AUTHOR: PALVAI GIRISH KUMAR MSc SOLID STATE PHYSICS OSMANIA UNIVERSITY SENIOR TECHNCIAL OFFICER INDIA Administrator for www.physicsdownloads.com & www.conceptualphysicstoday.comContact: firstname.lastname@example.org : AUTHOR: PALVAI GIRISH KUMAR MSc SOLID STATE PHYSICS OSMANIA UNIVERSITY SENIOR TECHNCIAL OFFICER INDIA Administrator for www.physicsdownloads.com & www.conceptualphysicstoday.comContact: email@example.com GEIGER MULLER (GM), SCINTILLATION & SEMICONDUCTOR
DETECTORS This document is propriety of www.conceptualphysicstoday.com & www.physicsdownloads.com RADIATION DETECTORS : RADIATION DETECTORS Geiger-Muller tubes
Semi conductors Few things about GM Detectors : Few things about GM Detectors Geiger Muller counter is a gas filled detector.
Works on the principle of ionization.
Works in the GM region of gas filled detectors.
Miniature in size. GEIGER MULLER TUBE : GEIGER MULLER TUBE GM Tube : GM Tube GEIGER MULLER TUBE : GEIGER MULLER TUBE WHAT IS GM REGION : WHAT IS GM REGION The region of operation of gas filled detector such that the gas multiplication factor is very high and the output pulse height is almost same irrespective of energy of incoming particle. GEIGER MULLER TUBE : GEIGER MULLER TUBE GM Tube Plateau Characteristic : GM Tube Plateau Characteristic GEIGER MULLER TUBE : GEIGER MULLER TUBE CONCEPT OF QUENCHING IN GM TUBE : CONCEPT OF QUENCHING IN GM TUBE Quenching :-
“Process of filling suitable gas to the counter for avoiding continuous discharge of GM tube, by charge transfer collisions”. TYPES OF QUENCHING IN GM TUBE : TYPES OF QUENCHING IN GM TUBE Organic Quenching
Halogen Quenching ORGANIC QUENCHING : ORGANIC QUENCHING Organic Quenching :
“ Addition of organic gas to counter for charge transfer collisions with ions. The requirement is that when ions of quench gas molecules are neutralized, the excess energy goes into discharge of complex molecules rather in liberation of free electron.”
Ex: Ethyl Alcohol & Ethyl Formate HALOGEN QUENCHING : HALOGEN QUENCHING Halogen Quenching :
“ Halogen gas addition has an advantage over Organic quenching in that these molecules are would again recombine after dissocation unlike Halogen molecules which permanently dissociate.”
Ex: Cl, Br GM Tube Dead Time : GM Tube Dead Time The dead time of Geiger tube is the period between the initial pulse the time and time at which a second Geiger discharge , regardless of its size begins. GM Tube Recovery Time : GM Tube Recovery Time The time interval required for GM counter to return to its original state to deliver full amplitude pulses is called “recovery time” of counter. GM Tube Applications : GM Tube Applications Survey meters
Area monitors Scintillation based detectors : Scintillation based detectors What is Scintillation? : What is Scintillation? A type of Luminescence.
Luminescence Fluorescence Phosphorous Slide 21: The property of emission of light when a energetic particle impinges on material (semiconductors) leading to creation of electron hole pairs and excitation of carriers. When these carriers come to their equilibrium states they emit light. What is luminescence? Slide 22: Three processes are involved in the Luminescence Phenomenon
Emission : Slide 24: Phosphorescence phenomenon :- conductor band Valence band ( a) (b) (c) (d) (e) Slide 25: An incoming photon with hν = Eg is absorbed creating an EHP
An excited electron gives up energy to the lattice by scattering until it nears the bottom of conduction band.
The electron is trapped by impurity level Et and remains trapped until it can be thermally re excited to conduction band.
The re-excited electron falls in a lower level of conduction band.
(e) Finally direct recombination occurs as the electron falls to an empty state in valence band giving off hv of approximately same band gap energy. Slide 26: (b) Non-Radiative recombination :- When the excited excess carriers reach equilibrium positions by emission of phonons due to surface / bulk defects / other defects it is said to be non-radiative recombination Slide 27: What Is Activator? Impurity atom occurring in relatively small concentrations in host material or a small stoichometric excess of one of constituents of material which exhibits the property of Luminescence. Slide 28: What Is Killer? Presence of certain type of impurity may also inhibit Luminescence of other centers, in which case they are referred as killers. Slide 29: Compounds which exhibit Luminescence in pure state.
