Chapter IV Experimental Technique 43-76
4.1.1 Neutron Sources 43
There are various neutron sources that can be used in the NAA method of sample analysis. These are,
A nuclear reactor,
An actinide such as californium which emits neutrons through spontaneous fission,
An alpha source such as radium or americium, mixed with beryllium; this generates neutrons by a (α, C+n) reaction,
A D-T fusion reaction in a gas discharge tube,
Farnsworth-Hirsch fusor, and
Combination of an alpha emitter and beryllium.
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In the present sturdy, nuclear reactor is used as the source of neutron as well as irradiator.
The major source of neutron is the nuclear reactor which operates on the principle of fission chain in 235U and 239Pu and is capable of producing by far the highest flux of neutron. The fission neutron energy spectrum in the reactor is very wide, ranging from 0.01 eV to 15 MeV, depending on types of the reactions and positions within a reactor.
4.1.1.1 Nuclear Reactor
In this experiment samples were irradiated by the 3 MW TRIGA MARK- II research reactors at the Atomic Energy Research Establishment (AERE), Savar, Dhaka. A partial view of the 3 MW TRIGA MARK- II research reactors is shown in Fig. 4.1. It is a multi- purpose reactor, capable of both steady state and pulsing operation which has been put into service in several disciplines since its commissioning.
The TRIGA MARK- II research reactor is a light water cooled graphite reflected reactor designed for continuous operation at a steady-state power of 3 MW and for pulsing with maximum reactivity insertions achieving a peak power of about 852 MW. The reactor is
Fig. 4.1 Partial view (Left) and schematic diagram of TRIGA MARK-II research (right) [2]
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housed in a hall of 20.11m x 23.46m having a height of 17.37m. The control room and the equipment for ventilation and other services systems are located in the adjoining four storied building having same height as the reactor hall [1].
4.1.1.2 Reactor Core
The reactor core, which is the bottom of the reactor tank, has a 0.63 cm thick wall having an inside diameter of 2 m and a depth of 8.2 m. Fig. 4.2 shows the cross-sectional view of TRIGA MARK- II reactor core. The reactor core and reflector assembly is a cylinder approximately 1.1 m in diameter and 0.89 meter high.
The reactor core consists of a lattice of fuel-moderator elements, graphite dummy element and control rods. A graphite reflector and a 5 cm thick lead gamma shield surround the core. The entire assembly is bolted to a stand that rest on the bottom of the reactor tank. The outer wall of the reflector housing extends 0.8 m above the top of the core to ensure retention of sufficient water for after-heat removed in the event of a tank drain accident. Cooling of the core is provided by natural circulation of up to 500 kW power level and by forced down flow circulation of tank water for higher powers, which is, in turn, cooled and purified in external coolant circuits. In case of loss of cooling water in the reactor tank there is a provision of emergency core cooling system with roof top backup system.
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4.1.1.3 Fuel-Moderation Elements
There are in total 100 fuel elements in the reactor core. The fuel is solid, homogeneous mixture of Eu-ZrH alloy containing 20% by weight of uranium enriched to about 19.7%235U and about 0.47% by weight of Erbium. The H/Zr ratio is approximately 1.6. Each element is clad with 0.051 cm thick stainless steel can. Two sections of graphite are inserted in the can, one above and one below the fuel, to serve as top and bottom reflectors for the core.
4.1.1.4 Reflector
The reflector of TRIGA reactors is a ring shaped block of graphite that surrounds the core radially. It is 30.5 cm thick radially, with an inside diameter of 45.7 cm and height of 55.9 cm. The graphite is protected from water penetration by a leak-tight welded aluminium can.
Fig. 4.2 Cross-Sectional and schematic diagram of the cross-sectional view of TRIGA MARK- II reactor core [2]
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4.1.1.5 Control Rod
The heat generating in the reactor is proportional to the fission rate which depends on the neutron density in the reactor core. The reactor starts up and shut down operation are included with control system, which is done by varying the neutron density in the core.
This is achieved by moving rods of a material of high neutron absorbing property.
Most of the time cadmium or boron which has very high neutron absorption cross- section is used as control rod to decrease the neutron density as well as reactor power.
The TRIGA MARK- II reactor is controlled by six boron carbide rods. Each control rod is a sealed aluminium tube containing powdered boron carbide as a neutron poison. The control rods are approximately 51 cm long.
4.1.1.6 Reactor Tank
The reactor tank consists of an aluminium vessel installed in the reactor shield structure.
The tank has an inside diameter of approximately 1.98 m and a depth of 6.25 m. A 2 x 2 inch aluminium channel used for mounting the ion chambers and underwater lights are attached to the top of the tank. The principal parameters of the TRIGA MARK- II reactor are given in Table 4.1.
