a hadron blind detector for the phenix experiment at rhic babak azmoun for the phenix collaboration...

A Hadron Blind Detector for the A Hadron Blind Detector for the PHENIX Experiment at RHICPHENIX Experiment at RHIC

Babak Azmoun for the PHENIX CollaborationBrookhaven National Laboratory

October 13, 2004

11/13/04 Babak Azmoun, BNL 2

The PHENIX HBD TeamThe PHENIX HBD Team Weizmann Institute of Science; Rehovot, Israel

– I.Tserruya, Z. Fraenkel, A. Kozlov, M. Naglis, I. Ravinovich, L. Shekhtman

Brookhaven National Lab; NY, USA– B.Azmoun, C.Woody

Stony Brook University; NY, USA– A. Milov, A. Drees, T. Hemmick, B. Jacak, A. Sickles

University of Tokyo; Tokyo, Japan– T.Gunji, H. Hamagaki, M. Inuzuka, T. Isobe, Y.

Morino, S. Oda, K. Ozawa, S. Saito, T. Sakaguchi

Waseda University; Tokyo, Japan– Y.Yamaguchi

Riken; Wako, Japan– S. Yokkaichi

KEK; Tskuba-shi, Japan – S. Sawada


OutlineOutline Motivation (Physics case) System specifications and detector concept GEM +CF4+CsI R&D Hadron blindness

Hadron rejection (alpha particles - in lab) Pion rejection (KEK) Detection efficiency

CsI quantum efficiency and expected N0

Optical Properties of CF4 / Effects of gas impurities Beam test @ PHENIX & KEK Summary


MotivationMotivation In ordinary matter, the Standard Model of particle physics states that Chiral symmetry, which is a symmetry between light quark flavors, is normally broken due to constituent quark masses. However, at high temperatures and/or high baryon densities, such as those produced in relativistic heavy ion collisions at RHIC, this symmetry may be restored.

Low mass e+e- pairs are the best probe for Chiral Symmetry Restoration

Effects of CSR may have been seen in CERN SPS results

The same effects are predicted to occur at RHIC, but will be difficult to detect

Nevertheless, the RHIC program would be incomplete without a low mass lepton pair measurement

PHENIX is the only experiment at RHIC that could perform this measurement


Low Mass Electron Pairs at RHICLow Mass Electron Pairs at RHIC• Strong enhancement of low-mass pairs persists at RHIC• Contribution from open charm becomes significant

• Possibility to observe in- medium modification of the intermediate vector mesons ():

• Dropping of invariant mass (Rapp & Wambach)• Broadening of vector meson () invariant masses (Brown et.al)• Thermal radiation from hadron gas (small…)

R. Rapp nucl-th/0204003

* e+e-


Experimental Challenges at RHICExperimental Challenges at RHICLarge combinatorial pair background due to copiously produced

photon conversions and Dalitz decays :

dN/dy dN/dypT > 0.2 GeV

e from charm 0.68 0.54

e from o e

e 8.4 1.1

e from e

e

(1.5 X/X0)18 2.25

Need rejection factor > 90% of e+ e - and e+ e -

Would like to improve S/B by ~ 100 - 200

e+ e -

e+ e -

S/B ~ 1/500

“combinatorial pairs”

total background

Irreducible charm background

all signal

charm signal

• Both members of the electron pair are needed to reconstruct the Dalitz pair or conversion.

•Single electron pair members contribute to background:

• Limited by:

– Low pT acceptance of outer PHENIX detector:

( pT > 200MeV)

– Limited geometrical acceptance of present PHENIX

configuration


Upgrade Concept: Utilize HBD Upgrade Concept: Utilize HBD to Identify and Reject background to Identify and Reject background

Hardware* Compensate magnetic field with inner coil to preserve (e+e-) pair opening angle (foreseen in original design B0 for r 50-60cm)* Compact HBD in inner region (possibly to be complemented by a TPC or other tracking detector in the future).

Strategy * Identify signal electrons (low mass pairs) with p>200 MeV in outer PHENIX detectors

* Identify low-momentum electrons (p<200 MeV) in HBD

* Reject pair if opening angle < 200 mrad (for a 90% rejection).

HBD

HBD

Specifications * Electron efficiency 90% * Double hit recognition 90% * Modest rejection ~ 200


HBD Layout: Windowless Cherenkov HBD Layout: Windowless Cherenkov DetectorDetector

e-

e+

Dilepton pairBeam Pipe

HBD Gas Volume: Filled with CF4 Radiator (nCF4=1.000620, rRADIATOR =50cm)

Cherenkov blobs on image plane(max = cos-1(1/n)~36 mrad rBLOB~3.6cm)

Triple-GEM micropattern readout detector, (8 panels per side)

Space allocated for services

Radiator gas = Working Gas

Electron Pairs produce Cherenkov light, but Hadrons with P < 4 GeV/c do not:

(Dileptons)>1/n

(Hadrons) <1/n

Pair Opening

Angle


The HBD The HBD

•Detector element: multi - GEM

High gain (CF4~104)

Reduced ion feedback

• Reflective CsI photocathode (CsI coating on top surface of upper-most GEM foil)

No photon feedback• Proximity focus detect blob

Low granularity (pad size~10cm2)

Concept:

• Windowless Cherenkov detector.

