Download 7D SAMPLE CONTAINERS 7E RADIATION TRANSDUCERS

7D SAMPLE CONTAINERS
Sample container are required all spectroscopic studies except emission
spectroscopy
Name : cells or cuvettes－ are made of materials that is transparent to
radiation in the spectral region of interest.
UV : quartz , fused silica (below 350 nm) , (180 nm ~ 3000 nm IR)
VIS : (1) glass container (silicate glasses : 350 nm ~ 2000 nm)
(2) plastic container
IR : NaCl or KBr (IR)
7E RADIATION TRANSDUCERS
7E-1 Introduction
The earliest detector: huma
ne
y
e
s→ ph
o
t
o
gr
a
ph
i
cpl
a
t
eorfilm.
Modern detector: convert radiant energy into electrical signal.
Properties of the Ideal Transducer
Ideal transducer :
1. High sensitivity
2. High Signal-to-Noise ratio (S/N)
3. Constant response over a considerable range of wavelength
4. Fast response time
5. Zero output signal in the absence of illumination ( no dark current)
6. S = KP
P : radiant power
S : electrical signal output transducer. I (current) or V (voltage)
For real transducer
S = KP + Kd
∵The dark current is usually a constant (Kd) over short measurement
periods. ` Give a compensating circuit can take Kd away ( re-zero 0)
∴S = KP
Types of Radiation Transducers
(1) Photon transducer (photoelectric or quantum detector)
－ used in UV , VIS , near IR
It is not popular for IR (need dry-ice or N2 to avoid interference from
thermal background noise)
(2) Heat detector ( thermal transducer )
－ used in IR
The energy of IR radiation is usually too small to excite the electron on
photon transducer. (longer wavelength means lower energy)
－ constant response to wavelength but lower sensitivity than Photon
transducer.
FIGURE 7-27 Relative response of various types of photoelectric transducers (A-G)
and heat transducers (H, I): A, photomultiplier tube; B, CdS photoconductivity; C,
GaAs photovoltaic cell; D, CdSe photoconductivity cell; E, Se/SeO photoconductivity;
F, silicon photodiode; G, PbS photoconductivity; H, thermocouple; I, Golay cell.
7E-2 Photon Transducers
Barrier-Layer Photovoltaic Cells
Photovoltaic cell (barrier-layer cell ) (FIGURE 7-28) :
－ radiation --> current
－ VIS region (FIGURE 7-27E) (similar to human eyes)
FIGURE 7-28
Schematic of a typical barrier-layer cell.
Ra
d
i
a
t
i
on→ s
e
mi
c
o
n
du
c
t
or→ covalent bonds are broken and form e()
and hole (+).
e – → e - migrate to silver film→ through external circuit
hole  Iron layer
I = 10 ~100 μA
Photocurrent is proportional to the radiant power that strikes the cell
(when R＜4
0
0Ω, R is the resistance of the external circuit)
Disadvantage :
(1) amplification of output is not easy.
(2) low sensitivity to low level of illumination.
(3) fatigue –output current decreases gradually during continued
illumination
Advantage: no external source of electrical energy is required,
simple,
rugged,
carriable, (portable)
low cost.
Vacuum Phototubes (FIGURE 7-29)
(Photoelectric effect) –more sensitive
Pho
t
onbe
a
m→ Ca
t
h
od
e(
p
h
ot
oe
mi
s
s
i
vema
t
e
r
i
a
l
, alkali metal) (－) →
electrons → a
no
de(high voltage difference) (＋)
Radiation (or light) own the characteristics of both electromagnetic wave
and particle. we call it photons or quanta
ne-
∝
radiant power of the beam (∝ : proportional to)
FIGURE 7-29 A phototube and op amp readout. The photocurrent induced
by the radiation causes a voltage drop across R, which appears as vo at the
output of the current-to-voltage converter. This voltage may be displayed on
a meter or acquired by data-acquisition system.
FIGURE 7-30
Spectral response of some typical photoemissive surfaces.
Photomultiplier Tubes (PMT) ( FIGURE 7-31)：
－ Can be used to detect the arrival of a single photon.
(light-tight compartment and low temperature)
－ nine electrode (dynodes)
dynodes 1 = 90 V
2 = 180 V
3 = 270 V
(Several electrons are emitted for each electron that strikes the surface of
each dynode.)
－ 106 ~107 electrons formed for each incident photon.
－ highly sensitive to UV and VIS radiation
(Good for low power radiation)
－ Extremely fast time response
limitation :
(1) dark-current. (-30℃ can remove it)
－ cause by thermal emission.
－ Intense light will cause irreversible damage to the photoelectric
surface. (Avoid daylight or other strong light)
(2) Only for measuring low-power radiation. (light-tight compartment)
FIGURE 7-31
Photomultiplier tube.