4.1.1.7 Reactor Shield
The reactor shield is a reinforced concrete structure standing 7.9 m above the reactor hall floor. The lower octagonal portion is 6.6 m across the flats. The beam ports are installed in the shield structure with tabular penetrations through the concrete shield and the reactor tank water, and they terminate either at the reflector assembly or at the edge of the reactor core. The radial shielding of the core is provided by a minimum of 2.29 m of concrete having a minimum density 2.75 g/cm3, 45.7 cm of water, 19 cm of graphite and 5 cm of lead.
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Table 4.1 Principal design parameters of the TRIGA MARK- II reactor [3]
4.1.2 Irradiation Facilities for Reactor
The TRIGA MARK- II reactor is designed to provide intense fluxes of ionizing radiation for research, training and isotope production. Experiments with the TRIGA reactor can be carried out using the following facilities:
Rotary specimen rack (Lazy Susan),
Pneumatic transfer system (Rabbit),
Central thimble,
Beam port facilities,
Maximum steady state power level 3 MW Fuel element design
Fuel- moderator material Uranium content
Uranium enrichment Burnable poison Shape
Overall length of fuel Outside diameter of fuel Cladding material
U-ZrH 20 wt % 19.7%U-235 0.47 wt % Erbium Cylindrical 38 cm (15 inch) 3.63 cm (1.43 inch) Type 304stain!ess steel
Number of fuel elements 100
Maximum excess reactivity 7.69 % k/k Reactivity loss due to equilibrium Xe 2.5 % k/k Number of control rods
Shim / safety Regulating Safety / transient
4 1 1
Total reactivity worth of control rods 12.785 k/k
Reactor cooling Forced down flow of pool
Water (above 500 kW)
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Triangular cut-outs in the core,
Dry Central irradiation tube, and
Thermal column for future use (presently filled up with heavy concrete blocks).
In the present study, Pneumatic transfer system (Rabbit) has been used irradiating samples.
4.1.2.1 Pneumatic Transfer System (Rabbit)
The pneumatic transfer system, which has a transfer time of about 4.6 sec, is used to irradiate monitors that produce short-lived radioisotope. Production of very short-lived radioisotope is accomplished by a pneumatic transfer system, which rapidly conveys a specimen to and from reactor core. When the polyethylene specimen capsule on “rabbit”
is ejected into the core, it comes to rest in a vertical position approximately at the mid- plane of the core. With automatic control, the specimen capsule is ejected from the core after a predetermined length of time.
Table 4.2 Values of neutrons flux (n.cm-².sec-1) at TRIGA MARK- ІІ Research Reactor, AERE, Savar, Dhaka [1]
4.1.3 Gamma Rays Spectrometry System
After irradiation the samples and standards are placed on the HPGe detector. For the detection of the gamma rays emitted from the experimental samples an experimental arrangement was established which includes a HPGe detector, a Digital Spectrum Analyzer DSA-1000 with Canberra Detector Interface Module (DIM) containing a high
Neutrons flux (n.cm-².sec-1)
Position Thermal Epithermal
DTC 7.53 × 1013 3.81 × 1012
RSR (Lazy Suzan) 1.39 × 1013 6.59 × 1011
Pneumatic transfer system 2.64 × 1013 1.23× 1012
Tangential beam port 2.40 × 107 -
Radial piercing beam port 1.2× 105 -
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voltage power supply, a pre-amplifier, amplifier, analog to digital converter (ADC) and PC based Multi-Channel Analyzer (MCA) coupled with software-Genie 2000, etc. The block diagram of the complete gamma ray detection arrangement is shown in Fig. 4.3.
The components of the experimental gamma ray spectrometry system are given below.
4.1.4 High Purity Germanium (HPGe) Detector
HPGe detector is one type of semiconductor detector. The most important advantage of the semiconductor detectors is their superior energy resolution ability to resolve the energy of particles out of a poly-energetic energy spectrum. Due to this high quality precision it is widely used for gamma spectroscopic measurement because of their superior resolution compared to NaI crystal [4]. Semiconductor detectors produce the available free charge carriers which can be used for the detection and measurement of incident radiation. It is a planer detector in which the electric field is fairly uniform and co-axial configuration in which the electric field varies inversely with the radial distance from the detector axis. HPGe detectors are available in two relatively simple geometries.
