Radiation Sensors: Photomultiplier Tubes and Photoemission

Radiation Sensors

• Radiation sensors are developed to detect emissions from matter.
• These sensors rely on the photoelectric effect.
• Radiation energy propagates through space in quanta.
• When radiation collides with matter, photons are emitted, reflected, or absorbed, depending on the material's properties.
• If a photon has energy hv (where h is Planck's constant and v is the radiation frequency), then hv = 1/2 mv^2 + Φe
  • 1/2 mv^2 is the energy of the emitted electron from the atom due to the photon's impact.
  • Φe is the energy required to release the electron (e is the electron charge, and ΦΦΦ is the material's work function).
• The kinetic energy of a photoelectron depends only on the incident photon energy transmitted to the electron.
• If the incident energy is high enough, the electron is detached and emitted from the material.
• Radiation sensors can also be called photosensors, photosensistors, or used for optical or photonic sensing.
• Types of radiations include infrared, ultraviolet, visible light, photon-type, X-rays, and nuclear radiations like β- and γ-rays.
• Earlier radiation sensor classifications:
  • Photoelectric cells (photoemissive)
  • Photoemf cells (photovoltaic)
  • Photoconductive cells (light-sensitive resistors)
• A photosensistor combines two electrodes in an electrolyte.
  • Electrodes change size/shape and the electrolyte changes to gas, liquid, or solid based on radiation range, frequency, or wavelength.

Basic Characteristics of Photodetectors

• Work function
• Quantum yield and quantum voltage
• Spectral sensitivity and spectral threshold
• Time lag
• Drift, fatigue
• Static and dynamic responses
• Linearity

Work Function

• Work function is a physical constant for a given material and is expressed in electron volts.
• Denoted by Φ, it represents the energy (E) needed to overcome the surface attractive forces.
  • Given by the equation E = Φe, where e is the electronic charge.
• Metallic elements exhibit smaller work functions for higher atomic numbers.
  • Cesium (1.54 eV) has the smallest work function and is a good choice for photodetectors.
• Alkali metals are electropositive, lose electrons easily, and are ideal, but vulnerable to atmospheric conditions.
• Alkali metals serve as surface layers on metal plates with higher work functions, such as Na, K, Rb, Cs, Ag, Be, Ta, Ni, Al, Cu, Ca, Zr, and W.
• Alkali metals have a single electron in their outermost orbit.
  • A low work function is enough to remove an electron from the atom.
  • When the number of electrons in the outermost orbit increases, the metal has a higher work function.

Quantum Yield and Quantum Voltage

• Is a ratio of emitted electrons from the sensistor cathode to the number of photons it receives.
• At any wavelength λ, the number of emitted electrons can be given by a number 6.242 x 10^18 * ℑ
  • ℑ is the sensitivity.
  • A flow of 6.242 x 10^18 electrons is required to produce 1A of current.
• The energy to free an electron: Eλ = 12395/λ eV = 1.9857 x 10^-8/λ ergs
• One watt (10^7 ergs/s) of power would release 5.036 x 10^14 λ electrons/s
  • Producing a current of (5.036 x 10^14)/(6.242 x 10^18) A/W
• Quantum yield: Qy = (6.242 x 10^18 ℑ) /(5.036 x 10^14 λ) = 12395 ℑ / λ
• The energy of a photoelection is acquired through photon impact
  • Expressed as quantum voltage.
  • Maximum Kinetic energy: Em = Eλ - Φ
• Threshold wavelength is defined as when Em = 0
• The value of the quantum voltage: Eλ = hν = hc/λe = 1.2395/λ eV

Spectral Sensitivity and Spectral Threshold

• Occurs at absolute zero when electron velocity is zero V = 0.
• If hv > Φe, then electron escape from the metal surface is possible with radiation.
• Threshold frequency in this case: νo ≥ Φe/h
• Wavelength is defined as: λo ≤ hc/Φe
• Velocity of light: λo = 1.2395/Φ µm
• For caesium, λo = 0.8045 µm
• The photoelectric emission from a metal surface occurs if the incident radiation's wavelength is less than the threshold frequency wavelength.
• The emission amount is proportional to the incident radiation's intensity.
• The spectral sensitivity of photosensistors is higher than the threshold frequency and is a function of the incident radiation.

Time Lag

• Time lag varies over a wide range for photosensistors.
• In photo-emissive cells it's very small, around *10^-8 s *, but quite large, around 5 x 10^-2 s in light-sensitive resistors.
• For gas-filled photocells, the time lag is around 10^-5 s.
• The time response of photosensistors is characterized by: Y = Yo (1 - e^(-αt))
  • Yo and α are constants.
  • α is large for photoemissive cells, medium for photovoltaic cells, and small for photoresistor types.

