CHAPTER 3
EXPERIMENTAL TECHNIQUES
47
CHAPTER 3
EXPERIMENTAL TECHNIQUES
3.1
Experimental Techniques for Measurement of Radon, Thoron and Progeny
A number of different techniques have been established by various researchers for
measuring the concentration of radon, thoron and their progeny as air pollutants in
dwellings and radon exhalation rates. These techniques are basically based on the
detection of emissions from radioactive decay of radon and its daughter products as a
result of radium- thorium decay chain. Most of these techniques are based on the
detection of alpha particles whereas, some are based upon the detection of gamma
emission and others utilize beta decay. In case of progeny the individual decay product
concentration or Potential Alpha Energy Concentration (PAEC) is measured. Some
measuring the short-term values are called active methods while others measuring the
integrated values are called passive methods. Due to low concentrations of radon and its
decay products in the environment, the precision and accuracy and detection efficiency of
the techniques used are of great importance. Most of the commonly used instruments/
methods for the measurement of radon concentration in the atmosphere are the ionization
chambers, scintillation cells, two-filter method, electrostatic collection method. But for
the study of diurnal variation of radon and its progeny concentrations in the atmosphere,
the sampling duration should be very small so that the activity could be determined
immediately (Nagaraja et al., 2006). For this purpose, the active devices are most helpful.
Some of the techniques required to measure radon, thoron, and their progeny in the
environment along with study of radon exhalation rates for some material samples have
been discussed here in brief as given below:
3.1.1 Techniques For Instantaneous Measurement of Radon
3.1.1.1 Rad7
The RAD7 was designed to detect alpha particles only. The DURRIDGE RAD7
uses a solid state alpha detector. A solid state detector is a semiconductor material
(usually silicon) that converts alpha radiation directly to an electrical signal. One
important advantage of solid state devices is ruggedness. Another advantage is the ability
to electronically determine the energy of each alpha particle. This makes it possible to tell
48
exactly which isotope (polonium-218, polonium-214, etc.) produced the radiation, so that
one can immediately distinguish old radon from new radon, radon from thoron, and signal
from noise. This technique, known as alpha spectrometry, is a tremendous advantage in
sniffing, or grabs sampling, applications. RAD7 measures radon gas concentration. Radon
daughters do not have any effect on the measurement. The RAD7 pulls samples of air
through a fine inlet filter, which excludes the progeny, into a chamber for analysis. The
radon in the RAD7 chamber decays, producing detectable alpha emitting progeny,
particularly the polonium isotopes. Though the RAD7 detects progeny radiation
internally, the only measurement it makes is of radon gas concentration.
3.1.1.2 Grab Sampling Technique
Grab sampling provides the instantaneous measurement of radon and its progeny
in air. In this technique, the air sample of radon is taken in a container for analysis (Lucas,
1957; George, 1976). Using this technique, the concentration of radon daughter products
in the dwellings is measured by collecting air- borne particulate on the filter paper for a
period of 2 to 10 minutes (Kusnetz, 1956; Raabe and Wrenn, 1969; Thomas, 1972; Rolle,
1972). The filter paper sample is obtained by sucking air through a 2.5 cm diameter
millipore filter type AA of 0.8 m pore size at the suction rate of 20 l/m for a time period
ranging from half an hour to one hour. Keeping in view the short half-lives of the radon
decay products, the analysis should be performed strictly within one hour after sampling.
The filter paper sample is counted using a ZnS (Ag) detector system directly coupled to a
nuclear counting system, which is able to record the counts due to disintegration of the
progeny of radon collected on the filter paper. Because of the simplicity, low cost and
minimal labour requirement, this technique is suitable for large scale surveys.
Disadvantage of grab sampling technique is that it does not give accurate information on
time averaged air concentration in dwellings. Since the values measured here fluctuate
widely depending on various factors, therefore, grab sampling techniques are used in
industrial monitoring (NCRP, 1988).
49
3.1.1.3 Ionization Chamber Technique
In this technique, there is a gas filled electrode system designed to detect the
presence of an ionizing particle (Wrenn et al., 1975). The ionizing particle creates a pair
of positive and negative ions on passing through the chamber. These ions get attracted in
opposite directions under the influence of an electric field produced on applying a
suitable potential difference. The ionization chamber is operated in the voltage range of
100 to 300 volt, so that the multiplication and recombination of the opposite pair of ions
is negligible. This technique can be used for measuring radon concentration down to 10
Bq m-3 (Pacer and Czarnecki, 1980).
