characterization and decontamination of buildings and ... documents...diamond wire cutter set to cut...

Characterization and Decontamination of Buildings and Associated Structures

Professional Training Programs

Objectives

• Become familiar with selected dismantlement activities

• Become familiar with some of the methods employed to characterize contamination at a facility

• Become familiar with some common portable survey meters used to characterize surface contamination

• Become familiar with some of the specialized instrumentation used to characterize surface contamination

• Become familiar with some of the more important decontamination methods

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Contents

PART I. Dismantlement of the Facility and its Components PART II. Characterizing and Decontaminating Materials and Equipment (M&E) Removed from the Facility PART III. Characterizing Facility After M&E are Removed PART IV. Decontamination of Surfaces PART V. Safety Issues Appendix A. Portable Survey Meters Appendix B. Specialized Instrumentation

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PART I

Dismantlement of the Facility and its Components

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Dismantlement Activities

General

• During the operational phase of the facility, most of the significant sources of radiation will have been identified.

• Licensed radioactive sources might be moved to a temporary storage location or disposed of.

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General

• Depending on the nature of the facility, it might be necessary to plan out the exact sequence of activities, the likely duration of these activities, and the pathways used by the M&E and workers.

• Initial activities might involve “preparing” the facility, e.g., remove non-radioactive equipment, furniture and facility components to improve access and/or facilitate subsequent activities.

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General

• Initial activities are often those that:

involve minimal exposure to workers

occur in areas without removable contamination

• Radioactive sources that contribute to external exposures often dealt with early on so as to minimize worker exposures during subsequent activities.

• As hotter sources are removed, lower activity sources become easier to detect.

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General

• Ductwork and most of the piping might be removed.

• Dismantlement of the facility electrical system has to be planned carefully.

• Portions of the structure might be broken up (e.g., volumetrically contaminated concrete walls and floors).

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Have a Disposition Plan in Place for M&E

• What materials and equipment (M&E) removed from the facility need to be characterized? If so how?

• What M&E might be disposed of immediately?

• What M&E might be stored temporarily on-site? If so, where and under what controls and safeguards?

• What M&E might be decontaminated onsite, or off-site? If so, how will it be decontaminated?

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Minimizing External Exposures

• Evaluate potential exposure rates due to gamma-emitting sources.

Do this in a manner that minimizes the exposures to those making the assessment. For example, use teletectors, robots, gamma cameras, etc.

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• Gamma imaging systems can generate digital images of gamma radiation fields at a considerable distance from the source.



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Minimizing Inhalation Exposures

• Conduct dismantlement in a manner that minimizes the generation of airborne material. For example:

Coat contaminated surfaces with strippable compound

Spray the area with water during the cutting and handling of contaminated material

Conduct operations inside temporary enclosures

Employ mobile ventilation systems with HEPA filtration and/or vacuum systems

• Employ suitable (pun intended) PPE

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Methods for Cutting Concrete and Metal

• Circular saws with diamond blades

• Diamond wire cutters

• Wet abrasive cutting

• Lasers

• Acetylene torches

• Jackhammers, backhoes, etc. can be employed to break up concrete and asphalt.

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• Piping embedded in concrete, or running below a floor, often must be removed.

• A concrete saw, similar to that shown here, might be used to remove the concrete above the piping.

http://www.fhwa.dot.gov/pavement/concrete/full5.cfm

Dismantlement Activities Concrete Saw

Breaking up Concrete Floor to Remove Drain Lines

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Using Heavy Machinery to Remove Floor

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Remotely Controlled Operations Reduce Exposures

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Moving debris with shovels

Protective Clothing (PPE)

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http://www.lanl.gov/projects/envplan/clean/protections/index.php?page=ta21


Water Sprayed During Demolition to Minimize Dust

Diamond wire cutter set to cut a bioshield plug

Shield plug cut and removed

This is a wire cutter that is similar to a band saw. These can be quite dangerous for nearby personnel.

Cutting Operation

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Steam generator being maneuvered inside containment building

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Steam generator being extracted from containment building

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Steam generator being lowered from containment building and transported by truck for disposal


PART II

Characterizing and Decontaminating

Materials and Equipment Removed from the Facility

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• Material and equipment (M&E) removed from the facility might have to be characterized and, if necessary, decontaminated.

• Characterization of potentially contaminated M&E might be performed according to the guidance in MARSAME (supplement to NUREG-1575).

• The M&E might be cut up or disassembled in order to make the potentially contaminated surfaces more accessible.

Characterizing and Decontaminating M&E

General

Piping cut up and characterized using hand-held instruments

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Scanning by Hand


• Hatch cover being assessed for surface contamination with gas flow proportional counter

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Scanning by Hand


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• Specialized instrumentation can help automate the characterization of contaminated M&E.

• These instruments generally detect gamma emissions from the contamination. Most are incapable of assessing contamination that consists of pure alpha or beta emitters.

