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 1
Light Guides Isotropic light emission 2
Light Guides Consider a phase space element for a photon in a light guide The canonically conjugate variables are taken to be x = transverse coordinate p = nsinα = angular divergence Liouville's theorem says Δx Δp 2Δx 1 sinα nsinα = 2 = 1 Δx Δx 2 1 2Δx sinα 1 2 nsinα 2 1 1 = Δx 2 Δp 2 Even for total internal reflection over all angles, if Δx 1 >> Δx 2 there will be substantial light loss 3
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 4
Wavelength Shifters Wavelength shifting bars Wavelength shifting fibers 5
ATLAS Tile Calorimeter ATLAS Tile Calorimeter 6
ATLAS Tile Calorimeter ATLAS Tile Calorimeter 7
ATLAS Tile Calorimeter ATLAS Tile Calorimeter 8
Outer Reflectors Usually the scintillator and light guide are wrapped/enclosed with an outer reflector Measurements at 440 nm, 9
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 10
Photon Detectors We ll be interested mainly in the visible region today 11
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 QE QE ( ) % (%) = N N pe γ = 124 and S S = ( ma/ W ) ( ) λ nm photocurrent incident power 12
Photomultiplier Tubes (PMTs) 13
Borosilicate typical Windows 14
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 E = hν γ > W = E G + E A Alkalai metals have a low work function e.g. bialkali is SbKCs 15
QE of bialkali PMT s Photocathodes 16
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 17
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 18
( E) σ ( N ) the number of ( E) ( N ) Energy Resolution σ = E N the probability distribution governing f ( n, ν ) = e n! σ N 1 so = = N N N for 3000 electrons arriving at the first dynode this is σ E = v n 1 ν 3000 photoelectrons is ; n is number, ν is mean = 2% the Poisson distribution 19
Electron Focusing 20
Dynode Structure The dynode structure multiplies the number of electrons Process is similar to photocathodes but here the incident radiation is electrons 21
Dynode Structure There are a variety of dynode structures including some that are position sensitive 22
Dynode Structure δ = number of secondary electrons number of incident electrons δ of 4-6 for most dynode materials And typically there are 10-14 stages (dynodes) gain G G = α4 G 10 6 = αδ 10 N 23
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 10-19 / 5 x 10-9 = 8 ma Of course the average current is much smaller 24
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 25
PMT Gain and HV Supply gain dg dv so d for N = 10 dg < 10% only if G Thus G = dg G = αδ NkV = N N N 1 d dv V d = d = kv = N d NG V d N dv V dv V < 1% the HV supply must be well a a a a regulated 26
PMT Gain and HV Supply Typical gain versus high voltage curve Rule of thumb is ΔV=100 gives ΔG=2 27
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 28
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 μa 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 29
PMT Base Using capacitors or transistors to supply charge 30
Magnetic Shielding ΔV between the dynodes is ~100-200V 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 31
Magnetic Shielding 32
PMT s There are a wide range of PMT types and sizes From Hamamatsu catalog 33
ATLAS Tile Calorimeter PMT 34