# Presentation: 479476

## A Hadron Blind Detector Upgrade for the PHENIX Experiment

- 13 November 2008
- PANIC

## Outline

- Motivation - the dielectron continuum.
- The PHENIX detector
- HBD operating principles
- Installation and performance in RHIC run 7
- Rebuilding and improvements for RHIC run 9

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## Motivation

- Leptons do not strongly interact with the RHIC medium and thus provide a “deep” probe with possible insights into:
- Chiral symmetry restoration
- Vector meson modification
- Thermal radiation
- Heavy quark bound states

- In the mass range between 150 and 750 MeV/c2 dielectrons are enhanced by a factor of 3.4 +/- 0.2(stat.) +/- 1.3(syst.) +/- 0.7(model) compared to the expectation from our model of hadron decays.

- The goal of the HBD is to improve the signal significance!

-  arXiv:0706.3034

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**Notes:**
Dielectrons are an interesting probe of the the RHIC medium...

## The PHENIX Experiment

- Currently, electrons are tracked by drift chamber and pad chamber
- The Ring Imaging Cherenkov Counter is primary electron ID device
- Electromagnetic calorimeters measure electron energy

**e****+**

**e******

****

****

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**Notes:**
And the challenge is pairing them up properly

## Background Sources

- Often only 1 electron from a  pair falls within the PHENIX acceptance.
-  The magnetic field bends the pair in opposite directions.
-  Some spiral in the magnetic field and never reach tracking detectors.

- To eliminate these problems:
- Detect electrons in field-free region near beampipe
- Need >90% efficiency

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## Separating Signal from “Background”

**Spectrum from photon conversion tightly peaked around 2m****e**

**Mass spectrum from pion Dalitz decays peaked around 2m****e**

- Opening angle can be used to cut out photon conversion and Dalitz decays
**→****MUST BE ABLE TO DISTINGUISH SINGLE HITS (“INTERESTING” ELECTRONS) FROM DOUBLE HITS (DALITZ AND PHOTON CONVERSION).**

**Heavier meson decays have large opening angles**

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**Notes:**
Besides being able to measure the electrons that previously escaped PHENIX measurement, we can separate “interesting” electrons from boring old dalitz and photon conversions and not mix the two - drastically reducing the combinatorial background.

## Gas Electron Multiplier (GEM)

- F. Sauli ,NIM A 386 (1997) 531

- Thin (~ 50 μm) Kapton insulator clad with copper on each side
- Holes are chemically etched into the GEM
- When a voltage is applied between the two sides, the high density electric field causes charged particles to avalanche

- 140 μm

- 70 μm

- 300-500V

- Gas gain 10-20

- C. Altunbas *et al*, NIM A, 490 (2002) 177-203

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**Notes:**
The fundamental detectors of the HBD to make these measurements possible are GEMs ...

## “Traditional” GEM operation

- Mesh drifts ionization trail **towards** GEM stack
- Usually operated in Ar/CO2
- ~300-400V across GEMs

- Mesh drifts ionization trail **away from** GEM stack, but allows Cherenkov photons
- CsI coating on top (gold) GEM
- Operate in CF4  - radiator and avalanche gas
- ~450-550V across GEMs

**Hadron Blind** GEM operation

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**Notes:**
The typical operation of GEMs uses a (triple) stack with a mesh above them to which we apply a voltage.

## Put It All Together

**Dilepton pair**

**Al coated mylar window with black kapton “skin” around beampipe**

**HBD Gas Volume: Filled with CF****4**** (L****RAD****=50 cm)**

**Cherenkov light forms “blobs” on an image plane**
**(d****blob**** = 3.6 cm****2****)**

**Triple GEM detectors**
**(10 panels per side)**

***Electrons radiate, but hadrons with ***
***P < 4 GeV/c do not***

**PCB pad readout (6.2 cm****2****)**

**5 cm**

**55 cm**

- e-

- e+

- 
- Pair Opening
- Angle

- CsI photocathode covering GEMs

**Space allocated for services (gas and HV)**

**Radiation Length<5%**

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**Notes:**
So we put these elements together

## HBD Performance at PHENIX

- Both HBD arms were installed prior to RHIC Run-7

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- Some Problems:
- Difficulty holding voltage - GEM power supply trips
- Trips led to mesh to GEM sparks
- Spark was bright enough that light output tripped other stacks
- Sparks damaged GEMs
- Insufficient active area in Run 7 running
- Forced us to dismantle and then repopulate the HBD with new GEMs.