3.1.1.4
Radon Emanometry: Scintillation Cell Technique
A radon emanometer is used to measure the alpha emanation rate from radon in
the gas fraction of a soil or water sample by pumping the gas into a scintillation chamber
using a closed circuit technique (Ghosh and Bhalla, 1966). This emanometer was the
earliest device to measure radon and is still widely used for quick radon surveys
(Alekseevs et al., 1959; Tewari et al; 1968 McCorkell and Card, 1978; Cheng and poritt,
1981; Virk, 1990; Virk and Walia, 2001). Another simple method used for the
measurement of radon concentration and exhalation rates of rocks and building materials
in the laboratory is the use of portable radon monitor named PRASSI. It has a built in
detector which is suitable for continuous or grab sampling measurements for radon gas
with scintillation cell technique. Basically, it consists of 1.83-liter cell coated with
zincsulphide activated with silver coupled to a low gain drift photo multiplier. The
instrument has two operating modes i.e continuous and grab sampling. In the continuous
mode, air is allowed to circulate in the measuring cell at a constant rate that is
electronically regulated at 3 lit/m. This monitor measures the counts in a preset time and
at preset intervals, from which the radon concentration can be determined in Bqm-3 or
pci/l. PRASSI has the ability to compensate for the count rate due to radon daughters
adherent to the cell walls showing that the resulting count rate is strictly proportional to
the concentration of radon gas only. In the grab sampling mode, air is flushed into the
measuring cell for few minutes and it is then closed by two electric valves. After a preset
time taken normally as 2.5h, the process of counting the period is repeated with the
sampled air. The sample air is pre filtered before reaching the measurement chamber and
50
its flow rate is regulated electrically to compensate for filter clog up. The radon
measurement is performed in a closed loop circuit as shown in Fig. 3.1. The exhalation
rates of radon from the samples have been determined by studying the growth of radon
activity in the bottle containing the sample of materials (El-Arabi et al., 2006). The
technique provides very high sensitivity to the system and found to be extremely useful in
the measurement of radon in solids or rocks.
3.1.1.5 Low Level Radon Detection System
For the study of diurnal variation of radon and its progeny concentration in the
atmosphere, the most simple and convenient method in the field measurements is the Low
Level Radon Detection System (LLRDS) (Srivastva et al; 1984; Nagaraja et al., 2006).
This system consists of sample collection chamber of 24 cm diameter and 11.5 cm height,
having a volume of about 5 liters with a provision of air inlet and out let. In this
technique,
LLRDS is evacuated and air is allowed inside the chamber for about 3-4
minutes so that air pressure inside the chamber becomes equal to the atmospheric
pressure. After collection of air sample in the chamber, at least a delay of 10 minute is
allowed for complete decay of thoron which may be present in the chamber. A negative
potential of about 800 V is applied for about 90 minute for saturation of radon daughter
atoms on the collection plate after which it is removed and counted for alpha activity.
3.1.2 Time-Integrated Techniques for Radon, Thoron and their Progeny
Measurement
Time-integrated schemes are passive and 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 either directly by detecting the alpha emission or indirectly by the detection of
the radioactive decay products of radon. Brief description of these techniques is given
below:
3.1.2.1 Charcoal Technique
In this technique, radon can be indirectly measured by determination of radon
decay products present in the sample (Cohen and Cohen, 1983).This method utilizes
activated charcoal to trap the radon which takes place at 18 0C having a typical duration
of about 10 days ( De-Jong et al., 2005). A charcoal sample with dimensions of the order
10cm can be used to collect
222
Rn over a period ranging from a day to a week. The
51
instrument consists of a small container filled with activated charcoal. The concentration
of gamma rays emitted from
214
Pb and
214
Bi, which are daughter products of
222
Rn, are
measured. With a lower limit of 0.2 pCi/l for sixty hour of exposure, this instrument is
suitable for measuring indoor radon levels. This technique is helpful for a period of more
than a week, because after this time period most of the
222
Rn collected at the beginning
will get exhausted due to its half-life period of 3.82 days.
3.1.2.2 Plastic Bag Technique
This is also the time-integrated technique for radon (Sill, 1969). The sampled air
collected for a period of two to three days is pumped into a bag, which is impermeable to
radon gas. For alpha counting, the integrated sample during analysis is transferred to the
scintillation flask. Concentration as low as 0.01pCi/l can be measured using this
technique.
3.1.2.3 Thermoluminescence Detection Technique
This is the phenomenon of emission of light by a substance when it is heated to a
suitable temperature, which can be attributed to the previous exposure to ionizing
radiation. When a crystal is exposed to ionizing radiation, the electron-hole pairs due to
ionization are created and some of which get combined with trapped charges.
Thermoluminiscence detectors used for radon measurement are very thin wafers (76 ms
of calcium sulphate doped with dysprosium in a matrix of teflon), which are mainly
sensitive to alpha radiation (Mc Curdy and Shiager, 1969; Pacer and Czarnecki, 1980).
