determination of the body burden with gamma emitters is one of the most important measurements in radiation protection for e.g. medical staff who work in domain of nuclear medicine , research staff or student who work with radioisotopes , etc . the whole body spectrometry is a well known technique that enables to get information about activity concentration and distribution of radionuclides in measurement subject . the most important aspect of activity determination when measuring human body is not to underestimate uncertainties connected with individual features ( e.g. size , shape , etc . ) and the position of each organ in human body [ 13 ] . a number of them , simulating different positions of the thyroid relative to the detector was done . it took place in the institute of nuclear physics polish academy of sciences ( ifj pan ) the thyroid is one of the larger and an odd endocrine glands , weighing 23  grams in neonates and 1860  grams in adults , and is increased in pregnancy . it is composed of two lobes connected by the isthmus of thyroid and , in some people , a small pyramidal lobe . thyroid volume measured by the ultrasound probe is : in women  to 18  cm , and in men  to 25  cm . the series of the measurements was performed at ifj pan using the whole body spectrometer ( wbs ) with 2 hpge detectors ( x , y 30  % relative efficiency each ) . one of them is pointing at patient s thyroid ( x ) and the other is located over abdomen ( y ) . the main part is shielding , made of 18 t of xix century steel , free from co-60 contamination . the outer dimensions of the chamber are 2.0  m    1.5  m    1.5  m with the width of the walls 15  cm . the patient is lying on a slightly curved bed so that the body is forming a light arc around detectors . a special phantom human thyroid and holder for it were designed . the anatomical shape of the phantom was made of an epoxy resin and consisted of two glands with small air cavities inside ( 16  cm ) . construction of the holder makes it possible to move the phantom in three directions ( x , y , z ) ( fig .  1 . ) . the ranges of shifts along x , y , z axis were respectively ( 5,5 ) , ( 2,2 ) , ( 2,2 ) ( in cm ) with a step of 1  cm . at the beginning of the measurement the point ( 0,0,0 ) was chosen . it was about 5  cm from x and 55  cm from y detector ( fig .  1 . ) . the mean time of each step was about 10  min with the dead time not higher than 0.2  % for both detectors.fig . 1a scheme of the whole body spectrometer ( wbs ) chamber with the ( 0,0,0 ) position of the thyroid phantom and distances between this point and detectors of the wbs a scheme of the whole body spectrometer ( wbs ) chamber with the ( 0,0,0 ) position of the thyroid phantom and distances between this point and detectors of the wbs the measurements were performed for thyroid phantom filled by standard solution of i with chemical composition 50  mg of ki in 1  dm of solution and activity amounted to 6  kbq on first day of the procedure . uncertainties which resulting from counting statistics for energy peak 364.5  kev were not higher than 2 and 10  % for x and y detector respectively . show the distribution of relative difference of efficiency ( eq.1 ) connected to shift of the thyroid phantom . results are presented as surfaces bordered by prescribed conditions for experiment1\documentclass[12pt]{minimal } 
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				 \begin{document}$$ \delta = \frac{{\varepsilon_{{\left ( { 0,0,0 } \right ) } } - \varepsilon_{{\left ( { x_{i } , y_{i } , z_{i } } \right ) } } } } { { \varepsilon_{{\left ( { 0,0,0 } \right ) } } } } \cdot 100\% $ $ \end{document}=0,0,0-xi , yi , zi0,0,0100%fig . 2the distribution of relative differences of efficiency versus thyroid phantom position shown in ( x , y ) plane for x detectorfig . 3the distribution of relative differences of efficiency versus thyroid phantom position shown in ( x , z ) plane for x detectorfig . 4the distribution of relative differences of efficiency versus thyroid phantom position shown in ( x , y ) plane for y detectorfig . 5the distribution of relative differences of efficiency versus thyroid phantom position shown in ( x , z ) plane for y detector the distribution of relative differences of efficiency versus thyroid phantom position shown in ( x , y ) plane for x detector the distribution of relative differences of efficiency versus thyroid phantom position shown in ( x , z ) plane for x detector the distribution of relative differences of efficiency versus thyroid phantom position shown in ( x , y ) plane for y detector the distribution of relative differences of efficiency versus thyroid phantom position shown in ( x , z ) plane for y detector \documentclass[12pt]{minimal } 
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				 \begin{document}$$ \delta $ $ \end{document}relative difference of efficiency , \documentclass[12pt]{minimal } 
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				 \begin{document}$$ \varepsilon_{{\left ( { 0,0,0 } \right ) } } $ $ \end{document}0,0,0efficiency of measurement for thyroid phantom geometry at ( 0,0,0 ) point , \documentclass[12pt]{minimal } 
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				 \begin{document}$$ \varepsilon_{{\left ( { x_{i } , y_{i } , z_{i } } \right ) } } $ $ \end{document}xi , yi , zi efficiency of measurement for thyroid phantom geometry at ( xi , yi , zi ) point , where : i  =  1 , 2 , 3  274 . the displacement of the phantom along x , y , z axis resulted in considerable changes of efficiency which reached up to the level of about 50  % for x and 30  % for y detector . the lower than anticipated value for y detector is due to the fact that the y detector is located further and it is less sensitive to the influences of the small displacement of the thyroid . the research allowed to determine the area in wbs at ifj pan which is predestined for thyroid measurements characterized by relatively small uncertainties dependent of distance changes . the two detectors system which was calibrated for thyroid gland geometry has the advantage of giving more information about distribution of i in human body . the higher results for y detector do not have any physical meanings besides the sole information , that i is observed somewhere else than in thyroid . the results of the presented studies confirmed that the uncertainties dependent of the position of the thyroid during the measurements are not negligible . the described series of the measurements allowed us to find the optimal area for the thyroid position . in this space small shifts of this organ result in the smallest observed changes of relative differences of efficiency .
the appropriate determination of the 131i which was absorbed into the human body , especially by thyroid , depends not only on individual features of each measurement subjects but also on reproducibility of their position or the thyroid s position in the human neck . 
 possible uncertainties caused by changes of the thyroid position relative to detector were studied in a series of measurements . 
 the research has shown that the dispersion of the results can reach up even to the level of 50  % .