Chapter 2 MATERIALS, INSTRUMENTS AND TECHNIQUES

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1 Chapter 2 MATERIALS, INSTRUMENTS AND TECHNIQUES 2.1 Introduction In the previous chapter environmental radioactivity has been discussed in atmospheric environment, water, and soil. The origin and role of Radon is also discussed in outer environment and indoor dwellings. How the metrological parameters affect the cause of radioactivity is also briefly described. In this chapter the stress is on the materials and instruments used in the present study. In order to study the applications of radon and environmental radioactivity detection, various types of materials and instruments have been used. A brief description of these materials and instruments is given below: 2.2 Plastic detectors Different polymers permit the detection not only of highly ionizing particles, but also of fast protons with energies up to several MeV and above a threshold of few tenths of an MeV (Cartwright et al., 1978) therefore they are classified to be more sensitive than inorganic detectors. The aqueous solution of KOH and NaOH are commonly used as track etchants for plastic detectors in all research laboratories engaged in SSNTD work (Fleischer et al., 1975). The following types of plastic detectors are employed in the present studies: Cellulose nitrate plastic detector The cellulose nitrate plastic detector, commercially known as LR-115 type II, manufactured by Kodak-Pathe, France is used for the alpha particle detection. It is a film made of clear polyester base 100µm thick on which a red layer of special cellulose nitrate of thickness 11.5 to 12.0µm is coated. The general formula of Cellulose dinitrate is well known and is shown below in Fig The most important property of the LR-115 type II is its alpha track registration only within a window of energy, typically from around 1.7 to 4.8 MeV (Jonsson, 1981; Abu-Jarad et al., 1980). 25

2 Accordingly, the plate out of radon daughters on the surface of LR-115 will not register because of their alpha energies (6.0 and 7.68 MeV from 218 Po and 214 Po, respectively) more than its upper threshold energy. The detection efficiency as determined by Nakahara et al. (1980), Damkjaer (1986) and Ramola et al. (1987) is about 50% for energies between 1.5 MeV and 4.8 MeV at normal incidence. These alpha sensitive plastic track detectors have been used for the measurement of integrated indoor radon concentration, radon exhalation rate and radium content in geological samples viz. soil and rocks. 2.3 Hot bath for Etching the recorded samples For enlargement of recorded tracks on above said detectors, chemical etching is required. This etching process enables the visibility of formed tracks due to alpha particle collision under the optical microscope. The etching of samples was carried out in a constant temperature bath (Fig. 2.2). 2.4 Optical microscope A Binocular microscope procured from Carl Zeiss (Fig. 2.3) was used to scan the chemically etched tracks in the samples. Using different combinations of objectives and eyepieces of various magnifications up to 1000 X can be made. It has motion along three mutually perpendicular directions. This microscope can be used in any stage of phase contrast, colour contrast and in dark and bright fields. It has also the facility for its operation in the reflection mode. The resolution of this microscope is 1µm. 26

3 CH2 (ONO2) (ONO2) (ONO2) O O (ONO2) O O CH2OH O Fig. 2.1: The structural formula of Cellulose Nitrate (LR-115) Plastic Track Detector 27

4 Fig. 2.2: Hot Bath for Etching LR Films 28

5 Fig. 2.3: Optical Microscope 29

6 2.5 Gamma- Scintillometer For the measurement of gamma ray activity in the fieldwork, scintillometer (Fig. 2.4) manufactured by electronic corporation of India ltd, Hyderabad was used. This instrument is lightweight portable radiation survey meter featuring solid-state design. This is ideally suitable for radiometric geophysical and environmental reconnaissance survey. The detector element is sodium iodide [nai(tl)], crystal optically coupled to the photomultiplier tube (pmt) with silicon oil as the coupling medium. The crystal photomultiplier assembly is rendered light tight. A 4.5v battery used for the eht supply to the photomultiplier tube. For operation, it has only one control with three positions denoted by low, off and high. The scale is calibrated in µrhr Techniques used to investigate/ estimation Uranium concentration Following techniques are used for uranium estimation depending upon the nature of the sample Laser flourimetery Fluorescence is a phenomenon in which many substances absorb light of one wavelength and in its place emit light of another wavelength. A visible flourescence occurs upon illumination by a mercury arc when the arc is used with a filter which transmits only ultraviolet radiation. For quantitative purpose the intensity of fluorescence is measured and compared with a solution of known concentration. Relatively strong ++ fluorescence in solutions is shown by uranyl compounds (containing the cation UO 2 ), but not by uranous or other uranium compounds. U 6+ also shows fluorescence in sodium fluoride and this behavior forms the basis of a widely used method for determination of uranium. 30

