Practicality of the thoron calibration chamber system at NIRS, Japan

Regulation and control methods for the unattached fraction of 220Rn progeny in a 220Rn progeny chamber

The unattached fraction of ²²⁰Rn progeny is one of the key parameters in lung dose evaluation models. To study the behavior of unattached ²²⁰Rn progeny and to prepare for calibration of the unattached ²²⁰Rn progeny monitors, a ²²⁰Rn progeny chamber that could stably regulate different unattached fractions of ²²⁰Rn progeny was established. A recirculation loop to eliminate the effects of the sampling and a loop connected to the dilution chamber to supplement the carrier aerosol were set up. Also, a theoretical model regarding the regulation of the unattached fraction of ²²⁰Rn progeny was established. Continuous measurements of the unattached fraction were conducted using the combination of a single-layer wire screen and a filter membrane. The experimental results showed that, under different aerosol levels, the relative standard deviation (RSD) of ²¹²Pb concentration during the continuous samplings was within 10.7% and the RSD of the unattached fraction of ²¹²Pb was within 9.9%. The relative deviation between the theoretical and the experimental values of ²¹²Pb concentration was within 16.7% and the model calculation values of the unattached fraction of ²¹²Pb were consistent with measured results.

Experimental facility for the production of reference atmosphere of radioactive gases (Rn, Xe, Kr, and H isotopes)

Radioactive gases are of great interest for environmental measurements and can be distinguished in two categories. The natural radionuclides such as the isotopes of radon (²²²Rn and ²²⁰Rn), and the anthropogenic radionuclides coming from fission products (isotopes of Xe and ⁸⁵Kr) and activation products (³H and ³⁷Ar). Gas monitoring in the environment is an important issue for radioprotection and for the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which both require metrological traceability of these gases. For this purpose, two gas chambers, of 42 L and 125 L, have been conceived and built at the LNE-LNHB to produce reference atmospheres of various gas mixtures. These chambers were created in order to provide any radioactive gas atmosphere with a wide range of activity concentrations (Bq·m^-3 to MBq·m^-3). The goal of this setup is to be representative of the different environmental conditions for detector qualification and to perform studies of radioactive gas absorption in materials of interest. As a result, the 2 chambers used in this experimental facility are designed to work from vacuum pressure to atmospheric pressure, with a constant activity concentration for any radioactive gas, and under dry to high humidity conditions. It can also be used in a static mode, in which the activity concentration will follow the radioactive decay of the gas. In this paper, the characterization of the chambers will be discussed. These two chambers are combined with different primary standards established by the LNE-LNHB. As the production of the reference atmosphere depends on the primary standard method, we present the details for each atmosphere production, which require a well-known volume, pressure or a direct activity concentration measurement.

Preparation and characterisation of ceramic-based thoron sources for thoron calibration chamber

The aim of this study is to explore the correlations between the properties of the source's material and the thoron flux produced. This means a complex procedure that involves morphological characterisation (the determination of specific surface area and pore size distribution) and thoron emanation and exhalation measurements as well. In this work, the preparation of 27 thoron sources has been carried out. Three types of ceramics with different morphological properties were used as a matrix material with three different thorium contents. Spheres were formed from the dollop, and they were fired at different temperatures (200, 600 and 900°C). The phase analysis of the samples was performed by powder X-ray diffraction. The pore size distribution was determined by mercury penetration. The thoron emanation was measured using an accumulation chamber; the measured thoron emanation coefficients were from 0.34 ± 0.03 to 7.69 ± 0.13 %. Based on the results, the preparation parameters of the thoron source optimised for the calibration procedure have been given.

222Rn+220Rn monitoring by alpha spectrometry

Controlled 222Rn+220Rn mixed atmospheres have been realised introducing calibrated sources in a stainless steel chamber. An electrostatic alpha monitor internal to the chamber has been used for an accurate discrimination of alpha peaks due to the products of the two isotopes. In the chamber, different specific activities are achieved in order to test the response of the internal reference instrument and to evaluate the possible interferences due to contemporary presence of both radon isotopes. Results show that: (i) the atmospheres are very stable, (ii) the monitor is adequate for their control because the various alpha lines are well evaluated and (iii) using Tyvek® filter, the efficiency of monitor is stable and constant vs. activity.

A method to simultaneously and continuously measure the 222Rn and 220Rn exhalation rates of soil in an open loop

This paper presents a process in which a radon monitor based on the electrostatic collection method is used to measure the (222)Rn and (220)Rn exhalation rates simultaneously and continuously employing a ventilation-type accumulation chamber. Generally, the radon exhalation rate can be measured by accumulation technique, but cannot be measured continuously. The advantage of this method using a ventilation-type accumulation chamber is that the radon exhalation rates can be measured continuously. Even though the environmental air is drawn into the chamber, the low atmospheric values of radon and thoron do not influence the measurement accuracy. The (222)Rn and (220)Rn exhalation rates error from the environmental air is less than 5% in this experiment.

Solid thoron source preparation in a porous mineral matrix

Thoron and progeny are decay products of (232)Th with a great impact on human health. The release of thoron gas from the mining and milling of thorite, monazite and other major thorium ores has been recognised as a potential radiological health hazard. For precise measurements, calibration is a very important factor. This paper describes a cheap and easy way of producing a stable thoron source made of thorium nitrate packed in a porous clay mineral matrix used as (220)Rn generator. The source should have a small spherical shape and be fired at 600°C; this will lead to a great pore volume, necessary for the thoron gas. High importance should be given to the water uptake. The exhalation power of (220)Rn was measured using a Lucas scintillation cell. Experimental efficiency values obtained ranged between 0.16 and 1.44 %.

Characteristic and performance of a simple thoron chamber

For calibration and intercomparison experiments, a thoron chamber with an inner volume of 300 l was designed based on a programmable constant temperature and humidity testing device in this work. The commercial lantern mantles enriched with (232)Th were used as the (220)Rn source and the mantles were set in 3×3×3 points of lattice style inside the chamber. Experimental studies showed that (220)Rn concentrations in the chamber could be easily controlled and adjusted from about 0.5 to 80 kBq m(-3) through manual settings of the relative humidity and temperature, and the spatial distribution of (220)Rn in the chamber was fairly homogeneous.

Generation and control of thoron emanated from lantern mantles

This paper describes the performance of a thoron ((220)Rn) flowthrough source made of a commercially available lantern mantle. This (220)Rn source is easy to construct and operate and has a negligible radon ((222)Rn) gas generated when the air was passed through the source. We studied the (220)Rn concentration generated from the lantern mantles in terms of the variability in concentration associated with both the total weight of the lantern mantles used in the source and the air flow rate used in the experimental chamber. We found that the concentration of (220)Rn generated in air ranged from 0.9 to 150 kBq m(-3) and exponentially depended on the absolute humidity. The (220)Rn concentration increased linearly with increasing total weight of the lantern mantles, but variations in the rate of air flow passed through the source had no influence on the observed (220)Rn concentration.