Radiometer for detecting alpha-emitting aerosols in indoor air..
TIKHONOV Andrey Anatolyevich
RADIOMETER FOR DETECTING ALPHA-EMITTING AEROSOLS IN INDOOR AIR
Traditionally, when the task of protecting an object from the possible use of radioactive substances for terrorist purposes arises, technical means are proposed that allow detecting radioactive sources by their external photon, alpha and beta radiation, neutron radiation [1]. However, a much greater danger, in terms of biological impact, is the use of radioactive gas-aerosol preparations for terrorist purposes.
Alpha particles, beta radiation, protons, deuterons and other heavy particles, causing a high ionization density, cause a strong damaging effect when entering the body by inhalation. It should not be forgotten that radioactive powders can be used as highly toxic chemical weapons. Isotopes of U and Pu have high chemical toxicity [2, 3].For example, the penetration of Pu into human internal organs leads to chemical poisoning (1 mg of plutonium is a lethal dose), radiation damage to the gastrointestinal tract, kidneys, liver, and brain. Such quantities of plutonium cannot be detected by their external radiation. It should be noted that the maximum permissible concentrations of Pu239 and U238 are determined based on their chemical toxicity [4].
The inhalation route of natural and artificial aerosols entering the body is recognized as one of the most important and dangerous. Dangerous concentrations of radioactive aerosols in the atmosphere of premises are created in various ways. This may be waste containing radionuclides of the uranium and/or thorium family. In the process of their radioactive decay, noble gases are formed — radon, thoron and actinon. The daughter products of the decay of these gases as a result of precipitation on particles suspended in the air form radioactive aerosols. Detection of artificial radioactive aerosols in the air is directly related to the control of radon and its daughter products of decay.
As shown in the work [5], one of the tasks of environmental monitoring in ensuring radiation safety is operational aerosol monitoring of the indoor atmosphere. The task of monitoring and detecting alpha-active aerosols in the indoor atmosphere is an important function of the radiation branch of the facility's safety system. To solve this problem, the Alpha-3 radiometer was developed (photo 1).
Photo 1. Alpha-3 radiometer
The Alpha-3 radiometer is a new development that has expanded the family of devices designed to measure the content of alpha-active aerosols in the air. The results of numerous calculation studies and experiments were used in the development of the radiometer, and specific requirements for modern safety equipment were taken into account. The radiometer units are placed in a regular case so that the purpose of the device is not obvious even to specialists.
The radiometer is based on the widely used aspiration measurement method [6]. In automatic mode, the radiometer measures the value of “latent energy” using the modified Markov-Ryabov-Stas method [7]. The radiometer has a semi-automatic mode, which allows implementing any method that involves taking a sample onto an AFA-RSP-3 filter with subsequent measurement of alpha activity [6]. Fig. 1 shows the block diagram of the Alpha-3 radiometer.
Fig. 1. Block diagram of the Alpha-3 radiometer”
Air sampling to the AFA-RSP filter, fixed in the filter holder 7, is carried out through the funnel 1. The air flow rate is controlled by the float pressure gauge 2. Sampling is provided by the blower 3, the operation of which is controlled by the switch 4. After sampling, the filter is moved to the detection device, consisting of the detector 5 and the amplifier 6. Normalized pulses after the discriminator 8 are fed to the input of the processing and control device 9. The radiometer is powered by an autonomous battery 11, and the stabilization of the supply voltages is carried out by the unit 10.
The Alpha-3 radiometer measures equivalent equilibrium volumetric activity (EEVArn) in the range from 5 to 10,000 Bq/m3; the limits of the permissible basic relative measurement error are ± 30% with a confidence level of 0.95. The radiometer measures the volumetric activity of long-lived alpha-active aerosols, starting from the maximum permissible levels.
The measurement time does not exceed 20 min. The intrinsic background of the measuring path is no more than 0.005 s-1. The sensitivity of the measuring path when recording the activity of standard solid sources of type 1P9 is 0.28 ± 0.02 (s-1Bk-1). The air blower capacity when taking an air sample to the filter is (2.5 ± 0.2)10-4 m3/s. The radiometer is powered from rechargeable batteries or from an external autonomous power source with a voltage of (12.5 ± 1.0) V. The maximum current consumed from the power source does not exceed 0.8 A. Without recharging, the battery life is enough for 100 measurements. Overall dimensions are no more than 105x480x360 mm. The mass of the equipped radiometer is no more than 7.5 kg.
The performance of the MP-12 blower is controlled by a float rotameter. The performance of the blower is adjusted manually by a flow regulator. The MP-12 blower consumes 450 mA when powered by 12 V.
It is known that variations in the natural background of alpha-emitting aerosols can reach values that differ by an order of magnitude [8]. The background depends on the season, time of day, weather conditions, building materials, etc. In this regard, control and detection of alpha-emitting aerosols involves regular monitoring of the content of natural aerosols in the indoor atmosphere, for example, by the value of “latent energy” [7]. The algorithm for generating an alarm signal involves comparing the current result with the average value of the “latent energy” calculated for the current moment. In addition, the current result of measuring the value of “latent energy” should be compared with the results of measuring the dose rate of photon radiation and the content of beta-emitting aerosols in the air.
A filter that has given a statistically significant anomalous result should be further investigated either with an alpha spectrometer or an Alpha-3 radiometer after a certain holding time required for the decay of natural short-lived alpha-active aerosols. The threshold for measuring the specific volumetric activity of aerosols of long-lived alpha-active nuclides is lower, the longer the sampling and sample calculation time. Thus, when collecting a sample of 1 m3, the sampling duration is ~ 70 min, when measuring the sample activity for 60 min, it is possible to determine the specific volumetric activity of aerosols of long-lived alpha-active nuclides at the maximum permissible level for plutonium ~ 0.03 Bq/m3 (~ 8×10-16 Ci/l).
The minimum detectable activity of long-lived aerosols, which can be measured promptly without long-term sample exposure, is estimated as follows:
here Vfon is the count rate due to RaA and RaC’; e is the detector sensitivity. If we take e=0.2, ERVOARn=100 Bq, and the pumping time is 5 minutes, then the minimum detectable activity is 30 Bq. That is, it is enough to have only 5×10-11 g of plutonium on the filter to confidently measure against the background of natural aerosols. At a pumping rate of 20 liters per minute, the minimum detectable volumetric activity of plutonium is ~ 300 Bq/m3.
Thus, regular measurements of alpha-active aerosols in indoor air, carried out using the Alpha-3 radiometer, make it possible to detect low concentrations of artificial alpha-emitting radionuclides. In addition, analysis of the results of joint measurements carried out as part of environmental monitoring helps to identify cases of the use of alpha-active aerosols for terrorist purposes and the degree of the threat posed.
In conclusion, the author thanks Doctor of Engineering Sciences B.V. Polenov, Cand. of Engineering Sciences V.I. Nikitin, Cand. of Engineering Sciences S.V. Volkov, and employees of Laboratory 192 of the Scientific Research Institute of Nuclear Physics V.G. Babich, V.N. Salnikov and E.V. Mityunina for useful discussions and assistance in the work.
Literature
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- Radiation protection instrumentation – Radon and radon decay product measuring instruments – Part 1: General requirements. IEC 61577-1 (2000-08).
- Markov K.P., Ryabov N.V., Stas K.N. Express method for assessing the radiation hazard associated with the presence of radon decay products in the air. Atomic energy, No. 4, 1962, p. 315.
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