PHYS C5

Why is radiation detection important

Radiation safety

Creating an image

Measuring ionising radiation dose

Measuring the spectrum of the radiation

How do machines detect ionising radiation

Ionisation in gases

Ionisation and excitation in certain solids

Changes in chemical systems (e.g. film blackening)

Heat generation in radiation absorbing material (calorimetry)

Radiation Detector Systems

• Luminescent materials convert radiation to visible light then to charge (Phosphor, Scintillator, Badge Dosimetry) *less efficient and lose info by converting to light

• Ionisation detectors convert radiation energy into charge (Geiger-Muller, Proportional Counters & Transmission Ionisation Chambers [gas filled detector - ion pairs] and Solid-State Detector [semiconductor - electron-hole pairs])

Best: Solid-State Detector because convert radiation directly to charge

Main Detector Types

Luminescence: incoming photon is absorbed and lower energy radiation is produced (film screens, scintillators)

Energy trapping: computed radiography image plates, badge dosimeters (TLDs & OSLs)

Digital radiography systems: solid state / semiconductor detectors

Ionisation chambers: Geiger-Müller and proportional counters

Forms of Detection

• Detect each photon - system operates in pulse-mode, some photons are ‘missed’ while the previous photon event is being ‘processed’ (dead time)

• Detect beam energy - system operates in current-mode, Output is integrated (or smoothed) over time

Ion pair

A secondary electron and the cation in was ejected from

detection efficiency

ability to count as many photons as possible - more interactions = more detection

What determines detection efficiency

Detector material:

More thickness = greater stopping power

High atomic number and density = more interactions and absorption

Entrance window = if too thick/dense stops low energy radiation before reaches detector

Large size = more radiation hit detector

What determines energy resolution

Detector material: less energy needed to produce charge carriers = less statistical variation

light collection - if lost or unevenly collected signal fluctuates

Why is detection efficiency and energy resolution competing priorites

they depend on mutually exclusive physical characteristics of detector:

Density and Atomic Number: high = better efficiency because more stopping power but poor energy resolution because poor light output/charge collection properties

Detector Volume vs. Signal-to-Noise Ratio: large, thick crystal more stopping power so good detection efficiency but increased capacitance (ability to store charge when voltage applied) which raises electronic noise, making peaks broader.

What determines count rate capability

Dead time

detector type - GM counters have longer recovery times whereas each microcell in a SiPM has its own quenching resistor

Gas Filled Detectors

Energy of incoming photon ionises gas atoms

There is a voltage between the plates of an ionisation chamber

Electric field collects electrons at anode and cations (ionised atoms) at cathode

Current proportional to enrgy of incident radiation

Ion Chamber Output

Regions: applied voltage

Recombination: 0-100V

Ionisation: 100-300V

Proportional: 300-800V

Geiger-Muller: 1000-1400V

Recombination region

charges recombine prior to collection

Ionisation Region

all ionisation events detected

measured signal directly proportional to number of ionisations produced by incident radiation

Secondary electrons

electrons ejected from their shell by radiation

Proportional region

charge collected is proportional to applied voltage and energy of incident radiation as fast-moving charged particles create secondary electrons

Geiger-Muller region

secondary electrons accelerated, causing avalanche - all molecules ionised at once

Avalanche

Cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions (within a fraction of a second)

Results in dead time, new events not detected, indicated count rate is lower than actual rate

Geiger-Müller Counters

Every detected event produces a current pulse of constant voltage (size of pulse independent of size of original ionisation event because of avalanche)

Cannot measure energy or differentiate between radiation types

Cannot quantify dose only detect radiation presence

Every type of radiation is counted (0.01 mm mica window for α particles)

Used in radiation safety / survey work to detect contamination

Output charge pulse is fed directly to a speaker or ammeter

Adding filters/absorbers allows estimation of energy, and type of radiation, but not precise spectral analysis

Calibrated to Cs-137

CPM

GM measure counts per minute (CPM): number of atoms in a given quantity of radioactive material that are detected to have decayed in one minute