**Notes:**
So we installed the HBD in PHENIX for Run 7 ...
Unfortunately as may be expected with a novel detector we had some operating difficulties

## HBD Performance

- Clear separation between hadrons and electrons (significant improvement in e-id!)
- But lacking a clear separation between single and double electron peaks, less than ideal photoelectron yield.

- The HBD response to electrons and hadrons (identified in the rest of PHENIX).

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- Analysis of peripheral Au+Au (50-92%) with only fraction of HBD operational:
- S/B (<150 MeV) = 15       250
- S/B (>150 MeV) = 1/25    1.25

**Notes:**
Despite these problems we were able to see the HBD response

## HV Improvements

- Added safeguards to HV to prevent Mesh-GEM sparks
- Improved trip procedure
- Adjust transfer gap V (between GEMs), allows higher gain w/out increasing GEM voltage
- Operating at a less severe reverse bias will allow us to maintain hadron blindness but improve the photoelectron yield.

- ~25% gain increase w/out more GEM stress

- Relative Gain

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**Notes:**
Besides the mesh-GEM spark prevention I mentioned earlier we’ve made some other improvements to the HV system.
Previously transfer gap V = dV =495

## Cosmic Ray Measurements

- Forward Bias

- Reverse Bias

- On the bench, we observe a MIP peak in forward but not reverse bias, and scintillation in both.

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**Notes:**
After all these improvements we rebuilt the detector with new GEMs to replace the damaged ones, and have been testing it on the bench with cosmic ray measurements.

## Presently

- HBD West installed on 6 November!
- Increased gas flow (2    5 l/m) could yield ~15% light gain
- HBD East undergoing testing on the bench.
- Looking forward to the upcoming RHIC run!

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## The HBD Team

-  Brookhaven National Lab:     B. Azmoun, R. Pisani, T. Sakaguchi,     A. Sickles, C. Woody
-  Columbia University:     C.-Y. Chi
-  Stony Brook University:     W. Anderson, Z. Citron, J. M. Durham,     T.Hemmick, J. Kamin, J. Sun, V. Pantuev
- University of California, Riverside:
-      A. Hollis, S. Rolnick, R. Seto
-  Weizmann Institute of Science:     A. Dubey, Z. Fraenkel, A. Kozlov, A.Milov,
-     M. Naglis, I. Ravinovich, D. Sharma,
-     I. Tserruya

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## Backups:

## MC Estimate of HBD Effectiveness

- Simulated detector response.
- Use (very) low mass region to fix single electron width.

- Improvement of signal significance

- More detailed analysis of the HBD response allows better separation between the single and double peak than appears to the naked eye. A data set with a fully operational detector will allow this.

**Notes:**
Even with what doesn’t look like great separation in the HBD electron response we are still able to gain information from it.

## Define the Effective Signal

- Equivalent sample size if there were no background

- with

- This is the quantity of merit for us.

- .12%

- .2%

## HBD Effect

- With f the increase in stats,  the electron pair efficiency, and R the reduction of the background.

- HBD will introduce:

## Measuring Light from a known Source

- Coincidence of SSB and pads

- We can measure CF4 scintillation at 160 nm.
- Gain is calibrated using an 55Fe source.

## MIP zoom-in

## The Evaporator

- _Evaporation Chamber_

- _Quantum Efficiency Station_

- Magnetically coupled driver for moving the GEMs inside the vacuum.

- on loan from INFN Roma

## The Evaporation Chamber

- _The Evaporation Chamber_

-  ~24 hrs to pump down vessel
-  vacuum ~10-7 mbar
-  **no water!!**
- Evaporate 4 GEMs simultaneously

- Molybdenum boats

- AC

-  Boats are in series so they must be brought up to temperature *slowly* (~10 min)
-  250 – 450 nm layer of CsI at rate of ~2 nm/sec

## Trips and Mesh to Top GEM Sparking

- R1

- R1

- R2

- R2

- R2

- R2

- R2

- 2R2

- R1

- Two separate power supplies per stackallows for flexibility between FB and RB

- R1 = 2.2 MΩ,   R2 = 10 MΩ,   Vin = 3720 V,   dV/GEM = 500V,   Ichain = 150 μA

- HV

- HV + bias

**TRIP!!**

-  Led to damage of top GEMs
-  Grid patterned “burns”   line up with mesh.

- Optical coupling between modules led to multiple stacks tripping simultaneously.

- Damage made GEM stacks inoperable
**→****Insufficient active area to do physics in Run 7.**
**→****Forced us into repopulating with new GEMs**

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**Notes:**
The most damaging problem was mesh to top GEM sparking.  We used two power supplies one for the mesh and one for a resistor chain to power the three GEMs.