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Study the effect of the size of crystal detector (scintillation) NaI(Tl) on the energy spectrum Inaam Hani Kadhim Department of Physics, College of Education for Pure Sciences, University of Babylon/Iraq
2014
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List of Contents &RQWHQWV >ŝƐƚŽĨŽŶƚĞŶƚƐ
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&KDSWHU2QHTheoretical ParW ϭ͘ϭ
Introduction
ϭϬ
ϭ͘Ϯ
Types of Radiation
ϭϭ
ϭ͘Ϯ͘ϭ
Alpha Particle
ϭ͘Ϯ͘Ϯ
Beta Particle
ϭϮ
ϭ͘Ϯ͘ϯ
Gamma-ray
ϭϯ
ϭϭ
ϭ͘ϯ
Ionization and Excitation
ϭϰ
ϭ͘ϰ
Radiation Sources
ϭϱ
ϭ͘ϰ͘ϭ
Natural or Background Radiation
ϭϱ
ϭ͘ϰ͘Ϯ
Human Made Radiation
ϭϲ
ϭ͘ϱ
Interaction of Gamma-ray with Matter
ϭϳ
ϭ͘ϱ͘ϭ
Photoelectric Effect
ϭϳ
ϭ͘ϱ͘Ϯ
Compton Scattering
ϭϴ
ϭ͘ϱ͘ϯ
Pair Production
ϮϬ
3
ϭ͘ϲ
Gama Ray Spectral Analysis
ϮϮ
ϭ͘ϳ
The Aim of the Present Work
Ϯϱ
&KDSWHU7ZR([SHULPHQWDO3DUW Ϯ͘ϭ
Gamma-ray Detectors
Ϯϳ
Ϯ͘Ϯ
Scintillation Counter
Ϯϳ
Ϯ͘ϯ
Mechanism of the Scintillation Counter
Ϯϵ
Ϯ͘ϰ
General Characteristics of NaI(Tl)
ϯϬ
Ϯ͘ϱ
The Detection system calibration
ϯϭ
Ϯ͘ϲ
Energy Resolution
ϯϮ
&KDSWHU7KUHH´Results and Discussion 3.1
Introduction
34
3.2
Study the effect of the size of crystal detector (scintillation) NaI(Tl) on the energy spectrum
34
3.2.1
Study the effect of voltage on the energy spectrum
34
3.2.2
Study the effect of amplification on the energy spectrum
41
3.3
Conclusions
47
References
48
4
List of Figures &ŝŐƵƌĞ
EŽ͘
WĂŐĞ
ĂƉƚŝŽŶƐ
EŽ͘
;ϭ͘ϭͿ
Penetrating ability for alpha, beta and gamma
ϭϰ
;ϭ͘ϮͿ
Photoelectric effect
ϭϴ
;ϭ͘ϯͿ
Compton scattering of a gamma-ray
ϭϵ
;ϭ͘ϰͿ
Pair production
;ϭ͘ϱͿ
Probability of pair production as a function of Ϯϭ gamma-ray energy
;ϭ͘ϲͿ
WĂƌƚƐŽĨƚŚĞƐLJƐƚĞŵĚĞƚĞĐƚ
;Ϯ͘ϭͿ
Detection system used
;Ϯ͘ϮͿ
Photomultiplier tube with possible resistance chain
ϯϬ
;Ϯ͘ϯͿ
calibration system energy
ϯϭ
;Ϯ͘ϰͿ
Formal definition of (FWHM) and peak centroid (n) in gamma-ray spectrum of Cs-137 by using NaI(Tl) detector
ϯϮ
;ϯ͘ϭͿ
The relationship between the voltage and the total area of the spectrum at Amplification (2) for both ϯϲ the detectors (1"x1.5") ,(2"x2") using the source of Cesium-137
;ϯ͘ϮͿ
The relationship between the voltage and energy resolution at Amplification (2) for both detectors ϯϳ (1"x1.5"),(2"x2") using a source of Cesium-137
;ϯ͘ϯͿ
The relationship between the voltage and the total area of the spectrum at amplification (2) for both ϯϵ the detectors (1"x1.5") ,(2"x2") using the source of Cobalt-60
Ϯϭ
Ϯϯ Ϯϴ
5
;ϯ͘ϰͿ
The relationship between the voltage and energy resolution at amplification (2) for detector ϰϬ (1"x1.5")using a source of Cobalt-60
;ϯ͘ϱͿ
The relationship between the voltage and energy resolution at amplification (2) for detector (2"x2") ϰϬ using a source of Cobalt-60
;ϯ͘ϲͿ
The relationship between amplification and the total area of the spectrum at voltage (500 volt) for ϰϮ both detectors (1"x1.5"),(2"x2") using a source of Cesium-137
;ϯ͘ϳͿ
The relationship between amplification and energy resolution at voltage (500 volt) for both detectors ϰϯ (1"x1.5"),(2"x2") using a source of Cesium-137
;ϯ͘ϴͿ
The relationship between amplification and the total area of the spectrum at voltage (500 volt) for ϰϱ both detectors (1"x1.5"),(2"x2") using a source of Cobalt-60
;ϯ͘ϵͿ
The relationship between amplification and energy resolution at voltage (500 volt) for detectors ϰϲ (1"x1.5")using a source of Cobalt-60
The relationship between amplification and energy ;ϯ͘ϭϬͿ resolution at voltage (500 volt) for detectors ϰϲ (2"x2")using a source of Cobalt-60
List of Symbols
^LJŵďŽů
Amp
Amplifier
T.A
Total area to Spectrum
Sc.A
Scattering area
ĞƐĐƌŝƉƚŝŽŶ
6
Ph.A
Photo peak
G
Gross
N
Net
F.W.H.M
Full Wdith at Half Maximum
C
Centroid
ER
Energy Resolution
HV
High voltage
List of Tables dĂďůĞ EŽ͘
ĂƉƚŝŽŶ
WĂŐĞ EŽ͘
;ϯͲϭͿ
Shows the accounts process for the source of radioactive Cesium -137 at Amplification (2) for both detectors ϯϱ NaI(Tl) 1"x1.5",(2"x2")
;ϯͲϮͿ
Shows the accounts process for the source of radioactive Cobalt-60 at amplification (2) for both detectors NaI(Tl) ϯϴ 1"x1.5",(2"x2")
;ϯͲϯͿ