• Same radiator and detector gas.

preferred option: CF4 (~50cm)

Large bandwidth and large Npe(Bandwidth not limited by a window)


R&D ProgramR&D Program

• GEM Systematics (using CF4): gain Vs voltage, energy resolution, gain stability, etc.• Optical properties of CF4: transmittance, effects of gas contaminants• CsI: quantum efficiency, compatibility with CF4, aging, etc.•Simulation studies •Prototype/Beam Test: N0 measurement (Npe), hadron blindness (photoelectron efficiency & -rejection), performance evaluation of GEM detector in high multiplicity environment.

TOPICS


Gain Curve: Triple GEM with CsI and CFGain Curve: Triple GEM with CsI and CF44::measured with Femeasured with Fe5555 and with UV lamp and with UV lamp

Fe55 x-ray

UV lamp

• GEMs work with CsI and CF4!

•Pretty good agreement between gain measured with Fe55 and UV lamp.

• Gains in excess of 104 are easily attainable.

• Voltage for CF4 is ~140 V higher than for Ar/CO2 but slopes are similar for both gases.

• Gain increases by factor ~3 for ΔV = 20V


AgingAgingCsI photocathode: * In spite of large ion back-flow there is no dramatic deterioration of the CsI QE. * For a total irradiation of ~10mC/cm2 , the QE drops by only 20%. (The total charge in 10 years of PHENIX operation is conservatively estimated to 1mC/cm2.)

GEM foils: * During the whole R&D period we never observed aging effects (e.g. loss of gain) on the GEM foils. Total irradiation was well in excess of 10mC/cm2 .

Stability measurements performed during day 3 (4 mC/cm2 ), day 4 (3 mC/cm2 ),

day 5 (2 mC/cm2 ).


Charge Collection in Drift Gap :Charge Collection in Drift Gap : Mean Amplitude

Rate

At ED = 0: - signal drops dramatically as anticipated. - rate also drops dramatically large hadron suppression


ED = 0

Single Photoelectron Detection EfficiencySingle Photoelectron Detection Efficiency measure detector response vs Emeasure detector response vs EDD at fixed gain at fixed gain

Very efficient detection of photoelectrons even at negative drift fields !!

IPE

pA

ET

ED (+)

ET

EI

G

G

G

T

T

I

D


At slightly negative ED, photoelectron detection efficiency is preserved whereas charge collection is largely suppressed.

Hadron Blindness (I): UV photons vs. Hadron Blindness (I): UV photons vs. particlesparticles


CsI absolute QE: set-upCsI absolute QE: set-up

Bandwidth: 6.2 – 10.3 eVPMT-2 and CsI have same solid angle

C1 optical transparency of mesh (81%)C2 opacity of GEM foil (83.3%)All currents are normalized to I(PMT-1)

Rotatable UV mirror

QE(CsI) = QE(PMT-2) x I(CsI) / I(PMT-2) x C1 x C2


CsI QE: results CsI QE: results

Cherenkov Spectrum:Output~1/2

Quantum Efficiency of CsI Deposited on to Metallic Substrates

0.1

1

10

100

1140 1200 1260 1320 1380 1440 1500 1560 1620 1680 1740 1800

Wavelength [Angstroms]

QE

[%

]

Smpl. 1: Al, 0.54micronsSmpl. 2

Smpl. 1: Cu+Ni+Au,0.66 micronsSmpl. 2

Substrate Material, CsI thickness

CsI QE tracks Cherenkov Spectrum into the deep UV-VUV regime

QE measurement is limited by dynamic range of Spectrometer (due to LiF windows):Spectrometer cutoff ~114nm ~10.8eV

So…

Folding the measured CsI QE into the Cherenkov

Spectrum gives an effective N0:

Measured in range 6.2-10 eV = 414 cm-1 in CF4

But…

N0 extrapolated to 11.5 eV (CF4 cutoff) = 915 cm-1

Optimum expected value ~ 940 cm-1

(Original PHENIX HBD proposal)


VUV Transmission in the presence VUV Transmission in the presence of gas Impuritiesof gas Impurities

Transmittance in 36cm of Ar Vs PPM's of O2

0

10

20

30

40

50

60

70

80

90

100

110

1100 1200 1300 1400 1500 1600 1700 1800 1900 2000


% T

ran

sm

itta

nc

e [

%]

