Instrumentation & Methods:
Alpha Scintillation Counter
Ra226, Ra228
Lynn West
Wisconsin State Lab of Hygiene
Method Review
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Radium 226 (EPA 903.1)
Radium 228 (EPA 904.0)
Alpha-Emitting Radium Isotopes
(EPA 903.0)
Radium Chemistry
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Chemically similar to Ca & Ba
+2 oxidation state in solution
Insoluble salts include: CO3, SO4, &
CrO4
Forms a complex with EDTA
z
Property used extensively in analytical
procedures
1
Radiochemical Characteristics
Isotope T1/2
223Ra
11.1 D
Decay
Mode
Alpha
224Ra
3.6 D
Alpha
226Ra
1622 A
Alpha
228Ra
5.8 A
Beta
Series
Actinium
(235U)
Thorium
(232Th)
Uranium
(238U)
Thorium
(232Th)
Radium 226 (EPA 903.1)
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Prescribed Procedures for
Measurement of Radioactivity in
Drinking Water
EPA 600 4- 80-032
August 1980
2
Interferences
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No radioactive interferences
The original method does not use a
yield correction
238U
decay series
903.1 Method Summary
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1 L acidified sample
Ra co-precipitated with stable Ba as
SO4
Precipitate is separated from
sample matrix & supernate is
discarded
3
Method summary cont.
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(Ba-Ra)SO4 is dissolved in EDTA
Solution Transferred to a “bubbler”
.
After
a period of ingrowth, 222Rn is
purged for sample & collected in
scintillation cell
A typical radon deemanation system
Scintillation cell
Vacuum gauge
Stopcock 5
Helium gas in
Stopcock 3
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Bubbler
Scintillation Cell
Vacuum System
& gauge
Avoid using Hg
manometer if
possible
Stopcock 4
Stopcock 1
Stopcock 2
Solution
level
Components
Vacuum applied
Support
Bubbler
O-ring joint
Sintered disc
Stopcock
Scintillation Cell
{ 222Rn
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from sample is
collected in the cell
Progeny establish
secular equilibrium
in about 4 hrs
The alpha counts
from 222Rn & its
progeny are
collected
Zn(Tl)S
Quartz Window
4
Alpha Scintillation Cell Counter
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Sample counted 4 hrs
after de-emanantion
Alpha particles
interact with Zn(Ag)S
coating & emit light
Light flashes are
counted on a scaler
Radon Cell Counters
Instrument Calibration
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Each instrument
system &
scintillation cell
needs to be
calibrated
Calibration
samples should be
prepared in the
same manner as
the samples.
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The entire deemanation system
effects the
calibration
measurement
Use NIST
traceable
standards
Perform yearly or
after repairs
5
Calculations
D=
G−B
λ ×t
1
1
×
×
×
2.22 × E × V × Y 1 − exp(−λ × t ) exp(−λ × t ) 1 − exp(−λ × t )
3
1
UNC =
2
3
1.96 × (G + B ) / t
1
1
λ ×t
×
×
×
2.22 × E × V × Y
1 − exp( −λ × t ) exp( −λ × t ) 1 − exp( −λ × t )
3
3
1
DL =
2
3
4.66 × B / t
1
1
λ ×t
×
×
×
2.22 × E × V × Y 1 − exp( −λ × t ) exp( −λ × t ) 1 − exp( −λ × t )
3
3
1
2
3
Calculations cont.
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Computer programs should be hand
verified
Decay constants and time intervals
must be in the same units of time
Minimum background count time
should be equal to the minimum
sample count time
Method Quality Control
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Per each batch of 20 samples,
analyze the following:
z
z
z
z
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Method blank
Laboratory control sample
Precision sample
Matrix spike sample
Established action limits for each
6
Method Quality Control, cont.
