Virtual radioactive source system for exercises modeling high doses

The goal is to enable the system to simulate any kind of ionizing radiation field indoors and outdoors using sensors of the virtual system. These techniques are collectively referred to as real-time locating systems (RTLS). The following technologies and standards were examined:

– radio frequency (RF) identification [19],

There are several products on the market that offer software and hardware solutions for implementing UWB -based RTLS.

The radio technology UWB shares many similarities with the other three technologies listed above. A significant difference is that UWB operates within the frequency range of 3.1–10.6 GHz, while the bandwidth of the signal may be >500 MHz. UWB is widely used in indoor ranging and positioning systems. The wide bandwidth renders the system highly robust and enhances its resistance to environmental obstacles [20]. The range of the UWB is approximately 20 m.

Since the development of such a system is an interdisciplinary task, RP experts and software engineers were involved. The VRSS is based on measuring the distance between the emitter and the transceiver.

The hardware and data-evaluation method developed for the operational calculations were investigated based on the Hungarian national standard and policies [10, 11, 13] as well as an international database [21, 22]. The control system, source code, interface, and display panel of the VRSS as well as the accessories of the scenarios were built.

The VRSS was developed using a UWB -based RTLS. By using this technology in the core hardware and software, the time of arrival (ToA) approach can be implemented by taking into consideration the propagation time (delay) of radio frequency (RF) signals to estimate distances. To implement the simulations, the following two approaches are used:

– Direct distance measurement (virtual mode)

The simulations are directly based on the distance measured between two UWB-enabled mobile devices, also referred to as tags. In this measurement scenario, one Master tag represents the detector/receiver, while the other the radiation source. The advantages of these tags are higher accuracy and simpler implementation.

The system uses a Raspberry Pi 3 computer, Raspberry PIOS based on Linux, 2 GB of RAM, and a Broadcom BCM2837 processor running at 1.2 GHz.

For the core software and hardware components responsible for UWB communication as well as raw distance measurements, modules, which act as tags and anchors, manufactured by Pozyx^® (Greenville, SC, USA) are used [24].

Measurements are continuously made, and the refresh time of the calculated dose as well as its rate displayed is 1.0 Hz following adjustments to be as accurate as real dosimeters [25]. The transceiver has a TFT LCD display, where the measured values are displayed for the users, a button to receive messages, and it is equipped with a loudspeaker and a vibrating motor. A built-in battery is also included in the transceiver, and the operating time can be increased with an external powerbank that can be attached via a USB cable. The transceiver–emitter has a display module, built-in amplifier, and antenna. The transceiver sends the data to the main control system, which calculates the dose rates and sends them back to the transceiver. The alarm levels of the device, tags, and transceiver can be adjusted so that the latter can provide the user with audio–visual signs.

Pozyx^® provides a hardware and software solution to implement UWB-based positioning. The exact position of the Pozyx^® tag or its distance from the other tags is sought.

In the virtual–virtual mode, a zero point should be set relative to which the coordinates of the placed. Anchors are determined by distance measurements. A diagram of the virtual–virtual mode is presented in Fig. 1.

Fig. 1.

When setting up the training site, the VRSS operator must choose the mode in which the measurements will be taken. It is also possible to set up the sampling intervals between measurements. The computer controlling the operating system needs a sufficient amount of time for the distance and position data to be acquired. Therefore, the sampling interval should be set with caution. The ideal sampling interval also depends on the chosen mode as well as the number of active detectors and sources. The recommended minimum sampling interval (T_R) in the distance measurement (virtual) mode can be calculated: (1) tR=100ms \[{{t}_{R}}=100\,\text{ms}\] (2) TR=D⋅Svir⋅tR \[{{T}_{R}}=D\cdot {{S}_{\text{vir}}}\cdot {{t}_{R}}\] where D denotes the number of active detectors, S_vir represents the number of virtual sources, and t_R stands for the recommended minimum sampling time.

The recommended minimum sampling interval (T_P) in the position measurement (virtual–virtual) mode can also be calculated: (3) tP=400ms \[{{t}_{P}}=400\,\text{ms}\] (4) TP=(D+Svir-vir)⋅tP \[{{T}_{P}}=\left( D+{{S}_{\text{vir-vir}}} \right)\cdot {{t}_{P}}\] where S_vir-vir denotes the number of virtual–virtual sources and t_P represents the recommended minimum sampling time.

In the virtual–virtual mode, the response time is slower if multiple detectors are used. Furthermore, it is time-consuming for the system to collect all the data from the different sources.

If the VRSS is used at the same time in the virtual + virtual–virtual modes, then the response time of the system is T_R + T_P.

The system uses microwaves and is based on devices operating on the principle of distance determination (transceiver–emitter) rather than that with regard to measuring the intensity of the signal [4].

