Issue
Radioprotection
Volume 57, Number 1, January-March 2022
Page(s) 67 - 70
DOI https://doi.org/10.1051/radiopro/2021036
Published online 12 January 2022

© SFRP, 2022

1 Introduction

Equipment designed to scan large volumes of cargo using fixed linear accelerators (Linacs) may pose a risk to the operators and its surroundings (Hupe and Ankerhold, 2007). Such equipment requires the construction of a structure with an area delimited by fixed protection barriers (NCRP, 2005). The regulations and technical requirements for the control and safety of the operators and the professionals involved in the cargo and container inspection system depend on a conceptual framework for radiological protection (Hupe and Ankerhold, 2007; Gomes et al., 2021).

In this study a comparison between the expected scan dose records and the doses from other applications was performed. The dimensionless factor R, which represents a ratio between two specific scanning quantities, is introduced as a reference for evaluating the radiological safety levels of the facilities. Procedure A, which is a comparison of two facilities (A and B) under a simple alternative safety assessment, seeks to offer an additional shell in the radiological protection process.

2 Methods

The experimental data acquisition was carried out on-site by simple and easily accessible instrumentation with the aim of spreading the culture of radiation protection (ICRP, 2007). Measurements were performed at a Brazilian cargo and container inspection facility using a Smiths Heimam particle accelerator model HCVP4029 with a maximum operating energy of 4.5 MeV (Silva et al., 2021). During on-site measurement activities at the port of Rio de Janeiro, Brazil, a Geiger–Müller type dose integrator (model MRAD 111) from the manufacturer Ultra Radac was used. The main characteristic of MRAD 111 is the measurement range of the ambient equivalent dose rate (H*(10)). Measurements from MRAD 111 falls into the range of 0.01 µSv/h < H*(10) < 2 Sv/h for the ambient equivalent dose rate, and 0.001 µSv < H*(10) < 9.99 Sv for the integrated ambient equivalent dose. The expected uncertainties for the measurements can be considered as ± 30% for ranges of 1 µSv/h < H*(10) < 2.0 Sv/h and energy dependence ± 40% for the range of 60 keV to 1.5 MeV.

The MRAD 111 monitor was fixed in the controlled area next to the scanner detector used for image formation after the direct beam (see Fig. 1). After thirty scanning operations (in ordinary inspection activity) the detector was removed for reading. The number of containers scanned was limited to 30 due to logistical reasons. The time for each operation was approximately 7 s of direct exposure to the direct X-ray beam.

The Geiger–Müller calibration was performed by comparison methods under strictly controlled conditions while considering a confidence level of approximately 95%. This procedure is standard, and the value of reference for H*(10) followed international standards as well. All the measures can be traced through the metrology network of the Brazilian National Laboratory of Ionizing Radiation Metrology (LNMRI). This metrology network is also part of the Secondary Standard Dosimetry Laboratories (SSDLs) network of the International Atomic Energy Agency (IAEA). The radiological risk was introduced, in the form of the R (ratio) factor, with the aim of facilitating the visualization of the data and weighting. The R factor represents the ratio between the number of scans needed to reach a specific dose for a specific effect and the average number of specific scans (Ss) of a facility. The mathematical formulation of the factor R, which must be seen as a tool more than a reference, is introduced by equation (1). The Ss is the ratio between the dose for the occurrence of the effect and the expected dose per scan per facility. (1)

The radiation monitors were in the radiation line inside the cargo scanning area. This methodological design may seem to conflict with an assessment of potential detriments to individuals in the public, given that this public is not expected to have access to this area. However, the objective of the work is to verify the safety conditions for both personnel and individuals from the public. Dose values in the scan line are expected to be in a range covering values for free area to those considered to be accepted by standards. As the study is developed conservatively, it was decided to use the dose values only in the irradiation lines. These doses will always be higher than those considered outside the scanning area, offering greater security in risk assessments and their communication to individuals in the public.

thumbnail Fig. 1

Scheme indicating the position of the direct beam transmitted to the detector and evaluation monitor.

3 Results and discussion

The integrated H*(10) ambient dose equivalent was measured as 3.4E-2 mSv. The H*(10) per scan was 1.12E-3 mSv/scan which was considered as a reference level for evaluating the H*(10). Comparisons with values ordinarily expected were made in order to facilitate the understanding of the range of the measurements. Table 1 shows some selected levels of radiation exposure.

Acute whole-body exposure to X or gamma rays implies biological effects that might result in death depending on the uniform dose (ICRP, 2007). Some of these known effects are listed in Table 2 with the respective dose range.

For a whole-body exposure to photons (X or gamma), the weight factor of both the radiation and the tissue (or organ), WR and WT, are unitary (ICRP, 2007). As the weight factors are dimensionless, the quantity absorbed dose (Gy) and the effective dose (Sv) are equivalent in this case. In its publication no103 (ICRP, 2007) the ICRP generated experimental data using linear accelerators (Linacs); thus establishing an estimate of dose thresholds for tissue reactions in the testis, ovary, lens and bone marrow of adult humans. Table 2 shows the comparison between some dose limits, biological effects, and the expected number of scans.

