Particle DetectorsParticle Detectors- PHY743 -- PHY743 -

Detect charged particles (e, , , K, …)

Detect the neutral particles (n and ) or separate neutral from charged

Determine the time referenceDetermine the position referenceDetermine the energy depositionIdentify the type of particle

Tools and instrument to scope and sense the microscopic elements

Detectors Based on EM Detectors Based on EM radiations Iradiations I

Excitation and followed by de-excitation

Example – Scintillation by charged particles1. Crystal (fast, good optics, less rad. damage, but expensive)

2. Plastic (fast, easy to shape, much cheaper for large volume)

3. Wave length shifter (Ultraviolet to blue/green)

4. 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.

ex.

g.s. (light), collected by detector

~10-9 sec

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)

Detectors Based on EM radiations I Detectors Based on EM radiations I – – Cont.Cont.

Recoil n

Knocked out p that causes scintillation

by the radiator

Undetectable

neutron (n)

Stationary proton (p)

in the radiator

Neutron detection – Head on collision

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

Detectors Based on EM radiations I Detectors Based on EM radiations I – – Cont.Cont.

Basic Structure of a Scintillation Detector

Scintillator: Convert energy loss to photonsLight 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)

Detectors Based on EM radiations I Detectors Based on EM radiations I – – Cont.Cont.

Scintillator

(Radiator)

Light Guide

PMTPMT

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 (0.6-0.8V with adjustable width ~10-100 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: RLRRT)

◦ 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

Detectors Based on EM radiations I Detectors Based on EM radiations I – – Cont.Cont.

SR

Disc

VthTR

Disc

VthTL SL

Charged Particle

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/(cTOF)} (Separation of particles with different masses)

Detectors Based on EM radiations I Detectors Based on EM radiations I – – Cont.Cont.

S1 Plan

e

S2 Plane

Charged particle with

known or within a range of momentumL – Length of

particle trajectory

Ti – Time of particle

detection

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

Detectors Based on EM radiations I Detectors Based on EM radiations I – – Cont.Cont.

Thin S1

Front

Thick S2

dE/dx

TOFTOF vs dE/dX can separates particles with different masses within a momentum range(Range Detector)

VETO n Counter Ray

Vetoed

Identified as n TOF for

p

Scint. Stack

Incident p

n beam

Example for Low Energy Photon Detection

Precision measurement of EM transitions of nuclei

Detectors Based on EM radiations I Detectors Based on EM radiations I – – Cont.Cont.

BGO

PMT BGO

Ge

Low energy

High energy

background

Ge Light Guide

Čerenkov radiation(charged particle)=v/c > o(speed of light in medium)=1/n

A B

EM radiation

Well formed wave front

Č radiation

Č radiation

Well formed wave front

cos = 1/(n)

Take place when n 1

Detectors Based on EM Detectors Based on EM radiations IIradiations II

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 = ALSin2 – Radiation angle w.r.t. particle trajectoryL – 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

Detectors Based on EM radiations II Detectors Based on EM radiations II – – Cont.Cont.

Gas Čerenkov Detector (n 1)

Typically used in hadronic beam lineSize is very large (large L to compensate

small )

Detectors Based on EM radiations II Detectors Based on EM radiations II – – Cont.Cont.

Gas cylinder Tilted reflection

mirror

Charged particle w/ small momentum and angular

ranges

PMT for particles with

lower (or higher mass)

PMT for particles with higher (or lower mass)

Threshold Čerenkov Detector

Low n solid radiator: Aerogel (n=1.01 to 1.06)

Detectors Based on EM radiations II Detectors Based on EM radiations II – – Cont.Cont.

Light diffusion box

Čerenkov Radiator (n)

PMT PMT

No Č from the radiator for particles with n < 1

For particle with n > 1 there is a Č from radiator, light diffused to PMT’s

Threshold Č Detector (Total Internal Reflection)

Detectors Based on EM radiations II Detectors Based on EM radiations II – – Cont.Cont.

PMT PMT

Radiator with c

Light absorption material

Particle with low ( < c)Radiation is absorbed

Particle with high ( > c)Radiation is transmitted by total internal reflection to PMT’s

(thus mass) threshold is set by critical angle c Suitable for near normal incident , small angular & momentum spread

Radiation Energy ( Sin2) Threshold Detector

Using 2-D vs light output size to separate particles with different masses

Detectors Based on EM radiations II Detectors Based on EM radiations II – – Cont.Cont.

PMT PMT

Light diffusion box

Čerenkov Radiator (n)

Particle with lower Smaller total light output

Particle with higher Larger total light output

Pair (e+e-) Production by Photon ()

Annihilation of e+ or e-

Continued process til E < 1.022 MeV – EM Shower

Detectors Based on EM Detectors Based on EM radiations IIIradiations III

E min. > 1.022 MeV

Nucleus e

+

e-

Generate scintillation or Č

radiationHeavy crystal is

better

e+ or e-

Nucleus

Two real photons (2)

Heavy material is better

Shower Counter by Pb-Glass (Crystal)

Radiation length and radiator thicknessTotal energy measurement and

precisionParticle Identification (PID)

Detectors Based on EM radiations III – Detectors Based on EM radiations III – Cont.Cont.