Shows the accounts process for the of source radioactive Cesium -137 at voltage on (500 volt) for both detectors ϰϭ NaI(Tl) 1"x1.5",(2"x2")
;ϯͲϰͿ
shows the accounts process for the of source radioactive Cobalt-60 at voltage on (500 volt) for both detectors ϰϰ NaI(Tl) 1"x1.5",(2"x2")
7
$ $EVWUDFW In this work are used two types of detectors scintillation NaI(Tl) sizes (2"x2") ,(1"x1.5")for a comparative study between them and are used two sources of radioactive cobalt Co-60 has two energy (1.33&1.73 MeV) , cesium has energy (0.662 MeV) . and calculating the total area of the spectrum space scattering and area of peak optical and portability energy analysis, found
the size large for crystal detector leads to probability
escape photons outside the crystals to be less because it can reveal again and recorded pulse with pulse recorded by the electron apostate so increase the value of portability energy analysis, and increase the volume of detectors cause increased probability for scattering Compton ,In other words the increase in the probability of interaction effect photoelectric be less than the increase in the probability of interaction scattering Compton so the Net Area under the peak at less an increase of scattering spectrum .
8
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9
1-1
Introduction Health physics is a systematic organization of knowledge about the
interaction between radiation and organic or inorganic mater. It is a versatile science that is based upon physics, chemistry, biology and medicine. The scientific and engineering aspects of health physics are concerned mainly with: (1) The physical measurements of different types of radiation and radioactive materials; (2) The
establishment of quantitative
relationships between radiation, exposure and biological damage; (3) The movement of radioactivity through the environments; and (4) The design of radiologically safe equipment, processes, and environments. As well as, this science deals with the protection of the individual and population groups against the harmful effects of ionizing and non-ionizing radiation [1]. Both the type of radiation to which the person is exposed and the pathway by which they are exposed influence health effects. Radiation and radiation emitters (radionuclides) can expose the whole body (direct exposure) or expose tissue inside the body when inhaled or ingested. Different types of radiation vary in their ability to damage different kinds of tissue. All kinds of ionizing radiation can cause cancer and other effects. The main difference in the ability of alpha particles, beta particles, gamma-rays and x-rays to cause health effects is the amount of energy they have, their energy determines how far they can penetrate into tissue, it also determines how much energy they are able to transmit directly or indirectly to tissues and the resulting damage [2]. The biological damage resulting from a given absorbed energy may be quite different for different tissues. For these reasons, it is necessary to measure the radiation with an instrument and then translate it into response of tissue. Since different detectors do not have the same efficiency or
10
sensitivity for all types of radiation and at all energies, there is no single instrument that can be used for alphas, betas and gammas [3]. One of the most popular detectors used to study and measure the concentrations of the radionuclides is scintillation detector. Since 1947, these detectors have come into the wide scale use, meeting demands for detectors capable of higher counting rates, higher efficiency for detecting low level radiation and shorter resolving time than were possible with existing instruments, and also making possible gamma-ray spectroscopy [4].
1-2 Types of Radiation Radiation is the name given to the energetic particles or waves emitted by an atom at the time of radioactive decay [5]. It is produced by radioactive decay, nuclear fission, nuclear fusion, chemical reactions, hot objects, and gases excited by electric currents. Nuclei can undergo a variety of processes which result in the emission of radiation. The most common forms of radiation are alpha and beta particles and gamma-rays [2].
