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.
Original language | English |
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Qualification | Doctor |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 2017 Jun 1 |
Place of Publication | Lund |
Publisher | |
ISBN (Print) | 978-91-7753-240-8 |
Publication status | Published - 2017 |
Bibliographical note
Defence detailsDate: 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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Subject classification (UKÄ)
- Natural Sciences