Silicon Photodiode Transducers
(FIGURE 7-32)
P type : ⅣA(Si, Ge) doped with ⅢA(Ga)
N type : ⅣA doped with ⅤA(As)
j62u04y7無電子
FIGURE 7-32 (a) Schematic of a silicon diode. (b) Formation of depletion
layer, which prevents flow of electricity under reverse bias.
(A DC voltage applied to form a depletion layer which can regenerate
hold and electron while photon strike.)
Sensitivity:
PMT ＞ Silicon diodes ＞ Vacuum phototube
bias: A DC voltage applied to a device to control its operation.
reverse bias
Bias on a PN junction that allows only leakage current (minority carriers)
to flow. Positive polarity on the n-type material and negative polarity to
the p-type material.
7E-3 Multichannel Photon Transducers
Early time --A photographic plate (or film strip) was placed along the length of
the focal plane of a spectrometer to record all the lines in a spectrum
simultaneously. (Sensitivity: down to 10 –100 photons)
Limitation: time required to develop the image of the spectrum.
Modern --PDA(1D), CID(2D) and CCD(3D) replace photographic plate (or film
strip)
Photodiode Arrays ( PDA) one-dimensional (FIGURE 7-33)：
Range: 64 ~ 4096 diodes － 1024 diodes is the most widely used,
FIGURE 7-33
top view.
A reverse-biased linear diode-array detector: (a) cross section and (b)
FIGURE 7-34
Schematic of a photodiode-array detector chip.
Charge-Transfer Devices
FIGURE 7-35 Cross section of a CTD in the charge integration mode. The positive
hole produced by the photon hv is collected under the negative electrode.
Charge- Injection Devices
FIGURE 7-36
Duty cycle of a CID.
Charge-Coupled Device
FIGURE 7-37
A CCD array: (a) arrangement of 512 × 320 pixels and
(b)schematic showing four of the individual detectors.
7E-4 Photoconductivity Transducers：
－ semiconductors (resistance decrease when they absorb radiation)
－ The most sensitive transducers for monitoring radiation in the
near-Infrared region (750 nm ~ 3000 nm)
－ Cds , Pbs (800 ~ 3000 nm , used in room temp. ∴used mostly),
PbSe
－ for-IR (operated in low temp.)
(cooling to suppress the noise arises from thermally induced transition.)
－ also apply in FT-IR.
Graph here
(Absorption of radiation by these materials promotes some of their bound
electrons into an energy stable in which they are free to conduct
electricity.)
7E-5 Thermal Transducers：
--- Theh
νc
omef
r
omI
Rr
a
n
gea
r
el
a
c
kofe
ne
r
gyt
ocause photoemission
of electron that phototransducers are generally not applied.
--- IR beam's power is 10-7 ~ 10-9 W
→ absorbing element must be as small as possible to produce a
detectable temperature change.
→ thermal noise from the surrounding must be avoided.
Characteristics of thermal transducers:
1. Tiny absorbing element. (a small black body)
2. Vacuum sealed detector (reduce thermal noise) and carefully shielded
from thermal radiation.
3. Chopped incident beam (minimize the effects of extraneous heat
source)
Three types of thermal detectors:
(a) Thermocouples ( voltage )
(b) Bolometers ( resistance )
(c) Pyroelectric Transducers (capacitance )
Thermocouples
Thermocouples detector
hν (IR) → T↑ → Vol
t
↑
A well-designed thermocouple transducer can detect the temperature
difference of 106 K which cause the potential difference of about 6 ~8
μV/μW .
FIGURE 7-38 Thermocouple and instrumentation amplifier. The output voltage vo
is proportional to the thermocouple voltage. The magnitude of vo is determined by the
gain resistor Rg’whi
c
hpe
r
f
or
mst
hes
a
mef
unc
t
i
ona
sR1/a in Figure 5-4.
Bolometers (specific application)
Bolometers transducer : a resistance thermometer made by metal (Pt or
Ni) or semiconductor (thermistors)
hν(
I
R) → Tc
ha
n
ge → Rc
ha
nge
Special application : (Ge) Bolometer－ operated at 1.5 K － an ideal
transducer for 5 ~ 400 cm-1 (2000 ~25 μm).
( most used IR range: 670 ~ 4000 cm-1)
Pyroelectric Transducers
Pyroelectric materials － dielectric material
Triglycine sulfate (NH2CH2COOH) 3‧H2SO4 is the most important
Pyroelectric material
Graph here
T ＞ 47℃ － Curie point for Triglycine sulfate loose residual
polarization.
Apply electric field
↓
Electric polarization (pyroelectric material)
(Electric field removed) ↓
Temperature-dependent capacitor polarization
IR radiant ~~~＞ ↓ T change
T change  change distribution
(change across the crystal material  create a current)
↓
Change conductivity (create a current)
Re
s
po
n
s
eTi
mef
a
s
t→ FT-IR detector (Time domain signal)
7F SIGNAL PROCESSORS AND READOUTS
7F-1 Photon Counting