GM Alarm

audible alarm at 100 CPM (above normal background radiation of 20– 60 CPM)

may alarm due to natural background radiation, medical isotopes from a person nearby, electronic interference (mobile phones), low battery, and a broken tube

Be concerned of extremely high readings and/or constant high readings

They will not provide accurate, high-level readings during an emergency

high sensitivity

more difficult to detect high penetrating power radiation as easily pass through low-density gas without interacting

Transmission Ionisation Chambers

Ionisation region

Measures overall dose because output proportional to number of ion pairs

Thin, transparent, gas-filled detectors placed in radiation beam

Monitors beam intensity, symmetry, and real time dose

Doesn’t significantly interrupt beam

Cannot distinguish between radiation types because produces a single, combined current (combination different radiation types) proportional to total energy deposited in the gas

Parallel Plate Ionisation Chamber

Photons and electrons <6 MeV

Used for electron percentage depth dose (PDD) distributions - how dose changes as it penetrates

Thin front window closely spaced to a rear collector

Guard rings define collection volume and improve accuracy by reducing wall effects

Cylindrical Ionisation Chamber

Photons and electrons >6 MeV

Used to get accurate reference dose

Conductive cylindrical shell is cathode, central rod is anode

Phantoms Def + Types

Used to replicate conditions to measure radiation with minimal risk to staff and patients

Slab phantoms

Water phantoms

Anthropomorphic Phantoms

Anthropomorphic Phantoms

a 3D shape with internal inhomogeneities approximating a human, contains holes for insertion of ionisation chambers

Water phantoms

used to measure absolute dose, readily available, inexpensive,

homogenous and tissue/muscle equivalent. Waterproof ionisation chamber placed on

movable arm within phantom

Slab phantoms

square blocks, varying thickness, different materials. Ionisation chambers placed in pre-hollowed holes to measure dose rates

Proportional Counters

Proportional region

Accelerate the charges produced in the initial ionisation so they have enough energy to cause secondary ionisations in a controlled, localized avalanche

produces a pulse proportional to energy deposited

distinguish between α and β particles (α has higher LET thus higher pulses), neutrons (using certain gases), x-rays (lowest ionisation), and gamma rays (lower ionisation than particles)

Used for spectrometry and measuring neutron radiation fields around linear accelerators

Spectrometry: Energy Discrimination - Single Channel Analyser (SCA)

receives electrical voltage pulse

Peak amplitude matched to a set voltage window, ΔV (set by lower and upper thresholds)

If amplitude is within the window, digital pulse is produced and counted to determine the number of events in that energy range

Energy Discrimination - Multi-Channel Analyser (MCA)

records a full energy spectrum (distribution of counts vs energy) rather than only counting

events within a single energy window

chain of SCAs arranged so the LL of each is the UL of the previous one

MCA Spectrum

number or counts per channel as a function of energy

each channel represents a certain energy interval

Air Ionisation Chambers (DAP Meter)

Ionisation chamber containing only air

used to measure incident radiation intensity in the form of exposure, E (C/kg), which can be converted to dose or dose rate

Measures DAP (Dose Area Product) - quantity independent of distance from focal spot (exact location of x-ray source)

Works in current mode

Luminescence

occurs when an outer-shell electron is raised to an excited state and returns to its

normal state with the emission of a light photon

Luminescence Types

Fluorescence– emission of light by a material (Scintillation)

Thermoluminescence– later emission of light due to heating of irradiated material (TLDs)

Optically stimulated luminescence– later emission of light following light exposure (OSLs)

Phosphorescence– light emission is delayed in time (CR cassettes)

Chemiluminescence– light emission after chemical reaction

Fluorescence

Electron shell transition

Energy difference is emitted as visible light (lower energy than incident energy)

X-ray film-screen fluorescence: an intensifier screen fluoresces (glows) green

Scintillators

convert x-rays to visible light via fluorescence

Used in:

fluoroscopy Image intensifiers

Digital radiography image receptors

PET scanners

Activator / Dopant + Traps

Trace amounts of an activator element (e.g Mg, C) create occupiable energy levels (tranps) within band gap

excited electrons move into positively charged traps on their way down from conduction band

electrons remain there until light or heat energy frees them

they then move through conduction band and recombine with a hole at dopant site to emit light

used to delay light emission (phosphorence)

