1. Recent Developments In Light Detection
Konstantinos Nikolopoulos
University of Athens
Experimental Methods I, 4th July 2005
Not Final Version

2. Why detect light? Usage of Light detection in High Energy Physics*
*short list of indicative examples
Light is used in many aspects of HEP experiments
Calorimetry
Tracking
Scintillating Fiber Trackers
Particle Identification
Cherenkov
Medical Imaging
PET
SPECT
Light Detectors are used in enormous numbers at HEP
Super – Kamiokande
Cherenkov Counters  PMTs
Pierre Auger Observatory
Fluoresence Detectors  PMTs
Light detection is crucial for every kind of high energy physics experiment.
Aim of this presentation is summarize current trends in light detection
Pierre Auger Prototype Fluorescence telescope camera (440 photomultiplier tubes)
Super – Kamiokande 20- inch PMT tube (Venetian Blind)
About the photo:
----Super-K----
The inner PMT has 20-inch diameter and is equipped with quasi-hemispherical window with bi-alkali photocathode. It is of the venetian blind type dynodes of 11 stages. (11,146 PMTs)
The outer part, which has 2 m in thickness, is used for anti-couters. All the surface of the outer counter is covered with white polyethylen sheets with high reflection. 1,857 8-inch PMT's are used for the outer counter.

3. Physics of Light Detection
Photoelectric Effect
When a photon impinges on the surface of any material, it can liberate an electron provided that the photon energy is higher than the photoelectric workfunction Φ. The liberated electron carries kinetic energy W=hν-Φ, which can be sufficient to bring the electron not only from the surface, but also from the volume of the material to the free space.
Semiconductors have a very small workfunction Φ.
Electron lift from Valence Band to Conduction Band in a Semiconductor
When an photon impinges on a semiconductor, then an electron from the valance band can be lifted to the conduction band. When the electron cannot recombine with the hole, due to the electric field of a silicon photodiode, it can be collected and the signal amplified.
The second process is far more efficient in light detection both in terms of detection efficiency and wavelength sensitivity, because it needs less energy O(1 eV) than the photoelectric effect one O(10 eV).

4. Main Characteristics of Light Detectors
Quantum Efficiency
The efficiency in single photon detection
Gain
Multiplication of the initial signal
Sensitivity
Minimum # of photons required for output signal
High – Voltage Supply
Timing and Positioning
Capability of providing time and position information of the incoming photon
Noise
Dark current
Nuclear counter effect (=The extra amount of charge produced in the photodiode by a charged particle directly hitting it, on the top of the charge produced by th scintillation light).
Experimental Conditions
B-field, Temperature, Radiation Levels, Particle Rate, e.t.c.
Complexity of construction
Physical Shapes of Detectors
Price

5. Types of Light Detectors
Vacuum Devices
Solid – State Devices
Hybrid Devices
Gaseous Detectors

6. Vacuum Devices
The first photoelectric tube was produced by Elster and Geiter in 1913.
The first photomultiplier tube was invented by the RCA laboratories in 1936.
Features:
Low noise amplification
Due to the dynode – chain amplification scheme. Only noise contribution is the stochastic nature of the secondary e- emission
High Gain O(106)
Output read by standard electronics  No need for amplification  No additional noise
Gain = f(# dynodes)
Good Energy resolution
Calorimeters with scintillating crystals
Single photon detection

7. Vacuum Devices II
Many dynode chain design exists, with different properties!
Linear Focused Type
Fast Response
Linearity
Large Output Current
Box - and - Grid Type
Consist of a series of quarter cylindrical dynodes.
Electron collection efficiency
Uniformity
Circular Cage Type
Compactness
High gain (with relatively low High—Voltage)
Venetian Blind Type
Large dynode area (large photocathode area)
Large Output current
Linear Focused
Box – and – grid
Circular Cage
Venetian Blind