<1ppm O2

~10ppm O2

~110ppm O2

Transmittance in 36cm of Ar Vs PPM's of H2O

0

10

20

30

40

50

60

70

80

90

100

110

1100 1200 1300 1400 1500 1600 1700 1800 1900 2000


% T

ran

smit

tan

ce [

%]

~10ppm H2O

~40ppm H2O

~200ppm H2O

Gas Transm ittance

0

1020

30

4050

6070

80

90100

110

1150 1250 1350 1450 1550 1650 1750

Wavelength [Angstrom s]

Tran

smis

ttanc

e [%

]

Ar (99.9980% pure),[O2]=5.6ppm, [H2O]=2.3ppm

CF4 (99.9990% pure),[O2]=0.6ppm, [H2O]=19.0ppm

(PMT: R6835)

Clear correspondence between water and oxygen levels and the degree of absorbance

Tolerance Level:

• [H2O]max ~ 15-20ppm

• [O2]max ~ 5ppm

Transmittance: T = Is/Ivac(PMT Current Ratio: Sample-gas scan/Vacuum-Ref. Scan)

Data in prevailing literature agrees with our data:Interaction Cross Sect. in H2OAttenuation Coefficient in O2


HBD Beam Test @ KEKHBD Beam Test @ KEK

Low number of pe’s is due to absorption within the gas (presumably because of O2 and H20 impurities)

It was therefore not possible to make a definitive measurement of N0

ED = +1 KV/cm

[cm][15.3]

S1100x45mm

S350x45mm

S2210x20mm

C1C2

[947] [845]

PbGlCal

Additional detectors including TPC, CNS-HBD, and an array of Silicon strips detectors

50cm CF4Radiator

Retractable Fe55 source

D2 lamp

HBD

GEM

(~98%), e- (~2%)

~19 e

Pions

Electrons

ED = -0.3 KV/cm

Pions

~3e

6 pe(efficiency ≠100%)

Electrons

35 e=26e (1.38*19e)

+ 9pe


HBD Response Simulation HBD Response Simulation

Total signal: 62 e = 29 dE/dx + 33 Cherenkov Blob size: single pad 12% more than one pad 88%

Normal case, no absorption in CF4, no lamp shadowing, realistic losses and

conservative N0 = 840 cm-1

Total signal: 38 e = 29 (dE/dx) + 9 (Cherenkov )Blob size: single pad response =78%

very similar to data

Includes 20 cm absorption length in CF4, lamp shadowing, realistic losses and conservative N0 =

840 cm-1


ED = -0.3 KV/cm

Applying a pion discriminator threshold @ ch.~60, greatly minimizes the number of pions detected, while maintaining ~100% of the pe signal

Pion rejection factor will depend on the threshold that one can safely apply without the loss of the photolectron signal, and in turn depends on the number of photoelectrons detected.

Pion Rejection Pion Rejection

Cut @ ADC

ch.~60

Expected electron spectrum ~ 33 pe(qualitatively simulated: yield

is not to scale)

Measured Pion spectrum

blob signal is shared among 3-4 pads ~10 pe’s per pad Pion signal is contained within a single padThe total pe signal will also receive a noise contribution from adding the noise from the 3-4 pads, the peak separation here may be a bit idealized in this respect


Test of triple-GEM detector in Test of triple-GEM detector in PHENIX IRPHENIX IR

Std. Conical, Std. Conical, Segmented Segmented

CERN FoilsCERN Foils

Active area of pads

~1.0x1.2cm2

Gain Stability @ Const. Gain

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

R un #

Ar/CO2 (360V)

CF4 (495V)

ADC pulse height distr. (Ar/CO2) in Lab

ADC Pulse height distr. (CF4) during Full Luminosity A-A

Collisions in RHIC

The triple GEM detector performed smoothly within the PHENIX IR using both Ar/CO2 (70/30) and CF4 working gases and exhibited

no sparking or excessive gain instabilities. The operation of the GEM and the associated electronics were not hindered by the presence of the ambient magnetic field generated by the central magnet within the IR. However, in close proximity to the beam pipe (50cm), the detector was sensitive to beam related background.The fundamental implication of these tests is that the incorporation of a GEM detector among the inner PHENIX detectors is quite feasible when considering how stable the GEMs’ performance was in such a high multiplicity environment.

Added Background

Gain Variation: +/- 10%, as in the lab

PHENIX IR


SummarySummary

GEM:• High gain/stable operation in pure CF4.• Applicable in the high multiplicity environment of PHENIX• Flexible design: provides surface for CsI evaporation, multi-stages are possible, and large active area with thin profile

CsI:• High Quantum efficiency in VUV region where the Cherenkov spectrum peaks

CF4:

• High N0 (Cherenkov radiator)…to be confirmed• Transparent in the VUV • High gain capability (Detector working gas)• Compatible with CsI

HBD: Exploits the concept that using GEMs, one can build a detector which is very efficient for detecting Cherenkov photons while being very insensitive to direct ionization from charged tracks.

