CHEM 524

CHEM 524 Course Outline (Part 8)

V. Detectors - Two main types Thermal and Photon

A. Characterization

Responsitivity (R = X/F--signal size) vs. Sensitivity (Q = dX/dF --change of signal with change of flux)

Linearity (region Q = const.), Dynamic Range (magnitude of Q variation measurable), Stability (time for const. R, Q), Degradation (long term stability), Hysterisis (power dependent change in R,Q)

Rise time (10-90% full response) and Time constant [t = (2pf_c)^-1 f_c--> R = 0.7 R_max]

Noise equivalent Power (S/N = 1)-- Detectivity (D*= DA_d^1/2 (Df)^1/2,D = (NEP)^-1)

Quantum efficiency (k(l) = # electron / # photon)

B. Thermal (energy) detectors -- increase in temperature creates electrical response (table)

1. Typically light irradiate blackened plate, heating it, causing repsonse in the sensor coupled to it

Thermocouple, thermopile (voltage vs. T),

Thermistor bolometer (resistance vs. T),

Pneumatic Golay (pressure vs. T)--

2. Pyroelectric -- e.g. TGS -- responds to dT/dt, change in T -- relatively fast time constant, integrates flux, flat response with wavelength, relatively inexpensive, can be made into arrays

C. Photon Detectors -- quantum response to # photons above threshold (table)

--D* will be limited by background radiation from room temperature windows, optics

--power respose will fall off in uv compared to IR, more energy per photon

1. Photo multiplier -- current source based on photo-electric effect

Photo cathode -- P-E effect -- modest quantum efficiency --source of spectral response--see curves
Multiplier -- gives internal gain / results in low light level sensitive --based on dynode chain, each with secondary electron emit

Dark current -- main source of noise at high temperature

Shot noise-- proportional to F^1/2-- more light better S/N

Can be pulse-counted -- best S/N at low light level

2. Variants: Channeltron, micro channel plate, intensifier

3. Photodiodes -- photo-voltaic (P-V) -- excite e^- to conduction band, act as current source

Sensitivity: quantum efficiency high but no internal gain, need significan amplification (I -->V)

Spectrum depends on material, Si-vis to 1.1 µ, Ge to 1.8 µ, InSb (near IR to 5 µ), MCT (Hg_xCd_{1- x}Te, mid IR, varies with x, normally to 12 µ)--see response curves

Time response depends on material (see handout)

4. Photoconductors -- dominate IR market -- effectively variable resistance of semi-conductor, operate down to band gap, needs bias voltage, together act as voltage source

PbS, PbSe, --near IR, cheap, room temperature, slow

InAs (to 3 µ), InSb (to 5 µ) -- fast response, lower noise as P-V

MCT -- Hgx(Cd)1-xTe -- variable mid IR, liquid N₂ cool

Doped Ge-- dopants vary band gap--need more cooling

D. Multichannel Detectors

Film, Vidicon, Diode Array (1-D, Si based)

CCD - Si based, 2D, high quantum yield

Intensifier can increase sensitivity