8. Vacuum Devices III Drawbacks: Q.E. typically constrained to ≈25%
Reflection of photons by the glass of the window
Passage of photons through the photocathode without interaction
The produced photoelectrons may stop inside the cathode material
Shape dictated by technical restrictions in the fabrication of the vacuum container
Hand work involved  Expensive to construct
Very sensitive to B – field
Defocusing of the e-

9. Further Innovations in Vacuum Devices
Sensitivity to B—field
Dynodes from meshes with small distance O(1mm) and high field between them.
Information on the position of the incoming photons
Metal – channel devices. The single dynode chain is replaced by many chains in parallel and there is a segmented anode.
New photocathode and window materials
Mesh Dynodes
Metal – Channel Device

10. Solid – State Devices
Photosensors made of semiconductor material gained much attention in recent years.
High Q.E.
Main negative contribution is the reflection at the surface
Insensitivity to magnetic fields
Theoretical Limit ≈ 15 T
Produced in standard fully – automated processes
Inexpensive
Fairly easy to be tailored for individual needs
In short time
Low mass
Small space consumption
Thickness < 1/2 mm
They open new areas of application!

11. PIN Photodiode
The PIN photodiode is a semiconductor device, typically fabricated using doped silicon. In a photodiode, absorption of an incoming photon produces a single electron-hole pair. A small electric field within the photodiode causes these charge carriers to migrate in opposite directions towards external electrodes. This results in a measurable current or voltage, which has a linear relationship to the incident light flux.
The silicon PIN diode is a very successful device. All major experiments in high-energy physics used it in big numbers in the last 2 decades. The operation is simple and reliable but since it has no gain it needs a charge-sensitive amplifier that adds to the cost and creates noise in the readout system. Single photon detection with silicon photodiodes is therefore not possible and the very good timing properties (1–2 ns) are destroyed by the signal rise time of the amplifier which is 10 ns or more.
Arrays of photodiodes are easy to produce and are commercially available
It has a rather simple structure and is produced by standard semiconductor processes: boron diffusion on one side and phosphor diffusion on the other side of a high purity silicon wafer and at the end contacts are made by aluminium deposition

12. Avalanche Photodiodes
Avalanche Photodiodes provide gain due to the high internal field at the junction of positively and negatively doped silicon.
The incoming photon produces an e-h pair.
The electron gain enough energy due to the high internal field to produce secondary electrons by impact ionization
An electron avalanche is created
Gain is 50 – 200.
Higher gains (up to 104) can achieved
Q.E. is of O(80%).
Large Dynamic range
Excellent energy resolution
Position information
Only with dedicated setup  position resolution O(0.3mm)
Drawback: Very stable environment of operation needed for gains over a few hundreds.
1% gain variation  δT= 0.1 K or δV/V=10-4
Schematic Diagram of an APD
Hamamatsu S8148
Position Information:APD 28 x 28 mm^2 mounted on layer of metal with well defined resistivity and amplifiers at the 4 corners comparison of signal amplitudes  position resolution O(0.3mm)
Environment (temperature + voltage)
Dimensions of S8148 -> 5mmx5mm active area

13. Avalanche Photodiodes in Geiger Mode
In this case the APD is operated at a bias voltage higher than the breakdown voltage.
Any photon or thermally liberated electron starts an avalanche.
The avalanche persists until the voltage is lowered actively or when the voltage drops on a properly chosen serial resistance.
The output signal is proportional to the over – voltage and the capacitance of the APD.
Drawback: There is a long dead time O(1 ms) after each breakdown.

14. Silicon Photomultipliers (SiPM)
The SiPM is a novel type of APD.
It is a multipixel silicon photodiode with a number of micropixels of size O(20 μm) joined together on common substrate and working on common load.
The operational bias voltage is ≈15% higher than the breakdown voltage.
The pixels are electrically decoupled from each other.
Each SiPM pixel operates in Geiger mode
Each SiPM pixel operates as a binary device
The SiPM as a whole is an analogue detector.
The SiPM intend to solve the problem of long dead time in the APDs functioning in Geiger mode.
The micropixels joined together on common substrate and working on common load.
The pixels are electrically decoupled from each other by polysilicon resistors (typical value of 500 kΩ) located on the same substrate.
Pixel gain determined by the charge accumulated in pixel capacitance.

