On-line Mobile in situ Gamma Spectrometry
(2004)- Abstract
- Mobile in situ gamma spectrometry is an important instrument in the emergency preparedness for nuclear accidents. This work contributes through the development of strategies and analysis methods for two applications: an on-line search for gamma-emitting orphan sources and on-line estimation of the depth distribution of 137Cs. A strategy and analysis method dedicated to the detection of orphan sources in the environment has been developed. The work has mainly been performed with 137Cs sources, but can be applied to any gamma-emitting radionuclide. The strategy is based on measurements with a short sampling time(approx. 1–5 s) at a constant velocity (~40–50 km/h). The analysis method is based on the ratio between an observed quantity from... (More)
- Mobile in situ gamma spectrometry is an important instrument in the emergency preparedness for nuclear accidents. This work contributes through the development of strategies and analysis methods for two applications: an on-line search for gamma-emitting orphan sources and on-line estimation of the depth distribution of 137Cs. A strategy and analysis method dedicated to the detection of orphan sources in the environment has been developed. The work has mainly been performed with 137Cs sources, but can be applied to any gamma-emitting radionuclide. The strategy is based on measurements with a short sampling time(approx. 1–5 s) at a constant velocity (~40–50 km/h). The analysis method is based on the ratio between an observed quantity from the most recently measured 15 seconds and the preceding 60 seconds. Hypothesis testing is applied to decide if a high ratio is likely to be caused by an orphan source present in the environment or by natural variation. Since consecutive measurements are serially correlated, the underlying statistical distribution of the natural variation was estimated experimentally. Several quantities have been evaluated as input to the analysis procedure. For 137Cs, the quantity showing the highest detection distance was the number of counts in the 137Cs window, corrected for the 40K in the spectrum (potassium-stripped counts). A 2 GBq 137Cs source was detected at a distance of 180 m from the road, using a 4129 cm3 NaI(Tl) detector. The depth distribution of 137Cs significantly affects the photon fluence distribution above ground, thus it is vital to know the depth distribution when estimating the true activity per unit area or unit mass by means of in situ gamma spectrometry. In this work, the peak-to-valley method (based on the ratio between the number of counts from primary and forward-scattered photons) was evaluated for on-line estimation of the depth distribution directly from the measured spectra. The depth distribution was described by a truncated Lorentzian distribution. The objectives were to evaluate the applicability of the peak-to-valley method in mobile in situ gamma spectrometry, and to determine a minimal radioactivity inventory giving reasonable statistical un-certainty within a counting time of less than ten minutes, for a large HPGe detector system (100% relative efficiency). The results show that the method can be applied when the activity concentrations of 137Cs is above ~100 kBq/m2 , accepting an intro-duced uncertainty of about 50% in the estimated true activity. (Less)
- Abstract (Swedish)
- Popular Abstract in Swedish
I beredskapen för kärnolyckor är mobila in situ gammaspektrometriska mätningar ett viktigt redskap; vilket betyder att mätinstrument placeras i ett fordon (tex bil, flygplan eller helikopter) och gammastrålningen från omgivningen mäts under körning. I detta arbete har strategier och analysmetoder för två olika användningsområden utvecklats: 1) sökning av försvunna radioaktiva strålkällor, så kallade herrelösa källor (eng. “orphan sources”) och 2) direkt från fordonet uppskatta hur djupt radionukliden 137Cs är nedträngd i marken. En strategi för sökning av försvunna radoaktiva strålkällor och analysering av mätdata har utvecklats. Metoden har utarbetats med bilburna system, men kan också användas... (More) - Popular Abstract in Swedish
I beredskapen för kärnolyckor är mobila in situ gammaspektrometriska mätningar ett viktigt redskap; vilket betyder att mätinstrument placeras i ett fordon (tex bil, flygplan eller helikopter) och gammastrålningen från omgivningen mäts under körning. I detta arbete har strategier och analysmetoder för två olika användningsområden utvecklats: 1) sökning av försvunna radioaktiva strålkällor, så kallade herrelösa källor (eng. “orphan sources”) och 2) direkt från fordonet uppskatta hur djupt radionukliden 137Cs är nedträngd i marken. En strategi för sökning av försvunna radoaktiva strålkällor och analysering av mätdata har utvecklats. Metoden har utarbetats med bilburna system, men kan också användas för andra fordon. Strategin är att köra med en konstant hastighet (~ 40–50 km/h) och kontinuerligt mäta gammastrålningens nivå. Resultatet avläses ungefär var tredje till femte sekund och analyseras on-line. Statistiska metoder används för att avgöra om en ökning av strålningsnivån i de senaste 15–20 sekunderna är orsakad av naturliga variationer, eller av att en radioaktiv strålkälla har blivit förbipasserad. Stora delar av arbetet har varit att optimera statistiska metoder och att undersöka hur mycket strålningsnivån normalt varierar. Den största delen av resultaten bygger på mätningar gjorda på ett stort öppet fält, genomkorsat av en rak grusväg, i Revingehed. En 137Cs strålkälla placerades med varierande avstånd från vägen, och ett flertal förbikörningar gjordes för varje avstånd. Det bästa resultatet (137Cs källan kan detekteras på längst avstånd från vägen) fås genom att först subtrahera signalen från det i marken naturligt förekommande 40K. En oskärmad 137Cs strålkälla med en aktivitet