Accurate gamma-ray spectrometry of environmental

samples: a challenge

O. Sima - Bucharest University

D. Arnold - PTB Braunschweig

C. Dovlete - ERL Bucharest

Accurate gamma-ray spectrometry of environmental samples: a challenge

• Introduction

• Problems in efficiency calibration of the spectrometer

• Coincidence summing effects

• Matrix effects

• Geometry effects

• GESPECOR

• Summary and conclusions

Introduction

• Assessment of radioactivity of environmental samples:– carefully designed sampling procedures

– appropriate sample preparation

– accurate sample measurement

– rigorous analysis of the results

• Modern requirements and conditions:– low detection limits– accurate evaluation of uncertainty– high number of samples, various types, matrices, available quantities– high efficiency detectors available

Problems in efficiency calibration• Low level activity + low detection limit:

– high efficiency measurement conditions

– volume sources

• Detection efficiency for volumic samples:– dependent upon sample matrix and density

=> Direct experimental calibration:

- limited number of matrices

- specific nuclides

- expensive, problems with the management of radioactive material

=> Additional procedures for a complete calibration required

• Detection efficiency for high efficiency measurements:– nuclide specific coincidence summing effects

Coincidence summing effects

• Are encountered in the case of measurement of nuclides which decay through the emission of coincident radiation (cascading photons, X-rays, annihilation photons etc)

• Depend on the details of the decay scheme:– Nuclide and peak specific effects

• Are much enhanced in high efficiency measurement conditions

• Effects: – summing out (coincidence losses from the peak) => decrease of

the apparent efficiency

– summing in (additional counts in the sum peak) => increase of the apparent efficiency

 SPECTRUM OF 22Na (WELL-TYPE DETECTOR)

           

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

0 500 1000 1500 2000 2500 3000

Energy in keV

Co

un

ts p

er c

han

nel

511 keV

1275 keV

1022 keV1786 keV

2297 keV

 

Coincidence summing:

- sample analysis

- efficiency calibration:

ex: 1 l Marinelli beaker

Co-60 => 1173 keV 0.926, 1332 keV 0.924

Y-88 => 898 keV 0.932, 1836 keV 0.920

ex: Well-type detector:

Co-60 => 1173 keV 0.445, 1332 keV 0.424

Y-88 => 898 keV 0.472, 1836 keV 0.390

=> Accurate procedures for the evaluation of the effects required

Matrix effects

• Matrix effects are encountered when the calibration source has a different composition and density than the sample of interest

• Depend on: – sample geometry– linear attenuation coefficient– photon energy– detector parameters

• Linear attenuation coefficient obtained from: – sample composition and density;– transmission experiments

d Sample

Detector

dEmptycontainer

Detector

- Transmission factor approximated by exp(-d) ?

Transmission factors (log scale). Sample: R=3.5, H=2 cm

Linear attenuation coefficient (1/cm)

1

0.1

0.01 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

exp(-d)

Relative error (%). Soil sample, R=3.5 cm, H=2 cm.

Energy (keV)

10 100 1000

30

20

10

0

- 10

- 20

Geometry effects

• For samples measured close to the end cap of a closed end coaxial detector efficiency very sensitive to geometry details

• For some samples (e.g. powder) it is difficult to assure exactly the standard geometry

• Detectors parameters may vary in time (e.g. the entrance window of the end cap)

Relative error of activity (A) (%). Soil sample, R=3.5 cmRelative error of activity (A) (%). Soil sample, R=3.5 cm

Energy (keV) 10 100 1000

0

- 2

- 4

- 6

- 8

- 10

GESPECOR• Monte Carlo based software dedicated to solve problems

in gamma spectrometry:

- computation of coincidence summing corrections

- computation of self-attenuation effects (matrix effects)

- computation of the efficiency

• Typical applications: environmental spectrometry

- detectors: HPGe (closed end or well-type), Ge(Li)

- sources: cylinder, Marinelli, point, parallelepiped, ring

- matrix: any (known composition) or known - nuclides: ~ 100 (for coincidence summing effects)

• Extension for very large samples: variance reduction techniques (focused photon emission, weighted emission point)

Direct efficiency calibration:- typical sources (cylinder, Marinelli, point sources)- special geometries:

Parallelepiped (Al-26 in meteorite samples)Spherical source (Rn-222 sources)In situ measurementsDrum waste containers

Efficiency transfer – less sensitive to detector details

Summary and conclusions

• Accurate assessment of the radioactivity of environmental samples – a challenge– Coincidence summing effects

– Matrix effects

– Geometry effects

• The GESPECOR software can solve typical problems required by an accurate assessment of the radioactivity of environmental samples