15. Silicon Photomultipliers II
Detection efficiency.
Q.E. is of O(80%).
εgeometrical=Ssensitive/Stotal, is of O(25%).
Gain O(106)
Independent of primary carrier number.
Excellent energy resolution.
Dynamic range O(10^3/mm^2).
<#photons/pixel> << 1 (signal saturation)
Electronics noise is negligibly small
High gain
Electronic noise contribution < 0.1 e-
Main noise contribution is the dark rate.
Originates from carriers created thermally in sensitive volume or due to effects of high electric fields.
O(1 room temperature
100 K
However, dark rate limits SiPM performance only in detection of very small light intensities O(1 ph.e.).
Single Photon Detection Efficiency of a SiPM, a APD and a classical PMT
εgeometrical can be improved by a factor of 2Single photon efficiency O(50%)

16. Silicon Photomultipliers (SiPM) III
Insensitive to B – fields
Excellent time resolution (≈100 ns) for single photo – electron detection
Time Resolution obeys the Poissonian law of 1/√Npixelsfired
Fast rise time ≈1ns
The discharge is contained within a limited region in the depletion zone (due to the voltage distribution) small duration
Does not suffer from nuclear counter effects
Stable operation in room temperature
Low gain variation with temperature
ΔΤ = 2.5 Κ for 1% gain variation.
Does not exhibit any serious radiation damage
Perhaps Neutrons are an exception to this rule
Response of SiPM to light of very low intensity. The number of photons can be counted for each event.
Timing:
Decay time constant: Cpixel*Rpixel=30 ns  Recovery time of single pixel < 100 ns  Recovery time of whole SiPM much smaller

17. Hybrid Detectors (HPD)
Schematic view of the Pixel HPD
These detectors combine the advantages of the vacuum devices and the solid – state devices
Pixel – HPD
Pad – HPD
The vacuum container and the photocathode are the same as in the classical PMT.
The photo – electrons are accelerated in a high electric field and are focused onto an anode:
A silicon PIN photodiode, an amplification of 4000 is achieved with an electric field of 15kV.
An APD, a gain of more than 105 can be achieved, allowing usage of simple readout electronic circuit.
Simulated potential distribution and electron trajectory
About the gain:
When reporting a gain of 4000, the thin protection layer on the surface of the silicon diode is neglected.
Pixel implies detector elements < 1mm

18. Hybrid Detectors (HPD) II
Q.E. is of 270 – 400 nm
εgeometrical is of O(80%).
Using electron optics.
Position information
When the anode is segmented.
The HPDs have a very good energy resolution, even for light of very low intensity.
Typical photoelectron spectrum
About Q.E.
Result including reflection at the entrance of the quartz window and transmission
About the geometrical acceptance:
The Photocathode area is de-magnified approx 5 to fit on the SiPixels array area

19. Visible Light Photon Counters (VLPC)
VLPCs are solid state photo – detectors, originally invented at Rockwell International and presently developed and manufactured by Boeing.
The operation principle of VLPCs is the effect of impurity band.
Occurs when a semiconductor is heavily doped with shallow donors or acceptors.
The impurity atoms are close together  electrical transport occuring by charge hopping from impurity site to impurity site!
The standard 1.12 eV gap of Si is used to absorb photons
The small gap with the impurity band is used for creating an electron – D+ avalanche multiplication.
Small gap O(0.05eV)  Relatively low field is required for avalanche the required field for producing an avalanche is low.
Localized and self– limiting avalanche(due to local field collapse, D+ have low mobility)
The drift region can been seen as an internal resistor in series with an ideal VLPC
Electric Field profile in a VLPC
Schematic Layout of a VLPC

