 0.5 ms 0.5 ms 4 ms 4 ms
Switching function in time domain = ten regular 18 ms quartets. Fast Fourier Transform (FFT) Odd multiples of 55.5 Hz FFT essentially assumes waveform goes on forever
Simulation for finite run times • The FFT does not properly account for finite run times • For this I took a test sinusoid, multiplied by the switching function and integrated over the run time • I stepped the frequency and integrated each frequency for the run time • The simulation shows the same “acceptance” frequencies as the FFT,but shows a sensitivity to “off resonance” frequencies for finite run times. • For very long run times, only signals coherent with the switching function remain
100 random (+ - - +) 18 ms quartets = 1.8 s run • Exactly equal + and – rejects DC • The 4 ms spin state rejects multiples of 250 Hz • The quartet structure rejects multiples of 111.1 Hz 111.1 333.3 555.5 777.7 222.2 444.4 666.6 888.8 250 500 750 1000
200 random (+ -) or(- +) 9 ms doublets = 1.8 s run • Exactly equal + and – rejects DC • The 4 ms spin state rejects multiples of 250 Hz • The doublet structure rejects multiples of 222.2 Hz 222.2 Hz 444.4 Hz 666.6 Hz 888.8 250 Hz 500 Hz 750 Hz 1000 Hz
400 random (+ ) or(- ) 4.5 ms singlets = 1.8 s run • Each spin state is integrated for 4 ms • 1/(4ms) = 250 Hz, so multiples of 250 Hz are rejected • States are randomly chosen, so in general there will notbe exactly the same number of + and -, and there will besome sensitivity to DC. 1000 Hz 250 Hz 500 Hz 750 Hz
A-B (Lumi-BCM), 25 mA, LH2, 2mm square raster, normal target cooling and pump speed 60 180 240 120 300 360
(A-B)/(A+B), 10 mA, LH2, 2mm square raster, normal target cooling and pump speed
Next 6 Months • Delivery of last preamplifiers to JLab (D. Ramsay) • Continue testing prototype sampling ADCs (P. King) • Support structure design • Full-scale glue-up (Yerevan, Mack) • Complete scintillation measurements (K. Kinsley) • Complete low gain divider design (Gericke) • Complete high gain divider design (Mack) • Procure dividers (Mack, Gericke) • Full-scale prototype (Yerevan, Mack)
Main Detector Summary • We’re making adequate progress. Need more designer help. • Full-scale optical assembly and cosmic tests by end of summer ’07. • Full-scale prototype module fall ’07. • Procurement of production module parts in early ’08. • Complete gluing and module assembly in summer ’08. • Production modules complete by Sept. 1, ’08. … followed by more QA and detailing until installation
Our 2-LED Technique • Changes in small AC signal due to shifts in DC load give access to the nonlinearity.Technique calls for: • one DC LED to provide the load • one AC LED to mimic a small signal • Doing the math: • Vanode = GIk(1 +βIk) • Ik(f) = IDC + IAC • Then to non-mixing order, • Vanode(f) = G(1+βIDC)IDC + G(1+2βIDC)IAC • The non-linearity is • (ΔVAC/VAC) / (ΔIDC/IDC) Using early 5-stage prototype: It works! Here, 2βIk= few x 10-4 Issue: Measures nonlinearity in the DC limit O(min-1) Annoyance: Precision is limited by the stability of the AC LED during the measurement, so serious measurements require 24 hours of burn-in.
Bias on <Q2> Due to Detector Response The PV asymmetry is proportional to Q2, so we need to understand <Q2> with an error << 2%. Our earlier estimates of <Q2> included the acceptance, cross section, and radiation, but neglected the detector bias. 5% bias Simulations indicate that our current mode experiment will give events far from the center of the bar about 5% higher weight. pe Because our higher Q2 events have a wider distribution, the weak correlation with yield increases the detected <Q2>. The new <Q2> = 0.02754. Y X The estimated detector bias on Q2 is +2.5%. This could worsen with radiation damage, and will be measured with wire chambers. Y
Use of a Pre-radiator: Trade-offs A shower-max preradiator could increase Signal/Background. But at what cost? Excess noise increases to 12%. Potential for increasing S/B > 30 A 2 cm Lead sheet in front of our quartz bars would increase S/B by > 30, but would require 390 additional hours, and increase the radiation dose to 3 MRad. Won’t need this if backgrounds are only 1%.
Photo-electron Count PDG formula predict ~900 photons above 250 nm cutoff: Simulation gives ~ 1000 photons on average for arbitrary path lengths. ~250 photons get to the cathode the rest is lost The photon is counted only if it makes a volume transition from the PMT window to the cathode. The mean number of photoelectrons per event: No Wrapping: ~40 Pes Millipore : ~50 Pes Original design criteria: > 10 PEs
Experiment Component Details Main Detectors R-3 VDC Target GEMs R-2 HDCs Pb Shielding Lumis R-3 Chambers & Rotation System Beam QTOR Mini-torus
Detector Design Elastic envelope on bar of Spectrosil 2000. Dimensions are 200 cm x 18 cm x 1.25 cm.