Low Radon Cleanroom for Underground Laboratories.

Aim of a low radon cleanroom technology is to minimize at the same time radon, radon decay products concentration and aerosol concentration and to minimize deposition of radon decay products on the surfaces. The technology placed in a deep underground laboratory such as LSM Modane with suppressed muon flux and shielded against external gamma radiation and neutrons provides "Zero dose" space for basic research in radiobiology (validity of the LNT hypothesis for very low doses) and for the fabrication of nanoelectronic circuits to avoid undesirable "single event effects." Two prototypes of a low radon cleanroom were built with the aim to achieve radon concentration lower than 100 mBq·m3 in an interior space where only radon-free air is delivered into the cleanroom technology from a radon trapping facility. The first prototype, built in the laboratory of SÚRO Prague, is equipped with a standard filter-ventilation system on the top of the cleanroom with improved leakproofness. In an experiment, radon concentration of some 50 mBq·m-3 was achieved with the filter-ventilation system switched out. However, it was not possible to seal the system of pipes and fans against negative-pressure air leakage into the cleanroom during a high volume ventilation with the rate of 3,500 m3·h-1. From that reason more sophisticated second prototype of the cleanroom designed in the LSM Modane uses the filter-ventilation system which is completely covered in a further improved leakproof sealed metal box placed on the top of the cleanroom. Preliminary experiments carried out in the SÚRO cleanroom with a high radon activity injection and intensive filter-ventilation (corresponding to room filtration rate every 13 s) showed extremely low radon decay products equilibrium factor of 0.002, the majority of activity being in the form of an "unattached fraction" (nanoparticles) of 218Po and a surface deposition rate of some 0.05 mBq·m-2·s-1 per Bq·m-3. Radon exhalation from persons may affect the radon concentration in a low radon interior space. Balance and time course of the radon exhalation from the human body is therefore discussed for persons that are about to enter the cleanroom.

Emission probability measurement of γ- and X- rays in Ra-226 and Pb-210 decay.

The emission intensities of 186.21keV, 46.54keV, XL(Bi) and XK(Rn) photons were measured with calibrated HPGe detectors. The efficiency was computed with the MCNP Monte Carlo code and validated using experimental points. Sources of Ra-226 were prepared from two different radioactive materials. The X-ray emission intensities in Ra-226 decay were determined without X-rays from the daughter decay products using a radon emanation source. The standard CMI source with Pb-210 was used and compared with a source prepared from the NPL solution delivered under the EMRP project "Metrology for processing materials with high natural radioactivity - MetroNORM".

Activity standardisation of 210Pb by 4πα liquid scintillation counting method.

The activity of 210Pb in a solution with 210Pb-210Po in the radioactive equilibrium was determined through its decay product 210Po by liquid scintillation counting (LSC), which has, after separation, as a pure α emitting nuclide, detection efficiency practically equal to 1. For the separation of 210Po from 210Pb solution, two methods based on precipitation of Pb, and Pb with Bi, leaving Po in the solution, were introduced. The first one was precipitation of Pb in the form of Pb(NO3)2 from a mixture of acetic acid, toluene, water and HNO3. The second one was based on co-precipitation of Bi and Pb with KCl from a mixture of organic solvents, where 210Bi with 210Pb was fixed to the precipitate. The relative standard uncertainty of 210Pb activity concentration was estimated to be lower than 0.45%.

Preparation of (228)Ra standard solution.

For the preparation of a standard solution of (228)Ra, (228)Ra was isolated from (232)Th salt. Two simple methods were developed for Th-Ra separation. Both are based on a very good solubility of thorium nitrate in organic solvents. The first one used Ra co-precipitation with Pb in the form of Pb(NO3)2 from acetic acid solution. The second method was based on solvent extraction, remaining Th in the organic phase, while Ra was concentrated in the aqueous phase. The activity of (228)Ra (up to 20kBq) in the standard solution was related to the (232)Th standard by means of gamma ray spectrometry measurement. The obtained uncertainty was less than 0.7% (k=1). The standard solution was free of (232)Th and contained the carrier in the usual concentration (1gL(-1) BaCl2, 10gL(-1) HCl).

Standardisation of 64Cu using a software coincidence counting system.

The activity of the radionuclide (64)Cu was determined by the efficiency extrapolation method applied to 4π(PC)-γ coincidence counting. The standardisation was performed by software coincidence counting-a digital method for primary activity measurement that simplifies the setting of optimal coincidence parameters. The γ-ray-energy window, characterised by identical gamma detection efficiency related to the sum of EC and to the sum of beta decay branches, was found. This setting ensured a linear and zero slope extrapolation curve.

Activity standardisation of ²²6Ra by 4πα liquid scintillation counting method.

Activity of (226)Ra in radium daughter products free solution was determined by 4πα liquid scintillation counting (LSC) method, where the detection efficiency of radium decay is practically equal to 1. The sources were prepared from solution with known (226)Ra mass concentration, from which, immediately before LS counting, (222)Rn and its daughter nuclides were removed by solvent extraction. LS counting results were corrected practically only for a <0.6% loss of radium from measured sample and for the ingrowth of (222)Rn and (218)Po concentrations in the sample after the separation was completed. The combined relative standard uncertainty was estimated to be lower than 0.34%.

Improvement of software coincidence counting system for standardisation of EC-ß(+) radionuclides.