1-2-1 Alpha Particle Alpha particles are positively charged particles emitted by a heavy radioactive element, such as radium, thorium, uranium, and plutonium. When the ratio of neutrons to protons in the nucleus is too low, certain atoms restore the balance by emitting alpha particles [2]. Alpha emitting atoms tend to be very heavy atoms (that is, they have high atomic numbers). With some exceptions, naturally occurring alpha emitters have atomic numbers of at least 82 (the element lead). The alpha particles are a
11
portion of the parent nucleus that contains two protons and two neutrons. After alpha particle emission, the daughter nucleus has an atomic number Z reduced by 2, and an atomic mass number reduced by 4, compared to the parent nucleus [5]. Alpha particle is identical to the nucleus of a He-4atom, and after it has lost its kinetic energy by collision with other atoms in a material it will capture two electrons and become an atom of 4He2 [6]. The emitted alpha particles are monoenergetic, their energy is in the range of a few MeV [7]. An alpha particle interacts strongly and has a very short range-a few cm in air. A single piece of paper can stop an alpha ray effectively [8].
1-2-2 Beta Particle A charged particle emitted from the nucleus of some radioactive atoms during radioactive decay. In this decay, the nucleus can correct a proton or a neutron excess by directly converting a proton into a neutron or a neutron into a proton. This process must involve another charged particle to conserve electric charge. The first process is known as negative beta decay or negatron decay and involves the creation and emission of an ordinary electron. The second process is positive beta decay or positron decay, in which a positive charge electron is emitted [6]. ŶїƉнĞͲɴͲĚĞĐĂLJ͙;ϭͲϭͿ ƉїŶнĞнɴнĚĞĐĂLJ͙;ϭͲϮͿ ,QWKHVHSURFHVVHV\HWDQRWKHUSDUWLFOHFDOOHGDQHXWULQRLQ ȕGHFD\ RUDQWLQHXWULQRLQȕ- decay) is also emitted, but it has no electric charge. In these process, Z and N each change by one unit, but the total mass number Z+N remains constant [9]. Since the number of protons in the
12
nucleus determines the element, the conversion of a neutron to a proton actually changes the radionuclide to a different element [10]. Beta particles are high-speed charged particles with a moderate penetrating power. Beta particles can pass through a sheet of paper, but may be stopped by a thin sheet of aluminum foil or wood [7, 8]. Beta particles lose their initial kinetic energy by collision with electrons in the atoms of the surrounding material. These atoms become ionized. After losing their energy the beta particles become free electrons [11]. 1-2-3 Gamma-ray Gamma-ray is an electromagnetic radiation emitted from the nuclei of some unstable (radioactive) atoms [12]. The nucleus has discrete energy levels, like those of the electrons in an atom. The nuclear force, however, is much stronger than the electromagnetic and hence transitions from one state to the other are characterized by the emission of photons of much larger energy-from a hundred KeV to a few MeV. Such photons are called gamma-rays [7]. When a nucleus decays by alpha particle emission, the nucleus is sometimes left in an excited energy state. It usually changes to a ground state, or lowest energy state, emitting one or more gamma rays [13]. Often, gamma ray emission accompanies the emission of a beta particle. When the beta particle ejection does not rid the nucleus of the extra energy, the nucleus releases the remaining excess energy in the form of a gamma photon [10]. Gamma radiation is very high-energy ionizing radiation because of their high energy, gamma photons travel at the speed of light and can cover hundreds to thousands of meters in air before spending their energy. They can pass through many kinds of materials, including human tissue
13
[9]. Very dense materials, such as lead, thick concrete are commonly used as shielding to slow or stop gamma photons [4,8].
Figure (1- 1) Penetrating ability for alpha, beta and gamma [2]
1-3 Ionization and Excitation Radiation is often separated into two categories, ionizing and nonionizing, to denote the energy and the danger of the radiation [5]. When sufficient energy is absorbed by an electrically neutral atom or molecule, an electron may be ejected from it, leaving behind a positively charged residue. This process-known as ionization, leads to the formation of an ion pair [12]. In some cases the energy received either by collision or by radiation absorption may not be sufficient to form an ion pair. The atom may then be left in a neutral, unionized but excited electronic state. It is sufficient now to note that an excited atom may radiate the excitation energy and return to its original, undisturbed state of lowest energy, the ground state [14]. Many forms of radiation such as heat, visible light, microwaves, or radiowaves do not have sufficient energy to remove electrons from atoms and hence, are called non-ionizing radiation [11].
14
Ionization or excitation may occur when either a photon or a charged particle, such as an electron, a proton or an alpha particle, collides with an orbital electron. This mechanism is of great importance in health physics because it is the avenue through which energy is transferred from radiation to mater. When living matter is irradiated, the primary event in the sequence of events leading to biological damage is either excitation or ionization [1]. 1-4 Radiation Sources The radiation comes from two sources: natural or background radiation and human-made radiation. 1-4-1 Natural or Background Radiation Natural or background radiation is the radiation emitted by radioisotope that exist on or inside the earth, as well as radiation incident upon the earth from outer space [3]. Natural background radiation comes from two primary sources: cosmic and terrestrial sources. 1-4-1-1 Cosmic Radiation The earth, and all living things on it, are continually bombarded by high-energy particles that originate in outer space called cosmic rays. These cosmic rays interact with nuclei of atmospheric constituents, producing a cascade of interactions and secondary reaction products that contribute to cosmic ray exposure, which decrease in intensity with the depth in the atmosphere [15]. The cosmic ray consists of 87% protons, 11% alpha particles, about 1% nuclei with low atomic number, and about 1% electrons of high kinetic energy. The cosmic rays have a high penetrating energy 1020 eV or more [16]. The interaction of cosmic ray in the atmosphere produces a number
15
of radionuclides called cosmogenic radionuclides, including 14
C,
3
7
H,
Be,
22
Na, etc [17].