på 2 GBq kunde då med ett 4-liters NaI system detekteras på ett avstånd av 180 m från vägen. Den uppmätta signalen i mobila mätningar från 137Cs i marken påverkas mycket av hur radionukliden är nedträngd i marken (djupfördelad). För att kunna göra en bra uppskattning av den totala aktivitetskoncentrationen av 137Cs i marken måste djupfördelningen uppskattas. I detta arbete har en metod (“peak-tovalley”-metoden) som hitintills använts för stillastående mätningar utvärderats för användning i mobila mätningar, för mättider på 5–10 minuter. Metoden baseras på att samtidigt bestämma mängden av fotoner från 137Cs som inte har växelverkat på vägen till detektorn och mängden av fotoner som har växelverkat i marken. Andelen fotoner som har växelverkat i marken ökar när radionukliden tränger djupare ned. Resultaten visar att metoden är användbar för mobila mätningar med en 100% HPGe detektor vid aktivitetskoncentrationer överstigande ca 100 kBq/m2. (Less)
Please use this url to cite or link to this publication:
https://lup.lub.lu.se/record/466943
- author
- Hjerpe, Thomas LU
- supervisor
- opponent
-
- Ph. D Hubbard, Lynn, Statens strålskyddsinstitut, Stockholm
- organization
- publishing date
- 2004
- type
- Thesis
- publication status
- published
- subject
- keywords
- radiobiologi, Kärnfysik, Nuclear physics, radiation protection., 137Cs, peak-to-valley method, depth distribution, orphan source search, orphan sources, emergency preparedness, mobile gamma spectrometry, nuclear accidents, Nuclear medicine, radiobiology, Nukleärmedicin
- pages
- 116 pages
- publisher
- Radiation Physics, Lund
- defense location
- Lunds universitetssjukhus, centralblocket förelsäsningssal 4
- defense date
- 2004-05-14 10:15:00
- ISBN
- 91-85313-00-9
- language
- English
- LU publication?
- yes
- additional info
- Article: Statistical Data Evaluation in Mobile Gamma Spectrometry: An Optimization of On-line Search Strategies in the Scenario of Lost Point Sources. Thomas Hjerpe, Robert R Finck and Christer Samuelsson. Health Phys 80(6):563-570; 2001. Article: A Comparision between Gross and Net Count Methods when Searching for Orphan Radioactive Sources. Thomas Hjerpe and Christer Samuelsson. Health Phys 84(2):203-211; 2003. Article: Accounting for the Depth Distribution of 137Cs in On-line Mobile Gamma Spectrometry through Primary and Forward-scattered Photons. Thomas Hjerpe and Christer Samuelsson. Radiat Environ Biophys 41:225-230; 2002. Article: Shielded and Unshielded Geometries in the Search for Orphan Sources. Thomas Hjerpe and Christer Samuelsson. Submitted for publication in Appl Radiat Isot 2004.
- id
- fc62d832-9547-4891-9c5d-37e3f7a6963e (old id 466943)
- date added to LUP
- 2016-04-04 11:14:20
- date last changed
- 2018-11-21 21:03:33
@phdthesis{fc62d832-9547-4891-9c5d-37e3f7a6963e,
abstract = {{Mobile in situ gamma spectrometry is an important instrument in the emergency preparedness for nuclear accidents. This work contributes through the development of strategies and analysis methods for two applications: an on-line search for gamma-emitting orphan sources and on-line estimation of the depth distribution of 137Cs. A strategy and analysis method dedicated to the detection of orphan sources in the environment has been developed. The work has mainly been performed with 137Cs sources, but can be applied to any gamma-emitting radionuclide. The strategy is based on measurements with a short sampling time(approx. 1–5 s) at a constant velocity (~40–50 km/h). The analysis method is based on the ratio between an observed quantity from the most recently measured 15 seconds and the preceding 60 seconds. Hypothesis testing is applied to decide if a high ratio is likely to be caused by an orphan source present in the environment or by natural variation. Since consecutive measurements are serially correlated, the underlying statistical distribution of the natural variation was estimated experimentally. Several quantities have been evaluated as input to the analysis procedure. For 137Cs, the quantity showing the highest detection distance was the number of counts in the 137Cs window, corrected for the 40K in the spectrum (potassium-stripped counts). A 2 GBq 137Cs source was detected at a distance of 180 m from the road, using a 4129 cm3 NaI(Tl) detector. The depth distribution of 137Cs significantly affects the photon fluence distribution above ground, thus it is vital to know the depth distribution when estimating the true activity per unit area or unit mass by means of in situ gamma spectrometry. In this work, the peak-to-valley method (based on the ratio between the number of counts from primary and forward-scattered photons) was evaluated for on-line estimation of the depth distribution directly from the measured spectra. The depth distribution was described by a truncated Lorentzian distribution. The objectives were to evaluate the applicability of the peak-to-valley method in mobile in situ gamma spectrometry, and to determine a minimal radioactivity inventory giving reasonable statistical un-certainty within a counting time of less than ten minutes, for a large HPGe detector system (100% relative efficiency). The results show that the method can be applied when the activity concentrations of 137Cs is above ~100 kBq/m2 , accepting an intro-duced uncertainty of about 50% in the estimated true activity.}},
author = {{Hjerpe, Thomas}},
isbn = {{91-85313-00-9}},
keywords = {{radiobiologi; Kärnfysik; Nuclear physics; radiation protection.; 137Cs; peak-to-valley method; depth distribution; orphan source search; orphan sources; emergency preparedness; mobile gamma spectrometry; nuclear accidents; Nuclear medicine; radiobiology; Nukleärmedicin}},
language = {{eng}},
publisher = {{Radiation Physics, Lund}},
school = {{Lund University}},
title = {{On-line Mobile in situ Gamma Spectrometry}},
year = {{2004}},
}