These detectors are typically exposed for 30 days in the uranium exploration program and
then processed for their thermoluminescence peaks.
52
Fig. 3.1: Closed loop circuit for sample measurement
53
3.1.2.4 Alpha Meter Technique
Alpha meter technique employs a solid-state alpha particle detector along with a
counter assembly. The alpha particle detector is a silicon-diffused junction with an active
area of 400 mm2. When an alpha particle enters the n-p junction, a number of electronhole pairs are generated which are proportional to the energy of alpha particle. The flow
of current is sensed and gets amplified hundreds of times to produce a pulse that may then
be counted. The alpha counts are displayed by the light emitting diode (LED) (Warren,
1977). The other instrumental method of measuring the radon flux includes the alpha
cards with a central collector for recording radon daughters (Card and Bell, 1979; Dyck et
al., 1983). The detector records the alpha particles emanated by radon isotopes and their
alpha emitting progenies. This instrument does not register any alpha particle with energy
less than 1 MeV.
3.1.2.5 Track Etch Technique
Track etch technique is one of the most widely used techniques for radon measurement
(Fleischer et al., 1975; Frank and Benton, 1977; Alter and Fleischer, 1981; Durrani and
Bull, 1987; Bhagwat, 1993; Virk, 1999; Eappen and Mayya, 2004; Kant et al., 2006; Nain
et al., 2008; M Amin et al., 2008; El-Zakla et al., 2008). In this technique, alpha track
film dosimeters, also known as alpha particle track etch detectors, are used. The principle
of detection in this technique consists of the damage imparted to the detector material by
alpha particles from radon and its decay products. The damage imparted to the detector is
observed under optical microscope. Heavily ionizing particles passing through insulating
media leave narrow trails of damage on an atomic scale (~30-100 Å) called as latent
tracks.
With each -particle producing a distinguishable track, these latent tracks can be enlarged
to microscopically visible size by the method of chemical etching or reaction due to the
which both the damaged as well as undamaged portion of the detector get dissolved when
the detector material having the latent tracks, is immersed in a suitable chemical solution.
Usually the damaged portion is etched preferentially and dissolves more rapidly. This is
because of the fact that a fraction of the energy dissipated by the charged particle is stored
in the material which puts the damaged region or material into a higher energy state
where it is more susceptible for chemical reaction to takes place. After the chemical
54
etching the latent tracks are enlarged sufficiently so that they can be viewed under an
optical microscope.
The parameters which influence the track etching can be broadly divided into the
following categories:
(i)
Detector material parameters (Molecular weight, density and chemical
composition)
(ii)
The incident particle parameters (charge, mass and velocity)
(iii)
The etching parameters (Nature of etching solution, concentration, time and
temperatures etc.)
The number of tracks per unit area in the detector is proportional to the average exposure
rate and exposure time. Exposure time can range up to a year or more, if desired, using
improved plastic track detectors which retain alpha tracks without fading for a very long
time at ambient temperatures. Though several detector materials have been developed but
LR-115 and CR-39 are the two most popular track detectors used in radon dosimetery and
named as Solid State Nuclear Track Detectors (SSNTDs).
SSNTDs: When a heavily ionizing nuclear charged particle passing through an
insulating solid, the physical and chemical properties of the solid along the path of the
particle changes and a narrow path of intense radiation damage is created. These paths are
called the latent tracks and these track recording materials are commonly known as solid
state nuclear track detectors.
The exact change in the physical and chemical properties of the damaged region
depends mainly on the incident particle parameter i.e. charge, mass and velocity of the
particle, the nature of the detector material, Pre irradiation storage condition and the
environmental condition like pressure and temperature during the exposure. However the
change in the properties of the damaged region is characterized as follows:
(i)
Reduced Molecular Weight
(ii)
Reduced Physical density
(iii)
Increased Solubility
(iv)
Presence of the new chemical species such as free radicals
55
General Features of SSNTDs:
(i)
Nuclear charged particle produce permanent record of damage tracks in these
detectors.
(ii)
These damage tracks (latent tracks) can be enlarged by suitable chemical etching.
The rate of chemical etching along the particle path depends on the damage
density produced by the charged particles.
(iii)
Relative sensitivity of different SSNTDs is different. Organic detectors such as
polymers are most sensitive whereas the glass and mineral detectors are least
sensitive.
(iv)
Fading of charged particle latent tracks occurs at high temperature.
The advantage of using SSNTDs in track etch technique is their simplicity, low
cost, non-destructive, small size, and having integrating capability for large scale studies
(Bochicchio, 2005). This technique does not require dark room, complicated etching
systems, etc., for processing and also does not involve any electronics (Durani and Ilic,
1997). SSNTDs method also have the advantage of moderately accurate and do not
require standards for their calibration due to which these are good tools for evaluating
alpha and beta activities by thoron and its progeny in mines, caves and building material
factories (Misdaq, 2002; Leung et al., 2007). Track etch technique has an extremely
useful property of recording a large number of events cumulatively over long periods of
time, in the form of tracks resulting from them.