The comparison between the results of two cargo irradiation facilities (A and B) showed that it is possible to apply the R factor as an alternative way to assess the levels of radiological safety. Figure 2 presents the results from equation (1) regarding the data presented in Table 2, which are referring to inspection facilities A and B.

Facility A has a demand of 150 scans per day and 1.12E-3 mSv/scan while facility B demands 1000 scans delivering 3.25E-3 mSv/scan. Considering the scanning time and the dose rate per scan in the cargo verification, it is possible to estimate the total dose received and then the biological expected effect. For R to be small, the number of specific scans (for each harm) must be large. This suggests lower dose values per procedure in a given facility (Eq. (1)). This condition allows us to infer that the procedures are optimized, thus improving the levels of radiological safety. Any communication to individuals in the public should consider this inverse relationship in the definition of R, current in the field of radiation protection (inverse square law). In this case, the values of R must be informed as a factor that when assuming high values indicates lower risks. This can be reported textually as well as hidden in an application capable of reversing the reasons when reporting in an infographic.

A consequence of applying the R factor is that the closer R comes to the unit, the higher is the risk level. Since fewer scanning procedures will be needed to reach a threatening dose level (see Tab. 2), the risk per scan rises. Facility B, which has a higher dose per scan, shows a lower R value (closer to 1) when compared to facility A. Changing the R factor may improve the knowledge about the safety levels. Since this method is based on information from dose scanning, it is possible to adjust the reference level in order to calculate R according to a selected biological effect. For example, a conservative assessment may consider dose limits for inducing leukemia since doses causing the effect are small. An important contribution in the field of radiation-induced leukemia can be found in the independent studies lead by Finch and Howe (Finch, 2007; Howe, 2007). However, as the same dose is being considered for both facilities, the results may negatively impact the industrial activity (lower number of scans).

Considering that the location of the monitoring system is in the cargo-scanning corridor, it may be relevant to use such a relationship to assist in the design of this type of facility. Such a project would have the objective of not removing the regulated area from the corridor. In this perspective, the R factor could be applied as an input to obtain the adequate thickness for different types of materials used for shielding purposes.

Table 1

Relationship between exposures by natural radiation and expected exposure in the inspection activity of scanning cargo and containers.

Table 2

Relationship between the estimation of the acute single dose limits (ICRP, 2007) and the number of scans.

thumbnail Fig. 2

Results from equation (1), based on the data presented in Table 2, referring to inspection facilities A (actual) and B (fictitious).

4 Conclusion

It was possible to assess the radiological safety conditions of a cargo and container inspection facility through a simple parameterization using official radioepidemiological data and the creation of a comparison factor (R). The R-values were calculated considering the exposure characteristics of each inspection facility based on equivalent scans (Ss). It is possible to establish Ss and consequently the factor R only considering the dose per scan. Although there is no fixed scale, the dimensionless R-value may also facilitate the personnel and general public’s perception of the risk levels. The factor R may be of value both to qualify the overall risk level of a cargo and container inspection facility and help guiding designers towards more effective shielding. Furthermore, the method is of low cost since it applies current-use detectors that are inexpensive and easy to operate.

Conflict of interest

The authors declare that they have no conflicts of interest in relation to this article.

Funding

There was no funding from any source for this study.

Ethical approval

This study did not carry out activities that would require approval by a research ethics committee.

Informed consent

This study did not carry out in vivo or in vitro experimentation activities of any kind.

Authors contributions

Samuel Q. Pelegrineli: conceptualization, methodology; Ademir X. Silva: writing original draft; Wilson S.S. Filho: investigation; Luciano S.R. Oliveira: visualization; Ricardo M. Stenders: writing-reviewing and editing; Juraci P.R. Junior: investigation; Wagner S. Pereira: investigation; Edson R. Andrade: writing original draft, investigation, and supervision.

Acknowledgements

The authors wish to thank the colleagues who have diligently contributed with many useful comments and suggestions.

References

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Cite this article as: Pelegrineli SQ, Silva AX, Filho WSS, Oliveira LSR, Stenders RM, Junior JPR, Pereira WS, Ramos de Andrade E. 2022. Evaluation of the radiological risk in cargo scanning by comparison with known biological consequences. Radioprotection 57(1): 67–70

All Tables

Table 1

Relationship between exposures by natural radiation and expected exposure in the inspection activity of scanning cargo and containers.

Table 2

Relationship between the estimation of the acute single dose limits (ICRP, 2007) and the number of scans.

All Figures

thumbnail Fig. 1

Scheme indicating the position of the direct beam transmitted to the detector and evaluation monitor.

In the text
thumbnail Fig. 2

Results from equation (1), based on the data presented in Table 2, referring to inspection facilities A (actual) and B (fictitious).

In the text

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