Drift, Fatigue

• When incident radiation fluctuates above 100Hz, the detector response does not follow the fluctuations faithfully.
  • This is prominent in LDRs and is called dynamic fatigue.
• For steady, high-energy incidence, the photodetector output is not always consistent with input
  • Called static fatigue.
• Drift is the transient response change during a short period after irradiation and is more common in photovoltaic cells.

Static and Dynamic Response

• Static response is the ratio of output to input under steady illumination.
  • Static sensitivity: Sst = ia/Φ1 where ia is the anode current.
• Dynamic response is given by dynamic sensitivity: Sdy = ∂ia/ ∂Φ1 = ∂i/∂t /∂Φ1/∂t

Linearity

• The linearity in the response of a photosensistor is not ideal, especially under loaded conditions.
• Photovoltaic cells produce a voltage and linearity between this voltage (V) and incident light flux (Φ1) is ideal.
• For photoemissive cells, linearity at R = Roe^(-βL)
  • R is much better.

Types of Photosensistors/Photodetectors

• Photoemissive cells and photomultipliers
• Photovoltaic cells including photodiodes
• Photoconductive cells and light detecting resistors

Photoemissive Cell and Photomultiplier

• This type of radiation sensor shows an external effect.
• A photoelectric cell consists of a pair of electrodes separated by a rare gas or vacuum.
• Light falls on a properly coated photocathode with a very low work function, releasing electrons that are attracted to the anode.
• The external circuit connects with a resistance, and changes in current indicate optical radiation intensity.
• A photomultiplication process is needed for larger current output because single-pair electrodes cause very small currents on the cathode.
• Dynodes, which emit secondary electrons, are used as secondary emitters of electrons.
• All electrodes are kept at a higher potential to attract electrons.
• A light shield is a grill connected to the photocathode.
• Optical radiation reaches the photocathode (P) through the light shield.
• Electrons from the photocathode attract to dynode 1, releasing secondary electrons upon impact.
• These successive impacts on other dynodes increase elections exponentially and this stream of elections are picked up by the anode (A).
• An external load may now connect to it to produce given current output
• The equation of the electric current is as follows
• I = IpK^n
  • Ip is the initial primary photoelectric current
  • K is a constant dependent on the duynode
  • n is the number of stages...
• The potential difference applied between successive stages is about 100-130 volts.
• A mica shield between the photocathode and multiplying stages prevents spurious electron emission.
• A vacuum photoemissive cell has a variation when filled with an inert gas at very low pressure.
  • With potential between the cathode and anode exceeding a certain critical value for the filling gas, the photoelection emitted gets accelerated and ionizes gas atoms into another election and a positive ion.
• The positive space charge close to the photocathode induces secondary election emission from it which partly neutralizes the positive ions.

Photoconductive Cell

• In intrinsic semiconductors, thermal energy excites a small number of valence band electrons to the first conduction band.
• The resulting holes and free electrons move in the crystal lattice, conducting electricity.
• This is called photoconductivity if it occurs due to photons.
  • It occurs when enough electrons shift into the conduction band after being irradiated by photons, changing material conductivity.
  • Devices include photoresistors or light-dependent resistors (LDR).

LDR

• Photoconductivity occurs when the photon energy is sufficient to shift the electrons.
  • The photoconductivity stops when the photon energy has a sharp cutoff when the photonic energy is equal to the semiconductor energy gap. Thermal vibrations cause fluctuations in the width of the energy gap
  • However low-energy photons may pass through the material without interacting with their electrons. Germanium is suitable for creating lenses suitable to these radiations.

Photovoltaic and Photojunction Cells

• A photovoltaic cell fabricated as photoemf cell which consists of a layer of a semiconductor on metal.
• It has a transparent thin of a precious metal layer coated on top
• The incident radiation at the top passes the semiconductor which releases electrons These electrons can flow inwards or outwards to the source of the radiation depending cell type.
• Photocells are of two types:
  • Backwall type: here the electric exposed to the radiation becomes positive
  • Frontwall type: here the electric exposed to the radiation becomes negative.
  • The barrier layer connects the interfaces of the electrodes and is where the charges move and develop. The selection of material determines the cell type: Uses copper oxide over copper Uses selenium over metal base The selenium cell has the properties close to that of pure selenium (p-type conductor) from the human eye.
• Usually this is aluminium or brass, over the Selenium thin film. Coating is then heated to create crystalline cadmium after which n-type layer is created.
• Barrier later is then developed

Photojunction Cells

• In forwarded bias, the pn junction is used as electric circuit where the diode has no effect and current flows naturally.
• Small leakages caused by the intrinsic carriers will thus begin.
• Forward bias increases the intrinsic carrier rate under irradiation in photodiodes.
• Such diodes are also called: Photojunction Cells

Position Sensitive Cell

• These cells are designed to be conductive based on where the resistance rests on the layer with photoconductive properties, and the position of the source of illumination. They include the following layers:
  • High resistance layer
  • Photoconductive layer
  • Conductive Insulted Level Any source near these would generate an electrical signal based on its nature

Other cells

• Phototransistors
  • Made to amplify photodiode current
  • Base connection allows it to operate similar to photodiode
• PhotoFETs
  • Similar to phototransistors in function although it yields a leakage current I(G)+I(P) because of the way its connections are configured.
• Photocouplers
  • Provides electrical installation between optical devices
  • Using a phototransistor with a light and an LED.
• Transfer Ratio
  • Uses optical transfer, the output current (I2) to input current (I2): ratio.
  • This determines how well in encapsulated.