• Appendix B contains more detailed information about some of these systems.

Specialized Instrumentation


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Example of specialized equipment: box monitor for assaying small contaminated objects


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• If necessary, M&E removed from the facility might be decontaminated (on-site or off-site) by some combination of:

Physical/Mechanical

Chemical

Electrochemical

Metal melt

Decontamination of M&E


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• Vacuuming, abrasion, brushing, grinding, strippable coatings, etc.

• Can be inexpensive

• Potential for generating airborne contamination

Physical/Mechanical Decontamination of M&E


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• Contaminated M&E soaked or rinsed in chemical solution

• Acids, bases, oxidizers, reducers, complexants

• Might involve heating, agitation, scrubbing, ion exchange

• Good for M&E of complicated geometries

• Not-so-good with porous materials

Chemical Decontamination of M&E


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• Contaminated material melted in a furnace

• Good for metallic M&E of complex geometry

• Contaminants end up in slag

• Clean metal ends up as ingot

Metal Melt Decontamination of M&E


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• Works with conductive metals

• Reverse of electroplating: object to be decontaminated is the anode and the walls of the acid-filled tank are the cathode.

• Might involve heating and agitation

Electrochemical Decontamination of M&E


PART III

CHARACTERIZING FACILITY AFTER M&E ARE

REMOVED

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Characterizing Facility After M&E are Removed:

General

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Characterizing Facility After M&E are Removed: General

Gamma-Emitting Sources Inside Building

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• Before M&E are removed from the facility, we might be unaware of the existence some gamma sources because they were masked by emissions from more intense sources.

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• For structural or other reasons, contaminated piping might be left in place.

• If the piping cannot be decontaminated, it might be filled with grout to fix the contamination in place.


Embedded Piping

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• Specialized instrumentation is available to characterize contamination inside such piping.

Examples are described in Appendix B.

• To prevent the detector from becoming stuck inside the pipe, the latter might be visually inspected with a video camera.


Embedded Piping

12 inch pipe crawler Pipe crawler for small pipes

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Embedded Piping

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• Soil beneath the building floor, typically a concrete slab, might be contaminated for several reasons:

- The building was constructed on top of contaminated soil

- Subfloor piping or holding tanks leaked

- Contamination penetrated cracks and/or joints in the floor

- Neutron sources (e.g., accelerators) produced long-lived activation products in the soil (e.g., H-3)


Subfloor Contamination

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• The location to be investigated might be identified on the basis of a gamma scan or professional judgment.

• A hole in the concrete floor might be created with a:

Concrete hole saw (corer)

Concrete circular saw

Jackhammer • Soil samples might be collected and analyzed, or a borehole

might be logged with a gamma detector.



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Subfloor Contamination • A jackhammer might be used to

produce hole in the concrete floor and/or collect samples of potentially contaminated concrete.

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Subfloor Contamination • Concrete hole saw (corer) might

be used to produce hole in the concrete floor and/or collect core of potentially contaminated concrete.

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http://mo.water.usgs.gov/epa/nh/Photos_files/geoprobe-inside.jpg

• This figure shows a Geoprobe® being employed to obtain a soil core from beneath a slab floor.

• Bucket augers, split spoon samplers, etc., might also be used to collect soil.

Characterizing Facility After M&E are Removed:

Surface Contamination

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• Scans

• Measurements

• Samples

• Smears

Characterizing Facility After M&E are Removed: Surface Contamination

Methods

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• Move detectors over surfaces to locate “hot spots”

• Employ beta or alpha detector as appropriate

• Keep probe within 1 cm of surface

• Move at 0.5–1 probe width per second

• Listen to audio output, preferably using headphones

• Even better if detector output and location is logged (e.g., each second)


Scans

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• If old contamination has been painted over, we might scan by looking for any emitted photons rather than for alpha or beta particles.

For example, at one time it was common to deal with alpha-contaminated surfaces by painting over them. In the case of the alpha emitter Am-241, contamination underneath paint might be located by scanning for its 59.5 keV gamma ray.


Scans

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• Examples of areas likely to be contaminated:

Horizontal surfaces facing up (e.g., overhead trusses)

Cracks and expansion joints in floors

Anchor bolts

Drains

Hard-to-clean areas (e.g., behind cabinets)

Work and storage areas


Scans

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• Fixed measurements are used to quantify alpha or beta activity (e.g., dpm/100 cm2 or Bq/cm2)

• Representative (unbiased) measurements can be analyzed statistically.

• Judgmental (biased) samples are used to investigate potential hotspots or assess hotspots identified by scan.


Fixed (Static) Measurements

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• To determine the count rate, divide the count (obtained using a scaler) by the count time.

• Avoid reading the count rate directly with a ratemeter.

• Count times of one minute are typical.

• If little to no removable contamination is present, the probe might be in direct contact with the surface.

