1 Light Collection  Once light is produced in a scintillator it must collected, transported, and coupled to some device that can convert it into an electrical signal (PMT, photodiode, …)  There are several ways to do this Plastic light guides

2 Light Guides Isotropic light emission

3 Light Guides  Even for total internal reflection over all angles, if  x 1 >>  x 2 there will be substantial light loss

4 Wavelength Shifters  Liouville’s theorem can be beat by decreasing the energy of the photons  Wavelength shifter can be used To collect light from large areas and transport it to a small PMT area To better match the PMT sensitivity To bend the light path

5 Wavelength Shifters Wavelength shifting bars Wavelength shifting fibers

6 ATLAS Tile Calorimeter

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9 Outer Reflectors  Usually the scintillator and light guide are wrapped/enclosed with an outer reflector Measurements at 440 nm,

10 Photon Detectors  Once light is produced in a scintillator we need to convert it into an electronic signal Vacuum based (this lecture)  Photomultiplier tubes (PMTs) Semiconductor (later lectures)  Photodiodes, APDs, SSPM, CCDs, VLPCs, … Hybrid  Vacuum+semiconductor Gas based (TEA, TMAE)  For Cerenkov detectors

11 Photon Detectors  We’ll be interested mainly in the visible region today

12 Photon Detectors  The main principle used is the photoelectric effect which converts photons into electrons (photoelectrons)  Important quantities characterizing the sensitivity are the quantum efficiency and radiant sensitivity

13 Photomultiplier Tubes (PMTs)

14 Windows  Borosilicate typical

15 Photocathodes  The important process in the photocathode is the photoelectric effect Photons are absorbed and impart energy to electrons Electrons diffuse through the material losing energy Electrons reaching the surface with sufficient energy (> W) escape  Alkalai metals have a low work function e.g. bialkali is SbKCs

16 Photocathodes  QE of bialkali PMT’s

17 Photocathodes  As you can see from the graph, the maximum QE is about 25% for current bialkali Photoelectron emission is isotropic  50% to first dynode, 50% to window Transmission losses  Bialkali photocathodes are ~40% transmissive 0.5 x 0.4 ~ 0.2

18 Energy Resolution  In gamma ray spectroscopy and other applications, the energy resolution is an important quantity  One contribution to the energy resolution is the statistical variance of the produced signal quanta  In the case of a PMT, the energy resolution is determined by the number of photoelectrons arriving at the first dynode

19 Energy Resolution

20 Electron Focusing

21 Dynode Structure  The dynode structure multiplies the number of electrons Process is similar to photocathodes but here the incident radiation is electrons

22 Dynode Structure  There are a variety of dynode structures including some that are position sensitive

23 Dynode Structure   of 4-6 for most dynode materials  And typically there are stages (dynodes)

24 Dynode Structure  Typical instantaneous current? Assume 10 3 photons at the photocathode Then there are 2.5x10 2 electrons at the first dynode Then there are 2.5x10 8 electrons at the anode And collected in 5ns gives a peak current of 2.5x10 8 x 1.6 x / 5 x = 8 mA  Of course the average current is much smaller

25 Dark Current  A small amount of current flows in the PMT even in completely dark state  Causes of dark current include Thermionic emission from photocathode and dynodes Leakage current (ohmic leakage) between anode and other electrodes Photocurrent produced by scintillation from glass or electrode supports Field emission current Cosmic rays, radioactivity in glass envelope, radioactivity (gamma) from surroundings (cement)  Dark current increases with increasing supply voltage

26 PMT Gain and HV Supply

27 PMT Gain and HV Supply  Typical gain versus high voltage curve  Rule of thumb is  V=100 gives  G=2

28 PMT Base  A voltage divider network is used to supply voltage to the dynodes Typical supply voltage is 2kV  The manufacturer usually supplies a circuit diagram and often sells the accompanying base

29 PMT Base  The HV supply must be capable of providing a DC current (to the divider network) as well as average and peak signal currents Typical signal current ~ 20 mA Typical average current ~ 20  A  It is possible that at high rates that the HV supply cannot provide enough current to the last dynodes and hence the PMT voltage will “sag” Additional charge can be supplied by using capacitors or transistors

30 PMT Base  Using capacitors or transistors to supply charge

31 Magnetic Shielding   V between the dynodes is ~ V  Low energy electrons traveling from dynode to dynode can be affected by small magnetic fields (e.g. earth B ~ 0.5 G) Effect is largest for head-on type PMT’s when the magnetic field is perpendicular to the tube axis  A magnetic shield (e.g. mu-metal) is used to reduce gain changes from magnetic fields

32 Magnetic Shielding

33 PMT’s  There are a wide range of PMT types and sizes From Hamamatsu catalog

34 ATLAS Tile Calorimeter PMT

35 Light Guides  Liouville’s theorem Phase space is conserved  Phase space density of photons cannot be increased  You can’t make a tapered light guide without losing light

36 Light Guides  So for a maximum output angle  2 the input angle  1 is limited  Even for complete TIR, if  x 1 >>  x 2 there will be substantial light loss

37 Light Guides  Transport light by total internal reflection (TIR) The air gap between scintillator and wrapping is important  Maximum angle for TIR at light guide output is  For light that does escape the light guide it can be recaptured using specular (Al foil) or diffuse (Tyvek) reflection

38 Light Guides

39 Light Guides  An example  Many people use Tyvek but one should do studies for each specific application

40 Light Guides  There will be some light loss even in the case of equal dimensions

41 Wavelength Shifter  Liouville’s theorem Phase space is conserved  Phase space density of photons cannot be increased  You can’t make a tapered light guide without losing light  One can get around Liouville’s theorem by using a wavelength shifter such as BBQ Light is absorbed and subsequently emitted (isotropically) at a longer wavelength

42 Wavelength Shifter