Pennsylvania Dept of Env Protection

Sample Collection & Preservation
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Entry Point
Representative
Composite
Total Activity

Sample Collection & Preservation
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Containers

Sub-microgram

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Plastic or Glass
Glass Only – tritium

Sample Collection & Preservation

Preservation


HNO
3
HCl


Done by laboratory

Within 5 days

Hold 16 hours
None – tritium and iodine

Sample Collection & Preservation

Holding time – Related to half life

8 Days ( 131 I)


6 Months

Tritium


Alpha/Beta
Radium

Gamma
1 – 4 Days ( 222 Rn, 224 Ra)

Pennsylvania Dept of Env Protection

Instrumentation – Detectors
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Gas proportional
Zinc sulfide (ZnS) scintillation
Liquid scintillation
Surface barrier
Lithium drifted germanium
(GeLi)
High purity, germanium (HPGe)

Instrumentation – Shielding
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Low level measurement
Decrease background
Protect from environment
Lead
Steel
Copper

Radioactivity Decay Review
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Alpha Particles
Beta Particles
Photons

Alpha
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Particle
Heavy – helium nucleus
Highly charged

Beta
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Particle
Light – electron
Moderately charged

Gamma
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Wave
No mass
No charge
Photon – like light but higher energy

Gas Proportional Counter
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Optional detector

Gas Proportional Counter
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response

Gas Proportional Counter
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Components
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Sample changer
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High voltage power supply
Detector
Preamplifier
Amplifier
Scaler
Timer
Data collection & output device

Gas Proportional Counter
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Sample
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Guard

Gas Proportional Counter
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Windowless
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Sample inside counting chamber
Thin Window
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Particle must penetrate window

Gas Proportional Counter
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Anti-coincidence
Cosmic radiation
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Background

Gas Proportional Counter
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Plateau
Instrument Background
Alpha Efficiency
Beta Efficiency

Gas Proportional Counter
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Operating voltage
Consistent count rate
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Alpha Plateau
Beta Plateau
“Knee”

Gas Proportional Counter
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Cosmic radiation
Electronic noise
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Natural radiation
Alpha
Beta
Background Subtraction

Gas Proportional Counter
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Counts / disintegrations
Detector area
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Geometry
Particle energy

Gas Proportional Counter
Beta
Carbon 14
Half life
5730 yrs
Energy (MeV)
0.156
Technetium 99 2.13X10
5 yrs 0.224
Strontium 90 29 yrs 0.546
Lead 210 22.26 yr 1.16

Gas Proportional Counter
Alpha Half life
Americium 241 432 yr
Polonium 210 138 days
Thorium 230 75,400 yr
Energy (MeV)
5.443, 5.486
5.304
4.688, 4.621

Gas Proportional Counter
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
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Reagent Background
Efficiency
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
Method
Self adsorption
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Alpha

Beta

Gas Proportional Counter
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
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Distance to detector
Window absorption
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Self absorption

Statistics
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Statistics

Statistics – Counting Error
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± 100 % at 95% confidence interval
1.96
σ
Where σ = standard deviation of net counting rate of sample

Statistics – Counting Error
 Standard deviation
σ =
R s  t s
R b t b where:
R s
= sample counting rate
R b
= background counting rate t s
= sample counting time t b
= background counting time

Statistics – Counting Error Example
R s
= 2.74 cpm
R b
= 1.50 cpm t s
= 50 min t b
= 50 min
C.E. = 1.96 [2.74/50 + 1.5/50] 0.5

Statistics – Counting Error Example
C.E. = 1.96 [2.74/50 + 1.5/50] 0.5
C.E. = 1.96 [0.055 + 0.030] 0.5
C.E. = 1.96 [0.085] 0.5
C.E. = 0.80 cpm
Result = 2.74
0.80 cpm

Statistics – Detection Limit

Statistics – Detection Limit
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
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LLD ~ (k
α
+ k
β
) σ o k
α
= false negative k
β
= false positive
σ o
= standard deviation of net counting rate of sample

Statistics – Detection Limit
 Generally use 95%
Confidence
 k
α
= k
β
= k = 1.645
At the LLD
 Sample count rate ~ background count rate

Statistics – Detection Limit
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σ o
= [ σ s
2 + σ b
2 ] 0.5
When R s
~ R b and t s
= t b
σ s
2 = σ b
2
σ o
= [2] 0.5
σ b
LLD = 2[2] 0.5
k σ b
LLD = 4.66 σ b
σ b
= [R b
/t b
] 0.5

Statistics – Detection Limit
 Time
 Volume
 Efficiency
 Self absorption
 Background