Otherwise, the count might need to be made with the detector at some specific distance above the surface (e.g., 0.5 or 1 cm).


Fixed (Static) Measurements

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• The same instrumentation might be used for performing both scans and fixed measurements.

• The focus is on assessing the contamination via alpha or beta emissions because detection efficiencies for charged particles are much higher than for gamma rays.

• If both alpha and beta contamination are present, it might be best to analyze the alpha and beta emissions separately. In other cases, it might be decided to use detectors that respond to both the alpha and beta particles.


Instrumentation for Scans and Fixed Measurements

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• The instrumentation must be properly calibrated if it is used for quantitative measurements.

• Each day that the instrument is used, it should undergo a QC performance (aka functional) check.

This might include a visual inspection, battery check, HV check, light leak check, background count, and response to check source (constancy check).



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• Beta contamination—typical portable instruments

- Pancake GM - Beta scintillator - Dual phosphor - Gas flow proportional • Alpha contamination—typical portable instruments

- Alpha scintillator (ZnS) - Dual phosphor - Gas flow proportional



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• Primarily used for beta detection; however, also responds to alpha particles and gamma rays

• Background approximately 20–50 cpm

• Small window area: approximately 17 cm2

• Useful for surveying small areas, but there are better beta detectors available for surveying larger flat surfaces


Pancake GM for Beta Contamination

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Pancake GM for Beta Contamination

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• Primarily used for beta detection; however, also responds to alpha particles and gamma rays

• Background approximately 200–5,000 cpm

• Large window area: approximately 50–100 cm2

• Prone to light leaks


Beta Scintillator for Beta Contamination

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Beta Scintillator for Beta Contamination

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• Operates in beta, alpha, or alpha and beta mode

• In the beta-only mode, it does not detect alpha particles but it does respond to gamma rays

• Background (beta mode): approximately 200–500 cpm


• Susceptible to light leaks

• Detection efficiency for low energy betas is not as good as that of a beta scintillator


Dual Phosphor Scintillator for Beta Contamination

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Dual Phosphor Scintillator for Beta Contamination

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• Operates in beta, alpha, or alpha and beta mode

• In the beta-only mode, it will not detect alpha particles but it does respond to gamma rays

• Background (beta mode): approximately 200–500 cpm


• Requires gas supply, typically P-10 gas

• Detection efficiency for low energy betas is very good


Gas Flow Proportional Counter for Beta Contamination

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Gas Flow Proportional Counter for Beta Contamination

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• Does not respond to beta particles and gamma rays

• Background: approximately 0–1 cpm


• Susceptible to light leaks


ZnS Scintillator for Alpha Contamination

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ZnS Scintillator for Alpha Contamination

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• Operates in alpha, beta, or alpha and beta mode

• Will not detect beta particles in the alpha mode

• Background (alpha mode): approximately 0–1 cpm




Gas Flow Proportional Counter for Alpha Contamination

Scanning and making measurements on building surfaces—Is it possible to determine whether this is before or

after remediation?

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Scanning and making measurements on building surfaces—Is it possible to determine whether this is before or

after remediation?

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• Position sensitive proportional counter with data acquisition system that logs the counts and location of the contamination

• Incorporates video camera

• Distinguishes alpha and beta contamination

• Performs scan in a quantitative fashion



Surface Contamination Monitor (SCM)

Wall mount detector in use at Rocky Flats Surface monitoring at INEEL

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Total Station (Trimble)

• Laser ranging used to pinpoint detector position

• Laser tracks reflector attached to detector

• Data logged on computer, as well as detector location

• Incorporates digital camera

• Can create digital three dimensional map of room

• Laser can direct technicians to coordinates of measurement locations

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• Georgia Tech reactor; storage pool to right

• Three dimensional image mapped out by Total Station

• Results of alpha-beta scan

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LARADS

• LARADS system similar to total station

• Results of alpha scan superimposed on digital photograph

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• Samples might be collected for laboratory analysis (e.g., paint samples, concrete cores)

• Used to determine if the material is contaminated and/or identify contaminants


Samples

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• Hammer drill used to collect sample of slag


Samples

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• Employed to estimate the amount of removable activity

• This might be done in order to determine if the fixed measurements can be performed with the probe in direct contact with the surface.

• Smears, often analyzed by liquid scintillation counting (LSC), might be the only practical way to determine the presence of difficult-to-detect nuclides (e.g., H-3).

• In some cases, they are used to demonstrate compliance with limits on removable contamination.


Smears

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• If demonstrating compliance with limits, smears are usually obtained by rubbing a “smear paper” over 100 cm2 of the surface.

• In some cases, the smear is moistened (i.e., a wet smear).

• Wet smears are most often used when assessing tritium.

• Smears for tritium should be placed in LSC vial as soon as possible.


