Ideal Detector Fast Cheap Rugged Responds to all

Ideal Detector • • Fast Cheap Rugged Responds to all wavelengths of light Can distinguish different wavelengths Sensitive Low LOD

Photoelectric Effect • Light consists of particles (photons). The energy of each is proportional to their frequency. • A certain minimum amount of energy (dependent on the material) is necessary to remove an electron from the surface of a zinc plate or another solid body (workfunction). • If the energy of a photon is bigger than this value, the electron can be emitted. • A large number of lower energy photons are not able to remove an electron.

Photoelectric detectors • Light is shone on a photoemissive cathode • The number of electrons emitted is proportional to the radiant power. • The electrons are collected at an anode (held at a more positive potential). • Both are sealed in an evacuated envelope. • Called a phototube.

• As the voltage difference between anode and cathode increases, more electrons are collected, • until • About 50 V where the signal no longer increases with V • Then the current is proportional to radiation intensity • The current is not sensitive to wavelength • (within certain limits)

Photomultiplier tube • Current produced in phototube is small – requires amplification • Add a series of DYNODES between cathode and anode – each at a successively higher V • Each emits several electrons when hit by an energetic (accelerated) electron • Cascade effect • Big signal

PMT’s • Fast response times (~10 -8 sec) • Show a dark current (a small background current even in the dark) • Mostly from thermionic emission – can overcome by cooling • Some from natural radioactivity and cosmic radiation

Photon Counting • Usually output of PMT is averaged over time. • But in low light the electrical pulses produced by each photon can be counted. • Improved S/N and precision • Good for low light – Fluorescence • Detector is more complex, expensive

P-type semiconductor • The addition of trivalent impurities such as • boron, aluminum or gallium to an intrinsic • semiconductor creates deficiencies of • valence electrons, called "holes". • It is typical to use B 2 H 6 (diborane gas) to diffuse boron into the silicon material.

N-type semiconductor • The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor. • Phosphorous may be added by diffusion of phosphine gas (PH 3).

Reverse bias

Reverse biasno current flows and there is a region formed without charge carriers Depletion layer

• When radiation hits the depletion layer, holes and electrons are formed and a current flows. • The current is proportional to P (power of incident radiation)

Photodiode array – a row of photodiodes – all on a chip

Diode arrays • Used with a grating which disperses the light so each ‘wavelength’ falls on a different photodiode. • Grating doesn’t move • No slits i. e. no monochromator • Spectral scans are very fast • Good as chromatography detectors or fast kinetics

Charge-Coupled Detector • A CCD is an integrated-circuit chip that contains an array of capacitors that store charge when light creates e-hole pairs. The charge accumulates and is read in a fixed time interval. • CCDs are used in similar applications to other array detectors such as photodiode arrays, although the CCD is much more sensitive for measurement of low light levels.