- According to Randall, such compounds should contain one ion or ion group per unit cell with an incompletely filled shell of electrons which is well screened from its sorroundings.
eg:- Manganous halides, Samarium, Gadolinium sulfate, and platino cyanides.
The Alkali halides activated with Thallium or other heavy metals.
ZnS and CdS activated with Cu, Ag, Au, Mn or with an excess of one of their constituents.
The silicate phosphors, such as Zinc Ortho silicate activated with Divalent manganese which is used for Oscilloscope screens.
Oxide phosphors, such as self activated ZnO and Al2O3 activated with Transition metals.
Organic crystals such as anthracene activated with napthacene ; these materials are often used in scintillation counters. Luminescent Crystalline Solids Slide 30: Dependence of luminescence efficiency on Activator concentration? Slide 31: REQUIREMENTS High light /Fluorescence yield
Linear conversion of ∆E into Scintillation photons
Transparent to scintillation photons
Short decay time of fluorescence
Good optical quality with sufficient size
Fluorescence wavelength matches spectral response of readout device. Types of Scintillators : Types of Scintillators In-Organic Scintillators:
eg:- Alkali halides
i) High Z
ii) High density
iii) Good quantum efficiency
iv) Linear light output
v) Slower decay time Organic Scintillators : Organic Scintillators Liquids and plastics
High hydrogen content -> Fast neutron detection
Fast decay times, pulse shape discrimination between different types of particles In-Organic scintillators : In-Organic scintillators As in semiconductor, primary radiation creates electron-hole pairs which can recombine in various ways depending on material, dopants or activators .
In contrast to Semi conductors , recombination and de-excitation through light emission is the goal here.
Impurities or activators are often added providing energy states in the forbidden gap through which electrons can de-excite back in the valence band.
e-h pairs can migrate as in semiconductors.
e-h pairs form excitons
Decay through activator sites by radiative decay. Photo Detectors : Photo Detectors Convert scintillation light into electrical signal
Photo multiplier devices
Photo diodes Photo Multiplier Tubes : Photo Multiplier Tubes High quantum efficiency:
Emitted photo electrons/ incident photons
Low work function
Thin layer (~ 30 nm) to allow photo electron to escape the layer Semiconductor detectors : Semiconductor detectors Why to go for semiconductor detectors? : Why to go for semiconductor detectors? Better energy resolution
Linear response over a large energy range
Fast time response
Simplicity of operation Comparison between semiconductor detectors and Gas filled Detectors : Comparison between semiconductor detectors and Gas filled Detectors Similarities:-
Charge generated due to ionization of atoms.
Collection of charge carriers is due to drift of charge carriers in electric field caused by external potential. Comparison between semiconductor detectors and Gas filled Detectors : Comparison between semiconductor detectors and Gas filled Detectors Differences:-
Small energy to generate charge carriers.
The mobility of both type of charge carriers is fast. Slide 49: Consider a block of intrinsic semiconductor. The block has a length of “L” and a cross-sectional area of “A” with a resistivity of Þ = 60K Ohm-cm. You can observe that I > i. Keeping I<<i is a challenge.
So a pure semiconductor cannot be used as direct radiation detector. Slide 50: Concept of Doping
Adding foreign atoms from adjacent groups in the periodic table.
Group V : eg:- Phosphorous -> Excess electrons -> donors -> n type
Group III : eg:- Boron -> Excess holes -> acceptors -> P type Concept of Semiconductor junction : Concept of Semiconductor junction Creation of charge carrier free area called as depletion region by combining n and p regions to form p-n junction due to diffusion of electrons and holes in opposite zones. Reverse bias operation of p-n junction : Reverse bias operation of p-n junction Applying reverse bias increases the depletion region inside the p-n type diode. Types of silicon detectors : Types of silicon detectors Diffused junction Silicon detectors:
Produced by diffusion of n-type impurity (typically phosphorous), on to the surface of a P-type Silicon crystal of high resistivity.
Typical depth of diffused n-type layer range from 0.1 to 2.0 µm.
Normally depletion region extends into P-type due to n+ layer.
Back contact is provided by p+ diffused layer(normally Boron used). Advantages : : Advantages : Rugged
Less susceptible to radiation damage.
Surface layer outside the depletion region represents a dead layer or window through which the incident radiation must pass before reaching depletion layer.
Cannot be used for Charged particle spectroscopy efficiently. Application of Silicon detectors : Application of Silicon detectors Alpha particle spectroscopy
Conversion electron spectroscopy
Charged particle timing Radiation Interaction Modes : Radiation Interaction Modes Ge-Energy spectrum : Ge-Energy spectrum