# Presentation: 249014

## Hadron Blind Detector for the PHENIX experiment at RHIC

**Takao Sakaguchi**
**Brookhaven National Laboratory**
**For the PHENIX Collaboration**

## Excellent QGP detector:  Thermal dileptons

**Thermal dileptons are an excellent probe for investigating the property of the hot and dense medium produced at RHIC.**
**Thermal dileptons are “brother” of “thermal photons”**
**No additional strong interaction in the medium produced**
**Provide information on temperature, dof, etc.**
**Signal to (Combinatorial random) Background is very small**
**Estimated to be ~1/200!**
**Mostly from Dalitz decay of neutral pions and photon conversions**
**Proven by the recent data from PHENIX (firstly shown at QM’05)**
**Need to identify the origin of electrons to reduce combinatorial background**
**We want to see S/B ratio of at least ~1/10**

- Recent data from CERES coll.

- PHENIX preliminary data

- Black: Foreground
- Red: Random Background
- Blue: Net signal

## Solution: Hadron Blind Detector

**Tag background electron pairs via opening angle **
**Veto electrons with partner in *****field free region***
***PHENIX has two (inner and outer) coils to make such a region***
**Electron pairs do not open up**
**Hadron Blind Detector (HBD) : **
**Proximity Focus Windowless Cherenkov detector **
**50cm radiator length, 2*135********, |y|<0.45**
**Radiator Gas=Working Gas**
**Pure CF****4**** radiator**
**n****CF4****=1.000620, ********th****=28 (~4GeV/c for ********) **
**CsI photocathode + Triple GEM with pad readout**
**Radiating tracks form “blobs” on the pad plane**
***max**=cos**-1**(1/n)~36 mrad*RBLOB~3.6cm)
**Dalitz pairs & conversions make two blobs, single electrons make one**

**Reduction**
**by ~100!**

## How to Blind Hadrons

**Absolute Quantum efficiency measured as a function of WL**

**Primary ionization is drifted away from GEM and collected by a mesh**
**UV photons produce photoelectrons on a CsI photocathode and are collected in the holes of the top GEM**
**Triple GEM stack provides gain ~ 10****4**
**Amplified signal is collected on pads and read out**
**Primary ionization signal is greatly suppressed at slightly negative E****d**** while photoelectron collection efficiency is mostly preserved**

**Hadron Blindness as a function of E****d**

**Wave length [nm****]**

## Detector Construction

**24 Triple GEM Detectors**
**  ****(12 modules per side)**
**Area = 23x27 cm****2**** **
** ****Mesh electrode**
** ****Top gold plated GEM for CsI**
** ****Two standard GEMS**
** ****Kapton foil readout plane**
**  ****One continuous sheet per side**
**  ****Hexagonal pads (a = 15.6 mm)**

- Very low mass (< 3% X0 including gas)

- Detector designed and built at the Weizmann Institute

## Gas system

**Photon transmittance of the Gas is monitored at the input and output of the detector**
**Water and oxygen level is also monitored**

- Transmittance as a function of H2O contamination

## HBD before installation into PHENIX

- _heating elements_
- 2 on each side, 2 on bottom (seen in next slide). 6 total on one hbd

- 38 x 25 cm
- 185cm2 35um thick
- Traces (assume Cu)

## HBD Installed in PHENIX

- HBD West (front side)
- Installed 9/4/06

- HBD East (back side)
- Installed 10/19/06

## GEM Performance

**All GEMs produced at CERN**
**133 produced (85 standard, 48 Au plated)**
**65 standard, 37 Au plated passed all tests**
**Good GPA! (~75%)**
**48 standard, 24 Au plated installed **
**Three GEMs in each stack are matched to minimize gain variation over the entire detector **
**All GEMs pumped for many hours under high vacuum (~ 10****-6****  Torr) prior to installation **
**Gain of each module was mapped for an entire sector**
**Resulting gain variation is between 5-20 %**

