# Presentation: 513002

**Why Do All Those Damned Detectors Look The Same?**
**Jim Thomas**
**Lawrence Berkeley Laboratory**
**January 11, 2004**

## ATLAS vs PHENIX vs ....

**They are about the same size**
**They are about the same shape**
**Are they really different?**

**PHENIX**

**ATLAS**

**Even fixed target detectors look like an angular slice of one of these detectors**

## References

_**Outstanding References**_
**Particle Properties Data Booklet **
**Particle properties**
**Excellent summaries of particle detection techniques**
_**[http://](http://pdg.lbl.gov/)**__**[pdg.lbl.gov](http://pdg.lbl.gov/)**_** to view the pages or order your own copy**
**Sauli’s lecture notes on wire chambers (CERN 77-09)**
**W. Blum and L. Rolandi, ****“Particle Detection with Drift Chambers”****, Springer, 1994.**
_**This talk relies heavily on additional resources from the Web**_
**C. Joram ****	****–   CERN Summer Student Lectures 2003**
**T.S. Verdee ****	****–   SUSSP 2003**
**S. Stapnes ****	****–   CERN School of Phyics 2002**

## The Oldest Particle Detector –  and a good one, too.

**High sensitivity to photons**
**Good spatial resolution**
**Large dynamic range 1:10****14**
**(Once upon a time)**** Used to tune cyclotron beams via scintillation light**

## What should a particle detector do?

**Note the scale pasted on the outside of the tube!**
**Glass scintillates and we “see” the effect on the electron beam**
**Today ... mean pt is 500 MeV so we need a meter of steel and concrete to stop the particle and make a total energy measurement.**

## First electrical signal from a particle

- E. Rutherford

- H. Geiger

- 1909

**The Geiger counter**

- pulse

## First tracking detector

- C. T. R. Wilson,
- 1912, Cloud chamber

- The general procedure was to allow water to evaporate in an enclosed container to the point of saturation and then lower the pressure, producing a super-saturated volume of air. Then the passage of a charged particle would condense the vapor into tiny droplets, producing a visible trail marking the particle's path.

## Detector Philosophy

**Particles are detected by their interaction with matter**
**Many different physical principals are involved**
**Electromagnetic**
**Weak**
**Strong**
**Gravity**
**Most detection techniques rely on the EM interaction**
**Although, all four fundamental forces are used to measure and detect particles**
**Ultimately, we observe ionization and excitation of matter.       In this day and age, it always ends up as an electronic signal.**

## Interaction of Charges Particles with Matter

**Coulomb Scattering**
**An incoming particle with charge ****z**** interacts with **
**a target of nuclear charge ****Z****. The cross-section **
**for this e.m. process is**
**Average scattering angle  ********θ******** = 0 **
**Cross-section for   ****θ**** ****→**** 0   is infinite !  **
**This implies that there will be many soft scattering events.**
**Multiple Coulomb Scattering**
**In sufficiently thick material layer ******** the particle will undergo **
**multiple scattering.  There will be **_**angular deflections**_** and **_**energy loss**_**.**

- Rutherford formula

**Radiation Length**

## How do particles lose energy in matter?

**Bethe-Bloch Formula**

**How do particles lose energy in matter?**

**density effect**

**ionization constant**

- “relativistic rise”

-  “minimum ionizing particles”    3-4*** ***

- “kinematic term”

**Notes:**
Discrete collisions with the atomic electrons of the absorber material.  
Consider only ionization.
Collisions with nuclei not important (me<<mN).
dE/dx in [MeV g-1cm2]
Bethe-Bloch formula only valid for “heavy” particles (mm).
dE/dx depends only on independent of m !
First approximation: medium characterized by electron density,      N ~ Z/A.

## dE/dx depends on 

**dE/dx depends only on ********and is********independent of mass**
**Particles with different masses have different momenta (for same ********)**
**dE/dx in [MeV g****-1****cm****2****]**
**in a gas detector this gets shortened to keV/cm.**
**First approximation: medium characterized by electron density,  N ~ Z/A.**

**Pep 4 TPC**

## Landau tails

**For ****thin layers**** ******** Few collisions, some with high energy transfer.**
****** ****Energy loss distributions show large **
**     ****fluctuations towards high losses: **
**    ****”**_**Landau tails**_**” ****due to**** “**_****__** electrons**_**”**

**For ****thick layers**** and high density materials ******** Many collisions**
****** ****Central Limit Theorem ******** Gaussian shape  distributions. **

