Protection From Contamination by 211At, an Enigmatic but Promising Alpha Emitter


 Purpose

 211At, a promising alpha emitter, can easily volatilize and contaminate the environment. To safely manage this unique alpha emitter, we investigated the permeability of four types of plastic films and gloves against 211At and identified suitable materials to avoid contamination by 211At.
Methods

Four types of plastic films, polyethylene, polyvinylidene chloride, polyvinyl chloride, and a laminated film, and two types of rubber gloves, latex and nitrile, were examined. Small pieces of filter paper were covered with these materials, and a drop containing 100 kBq of 211At was placed on them. The radioactivity of pieces of filter paper under the materials was evaluated by measuring counts using a gamma counter and obtaining autoradiograms 3.5 h later. These experiments were also performed using 225Ac, 125I, 111In, 201Tl, and 99mTc.
Results

 211At solution easily penetrated polyethylene, polyvinyl chloride, and latex rubber. Similar results were obtained for 125I, while other radionuclides did not penetrate films or gloves. These results suggest that halogenic radionuclides under anionic conditions are likely to penetrate plastic films and rubber gloves.
Conclusion

Our evaluation revealed that, when 211At solution is used, the protection by polyvinylidene chloride, a laminated film, or nitrile rubber would be more effective than that by polyethylene, polyvinyl chloride, or latex rubber.


Introduction
The recent outbreak of coronavirus disease 2019 (COVID-19) has drawn people's attention to the protection from virus contamination; however, protection from radioactive contamination has been essential for workers in the eld of nuclear medicine because the use of unsealed radioactive materials exposes them to unnecessary radiation. Protection measures against radionuclide contamination are sometimes different from those against viruses [1]. Moreover, the protection measures depend on the type of radionuclide. Currently, radionuclide therapy, in which particle-emitting radionuclides are used, is gaining popularity. In particular, radionuclide therapy using alpha emitters has attracted considerable attention from researchers and physicians in the eld of nuclear oncology. Protection from contamination Page 3/12 by alpha emitters is crucial because alpha particles can severely damage tissues in which they accumulate. Although alpha particles themselves can be blocked by a sheet of paper, the isolation of alpha emitters is not always easy. We recently pointed out that a solution containing 211 At, a promising alpha emitter for targeted alpha therapy (TAT), can easily penetrate latex gloves, which are the most effective personal protective equipment (PPE) against COVID-19 [2]. When we work with radioactive materials, we put on PPE and wrap radioactive materials by plastic lms to properly isolate them and avoid accidental internal radiation exposure. In this study, we investigated the ability of four types of plastic lms and two types of gloves to protect against contamination due to 211 At, compared with some other radionuclides, for the safe management of this enigmatic alpha emitter.

Materials And Methods
The 225 Ac is also a promising alpha emitter for TAT, and this radionuclide acts as a trivalent cation in a solution. 125 I is a halogen that acts as an anion in a solution. 111 In is a photon emitter that acts as a trivalent cation in a solution. 201 Tl is a photon emitter with a main energy of 70.3 keV, which is similar to that of characteristic X-rays emitted from 211 At [3]. 201  Four types of plastic lms and two types of rubber gloves were tested. Polyethylene (30 µm), polyvinylidene chloride (11 µm), and polyvinyl chloride (8 µm) and laminated lms of polypropylene, ethylene vinyl alcohol copolymer (EVAL TM , Kuraray, Tokyo, Japan), and polyethylene (104 µm) were used as plastic lms. The numbers in parentheses are the lm thicknesses. The rst three plastic lms are commercially available to wrap perishable food materials and keep them fresh. The laminated lm was developed to pack dried bonito akes so that they are not damaged by oxygen and high humidity. As rubber sheets, pieces of latex and nitrile rubber gloves were used. These gloves are currently used as PPE against COVID-19 [4]. The thickness of each glove was more than 130 µm for latex and 70 µm for nitrile.
A three centimeter square piece of lter paper was covered by a sheet of plastic lm or a piece of rubber cut out from a rubber glove. Fifty microliters of radionuclide solution whose radioactivity was adjusted to 100 kBq was dropped on a plastic lm or rubber ( Figure 1). The plastic lm or rubber was covered by a plastic Petri dish to minimize the evaporation of the radioactive solution. Each piece of lter paper under the plastic lm or rubber was picked up 3.5 h later. This interval is half the half-life of 211 At. These pieces of lter paper were placed on imaging plates (FUJIFILM, Tokyo, Japan) for 5 min and approximately 15 h. The imaging plates were scanned with an imaging plate reader (FLA-7000; FUJIFILM, Tokyo, Japan). The acquired images were analyzed using the ImageJ software (U.S. National Institutes of Health, Bethesda, MD, USA). The radioactivity of these pieces of lter paper was also measured using a gamma counter (2480 Wizard 2 ; PerkinElmer, Waltham, MA, USA). These experiments were repeated three times for each radionuclide.

