SOME QUESTIONS OF BETA-RADIATION PROTECTION BUILDING FOR OVERALLS

The issues of beta-radiation protection building are considered in order to select the optimal materials for overalls for work with sources of ionizing radiation. The electrons passage through assembly consisting of fabric layers of various types with addition of protective shielding materials has been studied. The attenuation coefficients of an electron beam from Sr-Y source in fabrics were experimentally determined. Computer simulation of the passage of electrons through heterogeneous media has been carried out, and recommendations have been given on the materials composition for protective overalls.


INTRODUCTION
The production of nuclear fuelthe nuclear fuel cycle (NFC), as well as the nuclear weapons complexis a continuous cycle of technological processes from uranium mining (mining uranium ore, process of cleaning it from impurities to obtain uranium concentrate, the enrichment processseparation into U-235 and U-238), the manufacture and use of nuclear fuel until the final disposal of radioactive waste.
Each of these processes, as well as carrying out repair, dismantling and emergency recovery work at all enterprises of the nuclear industry and energy, entails the need for personnel to stay in a zone of increased radiation hazardincreased doses of gamma-radiation, external neutron radiation, as well as betaradiation, which accompanies almost all production processes.
According to the requirements of the sanitary legislation [1,2] be chemically resistant to aggressive environments, which include radioactive substances; protective gloves should be strong, elastic, comfortable and easy to clean from radioactive contamination.
To protect against electromagnetic radiation (EMR), metallized and nonmetallized fabrics are produced. There are several types of such fabrics: synthetic fabrics that contain metallic copper or silver-plated threads; polyester and polyamide fabrics, which are sprayed with a copper or nickel coating, fabrics, on which nickel, copper, cobalt or silver coatings are applied by chemical deposition in a gaseous medium or solutions. Currently, nickel is the most commonly used metal coating. It is a ferromagnetic and well reflects the magnetic component of electromagnetic radiation. [3][4][5].
The search for materials that provide effective protection against ionizing radiation remains an important area of radiation physics [6]. In most practical problems, radiation protection is a heterogeneous mixture of different media. The calculation of such protection by analytical methods is very difficult, since the accumulation factors of heterogeneous media depend on a large number of task parameters: the incident radiation energy, thickness, material, number and geometry of layers, as well as their relative position.
In papers [7][8][9][10], the features of gamma quanta and electrons passage through heterogeneous layers of materials are considered, computer simulation data and experimental results are presented. It has been shown that the radiation protection efficiency of a heterogeneous assembly is better in the case when a light material is facing the source.
Protection of personnel from external gamma and neutron radiation can be ensured by reducing the duration of work ("protection by time"), by performing work using remote devices ("protection by distance"), as well as by using protective walls, screens, etc. [1,2]. All attempts to create PPE against these types of radiation have not been successful, since these radiations have a large penetrating power.
However, for protection against external beta radiation, PPE is very effective.
The thickness of the protective material should be from 0.3 to 0.7 g/cm 2 , depending on the energy of beta radiation [11], which corresponds to a protective suit weight of ~7-18 kg. The classification of PPE from external beta radiation is presented in Table 1. Protection against external alpha radiation is not relevant due to the low range of alpha particles in air (~5 cm), and the presence of PPE that provides protection against beta radiation fully provides protection against alpha radiation.
The aim of our work is to determine the attenuation coefficients of β-radiation from a 90 Sr-90 Y source in tissues and try to give recommendations on the composition of materials for protective overalls.

MATERIALS AND METHODS
All works were carried out in the metrological certified Laboratory of The energy spectrum of electrons from the 90 Sr-90 Y source [12] is shown in  Samples of materials were made in a circle form with a diameter of 6 cm.

RESULTS AND DISCUSSION
In the experiment, the source of ionizing radiation was located at a distance of ~1 cm from the detection unit. Protective materials were alternately placed between the source and the detection unit, and the flux density of β-particles was measured.
The attenuation of β-radiation is described by the formula: where I 0 is the flux density of β-particles without protection; μ(ρ, Z, Е) -linear attenuation coefficient of β-radiation, cm -1depends on the density ρ, the serial number of the substance Z, and also on the electron energy Е; d is the thickness of the protective layer.
Let us introduce the mass attenuation coefficient μ m = μ/ρ, then: where P=d•ρ is the surface density of the protective material, g/cm 2 . μ m = ln(I 0 /I)/P.
The results of measurements of radiation passage through fabrics are presented in table 2. The surface activity of the source of β-radiation 90 Sr-90 Y is I 0 =1775 particles/cm 2 ·min.
From Table 2, it is obvious that the use of classic fabrics without additional fillers (screens) for the manufacture of protective suits is inappropriate. Based on the requirements for overalls [11], the protective screen should be thin and elastic enough so as not to hinder the movement of personnel by more than 30%.
Heavy metals provide the best protection against ionizing radiation. However, an obstacle to their use in workwear is their toxicity. Let us evaluate the use of tungsten (tungsten carbide) for protection against β-radiation as part of PPE.
Tungsten is in 1.7 times denser than lead, chemically resistant and belongs to lowtoxic metals, unlike lead, nickel and copper.
Using the software package Geant, a computer simulation of β-radiation passage through a tungsten target was carried out. The 90 Sr-90 Y source was a circle 2 cm in diameter with the coordinates of electron emission equally probable in area [13,14]. A tungsten target was located close to the source, behind which was a total absorption detector. Table 3 shows the results of the escape of β-and γ-particles from tungsten with a thickness of 0.1, 0.2, and 0.3 mm per 10 5 events.  Assuming that the total weight of the protective suit should not exceed 20 kg, the maximum thickness of the tungsten carbide should be 200 μm. Then, taking into account density ρ(WC)=15.63 g/cm 3 , the surface density is P(WC)=d•ρ =3126 g/cm 2 ; and for density ρ(W 2 C)=17.2 g/cm 3 , the surface density is P(W 2 C)=d•ρ =3440 g/cm 2 ; In table 3 the data for solid sheet tungsten are presented. In protective suits, finely dispersed tungsten carbide powder should be used to ensure elasticity. Such a fraction is difficult to simulate. Let us calculate the yield of β-and γ-particles from tungsten plates with a total thickness of 0.2 mm, located at a distance of 10-20 μm from each other. The results are presented in table 4 for 10 5 events. It should be taken into account that when β-particles pass through the protective material, bremsstrahlung arises (Fig. 2), the output of which increases with increasing atomic number Z of the shield. Fig. 2 shows a peak at an energy of 60 keV, which corresponds to the K α1 =59.3 keV line of the characteristic X-ray emission for tungsten.
According to computer simulation (Tables 3,4 Table 5 shows that the introduction of tungsten powder in the amount of 1.768 kg/m 2 reduces β-radiation flux from 90 Sr-90 Y source by 3 times. And the use of a mixture of silicone and tungsten in ratio of 1 to 2 between the layers of velour and special fabric with a total density of 5.821 kg/m 2 provides a protection efficiency of 98.7%.

CONCLUSION
The paper analyzes experimental data and computer simulation data regarding the protective properties from beta-radiation of various materials for workwear. A combination of materials for the production of PPE for work with sources of beta radiation is proposed. This is a three-layer material consisting of an outer layer of special rubberized PVC fabrics, an interlayer of silicone with tungsten carbide powder and natural velour-type fabric. The protection efficiency of such combination of materials at an acceptable surface density reaches 98.7%.