1. Particle Detectors - PHY743 -
Detect charged particles (e, , , K, …)
Detect the neutral particles (n and ) or separate neutral from charged
Determine the time reference
Determine the position reference
Determine the energy deposition
Identify the type of particle
Tools and instrument to scope and sense the microscopic elements

2. Detectors Based on EM radiations I
Excitation and followed by de-excitation
Example – Scintillation by charged particles
Crystal (fast, good optics, less rad. damage, but expensive)
Plastic (fast, easy to shape, much cheaper for large volume)
Wave length shifter (Ultraviolet to blue/green)
Light output proportional to energy loss of the charged particle depending on radiator thickness and particle energy/momentum (if below minimum ionizing energy), i.e. Light intensity I  X and dE/dx character of the particle
Charged particle
EM energy ()
Molecular electron
ex.
g.s.
 (light), collected by detector
~10-9 sec
ex.
g.s.

3. Detectors Based on EM radiations I – Cont.
Nuclear collision followed by scintillation
Medium – Organic (plastic) scintillator (CH), i.e. rich amount of H
Need large thickness to compensate low collision cross section (thus low efficiency)
Neutron detection – Head on collision
Undetectable neutron (n)
Stationary proton (p) in the radiator
Recoil n
Knocked out p that causes scintillation by the radiator

4. Detectors Based on EM radiations I – Cont.
Direct absorption or Compton scattering of low energy 
Example – Ge-detector (heavy crystal)
E < ~2 MeV
Low rate capability (long signal process time)
Low efficiency
Extremely high energy resolution, E ~ 3 keV for 1 MeV 
Example – BGO-detector (crystal)
Higher  energy detection range than Ge-detector
Less energy resolution than Germanium scintillator
Both of them very expansive

5. Detectors Based on EM radiations I – Cont.
Basic Structure of a Scintillation Detector
Scintillator: Convert energy loss to photons
Light guide: Guide photons to Photomultiplier (PMT) (Issues: Cross section matching and collection efficiency – Ultra Violate Transmitting Lucite)
PMT: Convert photons to photo-electrons then amplifying them by ~105 to 108 times (Issue: Response function, gain, rise/transit times, and linearity for different applications)
Scintillator (Radiator)
PMT
Light Guide

6. Detectors Based on EM radiations I – Cont.
Illustration of Signal Process by a Single Scintillation Detector
Analog signals: SL and SR, i.e. V(t). [Si  Total energy loss, SL/SR  position (low precision). Issue: Signal/Noise Ratio]
Discriminator: Generate ( V with adjustable width ~ ns) logic signal Ti at the time Si passes Vth.
Ti registers the detection of charged particle and references the detection time
Coincidence of TL and TR – removes noise signals from PMT’s (Accidental coincidence rate: RLRRT)
Average of TL and TR: Meaning time – More precise detection time
TL /TR  position (better than SL /SR ration, still low precision – mms to cms)
Used as a VETO to separate charged and neutral particles
Charged Particle TL SL SR TR
Disc
Vth
Disc
Vth

7. Detectors Based on EM radiations I – Cont.
Example of Scintillation Detector for Charged Particles
S1.AND.S2 defines a charged particle detection (Real or Accidential - Rate)
Overlap configuration gives fast position determination (Limited precision but fast – high rate tracking)
Time of Flight (TOF) = T2 – T1 determines  { = L/(cTOF)} (Separation of particles with different masses)
Ti – Time of particle detection
S1 Plane
S2 Plane
Charged particle with known or within a range of momentum
L – Length of particle trajectory

8. Detectors Based on EM radiations I – Cont.
Example of Scintillation Detector for dE/dx to separate none minimum ionizing particles within a momentum range
Example of Scintillation Detector as VETO and Neutron Detection
TOF
Thick S2 dE/dx
Thin S1 Front
TOF vs dE/dX can separates particles with different masses within a momentum range
(Range Detector)
Incident p
n beam
Vetoed
VETO
n Counter Ray
Identified as n
Scint. Stack
TOF for p

9. Detectors Based on EM radiations I – Cont.
Example for Low Energy Photon Detection
Precision measurement of EM transitions of nuclei
BGO Ge
Light Guide
PMT
BGO Ge
Low energy 
High energy  background

10. Detectors Based on EM radiations II
Čerenkov radiation
(charged particle)=v/c > o(speed of light in medium)=1/n
Č radiation
EM radiation
Well formed wave front  A B
cos = 1/(n)
Take place when n  1
Well formed wave front
Č radiation

