Development and calibration of a portable detection device for in vivo measurement of high-energy photon emitters incorporated by humans

This work presents the evaluation of the applicability and sensitivity of a portable detection device specially designed for in vivo measurement of high-energy photon emitters in the human body. The calibration was performed at the InVivo Monitoring Laboratory of the IRD. The equipment consists of a lead-collimated NaI (Tl) 3”x3” scintillation detector assembled on a tripod. The detector and its compact associated electronics are connected via USB cable to a portable PC. Spectrum acquisition and analysis is controlled by specific commercially available software. The calibration was performed using a standard liquid source of Eu contained in 3 L polyethylene bottles. The evaluation of the system is based on the estimation of the minimum committed effective doses associated to the minimum detectable activities, calculated using current biokinetic and dosimetric models available in the literature. The dose detection limits for selected radionuclides of interest in an emergency scenario have shown to be far below 1 mSv allowing the system to be useful in accident situations.


INTRODUCTION
Many international organizations are worried about the nuclear terror events.In 2016, the International Atomic Energy Agency (IAEA) published the Information Circular in attention for Nuclear and Radiological Terrorism [1].That document recognizes the need for ensuring adequate nuclear emergency preparedness and response capabilities.Radiologic terrorism events include Radiological Dispersal Devices (RDD), referred as dirty bombs, which employ conventional explosives to disperse radioactive materials [2].In Brazil, a government body was created for this purpose including civil and military entities [3].The hospitals also should be prepared to identify and quantify intakes of radionuclides of interest in order to assess the committed effective doses associated to the internal exposure.In such situations a fixed whole body counting system may not be suitable to attend an increased demand of in vivo measurements [4].Therefore, portable detection devices are more versatile and effective equipments to be employed for prompt response to emergencies involving intakes of radionuclides.

MATERIALS AND METHODS
The detection system consists in a NaI(Tl)3"x3" detector assembled on a tripod and connected to the compact electronics.Spectrum acquisition and analysis is software-controlled.Firstly, the system was calibrated with a 152 Eu standard point source.The "Energy vs Channel" curve was obtained.After, an 152 Eu standard liquid source (7,177 kBq/g) was prepared to be apply in a thorax simulator composed by five 3 L polyethylene bottles.This simulator is low cost device, resistant and applicable to our purpose.The liquid source known mass (mass differences method) was transferred into a volumetric flask whose activity was calculated.Deionized water was introduced into this flask up to the 25 mL level being the activity per L calculated.The 4 mL volume was transferred to each polyethylene bottle contained 3 L deionized water volume.The bottle set activity was calculated which simulate a radionuclide incorporation.The bottle set contained radioactive liquid was positioned at 0,05 m distance from detector surface (Figure 1).Six Regions-of-Interest (ROI) were defined and a series of five 900-seconds counts were performed.The average net count was recorded in each ROI.The efficiencies were calculated as follows: where Nctg is the average count in each ROI, ∆t is count time, Ac is the source activity and Iγ is the relative intensity of the 152 Eu photon.Then, "Efficiency vs Energy" curve was obtained.For evaluation purposes, 137 Cs was selected as radionuclide possibly present in an emergency scenario.
Thus, the efficiency at 661.6 keV [5] in cps/dps was calculated from the Eff x Energy curve obtained previously.The calibration factor (CF) in cps/Bq was calculated from equation 2, where Iγ is the gamma relative intensity for 137 Cs at 661.6 keV: The evaluation of the system sensitivity is based on the estimation of the minimum committed effective doses associated to the minimum detectable activities [6].The minimum detectable activity (MDA) in Bq was obtained from equation 3: where NBG is the total background counts (BG) and tBG is the count time in the 661.6 keV region.It was considered 300-seconds to tBG.The MDI is a function of the MDA and depends on the exposure scenario and time elapsed between intake and in vivo measurement.It was calculated as follows (equation 4): where m(t) is the retention fraction in the compartment of interest, in Bq/Bq [7].The minimum detectable effective dose (MDED) in µSv was obtained from equation 5: where e(g) is the dose coefficient associated to the intake scenario adopted in the simulation [8].In this work, a single intake of 137 Cs by inhalation was assumed and the in vivo measurement performed 1 day after incorporation.

RESULTS AND DISCUSSION
The 152 Eu standard liquid source activity per mass was 7.2 kBq/g.The liquid source mass transferred to a volumetric flask was 2.615818 g.This resulted in 18.8 kBq (activity in the flask).In 25 mL, the activity per mL was 0.8 kBq/mL.In each bottle was transferred 4 mL wich mean 3.2 kBq.The activity of the all bottles was 16 kBq (0.4 µCi).The average net count was recorded in each ROI.Knowing the average net count, count time 900-seconds, activity of the bottles and gamma relative intensity ( 152 Eueach ROI) was calculated the efficiencies to each ROI, as follow in the Table 1.

CONCLUSION
The dose detection limits for the selected radionuclide of interest in an emergency scenario have shown to be far below 1 mSv allowing the system to be useful in accident situations or unexpected event.It is recommended to evaluate this technique considering other radionuclides of interest that could be used in malevolent actions.The proposed calibration and measurement protocols could be easily applied to other detection devices owned by civil and military medical facilities in order to improve prompt response capabilities in case of emergency situations.

Figure 1 :
Figure 1: Portable detection device and the thorax simulator.