82 J . A . Corbacho et al .: Radioprotection 2024 , 59 ( 2 ), 80 – 87
3 Results and discussion
3.1 Commissioning of the drone . Quantification of a radioactive debris container
Fig 1 . Cross-correlation between H *( 10 ) measurements from the Reuter-Stokes ionisation chamber and gross gamma counts from the drone mounted LaBr 3 ( Ce ) scintillation detector . Measurement conditions : 1 m above the ground on a flat surface without obstacles or vegetation within a radius of over 30 m . Homogeneous distribution of natural radionuclides at depth .
( Baeza and Corbacho , 2005 ; Tyler , 2004 , 2008 ; Arnold et al ., 2012 ; Ji et al ., 2019 ).
2.3
Correlation between H *( 10 ) dose rate and gross gamma counts
There is a direct relation between the ambient dose equivalent rate H *( 10 ) and the corresponding measured pulse height spectrum n ! defining the response vector v !
H ð10Þ ¼ v ! n ! :
There are several methods to derive area doses from pulse height spectra , which are described in the bibliography ( Toivonen et al ., 2008 ; Reginatto , 2010 ; Casanovas et al ., 2016 ; Drombrowski , 2014 ).
In this study , we have utilised an experimental method based on comparing the equivalent dose rate measured by a Reuter Stokes RS200 ionisation chamber and the gross gamma counts registered by the gamma detector mounted on the drone at various locations where the ambient equivalent dose rate is in the range of 0.050 – 0.6 mSv / h . The measurements were conducted on a flat surface with no obstacles and at a height of 1 m above the ground . The relationship between data can be fitted to a linear function ( Fig . 1 ). The RS200 ionisation chamber is considered a reference device for the measurement of ambient dose equivalent rate ( Duch et al ., 2008 ). The linear fitting parameters are as follows : slope : ( 4.61 ± 0.04 ) 10 �4 ; y-intercept : 0.016 ± 0.003 ; R 2 = 0.9998 . Therefore , this experimental calibration has yielded a dose rate conversion factor function :
H ð10ÞdroneðmSv = hÞ ¼4 : 61
10 �4 : Total Gamma countsðcpsÞ þ 0:016 ð4Þ
ð3Þ
The localisation of point sources ( orphan sources ) is based on the systematic sampling of an area with measurements using short integration times , typically in the order of 1 – 2s . However , the MDA increases significantly as the integration time decreases and distance from source increases . Consequently , once the source is located , it is important to increase the measurement time to improve the accuracy in the quantification of the activity of the source . Once the radioactive source has been identified , it is possible that it cannot be considered a point source due to its size . Consequently , quantifying its activity may not be feasible as the appropriate detector efficiency calibration is unavailable . Therefore , it may be useful to take the measurement far away from the radioactive source to consider it , for practical purposes , as a point source . For a point source , an efficiency calibration is available , and corrective factors are applied to account for attenuation due to distance and the layer of air between the source and the detector . In this sense , the inverse square law will apply where the distance , d , from the source is at least 10 times ‘ x ’, where ‘ x ’ is the longest dimension of the source ( Bevelacqua , 2004 ; Knoll , 2010 ). It should be noted that if other radioactive sources are present , their influence on the obtained activity results cannot be disregarded as the detector moves away from the source for which the activity is to be determined .
To test the ability , of the drone developed in this work , to quantify a non-point radioactive source , an outdoor test was conducted using a radioactive source of known activity . For this , several flights were carried out over a container with lowmedium activity material , specifically scrap metal from the decommissioning of the José Cabrera nuclear power plant ( Spain ). The characteristics of this container were as follows : 90 90 180 cm ( width height length ); mass 550 kg ; volume 1237 L . It had previously been characterized by the radioactivity laboratory located in the nuclear power plant itself . The mean activities of the radioactive material it contained were 137 Cs : ( 1.40 ± 0.12 ) • 10 5 Bq / kg and 60 Co : ( 1.67 ± 0.21 ) • 10 5 Bq / kg . The reference activity decay corrected values used for the identification and quantification of the radioactive source using the drone were 137 Cs : ( 75 ± 7 ) MBq and 60 Co : ( 81 ± 7 ) MBq .
This radioactive source can be classified as category 3 according to IAEA security guide No . RS-G-1.9 . ( IAEA , 2005 )
Aseriesofflights were conducted over the container at various heights ( 1 – 10 m ) and for different acquisition times ( 60 – 300 s ). To adjust the activity values for measurements taken when the detector is more than 1 m from the container , a correction factor has been applied which takes into account both the attenuation of the gamma photons in the air and the geometry . The expression used to calculate the efficiency , e d , of the detector for point sources placed along the detector axis at distances , d , greater than 1 m , is as follows :