11. Detectors Based on EM radiations II – Cont.
Features of Čerenkov radiation
Instantaneous (direct EM radiation by charged particles)
Well defined orientation: Cos = 1/(n) for n  1
Radiation energy  4
Low radiation power in general. Number of total photoelectrons from PMT photo cathode can be expressed as:
NPE = ALSin2
 – Radiation angle w.r.t. particle trajectory
L – Length of particle trajectory in the radiator
A – Characteristic constant that depends on light collection efficiency and quantum efficiency of the PMT. The typical A is about 100.
Application for Particle Identification

12. Detectors Based on EM radiations II – Cont.
Gas Čerenkov Detector (n  1)
Typically used in hadronic beam line
Size is very large (large L to compensate small )
PMT for particles with lower  (or higher mass)
PMT for particles with higher  (or lower mass)
Charged particle w/ small momentum and angular ranges
Gas cylinder
Tilted reflection mirror

13. Detectors Based on EM radiations II – Cont.
Threshold Čerenkov Detector
Low n solid radiator: Aerogel (n=1.01 to 1.06)
Čerenkov Radiator (n)
For particle with n > 1 there is a Č from radiator, light diffused to PMT’s
No Č from the radiator for particles with n < 1
PMT
Light diffusion box

14. Detectors Based on EM radiations II – Cont.
Threshold Č Detector (Total Internal Reflection)
Light absorption material
Radiator with c
Particle with high  ( > c)
Particle with low  ( < c)
PMT
Radiation is absorbed
(thus mass) threshold is set by critical angle c
Suitable for near normal incident , small angular & momentum spread
Radiation is transmitted by total internal reflection to PMT’s

15. Detectors Based on EM radiations II – Cont.
Radiation Energy ( Sin2) Threshold Detector
Using 2-D  vs light output size to separate particles with different masses
Čerenkov Radiator (n)
Light diffusion box
PMT
Particle with lower 
Smaller total light output
Particle with higher 
Larger total light output

16. Detectors Based on EM radiations III
Pair (e+e-) Production by Photon ()
Annihilation of e+ or e-
Continued process til E < MeV – EM Shower 
Nucleus e+ e-
Generate scintillation or Č radiation
Heavy crystal is better
E min. > MeV 
e+ or e-
Nucleus
Two real photons (2)
Heavy material is better

17. Detectors Based on EM radiations III – Cont.
Shower Counter by Pb-Glass (Crystal)
Radiation length and radiator thickness
Total energy measurement and precision
Particle Identification (PID)
PMT on each optically separated unit
Pb-Glass Array
A shower produces large overall light output by Č radiations
,e+, or e-
Heavy charged particles
Č radiation without shower process

18. Detectors Based on EM radiations III – Cont.
Shower Counter by Pb-Scint. Sandwich
Thin Pb plates sandwiched by scintillators
PMT
Light
guide
A shower from Pb produces large overall light output by Scintillation
,e+, or e-
Heavy charged particles
Scintillation (dE/dx) without shower process
Other Type:
Pb-Scint. Fiber Detector
Less energy resolution
but cheap in cost
Large size for high energy leptons

19. Your Exercise
A pair of scintillation counters forms a TOF hodoscope. If overall timing resolution is  = 100ps, what is the minimum path length needed to have a 4 separation for + and K+ at p=1.2GeV/c.
A Č radiator has n=1.5. A screen that views the Č radiation ring located 5cm behind the radiator. When +, K+, and p travel through them in the normal direction with p=1.3 GeV/c, what are the inner image ring diameters for the three types of particle?
If you have 1mm Pb plates and 5mm thick scintillator plates, to build a Pb-Scint. Sandwich shower counter with 10 radiation lengths (to ensure absorption of total energy), what will be the detector overall thickness?

20. Detectors Based on Ionization
Ionization by charged particle
Energy of ions in presence of electrostatic field
Secondary ionization and avalanche (r0)
Overall charge gain
Charged
Particle
Free Electron
Electric Field
Ion
Ionization
Charged Particle
Gas Molecule + -
- Ion
+ Ion
Charged Particle

21. Detectors Based on Ionization – Cont.
The simplest example: Geiger Counter
Ionization gas: Argon
Quench gas: Halogens
Detect ionization particles or photons by a short CASCADE (gas ionization) effect that gives an electric pulse
HV: Enough for CASCADE but not continued “breakdown”
Commonly used as radiation monitor for  particle, -ray, and even X-ray
Cathode Tube
Gas mixture
Anode Wire
Ionization particle or photon
Electronic Counter
Amplifier