In the case of a single-point source, the code calculates the dose rate of a selected nuclide as a function of distance based on the following formula: (5) H˙=dHdt=DCF⋅Ar2≅H*(10) \[\dot{H}=\frac{dH}{dt}=\text{DCF}\cdot \frac{A}{{{r}^{2}}}\cong {{H}^{*}}\left( 10 \right)\] where Ḣ (Sv/h) denotes the current dose rate at distance r, which is calculated by the program, H*(10) represents the measured dose rate, A (TBq) stands for the current activity of the virtual radioactive source as specified by the user, and r is measured by the system to represent the distance (m) between the source and detector. Furthermore, dose conversion factor, DCF (μSv/h/m²/TBq) refers to the isotope-dependent dose rate conversion factor specified by the user who chooses the isotope. The standard DCF values are listed in the Hungarian national standard [21] and the standard values are pre-programmed in the VRSS system [21, 22].

The operation of the VRSS is similar to that of real dose rate meters, only the measurement is different.

Visualization application

Visualization of the VRSS was developed based on the VRdose program code [3, 5]. The user (person, unmanned ground vehicle (UGV), or unmanned aircraft vehicle (UAV)) can actually move around the site with the virtual dose rate–measuring device. The system is capable of generating a map indicating the personal route of the user from which the virtual dose rates obtained along the route can be seen (Fig. 2).

Fig. 2.

The VRSS is programmed online using the interface.

Application of surface contamination in the VRSS

The virtual SC mode operates in a similar fashion to the dose rate mode, and only the main equation is different. The unit of measurement is given in activity per area, A_SC (Bq/cm²).

When decontamination of the radioactive surface needs to be simulated, the maximum surface activity can be defined for each of the decontamination steps. The system measures the distance between the emitter and transceiver as well as provides a distance, r. In Fig. 3, the virtual source is attached to the bottom of the table and the transceiver moved on the surface of the table. With this parameter, the surface activity can be calculated by the following formulae: (6) ASC=ASCmaxwhenr<R1 \[{{A}_{\text{SC}}}={{A}_{\text{SCmax}}}\,\text{when}\,r<{{R}_{1}}\] (7) ASC=ASCmaxR2−rR2−R1whenR1≤r<R2 \[{{A}_{\text{SC}}}={{A}_{\text{SCmax}}}\frac{{{R}_{2}}-r}{{{R}_{2}}-{{R}_{1}}}\text{when}\,{{R}_{1}}\le r<{{R}_{2}}\] (8) ASC=0whenR2≤r \[{{A}_{\text{SC}}}=0\,\text{when}\,{{R}_{2}}\le r\] where A_SC denotes the surface activity, A_SCmax represents the maximum surface activity (near the hotspot), and R₁ and R₂ are the predefined sizes of the contamination hotspot and splattering, respectively (Fig. 3).

Fig. 3.

Other equations can also be used, e.g., to determine the exponential dependence or inverse-square dependence, if requested by the user.

The SC mode was compared with measurements of a standard alpha source used for calibration.

All the operation modes of the VRSS were tested according to scenarios using real sources. During testing, the calculated and measured values using calibrated measurement devices were compared with those measured. The tests were performed in the laboratory of the Nuclear Security Department (NSD) [26]; moreover, distances were marked and different devices placed equidistantly. The response time of the VRSS was adjusted according to the response times of the real measurement devices.

Background measurements were tested. If no radioactive source is programmed or the dose rate meter is too far from the virtual source, the system automatically shows the natural background dose rate value, i.e., approximately 0.1 μSv/h. In that case, the system generates random numbers within the range of a relative standard deviation of ±40%. This function can be adjusted to higher levels of background radiation to imitate an area with SC where measurements are made by a drone. The increased background dose rate level imitation can also be used when such a device must be rebuilt, which does not function properly.

Comparative dose rate measurements were made using the virtual system vs. real sources and detectors. Data acquisition by the VRSS was compared with a 486.9 kBq ¹³⁷Cs source at distances varying from 5 cm to 50 cm. The theoretically calculated and real results as well as those measured by the VRSS (μSv/h) are shown in Fig. 4.

Fig. 4.

The results highlight that at short distances (<20 cm), the difference between the measured and calculated values observed is greater because within this range the volume effects of the source and detector are significant [27]. The virtual dose rate meter behaved in the same way as the real dose rate meters under the same conditions.

Regarding phases of alpha decontamination, in the system after each decontamination phase, the maximum SC can be preprogrammed by the user and the system will automatically calculate the SC area by Eq. (6) (Table 1). In the simulation test, a ²³⁹Pu source was used. The time intervals between the decontamination phases were 2–5 min. The steps are:

– Measuring the surface as well as defining the area and source of the contamination.

Other types of sources can be used which depend on the user.

More than one emitter can be placed to create several hot spots in the surface contaminated area and sample measurements from swabs can be recorded using another emitter.

The source of radiation was located by a drone with a virtual dose rate meter mounted on it. The measured values displayed on the main signal panel for participants of the Innovative Cluster for Radiological and Nuclear Emergencies (INCLUDING) project are presented in Fig. 5 [1]:

– The VRSS was used to train and make the final assessment regarding the suitability of candidates competing to be Hungarian astronauts at an outdoor training site as well as evaluate how the common procedure can be used in the event of an extraordinary event when a high-activity radioactive source becomes unshielded or its shielding is heavily damaged.

Fig. 5.