7 The flourimetric technique was established in its present form by Abbey (1964) for the determination of traces of uranium up to a level of 10 ppm. The sample in a nitric acid solution after complete decomposition by combination of HF or HNO 3 acid treatment and Na 2 CO 3 fusion, is extracted with ethyl acetate in the presence of high concentration of aluminum nitrate to separate and concentrate uranium. The latter is then transformed to an aqueous phase and an aliquot is fused with sodium fluoride under controlled conditions.the fluorescence of the solidified bead and that of standard and a blank treated in a similar way is measured with fluorimeter (Fig. 2.5). A calibration curve of fluorescence intensity versus concentration is established. From the calibration curve the amount of constituents in the sample is obtained Mass spectrometry (ICP-MS) In this technique different ions are separated according to their masses by the application of suitable electric and magnetic fields. Mass Spectrometer (Fig. 2.6) consists of an ion source, an analyser of ions and an ion detector.the whole system is enclosed in the vacuum. A gas or vapor is ionized in an evacuated chamber called ionization chamber. A discharge produced by a pulse radio-frequency potential between two conducting electrode rods of the sample materials, results in localized heating, vaporization and ionization of the solid. Electrons from a heater filament can ionize the atoms of a gas and then the gas beam is split according to the mass values of the nuclei.a combination of cylindrical electric and magnetic fields is found to be very effective in focusing ions of same mass at one point irrespective of the variations of their initial energy and direction. The mass spectra are recorded on special type of photographic plates. The darkening of the plate is related to the number of ions reaching the plate. For the measurement of intensity of lines on the photographic plate a microdensitometer is used. 2.7 Radium Measurement Techniques Radium along with uranium and thorium is found in natural materials (soil, rocks, plants and water etc.) and man-made products (construction materials, industrial wastes etc.) at very different levels of activity. The lowest limit of its detectability is obviously determined by the background noise and registration sensitivity of the method used. 31

8 Fig 2.4: Gamma-Scintillometer 32

9 Fig 2.5 Laser flourimetery 33

10 Fig. 2.6: Mass Spectrometer 34

11 In majority of the natural materials the alpha activity can reach an equilibrium concentration. Radium measurements can be done using gamma ray spectrometry and radon alpha method Gamma ray spectrometry Gamma ray spectrometry (Fig. 2.7) is the most widely used technique for the analysis of uranium, thorium, potassium, radium and cesium (Rao, 1974; Menon et al., 1982; Singh et al., 1986a; Singh et al., 2005c). The gamma ray radiations emitted from different radioactive nuclei are characterized by the spectral distribution. In geochemical studies the spectra are scanned by using a HPGe detector coupled to a photomultiplier. Gamma rays from a powdered sample filled in a plastic container placed on the detector, passing through the crystal produce scintillations which are converted into electric pulses by the application of high voltage of the phototube. After amplification they are fed to a single channel analyzer. In order to minimize the background, the sample is shielded by lead walls. The number of counts in a suitable window centered around the gamma line energy of interest is determined. The contents can be determined after evaluating a set of calibration constants for sample with known concentration of uranium, thorium, potassium, radium and cesium with the geometry of measurement of sample and the standard being the same. The gamma ray lines of 1.46, 1.76 and 2.62 MeV are generally employed for potassium, radium and thorium analysis. 2.8 Radon measurement techniques Because of its unique physical and nuclear properties, radon can easily be measured by numerous techniques. The techniques of radon measurement may be divided into two categories: 1. Instantaneous radon measurement techniques 2. Time integrated radon measurement techniques 35

12 2.8.1 Alpha-logger technique By connecting The Alpha-Guard PQ2000PRO, Alpha-Pump and a modified STITZ-soil gas probe, a complete soil gas measuring system was created. The system allows spot measurements as well as continuous soil gas monitoring. For exact soil gas measurements where also high concentration gradients shall be portrayed correctly, the Alpha-Guard (Fig. 2.8) has to be set to the flow through mode. Similarly RAD 7 (Fig. 2.9) is used to measure radon from water samples Time integrated radon measurement techniques Time integrated techniques involve the accumulation of radon over longer time periods from a few days to a week or more. In these techniques the radon is measured directly by detecting the alpha emission or indirectly by detecting the radioactive decay products of radon. A brief description of these techniques is given below: Track etch technique Solid State Nuclear Track Detectors have been used for a long time for radon measurements. The principle of detection is that every detectable alpha particle produces in a SSNTD a single trial of damage which, after chemical enlargement, turns into a narrow channel and is made visible under the microscope. Alpha track etch technique is one of the most widely used technique for radon detection (Fleischer, 1980; Durrani and Bull, 1987; Tommasino et al., 1992; Bhagwat, 1993; Ramola et al., 1998; Singh et al., 2001a, 2002, 2004, 2005a,b). The advantage of the track etch technique is its simplicity, low cost and feasibility in extensive radon surveys. The SSNTDs also have the advantage to be mostly unaffected by humidity, low temperatures, moderate heating and light. They present unique characteristics for long term integrating measurements of radon gas for large-scale surveys. The main demerit of this method is its poor sensitivity for integrated periods less than one month. In studies of indoor radon, monitoring periods of 3 months or longer are adequate. Measurements made quarterly with track etch detectors covering each season of the year are practical for the assessment of the average exposure of the occupants of the buildings. 36

13 Fig. 2.7: Gamma ray spectrometer 37

14 Fig. 2.8: Alpha Guard 38

15 Fig. 2.9: RAD 7 39

16 Water Quality Kit Water quality kit (Fig. 2.10) have been used for determination of TDS values of water samples. It is manufactured by Naina Solaries Limited, New Delhi. Its TDS measurement range in 0 to 200 mgl -1 in 5 ranges, resolution is 0.1 ppm. It is calibrated with standard solution before dispatch so it does not required daily calibration. It is calibrated only when the instrument is continuously off for more than 150 hours. 40

17 Fig 2.10: Water quality kit 41

Radionuclide Imaging MII Detection of Nuclear Emission

Radionuclide Imaging MII Detection of Nuclear Emission Radionuclide Imaging MII 3073 Detection of Nuclear Emission Nuclear radiation detectors Detectors that are commonly used in nuclear medicine: 1. Gas-filled detectors 2. Scintillation detectors 3. Semiconductor