1. Compton Lectures Autumn 2001
INVASIONS IN
PARTICLE PHYSICS
Compton Lectures Autumn 2001
Lecture 6
Nov

2. LECTURE 6
Particle Detectors

4. Bubble Chambers
“Yet a glass of beer reveals a remarkable interplay among gasses, liquids, and solids, temperature, pressure and gravity -- an interplay that is still not completely understood. Once you begin to learn about the nature of beer bubbles you will never again look at a glass of beer in quite the same way.” (Physics Today)

5. Bubble Chambers
A tank of liquid hydrogen at boiling temperature that is prevented from boiling by means of pressurization.
A beam of particles is allowed into the tank and after some short time (to allow for an interaction to take place) the pressure is released.
Charged particles that have crossed the chamber leave a trail of ionization behind them. With the release of the pressure, local boiling takes places within the tank. Bubbles of gas tend to form along the ion trail as the charged ions act as nucleation centers.
The bubbles are allowed to grow for a certain time, then a picture is taken which provides a record of the interaction.

6. Bubble Chambers + The interaction point is visible.
+ If placed in a magnetic field the momentum of the particles can be measured by the bending of the tracks.
+ The existence of neutral particles can be deduced if they decayed
into charged particles within the chamber.
- The chamber has to be re-set by compressing the liquid which bursts the bubbles i.e. there is a “down time”.
- There is no instant information and a great deal of extra equipment is needed to control the temperature, take the pictures, regulate the pressure...

9. Notice the trajectory of the spiraling lone electron, indicated by the arrow. This electron was knocked away from the atom by a high energy photon.

10. A B
An electron ( e - ) and a positron (e + ) leave the tracks shown in the picture.
The particle that deflects to the right ( track B) is an electron ( e- ).
The particle that deflects to the left ( track A) is a positron ( e+ ).

11. D C e- e+
One of the photons emitted at C travels to D where it interacts with a nucleus from the liquid and materializes into an electron/positron pair. To a good approximation, all of photon’s energy is shared by the e-/e+ pair.

13. What do we want (can) measure about a particle?
Identity Interaction with matter
Decay
Mass (from simultaneous measurements
of any two of |p|, E, |v|
Momentum Curvature in magnetic field
from E or |v| if m is known
Origin Tracking often together with
momentum measurement
Polarization Decays

14. Charged particles :detected via their em interaction. They signal
their passage by
ionization of gases or liquids: spark,proportional,drift and steamer chambers,liquid argon calorimeters…
excitation of scintillation light in gases, liquids or solids:scintillation counters
direct emission of radiation: particles travelling faster than c in a medium emit Cherenkov radiation; particles traversing interfaces of different dielectrics emit transition radiation
local heating of a liquid via excitation and ionization : bubble chambers
Neutrals can be detected by destroying their identity and converting them
into charged particles: energetic gammas have to be converted to e+e-
pairs, neutral hadrons decay or interact through em,weak or strong
interactions

15. Scintillation Counters
A charged particle passing through a scintillating material will cause the material to emit light
The light is produced by ionization of electrons which then recombine with an ion, by excitation of electrons within an atom or even by breakup of molecules depending on the material.
The light is guided to and detected by a PMT (photomultiplier tube) which puts out an electric signal.
The advantage is that the information is available within nanoseconds.

16. A PMT consists of a photocathode and a series of dynodes in an evacuated glass enclosure. When a photon of sufficient energy strikes the photocathode, it ejects a photoelectron due to the photoelectric effect. The photocathode material is usually a mixture of alkali metals. The photocathode is at a high negative voltage. The photoelectron is accelerated towards a series of additional electrodes called dynodes. Additional electrons are generated at each dynode. This cascading effect creates 105 to 107 electrons for each photoelectron that is ejected from the photocathode. The amplification depends on the number of dynodes and the accelerating voltage. Phototubes are similar to PMTs, but consist of only a photocathode and anode. Since phototubes do not have a dynode chain to provide internal amplification, they are used in less sensitive applications.

17. Example: 20,000 PMTs in SNO

18. Example: SLAC's cosmic ray detector

19. The Cosmic Ray Detector consists of three pairs of scintillator panels for muon detection. Sets A, B, and C are oriented with the flat surface of the panels horizontally, at 45°, and vertically, respectively.
The panels are shielded from light with aluminum foil, black plastic sheets, and black tape. When muons penetrate through these panels, chemicals within will scintillate (emit flashes of light).

20. Click here to go to SLAC’s cosmic ray detector

21. Cherenkov Detectrors
NASA

22. Cherenkov emission angle
Particles move with constant velocity in a medium
whose index of refraction, n(= c/c’) is greater than 1: The effective velocity of light in this medium is
less than the velocity of light in the vacuum.
If the particle velocity (v=bc)exceeds this effective velocity of light in this medium a shock wave, is setup in the medium.
c't
qcherenkov
(click here to see
a simulation) • • • • • vt
cos qcherenkov = c't/vt=1/bn
V>c’ and cos qcherenkov <1 if b>1/n

23. The angle of the emitted light depends on the particle’s velocity
The higher the velocity the higher the angle of radiation
For a beam of particles of known momentum the Cherenkov
counter allows you to identify particles of different mass.
Photon from Kaon
Photon from pion
Photon from proton
Beam
PMT

24. RICH: In these detectors, particles pass through a radiator, and the radiated photons are usually focused onto a position- sensitive photon detector by a mirror. The velocity is determined by a measurement of the radius r of the ring, on which the photons are detected

