Gamma-ray spectrometric data-processing techniques

gamma-ray spectrometric data-processing techniques

BMB PTOUCATK) COS5PACTUS

B.R.S. Minty

Bureau of Mineral Resources, Geology and Geophysics

DEPARTMENT OF PRIMARY INDUSTRIES & ENERGY BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS

A REVIEW OP AIRBORNE GAMMA-RAY SPECTROMETRIC DATA PROCESSING TECHNIQUES by B.R.S. Minty (Division of Geophysics)

AUSTRALIAN GOVERNMENT PUBLISHING SERVICE CANBERRA, 1988

DEPARTMENT OF PRIMARY INDUSTRIES & ENERGY Minister for Resources : Senator The Hon. Peter Cook Secretary : G.C.Evans

BUREAU OF MINERAL RESOURCES, GEOLOGY & GEOPHYSICS Director: R.W.R. Rutland AO

Published for the Bureau of Mineral Resources, Geology & Geophysics by the Australian Government Publishing Service

?Commonwealth of Australia 1988 This work is copyright. Apart from any fair dealing for the purpose of study, research, criticism, or review, as permitted under the Copyright Act, no part may be reproduced by any process without written permission. Copyright is the responsibility of the Director, Publishing & Marketing, AGPS. Inquiries should be directed to the Manager, AGPS Press, Australian Government Publishing Service, GPO Box 84, Canberra, ACT 2601.

ISSN 0084-7100

ISBN 0 6 4 4 0 8 1 4 9

Edited by A.G.L. Paine. Line drawings by N. Kozin. Word-processing by P. Burrell

P r i n t e d i n A u s t r a l i a by Graphic S e r v i c e s P t y . L t d .

CONTENTS

Page

ABSTRACT

V

INTRODUCTION

1

BASIC PRINCIPLES

Gamma spectrometry

2

Disequilibrium.

4

Background radiation.

.

5

Properties of airborne gamma-ray spectra...........

8

PROCESSING

Drift correction (or 'energy calibration1)

15

Dead-time correction

16

Background corrections

17

Terrain-clearance correction

19

Reduction to elemental countrates

2 0

Conversion of countrates to elemental abundances... 21

Filtering of statistical noise

22

DISCUSSION AND CONCLUSIONS

Automatic estimation of attenuating cover

2 3

Minimising errors: optimising the processing options 2 3

Background estimation

2 5

Coincident detection technique

28

RECOMMENDATIONS

2 9

ACKNOWLEDGMENTS

3 0

REFERENCES

3 0

APPENDICES

1. Background estimation using an upward-looking

detector

3 5

2. Temperature and pressure corrections

3 8

3. Reduction to elemental countrates

39

4. Inversion of airborne gamma-ray spectrometer data

42

iii

FIGURES

Page

1. Interaction of gamma rays with matter.

3

2. The radon daughter gamma energy spectrum.

6

3. The cosmic gamma energy spectrum.

7

4. The aircraft gamma energy spectrum.

8

5. The uranium line spectrum.

9

6. The thorium line spectrum.

9

7. The potassium gamma energy spectrum.

10

8. The uranium gamma energy spectrum.

11

9. The thorium gamma energy spectrum.

11

10. Pulse height vs energy response of an Nal detector. 13

11. Spectra from U ore covered by various depths of water. 14

12. Relative sensitivity of the 'up1 detectors to the

'down' detectors for terrestrial radiation.

2 6

13. Relative sensitivity of the 'up' detectors to the

'down' detectors for airborne radon.

27

14. Use of a coincident detection technique to allow

directional detection of radiation.

28

TABLES

2 38

1.

U decay series.

44

235

2.

U decay series.

45

232

3.

Th decay series.

46

4. Mass attenuation coefficients and half-thicknesses

for various gamma-ray energies in air, water, and

concrete.

47

5. Convential window settings for three channel

spectrometers.

4 8

iv

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