Smears

PART IV

DECONTAMINATION OF SURFACES

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Decontamination of Surfaces

Decontamination Methods

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• Needle gun

• Scabbler

• Concrete shaver (milling)

• Abrasive blasting

• Laser blasting

• Chemical decontamination

• Primarily used for cleaning concrete (and sometimes steel)

• Pneumatically powered

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Needle Gun

• Assembly of pointed rods (needles) used to hammer surface

• For cleaning concrete

• Usually pneumatically powered

• Surface hammered with assembly of steel or carbide bits on rotating drum or vibrating plate

• Some employ high voltage electric arc discharges

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Scabbler

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• Uses drum of rotating diamond-impregnated blades

• Shaves surface quickly, cuts through metal bolts

• Leaves smooth surface

• Large versions available

• Some are designed for operation on walls


Concrete Shaver (Mill)

• Scabbled concrete collected by vacuum system and deposited in waste drums (e.g., 13 or 55 gallon)

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Vacuum Systems

• Reduces worker doses due to inhalation

• Minimizes potential spread of contamination

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Vacuum Systems

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• Abrasive particles suspended in a stream of air or water are directed at high pressure at the surface being cleaned.

• Abrasive particles: steel shot, aluminum oxide grit, nut shells, glass, dry ice pellets


Abrasive Blasting

• Using dry ice pellets minimizes cleanup since they evaporate.

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• Some dry abrasive blasting units incorporate vacuum systems.

This reduces the generation of airborne material while completely eliminating the generation of waste liquids.


Abrasive Blasting

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• Standard or extremely high pressure systems, with or without chemical additives, can remove embedded contamination.


Pressure Washing

• These very high pressure (up to 20,000 psig) units can be capable of physically removing the surface layer of brick or concrete.

• The potentially large volumes of contaminated waste water might be a problem..

• A chemical decontamination agent (e.g., PENTEK 604) is applied to the surface. Later, the agent is then scraped or peeled off, leaving a clean surface.

• Might be $100 per gallon ($25 per liter) and one gallon might handle 100 ft2 (10 m2)

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Chemical Decontamination

• Does not damage the surface but is capable of removing contamination that might have penetrated the surface of porous materials such as brick, concrete, or wood.

• The potential generation of airborne material is minimized and liquid waste is produced that needs to be dealt with.

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Chemical Decontamination

Remote control scabbling unit Robotic arm used to seal waste container

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• Reduce external exposures and worker doses due to inhalation


Remote Control Systems

PART V

Safety Issues

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Safety Issues

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Site Condition

• Older sites can be in terrible shape (e.g., structurally unsound floors).

• Power might not be readily available so visibility can be a problem.

• Sites can be dirty. Animals can be present in buildings (alive and dead).

• Tripping hazards are often a problem.

• These sites are often in isolated areas.

• Conditions can change significantly from one day to the next, especially during dismantlement and decontamination.

Sites being decommissioned are not necessarily the nicest places to work.

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Safety Issues

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Radiological Hazards

• Prior to dismantlement and source removal, external exposures might be possible due to intense gamma sources.

• Internal exposures due to inhalation is a possible concern, especially during remediation—decontamination can generate airborne material.

• Radiological hazards should be minimal after remediation, although exposures might occur if waste and/or sources are stored on-site.

• Long-established safety protocols might no longer be in place (e.g., access controls might be compromised).

Safety Issues

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Non-Radiological Hazards

Cuts, tetanus, trips, falls from ladders, scaffolds, roof, etc., eye hazards (e.g., dust, debris), moving machinery, conditions changing from one day to the next, noisy environment that can make hearing more difficult, chemicals hazards, trenches caving in, insects, etc.

Safety Issues

Appendix A

Portable Survey Meters

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Radiation detected: Alphas Betas Weak response to photons

Window: 1.5 mg/cm2 mica

Window area: 15–20 cm2

Operating voltage: approximately 900 volts

Background: 20–50 cpm (0.3–1 cps)

Dead time: 20 μsec per count

Alpha efficiency: approximately 0.15 (15%)

Gamma response, Cs-137: 3,500 cpm/mR/hr (6,000 cps/mSv/hr)

Appendix A: Portable Survey Meters

Pancake GM

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• Shielded pancake probes reduce the background when surveying for surface contamination near gamma sources.

• The additional weight is a disadvantage when scanning for long periods of time and performing long measurements on vertical surfaces.