Main drawback of this technique is that we cannot obtain immediate or real time
results but have to wait for a considerable time to get statistically reliable results. In the
studies related with the measurement of radon a monitoring time of at least three months
is required for statistically reliable results. Variability of environmental conditions like
temperature and humidity, pre and post-irradiation storage history of polymeric detectors
and manufacturing procedures such as thickness control, processing treatment of different
batches etc., further add to the problems of precision and reliability (Durrani and Bull,
1987).
56
3.1.2.6 Twin Chamber Dosimeter Technique
This technique is most suitable for simultaneous measurement of radon, thoron,
and their progeny. In this method a twin cup dosimeter developed at Bhaba Atomic
Research Center (BARC), is used (Abdel and Somogy, 1986; Wafaa Arafa, 2002;
Ramachandran et. al., 2003; Eappen and Mayya, 2004).
The twin cup dosimeter
employed for simultaneous measurement of radon, thoron, and their progeny is made up
of a cylindrical twin chamber system, using 12µm thick, LR-115 type II, cellulose nitrate
based SSNTDs, manufactured by Kodak Pathe, France. The membrane mode measures
the radon concentration only as radon alone can diffuse through the membrane while the
glass fiber filter allows radon and thoron both to diffuse into chamber. Also, there is a
provision for bare mode in twin cup dosimeter to measure radon, thoron, and their
progenies in total.
3.2
Techniques Used in Present Study
3.2.1 Solid State Nuclear Track Detectors (SSNTDs)
SSNTDs in track etch technique has been used in the present study due to their
simplicity, low cost, non-destructive, small size, and having integrating capability for
large scale studies for the measurement of radon activity, and radon exhalation rates
studies in various samples.
The SSNTDs can also be used in radiobiological studies as the biological
effectiveness of densely ionizing radiation is of great interest for estimating the radiation
risk for the public resulting from exposure to radon and its daughters. This exposure is
characterized by low doses and, therefore, there are only few traversals of charged
particles through living cells. In order to study such effects invitro, experiments are
performed using cell monolayers that are irradiated with alpha particles of heavier ions of
defined energies. The cells are seeded on a substrate foil and here SSNTDs seem to be
suitable for using as substrate layer. It is because of the fact that the position of the
particle traversal has to be known to determine the cellular effect (Durante et al., 1996).
These have to fulfill specific requirements i.e. these must be very thin such that their
thickness is smaller than the range of ions, allowing the particles to pass across the
detector and hit the cells in the monolayer (Dorschel et al., 2003). Suitable detector
57
material having best registration efficiency for light ions is CR-39 (poly allyl diglycol
carbonate) but it is not available with the small thickness required for this purpose as
discussed above.
LR-115 Detector: Pristine LR-115 (cellulose nitrate, type-II, strippable, procured from
DOSIRAD, France) is alpha sensitive plastic track detector. It is a 12 m thick film red
dyed cellulose nitrate emulsion coated on inert polyester base of 100 m thickness as
shown in Fig. 3.2. It has maximum sensitivity for alpha particles, fission fragments and
ionizing particles with high enough linear energy transfer (LET). It is widely used for
detection and measurement of weak concentrations of ionizing particles, high-resolution
neutron radiographic uses, alpha radiography, cosmic ray investigations etc. The film can
be used to record the tracks of protons with an energy <100KeV and alpha particles with
energy  0.06 to 6 MeV (Tanti-Wipawin, 1975). It is insensitive to X-or - ray, photons,
electrons, high-energy protons and the sensitivity is claimed to be one of the best amongst
any other plastic detectors. For fast neutrons, it has low detection efficiency (10-5
track/neutron) (Khan, 1975). The passage of ionizing radiation through insulating solids
creates narrow trails of intense damage on atomic scale. These trails are called ‘Tracks’
which can be made visible under an ordinary optical microscope on being treated with a
suitable chemical etchant that preferentially attacks the damaged material and removes
the surrounding but undamaged portion at a slow speed (Enge, 1995).
Therefore, the etched tracks get enlarged and represent the sites of original
damaged regions. The track etching mechanism of LR-115 has been studied at different
temperatures ranging from 300C to 600C for different etching times and the calculated
value of activation energy is 0.1845 eV (Paul and Bose, 1980). The recommended etch
conditions given by the manufacturer are 2.5N NaOH, at 600C, 65 to 95 minutes (without
agitation). Another suitable etch condition reported is 2.5 NaOH, 600C, 60 to 70 minutes
with stirring (Costa-Riberio and Labao, 1975). The film has negligible background. Like
other cellulose nitrate plastics, the track recording properties are affected by pre-etch
environment of storage, chemical effects, thermal effects etc. The film provides a better
contrast and ease for counting the tracks when a green light source (by using a Kodak
Wratten Filter No. 40) complementary to the hue of the film is used.