Eg: NaI doped with Tl for scintillator cameras

Scintillation Crystals

produce light when hit by high-energy photon

light photons dispersed in random directions

Optical reflector surrounding crystal directs light photons to one side of the crystal

Light photons converted into electrons to produce a measurable current

number of light photons produced is proportional to energy of incident radiation

Photocathode

Converts light photons into electrons - photons strikes surface, photons transfer energy to electrons within the material, if greater than binding energy, electron is ejected

Electron energy proportional to frequency which is proportional to incident radiation energy

Older Scintillator Detectors

Doped scintillation material and a photomultiplier tube (PMT) which is a vacuum containing a photocathode (converts photons to electrons), 10-12 dynodes, and an anode (collects electrons at end of dynode chain) all enclosed within a light-tight housing

Dynode causes emission of more electrons which are attracted to next dynode, eventually amplifying current by ~10⁷

Newer Scintillator Detectors

scintillating material + solid-state detector (silicon photomultiplier [SiPM] or silicon PIN photodiode)

Radiation interacts with doped scintillator crystal (e.g. CsI:Tl/Na), producing light photons proportional to radiation energy

Solid-state detector interacts with the photons and produces electron-hole pairs

Silicon Photomultiplier (SiPM)

composed of thousands of microcells (each microcell contains an avalanche photodiode [APD] and a small quenching resistor) connected in parallel on a single silicon chip

APDs are reverse biased, ready to avalanche

When light photons hit SiPM an electron-hole pair is created, charges are accelerated by electric field, causes avalanche which makes silicon conductive and generates measureable pulses

Total electrical charge produced is proportional to total number of detected photons

if paired with scintillator it can distinguish radiation types

Quenching resistor stops the avalanche, allowing pixel to reset and detect subsequent photons

Silicon PIN Photodiodes

p-type, intrinsic (undoped semiconductor), n-type diode

reversed biased to widens intrinsic region and alllows efficient charge collection

Incident radiation interacts with intrinstic region creating e-h pairs that are collected by electric field producing charge signal at charge sensitive amplifier (CSA)

CSA integrates charge and produces a voltage

shaper amplifier (SA) amplifys and reshapes for quasi-Gauss shaped output, filters noise for better energy resolution, and enables signal to be sent to counting device

Does not distinguish radiation types

Scintillator & Types of Radiation

Use differences in energy deposition, ionisation density, light yield, and pulse shape to distinguish between radiation types

α-pulses deposit high energy in very short range but have longer tails than β-pulses

thin scintillators used for low energy rays and high-energy beta particles

thick scintillator for high energy rays - absorbs ray but doesn’t prevent produced light from being detected

Scintillator variation

Efficiency of conversion between energy and mass (light photon to electron) causes statistical variation

Measured output (pulse height) not always exactly proportional to photon energy as there is random fluctuation (peak broadens)

prevents separation of gamma rays with closely spaced energies

Phosphorescence - light emission is delayed in time

Europium-doped barium fluorohalide photostimulable phosphor (PSP) screen

X-ray photons strike PSP plate, electrons in move to higher energy state, become trapped by dopant,

latent image remains for hours, but decays over time

Plate is scanned by laser in a reader, stimulating trapped electrons to return to original state by emitting blue-green light

PMT converts blue light to electrical signal which creates visible image

Emitted light proportional to total energy - can not distinguish radiation types

After reading, plate is exposed to intense light to erase residual latent image for reuse

Solid-State/Semiconductor Detectors

direct conversion

radiation excites electrons craeting electron-hole pairs inside a semiconductor

electric field collects this charge, producing an electrical pulse proportional to deposited energy

highest energy resolution, high density/stopping power/sensitivity, MRI compatible

Valence band

outermost electron orbital that can donate an electron to conduction band when excited

nearly completely full so electrons are not mobile - can not flow as electric current

