Ir544 Barium sulphate method for consecutive determination of radium-226 and radium-228 on the same source

Barium sulphate method for consecutive determination of radium-226 and radium-228 on the same source

P Medley

Supervising Scientist Division

GPO Box 461, Darwin NT 0801

April 2010

(Release status – unrestricted)

How to cite this report:

Medley P 2010. Barium sulphate method for consecutive determination of radium-226 and radium-228 on the same source. Internal Report 544, April, Supervising Scientist, Darwin.

Location of final PDF file in SSDX Sharepoint

Supervising Scientist Division > PublicationWork > Publications and Productions > Internal Reports (IRs) > Nos 500 to 599 > IR544_Ra228 (P Medley) > ir544 Appended method Ra-226 and Ra-228 (Medley)

Location of all key data files for this report in SSD Explorer:

Nil data files

Authors of this report:

Peter Medley – Environmental Research Institute of the Supervising Scientist, GPO Box 461, Darwin NT 0801, Australia

The Supervising Scientist is part of the Australian Government Department of the Environment, Water, Heritage and the Arts.

© Commonwealth of Australia 2010

Supervising Scientist

Department of the Environment, Water, Heritage and the Arts

GPO Box 461, Darwin NT 0801 Australia

This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without prior written permission from the Supervising Scientist. Requests and inquiries concerning reproduction and rights should be addressed to Publications Inquiries, Supervising Scientist, GPO Box 461, Darwin NT 0801.

e-mail: [e-mail]

Internet: www.environment.gov.au/ssd (www.environment.gov.au/ssd/publications)

The views and opinions expressed in this report do not necessarily reflect those of the Commonwealth of Australia. While reasonable efforts have been made to ensure that the contents of this report are factually correct, some essential data rely on the references cited and the Supervising Scientist and the Commonwealth of Australia do not accept responsibility for the accuracy, currency or completeness of the contents of this report, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the report. Readers should exercise their own skill and judgment with respect to their use of the material contained in this report.

Printed and bound in Darwin NT by Supervising Scientist Division

Contents

Figures v

Equations v

Executive summary vii

1 Introduction 1

1.1 Radium in the environment 2

1.1.1 Radium in drinking water 3

1.2 Analysis of radium 3

2 Methodology 4

2.1 Gamma spectrometry method development 4

2.1.1 Source preparation 4

2.1.2 Gamma counting 6

2.1.3 Detection system calibration 7

2.1.4 Chemical recovery determination 10

2.1.5 228Ra activity determination 12

2.1.6 Quality control and quality assurance 13

2.2 Alpha spectrometry method development 13

2.2.1 Methodology 13

2.2.2 Source preparation for 228Th measurement 14

2.2.3 Alpha counting 16

2.2.4 228Ra activity determination 16

3 Method calibration and results 17

3.1 Gamma spectrometry method calibration 17

3.1.1 Efficiency determination 17

3.1.2 Chemical recovery comparison HPGe vs NaI 20

3.1.3 Detection limits 20

3.2 Alpha spectrometry method calibration 22

3.2.1 228Ra determination with alpha spectrometry 22

3.2.2 Detection limits 23

4 Conclusions 24

References 25

Appendix 1 Apparatus & method description for 228Ra determination 28

Appendix 2 Calibration of gamma and alpha spectrometers 29

A2.1 Preparation of sealed standards 29

A2.2 Preparation of unsealed standards 29

Appendix 3 Methods 30

A3.1 Preparation of sealed standards 30

A3.2 Preparation of unsealed standards 30

A3.3 Digestion of polypropylene filter, thorium extraction and deposition 30

Tables

Table 1 Details and dose conversion factors [Sv/Bq] of the four naturally occurring isotopes of radium 1

Table 2 A range of reported concentrations of 226Ra and 228Ra in different environmental sample types, highlighting the considerable range of activities found in the natural environment. Data are summarised from Iyengar (1990). 2

Table 3 Gamma decay energies and emission probabilities for relevant nuclides and decay lines in samples and standards prepared for 226Ra and 228Ra analysis via the BaSO4 method. All gamma energies are taken from Canet & Jacquemin (1990), except for 228Ac which are taken from Marten (1992). 7

Table 4 Efficiency calibration data 17

Table 5 Blank count rates after Compton and natural background subtraction of various blanks 19

Table 6 Chemical recovery data for a set of freshwater mussels using 133Ba as a tracer and counted on HPGe and NaI gamma spectrometers. 20

Table 7 Detection limit determination data 20

Table 8 General formulae for calculating various detection limits, taken from Currie (1968). бB is the standard deviation of the net count rate of the blank. 22

Table 9 133Ba chemical recovery for 3 standards after charring with concentrated sulphuric acid, digestion with concentrated hot nitric acid and hydrogen peroxide, then filtered through 0.45 µm. Methods used for gamma spectrometric (NaI) analysis are described in Medley et al (2005). 23

Figures

Figure 1 Illustration of mounting radium source discs in PVDF holders for gamma spectrometry. Polypropylene filters have a 17.5 mm active diameter, and a total diameter of 22 mm. 5

Figure 2 Time taken for 228Ac to reach equilibrium with 226Ra parent after radium separation.

Calculated using Equation 2 6

Figure 3 Spectrum of sample with 133Ba and 226Ra, counted on an HPGe spectrometer. Major peaks of these 2 isotopes can be seen to be distinctly separate from 228Ac decay lines at 911 and 969 keV. Spectrum produced at eriss. 7

Figure 4 Typical efficiency curves for a series (A, D, F, G & K) of gamma spectrometer configurations. Reproduced from Marten (1992). 8

Figure 5 Ingrowth of 228Th from the parent isotope 228Ra, based on Equation 1. Note that this ingrowth curve is valid regardless of the initial activity of 228Ra. 14

Figure 6 Flow diagram of standard method for thorium isotope analysis (Martin & Hancock 2004b). Steps highlighted have been omitted for this project. 15

Figure 7 Flow diagram of digestion procedure for 228Th ingrowth method 15

Figure 8 Combined net count rates of 228Ac decay lines at 911 and 969 keV of sealed standards vs. activity of sealed standards 18

Figure 9 HPGe gamma spectrum of the most active of the sealed standards. No additional lines close to the 911 and 969 keV lines of 228Ac can be seen, even in the logarithmic scale. 18

Figure 10 Recovery corrected combined net count rates of 228Ac decay lines at 911 and 969 keV of unsealed standards vs. activity of unsealed standards 19

Figure 11 Combined normalised net counts of 228Ac 911 and 969 keV peaks vs. %RSD for 6 unsealed standards 21

Figure 12 Measured vs actual 228Ra activity in unsealed standards as determined by ingrowth of 228Ra and measurement via alpha spectrometry 23