summary of what seen so far overview of charged or neutral particle interaction in matter overview...

Summary of what seen so far

Overview of charged or neutral particle interaction in matter

Overview of detectors providing precise time measurement -> scintillators Need them for

Overview of detectors providing precise space measurement ->gaseous tracking chambers

Need them for

triggerlifetime measurementidentification of particles

direction, angle measurementmomentum measurement identification of particles (using dE/dx differences)

Gaseous tracking chambers

Typical resolution ?

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.200 micrometersspace pointresolutionis quite typical

Gaseous tracking chambers

What is the typical size (radial, longitudinal) at a collider experiment?

Hint : what particle property do we want to measure ? and what polar angle distribution do we want to observe ?

Radial : momentum measurement


are needed to see this picture.

s=0.3 L2 B / 8 pT

e.g. s=0.15 cm for pT =10 GeV (150 micrometers is resolution)

so typically need L~ meters

Longit. : have as much acceptance as possible to measure eg. differential cross sections, etc.. Depends on the goals of experiment. Typically ~ meters



Gaseous tracking chambers: literature

W.Leo pages 119 - 146D. Green pages 151 - 176

Peter’s notes on ISIS web site (all lecture slides are there !)

Problem for today

QuickTime™ and aTIFF (Uncompressed) decompressor


electron

positron

B0

B0

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.BaBar detector at Stanford Accelerator PEPII

Ecms=10 GeVY(4S) -> BB=0.56

Problem will be about evaluation of BaBar detector design



Pros and cons ? Is the design appropriate to the physics goals?Can we suggest improvements?

Babar physics goals which concern us today :

- Measure very precisely the travel distance of the two B mesons- Measure very precisely the momentum of the particles comingfrom B meson decays

The B meson travels a distance L and then decays into particles a and c

> The impact parameter “b” particle “a” also carries information about the lifetimeof the B meson. so it is important to be able to measure that too. What is theexpected value for “b” ? (hint: assume small)

b

L

a

cr

z

> What is the resolution needed to observe the decay length “L” and the impact parameter “b” ? We are happy if L / error(L) is > 3

B mesons (hadrons containing b quarks) have a mean lifetime = 1.5 picoseconds.At the PEP collider B mesons are produced with a boost factor ~ 0.5 > This means that they will travel on average a distance “L” = ?

A:

b

L

L=average distance travelled in mean lifetime by B meson =

c = 0.56 * 1.5 ps * 3 108 m/s= 230 micrometers

b ≈ L if is small

= pT /p

of decay particle B ~ MB/2 / pB/2 ~ 1/()B

=> b ~ c = 450 micrometers

a

c

to observe L at least a 3 sigma significance , meaning that L/error(L) >3,

we need maximal resolution to be 70 micrometers. For b is 150 micrometers.

Asking for 3 sigma is really the minimum, one should need more.

So we need a different tracking device than the gaseousOnes, whose resolution is too coarse. Which one ?

We need

> Smaller resolution (electronic readout with higher granularity)> particles should loose little energy compared to initial energy> produce electronic signal high enough to detect particle and also fast enough to be readout before next collision event occurs



silicon

Which one?

Goals of the lecture

Silicon detectorsReference: D.Green, pages 177-201. W.Leo, pages

Example of silicon detectors in past and current experimentsReference: slides (and web links)

Exercise : Pros and cons of the BaBar detector?

Vertex reconstruction and kinematic fitting. Reference : slides (and web links)

Identification of heavy quarks Reference: slides (and web links)

Semiconductors devices

(besides book reference, veryy usefull to browse here

http://jas.eng.buffalo.edu/index.html )

http://jas.eng.buffalo.edu/index.html

Solid state or semiconductor detectors are made of crystallinesemiconductor material, typically silicon or germanium

Development really started in 1950’s

At first used for high resolution energy measurement and wereadopted in nuclear physics for charged particle detection and gammaspectroscopy

Last 20 years, gained attention in high energy physics for highresolution fast tracking detectors.

Basic operating principle is similar to gaseous devices: charged particleionizes and creates electron-hole pairs which are the collected by anelectric field. Photons will also be detected in solid state detectors,via photoelectric effect and then electron ionizes.

When isolated atoms are brought together to form a lattice, the discrete atomic states shift to form energy bands as shown below. Affects only the outer energy levels of atoms.

Basic SemiConductor properties



Intrinsic conductivity of semiconductors

Thermal excitation of charge carriers across gap

http://jas.eng.buffalo.edu/education/semicon/fermi/functionAndStates/functionAndStates.html

http://jas.eng.buffalo.edu/education/semicon/fermi/functionAndStates/functionAndStates.html

n = density of electrons in the conduction band = 1/V ∫ f(E) g(E) dE

Where QuickTime™ and a

TIFF (LZW) decompressorare needed to see this picture.


are needed to see this picture.(density of states)

nQuickTime™ and a

TIFF (LZW) decompressorare needed to see this picture.