Future: N0 measurement, Measure CF4Scintillation, Test of realistic prototype in beam


Additional SlidesAdditional Slides


Readout Board and PreampsReadout Board and Preamps

Hybrid Preampswith line drivers

~ ¾”honeycomb

Read pads ~ 3x3 cm2

Preamp signals to shaper + ADC

wires

GEMs

• Being developed by BNL Instrumentation

• Based on IO-535

•± input signal

•± 2.5 V output

• Need almost one full rack for the readout electronics


Charge Collection in Drift Gap:Charge Collection in Drift Gap:(I) Am(I) Am241 241 -spectra-spectra

ET

ED (+)

ET

EI

G

G

G

T

T

I

D

Am241

NP


CFCF44 Transparency to UV Photons Transparency to UV Photons

CF4 is transparent


Micropattern Readout Detector: Micropattern Readout Detector: GGas as EElectron lectron MMultiplier (ultiplier (GEMGEM))

General:•Invented by F. Sauli @ CERN in ~1995

•High precision micropattern readout

w/ high/stable gain operation•Convenient geometry: possibility of multiple stages•Applicable in high rate environments•Electron avalanche takes place inside GEM holes: NO photon feedback to limit gain

Structure: •50m thick Kapton foil with 5m copper cladding on both sides which act as electrodes. The foil is perforated with bi-conical shaped holes on the order of 50m in diameter and a 120m pitch distance.

Operation: •Amplification of charge from an incident electron is via an intense electric field inside GEM hole. •The field is generated by applying a potential between the electrodes of a few hundred volts.•The total charge collected from avalanche is on the order of thousands of times larger than charge of the incident electron, for a multi-stage GEM.•Geff ~ exp(VFOIL)

e- Avalanch (~40kV/cm)

Charge transfer/Collection (1kV/cm)


GEM R&DGEM R&D

Detector Box

The GEM detector was extensively tested in the lab using both pure CF4 and Ar/CO2 gas mixtures for comparison.

Set-Up Summary of Measurements Gain measured in various gases: although the volt. potential across CF4 is higher for the same gain w.r.t. other gases, CF4 still produces sufficiently high gains (~103 -104). Gain Vs Physical parameters: (e.g., temperature & pressure) vary the gain by up to ~100%. FWHM energy resolution: for CF4~38% in a triple GEM structure, and ~20% for Ar/CO2 (70/30) Gain Stability in Time: initial gain drift of ~50% is observed, but gain remains stable to within 10% after ~ 1hr. of continual operation (CERN GEM’s) Charge collection and transfer efficiency through GEM stack: measured w.r.t. GEM field config.optimized field config. Sparking probability in CF4 is low: discharge probability isn’t due to the gas, but to the quality of the GEM foils Ion feedback: high, but harmless to CsI Aging Studies: After accumulating 10mC/cm2, no significant change (~20%) in the CsI p.c. or the GEM’s behavior Gain uniformity across GEM surface: ~25% variation in gain using 10x10cm GEM foil

GEM foils of 3x3 and 10x10 cm2 produced at CERN


Ion Back FlowIon Back Flow

Ions seem to follow the electric field lines.In all cases, ion back-flow is of order 1!!!

Mesh

GEM1GEM2GEM3

PCB

1.5mm1.5mm

2mm

Absorber

CsI

pA

Hg lamp

E=0

Fraction of ion back-flow defined here as:

Iphc / IPCB

Independent of Et

Depends only on EI

(at low EI some charge is collected at the bottom face of GEM3)

Independent of gas


Total Charge in Avalanche Total Charge in Avalanche in Ar-CO2 and CF4 measured with Am241

When the total charge in CF4 exceeds 4 x 106 a deviation from exponential growth is observed leading to gain saturation when the total charge is ~2 x 107.

Charge saturation in CF4 !!!


Discharge ProbabilityDischarge Probability

vs. ΔVGEM

vs. Gain

• Stability of operation and absence ofdischarges in the presence of heavily ionizingparticles is crucial for the operation of the HBD.

• Use Am241 to simulate heavily ionizing particles.

In Ar-CO2, the discharge threshold is close to the Raether limit (at 108), whereas in CF4 the discharge threshold seems to depend on GEMquality and occurs at voltages VGEM 560-600V

CF4 more robust against discharges than Ar/CO2 .

HBD expected to operate at gains < 104 i.e. with very comfortable margin below the discharge threshold

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a hadron blind detector for the phenix experiment at rhic babak azmoun for the phenix collaboration...

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