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

Abstract: This thesis describes investigations made on mobile and stationary gammaspectrometry made both under laboratory conditions and in situ. The objective hasbeen to identify and quantify radionuclides in the form of point sources,contamination and widespread deposition in situations where the measurementgeometry is not known beforehand. Both scintillators and semiconductor detectors ofsimilar size were tested in a backpack configuration in the vicinity of an unmappedunderground waste storage in the Republic of Georgia. The results showed that a highefficiency HPGe detector (>100% relative efficiency) was the best choice with respectto sensitivity compared to LaBr3(Ce) and NaI(Tl) detectors. It was also the mostcumbersome system of them all in terms of field operability due to the liquidnitrogen. The evaluation method of plotting the full energy peak count rate on mapsworked well, especially when assessment of the maps was made offline by an externalbase support, which speeded up the field work significantly. A large part of the thesishas been focused on evaluating and using the so-called peak-to-valley method (PTVmethod). Measurements and simulations to investigate components in the pulseheight distribution contributing to the PTV ratio have been done both in a laboratoryand outdoors in a controlled radiation environment as well as in situ. The PTVstudies have been focused on investigating how well the method works forquantification of 137Cs, with the aim of either finding a reliable point source depth, ora 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, groundand the surrounding material of the detector, simulations in MCNP5 andmeasurements have been performed in various configurations.The simulations and measurements performed with a well-characterized HPGedetector in a low gamma background room, revealed a 25% difference in full energypeak efficiency between simulation and measurement for 662 keV photons from137Cs. The inner components of a detector appeared to have significant impact onagreement between simulation and measurement and components contributing tothis impact were identified.Regarding the PTV ratio three HPGe detectors were compared with respect to theirangular PTV ratio response, to prepare for a sensitive approach on estimatingdeposition penetration depth of 137Cs in situ. Detectors of sizes ranging from 18% to123% 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 showedthat a field of view of about 3 m in radius was a good choice presenting the possibilityto resolve whether the deposition is on the ground surface or has penetrated beneaththe surface.When the detector systems were brought to the fallout areas outside FukushimaDaiichi in Japan the evaluation of the 137Cs PTV ratios showed perturbation, whichwas ascribed the presence of 134Cs. The laboratory investigations of this perturbationshowed a significant disturbance down to a 134Cs:137Cs ratio in the deposition of1:100. To follow up on earlier results indicating an improvement in the reliability ofthe PTV ratio for both 137Cs and 134Cs, a collimator was applied at the in situmeasurement locations in the Fukushima Daiichi region. The collimation increasedthe PTV ratio significantly for 134Cs but not so for 137Cs when both radionuclideswere present. This indicated that the 134Cs PTV ratio should be used instead of thatfor 137Cs the first decade after an accident where both cesium radionuclides arereleased. If no 134Cs is present the collimator will successfully improve the 137Cs PTVratio due to the advantages of limiting incident angles to those close to the detector.

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