## Gain Stability of GEMs

**This appears to be a charging effect that has typically been seen in GEMs before, but the magnitude is large !**

**During gain mapping, a single pad is irradiated with a 8 KHz ****55****Fe source for ~ 20 min. Then all other pads are measured, and the source is returned to the starting pad.**
**Gain is observed to initially rise and then reach a plateau.**
**Gain increase has two components**
**Charge up: ~30% increase**
**Rate dep. Change: ~10%**

**First Layer is coated by CsI**

***Our HBD is operated at a very low rate (~ a few Hz) ************ Not a big problem***

## Hadron blindness of the detector

**Read 12 samples per trigger with 16 nsec interval for each channel (=200nsec, 2B.C.)**
**MIP distribution nicely fitted with a Landau distribution for Forward Bias**
**Derive detector gain from the mean of MIP distribution**
**Reverse Bias rejects ionization electrons almost perfectly -> Blinding Hadrons!**
**Hadron rejection power: ~15@90% elec. eff.**

***Full scale prototype result***

**Timing Sample (n-1, 16nsec step)**

**Raw FADC dist. (sample”0” is subtracted from all other samples)**

**Pulse Height: B=0, Reverse BIAS 505 V**

**Detector gain: 2500**

**Pulse height**

- MIP!

## Summary

**PHENIX installed a Hadron Blind Detector to reject the random combinatorial background by electrons and the hadrons**
**Identify electrons (single electron and photon converted electron pairs) in field free region**
**GEMs are used as readout modules of photo/dEdX electrons**
**Good GPA**
**On-Beam Test of Full scale Prototype demonstrated the basic hadron blindness properties of the detector, and also provided information helpful for constructing the final detector**
**Rejects primary ionization signals (dE/dx) of electrons and hadrons while keeping high photoelectron detection efficiency**

## Principle Players

- Weizmann Institute of Science
**A.Dubey, Z. Fraenkel, A. Kozlov, M. Naglis, **
**    ****I. Ravinovich, D.Sharma, L.Shekhtman, ****I.Tserruya***
- Stony Brook University
**W.Anderson, A. Drees, M. Durham, T.Hemmick, B.Jacak, J.Kamin, R.Hutter**
- Brookhaven National Lab
**B.Azmoun, A.Milov, R.Pisani, T.Sakaguchi, **
**     ****A.Sickles, S.Stoll, C.Woody ****(Physics)**
**J Harder, P.O’Connor, V.Radeka, B.Yu ****(Instrumentation Division)**
- Columbia University (Nevis Labs)
**C-Y. Chi**

- * Project Leader

## Backup

## The SB plant (I): CsI evaporation facility

** ****4 photocathodes produced per shot together with chicklets for QE monitoring**
** ****Excellent reproducibility. **
**  ****Excellent stability**

## Detector Gain

**MPV, Mean and Gain derived from the mip distribution of the different runs**

******V****GEM**** = 495 V up to  run # 203145**
**               ****505 V from run # 203146 **

**Notes:**
 FB RB at low gain and higher gain figures
Gain variation with time

## HBD before installation into PHENIX

- _heating elements_
- 2 on each side
- 2 on bottom at the centers of each front panel
- (6 total) on one hbd

- 38 x 25 cm
- 185cm2 35um thick
- Traces (assume Cu)

- 3.5m of 25mm copper tape
- 60um thick on one side
- and 1⁄2 length on the other
- Add 1⁄2 m for the faces
- Add 22gauge Cu wire twice the length
- (0.64mm in diam Cu +1.5mm Teflon)

## Present PHENIX Capabilities 

## HBD Detector Parameters

- Acceptance nominal location (r=5cm)                     || ≤0.45, =135o
-                     retracted location (r=22 cm)                || ≤0.36, =110o
- GEM size (,z)                                                         23 x 27 cm2
- Number of detector modules per arm                     12
- Frame                                                                      5 mm wide, 0.3mm cross
- Hexagonal pad size                                                 a = 15.6 mm
- Number of pads per arm                                         1152
- Dead area within central arm acceptance               6%
- Radiation length within central arm acceptance      box: 0.92%,  gas: 0.54%
- Weight per arm (including accessories)                   <10 kg