Pancake GM

Average energy (keV)

Maximum

energy (keV)

2𝝅𝝅 efficiency on contact

(c/d)

2𝝅𝝅 efficiency at 1 cm

c/d/100cm2

4𝝅𝝅 (total) efficiency on contact

(c/d)

4𝝅𝝅 (total) efficiency at 1 cm

(c/d)

Ni-63 17 66 0.0025

C-14 49 156 0.09 0.027 0.05 0.03

Tc-99 85 294 0.2 0.053 0.15–0.20 0.12

Tl-204 244 763 0.4 0.081 0.25

Cl-36 251 710 0.26

Sr/Y 90 565

(Y-90 and Sr-90)

2,280 (Y-90) 0.66 0.087 0.30 0.22

Ru-106 (Rh-106)

1,411 (Rh-106)

3,541 (Rh-106) 0.154 0.55

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Pancake GM

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Radionuclide Maximum beta energy

MDC (dpm/100 cm2)

MDC (Bq/cm2)

Ni-63 66 70,000 11.667 C-14 156 3,500 0.583 Tc-99 294 1,000 0.1667 Tl-204 763 670 0.112

Sr/Y-90 1415 550 0.092

Approximate minimum detectable concentration

From NUREG-1507. On contact measurements.


Pancake GM

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Beta Scintillator

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Radiation detected: Betas Alphas Weak response to photons (Photon response greater at lower energies)

Window: Aluminized Mylar®, ca. 1.2 mg/cm2 (a thicker window might be used to eliminate the alpha response)

Area: 50–126 cm2

Background: ca. 100–300 cpm (1–5 cps) (approximately 20 cpm/µR/hr or 10 cps/µSv/hr)

Dead time: ca. 10 µs


Beta Scintillator

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• Similar in construction to a zinc sulfide (ZnS) alpha scintillator except that the ZnS is replaced with a thin sheet of scintillating plastic

• Susceptible to light leaks (as is any scintillator with an aluminized Mylar® window)


Beta Scintillator

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Average energy (keV)

Maximum

energy (keV)


(c/b)


(c/d)

C-14 49 156 0.1 0.05

Tc-99 85 294 0.25 0.15

Tl-204 244 763 0.25

Cl-36 251 710 0.45 0.28

Sr/Y-90 565

(Y-90 and Sr-90)

2,280 (Y-90)

0.8 0.65

Approximate efficiency of typical beta scintillator


Beta Scintillator

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ZnS Alpha Scintillator

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Radiation detected: Alphas Has a weak response to neutrons

Efficiency: 10–20% (total 4 π efficiency)

The efficiency is usually assumed to be independent of the alpha energy

Window: Aluminized Mylar® (ca. 0.4–0.8 mg/cm2)

Window area: 50–100 cm2

Background: 0–2 cpm

Audio output: Speaker and/or headphones



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• Reflectors direct the flashes of light towards the photocathode of a photomultiplier tube (PMT).

• The PMT converts each flash of light into an electronic pulse.

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PMT

Reflector



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• Directly behind the Mylar® window is a thin layer of tiny zinc sulfide crystals.

• When an alpha particle penetrates the window, it interacts with the zinc sulfide and produces a flash of light scintillation.

Plastic coated with zinc sulfide crystals



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• Light leaks, often too small to be visible, can occur in the aluminized Mylar®.

• To test for light leaks, hold the probe up to a light source while listening to the audio for counts.

• The hole is often repaired with a dab of black paint.



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Dual Phosphor Alpha-Beta Scintillator

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Detector: ZnS and 0.25 mm thick plastic scintillator

Radiation detected: Alphas only Betas only (with weak photon response) Alphas and betas (weak photon response)

Window: aluminized Mylar®, 0.4–0.8 mg/cm2

Window area: 50–126 cm2 is typical

Background: 0–2 cpm (0–0.03 cps) in alpha mode 200–400 cpm (ca. 3–7 cps) in beta mode



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• Crosstalk: Some alpha pulses can be registered as beta counts and some beta pulses registered as alpha counts.

Alpha to beta crosstalk might be as high as 10%. Beta to alpha crosstalk only 1% or so. Depends on beta energy.

• Susceptible to pinhole light leaks in Mylar® producing high spurious counts



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PMT

Alpha Beta Aluminized Mylar®

ZnS Plastic scintillator

Reflector



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Gas Flow Proportional Counter

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Radiation detected: Alpha only Betas (with weak photon response) Alpha plus beta (photon response)

Window: 0.8 mg/cm2 aluminized Mylar®—most common 0.4 mg/cm2 windows for low-energy betas Thicker (beta only) windows also available

Window area: approximately 100 cm2

Alpha background: approximately 0–2 cpm (0–0.03 cps)

Beta background: approximately 300–400 cpm (5–6 cps)



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• Typically uses P-10 gas (90% argon, 10% methane)

• Typical flow rate: 40–60 mls/min

• Possible to disconnect from gas supply

If the detector is in good condition and is fairly small, it can be disconnected for an hour or so.

If the detector is leaking and is large, it can only be disconnected for a much shorter time.



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• Detector must be purged prior to use. The larger the detector, the longer the purge. Typically 15 min–1 hr.

• Detector efficiency can change when a different supplier of gas is used.

• During storage, connections to gas line should be capped to prevent dust getting inside.



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• Atmospheric pressure (elevation) and temperature can affect the detector efficiency, particularly the beta response.