**Real detectors ****(limited granularity)**** can not measure ****<dE/dx> .**** **
**They measure the energy ********E**** deposited in a layer of finite thickness ********x.**

- E

- <E>

## Ionization of gases

**Fast charged particles ionize the atoms of a gas.**
** **
**Often the resulting primary electron will have enough kinetic energy to ionize other atoms****. **

- Primary ionization

- Total ionization

- 10 - 40 pairs/cm
- E/pair ~ 25 eV

**Assume detector, 1 cm thick, filled with Ar gas:**

** ****1 cm**

**~ 100 e-ion pairs **

**100 electron-ion pairs are not easy to detect!**** **
**Noise of amplifier ******** 1000 e****-**** (ENC) !**
**We need to increase the number of e-ion pairs.**

## Gas Amplification in a Proportional Counter

**Consider cylindrical field geometry (simplest case):**

*C* = capacitance / unit length

**Electrons drift towards the anode wire**
**Close to the anode wire the electric field is sufficiently high (kV/cm), that the e****-**** gain enough energy for further ionization ********  **_**exponential increase**_** in the number of e****-****-ion pairs.**

## Avalanche form within a few radii or the wire and within t < 1 ns!
Signal induction both on anode and cathode due to moving charges (both electrons and ions).

- Signal induction both on anode and cathode due to moving charges (both electrons and ions).

**Signal Formation - Proportional Counter**

- Electrons are collected on the anode wire, (i.e. *dr* is small, only a few m). Electrons contribute only very little to detected signal (few %).

- Ions have to drift back to cathode, i.e. *dr* is big. Signal duration limited by total ion drift time !

- We need electronic signal differentiation to limit dead time.

- (F. Sauli, CERN 77-09)

- (F. Sauli, CERN 77-09)

## Multiwire Proportional Chamber

- (G. Charpak et al. 1968, Nobel prize 1992)
- Address of fired wire(s) only give 1-dimensional information.  This is sometimes called “Projective Geometry”.   It would be better to have a second dimension ....

**Multiwire Proportional Chamber**

- Normally digital readout:
- spatial resolution limited to

- ( d=1mm, x=300 ****m )

- Typical parameters:
- L = 5mm, d = 1mm, rwire= 20m.

## The Second Dimension ... 2D readout

-  Timing difference:
-  Segmented cathode planes:

**The Second Dimension ... 2D readout**

- Crossed wire planes. Ghost hits. Restricted to low multiplicities. 90 degrees or stereo planes crossing at small angle.

- Analog readout of cathode
- planes     100 m

## Timing Difference: Drift Chambers

**Drift Chambers : **
**Reduced numbers of readout channels**
**Distance between wires typically 5-10cm giving around 1-2 ********s drift-time**
**Resolution of           50-100********m achieved limited by field uniformity and diffusion**
**Perhaps problems with **_**occupancy**_** of tracks in one cell.**

- (First studies: T. Bressani, G. Charpak,  D. Rahm, C. Zupancic, 1969
-  First operation drift chamber: A.H. Walenta, J. Heintze, B. Schürlein, NIM  92 (1971) 373)

## Drift Chambers: Many Possible Designs

**Drift Chambers: Many Possible Designs**

- (U. Becker, in: Instrumentation in High Energy Physics, World Scientific)

**What happens during the drift towards the anode wire?**
**We need to know the drift velocity**
**Diffusion, too.**

## Drift and Diffussion in Gases

- Without external fields:
- Electrons and ions will lose their energy due to collisions with the gas atoms  thermalization
- Undergoing multiple collisions, a localized ensemble of charges will diffuse

*D*: diffusion coefficient

- With External electric field:

- multiple collisions due to
- scattering from gas atoms ****** **** **drift

- Typical electron drift velocity:** ****5 cm/********s**
- Ion drift velocities are  ~1000 times smaller

## 3D: The Time Projection Chamber

**3D: The Time Projection Chamber**

- Time Projection Chamber **** full 3-D track reconstruction
- x-y from wires and segmented cathode of MWPC
- z from drift time
- in addition dE/dx information

- Diffusion significantly reduced by B-field.