Gas Proportional Counter
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Time versus performance
Preset time
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Preset count

Minnesota Department of Health

EPA Method 900.0
+ +



Prescribed Procedures for
Measurement of Radioactivity in
Drinking Water
EPA-600/4-80-032 August 1980
Determination of Gross Alpha and
Gross Beta Radioactivity in Drinking
Water
-
  -

EPA Method 900.0
What we’ll cover
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Scope of the method
Summary of the method
Calibration

Determining operating voltage

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Determining system background
Determining efficiency calibration
Determining self-absorption factor
Quality control
Interferences
Application
Calculations

Activity

EPA Method 900.0
Scope
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The method is a screening technique for monitoring drinking water supplies

The solids are not separated from the sample

Solids concentration is a limiting factor in the sensitivity of the method

EPA Method 900.0
Alpha and Beta Procedure Summary
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
Sample is preserved in the field or at the lab with nitric acid

Lab preservation
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Within 5 days of collection
Hold for 16 hours after acidification
Homogeneous aliquot of preserved sample

Typically 250 mL or less

EPA Method 900.0
Alpha and Beta Procedure Summary
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Sample is evaporated to near dryness

If sample is evaporated to dryness in the beaker, re-start sample analysis
Add 10 ml 1N HNO dissolve solids
3 to beaker to

Additional nitric acid is added to convert chloride salts to nitrate salts

Chloride salts attack the stainless steel planchet

EPA Method 900.0
Alpha and Beta Procedure
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Sample is quantitatively transferred to a tared planchet
Sample is reduced to dryness on planchet
Sample residue is dried to constant weight
Analyzed for beta emissions

EPA Method 900.0
Alpha and Beta Procedure
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Planchet is flamed and stored for 3 days to allow for the ingrowth

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Flaming converts hygroscopic nitrate salts to oxides
Ingrowth for progeny of Ra-226
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Sample residue is reweighed to determine flamed residue weight
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Analyzed for alpha emissions

EPA Method 900.0
Alpha and Beta Procedure

EPA Method 900.0 Calibrations
(Determine Operating Voltage)
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Calibration Order
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Plateau
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Spillover Correction or Crosstalk
Background
Efficiency
Sample Self Absorption or Mass Attenuation

EPA Method 900.0 Calibrations
(Determine Operating Voltage)
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
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Determine appropriate (knee) operating voltage
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 alpha beta plateau
A plateau is generated by counting a source several times while increasing (stepping) the high voltage to the detector.
Alpha plateau = alpha activity
Beta plateau = alpha/beta activity
Generate an alpha/beta plateau after every
P10 gas exchange

Quality of the gas affects the plateaus and instrument performance

EPA Method 900.0 Calibrations
(Determine Operating Voltage)

EPA Method 900.0 Calibrations
(Determine Operating Voltage)

EPA Method 900.0
Alpha and Beta Gas Proportional Counters

EPA Method 900.0
Alpha and Beta Gas Proportional Counters

EPA Method 900.0
Alpha and Beta Gas Proportional Counters

EPA Method 900.0
Alpha and Beta Gas Proportional Counters

EPA Method 900.0
Alpha and Beta Gas Proportional Counters

EPA Method 900.0 Calibrations
(Spillover Correction or Crosstalk)
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Alpha beta discriminators should be adjusted to minimize false readings

Alphas counted as betas and betas counted as alphas

EPA Method 900.0
(Determine System Background)
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Contribution of the background must be measured
Measure under the same conditions, counting mode, and geometry as the samples
Count background longer than samples

Establish good statistics
Background determination is performed every time the P10 gas cylinders are changed

EPA Method 900.0
(Determine Efficiency Calibration)
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Calibrate to obtain relationship of count rate to disintegration rate.
Natural uranium and thorium-230 are approved as gross alpha calibration standards for evaporation methods and co-precipitation methods
Americium-241 is only approved for the coprecipitation methods.

40CFR part 141.25 Analytical methods for radioactivity. Footnote 11
Strontium-90 and cesium-137 are approved as gross beta calibration standards.

