Radioprotection No 59-2 | Page 25

84 J . A . Corbacho et al .: Radioprotection 2024 , 59 ( 2 ), 80 – 87

Table 2 . Activity levels of 40 K , 214 Pb , 214 Bi and 228 Ac measured by the drone . Ambient equivalent dose rate measured : ( 1 ) from the 226 Ra activity levels . ( 2 ) from gross gamma counts ( cps ) conversion factor . H *( 10 ) values recorded by RS200 ionisation chamber .

Soil activity levels Ambient equivalent dose rate H *( 10 )

Fig 2 . Correlation between H *( 10 ) values measured with the RS200 ionisation chamber and H *( 10 ) values measured with a LaBr 3 ( Ce ) scintillation detector mounted on a drone . Blue dots : H *( 10 ) values obtained from gross gamma counts ( cps ) to H *( 10 ) conversion factor . Red dots : H *( 10 ) values obtained from 226 Ra concentration activity measured in situ .

H ð10ÞðhSv = hÞ ¼ð0:0512 A K�40 Þ

þð0:749 A Th�serie Þ þð0:564 A U�serie Þ ð6Þ

The cosmic component of the H *( 10 ) was calculated from the altitude measurements , h in metres above mean sea level , using the model proposed by Rybach et al ., 1997 .

H ð10ÞðnSv = hÞ ¼37:0 e ð3 : 8 10�4 hÞ : ð7Þ

To measure the background radiation outside the study area , a control zone was selected ( 37.6144N ; �0.96998 ). At this point , three measurements were taken by the drone 1m above the soil ( acquisition time : 300 s ). The measured dose rate and error was : 0.078 ± 0.006 mSv / h

The H *( 10 ) measured values have been checked with those measured by RS200 ionisation chamber in the same place .

Table 2 shows the activity levels of 40 K , 226 Ra ( 214 Pb and 214 Bi ) and 232 Th ( 228 Ac ) measured by the drone . In addition ,

the H *( 10 ) values calculated both from the gross gamma counts ( expression ( 4 )) and from the activity levels ( expressions ( 6 ) and ( 7 )) is shown . The ambient dose rate measured by the RS 200 ionisation chamber is also shown . Furthermore , Figure 2 illustrates the correlation between H * ( 10 ) values measured by RS200 ionisation chamber and H * ( 10 ) derived from drone measurements .

Firstly , it should be noted that no net activity higher than MDA could be detected for 40 K and 228 Ac at all the points where gamma spectrometric measurements were carried out over the phosphate sludge ponds ( points 1 to 10 ). This is not the case for the control zone , which was located in an undisturbed place far from the phosphate sludge ponds and was considered as environmental background .

Secondly , a high concentration of 226 Ra progeny is observed . The average value and standard deviation of 226 Ra activity measured in sample points 1 to 10 corresponding

to the phosphate sludge ponds is of the order of 500 ± 200 Bq / kg in accordance with activity values measured in a previous study ( CSN , 20 09 ): 226 Ra : 860 ± 340 Bq / kg ; 40 K : 160 ± 100 Bq / kg ; 232 Th ( 228 Ac ): 50 ± 20 Bq / kg .

In relation to the values of ambient equivalent dose , H * ( 10 ), a high agreement may be observed between the values measured by the drone from the gross gamma counts , applied with the conversion factor to H *( 10 ), and those recorded bythe RS200 . The Pearson ’ s coefficient obtained is 0.978 . This is not the case with the dose rate calculated solely from the values of 226 Ra activity measured , which in some cases are lower by

40 % than the previous values ( points 3 to 6 ). The Pearson ’ s coefficient is : 0.869 . The contribution of 40 K and 232 Th to ambient dose rate over the phosphate sludge ponds is practically negligible . Consequently , the 226 Ra activity measured by the drone in points 3 to 6 has probably been underestimated compared to what actually exists . However ,