ارزيابي عدم قطعيت در اندازه گيري سزيم-137 موجود در آب با طيف سنجي گاما

Uncertainty assessment of measurement of Cs-137 in water using gamma spectrometry method

ﯿﺪه: ﯾﮏ ﻣﻌﯿﺎر ﻣﻔﯿﺪ در زﻣﯿﻨﻪي ﻣﻘﺪار اﻃﻤﯿﻨﺎنﭘﺬﯾﺮي ﻧﺘﯿﺠﻪﻫﺎي ﻃﯿﻒﺳﻨﺠﯽ ﮔﺎﻣﺎ، اراﯾﻪي ﻋﺪم ﻗﻄﻌﯿﺖ اﻧﺪازهﮔﯿﺮي اﺳﺖ. ﺑﻪ دﻟﯿﻞ اﻫﻤﯿﺖ و ﻟﺰوم ﭘﺎﯾﺶ ﭘﺮﺗﻮزاﯾﯽ در ﻣﻨﺒﻊﻫﺎي آب، ﻫﺪف از اﯾﻦ ﻣﻄﺎﻟﻌﻪ اﻧﺪازهﮔﯿﺮي رادﯾﻮﻧﻮﮐﻠﯿﺪﻫﺎي ﮔﺎﻣﺎﮔﺴﯿﻞ ﺑﻪ ﻫﻤﺮاه ﺑﯿﺎن ﻣﻘﺪار ﻋﺪم ﻗﻄﻌﯿﺖ اﻧﺪازهﮔﯿﺮي اﺳﺖ. ﺑﺮاي ﻣﺤﺎﺳﺒﻪي ﻋﺪم ﻗﻄﻌﯿﺖ اﻧﺪازهﮔﯿﺮي اﮐﺘﯿﻮﯾﺘﻪ در آب از ﻧﻤﻮﻧﻪاي ﮐﻪ ﻋﻨﺼﺮﻫﺎي ﭘﺮﺗﻮزاي آن از ﺷﺮﮐﺖ ﭘﻞ- اﺗﻢ ﺗﻬﯿﻪﺷﺪه ﺑﻮد، اﺳﺘﻔﺎده ﺷﺪ. ﺑﺮاي اﻧﺠﺎم ﻣﺤﺎﺳﺒﻪﻫﺎ از ﻗﻠﻪي واﻗﻊ در اﻧﺮژي keV 661 در ﻃﯿﻒ اﻧﺮژي رادﯾﻮﻧﻮﮐﻠﯿﺪ ﺳﺰﯾﻢ-137 در ﯾﮏ آﺷﮑﺎرﺳﺎز ژرﻣﺎﻧﯿﻢ ﺑﺎ ﺧﻠﻮص ﺑﺎﻻ )HPGe( اﺳﺘﻔﺎده ﺷﺪ. ﺑﺮرﺳﯽﻫﺎ ﻧﺸﺎن داد ﻋﺎﻣﻞﻫﺎي ﺗﺄﺛﯿﺮﮔﺬار ﺑﺮ ﻋﺪم ﻗﻄﻌﯿﺖ اﻧﺪازهﮔﯿﺮي ﺑﻪ دو ﺑﺨﺶ ﺷﻤﺎرش ﺑﺎ دﺳﺘﮕﺎه ﻃﯿﻒﺳﻨﺞ ﮔﺎﻣﺎ و آﻣﺎدهﺳﺎزي ﻧﻤﻮﻧﻪ ﺗﻘﺴﯿﻢ ﻣﯽﺷﻮد ﮐﻪ ﻫﺮ ﯾﮏ از اﯾﻦ دو ﺑﺨﺶ از ﭼﻨﺪﯾﻦ ﻣﻨﺒﻊ ﻋﺪم ﻗﻄﻌﯿﺖ ﻣﺎﻧﻨﺪ ﻋﺪم ﻗﻄﻌﯿﺖ ﻣﺮﺑﻮط ﺑﻪ ﺷﻤﺎرش، ﮐﺎﻟﯿﺒﺮاﺳﯿﻮن ﺑﺎزده، اﺣﺘﻤﺎل ﮔﺴﯿﻞ، ﺿﺮﯾﺐﻫﺎي ﺗﺼﺤﯿﺢ ﺷﻤﺎرش و اﻧﺪازهﮔﯿﺮي ﺟﺮم ﻧﺎﺷﯽ ﻣﯽﺷﻮد. ﻧﺘﯿﺠﻪﻫﺎي اﻧﺪازهﮔﯿﺮي ﻧﺸﺎن داد ﻋﺪم ﻗﻄﻌﯿﺖ ﻧﺴﺒﯽ ﺑﺮاي ﻧﻤﻮﻧﻪي آب ﺑﺎ اﮐﺘﯿﻮﯾﺘﻪ Bq 425 از ﺳﺰﯾﻢ-137 ﺣﺪود %3/7 )ﺿﺮﯾﺐ اﻃﻤﯿﻨﺎن = 1( اﺳﺖ.

A useful benchmark for the reliability of gamma spectrometry results is to provide measurement uncertainty. Due to the importance and necessity of radiation monitoring in water resources, this research has been oriented toward providing measurements of the gamma- emitting radionuclides along with introducing the expression for the uncertainty that appeared in these measurements. To calculate the uncertainty of the activity measurements in water, a sample, where its radioactive elements were obtained from the PolAtom Company, was used. The calculations were performed based on the 661 keV peak of 137Cs radionuclide and a high-purity germanium detector (HPGe). The investigations show that the factors affecting the measurement uncertainty are composed of two parts, including counting with the gamma spectroscopy system and sample preparation, where each of these parts comes from several sources of uncertainty such as uncertainties associated with the counting process, efficiency calibration, emission probability, the counting correction factors, and the mass measurement. The results of analyses show that the relative uncertainty is about 3.7% (Coverage factor = 1) for a water sample with an activity of 425 Becquerel from 137Cs.