20. Visible Light Photon Counters II
High Q.E. (80%).
High gain O(5* 10^4).
Low gain dispersion.
Operational in a high background radiation enviroment.
Drawback: Cryogenic temperatures needed!
In order to freeze out the intrinsic carriers!
VLPC Single Photoelectron counting capabilities

21. Gaseous Detectors
These detectors have been developed for cases that large areas are needed to be covered.
Their functionality principle relies on photo – conversion and subsequent multiplication in a gas avalanche.
The photo--conversion can be in the gas itself or in a CsI photocathode, which can be made in large areas.
Readout: MWPC e.t.c.

22. Field Distribution in the Gas – Electron Multiplier foil
Gaseous Detectors II
Main problem: Ion and photon feedback
Ion Feedback, ionization of the residual gas (Vacuum) or the gas used (gaseous detectors) in the detector. Collision of the gas molecules with electrons can cause the ionization of the gas then a pulse can be created
Positive ions striking the front dynodes or the photocathode may produce many secondary electrons  large noise pulse
This new pulse usually follows a true photo –electric pulse  after pulsing
Solved to some extent by “the gas electron multiplier (GEM) foil” technique
Field Distribution in the Gas – Electron Multiplier foil

23. The ATLAS Tile Calorimeter
A Torodial LHC ApparatuS is general purpose detector to study proton – proton √s=14 TeV (some Heavy – Ions Program is foreseen as well)
ATLAS is mostly known for its Muon System, however as a general purpose apparatus, ATLAS should be able to perform accurate calorimetry as well.
The ATLAS calorimetry system is quite complicated, incorporating different approaches
Liquid Argon Sampling Electromagnetic Calorimeter |η|<3.2
Liquid Argon Hadronic Calorimeter 1.5<|η|<3.2
Tile Hadronic Calorimeter |η|<1.7
Forward Calorimeter 3.2<|n|<4.9 (Liquid Argon)
The ATLAS detector
ATLAS Tile Calorimeter Schematic Layout
Forward Calorimeter Metal absorber, liquid argon in longitudinal rods

24. The ATLAS Tile Calorimeter II
Active medium: Tiles of polysterene (emission maximum ≈100 nm)
Absorber :Steel
Wavelength Shifter Fibers (≈100nm  ≈400nm)
Photo – Multiplier Tubes (10.500) (max sensitivity ≈420nm)
PMT characteristics
Min/Max wavelength = 300nm/650nm
Max 420nm
Window : Borosilicate
Photocathode : Bialkali
Gain = 3 105
Dark Current = 2 nA
Rise Time = 1.5 ns
Transit Time = 7 ns
Transit Time Spread = 0.26 ns
Number of dynodes = 8
Operation Voltage = 800 Volt
Tile Calorimeter: Operation Principle
ATLAS polysterene tiles
Hamamatsu R7877 :

25. The KLOE Electromagnetic Calorimeter
The main aim of KLOE is to do high precision CP – Violation studies in K0 decays a the Frascati Φ – Factory DAFNE.
The Electromagnetic Calorimeter
Sampling calorimeter: Scintillating Fibers are glued inside thin grooved lead layers. Volume ratio of the composite Pb:scint:glue of 42:48:10 and X0 =1.5 cm.
Total depth is 23 cm (15 X0) . Containment: 98% of 500 MeV e/m showers (Maximum shower energy at KLOE).
Scintillating fibers
Excellent time resolution for long detectors.
Relatively long attenuation lenght
Total fibers lenght is Km!
PMTs with Mesh dynodes are used
E/M Calorimeter inside B-field.
B- filed strength ≈ T at the readout.
The total number of PMTs is 4880.
π0 mass reconstruction using KLOE’s calorimeter
Density of the composite 5.0 g/cm3
Scintillating Fibers (diameter 1 mm ) are glued inside thin grooved lead layers (thickness 0.5 mm).
Fiber length is ranging from 2.0 to 4.3 m
e+e- collider 510 MeV each beam e+eγφ(1020MeV)(K+K- Br49.5%) (K0K0bar Br 34.3%)
Fiber Attenuation Length 4.5 m ( 2 m for a scintillator slab)
Excellent time resolution : given the small trapping angle of the light propagating on the fiber core.
Approximately 200 planes of lead/fibers are present
KLOE Electromagnetic Calorimeter