The dose from cosmic radiation varies in different parts of the world due to many factors including differences in elevation, the effect of the earth's magnetic field and local differences in terrain.
1-4-1-2 Terrestrial Radiation Radioactive material is found throughout nature. It occurs naturally in the soil, water, and vegetation [5].The primordial radionuclides whose half-lives are sufficiently long to have survived in detectable quantities since the formation of the earth, together with their radioactive daughters. The primordial radionuclides can be divided into those that occur singly and those that are components of chain series [18]. The major isotopes of concern for terrestrial radiation are uranium and its decay products, such as thorium, radium, and radon. Low levels of uranium, thorium, and their decay products are found everywhere. Some of these materials are ingested with food and water, while others, such as radon, are inhaled. The dose from terrestrial sources varies in different parts of the world. Locations with higher concentrations of uranium and thorium in their soil have higher dose levels [12]. 1-4-2 Human Made Radiation Natural and artificial radiation sources are identical in their nature and their effect [5]. Humans are using the radioactivity for one hundred years, and through its use, added to natural inventories. Its amount is small compared to the natural amount discussed above due to the short half-lives of many of the nuclides, and it demonstrate a marked decrease since the halting of above ground testing of nuclear weapons [16].
16
Two important categories of artificial radionuclides-include those, which appear in nuclear power plants from the fission of uranium and other nuclear reactions, and the radionuclides and labeled compounds, which are specially produced for application in various fields of human activity. Large varieties of radioactive nuclides are produced in minute amounts in nuclear research laboratories in the form of by products from the investigation of nuclear processes [19]. The most significant source of man-made radiation exposure to the general public is from medical procedures, such as diagnostic x-rays, nuclear medicine, and radiation therapy. Some of the major isotopes would be I-131, Co-60, Sr-90, Cs-137, and others. In addition, members of the public are exposed to radiation from consumer products, such as tobacco (polonium-210), building materials, televisions, luminous watches and dials (tritium), airport x-ray systems, smoke detectors (americium), etc. [5]. 1-5 Interaction of Gamma-ray with Matter Gamma-rays interact with matter in several ways, depending on their energy. Ordinarily, only three important processes are taken into account, these are the photoelectric effect, Compton effect, and the pair production [20]. 1-5-1 Photoelectric Effect The photoelectric effect is an interaction between a photon and a bound atomic electron. As a result of the interaction, the photon disappears completely and one of the atomic electrons is ejected as a free electron, called the photoelectron [3]. Generally, the photon will interact with the innermost or K-shell of the atom provided that it is energetic enough to ionize the K-shell [21].
17
In this reaction, energy of the photon (EȖ), minus binding energy of the electron (BE), is
given
to
the
free
electron
as
a
kinetic energy (T) [12]. This reaction is pictured schematically in figure (1-2). If the electron, which is free from the original atom, is an inner electron, an outer electron will take its place. An x-ray will be emitted with energy equal to the difference in binding energy of the two electron levels. This x-ray may act like a gamma-ray and free an electron from another atom by the photoelectric effect, and the process will be repeated until all the x- rays have too little energy to free any other electrons [20].
Figure (1-2) Photoelectric effect [12]
The photoelectric effect is stronger for lower energy gamma-rays and for absorber materials of high atomic number [21]. The result of a single gamma ray causing the photoelectric effect is thus a large number of ionized atoms, each with outer electron removed, and a large number of free electrons. 1-5-2 Compton Scattering Compton scattering is an interaction between gamma-rays and free or only weakly bound electron [9, 19]. In this process, the incoming gamma-UD\ SKRWRQ LV GHIOHFWHG WKURXJK DQ DQJOH ș ZLWK UHVSHFW WR LWV original direction. The photon transfers a portion of its energy to the
18
electron and is scattered at a lower energy, which is then known as a recoil electron. This is illustrated in figure (1-3). Because all angles of scattering are possible, the energy transferred to the electron can vary from zero to a large fraction of the gamma-ray energy [4, 22]. The expression, which gives the interaction, can be simply derived by writing simultaneous equations for the conservation of energy and momentum. Using the symbols defined in the sketch below, we have [22] Kȣǯ Kȣ>KȣPoc2) (1- FRVș @
«-3)
Where, moc2 is the rest mass energy of the electron (0.511 MeV).
