Chapter III Theoretical Aspects 26-42
3.7 Limitations of NAA 36
NAA has some limitations, such as the dependence on an irradiation facility preferably a nuclear reactor and the threshold imposed by legal safety regulations for the manipulation of radioactive materials. It is associated with comparatively lengthy analysis procedure. In extreme trace analysis if long-lived radionuclides are involved, irradiation periods of many hours or even days and measuring periods of tens of hours (per sample) are often required. Considerable decay periods are occasionally necessary to reduce disturbing (matrix) activities, when non-destructive work is carried out. The way to remedy these inconveniences is by turning to larger samples, higher neutron fluxes and more efficient counting devices (i.e., larger HPGe detectors) [11].
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3.8 Theoretical Aspects of Measuring Natural Radioactivity
An element that spontaneously emits this kind of radiation which includes the emission of alpha particle, beta particles, gamma rays and conversion electrons is considered radioactive and the radiation it emits is known as natural radioactivity. Radiation means any sort of energy from spreading out from a centre. Radio waves, light, infrared light and microwaves are all examples of radiation. These types of radiation are all related, they are members of the electromagnetic spectrum and all travel at the speed of light, 3 x 108ms-1. The radiation associated with radioactivity is more violent as they are more energetic. This radiation is classed as ionizing radiation as the radiation from the nuclei can easily destroy molecules by stripping away electrons from their atoms and ionizing them. Other ionizing radiation includes UV light and X-rays. Both of these are members of the electromagnetic spectrum. The radiations from nuclear processes are not necessarily members of the electromagnetic spectrum. Two forms are particles also which come from the nucleus itself as the change occurs [8-11].
3.9 Radioactive Equilibrium
To explain nuclear equilibrium let us consider a typical nuclear reaction
A B C
Where, A is the parent, B and C are the successive daughter products. The quantity of radionuclide B when secular equilibrium is reached is determined by the quantity of its parent A and the half-lives of the two radionuclides. This can be seen from the time rate of change of the number of atoms of radionuclide B,
Where, λA and λB are the decay constants of radionuclide A and B.
Radioactive equilibrium in general is two types: secular equilibrium and transient equilibrium.
3.9.1 Secular Equilibrium
Secular equilibrium is attained when the parent’s half-life is much longer than that of the daughter i.e λA<< λB .
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Secular equilibrium occurs when,
, or .
In equilibrium state quantity of radionuclide B declines in turn. For times short compared to the half-life of A,
λ
A<<1
and the exponential can be approximated as 1.3.9.2 Transient Equilibrium
The transient equilibrium is a situation in which equilibrium is reached by a parent- daughter radioactive isotope pair where the half-life of the daughter is shorter than the half-life of the parent. In contrast to secular equilibrium, the half-life of the daughter is not negligible compared to parent's half-life. Transient equilibrium attains when the parents half-life is longer than that of the daughter i.e. λA< λB. The ratio of daughter-to- parent activity is given by,
=
Both A and B decay while remains constant. This type of equilibrium is called transient equilibrium. Whether any equilibrium is secular or transient depends upon the duration of our observation of the sample [12].
3.10 Radionuclides in Nature
Radionuclides occur both naturally and are made artificially using nuclear reactors, cyclotrons, particle accelerators or radionuclide generators. There are about 650 radionuclides with half-lives longer than 60 minutes [13]. Of these, 34 are primordial radionuclides that existed before the creation of the solar system, and there are another 50 radionuclides detectable in nature as daughters of these or produced naturally on earth by cosmic radiation. More than 2400 radionuclides have half-lives less than 60 minutes.
Most of these are only produced artificially, and have very short half-lives. For comparison, there are about 254 stable nuclides [14, 16].
3.11 Interaction of Gamma Rays with Matter
Gamma ray spectroscopy is a powerful technique for quantifying the spectrum of emitted gamma rays from a radioactive source. The method is well-known for its sensitivity for
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the detection and determination of a large number of elements. The method consists of production of artificial radionuclides from stable elements by irradiation of a sample in a neutron flux and measuring the gamma radiation emitted by radionuclide formed in the process [17]. The emission of gamma rays is regarded simply as a result of transformation of a nucleus from an excited state to a less excited one. For many radioactive elements the emission of α or β particle from a nucleus is immediately followed by the emission of a γ-ray. Due to very short wavelength (even less than ordinary x-rays) γ-rays are highly energetic and hence possess a very high penetrating property [18].
