# Presentation: 466434

## PHENIX Drift Chamber operation principles

- Modified by Victor Riabov
- Focus meeting
- 01/06/04
- Original by Sergey Butsyk
- Focus meeting
- 01/14/03

## Outline

- Drift chamber (DCH) design
- Construction and assembling
- Operation principles
- Calibration aspects
- Trackfinding principles
- Future goals

## Drift Chamber for PHENIX(basic information)

- Main purpose:
- Precise measurement of the charged particle’s momentum
- Gives initial information for the global tracking in PHENIX
- Acceptance:
- 2 arms 90o in  each
- ±90 cm in Z
- 0.7 units of 
- Location:
- Radial  :2.02<R<2.48 m
- Angular:
- West:  -34o < o
- East :  125o < o

## Drift Chamber design

- Made of titanium
- Conists of 20 identical keystones
- Weight ~ 1.5 tons
- Total tension of wires ~ 3 tons

## Lay-out of one keystone

- 6 radial layers of nets (X1,U1,V1,X2,U2,V2)

**Stereo nets start in one keystone ****(n)**** and end in the neighbouring keystone e.g. ****(n+1)**** for U, ****(n-1)**** for V**
**The tilt of UV nets along ******** allows measurement of Z component of  the track**

## Wire net configuration

**Jet-type **wire structure
**X nets:** 12 anode wires** UV (stereo) nets: **4 anode wires
- Total of **80 anode** nets of each type are evenly distributed in 
- ~**6400 readout channels** in every DC
**Independent signal readout** from both sides (North, South)

## Construction and assembling

***Mechanical design and production*** – PNPI (Russia)
***Front Electronics*** – SUNYSB
***Wire net production, assembling*** -  PNPI,SUNYSB

## DCH Operation Principles

**To reconstruct charged particle track DC samples a few points in space along the path of the particle. One such point is called a “HIT”**
**Registration of one HIT is based on a few physical processes:**
**When charged particle transverse the gas volume of the DC it creates clusters of primary ionization on its way**
**Electrons of primary ionization drift from the point of ionization to anode wires along electric field lines**
**Electrons of primary ionization create avalanches in the vicinity of anode wires **
**Back drift of posistively charged ions generate measurable signal on anode wires which is amplified, shaped and discriminated**
**To register a HIT in the DC:**
**Carry out *****drift time***** measurements: Start - collision time measured by BBC; Stop - time when signal appears on the anode wire**
***Drift time***** (t) can be tranformed into *****drift distance***** (x) if calibration curve is known x = x(t)**
**Working gas is chosen to have an uniform drift velocity in the active region ******** linear xt relation can be used x = V****dr**** · t**

## Gas mixture choice 

- 50% Ar -  50% C2H6 mixture is chosen for operation based on:
- uniform drift velocity at E~1 kV/cm
- High Gas Gain
- Low diffusion coefficients
- In Year2 ~1.6% Ethanol was added  to the mixture to reduce ageing and improve HV properties of the nets

## Drift field configuration

- Necessary electric field configuration is created by       5 groups of wires:
**Cathode**** Wires** – Create uniform drift field between anode and cathode
**Field**** Wires** – Separate adjacent anode wires and help to control gas gain
**Back**** Wires** – Stop drift from one side of the anode wire
**Gate ****Wires** – Localize the drift region width
**Termination ****Wires** – Help to reduce boundary affects, make gas gain uniform along the plane

## Drift Field Configuration (II)

- Here is what happens when the charged particle passes through the wire cell
- Note that only even wires collect charge due to the **back wires** that block the odd anode wires !
- Back wires solves left-right ambiguity  problem

## DCH Performance (Run03)

***Single wire efficiency***  ~ 90-95%
***Back efficiency (probability to get hit from the back closed side)***  < 10%
***Spatial Resolution***  ~ 120 um
***Angular resolution*** *d****/*** ~ 1 mrad

## Calibration aspects

**Idea: Substitution of real calibration dependence with a line: x(t) = Vdr·(t-T0), where**
- T0  -  effective time
- Vdr – effective drift velocity in the drift region
**Once we know these parameters <-> we know everything!**

## Global calibration

- Timing distribution for each arm have a characteristic shape
- By fitting the leading and trailing edge of the distribution with Fermi function we obtain time at 1⁄2 height. t0 and t1
- t0 is assumed to be global **T0** for the arm
**Vdr** = <distac>/(t1-t0) is global drift velocity

## Other calibration effectstaken into account

**Slewing corrections** – dependence of measured arrival time on the signal width
**Shape of the drift region** – the wires close to the mylar windows experience distortion of the electric field
**Wire-by-wire t****0**** corrections **– includes geometrical shifts of the anode wires within the  net, electronics channel-by-channel variations e.t.c
**Plane-by-plane V****dr**** corrections** – Vdr is electric field dependant, thus it changes significantly on the side-standing wires where edge effects distorts the field strength
**Global alignment to the vertex **– center of the arm can be shifted from the vertex location. Translation of the arm center can be found by centering a distribution of the vertexes around zero in field-off data

## Residual distributions

- Pick hits of the track on 3 neighboring wires (i.e. 1,3,5). Residual for wire #3 is:   **x3 = (x1 + x5)/2 – x3**
- Residual is equal to **ZERO **if hits are perfectly aligned
- Basic idea of local calibrations is to center all residuals at zero for all the parameters (i.e. align ever three neighboring wires)

## Slewing corrections

- Look at residuals vs. width

## Wire-by-wire t0 corrections

- Study residuals shifts within one net
- We can determine t0 shifts that zero the mean of residuals distributions

## Shape of the drift region

- The shape of the drift region was simulated in GARFIELD and parameterized in the offline software

## Tracking principles

- Main assumptions:
-  Track is straight in the detector region
-  and  variables defined on the figure
- Use hough transform – calculate  and  for all possible combinations of   hits and bin those values into hough array – 2D histogram on  and 
-  Look for local maxima in hough array that surpass the threshold

## Track Candidates

- The  results of the hough transform are track candidates
- First we look for tracks with X1 and X2 hits
- Remaining unassociated hits goes into X1 only and X2 only tracking
- Finally we left with the  following tracks
- Z coordinates of tracks are dedined by PC1-UV-vertex tracking

## Final results

## Future goals

- Calibration of the detector is a main contributor to the momentum resolution  need to improve the absolute calibration methods
- Use outer detector’s matching
- Online Calibration
- Improve HV stability over the run
- Control gas mixture  properties during the run
- Improve UV reconstruction