Pb-Glass Array

PMT on each optically

separated unit

,e+, or e-

A shower produces large

overall light output by Č radiationsHeavy charged

particles

Č radiation without shower process

Shower Counter by Pb-Scint. Sandwich

Detectors Based on EM radiations III – Detectors Based on EM radiations III – Cont.Cont.

Thin Pb plates sandwiched by scintillators

PMT PMT PMT

Lightguide

,e+, or e-

A shower from Pb produces large overall

light output by Scintillation

Heavy charged particles

Scintillation (dE/dx) without shower process

Less energy resolutionbut cheap in cost

Other Type:Other Type:Pb-Scint. Fiber

Detector

Large size for high energy leptons

Your ExerciseYour Exercise

1. 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.

2. 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?

3. 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?

Ionization by charged particle

Energy of ions in presence of electrostatic field

Secondary ionization and avalanche (r0)Overall charge gain

Detectors Based on Detectors Based on IonizationIonization

ChargedParticle

Free Electron

Electric Field

Ion

Ionization

+-

Charged Particle

Charged Particle

Gas Molecul

e

+ Ion

- Ion

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

Detectors Based on Ionization Detectors Based on Ionization – – Cont.Cont.

to computer

HV Supply

Anode Wire

Cathode Tube

Amplifier

Gas mixture

Ionization particle or

photon

Electronic Counter

Multi-Wire Proportional Chamber – Drift Chamber

Detectors Based on Ionization – Detectors Based on Ionization – Cont.Cont.

Cross Section View of One Coordinate

Plane

Cathode Foils at -HV

Field Wires at -

HV

Sense Wires at Ground

Gas Mixture: Ar (Ionization) + Ethane (Quench)

One Cell

Electric Field Lines

Charged Particle & Initial Ionization

(Few Pairs)

+-

- +

Avalanche takes place

near the sense wire (r ~ 0)

Gas Mixture Argon – Maximize the ionization rate Ethane – Larger molecule, collision rate for constant

velocity, quench

Drift Time

Basic Electronics

Detectors Based on Ionization – Detectors Based on Ionization – Cont.Cont.

Position

Drift Time Slope -

Velocity

Sense wire Pre-

Amplifier

Long distance bus

Amplifier - Discriminator

To DAQ

Detectors Based on Ionization – Detectors Based on Ionization – Cont.Cont.

Initial ionization: 2-10 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: 0.2-0.3 mm (single cell)

Basic Position Determination by Drift Chamber

Off-Set Planes to Remove Left-Right Ambiguity

Detectors Based on Ionization – Detectors Based on Ionization – Cont.Cont.

Single Drift Chamber Plane

-HV PMT Disc.

TDC Start

Amp

Disc.

TDC Stop

T0 references ZERO drift

TDC - T0 gives the position

Left – Right ambiguity

TL + TR = Constant

X - PlaneX’ - Plane

Track Particle Trajectory by Multiple Planes

2 Fitting to Determine a Straight Trajectory Line

Detectors Based on Ionization – Detectors Based on Ionization – Cont.Cont.

Two Separated Sets

Z

X, X’

U, U’V, V’

Each set measures x 6 times and y 4

times

Example: Application of Cylindrical Drift Chamber

Detectors Based on Ionization – Detectors Based on Ionization – Cont.Cont.

Solenoid York IronConstant Axesial B Field

Cylindrical Chambers

Inner Fast Detectors

Circular Motion Due to Transverse Momentum

Other Type of Gas Chamber – MWPC

Small wire spacing and gap (w/o field wires)Position determined by wire position – low

precisionFaster (smaller cells and none-constant drift

velocity)Can be 100 times higher rate per wire than DCCheaper than other high rate tracking devices

Detectors Based on Ionization – Detectors Based on Ionization – Cont.Cont.

Single MWPC Plane

- HV on the cathode

TDCAmp

Disc

Anode on ground

Field Lines

Other Type of Gas Chamber – Vertical DC (VDC)

Large Gap – Vertical/constant v for secondary charge drift

Measure several drift times for each particleProvide position and angles at the same timeParticles must incident with anglesHigh precision and effective but low rate

capability

Detectors Based on Ionization – Detectors Based on Ionization – Cont.Cont.

Single VDC Unit

- HV on the cathode

TDCAmp

Disc

Anode on ground

Field Lines

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

Detectors Based on Ionization – Detectors Based on Ionization – Cont.Cont.

http://gdd.web.cern.ch/GDD/

-V

~400v0.002”

GEM

To computer

Solid State Tracking Detector – Silicon Strip Detector (SSD)

Detectors Based on Ionization – Detectors Based on Ionization – Cont.Cont.

• Fast, thus high rate capability• Fine pitch, thus high precision• Compact• Much More expansive for large size• Radiation Damage

Energy Loss – such as dE/dx effectMultiple (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.

Effect to Particles by Detectors Effect to Particles by Detectors Other Than DetectionOther Than Detection

0

00 /ln 038.01/ 6.13

0

XxXxzcp

MeV

Put It All Together: A Detector Put It All Together: A Detector SystemSystem

Example by Hall C HMSExample by Hall C HMS

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.

Summary of Particle DetectorsSummary of Particle Detectors