And similarly for holes

(Reference : http://britneyspears.ac/physics/basics/basics.htm)



http://jas.eng.buffalo.edu/education/semicon/fermi/levelAndDOS/index.html

http://britneyspears.ac/physics/basics/basics.htm







ni = AT 3/2 e (-Eg/2KT) ni= concentration of e (holes). Eg= energy gap at 0 Kelvin

Constantly :• e/h pairs are generated by thermal energy.• e and holes recombine.

equilibrium

T=0, no conduction T=300 K, pure Si, 1.5 10 10 cm-3 (Remember there are 1022 atoms cm-3)

-> Silicon is a poor conductor

e -Eg/2KT ~ 10-9

n electron= n holes in pure semiconductor

?

If one applies a electric field E to a semiconductor, e and holes start moving.

Drift velocity :ve = e E , vh = h E =mobility=f(E,T)

T=300K , E<103 V/cm : is constant E ~ 103 - 104 V/cm : ~ E-1/2

E >104 V/cm : ~ 1/E

saturation v=107 cm/s

~ T-m m=2.5 for e, 2.7 for holes in Si

e = 1350 cm2/Vs in Silicon -> v= 1.3 106 cm/s (gas was 105 cm/s)

J = current = e ni (e + h ) E

Conductivity ~ 1/ resistivity

Recombination and trapping

e can fall back into valence band, but need exact energy -> rareNonetheless lifetime for e and holes is ~ ns -> what happens ?

Impurities or defects in the semiconductor !

additional levels in the forbidden gap

time electron is free should be >> time takes to collect electron out of detector-> impurity concentration should typically be < 10 10 impurities cm-3

Recombination centre:This center can capture electron from conduction band and either release it back to the conduction band after a while or collect also a hole and e-hole annihilate

Trapping center:This center can only trap an electron or a hole. They hold it and then release it after a while.

http://jas.eng.buffalo.edu/education/semicon/recombination/indirect.html

http://jas.eng.buffalo.edu/education/semicon/recombination/indirect.html

N-Type

P, As, Sb5 electrons in the M-shell1 electron with binding energy 10-50 meV

B, Al, Ga3 electrons in the M-shell1 electron missing

P-Type

Doped SemiConductors

.. When doping is actually good :)



0.05 eVin Si

Amount of dopant is quite small typically (10 13 cm-3).

ND + n= NA + pIn n type NA=0 , ND~n p= ni

2 / ND

-> conductivity is = e ND e

Fermi levelmuch closer to conductive bandor valence band

Donor concentration determinesconductivity

… but how can we use a piece of Silicon fordetecting a high energy particle … ?

+ +

- - VIs this going towork?

Can you foresee anyProblems ?

Intrinsic silicon will have electron density = hole density ~ 1010 cm-3

In the volume above 4.5 108 free charge carriersBut : only 3.2 104 produced by MIP (dE/dx in 300um Si divided by 3.6 eV).

So, to use silicon as particle detector, we need to decrease number of free carriers

How?

We don’t like the thermal current !

- Reduce temperature ( need cryogenics, more expensive)- Create a free zone in the semiconductor

Reverse pn junction

There must bea single Fermilevel

Deformation ofband level

Potentialdifference

http://jas.eng.buffalo.edu/education/pn/pnformation3/index.html


Difference in concentrationstarts diffusion

Perfect candidate fordetector region



Solar cell

Do we know an example of what a pn junction can be usefull for?

http://jas.eng.buffalo.edu/education/pnapp/solarcell/index.html

Field in a p-n junction is not intense enough to provide efficient charge collection

thickness of the depletion zone will not be enough to detecthigh energy particles

2/1

2

−∝=

∝

VdA

C

dV

ε

V is potential in figure f) of pn junction


Solution: By applying an external voltage, we can enlarge the depletionzone and therefore the sensitive volume for radiation detection.The capacitance, hence the electronic noise, will also decrease




Reversed biased junctions

http://jas.eng.buffalo.edu/education/pn/biasedPN2/BiasedPN2.html

http://jas.eng.buffalo.edu/education/pn/biasedPN2/BiasedPN2.html

The higher external voltage also helps increasing efficiency of charge collection.

Max voltage appliable depends on the resistivity of the semiconductor. At some point junction will breakdown and begin conducting.

In Si n-type, with V=300V a depletion d=1mm can be obtainedBigger d bigger resistivity (to postpone breakdown)

1 m Al

+V

DepletedLayer

1 m Al

Electrons

Holes~1018/m3

-V

p+ implant

n+ implant

Si (n type)

Basic scheme for operating a pn junction

p+n junction, depletion region all in the n region (as seen)

To collect charge, electrodes must be placed on both ends. But the ohmic contactcannot be made by directly depositing metal on the semiconductor (else arectifing junction extending into the semiconductor is formed).So heavily doped layers of n+ or p+ are used between the semiconductor and themetal.

SignalfromincomingparticleIs readout

Typically, a preamplifier of charge-sensitive type, with low noise characteristics, is used to collect the charge out of the detector (~ 30000 eh pairs in 300 micrometers, need ampl.)