Cesium-137 is volatile
NIST traceable standards

EPA Method 900.0
(Determine Efficiency Calibration)

EPA Method 900.0
(Determine Efficiency Calibration)

EPA Method 900.0
Alpha/Beta Self-Absorption Factors
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Determined by graphing residue weight
(mg) vs. the efficiency factor (dpm/cpm)
Multiple aliquots
Constant alpha and beta activity using calibration standards
Varying solids concentration
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2-inch diameter counting planchet (20 cm 2 )
0 and 100 mg for alpha
0 and 200 mg for beta

EPA Method 900.0
Alpha Self-Absorption Factors
Planchet #
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
0.0316
0.0335
0.0389
0.0659
0.0834
0.0980
0.1087
0.1219
Solids (g)
0.0087
0.0092
0.0116
0.0143
0.0180
0.0202
0.0241
0.0260
0.0300
Th-230
44.32
46.00
39.47
27.23
26.34
21.11
17.96
16.39
61.32
53.61
50.75
43.36
46.74
cpm
72.03
72.83
69.38
64.32
Decay Corrected
Counts
375.14
375.14
375.14
375.14
375.14
375.14
375.14
375.14
375.14
375.14
375.14
375.14
375.14
375.14
375.14
375.14
375.14
0.1181
0.1226
0.1052
0.0726
0.0702
0.0563
0.0479
0.0437
Efficiency
0.1920
0.1941
0.1849
0.1715
0.1635
0.1429
0.1353
0.1156
0.1246

EPA Method 900.0
Alpha Self-Absorption Factors

EPA Method 900.0
Quality Control
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Instrument efficiency check
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Analyzed daily
Control chart
Establish action limits
Low background check
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Analyzed daily
Control chart
Establish action limits
Analytical Prep Batch
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Laboratory Reagent Blank (LRB)
Laboratory Fortified Blank (LFB)
Sample Duplicates at a 10% frequency
Sample Spikes at a 5% frequency
Control chart
Establish action limits

EPA Method 900.0
Interferences
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Moisture obstructs counting and self–absorption characteristics
Non-uniformity of the sample residue in planchet

 accuracy precision
Sample density on the planchet area should not be more than 5 mg/cm 2
(< 100 mg) alpha for gross alpha
Sample density on the planchet area should not be more than 10 mg/cm 2
(< 200 mg) for gross beta

EPA Method 900.0
Application



The National Primary Interim Drinking
Water Regulations (NIPDWR) require the following detection limits


Gross Alpha 3 pCi/L
Gross Beta 4 pCi/L
Maximum Contamination Level (MCL)
Gross alpha 15 pCi/L
>15 pCi/L run uranium determination

EPA Method 900.0
Calculations

Alpha radioactivity

Alpha
(pCi/liter)
= A * 1000
2.22 * C * V
Where:
A= net alpha count rate (gross alpha count rate minus the background count rate) at the alpha voltage plateau
C= alpha efficiency factor, read from graph of efficiency versus mg (cpm/dpm)
V= volume of sample aliquot, (ml)
2.22= conversion factor from dpm/pCi

EPA Method 900.0
Calculations


Beta radioactivity
If there are no significant alpha counts when the sample is counted at the alpha voltage.

Beta
(pCi/liter)
= B * 1000
2.22 * D * V
Where:
B= net beta count rate (gross beta count rate minus the background count rate) at the beta voltage plateau
D= Beta efficiency factor, read from graph of efficiency vs. mg (cpm/dpm)
V- volume of sample aliquot, (ml)
2.22= conversion factor from dpm/pCi

EPA Method 900.0
Calculations


Beta radioactivity
Beta counting in the presence of alpha radioactivity.

Beta
(pCi/liter)
= (B – AE)* 1000
2.22 * D * V
Where:
B= net beta count rate (gross beta count rate minus the background count rate) at the beta voltage plateau
A= net alpha count rate (gross alpha count rate minus the background count rate) at the alpha voltage plateau
E= alpha amplification factor, read from the graph of the ratio of alpha counted at the beta voltage/alpha counted at the alpha voltage vs. sample density thickness
D= Beta efficiency factor, read from graph of efficiency vs. mg
(cpm/dpm)
V- volume of sample aliquot, (ml)
2.22= conversion factor from dpm/pCi

EPA Method 900.0
Calculations
Alpha and beta radioactivity
A
(pCi/L)
= (G-B)((SAF*g)+1)/(2.22*E*T*V)
Where:A = gross alpha/beta activity in pCi/L
B = background counts per minute
E = efficiency of detector
G = gross counts per minute
SAF = alpha/beta self-absorption efficiency factor
T = count time
V = sample volume, (liters) g = net weight of solids, (grams)
2.22 conversion factor, dpm/pCi

EPA Method 900.0
Method SOP Main Sections
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Scope and Application
Summary of Method
Definitions
Regulatory Deviations
Interferences
Safety
Equipment and Supplies
Reagents and Standards
Calibration and Standardization
Procedure
Data Analysis and Calculations
Method Performance
Pollution Prevention
Waste Management
References
Diagrams, Flowcharts, Validation Data