Simultaneous Measurement Of Radon And Thoron Exhalation Rate From Soil And Building Materials

Simultaneous measurement of radon and thoron exhalation rate from soil and building materials

C. Cosma, O. Cozar , T. Jurcut, C. Baciu , I. Pop

Babes-Bolyai University, Physics Department, 3400-Cluj-Napoca, Romania

University of Oradea, Sciences Department, 3700, Oradea, Romania

Babes-Bolyai University, Geology Department, 3400-Cluj-Napoca, Romania

Technical University, Physics Department, 3400, Cluj-Napoca, Romania

Our paper presents two methods for simultaneous measurement of radon and thoron exhalation from soil and building materials: (1)-charcoal adsorption, respectively (2)-Lucas cell method.

The both methods are applicable especially in the case of a soil flux enhanced in thoron gas. In the Lucas cell case a short accumulation time was used (10-15 minutes). The thoron concentration under accumulation volume was measured immediately after the sample gas extraction and a regression equation is used for determining equilibrium thoron concentration.

In the case of charcoal method the thoron was measured 4 hours after a special degassing of the sample. Using the LUK-3A device for the building materials, the thoron flux was measured only on a special enhanced thorium+radium sample

1. INTRODUCTION

The radon isotopes 220Rn (thoron) and 222Rn (radon) were intensely studied in the last time due to their involvement in the lung cancer risk [1] and also for geological purposes [2]. The main sources of the indoor radon are the soil radon and radon exhalation from building materials [3]. The active charcoal was often utilized for radon measurement especially for indoor radon [4-5]. The method of the soil radon and exhalation measurement from the soil and building materials using adsorption in active charcoal was also used [6-8] with good results but in this case corrections regarding charcoal humidity related of the break point of charcoal are needed [9]. In all these cases the radon adsorbed in charcoal is measured by gamma spectrometry, commonly with NaI(Tl) detectors. For economical and practical reasons, the charcoal canisters must be degassed and re-used of many times. Commonly, the charcoal cleaning is made by degassing the charcoal at 120-140п‚oC during 12-14 hours, but in the case of high radon content adsorbed in charcoal, for a complete desorption of radon, 2-3 steps are necessary [10]. The method for charcoal degassing proposed in this work has the advantage to be more rapid (10-15 minutes) and it produces a much better degassing. On the other hand, the manner of degassing allows the thoron determination from charcoal even in the case when it is measured by low gamma spectrometry using NaI (Tl) detectors. Generally, the measurement methods for thoron determination require either alpha or high gamma resolution spectroscopy.

This work presents the method for simultaneous measurement of radon and thoron from soil using charcoal adsorption. Exhalation of radon from some Romanian buildings materials obtained by charcoal adsorption method was verified in several cases by Lucas cell method. In the case of one sample the result was compared with those obtained in the frame of ERRICCA Intercomparison Exercise from Athens in April 1999 [11]. Finally, the both radon and thoron exhalation from a special concrete sample made in our laboratory, artificially enhanced in 226Ra and also in 224Ra(Th[NO3]2) was made.

2. EXPERIMENTAL METHODS

A brass cylindrical vessel of 0,7 L, Fig. 1, was used for charcoal degassing. In the upper part it is connected to a vacuum pump. After filling (250 g of charcoal) and closing using six screws and a lead fitting, the vessel is put into another larger vessel with water. The later is placed on an electric boiling. When reaching the water boiling temperature, the pump is started for 15 minutes. The charcoal is then pulled out and can be used for radon-thoron adsorption in the next exposure.

Measuring the charcoal radioactivity after this operation, the control of degassing efficiency is made. After about 3,5 hours from the end of the degassing process, the gamma radioactivity is

Figure 2. The device for radon

Figure 1. The brass device for radon degassing flux gathering

reduced to the background radioactivity of the charcoal. For radon exhalation gathered from soil was used the device from Fig. 2, which consists from a frame box of 0.23 m2 (37x62 cm), and about 200g degassed charcoal. After exposure (as a rule, 4-12 hours) the charcoal is introduced into the Marinelli vessel and gamma radioactivity is measured over the whole spectrum with an NP424 gamma spectrometer equipped with a large NaI(Tl) detector (76x45 mm) enclosed in a lead castle of 30 mm wall thickness to reduce the background. In the case of thoron absence from the charcoal, the radon exhalation rate пЃ†Rn can be calculated following equation:

(Bq/m2s) (1)

where C=1.412*10-3 is the calibration factor, A0-counts per second under integral spectrum (E0>60keV); te- exposure time (usually 4-12 hours); S=0.23m2 the collecting area; T =3.825 days is the half-life time of radon and tc the time interval between the middle of the exposure time and the beginning of the measurement. A time interval longer as 3.5 hours must be considered between the end of sampling and the start of measurement.

In the case of an important thoron exhalation that accompanies the radon flux, this equation can be used only if tc is 5-6 times greater than the half time of thoron descendants (10.65h) that is about two days. In this time the charcoal from Marinelli vessel must be very well sealed so that the radon must to rest in this container. The thoron exhalation can be determined if the exposed charcoal is firstly degassed as above using the device from Fig.1 and the sample is gamma measured after 4 hours when the radon progeny decreased of about 500 times. Fig.3 represents such experiment. Therefore the initial sample must be divided in two: one for radon and other for thoron measurement.

Figure 3. Thoron progeny decreasing four hours after the exposed sample was degassed.

(T1/2 = tgпЃЎ = 11.3