The basic element of HPGe detector is a single crystal of germanium semiconductor. It is mainly a cylinder of germanium with an n-type contact on the outer surface and p-type
Precision Pulse Generator
Pre-amp High
Voltage
Filter Detector
Oscilloscope MCA
Detector Bias Supply
Count Rate Meter
Amplifier
Fig. 4.3 Block diagram of gamma ray detection arrangement
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contact on the inner surface of an axial well. The n and p contacts or electrodes are typically diffused lithium and gold surface barriers respectively. The outer n-type diffused lithium contact is about 40 mg/cm2 of evaporated gold. The outer n-type diffused lithium contact is about 0.5mm thick as shown in Fig 4.4. In HPGe detector the intrinsic region is created by depletion of charge carriers applying a reverse bias across the diode which is sensitive to ionizing radiation particularly x-ray and -rays. When gamma ray photon interacts within the active region free charge carriers (electron-hole) pair is produced. These carriers are collected by the electric field to their respective collecting electrodes. The resultant charge is integrated by a charge sensitive pre- amplifier and converted to a voltage pulse with amplitude proportional to the original photon energy. The performance of a detector depends on its depletion depth which is inversely proportional to the net impurity concentration in the detector material. Thus extremely pure material is required to obtain wide depletion depth.
Typical detectors are mounted in a vacuum cryostat with a copper cold finger immersed in liquid nitrogen. The crystal is clamped on the other end of the copper cold finger as shown in Fig. 4.5. Since, germanium has relatively low band gap (0.74eV) the detector must be cooled in order to reduce the thermal leakage current to an acceptable level because leakage current can destroy the energy resolution of the detector. Liquid Fig. 4.4 Cross sectional view of a co-axial
germanium crystal [5]
Fig. 4.5 Configuration of the HPGe detector
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nitrogen (LN2) which has temperature of 77 K (-196oC) is the common cooling medium for such detectors. The detector is mounted in a vacuum chamber which is attached to or inserted into an LN2 Dewar. The sensitive detector surfaces are thus protected from moisture and condensable contaminants.
4.1.4.1 High Voltage Unit
Most of the detectors require extremely high voltage power supplies. The Canberra model-3105 is a high voltage power supply unit. A standard NIM-BIM power supply model-2000 is used to the input. Moreover, the detector system has a high voltage filter containing capacitor capable of delivering a dangerously high current for a brief time while being discharged (even if the bias supply has been disconnected). In this experiment +4000 volts was applied for the biasing of the detector.
4.1.4.2 Preamplifier and Amplifier
Absorption of photon by detector produces a current pulse at the preamplifier input.
These pulses are too small to measure without amplification of the electric signal. The preamplifier has been located as close as possible to the detector to minimize the signal from noise and captive loading. It also serves as an impedance matcher, presenting high impedance to the detector to minimize loading, while providing a low impedance output to drive succeeding components. Canberra Model 2022 used in the work has the following characteristics.
4.1.4.3 Multi-Channel Analyzer (MCA)
A multi-channel analyzer is a device that can simultaneously analyze pulses within many different channels or intervals. The MCA covers entire voltage range at a time and displays the information of all the collected pulses in real time. It contains a memory which stores a list of numbers corresponding to the number of pulses at each discrete voltage. After storage the data can be displayed graphically by MCA.
4.1.4.4 Analog to Digital Converter (ADC)
An analog-to-digital converter or ADC is an electronic system that converts analog signal to digital one. The Canberra model-8075, a single width NIM (Nuclear Instrumentation
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Module) style analog-to-digital converter was placed between the amplifier and the MCA.
This type of ADC has huge applications in nuclear and x-ray spectroscopy.
4.1.4.5 High-Permit PC Spectroscopy Software
Two types of softwares were used for gamma spectra acquisition: one is ORTEC DspecJrTM with Maestro-32 acquisition software; another one is CANBERRA DSA with Genie-2000 acquisition software.
4.1.5 Background Radiation Effect
Background effect plays an important role in radioactive analysis. All radiation detectors record some background signal due to the cosmic radiation that continuously bombards the earth’s atmosphere and the existence of natural radioactivity in the environment. The background effect is very important for the present work, in detecting gamma rays by an HPGe detector. The effect of this background varies greatly with size and type of detector and with the extent of shielding that may be placed around it. Sources of background radiation are conveniently grouped into five categories:
The natural radioactivity of the constituent materials of the detector itself,
The natural radioactivity of the ancillary equipment, supports, and shielding placed in the immediate vicinity of the detector,
Radiations from the activity of the earth surface (terrestrial radiation) walls of the laboratory, or other far away structure,
Radioactivity in the air surrounding the detector, and
The primary and secondary components of cosmic radiation.
As the magnitude of the background ultimately determines the minimum detectable radiation label, it is very significant in those applications involving radiation sources of low activity.