• The magnitude of this effect should be assessed if the detector is used at widely varying temperatures and pressures.

• It might be determined that it is necessary to employ different beta operating voltages for the different temperatures and pressures.

• For example, the typical ORAU beta operating voltage is 1,750 volts for temperatures above 35°F and 1,775 volts below 35°F.



Average energy

Maximum energy


2𝝅𝝅 efficiency at 1 cm


4𝝅𝝅 (total) efficiency

at 1 cm

Alpha 4–8 MeV na 0.35 0.25 0.15

Ni-63 17 keV 66 keV 0.04 0.007

C-14 49 keV 156 keV 0.225 0.28 0.1 0.065

Tc-99 85 keV 294 keV 0.38 0.3 0.25 0.19

Tl-204 244 keV 763 keV 0.55 0.46 0.34 0.28

Sr/Y-90 565 keV

(Y-90 and Sr-90)

2,280 keV (Y-90) 0.64 0.5 0.64 0.4

Ru-106 (Rh-106)

1,411 keV (Rh-106)

3,541 (Rh-106) 0.72 0.55

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Radionuclide Maximum beta energy

MDC on contact

(dpm/100 cm2)

MDC on contact (Bq/cm2)

MDC at 1 cm (dpm/100 cm2)

MDC at 1 cm (Bq/cm2)

Ni-63 66 2,000 0.3333 10,000 1.6667 C-14 156 700 0.1167 1,100 0.1833 Tc-99 294 300 0.05 370 0.0617 Tl-204 763 200 0.0333 250 0.0417 Sr/Y-90 1,415 160 0.0267 180 0.03 Alpha NA 25 0.0133 40 0.0067

Approximate minimum detectable concentration

Based on data from NUREG-1507 Window thickness of 0.8 mg/cm2

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• The gas flow proportional counter is probably the best instrument for measuring surface contamination over large flat surfaces.

• The detector of the floor monitor (shown to left) can be disconnected and used to scan vertical or other horizontal surfaces.

• Floor monitors generally used for non-quantitative scanning purposes.



127

• Windowless gas flow proportional counters are marketed as capable of detecting tritium surface contamination.

• The betas have no window to penetrate.

• Used to perform fixed measurements, not scans


Windowless Gas Flow Proportional Counter

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• Residual air must be purged before count.

• Surface must be flat. A flexible apron is sometimes used to provide a better seal between the probe and the surface.

• Surfaces with static charge and dust getting inside the detector chamber can produce spurious counts.


Windowless Gas Flow Proportional Counter

129

Primarily used to measure gamma exposure rates in the mR/hr (or µSv/hr – mSv/hr) range


Side Wall/Window GM Detector

130

Radiation detected: Photons Betas (shield open) over 300–400 keV

Window: 30 mg/cm2 steel

Operating voltage: 800–900 volts

Background: 20–50 cpm (0.3–1 cps)

Dead time: 100–200 µs/count




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• Rugged, no thin window to damage

• In high-intensity radiation fields, will under-respond and even saturate (i.e., read zero).

• Over-responds (reads too high) with low-energy (ca. <250 keV) photons if calibrated at higher energies

• As a rule, the mR/hr scale is used with the shield closed.

• Use cpm scale when measuring surface contamination with the shield open.



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• Energy-compensated GMs use a bi-metal filter to selectively attenuates the low-energy photons that cause the over-response.

This results in a flat energy response.

Shield closed Shield open



133


NaI Gamma Scintillator

134

Radiation detected: Photons

Typical crystal sizes: 1–2 inch diameter

Background: Increases with crystal size Typically 800–4,000 cpm (10–70 cps)

Operating voltage: 800–1,000 volts




135

NaI crystal (hermetically sealed)

PMT

Probe housing

• The NaI crystal is housed in a hermetically sealed metal container with a quartz/glass window at one end.

• The window is in contact with the photocathode of a photomultiplier tube.

• The photons produce flashes of light (scintillations) from the crystal that are converted into electronic pulses by the photomultiplier tube.



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• Fragile: subject to mechanical shock and thermal shock (rapid large changes in temperature can crack the crystal)

• If the integrity of the container is compromised, the crystal can absorb moisture (and turn yellow).

In some cases, the damaged surface layer of the crystal can be removed and the crystal continue to be used.

• Damaged crystals still produce a signal, but the performance deteriorates. In many cases, the damage is not recognized and the detector is still used.



137

• With some NaI meters, it is possible to set a “window” so that they only respond to pulses in a specified size range.

For example, if both Cs-137 and Co-60 are present but only the cobalt is of interest, a window could be set up about the Co-60 pulses, reducing the interference from the Cs-137 and other nuclides that contribute to background.



138

• In this instrument, Channels 1 and 2 are preset windows.

• When switched to Channel 1, the meter only responds to small pulses.

• When set to Channel 2, it only responds to the large pulses (e.g., Co-60).