25. Cherenkov-ring radii of in a CF-Ar (75:25) RICH-counter
Cherenkov-ring radii of in a CF-Ar (75:25) RICH-counter. The solid curves show the expected radii for an index of refraction of n =
(BNL)

26. SuperK

27. (LHCb)

28. SuperK

29. SuperK

30. Wire Chambers Cathode plane (-) Electrons drift towards sense wires
Anode “sense wires”
Cathode plane (-)
(enclosed in a
container filled with
low pressure gas)

31. Wire Chambers
A wire chamber works in the proportional mode when the signals recorded by the detector are proportional to the energy loss dE/dx of the particle traversing it. This happens when the accelerating electric field is high enough to cause the electrons generated by the ionization in the gas to produce more ion-electron pairs in the gas a phenomenon called gas multiplication or avalanche effect . The freed electrons drift towards the anode and produce an analogue signal that can be used for position and energy loss measurement.
A signal can be produced within nanoseconds
of the particle passing through the detector.
By using a series of such detectors with the wire
planes at angles with each other it is possible to
reconstruct the 3-D path of the particle.

32. also used in medical imaging
A five-thousandth-of-a-millimeter-thick slice of a rat brain. The colors illustrate the concentration of molecules sensitive to radiation. With MWPC detectors this picture could be produced in a day compared to three months with traditional methods.

33. Drift Chambers Drift chambers measure the time
taken for the ions to arrive at the sense wires.

34. Starts the clock running when the particle enters the chamber
Low drift velocities lead to large drift times and thus better position resolution because of the improved timing resolution, while high drift velocities are important in experiments where the count rates are expected to be high and therefore deadtime becomes important
Starts the clock running when
the particle enters the chamber

35. Drift velocity-Choise of gas

36. Drift Chambers in magnetic field
Allows not only the determination of the particle’s
path, but by measuring the bend, it gives a measurement
of the particle’s momentum.

38. aging

39. Silicon Detectors
Silicon is a semiconductor.It can be fabricated in two forms; n type, with a surplus of electron sites in the crystal lattice, and p type, with a deficit of electron sites in the crystal lattice

40. When a charged particle passes through a silicon detector it creates ionization in the bulk of the silicon. This frees electrons from the atoms of the silicon and leaving these atoms with an electron vacancy. These vacancies are referred to as "holes".
 The "holes" "drift" in the electric field towards the negatively charged p type strips. The electrons "drift" towards the positively charged back plane.
 When the "holes" reach the p type strip they are collected and induce a measurable charge on the associated aluminum strip. The aluminum strips are connected to sensitive electronic read out channels.
 By recording which electronic channel fired, it is possible to determine where the charged particle passed through the detector very fast and with great accuracy

41. Silicon Pixel Detectors

42. Silicon Pixel Detectors

43. Electromagnetic Calorimeter
Used to identify and precisely measure the energies and location of electrons and photons. electrons and photons entering the EM calorimeter material (e.g lead crystal) interact and produce ``showers'' of secondary particles. Each shower itself consists of many electrons and photons, which eventually get absorbed.

44. EM Shower example

45. Hadronic Calorimeter
Discrimination, often at the trigger level, between electromagnetic and hadronic showers is a major criterion for a calorimeter; It important to contain electromagnetic showers over a short distance, without initiating too many hadronic showers

46. CDF showcase calorimeters Muon chambers tracker
Go to the CDF public web page

48. electron muon A shower in the electromagnetic calorimeter
The shower is narrow and has a well defined
shape both longitudinally and transversely.
A track must point to the shower; The momentum
of the track has to match the energy of the shower.
muon
penetrates both EM and hadron calorimeters and leave
a track(hits) in the muon chambers. A track in the tracking
chamber has to match the track in the muon chamber.
A muon typically deposits a few hundred MeV in the em
calorimeter and several GeV in the hadron calorimeter.

49. neutrino tau Missing energy; calorimeter energy imbalance
Taus decay primarily into one or three charged pions with
accompanying p0s. A high momentum single track or
triad of tracks isolated in that there is little energy in the
calorimeter around the tau candidate cluster and no other
tracks nearby is the signature of a hadronically decaying tau

50. light quarks and gluons: JETS
Identified with a cone algorithm to cluster energy in the
h-f space
charm quarks
tagging using displaced vertices algorithms
bottom quarks
tagging using displaced vertices and leptons from
the semileptonic decays
photons
same as electron without track

51. The detector is simulated
Click here to see the “Anatomy of a detector” movie
And here to see the P2K/NASA TV movie

52. Trigger Subtleties Fakes Instrumental background
Multiple interactions/crossing
Energy calibrations
etc

53. Cross Section Detector Background Machine L is the Luminosity$
Machine
L is the Luminosity
e is the acceptance
B is the Background
s is the cross section
CANNOT WRITE OUT ALL EVENTS
NEED TO TRIGGER ON THE INTERESTING ONES
Physical
Cross Section

54. Physics processes @ Tevatron
Zzzz

55. CDF Deadtimeless Trg&DAQ
Calorimeter energy
Central Tracker (Pt,f)
Muon stubs
Cal Energy-track match E/P, Silicon secondary vertex
Multi object triggers
Farm of PC’s running
fast versions of
Offline Code  more
sophisticated selections

56. DETECTORS

57. Click here for the ATLAS movie