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Instrument operating procedure
should describe
z
z
z
Daily control charts and acceptance
limits
Required action
Preventative maintenance
Method SOP main sections
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SCOPE AND APPLICATION
SUMMARY OF METHOD
REGULATORY DEVIATIONS
METHOD PERFORMANCE
SAFETY
SAMPLE HANDLING &
PRESERVATION
INTERFERENCES
DEFINITIONS
EQUIPMENT
REAGENTS
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METHOD:
DETERMINATION OF 226RA
CALIBRATION OF
SCINTILLATION CELLS
CALCULATIONS
QUALITY CONTROL
WASTE DISPOSAL
POLLUTION PREVENTION
REFERENCES
FIGURES
Radium 228 (EPA 904.0)
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Prescribed Procedures for
Measurement of Radioactivity in
Drinking Water
EPA 600 4- 80-032
August 1980
7
Interferences
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The presence of 90Sr in the water
samples gives a positive bias to the
measured 228Ra activity.
Due to the short half-life of 228Ac, a β
emitter of similar energy is substituted
during instrument calibration. A high or
low bias may result depending on which
isotope is selected.
Natural Ba may result in falsely high
chemical yield.
232Th-
decay series
904.0 Method Summary
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228Ra in a drinking water sample is
co-precipitated with Ba & Pb as SO4
The (Ba-Ra)SO4 precipitate is
dissolved in basic EDTA. The
progeny, 228Ac, is chemically
separated from its parent by
repeatedly forming the (Ba-Ra)SO4
Allow at least 36 hrs for the
ingrowth of 228Ac & secular
equilibrium
8
904.0 Method Summary, cont.
{ 228Ac
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is then separated from 228Ra
by precipitation as a OH-. (Save
supernate)
This is the end of ingrowth & the
beginning of 228Ac decay
228Ac is co-precipitated with Y as
(Ac-Y2(C2O4)3)
904.0 Method Summary, cont.
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Transferred to a planchet & β
counted on a low-background α/β
proportional counter
The Ba carrier yield is found by
precipitating the Ba from the
supernatant as BaSO4
Instrumentation
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Low background gas flow
proportional counter
z
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P-10 counting gas (10% CH4 & 90%
Ar)
Due to short half-life of 228Ac, a
multi-detector system is desirable
z
z
6.13 hr
Processing time from start of decay to
count is about 250 m
9
Gas flow proportional counter
window assembly
Instrument Calibration
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{
Each instrument
system needs to
be calibrated
Calibration
samples should be
prepared in the
same manner as
the samples.
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Use isotope with
beta energy
approximately
equal to 0.404 keV
Use NIST
traceable
standards
Perform yearly or
after repairs
Calculations
D=
G−B
1
1
λ ×t
×
×
×
2.22 × E × V × R 1 − exp(−λ × t ) 1 − exp(−λ × t ) exp(−λ × t )
2
2
UNC =
3
1.96 × (G + B ) / t
λ ×t
1
1
×
×
×
2.22 × E × V × R 1 − exp(−λ × t ) 1 − exp(−λ × t ) exp(−λ × t )
2
2
2
DL =
1
3
1
4.66 × B / t
λ ×t
1
1
×
×
×
2.22 × E × V × R 1 − exp( −λ × t ) 1 − exp( −λ × t ) exp( −λ × t )
2
2
2
3
1
10
Method Quality Control
{
Per each batch of 20 samples,
analyze the following:
z
z
z
z
{
Method blank
Laboratory control sample
Precision sample
Matrix spike sample
Established action limits for each
Method Quality Control, cont.
