References Hans Kuzmany : Solid State Spectroscopy (Springer) Chap 5 S.M. Sze Physics of semiconductor devices (Wiley) Chap 13 PHOTODETECTORS

Detection of Electromagnetic radiation Signal and Noise Photomultipliers Photoelectric Detectors

Signal and Noise for photon counting Low signal/noise ratio 23/4 = 6.5 Scattering experiment S= signal N = noise B = background D = dark level

Signal and Noise Low probability event Signal = photon absorption Scattering Absorption Probability of absorption, i.e. contribution to signal= p Probability = q =1-p For coins p=q=1/2 For photons……. p <<q Origin of Noise at detector

Signal and Noise Signal = photon absorption n incident photons Probability of no absorption q =1-p Binomial distribution k adsorbed photons Probability of 1 photon absorption = p Probability absorption of k photons Or no contribution to signal

Signal and Noise For photons……. p <<q np = expected value Poisson distribution n = 200 p = 0.05  k  = 10  k  = mean value of k

If I have 200 photons every sec, for every second the absorbed photon number might be 7,9,10,15, depending on the probability distribution, but on average I have 10 photons absorbed per sec. For large n, np   k  So is the average magnitude of the signal By increasing the measuring time (or equivalently increasing the number of incident photons ) the magnitude increases linearly, and the noise increases as square root, so the signal to noise ration gets better as  T The noise intensity is defined as the variance  of the Poisson distribution So the signal is on average 10  

PHOTOMULTIPLIERS Elements: Photocathode Dynodes Anode e-e- Operation: Photocathode animation Photon in Photocathode e - emission e- on dynode Secondary e - emission Current on Anode

Photocathode Material to emit electrons by photoelectric effect Key property: low work function to allow extraction of e - The photon absorption depend on the material Hence the photocathodes are sensible to some part of the light spectrum Quantum efficiency

Radiation sensitivity Typical 80 mA/W I c = current at photocathode P = incident light power

Dark current Due to thermal emission of electrons M = material dependent factor (  0.5) T = temperature W = material work function (1.5-3 eV) J(T) increases rapidly with T, so photocathode needs to be cooled if you need to observe few e/s

Dynodes The dynodes work by employing secondary electron emission (SEE) SEE: When a primary beam hits a surface, then it generates electrons that are either emitted either travel into the solid and generate more electrons

Secondary Electron Eemission Physical principle: ionization of a solid (atom) by an electron with kinetic energy E 0 Each scattering event might generate one or more e - Secondary Electron Yield I 0 = incident beam current I S = secondary current (I emitted from surface) E 0 =  [1  10 6 eV]

Secondary Electron Eemission Contributions I e = elastically scattered e - I 0 = incident beam current I S = secondary current (I emitted from surface) E 0 =  [1  10 6 eV] I r = rediffused e - I ts = true secondary e -

Collect the current by applying a voltage V so that only e - with E K  E = eV arrives at detector The signal is the sum (integral) over the electrons up to the maximum E K Usually we are interested in the value of S for a range of energy and to get N(E) we must differentiate the signal Electron Energy (eV) N(E) E0E0 E 0 + ΔE

For dynodes all the current originated from secondary emission is used The number of dynodes n provides the multiplication factor G (gain) of the photomultiplier Typical values  = 5, n = 10 G = 5 10  10 7 G depends on the voltage because the voltage sets the primary energy of the incident e - generated in the dynode

Slab of semiconductor between two electrodes PHOTOELECTRIC DETECTORS Generation of carriers: intrinsic  = mobility n,p = concentration q = charge For < c incident radiation is adsorbed Generation of carriers: extrinsic The cutoff is determined by the energy of donor and acceptor states Performance detemined by: gain, response time, sensitivity

Principle of operation n 0 = density of carriers generated by a photon flux at t=0 Recombination processes P = optical power n(t) = density of carriers at time t  = carrier lifetime At steady state, the carrier generation rate is equal to recombination rate 1/  = recombination rate Total number of photons impinging on the surface/unit time is P/h  Generation rate  = quantum efficiency PHOTOELECTRIC DETECTORS Steady, uniform photon flux on A=wL

The current due to photon absorbption The gain of the device depends on carrier lifetime and carrier velocity Defining primary photocurrent as Carrier Transit time

 depends on absorption coefficient PHOTODIODES Depleted semiconductor High E to separate photogenerated e - -h pairs Depletion region small to reduce t r Depletion region large to increase  Reverse bias to reduce t r