Transcript An Improved Method for Determination of Ra-228

An Improved Method for
Determination of Ra-228*
“The Double-Pass Approach”
Jamie Christoff & Bill Burnett
Department of Oceanography
Florida State University
*Research
funded by the PG Research Foundation
Naturally-Occurring Radium Isotopes
Isotope
Ra-223
11.4 d
Ra-224
3.66 d
Decay Decay Energy
Chain Mode MeV

U-235
5.61
5.72

Th-232
5.69
Ra-226
1600 y
U-238

4.78
Th-232

0.046
Ra-228
t1/2
5.75 y
-
Problems in the Assay of Radium
Approaches
Problems
Rn emanation
Ra-226 only
no tracer
tedious
reagent blanks
inadequate
selectivity (Ca
interference)
BaSO4, PbSO4
coprecipitation
standard
ion-exchange
Flow Chart - Ln•Resin Method
1
2
0.5-2 L
acidified
sample
Ba-133
3
Ln
BaSO4 ppt
•
2
Rinse 0.09M HNO3
3
0.35M HNO3
226Ra
3
1
via Rn emanation
2
223,224Ra
0.09M HNO3
g-ray measurement Ba-133
Load sample in 0.09M HNO3
Resin
Conversion to
BaCO3
{hold for ~30 hrs.}
1
228Ra
via -spectrometry
via 228Ac
{proportional or HPGe counter}
Burnett et al., 1993
Calculations
228
Ra (pCi / L) =
A
t 2

2.22  E  Y  e -t 1  V
1 - e -t 2
A = net cpm;
E = detector efficiency;
Y = yield;
 = decay constant of 228Ac;
V = volume;
t1 = time from separation until start of counting; and
t2 = counting time
Elution Curve — Ln•Resin
The Ac fraction is collected and a CeF3
precipitate prepared for low-level gas-flow
proportional counting.
Problems/Improvements
• BaSO4 conversion (metathesis) time-
•
•
consuming and not quantitative;
Samples heavily contaminated with Sr90 (Y-90) have produced false
positives;
Ln•Resin and TRU•Resin approach for
Ra-228 not suitable for soil samples —
additional clean-up required.
New “Combined” Approach
•
•
•
•
•
•
Water samples — ppt MnO2 —> scavenges
actinides, Ra, Ba (Sr stays in solution)
Soil samples: convert matrix to 2M HCl
Pass sample (2M HCl) over Actinide Resin to
remove actinides — collect load/rinse (Ra, Ba)
Process Actinide Resin fraction for Pu, Am, etc.
Store load/rinse ~30 hrs. for Ac-228 ingrowth; pass
over 2nd column for Ac separation
Several options for counting including direct
counting of Actinide Resin via LSC
Uptake of Ac
via
Actinide Resin
At 2-6M HCl, Ac and
Ra have k' values
differing by ~5 orders
of magnitude ensuring
complete separation
The “Double-Pass” Approach
First Pass:
Second Pass:
1
1
2
2
1
Actinide
2
•
Process
Actinide
Elements
Load sample in 2M HCl
Rinse 2M HCl
1
Actinide
2
•
Load sample in 2M HCl
Rinse 2M HCl
Resin
#2
Resin
#1
1
1
2
•Collect
•Ba-133 yield
•Hold >30 hrs.
Extrude resin into
plastic vial; add
cocktail, count via
LSC
2
226Ra
via Rn emanation
223,224Ra
via
-spectrometry
2nd column options: TRU.Resin — load 2.5M HNO3; elute Ac 1M HCl, ppt CeF3, count
Diphonix — load 2M HCl; elute Ac 0.5M HEDPA, evap., count
Water Samples: MnO2 ppt
Seawater
MnO2
Suspension
centrifuge/
filter
MnO2 ppt
(Pu, Am, Ra, Ba)
• Seawater, 100-400 liters
• Acidify to pH 2, add Pu/Am tracers, stir/hold
• For 100L sample, add 35 mL sat KMnO4 (~2.1g);
Pu-->Pu(VI), org oxid, purple color
• Adjust pH to 8-9 with NaOH
• Add 0.5M MnCl2 (2x vol of KMnO4); --> MnO2 ppt,
dark brown
2MnO4- + 3Mn2+ + 2H2O = 5MnO2 + 4H+
•Re-adjust pH to 8-9 as necessary
•Stir occasionally to keep MnO2 suspended for few hours
•Allow Mn02(Pu, Am) ppt to settle overnight
•Pump supernatant into clean tank for Cs, Sr processing
•Drain MnO2 slurry from bottom tap
supernatant
Cs, Sr,...
Hold-back of 90Sr
120
Reference
Sr-90 Solution %
100
80
60
MnO2 ppt
40
20
0
0
0
10
15
25
Sr (mg/L)
50
75
100
Large Volume Seawater Samples
Supernatant seawater transferred from one
plastic tank to another via pumping — this
will be used for 90Sr and 137Cs.
MnO2 suspension withdrawn from
bottom of conical-shaped plastic
tanks — processed for Am and Pu.
Smaller-Scale MnO2 ppt
MnO4 (purple) is reduced by added MnCl2 to
precipitate MnO2 (brown). MnO2 precipitate settles
relatively quickly.
Count Overnight
5.00
y = -0.002040x + 4.6462
r2 = 0.9696
n=72
half life = 5.66 hrs.
Ao = 104 cpm
Ln net cpm
4.50
4.00
Analysis of rate of decay
indicated a half-life ~8%
too low
3.50
Ac-228 t1/2 = 6.13 hrs.
3.00
2.50
0
200
400
Time (min)
600
800
Decay Component Analysis
150
Ao = 135 cpm
100
2 = 0.001878 min -1
t1/2 = 6.13 hrs
Net cpm
Ao = 95.4 cpm
50
1 = 0.00292min-1
t1/2 = 24 min
0
0
100
200
Time (min)
300
400
Ba Yields: MnO2 ppt
Ba-133 Yield (%)
Sample
0
A
B
C
D
E
F
G
H
I
J
K
I
25
50
75
100 125
Sample spilled
No column
After column
Ra-228 Test Results
Expected
Measured
Ra-228 (dpm/L)
400
300
200
100
0
N
O
P
Sample
Ra-1
Ra-2
Summary
• MnO2 ptt effectively scavenges Ra isotopes
•
•
•
as well as actinides;
Radium recoveries very high, dissolution of
MnO2 simple;
“Double Pass” approach provides way to
combine Ra-228 with actinide element
analysis; and
Direct counting of Actinide Resin via LSC
provides high efficiency & ease of analysis.