Measuring
Radionuclides in the
Environment
Dr. Bob Johnson
Argonne National Laboratory
April 2011 | Argonne National Laboratory, USA
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Decision-Making Requires Data
Instrumentation and analytical methods are the sources
of data:
 “Real-time” versus more traditional methods
 Radionuclide-specific versus gross activity
measurements
 In situ versus ex situ measurements
 “Cheap, fast, qualitative” versus “expensive, slow,
definitive”
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Ionizing Emissions Serve as the Basis for Most
Radionuclide Measurement Systems
 Radioactive decay can involve a variety of emissions,
including alpha particles, beta particles, and gamma rays
 Gamma rays are typically the preferred emissions for
measuring activity concentrations in soils because they are
easiest to measure
 Radionuclides that are not gamma emitters are often mixed
with other radionuclides that are, and can be used as
surrogates
 For the decay of a particular radionuclide of concern, such as
radium-226 or its daughter products, emissions are
characterized by their specific energy and abundance
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Measurement Challenges
 Alpha radiation (positively charged particle)
– Easily shielded, even by air
– Place detector on or very near surface
 Beta radiation (negatively charged particle)
– Shield/discriminate alpha, measure beta-gamma
– Use at close range
 Gamma radiation (photon, no mass)
– Field of view, collimators, shine, source geometry, self-shielding
– 1-m height for dose rate measurements
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Field of View
 “Field of View” refers to what a detector “sees”
 Field of view can vary dramatically from detector to detector
 Field of view is controlled by height of detector off ground,
detector geometry, and collimation
 Shine refers to sources of radiation that affect a measurement
without being in the presumed field of view
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Energy Spectra
Characteristic
photons
Resolution of energy
peaks
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Photon energy
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Measurements Often Target Daughter Products
or Progeny
Th-232
U-238
Th-228
U-234
 4.5 x 109 yrs

1.2 mins
 14 x 109 yrs
6.1 hrs
 240,000 yrs
Uranium-238
Decay Series
Pa-234

24 days
Th-234

Thorium-232
Decay Series
 1.9 yrs
Ac-228

Th-230
Ra-224
5.8 yrs
 77,000 yrs
Ra-228
 3.6 days
Ra-226
Rn-220
 1,600 yrs
Po-212
 55 sec
Rn-222
Po-214

61 mins
Po-210
Po-216
 3.8 days


20 mins
5 days

Po-218
Bi-214
Pb-214

Bi-210

22 yrs

27 mins
Pb-210
11 hrs
 140 days
Pb-208
 61 mins

3.1 mins
Pb-212
Pb-206
(stable)
April 2011 | Argonne National Laboratory, USA
Bi-212
 0.15 sec
160 sec
 3.1 mins