Results
When the [ 211 At]NaAt solution was dropped on pieces of plastic lm and sheets of rubber, strong radioactivity was detected in the lter paper under polyethylene lm, polyvinyl chloride lm, and latex rubber, in that order. No hot spots were detected in the pieces of lter paper under polyvinylidene chloride lm, the laminated lm, or the nitrile rubber ( Figure 2). The radioactivity of pieces of lter paper is summarized in Figure 5, and the originally measured data are shown in the Supplementary Table. Discussion 211 At, a promising alpha emitter for TAT, is popular in Japan because of its wider availability. Many researchers in Japan have been engaged in studies using radionuclides. However, this element is regarded as an enigmatic element [5] and most of its characteristics remain unclear. Previous studies have reported that 211 At can easily volatilize and contaminate the environment [6]. Therefore, strict protection and shielding measures are essential when using this enigmatic alpha emitter. In this study, we revealed that the permeability of the [ 211 At]NaAt solution is dependent on the type of shielding material. 225 Ac is another popular alpha emitter, and the [ 225 Ac]AcCl 3 solution penetrated lms and rubber little.
Since the numbers of alpha particles emitted during the single decay of 211 At and 225 Ac atoms are one and four, respectively, 225 Ac is a stronger alpha emitter than 211 At. Considering these ndings, we believe that the penetration of the [ 211 At]NaAt solution of lms and rubber was not induced by the direct destruction of materials by emitted alpha particles. The penetration depends on the chemical properties of the shielding materials. 225 Ac, which is cationic in a solution, and other cationic radionuclides 111 In and 201 Tl also failed to penetrate the shielding materials. In contrast, 125 I, which is a halogen and is anionic in [ 125 I]NaI solution as 211 At, showed a similar trend to that of 211 At. However, another popular radioactive anion, [ 99m Tc]TcO 4 -, did not penetrate the material. These results suggest that halogenic radionuclides under anionic conditions are likely to penetrate plastic lms and rubber.
Among the materials examined in this study, two types of plastic lms made of polyethylene, polyvinyl chloride, and latex rubber glove were penetrated by 211 At, and they were considered ineffective in protecting the contamination due to 211 At. However, the polyvinylidene chloride lm, the unique laminated lm, and the nitrile rubber glove were resistant to the penetration by the [ 211 At]AtNa solution. Although the details of the differences in chemical properties of materials between these two groups, easily permeable and non-permeable for 211 At, were unrevealed, the permeability of the [ 211 At]AtNa solution was correlated with that of gas or water according to open data [7]. It is di cult to clearly explain the mechanism of the permeability of 211 At for plastic lms and rubber gloves through these data, and further investigation is needed to determine the optimal methods to protect the contamination by 211 At. However, polyethylene is cheap, and bags made of this material are sold for radioactive waste stuffs, and they are actually used in experiments with 211 At [8]. Latex rubber gloves are also the most popular PPE. Considering such situations, warnings should be given to people who deal with the 211 At solution. Hence, we must put on nitrile gloves and wrap or cover specimens with polyvinylidene chloride lm or a laminated lm while working with compounds including 211 At to avoid unnecessary internal radiation exposure.

Conclusion
Our preliminary experiments indicated that the 211 At anion can easily penetrate at least two types of plastic lms, polyethylene and polyvinyl chloride, and latex rubber. The permeability of 211 At may depend on its chemical properties as a halogen that becomes an anion in a water solution. When we deal with the 211 At anion, we must put on nitrile gloves and wrap or cover specimens using a polyvinylidene chloride lm or a laminated lm.  The schema of the experiments to evaluate the permeability of lms and rubber. a The photograph indicating the con guration of a radioactive drop, sheet of lm or rubber, piece of lter paper, and so on. b The schema of the con guration. 1: lm or rubber, 2: ltering paper, 3: radioactive drop, 4: aluminum ring (to keep the drop at the same position), 5: plastic plate (to avoid contamination by volatilized radionuclides), 6: water (to avoid vaporization of drop). c The schema of the section of the con guration.   The permeability of the [125I]NaI solution through lms and rubber. The autoradiogram obtained after 5min exposure to imaging plates. a Full scale image, b overexpressed image.