Download H Detectors

Ideal Detector
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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.