Transcript H Detectors
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 B2H6 (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 (PH3). 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 for 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.