26. The CMS Electromagnetic Calorimeter
CMS is a general purpose detector for the Large Hadron Collider
CMS Electromagnetic Calorimeter
Homogenous calorimeter
PbW04 scintillating crystals used.
Calorimeter dimensions: Longitudinal 25 X0 = 22.2 cm Transverse 1 RM = 2.2 cm
Operation in high B-field  4T
Very High radiation leves
One APD mounted on each side of the crystal depth of interaction.(Sufficient pulse height from the APDs)
Many sources to be kept under control:
Longitudinal uniformity of light collection
Strong light yield variation with temperature (-2.3%/0C)
APD gain variation with applied tension (-3%/Volt) and termperature (-2.3%/0C)
Light collection uniformity
light transmission due to radiation damage
Leakage front and rear
The PbW04 scintillating crystals used by CMS ECal
The CMS apparatus
Energy Resolution measured with 180 GeV electrons in 3x3 crystal matrix with 2 APDs/crystal readout
95% of the shower contained in 2 RM
Schematic layout of the CMS ECAL

27. LHCb Ring Imaging Cherenkov
LHCb is a single arm forward spectrometer designed to perform various B – physics measurements in the LHC.
2 Ring Imaging Cherenkovs
RICH 1  Aerogel n = ( 2 – 10 GeV) C4F n = (10 – 60 GeV)
RICH 2  CF n = (16 – 100 GeV)
HPDs  61 pixel/HPD
Magnetic Field, High Radiation Dose
LCHb Schematic Layout
Cherenkov rings produced by 120 GeV π-,
radiator C4F10 .The circle is guide to the eye.
Typical Event from RICH1
Schematic Layout of RICH1

28. Schematic view of the D0 apparatus
The D0 SciFi Tracker
D0 is a general purpose apparatus to study √s=2 TeV proton – antiproton collisions at Tevatron.
Central Scintillating Fiber Tracker
Consists of multi – clad scintillating fibers
refraction index 1.591.491.42 for improvement of light trapping
VLPCs
Q.E. 70%,
Gain
Rate capability > 10 MHz
Noise Rate <0.1 %
Impurity band 0.05 below the conduction band
Operation at 9K
Schematic view of the D0 apparatus
VLPC wafer
Silicon Tracker  channels
SciFi multi –clad 3 materials (polysterene  acrylicfluoro – acrylic) refraction index 1.591.491.42 for improvement of light trapping the wave guides  VLPCs in cryostats on the d0 platform under the central calo

29. The GlueX Barrel Electromagnetic Calorimeter
The goal of the GlueX experiment at Jefferson Laboratory is to search for gluonic excitations manifested in exotic hybrid vector mesons with masses up to 2.5 GeV, using linearly polarized photons of 8 – 10 GeV energy.
Electromagnetic Barrel Calorimeter
Sampling calorimeter
Scintillating fiber
Layout of the GlueX apparatus

30. Conclusions PMT APD HPD SiPM γ- Detection Efficiency Blue 20% 50% 12%
Green –yellow
A few %
60 – 70%
15%
Red
<1%
80%
Gain
103
106
High – Voltage
1-2 kV
100 – 500 V
20 kV
25 V
Operation in B-field
problematic
O.K.
Threshold Sensitivity S/N>>1
1 ph.e.
≈10 ph.e.
Timing / 10 ph.e.
≈100 ps
A few ns
30 ps
Dynamic Range
≈106
large
≈103/mm2
Complexity
High
(vacuum,HV)
Medium
(low noise electronics)
Very High
(hybrid technology, very HV)
Relatively low