Figure (1-3) Compton scattering of a gamma-ray [22]
Two extreme values are important in Compton scattering. The first is the minimum energy of the scattered gamma-ray that occurs at a VFDWWHULQJ DQJOH ș °. This minimum of gamma-ray energy is often called the backscattered gamma-ray and its energy (EǯȖ%&) given by: EǯȖ%& = (moc2/2)/ [1+ (moc2/2)/EȖ]
«-4)
19
From equation (1-4) it can be seen that the energy of the backscattered gamma-ray cannot exceed 0.255 MeV even for very high energy incident gamma-rays [23]. The second extreme value of interest is the energy of the electron UHFRLOLQJ DW ij °, often termed the Compton edge energy (EeCE). This forward scattered electron accompanies the 180 ° backscattered gamma-ray since there can be no transverse component of momentum in this case and its energy is easily found from conservation of energy to be [23] EeCE (Ȗ>Poc2/2)/EȖ]
«-5)
From equation (1-5) it can be seen that the Compton edge energy is always less than the incident gamma-ray energy. The probability of Compton scattering depends upon the number of available (weakly bound) electrons as well as on the cross-section and so it increases with both the atomic number of the scatter and the incident gamma-ray energy [18]. 1-5-3 Pair Production Pair production is an interaction between a photon and nucleus coulomb field. As a result of the interaction, the photon disappears completely and an electron-positron pair appears, although the nucleus dose not undergo any change as a result of this interaction [3]. This reaction is only energetically possible provided that the gamma-ray energy exceeds twice the rest mass energy equivalent of the electron (1.022 MeV) [20]. The positron will, after being slowed down to rest, annihilate with the nearest available electron and so release two gamma-ray quanta each of 0.511 MeV as shown in figure (1-4) [24].
20
Figure (1-4) Pair production [3]
The probability of pair production is zero for gamma-rays with energy below 1.022 MeV, but it increases rapidly with the increase of energy above this threshold as shown in figure (1-5). So for high energy gamma-rays, pair production is much more probable than either Compton scattering or photoelectric effect. Both the positron and electron will cause ionization by scattering with atomic electrons until they have lost most of their kinetic energy [21,22].
Figure (1-5) Probability of pair production as a function of gamma-ray energy [24]
Thus, the pair production is a process by which a single high-energy gamma-ray may be converted into lower energy gamma-rays and also cause ionization [13].
21
1-6 Gama Ray Spectral Analysis Gamma-ray spectroscopy is the quantitative study of the energy spectra of gamma-ray sources, in the nuclear laboratory and nuclear process,
and
in
geochemical,
astrophysical
and
other
radiation
measurement contexts. Most radioactive sources produce gamma rays of various energies and intensities. When these emissions are collected and analyzed with a gamma-ray spectroscopy system, a gamma-ray energy spectrum can be produced [25]. A detailed analysis of this spectrum is typically used to determine the identity and quantity of gamma emitters present in the source. The gamma spectrum is characteristic of the gamma-emitting nuclides contained in the source, just as in optical spectroscopy, the optical spectrum is characteristic of the atoms and molecules contained in the sample͘ The equipment used in gamma spectroscopy includes an energysensitive radiation detector, electronics to collect and process the signals produced by the detector, such as a pulse sorter (i.e., multichannel analyzer), and associated amplifiers and data readout devices to generate, display, and store the spectrum. Other components, such as rate meters and peak position stabilizers, may also be included. The most common detectors include sodium iodide (NaI) scintillation counters and high-purity germanium detectors [18]. Gamma spectroscopy detectors are passive materials that wait for a gamma interaction to occur in the detector volume. The most important interaction mechanisms are the photoelectric effect, the Compton effect, and pair production. The photoelectric effect is preferred, as it absorbs all of the energy of the incident gamma ray. Full energy absorption is also possible when a series of these interaction mechanisms take place within the detector volume. When a gamma ray undergoes a Compton interaction or pair production, and a portion of the energy escapes from
22
the detector volume without being absorbed, the background rate in the spectrum is increased by one count. This count will appear in a channel below the channel that corresponds to the full energy of the gamma ray. Larger detector volumes reduce this effect [25]. The voltage pulse produced by the detector (or by the photomultiplier in a scintillation counter) is shaped by a multichannel analyzer (MCA). The multichannel analyzer takes the very small voltage signal produced by the detector, reshapes it into a Gaussian or trapezoidal shape, and converts that signal into a digital signal. In some systems, the analog-to-digital conversion is performed before the peak is reshaped. The analog-to-digital converter (ADC) also sorts the pulses by their height. ADCs have specific numbers of "bins" into which the pulses can be sorted; these bins represent the channels in the spectrum. The number of channels can be changed in most modern gamma spectroscopy systems by modifying software or hardware settings. The number of channels is typically a power of two. The choice of number of channels depends on the resolution of the system and the energy range being studied [24]. The multichannel analyzer output is sent to a computer, which stores, displays, and analyzes the data. A variety of software packages are available from several manufacturers, and generally include spectrum analysis tools such as energy calibration, peak area and net area calculation, and resolution calculation.
Fig. ( 1-6 ) Parts of the system detection
23
Photopeak The photopeak is the peak formed by the case where the gamma ray deposits all of its energy in the detector. This can happen because the gamma interacts once through the photoelectric effect. It can also happen if the gamma initially interacts through Compton scattering or pair production, but the secondary particles then are also completely absorbed. High voltage The term high voltage usually means electrical energy at voltages high enough to inflict harm or death upon living things. Equipment and conductors that carry high voltage warrant particular safety requirements and procedures. In certain industries, high voltage means voltage above a particular threshold [26]. Amplifiers Amplifier, or (informally) amp is an electronic device that increases the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude. In this sense, an amplifier modulates the output of the power supply. Amplifiers are described according to their input and output properties.[2] They exhibit the property of gain, or multiplication factor that relates the magnitude of the output signal to the input signal. The gain may be specified as the ratio of output voltage to input voltage (voltage gain), output power to input power (power gain), or some combination of current, voltage, and power. In many cases, with input and output in the same unit, gain is unitless (though often expressed in decibels (dB)) [27].