Conduction band

energy band that excited electrons enter from valence band, here electrons move freely producing electric current

nearly completely empty so electron can flow freely

Band gap

difference in energy between valence and conduction bands

determines minimum energy required for excited valence electron to enter conduction band

size determines whether the material is a conductor (no band gap), a semiconductor, or an insulator (large band gap)

electron-hole pairs

when electron is excited by incident energy and moves to conduction band a vacant electron position (hole) is left in valence band

hole that can flow as current like a charged particle

N-type Semiconductors

Ge and Si crystalline solids are covalently bonded through a tetra-valent scheme (4 inter-atomic bonds)

addition of Penta-valent atoms provide ‘hanging’ electrons that are not bonded

P-type Semiconductors

addition of tri-valent atoms results in a deficiency of 1 electron (hole)

hole can be filled by electron from neighboring bond

p-n Junction

Combining n-type and p-type regions makes p-n junction used in diodes

Amount of diffusion at junction determines amount of current - measured to give incident energy

diffusion current

caused by concentration difference

occurs without incident light

holes diffuse to n-side, electrons diffuse to p-side creating diffusion current

Depletion region

area where holes and electrons cancel each other out so there is no free charge carriers

positive holes left on n-side and electrons left on p-side

creates electric field from n-side to p-side

Photocurrent

Incident photons of energy >1.1 eV ionize covalent bonds, creating electron-hole pairs

Electric field sweeps the new holes to p-side (anode) and new electrons to n-side (cathode)

Indirect Conversion Detectors - Examples

Scintillators:

CsI

GOS

LYSO

Direct Conversion Detectors - Examples

Silicon (Si)

Cadmium telluride (CdTe)

Cadmium zinc telluride (CZT)

Semiconductors radiation differentiation

Radiation type inferred from:

Energy deposited

Interaction pattern and behaviour

Spatial distribution

Penetration depth

Alpha particle pulse

Since lots of energy is deposited densely - short range

gamma ray pulse

Low ionisation density - spread out energy deposition (range)

Discrete energy (photopeak) - most of its energy is deposited in one event if all energy is absorbed

Optically Stimulated Luminescence (OSL)

Electrons excited by radiation get trapped in defects

Green light used to release them

Blue light produced is directly proportional to number of trapped electrons thus radiation dose

Decay Curve

Intensity of produced light tails off with simulation time

Converting OSL Signal to Dose

Compare intensity of emitted light with standardized decay curves

Thermoluminescence Dosimetry (TLDs)

Electrons excited by radiation get trapped in defects

Lithium fluoride as active ingredient

Material heated to ~200C

Trapped electrons raise to conduction band and emit light as they fall back to valence band

Light output directly proportional to radiation dose

Why is BeO used in OSLs and TLs

acoustic impendance close to tissue

Insoluble in water

Insensitive to most chemicals

Low cost

High availability

For TL - high thermal conductivity

Badges and Radiation Types

Badges have 4 shielded detector elements (open window, plastic, aluminium, copper) that distinguishes between radiation types

Open window/thin filter - allows beta particles and low energy photons to penetrate, measures shallow dose

Aluminium/Copper - attenuates low energy radiation so only gamma and x rays can penetrate, measures deep dose

Badges Static vs Dynamic Exposure

Determines if badge was worn properly or left in radiation field

Dynamic - even, blended pattern or from various incidedent angles and distances

Static - clear, sharp pattern corresponding to filter pattern

Solid-State Personal Dosimeter

Real-time radiation monitoring devices

Silicon PIN photodiode

Audio alarm and digital data logging

Measures all radiation types

Compact

Retrospective/Emergency Dosimetry

Used when someone with suspected overexposure does not wear dosimeter or exposure registered on badge does not reflect wearer’s dose

OSL used on electonic components from everyday portable devices (smartphones)

alumina-based ceramic substrate traps

Cytogenetic Dosimetry

Detecting presence or absence of dicentric chromosomes in cells can indicate recieved dose

Compare blood samples of exposed person to non-exposed people