58
Track Formation Mechanism in Organic Detectors
In case of organic detector such as polymers, the atoms are arranged in a chain
like structure. The passage of the charged particle causes breakage of chains as shown in
Fig. 3.3. Thus new chain ends and other chemical reactive sites are formed. These regions
are preferentially attacked by etchants and hence the etched tracks are produced.
Necessary conditions for track formation
(i)
The tracks are formed easily in materials of low mechanical strength, low
dielectric constant and of close inter-atomic spacing.
(ii)
The positive ions produced in the channel should not recombine within 10-13 sec
because approximately so much time is required for the ions to be removed from
the lattice points. This is found only in those substances which have the
conduction electron density ~1020/cm3.Thats why metal cannot record tracks. For
metals conduction electron density ~1022/cm3.
(iii)
Materials of resistivity greater than 2x105 Ωm are known to be capable of
recording and storing the tracks. Materials of lower resistivity apparently do not
have this capability (Fleischer et al., 1965).
(iv)
The detector material must have low ion mobility. This is because of the fact that
the ionized region along the track consist essentially of a high concentration of
positive ions and their movement away from the track might suppress the
permanent track formation.
(v)
Only those particle can register etchable tracks whose energy loss rate exceeds a
critical value (dE/dx)c called the critical energy loss for etchable track formation
in a particular detector material. For different detector materials, The value of
(dE/dx)c will be different. The (dE/dx)c value for plastic is minimum and for
crystal is maximum since plastic can record even protons whereas in crystals the
particles below 18Ar40 cannot produce etchable tracks.
59
Fig. 3.2: (a) LR-115 sample (b) Schematic of a strippable LR-115 detector
Fig. 3.3: Track formation in organic detector
60
3.2.2 Canister Technique
For the measurement of radon concentration and exhalation rates from soil and
sand samples the “Canister Technique” (Abu-jarad, 1988) used by many groups (Khan et
al., 1992; Chauhan and Chakarvarti, 2002; Sonkawade et al., 2008; Mahur et al., 2008;
Gupta et al., 2009; Nain et al., 2010; Gupta et al., 2010) has been adapted. The exhalation
rate is the amount of radon emanated from a given sample per unit mass (for mass
exhalation rate) or per unit surface area (for surface exhalation rate) per unit time. The
materials samples under study from different locations were dried in oven at 110 0C for
24 hours to remove the moisture. The samples were finely powdered and sieved through
100-mesh sieve. A known amount 100g of fine powder of the samples was placed in
different cylindrical canister having dimension similar to that used in calibration process
by Singh et al., (1997).The LR-115 plastic track detector is fixed on the bottom of the lid
of each can with tape such that sensitive side of the detector always faced the specimen.
The can is tightly closed from the top and sealed as shown in Fig. 3.4. At the end
of the exposure time (~100 days), the detectors are removed and subjected to a chemical
etching process in 2.5 N NaOH solution at 60±0.50C for 90 minutes as shown in Fig. 3.5.
The etched detectors are thoroughly washed and then immediately after the
completion of washing, the red sensitive layer is stripped because of spark counting (not
in case of optical counting). The stripping is quite easily done by pinching one of the
corners of the film between the thumb and the index finger. All that is then required is to
pull the sensitive layer in a direction parallel to the base until the two are completely
separated. The dried detectors are then used for alpha counting using a spark counter. The
detectors were first pre-sparked at 900 V twice and then tracks were counted at 500 V
twice.
From the track density (track/cm2/day), the radon activity was obtained in Bqm-3,
using the calibration factor of 0.056 tr. cm-2 d-1/Bqm-3 obtained from an earlier calibration
experiment (Singh et al. 1997).The Fig. 3.6 shows the alpha tracks in LR-115 plastic track
detectors produced by alphas from radon decay with etching condition, 2.5 N NaOH at
60±0.50C, 90 minutes.
61
Fig-3.4: The plastic can used for the measurement of equilibrium radon
concentration and exhalation rates from different samples.