4.1.6 Shielding Arrangement around the HPGe Detector
The background radiation can cause great harm to the devices of the gamma ray spectrometry system. Therefore shielding of the detector is essential. A second purpose
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of detector shielding is to provide a degree of isolation in laboratories where other radiation sources may be used or moved about during the course of a measurement. The shielding not only reduces the background resulting from cosmic radiation and from natural radioactive traces in the building materials or in the surface of the earth, but also from nearby nuclear facilities and other radiation sources like air which presumably contains trace or radioactive gases, 222Rn and 220Th and so on. For low background, the conventional shielding materials are lead, steel, mercury, and concrete. In our experiment, lead is used as the shielding material around the HPGe detector. Because of its high density (11.4gm/cc) and large atomic number (Z=82) lead is the most widely used material for the construction of detector shields. A brief description of shielding arrangement used in the experiment is summarized in Table 4.3.
Table 4.3 Description of shielding arrangement around the HPGe detector
4.2 Experimental Procedure
This section describes the determination of different nutritional and toxic elements in baby food (powder milk and cereals) by employing NAA method using the 3MW TRIGA Mark-II research reactor at AERE, Savar, Dhaka. In this experiment the following steps are involved.
Sample collection,
Sample preparation,
Irradiation,
Gamma ray counting and peak analysis,
Quality control analysis, Low Background shielding
Material Lead (Pb)
Form Square
Length 14.5 cm
Height 12.5 cm
Thickness 4 cm
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Estimation of uncertainty budget, and
Experimental measurements.
4.2.1 Sample Collection
In this study, we have investigated 7 baby cereals and 8 powdered milks including lactogens and adult milk which are produced locally. All of the samples were bought from Savar areas, Dhaka. The significance of an analysis depends on a large extent on the sampling program.
Cereals
Lactogens
Powder milk
Joiner Horlicks
Table 4.4 Sample description and identification of cereals
Serial no. Types of sample Materials in the mixture Sample ID
1 Cereal-1 Rice and Mango with Milk C.1
2 Cereal-2 Wheat and Honey C.2
3 Cereal-3 Wheat and Fruit C.3F
4 Cereal-3 Wheat and Vegetable C.3V
5 Cereal-4 Rice and Potato with Chicken C.4
6 Cereal-k Rice with Bins C.K
7 Junior Horlicks Wheat, burly, sugar etc. J.H
Each sample was kept separated with care to avoid the risk of contamination because the concentrations of most trace elements are in the ppm level. All the samples were dried for 48 hours. After that the samples were massed to fine powder in Atomic Energy
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Research Establishment, Savar, Dhaka. Before the proper investigation the sample identification is essential. The descriptions of all the samples are given below.
Table 4.5 Sample description and identification of powder milk
Serial no. Types of sample Materials in the mixture Sample ID
1 Anchor Powder Milk A.1
2 Dano Powder Milk D.1
3 Diploma Powder Milk D.2
4 Lactogen-1 Powder Milk L.1
5 Lactogen-2 Powder Milk L.2
6 Marks Powder Milk M.1
7 Nido Powder Milk N.1
8 Red Cow Powder Milk R.C
After collection of the raw samples, the preparation of samples for irradiation was done in a clean room to avoid contamination.
4.2.2 Sample Preparation
In both the cases of cereals and powder milk, samples were dried for 48 hours. After that these were massed to fine powder. The used tools were cleaned by de-ionized water and acetone to avoid contamination with the next sample. Two types of samples were prepared for this research: one for neutron activation analysis and another for natural radioactivity measurements.
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Fig 4.6 Sample for irradiation first packing (left) final packing ultraclean polyethylene (right)
4.2.2.1 Sample Preparation for NAA
Fifteen party dishes were cleaned by de-ionized water and acetone and then dried by the oven. Sample identification (ID) numbers were given on the dishes accordingly. All the samples were dried for 48 hours. For NAA investigation, 2 sets of samples with CRMs were prepared for short and long irradiations. In each case about 80 mg was taken and weighed using a digital micro balance to a clean irradiation type polyethylene envelops and then heat sealed as shown in Fig. 4.6. Double encapsulation was used so as to remove the outer one after irradiation to avoid the contamination of the detector.
4.2.2.2 Sample Preparation for Natural Radioactivity
Fifteen party dishes were cleaned by de-ionized water and acetone and then dried by the oven. The sample identification (ID) numbers were given on the dishes accordingly. The experimental samples were poured to the dishes and then dried into the oven. After that, 19 plastic pots having the same geometry were cleaned by same way and dried naturally.
After drying, the pots were weighed and leveled by sample ID. For natural activity calculation, about 75 gm of each sample was poured into the pot keeping the sample geometry identical. These were then made air tight. All the sample pots were kept undisturbed for four weeks before counting to assure the parent-daughter equilibrium in the natural decay series. For example, 222Rn from the 238U decay series can easily be escaped by diffusion from the samples during preparation. Since the half-life of 222Rn is