Methods for localizing and quantifying radionuclide sources and deposition using in situ gamma spectrometry: Critical review of the peak-to-valley method based on experimental studies and applications in Georgia and Japan

Research output: ThesisDoctoral Thesis (compilation)

Abstract

This thesis describes investigations made on mobile and stationary gamma

spectrometry made both under laboratory conditions and in situ. The objective has

been to identify and quantify radionuclides in the form of point sources,

contamination and widespread deposition in situations where the measurement

geometry is not known beforehand. Both scintillators and semiconductor detectors of

similar size were tested in a backpack configuration in the vicinity of an unmapped

underground waste storage in the Republic of Georgia. The results showed that a high

efficiency HPGe detector (>100% relative efficiency) was the best choice with respect

to sensitivity compared to LaBr3(Ce) and NaI(Tl) detectors. It was also the most

cumbersome system of them all in terms of field operability due to the liquid

nitrogen. The evaluation method of plotting the full energy peak count rate on maps

worked well, especially when assessment of the maps was made offline by an external

base support, which speeded up the field work significantly. A large part of the thesis

has been focused on evaluating and using the so-called peak-to-valley method (PTV

method). Measurements and simulations to investigate components in the pulse

height distribution contributing to the PTV ratio have been done both in a laboratory

and outdoors in a controlled radiation environment as well as in situ. The PTV

studies have been focused on investigating how well the method works for

quantification of 137Cs, with the aim of either finding a reliable point source depth, or

a factor to correct the estimated surface equivalent mean deposition in situ,

compensating for the shielding effects brought on by the ground penetration of 137Cs.

In order to better understand the scatter processes of 137Cs photons in the air, ground

and the surrounding material of the detector, simulations in MCNP5 and

measurements have been performed in various configurations.

The simulations and measurements performed with a well-characterized HPGe

detector in a low gamma background room, revealed a 25% difference in full energy

peak efficiency between simulation and measurement for 662 keV photons from

137Cs. The inner components of a detector appeared to have significant impact on

agreement between simulation and measurement and components contributing to

this impact were identified.

Regarding the PTV ratio three HPGe detectors were compared with respect to their

angular PTV ratio response, to prepare for a sensitive approach on estimating

deposition penetration depth of 137Cs in situ. Detectors of sizes ranging from 18% to

123% in relative efficiency showed similar PTV ratios for incident angles between 0°

and 90° when using a 30 keV valley interval. The point source measurements showed

that a field of view of about 3 m in radius was a good choice presenting the possibility

to resolve whether the deposition is on the ground surface or has penetrated beneath

the surface.

When the detector systems were brought to the fallout areas outside Fukushima

Daiichi in Japan the evaluation of the 137Cs PTV ratios showed perturbation, which

was ascribed the presence of 134Cs. The laboratory investigations of this perturbation

showed a significant disturbance down to a 134Cs:137Cs ratio in the deposition of

1:100. To follow up on earlier results indicating an improvement in the reliability of

the PTV ratio for both 137Cs and 134Cs, a collimator was applied at the in situ

measurement locations in the Fukushima Daiichi region. The collimation increased

the PTV ratio significantly for 134Cs but not so for 137Cs when both radionuclides

were present. This indicated that the 134Cs PTV ratio should be used instead of that

for 137Cs the first decade after an accident where both cesium radionuclides are

released. If no 134Cs is present the collimator will successfully improve the 137Cs PTV

ratio due to the advantages of limiting incident angles to those close to the detector.

Details

Authors
  • Karl Östlund
Organisations
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Natural Sciences

Keywords

Original languageEnglish
QualificationDoctor
Awarding Institution
Supervisors/Assistant supervisor
Award date2017 Jun 1
Place of PublicationLund
Publisher
  • Lund University, Faculty of Science
Print ISBNs978-91-7753-240-8
Publication statusPublished - 2017
Publication categoryResearch

Bibliographic note

Defence details Date: 2017-06-01 Time: 09:00 Place: Room 2005-7, Medical Radiation Physics, Inga-Marie Nilssons gata 49, Malmö External reviewer Name: Sanderson, David C.W. Title: Professor Affiliation: Scottish Universities Environmental Research Centre, University of Glasgow, Scotland ---

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