Radioprotection No 59-2 | Page 24

J . A . Corbacho et al .: Radioprotection 2024 , 59 ( 2 ), 80 – 87 83

Table 1 . Experimental activities measured with the LaBr 3 ( Ce ) detector mounted on the drone at various heights and for different measurement times .

1 60

2 60

4 300

10 300

8 9

< e d ¼ e e ð�m ar a dÞ

=

1 m

4pd 2 ; d > 1 :

; ; ð5Þ e 1m ; d ¼ 1

where e 1m is the experimental efficiency at a distance of 1m , m a is the mass-attenuation coefficient in air ( 0.661 keV : 0.07538 cm 2 / g ; 1173 keV : 0.05840 cm 2 / g ; 1332.5 keV : 0.05518 cm 2 / g ), r a is the air density : 0.012923 g / cm 2 , and d is the separation in the air between the source and the detector .

It is important to note that expression ( 5 ) is a simplification , as it does not consider other parameters that also influence detector efficiency , such as the angle of incidence of photons at distances beyond 1 m ( Ritter , 2021 ).

Detection limits have been determined using the L . Currie criteria ( Currie , 1968 ).

Table 1 shows the activity values , detection limits , and its comparison to the reference values obtained from the different measurements .

The measurements carried out at distances close to the container ( 1 – 2 m ) give lower activity values than reference values . These results were to be expected because the efficiency calibration used was for a point source and the size of the radioactive source is too large to be considered a point source . However , for the measurement carried out at a distance of 10 m between the container and the drone , the measured activity values are of the same order of magnitude as reference values , despite the relative difference between measured activity and the reference activity still being significant (~ 15 %). However , this is considered to be a very satisfactory result for the objectives of using this type of device for in situ measurements with short acquisition time and applying a simplification to determine the efficiency of the detector at distances greater than 1 m .

3.2 Radiological characterisation of phosphate sludge landfill

The study area chosen to carry out a radiological characterisation using the measurements provided by the drone system developed in this work was a former phosphate production plant located in the southeast of Spain ( 37.61 N ; �0.965 ). Today , process wastes are still present , consisting mainly of phosphate sludge . Between 2005 and 2006 , a detailed radiological study of these areas was carried out ( CSN , 2009 ). The study area consists of two parts . One , where the buildings were located ( now demolished ), with a total surface of 180 000 m 2 . The second area , where there are three phosphate sludge ponds and various phosphate waste drifts has a total surface of 150 000 m 2 . Both areas are at an elevation between 9 and 21 metres above mean sea level .

Measurements were taken in a grid of approximately 10 10 m over the phosphate sludge ponds . The flight altitude was 1 m . These are optimal conditions for a suitable in situ gamma spectrometry measurement , since most of the photon flux reaching the detector , which is located 1 m above the ground , comes from an area equivalent to a circle with a radius of 10 m ( ICRU , 1994 ). At each point , a 30 s measurement was taken . The efficiency calibration for an infinite plane source with a homogeneous distribution in depth was used to determine the activity levels for natural radionuclides : 40 K , 226 Ra ( from its daughter : 214 Pb and 214 Bi assuming they were in equilibrium with their parent radionuclide ) and 232 Th ( 228 Ac ). In the event of an emergency , the most appropriate distribution would be an infinite plane where the radionuclides have been deposited in the surface layer .

The MDA values for a 30 s acquisition time and 1 m flight altitude are of the order of : 40 K : 300 Bq / kg ; 214 Pb : 30 Bq / kg ; 214 Bi : 30Bq / kg ; 228 Ac : 40 Bq / kg .

The conversion factor from gross gamma counts to H *( 10 ) detailed in Section 2.3 was also used to measure the ambient dose equivalent rate H *( 10 ) throughout the area .

Furthermore , the H *( 10 ) values was also calculated from the activity measurements of the 40 K , 226 Ra and 232 Th natural radionuclides measured by the drone-based system . For this purpose , the terrestrial component of the H *( 10 ) was calculated using the following expression ( Lemercier et al ., 2018 ).