SYSTEM OF MONITORING THE ATMOSPHERICAL RADON WITH AN IONIZATION CHAMBER DETECTOR TYPE IN PULSE MODE

Transcription

1 SYSTEM OF MONITORING THE ATMOSPHERICAL RADON WITH AN IONIZATION CHAMBER DETECTOR TYPE IN PULSE MODE Marian Romeo Călin, Adrian Cantemir Călin Horia Hulubei National Institute of Physics and Nuclear Engineering IFIN-HH, Bucharest Magurele, POB MG 6, R , Romania, Abstract: Measuring atmospheric radon is a highly interesting area of environmental radiation protection. As is well known, radon penetrates the human organism along with atmospheric air through breathing. The element is emitted from the ground due to processes involved in chain nuclear disintegration and can also be produced by other sources such as building materials, which are part of the buildings, underground stations, coal pits, salt works, quarries, etc. In the circumstances, it is quite important that the volume activity of radon and its progenies be checked, which accounts for the broad diversity of methods that have been developed for this purpose. One of the most widespread such methods is based on measuring the average ionization current generated by radon circulation through an ionization-chamber detector. This paper presents the development of a lab device for monitoring radon concentration using a radiation detector of ionization chamber type (in 3 different variants). It is a pulsed mode device, in which the measurement of the average value of the ionization current is complemented with, or replaced by, a recording of the ionization current pulses generated by alpha disintegration within the useful volume of the detector. The ionization chamber thus operates as a 4π counter, in which radon and its progenies are the alpha source. The paper provides a mathematical evaluation of the efficiency of the method and of the measuring device, based on a data acquisition system and a special calculation program, which make it possible to study the time and amplitude distributions of the pulses produced by alpha radiation in the ionization chamber. Early results have indicated the volume activity of radon and its descendents can be measured with a 5% 6% uncertainty margin.

2 1. Introduction The counting of pulses due to individual alpha particles by the ionization chamber [1], [2], offers an excellent possibility for the measurement of the activity of gases including the atmospheric Radon. For an internal gaseous alpha radioactive source, the ionization chamber could detect, in principle, almost all emitted alpha-particles. The alpha-particle counting efficiency for a gaseous radioactive source depends, however, in practice, on the ionization chamber size and on the counting threshold, because alpha-particles emitted by nuclei situated near the chamber wall (or near the collecting electrode) might be not fully stopped in the sensitive volume of the chamber and their pulse amplitudes could get below the threshold. By supposing an ionization chamber with a diameter value equal to or larger than the range of the 2500 number of events alpha particle energy Figure 1. Calculated alpha particle energy distribution Alpha particle, the fraction of the small amplitude pulses should be low because even for the nuclei placed near the chamber wall the fraction of alpha particles emitted perpendicularly to the wall is relatively low. In order to get an estimation of the effect of the counting threshold on the alpha particle counting efficiency we have to calculate, first of all, the expected spectrum of the alpha particles. For a monoenergetic alpha transition, for instance the alpha decay of 222 Rn, beside the line corresponding to the maximum energy of the transition, the alpha spectrum should contain a continuous distribution of pulses due to alpha particles not fully stopped in the sensitive volume of the chamber. We performed such calculations by using a Monte Carlo program for a 3 - inches diameter cylindrical ionization chamber with a thin wire as collecting electrode (the radius of the chamber is about the range of alpha particles in air at the atmospheric pressure). The obtained spectrum is shown in Figure 1. As can be seen, the spectrum is well concentrated at the maximum alpha energy. The efficiency of alpha particle counting as a function of the energy threshold is presented in Figure 2 (curve 1). For alpha energy thresholds equal to 5-10% of the maximum energy of the 222 Rn alpha particles the alpha particle counting efficiency is about 93-95%. The alpha particle counting efficiency is considerable higher in the case of Radon including its main daughters (for 222 Rn