Since 2000 the software coincidence counting (SCC) system has been used for activity standardisation of about 15 radionuclides. Their activities were determined by the efficiency extrapolation method applied to 4π (PC)-γ coincidence. Some standardisations, mainly standardisations of EC-ß(+) radionuclides, required optimal setting of coincidence parameters, for which new procedures based on "the coincidence to total PC count ratios", "a multiple gamma window combined from subwindows fractions" and "a source test for PC detection efficiency " were introduced. The paper summarises the development of SCC system related to findings from detailed analyses of recorded data. These findings are not practically achievable by a conventional approach.

Standardisation of 124SB and 152EU using software coincidence counting system.

Activities of the radionuclides (124)Sb and (152)Eu were determined by the efficiency extrapolation method applied to 4pi(PC)-gamma coincidence counting. The (124)Sb sources were prepared from a solution with the chemical form of 50 microg g(-1) SbCl(3) in 2 M HCl. To inhibit the volatility of antimony chlorides, the sources were slowly dried in a H(2)S atmosphere with relative humidity of 76% for about 48 h. This procedure increased the beta detection efficiency up to 0.98, which simplified the standardisation. In the (152)Eu standardisation, the optimal gamma-ray energy window setting to achieve a linear dependency and the correct slope of the extrapolation curve were derived by means of software coincidence counting system using offline evaluation of data with different coincidence parameter settings. The results obtained by the software coincidence counting system were compared with those obtained by the conventional coincidence method.

Radon-in-water standard.

The radon-in-water standard installed at the Czech Metrology Institute in 1994 is based on a generator producing radium-free radon solution and on a measurement system for generator calibration and stability checking. The generator consists of about 6L cylindrical vessel with a solid phase (222)Rn source with 99.9% air emanation power and an external circuit for solution homogenisation. Standard solutions are prepared by charging water in the vessel with radon for an appropriate period; radon volume activity of the solution may vary from 300 to 2000 Bq/L; its uncertainty is less than 2.5%.

Standardisation of (56)Co and (57)Co using software coincidence counting system.

Activities of the radionuclides (56)Co and (57)Co were determined by the efficiency extrapolation method applied to 4pi(PC)-gamma coincidence counting. Solutions of (56)Co usually contain a significant amount of (57)Co and (58)Co, so the measured activity of (56)Co requires correction. When the conventional coincidence method is used for (56)Co standardisation, the corrections are derived from the dependence of Proportional Counter (PC) detection efficiencies for (57)Co and (56)Co measured using sources with mixture of (56)Co and (57)Co, which is complicated. These difficulties were reduced by means of a software coincidence method, with a HPGe detector in gamma channel, where the detection efficiencies were evaluated directly from the records of coincidence measurements of standardised sources. In the case of (57)Co standardisation, the software coincidence counting system was applied for the evaluation of optimal setting of coincidence parameters. The results obtained by software coincidence counting system were compared with those obtained by the conventional coincidence method.

Monte Carlo calculations of calibration and geometry correction factors of a radionuclide calibrator.

The activity of radioactive pharmaceuticals administered to patients in nuclear medicine is usually determined using well-type high-pressure ionization chambers. For the Bqmeter chamber (Consortium BQM, Czech Republic) a Monte Carlo model was created using the MCNP4C2 code. Basic chamber characteristics for two sample containers of various geometry (a vial and an ampoule) were calculated and compared with measurements. As the pharmaceuticals are often measured in various syringes, the chamber response for samples in syringes was also studied.

Standardisation of 54Mn and 65Zn using a software coincidence counting system.

The activities of 54Mn and 65Zn have been determined by 4pi(PC)-gamma coincidence counting, with efficiency variation performed by the conventional method of altering the self-absorption in the sources as well as by the computer discrimination method. The standardisation of 65Zn presents some complications requiring optimisation of the gamma-ray energy window settings to achieve a linear efficiency-extrapolation curve. Determination of these optimal settings by the conventional coincidence method is a tedious task. These difficulties have been reduced by the utilisation of a software coincidence counting system that records time and amplitude information of individual pulses from coincidence measurements, where the coincidence parameters are set after the data collection process has completed, facilitating multiple data evaluations on a single data set. The optimal gamma-ray energy window settings for the 65Zn standardisation were derived from the results of the 54Mn standardisation, as well as from studies of the 65Zn data itself. The setting of the PC channel thresholds for K and both (K+L) electrons is also discussed. The results are compared with those attained using conventional coincidence counting.

Application of pulse mixing method in software coincidence counting.

In recent years the software coincidence counting system, designed for absolute activity measurement, has been developed in the Czech Metrology Institute. In this system a true coincidence count rate is calculated from the records of time and amplitude data of individual pulses and may be determined by two different methods. The first one uses a coincidence resolving time, in a manner similar to a classical coincidence measurement. The second method applies the pulse mixing method formulae, so that it does not use the resolving time and the correction for accidental coincidences. Both methods have been tested on the same data from a 4pibeta (PC)-gamma coincidence measurement of 60Co sources. The difference between the results obtained from both calculation methods applied on the data from the same measurement did not exceed 0.015%. The details of both methods and the results of their comparison are presented.

Software coincidence counting.

A system designed for absolute activity measurement is described using a digital method. The system is based on data recording from a coincidence measurement and subsequent software processing of the data records. The data acquisition device collects amplitudes of individual pulses from analogue-to-digital converters and supplies them with time information. Software processing of data records from this system offers many benefits in comparison to conventional coincidence counting, for example it enables to perform time and pulse height analysis and setting of coincidence parameters by using a wide variety of evaluation methods to one data record. The digital system was tested with a 4pi beta-gamma coincidence detectors arrangement consisting of a proportional counter and two NaI(Tl) scintillation detectors. The results obtained with a 60Co source are presented.