62
Fig-3.5: (a) Constant temperature water bath
Fig-3.5: (b) Sample etching arrangement in water bath
63
3.2.3 Counting of Tracks by Spark Counter
The basic information needed is the measurement of track densities. Alpha tracks
developed on LR -115 cellulose nitrate films are counted using a spark counter of Poltech
Instruments, Mumbai. The spark counting technique is the most successful and widely
used technique for counting etched tracks in plastic track detectors. It was first invented
by Cross and Tommasino (1970) and has been developed and discussed by many workers
(Somogyi et al., 1978; Azimi-Garakani et al., 1981; Tommasino et al., 1986, Durrani and
Bull, 1987). The etched sensitive layer of LR-115 detector acts as an insulating material,
is placed between two electrodes of the spark counter forming a capacitor and covered
with an aluminized plastic foil (a very thin layer of aluminium evaporated onto a Mylar
backing). The aluminized side of the plastic foil is in contact with the thin detector. The
thick conductive electrodes of a spark counter are commonly made of brass. Normally a
heavy weight is put on the top of the plastic foil to have an intimate contact between the
thin detector and the electrodes. Polltech spark counter has this design for easy operation
using electronic control panel with display and provisions for data storage and data
transfer facilities. When a high voltage is applied across the capacitor C, an electrical
discharge or spark takes place through a track-hole. The voltage pulse produced across
the resistor, R, can easily be counted electronically by a scalar (Azimi-Garakani et al.,
1981).
The spark passing through a track hole has enough energy to eliminate the thin
layer of aluminium coating and to produces a much larger hole in the aluminium
electrode. Because of the elimination of the aluminium there exist a short-circuit in the
electrode, and hence second spark in the same track hole is avoided. As a result the spark
is stopped, the capacitor C is charged again. Consequently, the spark shifts randomly
from one track hole to another until all track holes are counted. The evaporated spot on
the aluminium, which have the diameter of about 100 μm are equal to the number of
sparks and hence to the number of track holes in the plastic track detector. The schematic
diagram and the Polltech spark counter unit are shown in Fig. 3.7 (a) and (b). The
operating voltages are stored in a microprocessor for operational ease. The pre-sparking
and counting voltage are adjustable from 100V to 1000V DC through digital
potentiometers. The pre spark voltage is 900 V and spark voltage is 500 V in our case.
64
Fig-3.6: Alpha tracks in LR-115 plastic track detectors produced by -particles
from radon decay.
Fig. 3.7: (a) Schematics of a spark counter
65
(b) Polltech spark counter unit
The spark counting time called the Gate Window time is adjustable from 1 to 10 seconds
which is 8 sec for spark counter used in present work.
3.2.4 Technique for Measurement of Radon, Thoron and their Progeny in
Dwellings.
3.2.4.1 Twin Chamber Dosimeter Cups
Several methods are in use for measuring the radon and its daughter elements in
dwellings. For the measurement of radon, thoron and their progeny, twin chamber
dosimeters cups has been employed. In this technique, LR-115 type II, strippable plastic
track detector films of size (2cm x 2cm) are used. The specially designed twin cup
dosimeter used here consists of two chambers of cylindrical geometry separated by a wall
in the middle with each having a length of 4.5 cm and radius of 3.1 cm (Eappen and
Mayya, 2004; Reddy et al., 2005) as shown in Fig. 3.8. Also, there are large numbers of
fine holes at the ends i.e. perforated ends are there so that free entry of air inside the
chambers is possible. This dosimeter employs three SSNTDs out of which two detectors
were placed in each chamber and a third one was placed on the outer surface of the
dosimeter. One chamber is fitted with glass fiber filter so that radon and thoron both can
diffuse into the chamber while in other chamber, a semi permeable membrane made of
latex or cellulose nitrate, having a thickness of 25 µm is used (Abdel and somogy, 1986;
Wafaa Arafa, 2002; Ramachandran et al., 2003; Eappen and Mayya, 2004). The
membrane mode measures the radon concentration alone as it can diffuse through the
membrane but suppresses the thoron. The twin cup dosimeter also has a provision for
bare mode enabling it to register tracks due to radon, thoron and their progeny in total.
Therefore, using this dosimeter we can measure the individual concentration of radon,
thoron, and their progeny at the same time.
The plastic detectors like cellulose nitrate, commonly known as LR-115 type II
size (2cm x 2cm) are loaded in the twin chamber dosimeters which provide three modes
of exposure to determine radon, thoron and their progeny simultaneously (Dwivedi et al.,
2001; Ramachandran et al., 2003).The selection of the detector LR-115, is based on the
fact that these detectors do not develop tracks originating from the alpha particles of the
progeny deposited on them and hence ideally suited for air concentration measurements
(Durrani, 1997) . In the present work, we used these dosimeters and installed them in the
66
dwellings of Chandigarh, Mohali, Rupnagar and Ambala for an exposure period of three
months. The dosimeters were installed in the dwellings at the average human height away
from the walls for the period of three months. Following the environmental exposure for a
stipulated period, the detectors were removed and chemically etched for 1.5 h without
stirring in 2.5 N NaOH solution at a temperature of 600C for developing the tracks
registered on them. Experimental set up used for chemical etching of LR-115 plastic track
detectors is shown in Fig. 3.5.
