. 2 RCRS provides: Pick-up and delivery of meters Required 9-month calibration Preventive...
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Transcript of . 2 RCRS provides: Pick-up and delivery of meters Required 9-month calibration Preventive...
UNIVERSITY OF FLORIDARadiation Safety Short Course
Radiation Detectors &
Survey Instrumentation
2
Survey Meter Support Services
RCRS provides: • Pick-up and delivery of
meters• Required 9-month
calibration• Preventive maintenance
and repairs• Shipping for extensive
repairsRCRS Contact:
John ParkerNuclear Science
Center392-7359
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Types of Surveys Direct measurements
oPortable survey instruments
Indirect measurementsoSwipes and use of counters
(Liquid Scintillation and Gamma)
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Use of Portable Survey Instruments
Read the instrument’s operating manual
Check the batteries before each use
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Use of Portable Survey Instruments
Use a check source to determine operability of detector
Determine efficiency of detector
Determine the instruments response time
Determine the operating background
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Geiger Mueller (GM) Detectors
GM are tubes filled with a mixture of Q-gas
Used for radioactive contamination monitoring
Detects high energy beta radiation
GM measures each individual interaction inside the detector
Do NOT identify the radioisotope or specific energy
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Sodium Iodide (NaI) Probe
Probe has a NaI crystal and a photomultiplier tube
Detection is based on emission of photons
Used for radioactive contamination monitoring
Detects low energy x-rays
Does NOT identify the radioisotope
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Ionization Chambers
Ion Chambers are filled with air
Used for measuring radiation field intensity (exposure rate)
Detect x-rays and gamma radiation
Measure average current produced over many interactions
Do NOT identify the radioisotope or specific energy
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Ionization Chambers
A.K.A. - Dose Rate Meter
Roentgen (R): a unit of exposure to x-rays or gamma rays. One roentgen is the amount of gamma or x-rays needed to produce ions carrying 1 electrostatic unit of electrical charge in 1 cubic centimeter of dry air under standard conditions.
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Liquid Scintillation Counter (LSC)
Detection is based on emission of visible or near visible light (photon)
Sample vial must contain scintillation cocktail
Interaction between radiation and cocktail causes photons to be emitted
Photomultiplier tubes are used to detect and amplify the photons
It can identify the activity and energy of a beta emitter in a sample vial
It is much more efficient than a portable survey meter
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The Scintillation Process
Radioactive molecule
Solvent molecule
Fluor molecule
Photomultiplier tube
Beta particles are emitted, which cause solvent molecules to become excited.
The energy of the solvent molecule is transferred to the fluor molecule, which in turn emits light.
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Gamma CountersDetection is based on emission
of visible or near visible light (photons)
Uses solid scintillation (no scintillation cocktail required)
Interaction between radiation and a solid scintillator causes photons to be emitted
Photomultiplier tubes detect and amplify photons
Used to determine the activity of a x-ray or gamma emitter in a sample vial
Gamma counters can identify activity and energy of gamma emitters
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Choosing the correct instrument
?
Radiation Field / Dose Rate (mR/hr)Ion chamber
Activity / Contamination amountsBeta emitters
Low and Mid energyLSC only
High energyGM DetectorLSC (for documentation purposes)
Gamma emittersSodium Iodide Gamma counter (for documentation purposes)
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Useful Formulas/Conversions
Hmm…a conversion might
help here!
1 µCi = 2.22 x 106 dpm
net cpm = gross cpm – background
efficiency = (net cpm) / (dpm)
dpm = (net cpm) / (efficiency)
cpm = (dpm) x (efficiency)
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GM Tube
Ion Chamber NaI Probe GC LSC
P-32
x-ray
I-125
Tritium
Cr-51
Radiation Field
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Sample Problem 1
Hint:5800
counts / 5 minutes = 1160 gross
cpm
You have a swipe that was counted for 5 minutes that yields 5800 counts.
If the background is 125 cpm and the counter efficiency is 80%, what is the dpm of the swipe?
Hint:1160 gcpm –
125 background cpm = 1035
net cpm
Big Hint:1035 net
cpm / 0.80 efficiency = 1294 dpm
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Sample Problem 2
Hint:120,000
counts / 2 minutes =
60,000 gross cpm
You have a standard that has an activity of 105,000 dpm.
After 2 minutes of counting in a gamma counter you get 120,000 counts.
If the background is 350 cpm, what is the efficiency of the gamma counter?
Hint:60,000 gcpm –
350 background
cpm = 59,650 net cpm
Big Hint:59,650 ncpm / 105,000 dpm = 57% efficiency
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Sample Problem 3
You are using P-32 as a tracer and determine that 20% of fed material ends up in your final sample.
If you need a count rate of 2,000 cpm in a sample, determine how many microcuries you must use given the fact that your LSC has an efficiency of 50% for P-32.Hint:
2,000 cpm / 0.20 =
10,000 cpm Hint: 10,000
cpm / 0.50 efficiency = 20,000 dpm
Big Hint: 20,000
dpm / 2.22 x 106 dpm/µCi = 0.009 µCi
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Life is not measured by the
number of breaths we take, but by the moments that take our breath away…