Measuring Radionuclides in the Environment Cours… · Measuring Radionuclides in the Environment ... mapping tools such as GIS, ... Concept of sensitivity,

April 2011 | Argonne National Laboratory, USA

Measuring Radionuclides in the Environment

Dr. Bob JohnsonArgonne National Laboratory


IDN and ENVIRONET Training Course

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”



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



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



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



Energy Spectra

Photon energy

Characteristicphotons

Resolution of energypeaks



Measurements Often Target Daughter Products or Progeny

Pb-210

U-238

Th-234

U-234

Th-230

Ra-226

Po-218

Pb-214

Bi-214

Po-214

Bi-210

Po-210

22 yrs27 mins140 days

160 sec5 days20 mins

3.1 mins

3.8 days

1,600 yrs

77,000 yrs

240,000 yrs1.2 mins

24 days

4.5 x 109 yrs

Pa-234

Pb-206

(stable)

Rn-222

Uranium-238Decay Series

Thorium-232Decay Series

Th-232

Ra-228

Th-228

Ra-224

Rn-220

Po-216

Pb-212

Bi-212

Po-212

Tl-208

3.1 mins

11 hrs61 mins

300 nsec61 mins

0.15 sec

55 sec

3.6 days

1.9 yrs6.1 hrs

5.8 yrs

14 x 109 yrs

Ac-228

Pb-208

(stable)



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



Basic Measurement Choices

Soil samples with laboratory analyses

Soil samples with field analyses

Direct measurement techniques

Scanning or survey techniques



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



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



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



Radiation and Detectors

Alpha emitters

Beta-gamma

Low-level gamma

Higher-level beta-gamma

Gas Proportional ZnS scintillator Phoswich scintillator

Gas Proportional

G-M NaI (FIDLER, 1”×1”, 2” ×2”)

G-M, PIC, NaI, plastic scintillator, LaBr3



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



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



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



Scanning Systems Use a Variety of Platforms



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



Example Output from Gross Gamma ScanThis 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).




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.




This walkover was done using a FIDLER NaI detector, contaminants of concern were Ra-226, U-238, and Th-232.



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

Measurement Sensitivities



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



Example Data Sets…• 36 background samples analyzed

for Am-241

• average: -0.003 pCi/g, stdev: 0.041 pCi/g

• 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/g0

2

4

6

8

10

12

<-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

0

50

100

150

200

250

300

350

20K -21K

21K -22K

22K -23K

23K -24K

24K -25K

25K -26K

26K -27K

27K -28K

28K -29K

• 938 measurements from a background area

• 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




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).



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



Documents

Measuring Radionuclides in the Environment Cours… · Measuring Radionuclides in the Environment ... mapping tools such as GIS, ... Concept of sensitivity,