Health, Safety and Environmentwww.hse.ubc.ca

Radiation Detection Systems

8. Laboratory Radiation Surveillance

Direct Survey Meters• Geiger-Mueller

• Scintillation Counter

• Measure surfaces directly

• Main use for contamination control

Radiation Survey Meters Maintenance

• Per use:– Battery power– Check source– Check background

• Calibration:– Yearly– After maintenance or repairs

Geiger-Meuller Tube

Low Energy Gamma Scintillator (LEGS)

Survey Instrument Comparison• Geiger-Muller

– Detection through window

– Detects rays (photons)– Detects a few particles– Shields allow

differentiation between particles & photons

– Designed to measure activity

– Can be less sensitive to low counts

• Scintillation Counter– Much more sensitive

than Geiger-Muller– Widespread detection

Indirect Survey Methods• Liquid Scintillation Counter

• Gamma Counter

• Wipe of surfaces

• Detect contamination on wipes

Gamma Counter

No internal radioactive standard.

May generate small, negative numbers when

counting low activity samples: ie wipe tests.

Wipe test criterion of 100 cpm above bkgnd

still applies!

Scintillation Counter

Distintegrations Per Minute = Counts Per Minute / % Efficiency

Scintillation Counter

Sample 124

800 cpm

Eff=39%

Distintegrations Per Minute = Counts Per Minute / % Efficiency

Sample 123

1000 cpm

Eff=50%

Activity / CalibrationA ~ 2.22 MBq

N = Activity x (Efficiency x Geometry Factor) • A ~ 2.22x106

dps• Efficiency ~ 50 %• GF ~ 0.5

Amp

Counter N

N =

Activity / CalibrationIf you detect 555,000 cps, is the If you detect 555,000 cps, is the activity of the source 2.22 MBq?activity of the source 2.22 MBq?

Consider other contributing Consider other contributing factors :factors :

Radiation Sources in the Workplace

9. Radiation Protection Principles

Radiation

Transfer of energy, in the form of waves or particles, from one point

in space to another point in space.

• Time

• Distance

• Shielding

• Contamination Control

Time

Minimize the time spent in a radiation field.

Example:

You are working in front of a fume hood where the field is 18 Sv/h.

What is the dose you would receive after 90 minutes?

after 10 minutes?

Inverse Square Law

The radiation intensity, I, is proportional to one over the distance squared:

The source is assumed to be small compared to the distance.

Distance

ID21

1 4 9

Inverse-Square Law

0 1 2 3

If I α 1 (D)2

What is the intensity at twice the distance?

I1 = (D2)2

I2 (D1)2

Let D2 = 2D1 I2 = I1/(D1)2 / (2D1)2

I2 = I1 (D1)2 / (D2)2OR

I2 = I1 / 4

Distance ExampleAt 10 cm you measure the field intensity to be 160 μSv/ h.

What is the field intensity at 1 m?I1 = D1 =

I2 = D2 =

ShieldingMaterial placed between yourself and the source will reduce your exposure to radiation.

The amount of reduction will depend upon the material and the radiation.

• Material density and thickness

• Radiation type: α, β, γ, or x-ray

• Radiation energy

Half-value Layer

20 Sv/hr

Half-value Layer

Sv/hr

Half-value Layer

Sv/hr

Half-value Layer

Sv/hr

Recommended Shielding• 32 P 12 mm Plexiglas

• 14 C Glass containerPlexiglas

• 125 I 1 mm Lead sheet

• 99m Tc 12 mm Lead

Contamination Control

Wipe Test

Survey Meter

Combination

Purpose is to ensure that all work and non-work surfaces do not pose a risk to health

Wipe tests

Use filter paper/tissue etc.

Wet with appropriate solvent.Standard surface area to cover is 100 cm2 for each wipe.

Place in vial with scintillation cocktail, count.

Always include a background.Action level for contamination is 100 cpm above bkgnd.

Spurious counts may be due to static, or fluorescence not from radioactive source.

Be suspect of zeroes!

END DAY 1