electromagnetic radiation measurement

1. MEASURMENT OF ELECTROMAGNETIC
RADIATION
BY
MUNA BUDHATHOKI
CMCTH, CHITWAN

2. Solid state detector
• It is of 3 types;
1) Scintillation detector
2)Thermo-luminescence detector
3)Semi-conductor detector

3. scintillator
• Detector based on scintillation.
• Certain organic & inorganic crystal (phosphor)
emit scintillation upon absorption of radiation.
• In TLD and scintillation detector the
fluorescence and phosphorescence is
measured.
• In semi-conductor detector, the ionization and
corresponding current and voltage is
measured.

4. scintillator

5. Construction of scintillator
• It is cylindrical in shape.
• Contains certain organic like anthracene, stilbene and
plastic scintillator as well as thallium activated
inorganic phosphors such as NaI(TI) or CsI(TI).
• These crystal are is for detection of the rays and for the
change the x-rays and Gamma rays to the light .
• High atomic no phosphor for gamma rays & plastic
scintillators for beta particles.

6. Cont…
• Mostly the crystal is of the alkali halide as the
photoelectric interaction is increased by the
presence of the high-atomic halide component.
here NaI is prefered to CsI and KI because of
high cost.
• The alkali halide crystal should be activated with
an impurity which is mostly thallium iodide at a
concentration of about 0.1% . And crystal are
denoted as NaI(T1),CsI(T1),or KI(T1).

7. Working principle
• As the x and y ray interacts within a scintillation
crystal electrons are raised from lower energy to
a state of higher energy.later the electrons return
back to the lower energy state releasing the light
photons.
• Then light photons is directed to the
photocathode of PMT. and electrons is ejected.
• The ejected electrons are amplified and
multiplied by PMT of 6 to 14 dynodes with
applied pd of 100 to 500v between successive
dynodes.

8. cont
• Lastly the collected electrons by the anode are
collected to voltage pulse with an amplitude
of a few millivolts to a few volts.then these
pulse is delivered to the preamplifier,which
often is mounted on the photomultiplier tube.

9. Thermo luminescence detector
• Some materials absorbed radiation and retain in
metastables states.when this radiation is released
in the form of UV,visible or infrared light,then this
phenomenon is called luminescence.
• Luminescence are of two type ;a)fluorescence
b)phosphorescence
• Thermo luminescence material

10. TLD

11. cont
• Most commonly used TLDs are LiF:Mg,Ti,
LiF:Mg,Cu,P & Li2B4O7 because of their tissue
equivalent. and beside these CaSo4:Dy,
Al2O3:C & CaF2:Mn are used because of
their high sensitivity.
• Before TLD is used it need to be annealed
• The free electrons and holes produced by
radiation

13. cont
• Traps are of two types ; a) storage trap
b)Recombination centre
• A charge carrier released from a storage trap
may recombine with a Trapped charge carrier of
opposite sign in a recombination centre
(luminescence centre).
• The recombination energy is at least partially
emitted in the form of ultraviolet, visible or
infrared light that can be measured with
photodiodes or photomultiplier tubes (PMTs
bination centres.

14. Cont..
• The electron or energy is trapped in the electron
traps after exposure for month , years unless
energy is supplied.
• The electrons released due to the radiation goes
to conducting band and after heating its returns
to valance band releasing light and light is
directed to the photocathode of PMT
• The amount of light striking the photocathode is
propertional to the energy absorbed in the
phosphor during irradiation.

15. • The light photons is change in to the electrons
in the photocathode of the PMT which is
amplified and measured.

17. Semiconductor detector
• Device that uses for the detection of ionizing
radiation

18. Detection mechanism
• Ionizing radiation measured by the no of charge
carrier set free in the detector material
• Radiation incident on the semiconducting
junction produces electron hole pair as it passes
through it
• Electrons and holes are swept away under the
influence of electric field , where they result in a
pulse that can be measured in an outer circuit
•

19. • Types
i)silicon detector
ii) Diamond detector
iii)Germanium detector
iv) Cadmium telluride detector

20. Silicon detector
• Work in principle by dopping narrow strip of
silicon (100micro meter)
• As charge particle passes through this stripe ,
they cause small ionization current that can be
detect and measured

21. Germanium detector
• Used for gamma and x-ray spectroscopy
• Crystals are dopped with lithium
• Also called high purity germanium detector
• Liquid nitrogen used to cool the
semiconductor

22. MOSFET
• Transistor used for amplifying or switching
electronic signals
• Miniature silicon transistor
• Based on the measurement of the threshold
voltage(0.45v)
• Poses excellent spatial resolution
• Offers little attenuation of the beam
• Particularly useful in vivo dosimetry

24. • Consist three terminals ;source
Gate
Drain
• Charge carrier enters from the source and exist
from the drain
• The channel width is controlled by the voltage on
an electrode called gate

25. Types ; single bias(single MOSFET)
Dual bias (dual MOSFET)

26. Single bias
Temperature dependence
Instability of response
Dual bias
Two MOSFET in same silicon chip operating at two
different gate biases
Better sensitivity , and stability than single MOSFET
Minimal temperature effect