X-Ray and Nuclear Radiation Sensors

• Use radiations to find their different units to determine measurements, using energies detected at different stages.
• Roentgen
  • intensity of air radiation that looks at per cc levels and air pressure at degrees of 1 atm.
  • The RBE or Relative Biological Effectness refers to the rate of the x-rays, gamma ray effect on the rate of detection.

Sensor Types

• Geiger - Muller counter
• Proportional counters are more sensitive and often used for detecting sources like Alpha, Beta and Gamma radiation.
• Ionisation or Scollintation chambers
• Electron Multiplier Tubes
• A non-dispersive detector that does not contain the radio isotopes.

Particle Emissions

• Nuclear omissions from radiation include:
  • Alpha particles
  • Beta particles
  • Gamma Radiation
• They radiate via neutrons in X-rays.
• Non ionising includes:
  • UV radiation - Infrared radiation.
  • Low frequency radiation, radio frequencies, and microwaves.

Ionization and Radiation Characteristics

• Gamma and neutrons are indirect, with p+n junction used for films and diodes.
• Alpha are positive and high with high ionisation however have low penetration that require semi conductors along with plastic
• Beta is electron-based, requiring Geiger plates

Ionisation Chambers

• Using a central electrode separate from chamber this is a basic chamber is designed
• This basic chamber is gas filled.
• Voltage passes the electiode
• Electrode has the following properties:
  • V = Q/C: this results with chamber of ion detection There will therefore be ionese of three types namely: electrons, positives molecules, and negative molecules
• Smaller time will be required for the voltages.
• When there is re-comination of the molecules this gives rise to certain densities,

Positive and negative charge densities

- dest/dt = ded-/dt = p edt ed-

• P is the recombination coefficient of value between to 10^-6 or 10^-5 for gasses. The elections in a chamber drift and therefore voltage is: V = (M+)(Ef/p )

For specific ionization parameters:

• number of atoms:
• energy
• interval

Proportional Counters

• These sensors require:
• High degree of sensitivity and are used to find the rate of Alphas, Betas and Gamma radiation.
• Chambers need to be gas filled.
• Gas amplification allows for a greater ionisation
• Multiplying of the gases in the chamber can change between to ^3, 10^4 and ^5, 10^6
• Applying good field designs while number.

Proportional counters properties:

• Diameter collections, outer electiode
• Number of atoms involved by formula: vp (t) ((noe)/c (ln (do/dn) / (ln(doldn))* - (ln(dp/dn) / (ln (doldn)) where is the + or - relationship,

The velocity of positive ions is defined as:

• Vt = d/dt (dp) = M(Ef/p) with supply being the electrodes,

Geiger counters

• Detects radiations of three types using a wire of gas where the radiations terminate at the sides:
• They possess high sensitivity and high throughputs. Properties that affect the counts:
• What type the ends are eg needles
• What type the sides are, glass and metal
• How the material is made, glass and metal
• How the radiation is passing, through electrons, through ionisation.
• Type of gas used, argon and nobel gas mixtures.
• Use insert length to ensure count length is the same

Scintillation Detector

• Uses electrical pulses activated by: crystals, liquids or gas.
• The components include: light pulse, photodectors with amplifier shaping capabilities and scalers where radiation passes the crystal

Formula

Varies, n = K+K2 E + K, shows dimensional constant Solid states possess a type of semi conductor like Si and GE to detect a great number of radiation Highs level of radiation: is the most sensitive part. Films come in plastics and are the best, using flourite can and does Solid sheets detect the radiation via LiF and Caf2.

Fiber Optic Sensors

• Fibres send:
  • Communicate via channels or external effects
  • Classify by groups.
  • Transmitters through the fibers modulate sensor output.
• There are multiple types of Fibre Optics
• Thermistors (temperatures) with radiation and lasers use 2 sensors that detect a function within a range of different temperatures.
• Fibres need to detect levels.
• Fibre needs to work at what is specified.

Fibre optic sensors

• Optic fibre works on total reflection where incident lights detect any reflection
• This level stops depending on the medium or level of liquid
• Bottom ends need to be long Fluid flows need to be transverse on optical fibers Acoutstics and multimodes need to be used due to bending to see any light passing the fibre

Explore the principles of radiation sensors, focusing on photomultiplier tubes and photoemission. Understand the function of dynodes and mica shields in photomultiplier tubes. Investigate the impact of work function on photoemission and the effects of inert gases in vacuum photoemissive cells.