{
Instrument operating procedure
should describe
z
z
z
Daily control charts and acceptance
limits
Required action
Preventative maintenance
Method SOP main sections
{
{
{
{
{
{
{
{
{
{
SCOPE AND APPLICATION
SUMMARY OF METHOD
REGULATORY DEVIATIONS
METHOD PERFORMANCE
SAFETY
SAMPLE HANDLING &
PRESERVATION
INTERFERENCES
DEFINITIONS
EQUIPMENT
REAGENTS
{
{
{
{
{
{
{
{
METHOD:
DETERMINATION OF 228RA
CALIBRATION OF
INSTRUMENT
CALCULATIONS
QUALITY CONTROL
WASTE DISPOSAL
POLLUTION PREVENTION
REFERENCES
FIGURES
11
Alpha-Emitting Radium Isotopes
(EPA 903.0)
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Prescribed Procedures for
Measurement of Radioactivity in
Drinking Water
EPA 600 4- 80-032
August 1980
Interferences (EPA 903.0)
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{
Natural Ba may result in falsely high
chemical yield
Ingrowth of progeny must be
corrected for
z
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Method only corrects for
226Ra
progeny
Does not accurately measure 226Ra
if other alpha emitting isotopes are
present
Calibration based only on 226Ra
Th-228
1.90 y
Th-232
1.4×1010 y
Atomic
number
(Z)
Ac-228
6.13 hours
Ra-228
5.75 y
Mass
number
(N)
Ra-224
3.64 days
alpha decay
Rn-220
54.5 s
beta decay
Po-216
158 ms
Po-212
300 ns
67%
Bi-212
60.6 m
Pb-212
10.6 hours
Th- decay series
232
Pb-208
33% stable
Tl-208
3.1 m
12
U-234
2.48×105 y
U-238
4.4×109 y
Pa-234
1.18 m
Atomic
number
(Z)
Th-230
8.0×104 y
Th-234
24.1 d
Mass
number
(N)
Ra-226
1622 y
alpha decay
238
U decay series
Rn-222
3.825 d
beta decay
Po-210
138.4 d
Po-214
1.6×10-4 s
Po-218
3.05 m
Bi-210
5.0 d
Bi-214
19.7 m
Pb-210
22 a
Pb-214
26.8 m
Pb-206
stable
U-235
7.3×108 y
Pa-231
3.48×104 y
Th-231
25.6 h
Atomic
number
(Z)
Th-227
18.17 d
Ac-227
22.0 y
Mass
number
(N)
Ra-223
11.7 d
235
U decay series
Fr-223
22 m
alpha decay
Rn-219
3.92 s
At-219
0.9 m
beta decay
At-215
10-4 s
Po-211
0.52 s
Po-215
1.83×10-3 s
Bi-215
8m
Bi-210
5.0 d
Bi-211
2.15 m
Pb-207
stable
Pb-211
36.1 m
Tl-207
4.79 m
903.0 Method Summary
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1 L acidified sample
Ra co-precipitated with stable Ba &
Pb as SO4
z 223Ra
z 224Ra
z 226Ra
{
Precipitate is separated from
sample matrix & supernate is
discarded
13
903.0 Method Summary, Cont.
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Progeny ingrowth starts with the
final (Ba-Ra)SO4 precipitation.
z
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Since a correction factor is applied to
correct for ingrowth, care needs to be
taken to avoid disturbing the radon
progeny ingrowth after this step
Transfer to tared planchet & dry
under infra-red heat lamp
Instrumentation (EPA 903.0)
{
Low background gas flow
proportional counter
z
{
P-10 counting gas (10% CH4 & 90%
Ar)
Alpha scintillation counter
Instrument Calibration (EPA 903.0)
{
{
Each instrument
system needs to
be calibrated
Calibration
samples should be
prepared using
226Ra
{
{
Use NIST
traceable
standards
Perform yearly or
after repairs
14
Calculations (EPA 903.0)
D=
G−B
2.22 × E × V × I × R
UNC =
DL =
1.96 × (G + B) / t
2.22 × E × V × I × R
4.66 × B / t
2.22 × E × V × I × R
Method Quality Control (EPA 903.0)
{
Per each batch of 20 samples,
analyze the following:
z
z
z
z
{
{
Method blank
Laboratory control sample
Precision sample
Matrix spike sample
Established action limits for each
Demonstration of capability
Method Quality Control, Cont. (903.0)
{
Instrument operating procedure
should describe
z
z
z
Daily control charts and acceptance
limits
Required action
Preventative maintenance
15
Method SOP main sections (903.0)
{
{
{
{
{
{
{
{
{
{
SCOPE AND APPLICATION
SUMMARY OF METHOD
REGULATORY DEVIATIONS
METHOD PERFORMANCE
SAFETY
SAMPLE HANDLING &
PRESERVATION
INTERFERENCES
DEFINITIONS
EQUIPMENT
REAGENTS
{
{
{
{
{
{
{
{
METHOD:
DETERMINATION OF 228RA
CALIBRATION OF
INSTRUMENT
CALCULATIONS
QUALITY CONTROL
WASTE DISPOSAL
POLLUTION PREVENTION
REFERENCES
FIGURES
16