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.