24
A preamplifier (preamp) is an electronic amplifier that prepares a small electrical signal for further amplification or processing. A preamplifier is often placed close to the sensor to reduce the effects of noise and interference. It is used to boost the signal strength to drive the cable to the main instrument without significantly degrading the signal-tonoise ratio[28]. 1-7 The Aim of the Present Work The aim of the present work is study a comparison of the detector (scintillation) sizes (2"x2") ,(1"x1.5") in terms of efficiency and Severability energy resolution and note the effect change voltages and amplification on the power spectrum by calculating the total area of the spectrum space scattering and area of peak optical.
25
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([SHULPHQWDO3DUW
26
2-1 Gamma-ray Detectors All nuclear radiation detection is based on the interaction of radiation with matter. The detection methods are in general based on the process of excitation or ionization of atoms in the sensitive volume of the detector by the passage of charged particles [4]. The types of gamma detectors of greatest interest for safeguard work are the gas-filled tube, the sodium iodide (NaI) scintillation and semiconductor detectors [16]. The methods of gamma-ray interaction with matter were discussed in detail in chapter one. Here we will discuss the NaI(Tl) detector used in some detail. 2-2 Scintillation Counter The scintillation counter is comprised of two main components: firstly, a scintillator that absorbs incident radiation and converts the energy deposited by ionization into a fast pulse of light and, secondly, a photomultiplier. This second component converts the light pulse into a pulse of electrons and also amplifies the electron pulse by a very large factor by means of a sequence of secondary emission stage [26]. Common scintillators include thallium-doped sodium iodide (NaI(Tl)) often simplified to sodium iodide (NaI) detector, because photomultipliers are also sensitive to ambient light, scintillators are encased in light-tight coverings. Thallium doped Sodium Iodide NaI(Tl) is the most widely used scintillation material. NaI(TI) is used traditionally in nuclear medicine, environmental measurements, geophysics, medium-energy physics, etc. The fact of its great light output among scintillators, convenient emission range (in coincidence with maximum efficiency region of photomultiplier (PMT) with bialkali photocatodes), the possibility of large-size crystals production, and their low prices compared to other scintillation materials compensate to a great extent for the main Nal(TI) disadvantage. Which is
27
namely the hygroscopicity, on account of which NaI(TI) can be used only in hermetically sealed assemblies. Varying of crystal growth conditions, dopant concentration, raw material quality, etc. makes it possible to improve specific parameters, e.g., to enhance the radiation resistance, to increase the transparency, and to reduce the afterglow. For specific applications, low-background crystals can be grown. NaI(TI) crystals with increased dopant concentration are used to manufacture X-ray detectors of high spectrometric quality. NaI(TI) is produced in two forms: single crystals and polycrystals. The optical and scintillating characteristics of the material are the same in both states. In some cases of application, however, the use of the polycrystalline material allows coping with a number of additional problems. First, a press forging makes it possible to obtain crystals with linear dimensions exceeding significantly than those of grown single crystals. Second, the polycrystals are ruggedized, which is important in some cases. Moreover, NaI(TI) polycrystals do not possess the perfect cleavage, so the probability of their destruction in the course of the use is reduced. The use of extrusion in converting NaI(TI) into the polycrystalline state makes it also possible to obtain complex-shaped parts without additional expensive machining [17,20]. In the present work , the nuclear detection system , figure ( 2-1 ) , type
( UCS 30) (Spectrum Techniques LLC ) with NaI(Tl) size of crystal
(2"x2") ,(1"x1.5")cm.
Fig. ( 2-1 ) Detection system used [27]
28
2-3 Mechanism of the Scintillation Counter When a gamma ray interacts with scintillation crystal, light is released by the process previously described. The photons of light are transmitted through the transparent crystal and are directed upon the photosensitive cathode (photocathode) of a photomultiplier tube [26]. Photomultipliers are constructed from a glass envelope with a high vacuum inside, which houses a photocathode, several dynodes, and an anode. Incident photons strike the photocathode material, which is present as a thin deposit on the entry window of the device, with electrons being produced as a consequence of the photoelectric effect. These electrons are directed by the focusing electrode toward the electron multiplier, where electrons are multiplied by the process of secondary emission [23]. The electron multiplier consists of a number of electrodes called dynodes. (DFK G\QRGH LV KHOG DW D PRUH SRVLWLYH YROWDJH E\ § 9ROWV than the previous one. A primary electron leaves the photocathode with the energy of the incoming photon, or about 3 eV for "blue" photons, minus the work function of the photocathode. As a group of primary electrons, created by the arrival of a group of initial photons, moves toward the first dynode they are accelerated by the electric field. They HDFK DUULYH ZLWK §00 eV kinetic energy imparted by the potential difference. Upon striking the first dynode, more low energy electrons are emitted, and these electrons in turn are accelerated toward the second dynode. The geometry of the dynode chain is such that a cascade occurs with an ever-increasing number of electrons being produced at each stage. For example, if at each stage an average of 4 new electrons are produced for each incoming electron, and if there are 12 dynode stages, then at the last stage one expects for HDFK SULPDU\ HOHFWURQ DERXW A § A electrons. This large number of electrons reaching the last stage, called the anode, results in a sharp current pulse that is easily detectable, signaling
29
the arrival of the photon at the photocathode about a nanosecond earlier[18].