The detectors were rinsed and then stripped. The tracks produced by alpha
particles, were counted using a spark counter. Experimental set up used for counting of
tracks in detectors is shown in Fig. 3.7. Using suitable calibration factor and formulae to
be discussed in Chapter-5, the alpha track density was converted into concentration of
radon, thoron and their progeny.
3.2.4.2 Single Entry Pin hole based Radon-Thoron Dosimeters
The pin hole based radon thoron dosimeters (PRTMs) consist of two
compartments, each compartment is cylindrical having a length of 4.1 cm and radius 3.1
cm, internally coated with metallic powders to have zero electric field inside the
compartment volume so that the deposition of progenies formed from gases will be
uniform throughout the volume. The two compartments are separated by a central
pin‐holes disc, acting as
220
Rn discriminator. The disc has four pin holes each with
dimension of 2 mm length and 1 mm diameter. There is a single entry for the gas in the
dosimeter cups. The gas enters in first compartment namely “radon + thoron”
compartment through a glass fiber filter paper (pore size 0.7 μm) and diffuses to second
compartment namely “radon” compartment through pin‐holes cutting off the entry of
220
Rn into this compartment. The schematics of the pin hole dosimeter is shown in Fig.
3.9. LR-115, Type-II, strippable, SSNTDs of size 3 cm x 3 cm were fixed at opposite
ends of the entry face in the each compartment. The detector of “radon + thoron”
compartment measure the tracks produced by the alphas emitted from both the radon and
thoron, while the detector of the second compartments measured the tracks due to radon
only.
67
Fig. 3.8: (a) Schematics of twin chamber dosimeter cups
Fig. 3.8: (b) Twin chamber dosimeter cup
68
The dosimeters were installed in the dwellings at the average human height away
from the walls for the period of three months. Following the environmental exposure for a
stipulated period, the detectors were removed and chemically etched for 1.5 h without
stirring in 2.5 N NaOH solution at a temperature of 600 C for developing the tracks
registered on them. Experimental set up used for chemical etching of LR-115 plastic track
detectors is shown in Fig. 3.5. The detectors were rinsed and then stripped. The tracks
produced by alpha particles, were counted using a spark counter. Using suitable
calibration factor and formulae to be discussed in Chapter-5, the alpha track density was
converted into concentration of radon, thoron and their progeny.
Advantages for pin holes based dosimeters for radon thoron measurement
The single entry of air in pin hole based radon thoron dosimeter has advantageous
over the conventional double entrance type twin cup dosimeter Sahoo el. al (2013). There
is a common entrance for the radon gas so the concentration of the gas is same always in
both the chambers. Since the entrance port is downside, when installed, the effect of
turbulance from horizontal wind velocity is minimal. The metallic coating on inner side
of the cup provide a net zero electric field which gives uniform deposition of radon and
thoron progeny inside the cup.
69
Fig. 3.9: (a) Schematic diagram of single entry radon, thoron dosimeter
Fig. 3.9: (b) Single entry radon, thoron dosimeter
70
3.2.5 Technique for Measurement of Bulk Etch Rate, Track Etch Rate and Track
Sensitivity.
The LR-115 detector is a commonly used SSNTD for the detection and
measurement of radon, thoron and its progeny through track formation. Mostly the
chemical etching technique was used for the revelation of these tracks. Etching converts a
latent track into a visible structure, whose information can be read under an optical
microscope. So bulk attack by the etching solution is a necessary condition and the bulk
etch rate of the polymeric sample is an important factor that controls track formation in
SSNTDs. The bulk etch rate (Vb) for the LR-115 SSNTD was calculated by two methods
i.e. by measuring the removal of the thickness of the polymeric sample for different time
intervals at a constant temperature and by the method of % mass change of the polymeric
sample. Etch rate was determined by measuring the foil thickness using a digital
micrometer (Mitutoyo, No. 293-821) having least count of 1 µm shown in Fig. 3.10. Etch
rates were also determined by weighing the films before and after chemical etching using
a digital weighing balance (Sartorious, Germany with a least count of .01mg) as shown in
Fig. 3.10.
Normally the indoor radon monitoring is done by employing the detector in the
natural environmental conditions that can change the detectors properties. These
environmental effects are more prominent in countries like India, where temperature
levels are high and solar radiations are very intense. So the study of surface chemical
etching behavior of this polymer by giving pre-UV and post-UV exposure to alpha
irradiation on it is an important aspect. The measurement of radon values depends upon
the exact counting of the tracks produced by alpha particles on these detectors. The
counting of these tracks is normally done by an optical microscope of suitable
magnification. So the quality of visualization of these tracks is also an important factor.
