Measurement and Detection of Ionizing Radiation

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Measurement and Detection of Ionizing Radiation

• Ionizing radiation is invisible
• Many methods are available for detection and
  measurement, including
• Ionization detectors
• Scintillation detectors
• Biological methods
• Thermoluminescence
• Chemical methods free radicals produced
• Measurement of heat- energy dissipated

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Ionization

• Devices contain a gas that can be ionized
• A voltage is applied to the gas
• Specific instrumentation and types of measurement
  depend on amount of voltage applied to the gas.
• Three types of instruments
• Ion chambers
• Proportional counters
• Geiger-Mueller counters

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Log of electrical signal vs. voltage
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Radiation ionizes the gas. Ions move toward
electrodes, creating current.
http//www.science.uwaterloo.ca/cchieh/cact/nucte
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Ion chamber continued

• Voltage is high enough that ions reach the
  electrodes, produce current.
• Proportional to energy the more energy, the more
  current.
• Generally requires some amplification of the
  signal.
• Example of use pocket dosimeters

http//www.ludlums.com/images/dosimeter.jpg
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Proportional counters
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• Each ionization electron is accelerated by the
  voltage so that it ionizes more of the gas.
• The higher the energy of the radiation event, the
  greater the avalanche, the higher the current
• Each ionization event detected separately.
• Used for neutrons

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Geiger Mueller counters
http//www.pchemlabs.com/images/eberline-rm20-geig
er-counter-a.JPG
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How Geiger counters work

• Voltage is high enough that every radiation event
  triggers a complete avalanche of ionized gas
• Does not discriminate among different energy
  levels
• Each event is registered
• A quenching agent stops the reaction, resets gas
  for next event
• Slow response time (comparatively) but simpler
  circuitry.
• Good for simple, sturdy, instruments
• Best for gamma low efficiency for alpha, beta.

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More Geiger details
Higher voltage leads to constant avalanches
instrument pegs. Improved efficiency with
pancake probe collects more radiation due to
geometry.
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Proper use of Geiger counters as survey meters

• http//orise.orau.gov/reacts/guide/index.htm
• Units of radioactivity and radiation
• Radiation detection instruments and methods
• First check battery and check source
• Enclosed radioactive material of known amount
• Check level of background radiation
• Survey area in question
• Move survey instrument slowly
• Keep constant distance from object being
  surveyed do not make contact.

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Solid scintillation counters

• Crystal-based
• Radiation hits crystal which releases visible
  photons
• Photons amplified by photomultiplier tube,
  converts to electrical signal
• Zinc sulfide
• Good detection of alpha particles, rapid response
  time
• Sodium iodide w/ thallium
• Good for detection of gamma
• New ones showing up

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/Radiation20and20Radioactivity_files/image018.gi
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Liquid Scintillation counters

• Workhorse in biology labs for many years
• Very useful for beta emitters, some alpha
• Modern equipment
• Computer driven

http//www.gmi-inc.com/Genlab/Wallac20141420LS.j
pg
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Basic principles

• Radioactive sample is mixed with organic solvents
  (cocktail)
• Toluene replaced with biodegradable solvents
• Detergents allow up to 5 aqueous samples
• Radiation hits solvent, energy is absorbed by
  solvent Energy passed to one or more fluors
• Fluor emits visible light which is detected
• By fluorescence
• Amplified by photomultiplier, converted to
  electrical signal.

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Coincidence circuitry

• Photomultipliers very sensitive
• Inside of instrument completely dark
• Tubes give off thermal electrons
• Result would be very high background counts
• Coincidence circuitry compares results from 2
  photomultipliers
• Event not detected by both thermal electron
• Ignored
• Event detected by both is affect of beta particle
• Counted.

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Counts and energy discrimination

• As radiation travels through solvent, it gives up
  energy
• The more energy it has, the more fluor molecules
  get excited and release photons
• Thus, the higher the energy, the brighter the
  flash
• The higher the electrical pulse sent from the PMs
• Instruments can be electronically adjusted
• Discriminators set for different pulse height
• Able to count betas from H-3 vs. C-14 vs. P-32

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Beta energy spectra
cpm
Pulse height
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Summary of capabilities

• Pulse height
• From brightness of flash the more energetic the
  radiation, the brighter the flash.
• Discriminators (gain) in the instrument can be
  set so you determine what energy you want
  counted.
• Number of pulses
• Corresponds to how many flashes, that is how many
  radiation events (decays) the amount of
  radioactivity.

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Difficulties with LSC

1. Static electricity causes spurious high counts,
   esp. when humidity is low
2. dont wipe outside of vials!
3. Chemiluminescence chemical reactions in sample,
   from overhead lights, glass.
4. Suspiciously high counts can be redone
   chemi-induced high counts subside over time.
5. Quench
6. Anything that interferes with counting
   efficiency.
7. Measured counts per minute (cpm)
8. Desired decompositions per minute (dpm)

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Counting efficiency

• Because samples are usually dispersed in clear
  containers, geometry is favorable for energy
  transfer in all directions and good light
  emission
• Not all decay events will get registered,
  however, because no system is 100 efficient
• We seek to know the of decompositions per
  minute (dpm) but measure the counts per minute
  (cpm).
• Using standards helps determine efficiency.

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Effect of Quench
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All about quench

• Chemical quench
• Acids, bases, high salt, any chemical that
  interferes with transfer of energy from the
  solvent to the fluor.
• Result fewer activated fluor molecules, less
  intense flash, interpreted as a lower energy
  event.
• Color quench
• Colored material absorbs visible light from fluor
• Less intense flash, appears as lower energy event

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About quench -2

• Self absorption
• If particulate matter not well suspended, energy
  not absorbed by fluor, not detected as well. Both
  lowering of cpm and forcing into lower energy
  range.

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Counting statistics

• Radioactive decay is a random event
• To be sure results are reliable, a minimum number
  of decay events must be recorded.
• Reliability depends on total number of counts!
• Example
• Statistical significance is the same in these two
  cases
• 10 minute count yielding 500 cpm
• 1 minute count yielding 5000 cpm.
• Both have total of 5000 counts
• Instruments have settings for stopping count when
  a certain statistical threshold is reached.