an improved method for determination of ra-228 *
DESCRIPTION
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. Problems in the Assay of Radium. - PowerPoint PPT PresentationTRANSCRIPT
An Improved Method for Determination of Ra-228*
Jamie Christoff & Bill Burnett
Department of Oceanography
Florida State University
“The Double-Pass Approach”
*Research funded by the PG Research Foundation
Naturally-Occurring Radium Isotopes
Isotope t1/2 DecayChain
DecayMode
EnergyMeV
Ra-223 11.4 d U-235 α 5.615.72
Ra-224 3.66 d Th-232 α 5.69
Ra-226 1600 y U-238 α 4.78
Ra-228 5.75 y Th-232 β- 0.046
Problems in the Assay of Radium
Approaches Problems
Rn emanation Ra-226 onlyno tracer
BaSO4, PbSO4coprecipitation
tediousreagent blanks
standardion-exchange
inadequateselectivity (Cainterference)
Flow Chart - Ln•Resin Method
1 2
Load sample in 0.09M HNO3
Rinse 0.09M HNO3
1
2
3 0.35M HNO3
3
3
Ln•
Resin
1
2
Burnett et al., 1993
0.5-2 L acidified sample
Ba-133
BaSO4 ppt
Conversion to BaCO3
{hold for ~30 hrs.}
0.09M HNO3
-ray measurement Ba-133
228Ra via 228Ac
{proportional or HPGe counter}
226Ra via Rn emanation
223,224Ra via α-spectrometry
Calculations
228Ra (pCi/ L) =
A
2.22 • E • Y • e-λt1 • V •
λt21 - e-λt2
A = net cpm;
E = detector efficiency;
Y = yield;
λ = 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 Sr-90 (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
1 2
Actinide•
Resin#2
1
2
226Ra via Rn emanation
223,224Ra via-spectrometry
1 2
Rinse 2M HCl1
2
Actinide
•
Resin#1
1
2
Load sample in 2M HCl
•Collect•Ba-133 yield•Hold >30 hrs.
Process Actinide Elements
First Pass:
Rinse 2M HCl
1
2
Load sample in 2M HCl
Extrude resin into plastic vial; add
cocktail, count via LSC
Second Pass:
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, 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
Seawater
MnO2
Suspension
MnO2 ppt(Pu, Am, Ra, Ba)
supernatantCs, Sr,...
centrifuge/filter
Hold-back of 90Sr
0
20
40
60
80
100
120
0 0 10 15 25 50 75 100
Sr (mg/L)
MnO2 ppt
Reference
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
Analysis of rate of decay indicated a half-life ~8%
too low
Ac-228 t1/2 = 6.13 hrs.
Decay Component Analysis
Ao = 135 cpm
Ao = 95.4 cpm
Ba Yields: MnO2 ppt
Sample spilled
Ra-228 Test Results
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.