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Microrem (microsievert) Meter

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Detector: Plastic scintillator inside case

Lowest scale: 0–20 µrem/hr (0–0.2 µSv/hr)

Highest scale: 0–200,000 µrem/hr (0–2,000 µSv/hr)

Audio: If present, it is an artificial digital version



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• Flat energy response; unlike the Micro R meter, it reads accurately at low energies even if calibrated at high energy.

• Convenient to carry since no cable to deal with

• Somewhat lighter and more rugged than Micro R meter

• One version comes with thin window to improve its response to low energy x-rays



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Detects low-energy photon emitters. Most commonly used to detect the 59.5 keV gammas of Am-241. Sometimes used to detect U-238 via the low-energy photons emitted by Th-234.


FIDLER

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Field instrument for the detection of low energy radiation


Detector: Thin (ca. 2 mm) NaI crystal

Window: Beryllium (0.25 mm thick)

Aluminum or other material might be used


FIDLER

144

• Heavy

• Expensive

• Typically used to make fixed measurements (facing down and at 30 cm above the ground)

• To reduce background and increase sensitivity, it is common to set up a window about the pulse sizes of interest.


FIDLER

PMT

Low-energy photon

High-energy photon

Thin NaI crystal

Thin window (beryllium/aluminum)

• The thin beryllium window increases efficiency for low-energy photons.

• The thin crystal reduces the efficiency for high-energy photons without affecting efficiency at low energies. This minimizes background.

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FIDLER

Used to measure exposure rates (mR/hr, R/hr)

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Ion Chamber

147

Radiation detected: Photons Betas and/or alphas (open window)

Chamber volume: 200–300 cm3

Chamber gas: Typically air Possibly nitrogen or argon

Window thickness: 1.5–7 mg/cm2

Lowest scale: 0–0.5, 0–5.0, or 0–25 mR/hr (0–5, 0–50, or 0–250 µSv/hr)

Highest scale: 0–5, 0–50, 0–500, or 0–1,000 R/hr (0–0.05, 0–0.5, 0–5, or 0–10 Sv/hr)


Ion Chamber

148

• Operates in current mode

• No audio

• Slow to respond; might need 1–2 minutes for reading to stabilize

• Some use chambers that are open to the atmosphere

• Some employ sealed chambers

• If sealed, the gas might or might not be pressurized (e.g., the Fluke 451P chamber is sealed and pressurized with air to 6 atmospheres).


Ion Chamber

149

• If open to the atmosphere, the response is affected by temperature and pressure. Corrections might be necessary.

• If the chamber is open to the atmosphere, the air passes through a desiccant (typically indicating silica gel) prior to entering chamber.

• When dry, silica gel is deep rich blue. When it has absorbed too much moisture to be useful, it turns pink and must be replaced.


Ion Chamber

150

• Ion chambers often have windows.

• Depending on the window thickness, the ion chamber response can indicate the presence of beta or alpha contamination when the shield is in the open configuration.

• Exposure rate measurements are usually made with the shield covering the window because the exposure rate (in mR or R/hr) is only defined for photons (not betas).


Ion Chamber

151

• It is possible to estimate the beta dose rate using the difference between the open and closed readings. The difference reflects the instrument response to the betas.

• The difference in mR/hr is multiplied by a factor (typically 4–5) derived from measurements on a uranium slab to get the beta dose rate in mrad/hr.


Ion Chamber

Appendix B


This section is not intended to be comprehensive. Rather, it is

meant to give an indication of the range of specialized equipment that is commercially available.

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153

Appendix B: Specialized Instrumentation

GammaCam (Exelis – ITT)

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• Consists of a sensor head mounted on tripod and a portable personal computer (PC)

• Sensor head incorporates an electronic camera and a gamma ray detector of high-density terbium-activated glass

• Sensor head usually positioned 15–30 feet away from area being characterized

• Measurement period might be a fraction of a second or many hours, depending on the activity of the source



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• Radiation intensity mapped out in color; red is highest, blue is lowest

• Radiation field superimposed on a black and white photo of the area being characterized

• Uniform background is subtracted out of image

• Of most value during characterization phase of decommissioning when dealing with high radiation levels

• Comparable to Babcock’s Radscan 900



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• Remote characterization can reduce surveyor exposures and contamination

• Provides real-time image of gamma radiation intensity

• Provides an electronic visual record

• Can reduce time needed to characterize radiation fields

• Facilitates characterization of difficult to reach areas (e.g., upper walls)

• Training in the setup, use, and data interpretation can be completed in a day



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RadScan 900 (Babcock International)

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• Detector head mounted on stand coupled to portable PC

• Incorporates gamma ray detector and electronic camera

• Detector: NaI(TL) scintillator

• Light converted to electronic pulses by photodiode

• Head rotates 340° and tilts from -40° to +90°

• Scanning speed: 2–4° per second

• Can be positioned 1–50 meters away from area being characterized

http://babcockinternational.com/capabilities/infrastructure/nuclear/contaminated-land-and-land-quality-management/



