Instrumentation & Methods:
Gas Proportional Counters
Richard Sheibley
Pennsylvania Dept of Env Protection
Instrumentation – Detectors
{
{
{
{
{
{
Gas proportional
Zinc sulfide (ZnS) scintillation
Liquid scintillation
Surface barrier
Lithium drifted germanium
(GeLi)
High purity, germanium (HPGe)
Instrumentation – Shielding
{
{
{
{
{
{
Low level measurement
Decrease background
Protect from environment
Lead
Steel
Copper
1
Radioactivity Decay Review
{
{
{
Alpha Particles
Beta Particles
Photons
Alpha
{
{
{
Particle
Heavy – helium nucleus
Highly charged
Beta
{
{
{
Particle
Light – electron
Moderately charged
2
Gamma
{
{
{
{
Wave
No mass
No charge
Photon – like light but higher
energy
Gas Proportional Counter
{
{
{
Alpha particles
Beta particles
Photons (gamma)
z
Optional detector
Gas Proportional Counter
{
{
{
Ion Pair formation
Voltage Pulse
Proportional response
3
Gas Proportional Counter
{
Components
z
z
z
z
z
z
z
z
Sample changer
High voltage power supply
Detector
Preamplifier
Amplifier
Scaler
Timer
Data collection & output device
Gas Proportional Counter
{
Two Detector System
z
Sample
z
Guard
Gas Proportional Counter
{
Sample Detector
z
Windowless
{
z
Sample inside counting
chamber
Thin Window
{
Particle must penetrate window
4
Gas Proportional Counter
{
Guard Detector
z
z
z
Anti-coincidence
Cosmic radiation
Background
Gas Proportional Counter
{
Instrument Performance
verification
z
z
z
z
Plateau
Instrument Background
Alpha Efficiency
Beta Efficiency
Gas Proportional Counter
{
Plateau
z
z
z
z
z
Operating voltage
Consistent count rate
Alpha Plateau
Beta Plateau
“Knee”
5
Gas Proportional Counter
{
Instrument Background
z
z
z
z
z
z
Cosmic radiation
Electronic noise
Natural radiation
Alpha
Beta
Background Subtraction
Gas Proportional Counter
{
Instrument Efficiency
z
z
z
z
Counts / disintegrations
Detector area
Geometry
Particle energy
Gas Proportional Counter
Beta
Half life
Energy (MeV)
Carbon 14
5730 yrs
0.156
Technetium 99 2.13X105 yrs
0.224
Strontium 90
29 yrs
0.546
Lead 210
22.26 yr
1.16
6
Gas Proportional Counter
Alpha
Half life
Energy (MeV)
Americium 241 432 yr
5.443, 5.486
Polonium 210
138 days
5.304
Thorium 230
75,400 yr
4.688, 4.621
Gas Proportional Counter
{
Method QC
z
z
z
Reagent Background
Efficiency
{ Method
Self adsorption
{ Alpha
{ Beta
Gas Proportional Counter
{
Sample count rate factors
z
z
z
Distance to detector
Window absorption
Self absorption
7
Statistics
{
{
{
{
Poisson Statistics
Random
Chi-square test
Standard deviation
Statistics
Statistics – Counting Error
{
Drinking water – defined in
40 CFR 141.25(c)
z
z
z
± 100 % at 95% confidence
interval
1.96σ
Where σ = standard deviation
of net counting rate of sample
8
Statistics – Counting Error
Standard deviation
σ=
where:
Rs = sample counting rate
Rb = background counting rate
ts = sample counting time
tb = background counting time
{
Rs Rb
+
ts tb
Statistics – Counting Error Example
Rs = 2.74 cpm
Rb = 1.50 cpm
ts = 50 min
tb = 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
9
Statistics – Detection Limit
Statistics – Detection Limit
{
{
{
{
LLD ~ (kα + kβ) σ o
kα = false negative
kβ = false positive
σ o = standard deviation of net
counting rate of sample
Statistics – Detection Limit
Generally use 95%
Confidence
{ kα = kβ = k = 1.645
At the LLD
{ Sample count rate ~
background count rate
{
10
Statistics – Detection Limit
{
{
{
{
{
{
{
σ o = [σ s2 + σ b2]0.5
When Rs ~ Rb and ts = tb
σ s 2 = σ b2
σ o = [2]0.5 σ b
LLD = 2[2]0.5 k σ b
LLD = 4.66 σ b
σ b = [Rb/tb] 0.5
Statistics – Detection Limit
{
{
{
{
{
Time
Volume
Efficiency
Self absorption
Background
Gas Proportional Counter
{
Counting interval
z
z
z
{
{
Time versus performance
Preset time
Preset count
Detection limit
Counting error
11
Instrumentation & Methods:
Gross alpha & beta
Jeff Brenner
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
-
α
β-
EPA Method 900.0
What we’ll cover
{
{
{
Scope of the method
Summary of the method
Calibration
z
z
z
z
{
{
{
{
Determining
Determining
Determining
Determining
operating voltage
system background
efficiency calibration
self-absorption factor
Quality control
Interferences
Application
Calculations
z
Activity
12
EPA Method 900.0
Scope
{
{
{
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
{
Sample is preserved in the field or at the lab
with nitric acid
z
{
Lab preservation
{ With in 5 days of collection
{ Hold for 16 hours after acidification
Homogeneous aliquot of preserved sample
z
Typically 250 mL or less
EPA Method 900.0
Alpha and Beta Procedure Summary
{
Sample is evaporated to near
dryness
z
{
If sample is evaporated to dryness in