22. Detectors Based on Ionization – Cont.
Multi-Wire Proportional Chamber – Drift Chamber
Charged Particle & Initial Ionization (Few Pairs) + -
Electric Field Lines
Cross Section View of One Coordinate Plane
Cathode Foils at -HV
One Cell
Avalanche takes place near the sense wire (r ~ 0)
Sense Wires at Ground
Field Wires at -HV
Gas Mixture: Ar (Ionization) + Ethane (Quench)

23. Detectors Based on Ionization – Cont.
Gas Mixture
Argon – Maximize the ionization rate
Ethane – Larger molecule, collision rate for constant velocity, quench
Drift Time
Basic Electronics
Position
Drift Time
Slope - Velocity
Sense wire
Pre-Amplifier
Long distance bus
Amplifier - Discriminator
To DAQ

24. Detectors Based on Ionization – Cont.
Initial ionization: pairs (path length & pressure)
Positive ions:
>2000 times slower than negative ions
Detector rate capability and efficiency depends on the + charge collection and gas recover/refresh speed
Negative ions (electrons):
Accelerate alone the field direction and gain kinetic energy
Continued secondary ionization (amplification ~10)
Consecutive “collisions” and ionizations make the drift velocity near constant
Avalanche near sense wire: charge amplification ~103
Overall charge gain: ~104 (an electric pulse)
Drift time gives the position Resolution:   mm (single cell)

25. Detectors Based on Ionization – Cont.
Basic Position Determination by Drift Chamber
Off-Set Planes to Remove Left-Right Ambiguity
T0 references ZERO drift
TDC - T0 gives the position
-HV
PMT
Disc.
TDC Start
Single Drift Chamber Plane
Amp
Disc.
TDC Stop
Left – Right ambiguity
X - Plane
X’ - Plane
TL + TR = Constant

26. Detectors Based on Ionization – Cont.
Track Particle Trajectory by Multiple Planes
2 Fitting to Determine a Straight Trajectory Line
Two Separated Sets
X, X’
U, U’
V, V’ 
Each set measures
x 6 times and y 4 times Z

27. Detectors Based on Ionization – Cont.
Example: Application of Cylindrical Drift Chamber
Solenoid York Iron
Constant Axesial B Field
Cylindrical Chambers
Inner Fast Detectors
Circular Motion Due to Transverse Momentum

28. Detectors Based on Ionization – Cont.
Other Type of Gas Chamber – MWPC
Small wire spacing and gap (w/o field wires)
Position determined by wire position – low precision
Faster (smaller cells and none-constant drift velocity)
Can be 100 times higher rate per wire than DC
Cheaper than other high rate tracking devices
TDC
Amp
Disc
Anode on ground
Field Lines
Single MWPC Plane
- HV on the cathode

29. Detectors Based on Ionization – Cont.
Other Type of Gas Chamber – Vertical DC (VDC)
Large Gap – Vertical/constant v for secondary charge drift
Measure several drift times for each particle
Provide position and angles at the same time
Particles must incident with angles
High precision and effective but low rate capability
Field Lines
TDC
Amp
Disc
Anode on ground
Single VDC Unit
- HV on the cathode

30. Detectors Based on Ionization – Cont.
High Rate Chamber – Gas Electron Multiplier (GEM)
Still Gas Ionization and Avalanche, again, but…
A different way to get an intense electric field,
Without dealing with fragile tiny wires, and
Release + ions much faster -V
~400v
0.002”
GEM
To computer

31. Detectors Based on Ionization – Cont.
Solid State Tracking Detector – Silicon Strip Detector (SSD)
Fast, thus high rate capability
Fine pitch, thus high precision
Compact
Much More expansive for large size
Radiation Damage

32. Effect to Particles by Detectors Other Than Detection
Energy Loss – such as dE/dx effect
Multiple (Coulomb) Scatterings
MCS theory is a statistical description of the scattering angle arising from many small interactions with atomic electrons.
MCS alters the direction of the particle.
Most important at low energy.
 is particle speed, z is its charge, and X0 is the material’s Radiation Length.   

33. Put It All Together: A Detector System Example by Hall C HMS

34. Summary of Particle Detectors
Detect Particles by Letting them Interact with Matter within the Detectors.
Choose appropriate detector components, with awareness of the effects the detectors have on the particles.
Design a System of Detectors to provide the measurements we need.