The detection of γ-rays therefore depends on how the gamma-ray photons undergo an interaction that transfers all or part of the photon energy to an electron in the absorbing material. Although a large number of possible interaction mechanisms are known for γ- rays in matter, only three major types play an important role in radiation measurements.
They are,
Photoelectric effect,
Compton scattering, and
Pair production.
Fig. 3.3 The interaction mechanisms for gamma- rays in matter [19]
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It can be observed from Fig. 3.3 that at low energies photoelectric interaction probability is largest, Compton scattering is the principal absorption mechanism in the intermediate energy ranging from 100 keV to 10 MeV. On the other hand, when gamma energy exceeds 1.02 MeV, the probability of pair production becomes dominant [19].
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References
[1] Hevesy, G., Levi, H., “Radio analytical Chemistry in Denmark”, A Bibliography, 34, 145, (1936-1977).
[2] Amiel, S., “Nondestructive activation analysis, studies in analytical chemistry”, Elsevier Scientific Publication, Amsterdam-oxford-New York, 3(2), 44-48, (1981).
[3] Asres, Y. H., Chaubey, A.K., Awoke, T. H., Dilbetigile, A.M., “Thermal Neutron Activation Analysis Technique of Rock samples from Choke Mountain Range, East Gojjam, Ethiopia”, Int. J. of Basic and Applied Science, 1(4), 694-704, (2013).
[4] Laul, L. C., “Neutron activation analysis of geological materials”, Atomic Energy Rev., 17(3), 603-695, (1979).
[5] Awoke, T. H., Chaubey, A.K., Dilbetigile, A.M., Asres, Y. H., “Application of Instrumental Neutron Activation Analysis for the Elemental Analysis of various Rocks from Areas around Debre Birhan City, Ethiopia”, IJRRAS, 12(1), 115-125, (2012).
[6] http://www.intechopen.com/source/html/43467/media/image1.png
[7]Filby, R. H., “Isotopic and nuclear analytical techniques in biological systems: A critical study-IX (Neutron activation analysis)”, Pure & Appl. Chem., 67(11), 1929- 1941, (1995).
[8] Susan, J.P., “Activation Spectrometry in Chemical Analysis”, 10-17, (1991).
[9] David T.W., “Neutron Activation Analysis (NAA)”, AU J.T., 8(1), 8-14 (2004).
[10] Yusuf, A. M., Adamu, N. B., Musa, M., “Assessment of Indoor Cancer Linked Radionuclides in Sokoto Urban Dwelling” Journal of Natural Sciences Research, 4(1), (2014).
[11] Currie, L.A., “Limits for qualitative detection and quantitative determination- Application to radiochemistry”, Anal. Chem. 40, 586-593. (1968).
[12] C. L. Arora, “B. Sc. Physics”, 3, 53-55.
[13] Arthur B., Concepts of Modern Physics, Sixth Edition, 422.
[14] Quindos, L. S., Rodenas, C., Fernandez L., Soto, J., “Estimate of External Gamma Exposure Outdoors in Spain”, Radiation Protection Dosimetry, 45, 527-531, (1992) [15] Loveland, W., Morrissey, D., Seaborg, G.T., “Modern Nuclear Chemistry”, Wiley- Inter Science., 57-62, (2006).
[16] Eisenbud, M., Gesell, Thomas, F., “Environmental Radioactivity: From Natural, Industrial, and Military Sources”, 134-139, (1997).
[17] Evans, R.D., “The Atomic Nucleus”, Mc. Grawhill, New York, 673, (1952).
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[18] Ragheb, M., “Gamma-Ray Interactions with Matter”, (2013).
[19] https://en.wikipedia.org/wiki/Gamma_ray
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Chapter IV
Experimental Technique
4.1 Experiment Setup
The basic instrumentation of NAA includes a neutron source for producing neutron flux which triggers several nuclear reactions and a gamma spectroscopy system for detecting and analyzing the characteristics gamma-rays released from the radioisotopes during (n,γ) reaction. Nuclear reactor is the main mechanism of neutron source and nuclear reaction. The present research work is done by TRIGA MARK-ІІ Research Reactor of Bangladesh Atomic Energy Commission (BAEC). The reactor produces neutron flux and induces (n,γ) reactions. A gamma spectroscopy system including HPGe detector, multi- channel analyzer (MCA), preamplifier, amplifier, analog to digital converter (ADC) and other associated electronics is used for gamma ray detection and analysis. The instruments used in the present study are divided under the following categories:
Arrangement of neutron sources
Irradiation facility
Gamma ray spectrometry
4.1.1 Neutron Sources
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