300 nsec
Tl-208
(stable)
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Secular Equilibrium Assumptions Are Often Important
 Secular equilibrium: decay products are at the same activity
concentration as parents
 Occurs when undisturbed in-growth has taken place over several
half-lives for daughter products
 If assumption holds, allows one to measure a daughter and infer the
activity concentration of the parent radionuclide
 A very common assumption made by laboratory gamma spec
 Can be an issue when daughters are mobile and samples are
disturbed (e.g., Ra-226 decay chain)
 For data quality reviews, important to understand exactly what a lab
is measuring
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Basic Measurement Choices
 Soil samples with laboratory
analyses
 Soil samples with field
analyses
 Direct measurement
techniques
 Scanning or survey
techniques
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Soil Sampling and Laboratory Analyses
 Typically ~400 grams of soil
collected
 Laboratory uses alpha or
gamma spectroscopy or beta
scintillation for analysis with
accurate activity concentrations
returned
 Costs are on the order of $200
per sample or more
 Turn-around times usually
weeks
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Direct Measurements: In Situ HPGe/NaI Systems
• Works for gamma-emitting radionuclides
(Ra-226, Th-232, Cs-137 and U-238)
• With proper geometry assumptions,
provides activity concentration
estimates
• Measurement times on order of
15 minutes with results immediately
available
• Field of view considerations important
• Per measurement cost on order of
$100
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Field Analytical Methods: XRF
• Designed for inorganic
analysis
• Measures total U and
other metals (ppm)
• 2-minute count times
or less
• Per measurement
costs on order of $40
or less
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Radiation and Detectors
 Alpha emitters
 Gas Proportional
 ZnS scintillator
 Phoswich scintillator
 Beta-gamma
 Gas Proportional
 Low-level gamma
 G-M
 NaI (FIDLER, 1”×1”, 2” ×2”)
 Higher-level betagamma
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 G-M, PIC, NaI, plastic
scintillator, LaBr3
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Quick Soil Screening: Geiger-Mueller Detector
 General-purpose detector
– Beta-gamma surveys
– Surface contamination
– Exposure rate
measurements (50 µR/h)
– Health and Safety
applications
– Screening areas/materials
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Gross Activity Area Scans: NaI-Based Systems
• Thin window mini-FIDLER with NaI
crystal for gross counts
• Shielded housing to lower background
and control field of view
• Rate meter for measuring output and
interfacing with data logger
• Can be combined with GPS unit for
locational control
• 2 second acquisition time
• Pennies per measurement
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Lanthanum Bromide Detectors
 First appeared in 2001
 Higher energy resolution than
NaI (allows for better
radionuclide identification)
 No requirement for cooling
such as for HPGe (better for
field deployment)
 Cost between NaI and HPGe
 Currently primarily used in
support of homeland security
 Will become more common in
environmental applications
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Scanning Systems Use a Variety of Platforms
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Example Output from Gross Gamma Scan
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Example Output from Gross Gamma Scan
• GPS-based NaI scans
provide detailed information
about spatial distribution of
contamination in soils
• When coupled with
mapping tools such as GIS,
provide excellent means for
characterizing soil surfaces
This walkover was done using a 2x2
NaI detector, contaminants of
concern were Th-230, U-238 and
Ra-226, detection sensitivities
around 2 pCi/g for Ra-226
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Example Output from Gross Gamma Scan
This walkover was done using a
FIDLER NaI detector, contaminants
of concern were U-238. Apparent
sensitivities on the order of 5 pCi/g
U-238, but these were enhanced by
slightly elevated collocated Ra-226
and Ra-228 (approximately 1 pCi/g
total above background conditions).
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Example Output from Gross Gamma Scan
This walkover was done using a
mini-FIDLER NaI detector,
contaminants of concern were
NORM (Ra-226). Sensitivities on the
order of 1 pCi/g above background
conditions.
April 2011 | Argonne National Laboratory, USA
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Example Output from Gross Gamma Scan
This walkover was done using a
FIDLER NaI detector, contaminants
of concern were Ra-226, U-238, and
Th-232.
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Measurement Sensitivities
 Concept of sensitivity, or detection limits, applies both to
laboratory radionuclide-specific analyses and scanning
measurements
 Basic concept hinges on the idea that a “detection” should
correspond to a result that one is confident is not “zero” or the
absence of contamination
 In the case of a laboratory measurement, this means a
measurement result that is indicative of the presence of a
particular radionuclide
 In the case of a scanning measurement, this means a
measurement result that is inconsistent with background
readings
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MARSSIM’s Approach…
• Lc – level above which something has been identified
inconsistent with background
• LD – Level at which something will be reliably identified
as present
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Example Data Sets…
12
• 36 background samples analyzed
for Am-241
10
8
• average: -0.003 pCi/g, stdev: 0.041
pCi/g
6
4
2
0
<-0.05
-0.05 to 0.03
-0.03 to 0.01
-0.01 to 0.01 0.01 to 0.03 0.03 to 0.05
>0.05
• using a 3 standard deviation rule,
the LC in this case would be 0.123
pCi/g, and the LD would be 0.246
pCi/g
350
• 938 measurements from a
background area
300
250
• average: 23.6K cpm; stdev:1.2K cpm
• using a 3 standard deviation rule, the
LC in this case would be 27.2K cpm,
and the LD would be 30.8K cpm
200
150
100
50
0
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20K 21K
21K 22K
22K 23K
23K 24K
24K 25K
25K 26K
26K 27K
27K 28K
28K 29K
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Example Output from Gross Gamma Scan
This walkover was done using a 3x3
NaI detector, contaminants of
concern were Th-230 comingled with
very low levels of U-238. Variations
in soil types and natural variations in
gross activity complicated
interpretation of gamma walkover
survey (i.e., survey was as
misleading as it was informative).
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Reducing Detection Limits…
 For laboratory measurements, this means increasing measurement
times and/or using better detectors and/or switching analytical
methods
– Rule of thumb: should try to achieve a detection limit that is 10%
of either background or DCGL standards
 For scans, this means reducing background “noise”
– Switching detectors
– Collimating detectors
– Differentiating between different types of background materials
– Increasing measurement time typically has little beneficial impact
for gross activity measurements
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April 2011 | Argonne National Laboratory, USA