Leakage current

Reverse biased pn junction does not conduct, ideally.In reality a small current always exists : leakage current.Appears as noise at the detector output.

Sources:1. Movement of minority carriers (nanoAmpers/cm2)2. Thermally generated e/h due to impurities in depletion region (microAmp/cm2)3. Largest source: leakage current through surface channels. depends on a lot of factors (surface chemistry, contaminants, etc.) clean encapsulation is usually required

http://jas.eng.buffalo.edu/education/pn/biasedPN/index.html

http://jas.eng.buffalo.edu/education/pn/biasedPN/index.html

Intrinsic efficiency and sensitivity

Basically 100%. Limiting factor on sensitivity is noise from leakage current (I)and noise from associated electronics ( C ) and thermal noise ( KT/R )which sets a lower limit on the amplitude that can be detected

Very important to choose correct depletion thickness, to ensure good signal



often require cooling to be operated, adds to material budget of detector

To summarize

Silicon based detectors

Silicon microstrip detectors

pitch

Voltage roughly 160 V

Q: Formula for resolution on position

strip detector with pitch P=50 micrometers

Q: what is the position resolution if the information saved is: which strip is hit ?

Q: If one saves also the information: charge collected at each strip ,can one think of improving the resolution ?

y

P

y

P

2 <(y - <y>)2 > = ∫ (y - <y>)2 dy / ∫ dy between -P/2 and P/2 (continuous form)

assume uniform illumination given <y>=0

2 = ∫ y2dy /∫ dy = P2 /12

So if P=50m, then = 15m

A:

Reading out amplitude (of charge signal) at each strip, and weigthingpositions with this, we can get better precision on position

The position of the particle = the center of gravity of the charges collected at several readout strips.

Charge liberated by a charged particle is collected at the electrodes within 10 ns .

Signals picked up at the strips measure the position with a precision dependent on the pitch of the strips.



Detector with 20m pitch, readoutevery 6 or 3 strips. Resolution

r is respectively :

When Magnetic field applied !

One can improve by reading out more strips (every one, eg.).

Simplified readout in this case if possible, to put on detector the electronics associated with each strip

Magnetic field (typically applied in high energy particle physics detectors) worsens the resolution and introduces a bias.

Holes less mobile -> less angle

Not optimal



How to get2-dim information

realfake

Solid state pixel detectors Avoids problem with combinatorics and gives precise 3-D information

Precise 3D information : 20 x 20 x 20 micrometers pixels

Indeed we can clearly resolve decay distance “L” and impact parameter “b”

Disadvantage:Added material due tocryostat

Reconstructed B decays

Other examples of silicon detectors

DELPHI

In both SLD and DELPHI detector we have mentioned the resolution on impact parameter “b” seen at start

b

L

a

c

b = a + const/ (p sin 3/2)

- do we understand why ?- how does the resolution on “b” influence the choice of design ?

r

z

r2

zb

r1

z1 z2

(z2-b) / r2 = (z2-z1) / (r2-r1)

b= (r2 z1 -r1 z2)/(r2-r1)

b 2= (r2 2 z1

2 + r12 z2

2) / (r2 -r1) 2Resolution on b

indeed resol. on b is a constant, depends on point resolution of detector

how close one can get depends on radiation damage suffered (see later)

if after z1 particle suffers multiple scattering then z2’ = z2 + (r2-r1)* ms so z2

2 => z2 2 + (r2-r1)2 const / p2 so we get now the term on

b dependent on p

If r1=1cm and r2=1m and z1 =10microns and z2 = 200 microns (case of SLD vertex detector and gaseous tracking chamber)

then clearly b is good because the “near” measurement is good.

b 2= (r2 2 z1

2 + r12 z2

2) / (r2 -r1) 2

-> it is a good idea to insert a high resolution detector close to the interaction point and eg. B decay point

DATA From H.F-W. Sadrozinski, UC-Santa Cruz

50

cost

/are

a (

$/cm

2)

Moore's Law for Silicon Detectors

Blank wafer price 6''

< 2 $/cm2

1

2

10

4''

6''Wafer size

Now affordable also to cover large volume with silicon.

Any disadvantages in using only silicon for tracking devices atcollider experiments ?

:( more multiple scattering

:( more material, more energy loss

:( Probability of brehmstrahlung for electrons is higher in Si (~Z^2 vs ionization that goes like Z), and also photons will convert in pairs more easily

CMS

All silicon

Inner tracking

Detector

(two single sided strip

detectors, mounted

back to back)

Radiation damage

At the moment silicon detectors are used close to the interaction region in most collider experiments and are exposed to severe radiation conditions (damage).

The damage depend on fluence , particle type (,,e,n,etc)

and energy spectrum. It affects both sensors and electronics.

Three main consequences seen for silicon detectors :

(1) Increase of leakage current

(2) Change in depletion voltage, problematic

(3) Decrease of charge collection efficiency (less and slower signal)

summary of what seen so far overview of charged or neutral particle interaction in matter overview...

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