Transcript Slide 1

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Measurement and Detection of
Ionizing Radiation
Methods of detection
Electronical instruments
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• Ionizing radiation is invisible
• Many methods are available for detection and
measurement, including
– Ionization detectors
– Scintillation detectors
– Biological methods
– Thermo luminescence
– Chemical methods – free radicals produced
– Measurement of heat- energy dissipated
Ionization
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• All detecting methods are based on the
interaction of the radiation with matter. If a
radiation does not interact with any matter, we
have no method of the detecting it. Since
ionization is an important process for
radioactivity, most detectors exploit the signals
generated due to ions and electrons .
• on pairs in a gas produced by ionizing radiation
do not recombine until the energies of electrons
have dissipated. In a gas, ions and electrons
move freely
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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Ionization Chamber
• The key components of an ionization chamber are
shown here. It consists of a detector chamber, a
voltage supplier (battery), an ampere meter, and a
load resister . Ionizing radiation enters the detector
chamber and ionizes the mixture of gas in it. The
electrons drift towards the positive electrode and ions
move towards the negative electrode. Thus, ampere
meter detects a current.
• The number of ion pairs is proportional to the number
of ionizing particles entering the detector chamber.
Thus, the current is proportional to the intensity of
ionizing radiation.
• The light electrons drift 100,000 times faster than the
heavy ions. The motion of electrons is mostly
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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
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• At some hundreds volts, the improvement in sensitivity is more than
collecting all the When voltages applied to electrodes of ionization
chambers increase, the sensitivities increase. Electrons and ions on the
electrode. The currents corresponding to multiples of ions and electrons
produced by radioactivity. To distinct them from simple ionization
chambers, these detectors are called proportional counters.
• In proportional counters, the high voltage applied to the electrodes
created a strong electric field, which accelerate electrons. The electrons,
after having acquired the energy, ionize other molecules. Production of
secondary ion pairs initiates an avalanche of ionization by every primary
electron generated by radiation. Such a process is called gas
multiplication.
• The gas multiplication makes the detection much more sensitive. Yet, the
current is still proportional to the number of primary ion pairs.
• When voltages applied to proportional counters get still higher, sparks
jump (arcs) between the two electrodes along the tracks of ionizing
particles. These detectors are called spark chambers, which give
internal amplification factors up to 1,000,000 times while still giving an
initial signal proportional to the number of primary ion pairs.
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.
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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
• 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://www.orau.gov/reacts/measure.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
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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
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• 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.
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.