3 alpha counting efficiency Radon only 2 Radon and main daughters Figure 2. Calculated efficiencies for a 3 diameter ionization chamber the main daughters are 218 Po and 214 Po, the other daughters being rejected by the lifetimes values) due to the dynamics of the Radon main daughters. Let s assume the outer electrode to be negatively biased. As has been observed in [2], after the alpha decays of 222 Rn and 218 Po the daughters 218 Po and, respectively 214 Po, are forming, at equilibrium, positive ions which are collected by the outer electrode. As the collecting time is much smaller than the time interval between the Radon sampling and the measurement, the Radon daughter s alpha decays take place from the chamber wall. There are two consequences: 1) All emitted alpha particles are of maximum energy. 2) The alpha particle counting efficiencies for 218 Po and 214 Po are equal to 100%. The resulting alpha particle counting efficiency for the measurement of the Radon and its main daughters is represented by curve 2 in Figure 2. In this case, the alpha particle counting efficiency exceeds 95% up to alpha energy thresholds of about 15% of the maximum energy of alpha particles. Let s emphasize the maximum alpha energy threshold, for a given efficiency, would increase with the diameter of the ionization chamber, because the percentage of maximum energy alpha particles is increasing. The total efficiency, defined as the ratio between the alpha particle counting rate and the total activity of the Radon and its main daughters at equilibrium, is equal to 2/3 of the alpha particle counting rate efficiency, because only 50% of alpha particles emitted by 218 Po and 214 Po are entering the sensitive chamber volume. 2. Experimental arrangement alpha energy threshold (E/E Rn ) The block-scheme of the instrumentation is shown in Figure 3. The analog signal, proportional to the ionization current, delivered by the preamplifier PA, coupled to the ionization chamber, is sampled at a given frequency and converted to a digital signal by the data acquisition system DAS The sampled values are processed in a personal computer PC and the final results are displayed on the PC screen. As preamplifier we used a ICH 8500A integrated scheme and a feedback RC parallel circuit, with a resistance of Ω and a capacitance of pf, equal to the inputoutput capacitance of the integrated scheme.

4 Figure 3. Block-scheme of the instrument IC PA DAS 8001 PC ionization chamber preamplifier data acquisition system personal computer For alpha counting, a sampling frequency of: f s = 20 Hz has been used and the total number of samples was: n = , depending on the measured alpha counting rate value. The alpha peaks exceeding a given alpha energy threshold were counted by a subroutine of the data processing program Output (V) Time (ms) Figure 4. Time response of the electronic scheme 3. Results Having in mind the importance of the dead time as a main parameter in the alpha particle counting, we initially proceed by measuring the time response of the electronic scheme. For this purpose we applied a 2V-step pulse from a generator, through a Ω resistance, to the input of the preamplifier, separated from the ionization chamber. The time response of the electronic scheme is shown in Figure 4. A least square fit with an exponential function lead to a value for the time constant of the electronic scheme equal to: τ = 210 ms. A typical alpha spectrum obtained with the scheme presented in Figure 3 is shown in Figure 5. As should be expected, there are very few pulses of low energy. By using a statistics of about 1500 pulses we obtained a 5% pulses with amplitudes less than the 0.25 pa threshold, corresponding to a 95% efficiency for the alpha counting, in good agreement with the previous estimations.

