UNIVERSITY OF FLORIDARadiation Safety Short Course

Radiation Detectors &

Survey Instrumentation

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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…