Figure (2-2) Photomultiplier tube with possible resistance chain [19]
2-4 General Characteristics of NaI(Tl) The photoelectric effect is quite marked in NaI and is mainly due to the iodine which has a Z of 53. Higher atomic number atoms have a large photoelectric cross-section than low atomic number atoms [26]. The high density of the scintillator permits the total absorption of many radiations within the crystal. With total absorption, the light output (LO) may be quite accurately proportional to the energy of the incident radiation, and the device may be used as an energy spectrometer [16]. LO is the coefficient of conversion of ionizing radiation into light energy. The LO of NaI(Tl) is taken to be 100%. LO of other scintillators is determined relatively to that of NaI(Tl) percentage [19]. NaI(Tl) has short decay time for the emission of light so that fast counting is possible and negligible after-glow. It is also so hygroscopic which means it is easily damaged when exposed to moisture in air at humidity levels [3].
30
2-6 Energy Resolution Energy resolution (R) shows the ability of a detector to distinguish gamma sources with slightly different energies. The R of the detector is defined as the full width at half maximum (FWHM) divided by the location of the peak centroid (n) [30]. A formal definition of FWHM and peak centroid (n) are shown in figure (2-4), where R is expressed as percentage [17] E.R = FWHM î«2-2) n F.W.H.M : Full width at high maximum . Ch .no : photo peak position In the present work, the resolution of the detector is found about 8.7% measured by using Cs-137 (662 MeV) source (Sabharwal,et.al.,2008).
Figure (2-4) Formal definition of (FWHM) and peak centroid (n) in gamma-ray spectrum of Cs-137 by using NaI(Tl) detector [31]
32
&KDSWHU7KUHH
5HVXOWVDQG'LVFXVVLRQ
33
3-1 Introduction This study used two types of detectors scintillation NaI(Tl) sizes (2"x2") ,(1"x1.5") as well as used two sources of radioactive Cesium has energy (0.662 MeV) HIIHFWLYHQHVV UDGLRORJLFDO ȝ&L DQG (30.07 year), and
half-life
Cobalt Co-60 has two energy (1.33&1.73 MeV),
HIIHFWLYHQHVV UDGLRORJLFDO ȝ&L DQG KDOI-life (5.27 year) [32], with the installation of the time on (200) sec and the distance between the radioactive source and detector NaI(Tl) (15) cm , by changing the voltage (V) (500-700) volt and changing the amplification(Amp) (1-64).
3-2
Study the effect of the size of crystal detector (scintillation) NaI(Tl) on the energy spectrum For the purpose of knowing the impact of crystal size detector
(scintillation) on the resulting spectrum was calculated the total area of the spectrum (T.A) , a scattering area (Sc.A) and Photo peak (Ph.P) and N, G refer to the Net and Gross respectively.
3-2-1- Study the effect of voltage on the energy spectrum To find out the effect of voltages on the energy spectrum will explain the tables (3-1),(3-2) calculations for both the detectors NaI(Tl) and the sources of the radioactive Cesium -137 and Cobalt -60 respectively at amplification on (2) by changing the voltage (V) (500-700) volt.
34
Table (3-1) Shows the accounts process for the source of radioactive Cesium -137 at Amplification (2) for both detectors NaI(Tl) 1"x1.5",(2"x2") 2Amp Detector 1"x1.5" T.A
Sc.A
Ph.P
F.W.H.
V
Cs-137
G
N
G
N
G
N
M
C
E.R 7.59
500
25857
23485 19372 18035 5709
4859
40
527
550
30477
25178 24556 22379 5769
4835
88
1343 6.552
600
31527
27378 25921 22712 5782
4864
109
2920 3.733
620
32207
28840 26559 26018 5789
4820
112
3824 2.929
Detector 2"x2" V
T.A
Sc.A
Ph.P
F.W.H.
C
E.R
²
²
²
²
²
²
²
3641 7541
6188
12
123
9.756
G
N
G
N
G
N
M
500
1114
993
²
²
²
²
550
9508
9148
²
²
²
600
14413
13939
4670 2279
650
22230
21750
10990 7776
6213
18
210
8.571
700
30370
29363 20424 18019 8059
6745
27
333
8.108
From table (3-1) we see that increased voltages lead to increasing every (T.A) and (Sc.A) as in Figure (3-1), which shows the relationship between voltage and the total area of both detector by using
source
Cesium-137 at amplification (2), and notes in detector 2"x2" be the spectrum does not appear at first clearly but gradually begins to emerge while the energy resolution decreases with increasing voltage for both detector as shown in figure (3-2).