5 Ionization current time spectrum (Vol. Activ.=50 Bq/m3) I (pa) CHANNELS (80 ms/channel) Figure 5. Experimental ionization current time spectrum 4. Conclusions From the obtained results we may conclude that the proposed scheme is an acceptable alpha counter. Let s take into account the possibility to use such schemes based on ionization chambers as particle detectors for the measurement of the Radon volumic activity. In order to connect the counting rate of alpha particles, emitted by Radon and its progeny, to the Radon activity (actually the volumic activity) the following aspects must be taken into consideration: 1) The detection efficiency of alpha particles emitted by different radionuclides involved in the radioactive chain of Radon. 2) The time dependence of the activities of the involved radionuclides. 3) The relative intensities of the alpha transitions of different energies in the disintegration schemes of these radionuclides. To be more specific let s analyze in the following these three aspects in the case of the 222 Rn isotope, as being the most important for radioprotection applications. The detection efficiency, as we have seen before, is different for the mother 222 Rn and for the daughters 218 Po and 214 Po due to the dynamics of the daughter ions in the ionization chamber. As has been reported in [2], after about 1 hour from the 222 Rn sampling, an equilibrium state, with the 218 Po and 214 Po ions lying on the ionization chamber wall, is reached. In this way, a 50% of the alpha particles emitted by the main daughters are detected, while the alpha particles emitted by the 222 Rn in the whole ionization chamber volume are detected with more than 95% efficiency (see curve 1, in Figure 2). The time dependences of the activities of all radionuclides descending from 222 Rn, calculated by using the chain disintegration equations, are shown in Figure 6. As may be observed in Figure 6 and more detailed in Table 1, the difference between the activity corresponding to the measured alpha counting rate and the ideal activity when all three radionuclides of interest would have the Radon lifetime, is less than 1% after about 3 hours from the Radon sampling (after 2 hours the same difference

6 do not exceed 4%), while the activities of other daughters are negligeable, still 250 hours from the Radon sampling. Thus, a counting rate measurement at 2-3 hours after the Radon sampling satisfies, simultaneously, the radioactive equilibrium as well as the Radon daughters ions dynamics, conditions. Activity (10 log A v ) Rn(α) 218 Po(α) 214 Pb(β) 214 Bi(β) 214 Po(α) 210 Pb(β) 210 Bi(β) 210 Po(α) Time (h) Figure 6. Activity time dependence for all radionuclides descending from 222 Rn As concerns the relative intensities of the alpha transitions of different energies, in the Radon and its daughters disintegration schemes, the alpha disintegrations of the three radionuclides of interest take place predominantly on the ground states of the daughter radionuclides (the most intense satelite alpha line, in the case of 222 Rn, do not exceed 0.1% from the principal alpha line). By summarizing, the pulse ionization chamber method may be used for the Radon activity absolute measurements. The estimated uncertainties are as follows: Table 1. Activity time dependence for the Radon and its main daughters t (hours) A 1( 222 Rn) A 2( 218 Po) A 5( 214 Po) A 8( 210 Po) (x10-6 ) A 1+0.5(A 2+A 5) A/2A alpha detection threshold: 3%; - daughters ions dynamics: 3%; - alpha counting statistics: 4%, with an overall uncertainty of measurement of about 6%. For an arbitrary daughters ions dynamics, by taking into account the limits, represented by full deposition of the daughters ions on the chamber wall and by none deposition on the chamber wall, a 20% daughters ions dynamics uncertainty have to be introduced and this component becomes predominant in the overall uncertainty.

7 Figure 7. Pulsed-mode ionization chamber-type detector (V = 3.5 liter) 1-casing; 2-voltage electrode; 3-lid; 4-flanges with filter and grates; 5-collecting electrodes; 6-wire to collecting electrode; 7-insulator; 8-connecting screw of voltage electrode; 9-wire to voltage electrode; 10-collecting electrode connector; 11-voltage electrode connector; 12-spacers; 13-outer shield; 14- coupling; 15-preamplifier-integrator system; 16-stand. Let s emphasize that a 20% uncertainty is still acceptable for many Radon radioprotection measurements. The experimental data listed above have been obtained with the Pulsed-mode ionization chamber-type detector (V = 3.5 liter) shown in Figure References [1] R. D. Bolton, D. W. McArthur, U. S. Patent No. 5,550,381/ [2] R. Colle, J. M. R. Hutchinson, M. P. Unterweger, The N.I.S.T. Primary Radon-222 Measurement System, Journ. Res. Natl. Inst. Stand. Technol. Vol. 95, No.2, 155 (1990)