The ultrasonic beam is employed to the etchant to find its use as a tool for enhancing the
revelation of the alpha particle tracks in the detector. The track sensitivity describes the
shape and size of the track and depends on both bulk etch rate and track etch rate.
Pristine LR 115 type-II (cellulose nitrate, non-strippable) was procured from DOSIRAD,
France in the form of thin films of active layer thickness of 12 µm on a clear 100 µm
polyester base. Several samples of size 1.5’ x 1.5’ were cut from the films. The thickness
71
of each sample was measured before etching on a specified area using a digital
micrometer (Mitutoyo, No. 293-821) having least count of 1 µm.
Alpha irradiation: All alpha irradiation were carried out using accessories (having
a chamber to create vacuum so that collimated particles were incident normal to the
surface of the sample) provided with PHOENIX (physics with home-made equipment &
innovative experiments) interface developed by Inter University Accelerator Centre
(IUAC) New Delhi India (Kumar et al., 2009). The Am241 source was used as a point
source having alpha emission of 5.485 MeV and was placed at a distance of 1 cm from
the LR-115 samples kept in a sample holder as shown in Fig. 3.11. All the samples were
irradiated for same time period.
UV irradiation: All the samples of LR-115 were exposed to UV light for two
hours in a UV chamber using a black lamp (SAMSON made) of 125 W at λ = 320 nm as
shown in Fig. 3.12(a). The UV chamber was having the facilities of a black lamp (λ = 320
nm), mercury lamp of 125 W (mixed light), UV tubes of 60 W (λ = 254 nm) and a simple
white light tube of 30 W. The samples were fixed in a sample holder that was placed at a
distance of 10 cm (so that spreaded UV light falls on the samples for the uniformity)
directly below the UV lamp. The whole arrangement was suitable to study the photooxidation of the polymeric samples under the UV light.
Ultrasonic Etching: All the samples of LR-115 used to study ultrasonic’s effect
were etched under the application of ultrasonic beam in a temperature controlled
ultrasonic cleaner shown in Fig. 3.12 (b) (Model No. GB-5000B) of 5.0 l capacity with a
power output of 80 W. The samples were etched in a beaker (containing etchant)
submerged in the water of the ultrasonic cleaner tank. The ultrasonic were passed through
the water of the ultrasonic tank to the etchant of the beaker. The etching was done at a
temperature of 60°C ± 1°C. Due to the passage of ultrasound through the etching
medium, the temperature of etching medium rises. The rise in temperature depends on the
acoustic power of the ultrasound. We have tried to isolate this effects produced by
ultrasound by monitoring the temperature continuously and adding the cold water in the
tank periodically. To avoid this effect we have also taken small time for the etching
cycles of 20 min. each.
72
Fig. 3.10: (a) Digital micrometer (Mitutoyo, No. 293-821)
Fig. 3.10: (b) Digital weighing balance (Sartorious, Germany)
73
Fig. 3.11: (a) PHOENIX interface (b) Vacuum chamber with source and sample
Fig. 3.12: (a) UV chamber
(b) Ultrasonic cleaner
74
Fig. 3.13: Optical microscope with a camera.
75
Thickness and Optical Measurements: The thickness measurements for a
particular sample were taken after etching on the specified area as discussed earlier.
These samples were viewed through an optical microscope fitted with a camera as shown
in Fig. 3.13 (Model No. 7001 IMS, Vaiseshika Ltd.). The photomicrographs of revealed
alpha tracks in these detectors were taken using that microscope at 200X magnification.
3.2.6 Technique for in situ Measurement of Gamma Dose
Beta-Gamma radiation survey meter (RM707), manufactured by Nucleonix Systems,
Hyderabad has been used for the in situ measurement of the gamma dose in the dwelling
of the study area. It has a miniature halogen quenched GM detector with energy
compensating filter. It is a battery operated radiation survey meter which works on 7.5V
(5 x 1.5V) AA size battery). Internally the regulated power supplies of +5V and +5V/6V
are generated by two low dropout regulators for powering the pulse processing and
display section and the high voltage section. The high voltage section generates an EHT
of +500V @ 75mA for biasing the GM detector which is used as the radiation sensor. It is
a micro-controller based unit which serves as a general purpose alarming survey meter for
use in medical, agricultural, industrial & other installations where radioactive isotopes are
used for a variety of applications. It is also used in measuring radiation levels in Reactor
buildings, nuclear installations, Radio-chemical plants, reprocessing plants, etc. The block
diagram and the actual unit of beta gamma radiation survey meter are shown in Fig. 3.14
(a) and (b).
76
Fig. 3.14: (a) Block diagram of Beta-Gamma radiation survey meter
Fig. 3.14: (b) Beta-Gamma radiation survey meter (RM707)
77