- Requires precise knowledge of vD  LASER calibration + p,T corrections

- Drift over long distances  very good gas
- quality required

- Space charge problem from positive ions, drifting back to midwall  use a _gated grid_

- Gate open

- Gate closed

- Vg = 150 V

- ALEPH TPC
- (ALEPH coll., NIM A 294 (1990) 121,
- W. Atwood et. Al, NIM A 306 (1991) 446)

- Ø 3.6M, L=4.4 m

- Rm
- zm
- (isolated leptons)

**STAR TPC**

## Momentum Measurement in a Uniform Field

***( N > 10 )     ***

**Momentum Measurement in a Uniform Field**

## Momentum Resolution: the STAR Magnet + TPC

**Momentum resolution is only limited by the strength of the magnetic field and is independent of the mass of the particle at high P****T **
**Momentum resolution at low P****T**** is determined by multiple coulomb scattering (MCS)**

**MCS Contribution**

**B Field limited**

## Semiconductor Detectors: Silicon

**Interaction with ionizing radiation**

**Reading out the pixels**

**The typical Semiconductor detector is a based on a Si diode structure**

## The Properties of Silicon

**Si Structures are small and can be mass produced in large arrays**
**Ideal for locating a point on the track of a particle**

## Scintillation Light: Inorganic Scintillators

- PbWO4 ingot and final polished CMS ECAL scintillator crystal from Bogoroditsk Techno-Chemical Plant (Russia).

## Inorganic Scintillators: NaI, BGO, PbWO4, ...

- Excitation of electrons into the conduction band allows light to be produced during relaxation to the ground state.
- Inorganic scintillators are usually high density and high Z materials
- Thus they can stop ionizing radiation in a short distance

## Scintillation Light: Organic Scintillators

**Liquid and plastic organic scintillators are available**
**They normally consist of a solvent plus secondary (and tertiary) fluors as wavelength shifters. **

## Scintillator Readout Schemes

**PhotoDetector**

**Scintillator Readout Schemes**

**Geometrical adaptation:**
- Light guides: transfer by total internal reflection  (+outer reflector)
**Wavelength shifter (WLS) bars**

**“****fish tail”**

## Photo Multiplier Tubes  (PMT)

- Main phenomena:
- photo emission from photo cathode.
- secondary emission from dynodes.
- 	dynode gain *g *= 3-50 (f(E))
- total gain
- 	10 dynodes with g=4
- 	*M* = 410  106

- (Philips Photonic)

## Scintillator Applications

- Tracking
- Charged particle passing through a stack of scintillating fibers
- (diam. 1mm)

- Sampling Calorimeters
- Absorber + detector separated   additional sampling fluctuations

- Time of Flight
- Measure the time of flight of a particle between a thin, flat,  “start” counter and a thin “stop” counter.

## Measurement of Energy: Calorimeters

- Big European Bubble Chamber
- filled with Ne:H2 = 70%:30%
- 3T field, L=3.5m, X0=34 cm
- 50 GeV incident electron

## Electromagnetic Cascade

## Electromagnetic Cascade: Longitudinal Development

## Radiation Length and the Moliere Radius

## Hadronic Cascade

## 150 GeV Pion Showers in Cu

- Hadron shower not as well behaved as an em one

- Hadron calorimeter are always sampling calorimeters

## Hadronic Cascade: Profiles

## Lets Design a Detector:  Requirements 

**Very good particle identification**
-     trigger efficiently and measure ID and momentum of all particles
**High resolution electromagnetic calorimetry**
**Powerful inner tracking systems**
-     Improves momentum resolution, find tracks of short lived particles
**Hermetic coverage**
-     good rapidity coverage, good missing ET resolution
**Affordable detector**

## ‘Cylindrical Onion-like’ Structure of  HENP Detectors

## The STAR Detector at RHIC

**Not Shown:  pVPDs, ZDCs, PMD, and FPDs**

## Conclusions

**We have taken a random walk through a variety of detector technologies and put the pieces together into a detector**
**You can repeat this exercise using the PDG booklet (!)**
**It contains a wealth of information**
**It is extremely well written and only contains the most essential information**
**The design of HENP detectors is driven by the desire to measure the ID and momentum of all particles in the range from 100 MeV to 100 GeV.**
**all 4 components of the momentum 4-vector  (E, p****x****, p****y****, pz)**
**all 4 components of the spacial 4-vector  (ct, x, y, z)**
**If you can afford to do this with full 4******** coverage, then your detector will end up looking pretty much like all the other big detectors.  However, there are big differences in the details and cost effectiveness of each detector design.**

## Two “Large” Detectors at RHIC

**Two “Large” Detectors at RHIC**

## Two “Small” Experiments at RHIC