Transcript energy

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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http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/int
eractdetector.html
Radiation ionizes
the gas. Ions
move toward
electrodes,
creating current.
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
• 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-geiger-counter-a.JPG
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How Geiger counters work
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• 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.
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”
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• 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.
Solid scintillation counters
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• 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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http://www.fnrf.science.cmu.ac.th/theory/radiation/Radiation%20and%20Radioactivity_fil
es/image018.gif
Liquid Scintillation counters
• Workhorse in biology labs for many years
• Very useful for beta emitters, some alpha
• Modern equipment:
– Computer driven
http://www.gmiinc.com/Genlab/Wallac%201414%20LS.jpg
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Basic principles
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• 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
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• 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
c
p
m
Pulse height
Summary of capabilities
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• 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.
Difficulties with LSC
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1. Static electricity: causes spurious high counts,
esp. when humidity is low;
1. don’t wipe outside of vials!
2. Chemiluminescence: chemical reactions in
sample, from overhead lights, glass.
1. Suspiciously high counts can be redone; chemiinduced high counts subside over time.
3. Quench
1. Anything that interferes with counting efficiency.
1. Measured: counts per minute (cpm)
2. Desired: decompositions per minute (dpm)
Counting efficiency
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• 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.
Effect of Quench
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All about quench
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• 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
About quench -2
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• 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.
Counting statistics
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• 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.