31. Conclusions (continued)
As concluding remarks we would like to focus on the following:
Light detection is crucial for every kind of High Energy Physics experiment.
One should also note that we didn’t mention at all a very closely related topic, that of light detection for medical purposes (PET e.t.c.).
The progress in the field of light detection is enormous. Today, the market offers light detectors for almost every need and funding.
Semiconductor devices are a very promising development in the field. It is expected to be further improved.
Further progress in the field can be expected.

32. References
General
D. Renker, Photosensors, Nucl. Instr. And Meth. A 527 (2004), p.15
Hamamatsu Photonics ,
Vacuum Devices
Hamamatsu Photonics, Photomultiplier Tubes (PMTs) – Construction and Operating Characteristics,
APDs
I. Britvitch et al., Avalanche photodiodes now and possible developments, Nucl. Instr. And Meth. A 535 (2004), p.523
SiPM
HPD
E. Albrecht et al., Nucl. Instr. And Meth. A 442 (2000), p.164
L. Somerville, Pixel hybrid photon detectors for the ring imaging Cherenkov, Nucl. Instr. And Meth. A 546 (2005), p.81
A. Braem et al., Nucl. Instr. And Meth. A 504 (2003), p.19
VLPCs
M.D. Petroff, et al., Appl. Phys. Lett. 51 (1987) p. 406
A. Bross et al., Nucl. Instr. And Meth. A 477 (2002), p.172
Gaseous
Experiments
The ATLAS experiment,
The CMS experiment,
The LHCb experiment,
The KLOE experiment,
The D0 experiment,
The GlueX experiment,
The Pierre Auger Obervatory,
Super-Kamiokande, US Collaboration homepage,

33. Back-Up Slides

34. Positron Emission Tomography
PET produces images of the body by detecting the radiation emitted from radioactive substances. These substances are injected into the body, and are usually tagged with a radioactive atom, such as (C-11, F-18, O-15, or N-13), that has a short decay time. (bombardment of normal chemicals with neutrons)
PET detects the gamma rays given off at the site where a positron emitted from the radioactive substance collides with an electron in the tissue.
PET provides images of blood flow or other biochemical functions, depending upon the type of molecule that is radioactively tagged. For example, PET can show images of glucose metabolism in the brain, or rapid changes in activity in various areas of the body. However, there are few PET centers in the country because they must be located near a particle accelerator device that produces the short-lived radioisotopes used in the technique.
Schematic of a PET scanner
In a PET scan, the patient is injected with a radioactive substance and placed on a flat table that moves in increments through a "donut" shaped housing. This housing contains the circular gamma ray detector array, which has a series of scintillation crystals, each connected to a photomultiplier tube. The crystals convert the gamma rays, emitted from the patient, to photons of light, and the photomultiplier tubes convert and amplify the photons to electrical signals. These electrical signals are then processed by the computer to generate images. The table is then moved, and the process is repeated, resulting in a series of thin slice images of the body over the region of interest (e.g. brain, breast, liver). These thin slice images can be assembled into a three dimensional representation of the patient's body.

35. Single Photon Emission Computed Tomography
Single Photon Emission Computed Tomography (SPECT) SPECT is a technique similar to PET.
The radioactive substances (Xenon-133, Technetium-99, Iodine-123) have longer decay times than those used in PET
Single photon emittance instead of double gamma rays.
SPECT can provide information about blood flow and the distribution of radioactive substances in the body.
Its images have less sensitivity and are less detailed than PET images.
SPECT technique is less expensive than PET.
SPECT centers are more accessible than PET centers because they do not have to be located near a particle accelerator.