# Presentation: 670168

## Calibration Of A TPC With Krypton-83

- Tobias Eggert, MPI für Physik, München

**Overview**

-                       and
-  Principles Of Time Projection Chambers
-  The STAR-FTPC
-  Calibration With Krypton-83
-  Status And Outlook

## Relativistic Heavy Ion Collider: Au + Au collisions with Ec.m. = 200 GeV per nucleon pair 

- Relativistic Heavy Ion Collider: Au + Au collisions with Ec.m. = 200 GeV per nucleon pair
- The Solenoidal Tracker At RHIC (STAR) searches for signatures of the quark gluon plasma
- Charged particles produced in a cen-tral collision  104
- B = 0.5 T
- MPI für Physik, München designs and constructs the two Forward Time Projection Cham-bers (FTPCs)
- Cover forward region 2.5<|**|<4 (2°<|**|<9°)

## The Quark Gluon Plasma

**The Quark Gluon Plasma**

- A new state of nuclear matter where quarks and gluons are free and not bound into nucleons
- Big bang standard model: QGP at *t *= 10-5 s
- Restoration of chiral symmetry (*m*q  0)
- Signatures of QGP
- Strangeness enhancement
- J/ suppresion
- Direct photons and leptons

## Principles Of TPCs

**Principles Of TPCs**

- Charged particles crossing a gas volume produce pairs of electrons and ions along their tracks
- The electron clouds are drifted to the anode (or sense) wires by an electric field where they are amplified (like in a proportional counter)
- The positive charge produced by the amplificationis detected by the nearby pads
- The signals from these pads are amplified, shaped, and digitized
- The position of the pads gives the *x* and *y* coordinates
- The *z* coordinate is given by the arrival time

## The STAR-FTPC

**The STAR-FTPC**

- Conventional TPCs:
**Axial** drift field; thus readout at the endcaps
- Gas: Argon/methane or Ar/CO2 mixture ratio approx. 9:1
- High multiplicity and track density requi-res **radial** drift field which provides bet-ter position and two-track resolution
- Bent readout chambers at the cylin-der’s surface
- “Cool gas”: Ar/CO2 mixture ratio 1:1 for smaller electron diffusion whichalso reduces deflection due to Lorentz force

## Why Radial Drift?

**Why Radial Drift?**

- View from the endcaps
- Short tracks ( 1 cm) as the par-ticles are moving towards the end-caps
- Two-track resolution > 1 cm
- View from the cylinder’s surface
- Longer tracks ( 1 m)
- Much better two-track separation (if diffusion is low): 1 ... 3 mm
- Disadvantages
- Needs bent readout chambers
- Axial B-field + radial E-field  Lorentz force
- Thus deflection of drifting electrons

- E

- v

- B

## Why Do We Need A Calibration?

- Applications for a calibration with radioactive krypton
- Precise calibration of electronics and gain variations
- Cross-check of the pulser calibration of electronics response
- Check of detector linearity over a wide energy range
- Investigation of the long term detector stability by compari-son of gain over longer periods of time
- Advantage compared to pulser calibration
- Pulsing the sense wires induces a signal on all pads which produces a large current load on each FEE channel
- Single tracks and Kr-events produce a signal on 2 to 6 adja-cent pads only  lower current demand on electronics

## Why Krypton-83?

- Produces electron energy deposition in the important en-ergy range of 9 to 42 keV which corresponds to the ex- pected energy deposition of particles produced in heavy ion collisions (primarily protons and pions)
- Is useful for large chambers because it is a gas which can be distributed over the volume with the existing gas system
- Mother isotope Rb-83 has a long lifetime (** = 124 d), is a solid, and thus may be placed into the gas stream via a bypass line
- Short lifetime (** = 2.6 h) of Kr-83 ensures that no disposal of radioactive gas is necessary since the chamber may be operated normally after about three half-lives have passed subsequent to the cut-off of krypton into the chamber

## Krypton Properties

**Krypton Properties**

- Produces electron energy deposition in the important energy range of 9 to 42 keV which corresponds to the expected energy deposition of particles produced in heavy ion collisions at STAR
- Isomeric 42 keV level decays entirely to the 9 keV level by IC
- In addition, Auger electrons and X-rays are produced

- Simulation

- Rb-83

- Kr-83

- 571

- 562

- 42

- 9.4

- 0

- 30%

- 62%

-  6%

- keV

- 26%       74%

- 1.83 h
- 147 ns

- 86.2 d

**keV**

## Calibration With Kr-83

**Calibration With Kr-83**

- Detect decays of 83mKr and compare measured with known spectrum  energy calibration
- NA 49 Krypton calibration (so called kalibration) result:

## The Test-Setup

**The Test-Setup**

- Ar/CO2 gas input

- bypass line         solid Rb-83

- Kr-83 particles

- decaying Kr-83 particle

- e- are drifted to the anode wires

- FEE card

- flange

- sense wires and pads

- gas output

- Same conditions (materials, gas mixture, voltages) as in the FTPCs except drift field geometry
- 32 channels compared to 19,200 of the two FTPCs

## Status And Outlook

- RHIC is not yet running
- FTPCs are under construction
- Scheduled to install one FTPC in March 2000
- Calibration of a small test setup is in process