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• Radiation field superimposed on photo of the area being characterized

• Radiation intensity mapped in color

• Distinguishes radionuclides via gamma ray energy

• Most useful during characterization of facility when dealing with high radiation levels



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RadMIC Internal Contamination Pipe Probe (BIC Scientific)

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• Detector: alpha and beta scintillators in various combinations

• Available with outer diameters of 2, 4.5, and 9 cm

• Various spring loaded spiders for different diameter pipes

• Pushed by controller through pipe up to 4 m in length


RadMIC Internal Contamination Pipe Probe (BIC Scientific)

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• Two pair of pancake GM tubes

• Offset so as to achieve near 360 degree coverage

• Pushed or pulled through pipe


ORAU Pipe Inspection System

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• Computer controlled monitor for 0.5–2.75 inch (1–7 cm) diameter pipes

• Contaminated pipe is fed into monitors, not designed for in situ characterization

• Uses multiple plastic scintillators

• Sensitivity: 5,000 dpm in a 20 µR/hr field from Co-60 (8,300 Bq in a 20 µSv/hr field)


CPM-402A Pipe Monitor (TSA Systems)

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• Computer-controlled monitor for materials and small equipment, containerized waste

• Uses multiple plastic scintillators; He-3 neutron detectors optional


ABM-486 Box Monitor (TSA Systems)

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• IonSens Alpha Pipe Monitor, IonSens Large Item Monitor, IonSens Conveyorized Survey Monitor

• Evaluates alpha activity on exposed surfaces of items with complex geometry

• Draws air through unit in which contaminated material is enclosed; measures ionization in collected air

• Five minute count times and 15 Bq minimum detectable activity (MDA)


IonSens (Babcock International)

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• Large area detector for alpha and/or beta contamination on flat, hard surfaces (e.g., floors)

• Gas flow proportional (P-10)

• Window area: 582 cm2

• Detector can be detached for use on walls

• Window thickness: 0.8 mg/cm2 standard; 0.4, 3.9, and 7.9 mg/cm2 also available


239-1F Floor Monitor (Ludlum)

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• Floor monitor for alpha and beta contamination

• Plastic scintillator

• Background subtract capability

• Includes alarm settings

• 600 cm2 window

• Mylar® window expensive to replace


FLM3 Floor Monitor (Thermo Scientific)

BFM-1

AFM- 2

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BFM-1 Beta Floor Monitor:

• 3 GM detectors (windows 30 mg/cm2)

• 24 inch (60 cm) coverage

AFM-2 Alpha Floor Monitor: • ZnS alpha scintillators

• 144 in2 (930 cm2) area


Floor Monitors (Technical Associates)

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ABFM-5 Alpha-Beta Floor Monitor: • Twelve pancake GMs • 1.5 mg/cm2 windows

ABFM-6 Alpha-Beta Floor Monitor: • Plastic scintillator • 0.8 mg/cm2 windows

ABFM-7 Alpha-Beta Floor Monitor: • Gas flow proportional counter • 144 square inch area • 0.08 mg/cm2 windows


Floor Monitors (Technical Associates)

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• Computer-based data acquisition and management system for characterizing alpha and beta contamination on flat surfaces

• Position-sensitive proportional counter on mobile platform (cart)

• Moves in a straight line at a controlled speed; this permits the position of the detector to be known at any given time

• Various detector lengths available (e.g., 1–2 meters)


Surface Contamination Monitor (Millennium Services)

• The signal is monitored at each end of the detector anode. The time difference between the arrival of the signal at each end is used to determine the point where the incident radiation penetrated the detector window.

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Three dimensional and color coded displays of data 176



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Background differences in asphalt areas

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• LARADS – computer-based data acquisition and management system for interior and exterior surveys

Eberline Services Inc.

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Laser Assisted Ranging and Data System (Eberline)

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• Probe position determined within centimeters by laser (automatic tracking system)

• Conventional detectors with reflector attached to probe

• Detector output transmitted to portable PC via radio modem


Laser Assisted Ranging and Data System (Eberline)

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ISO-CART Mobile Assay System (Amtek-Ortec)

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• Cart-mounted high purity germanium detector

• Employs ORTEC’s digiDART gamma spectroscopy system and ISOTOPIC software

• Identifies and quantifies radionuclides in a variety of common geometries (e.g., pipes, drums, boxes, ceilings, walls, soil, points, disks)

• Three different collimators

• Calibrated with multi-nuclide point source



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• Distance to object determined via laser

Canberra

185


In-Situ Object Counting System (Canberra-Areva)

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• High purity germanium gamma spectroscopy system for identifying and quantifying volumetric concentrations of gamma emitters in situ

• Counting efficiency calculated by ISOCS software


In-Situ Object Counting System (Canberra-Areva)

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