the beaker, re-start sample analysis
Add 10 ml 1N HNO3 to beaker to
dissolve solids
z
z
Additional nitric acid is added to
convert chloride salts to nitrate salts
Chloride salts attack the stainless steel
planchet
13
EPA Method 900.0
Alpha and Beta Procedure
{
{
{
{
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
{
Planchet is flamed and stored for 3 days
to allow for the ingrowth
z
z
{
{
Flaming converts hygroscopic nitrate salts to
oxides
Ingrowth for progeny of Ra-226
Sample residue is reweighed to determine
flamed residue weight
Analyzed for alpha emissions
EPA Method 900.0
Alpha and Beta Procedure
14
EPA Method 900.0 Calibrations
(Determine Operating Voltage)
{
Calibration Order
z
z
z
z
z
Plateau
Spillover Correction or Crosstalk
Background
Efficiency
Sample Self Absorption or Mass Attenuation
EPA Method 900.0 Calibrations
(Determine Operating Voltage)
{
Determine appropriate (knee) operating
voltage
z
z
{
{
{
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
z
Quality of the gas affects the plateaus and
instrument performance
EPA Method 900.0 Calibrations
(Determine Operating Voltage)
15
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
16
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
17
EPA Method 900.0 Calibrations
(Spillover Correction or Crosstalk)
{
Alpha beta discriminators should be
adjusted to minimize false readings
z
Alphas counted as betas and betas
counted as alphas
EPA Method 900.0
(Determine System Background)
{
{
{
Contribution of the background must be
measured
Measure under the same conditions,
counting mode, and geometry as the
samples
Count background longer than samples
z
{
Establish good statistics
Background determination is performed
every time the P10 gas cylinders are
changed
EPA Method 900.0
(Determine Efficiency Calibration)
{
{
{
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.
z
{
Strontium-90 and cesium-137 are approved as
gross beta calibration standards.
z
{
40CFR part 141.25 Analytical methods for
radioactivity. Footnote 11
Cesium-137 is volatile
NIST traceable standards
18
EPA Method 900.0
(Determine Efficiency Calibration)
EPA Method 900.0
(Determine Efficiency Calibration)
EPA Method 900.0
Alpha/Beta Self-Absorption Factors
{
{
{
{
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
z
z
z
2-inch diameter counting planchet (20 cm2)
0 and 100 mg for alpha
0 and 200 mg for beta
19
EPA Method 900.0
Alpha Self-Absorption Factors
Th-230
Solids (g)
cpm
Decay Corrected
Counts
Efficiency
1
0.0087
72.03
375.14
0.1920
2
0.0092
72.83
375.14
0.1941
3
0.0116
69.38
375.14
0.1849
4
0.0143
64.32
375.14
0.1715
5
0.0180
61.32
375.14
0.1635
6
0.0202
53.61
375.14
0.1429
7
0.0241
50.75
375.14
0.1353
8
0.0260
43.36
375.14
9
0.0300
46.74
375.14
0.1246
10
0.0316
44.32
375.14
0.1181
11
0.0335
46.00
375.14
0.1226
Planchet #
12
0.0389
39.47
0.1156
375.14
0.1052
13
0.0659
27.23
375.14
0.0726
14
0.0834
26.34
375.14
0.0702
15
0.0980
21.11
375.14
0.0563
16
0.1087
17.96
375.14
0.0479
17
0.1219
16.39
375.14
0.0437
EPA Method 900.0
Alpha Self-Absorption Factors
Self- Absorption Curve
0.2500
0.2000
0.1500
0.1000
0.0500
0.0000
0.0000
0.0200
0.0400
0.0600
0.0800
0.1000
0.1200
0.1400
S olids (gra ms)
EPA Method 900.0
Quality Control
{
Instrument efficiency check
z
z
z
{
Low background check
z
z
z
{
Analyzed daily
Control chart
Establish action limits
Analyzed daily
Control chart
Establish action limits
Analytical Prep Batch
z
z
z
z
z
z
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
20
EPA Method 900.0
Interferences
{
{
Moisture obstructs counting and
self–absorption characteristics
Non-uniformity of the sample residue in
planchet
z
z
{
{
accuracy
precision
Sample density on the planchet area
should not be more than 5 mg/cm2
(< 100 mg) alpha for gross alpha
Sample density on the planchet area
should not be more than 10 mg/cm2
(< 200 mg) for gross beta
EPA Method 900.0
Application
{
The National Primary Interim Drinking
Water Regulations (NIPDWR) require the
following detection limits
z
z
{
{
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
z
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
21
EPA Method 900.0
Calculations
Beta radioactivity
{
If there are no significant alpha counts when the sample
is counted at the alpha voltage.
{
Beta
z
(pCi/liter)
Where:
=
B * 1000
2.22 * D * V
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.
z
Beta
Where:
(pCi/liter)
=
(B – AE)* 1000
2.22 * D * V
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
22
EPA Method 900.0
Method SOP Main Sections
{
{
{
{
{
{
{
{
{
{
{
{
{
{
{
{
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
23