35
Table (3-2) Shows the accounts process for the source of radioactive Cobalt-60 at amplification (2) for both detectors NaI(Tl) 1"x1.5",(2"x2") 2 Amp Detector 1"x1.5" T.A
Sc.A
Ph.P1
Ph.P2
F.W.
V
F.W. C
E.R1
E.R2
33
1040
4.048
3.198
1015
45
1804
2.61
2.482
1493
49
2509
2.148
1.952
C
E.R1
E.R2
C N
G
N
G
N
H.M
500
22867
20328
20193
16696
1413
741
37
530
33461
29721
29256
21400
2342
1483
550
51891
47985
43425
32508
4872
2870
Co 60
G
G
N
H.M
914
1147
770
42
1609
1703
48
2234
2436
Detector 2"x2" T.A
Sc.A
Ph.P1
Ph.P2
F.W.
V
F.W.
C G
N
G
N
G
N
H.M
G
N
H.M
500
7595
7465
²
²
²
²
²
²
²
²
²
²
²
²
550
15893
15610
8257
5477
3877
2160
10
132
2803
1435
11
156
7.567
7.143
600
21100
20925
13766
9932
3898
2208
16
235
2990
1950
17
270
6.809
6.296
650
28886
28519
21754
17083
3957
2253
24
386
3021
2145
23
442
6.218
5.204
700
36648
35649
29220
23099
4070
2286
38
602
3053
2232
30
677
6.312
4.431
Table (3-2) shows, as stated in Table (3-1) that the total area of the spectrum (T.A) and scattering area (Sc.A) increases with voltage as in the figure(3-3), where the energy resolution in the first photo peak in Cobalt be
bigger than energy resolution in second photo peak as shown the
figure (3-4) ,(3-5).
38
voltages on the spectrum energy and his found that the (T.A) and (Sc.A) increasing with increase voltages as well as increase the photo peak leading to an offsets peak from location to the location of the top of the channel axis while energy resolution decreases due to the increased width of the channel. 3-2-2- Study the effect of amplification on the energy spectrum To find out the effect of amplification on the energy spectrum show tables (3-3),(3-4) Practical calculations for both detectors by using sources of the radioactive Cesium-137 & Cobalt-60 respectively with the installation of the time on (200) sec and the distance between the radioactive source and detector NaI(Tl) (15) cm at voltage on (500 volt) by changing the amplification (1-64) (Amp). Table (3-3) Shows the accounts process for the of source radioactive Cesium -137 at voltage on (500 volt) for both detectors NaI(Tl) 1"x1.5",(2"x2")
Cs-137
500 volt A m 1 2 4 8 16
T.A G 18271 25857 29833 31914 32648
N 16590 23485 27348 28066 31304
Detector1"x1.5" Sc.A Ph.P G N G N 11817 10286 5424 4136 19483 18210 5411 3924 23443 22038 5491 4364 26047 24302 5846 4674 26700 25724 5979 5236
F.W. H.M 23 40 58 91 139
C
E.R
260 526 1022 1960 3750
8.846 7.605 5.68 4.643 3.707
C
E.R
² ² ² 163 338 658
² ² ² 9.202 8.876 7.903
Detector 2"x2" A m 1 2 4 8 16 32
T.A G N 72 50 1114 993 10100 9935 18366 17935 30752 29601 41414 38778
Sc.A G N ² ² ² ² ² ² 8266 7162 20704 18513 31622 28834
Ph.P G N ² ² ² ² ² ² 7795 5464 8012 6127 7915 5968
F.W. H.M ² ² ² 15 30 52
41
Table (3-4) shows the accounts process for the of source radioactive Cobalt60 at voltage on (500 volt) for both detectors NaI(Tl) 1"x1.5",(2"x2")
500V Detector 1"x1.5" T.A
Sc.A
Ph.P1
1
Ph.P2
F.W.
V
F.W. C
E.R1
E.R2
21
522
4.113
4.023
C G
N
G
N
G
N
H.M
16436
14689
13703
10427
1413
752
19
G
N
H.M
462
1154
860
22867
20328
20193
16696
1413
741
29
914
1147
770
39
1040
3.173
3.75
26126
23056
23410
19114
1545
515
39
1748
1071
302
43
2002
2.231
2.148
8
52918
46987
44430
34203
4961
3296
42
3283
3366
2355
47
3799
1.279
1.237
C
E.R1
E.R2
²
²
²
Co-60
2 4
Detector 2"x2" T.A
Sc.A
Ph.P1
1
Ph.P2
F.W.
V
F.W.
C G
N
G
N
G
N
H.M
79
62
²
²
²
²
²
²
G
N
H.M
²
²
²
2
7595
7465
²
²
²
²
²
²
²
²
²
²
²
²
4
16776
16696
9446
6546
4603
1465
10
147
2343
1053
11
171
6.803
6.433
8
24929
24642
17692
12871
3733
2025
22
304
2773
1541
22
349
7.237
6.304
16
36720
35874
29786
24998
3637
1984
32
608
2731
1733
33
695
5.263
4.748
32
47365
43123
40239
32963
3822
2382
38
1159
3053
2232
39
1321
3.278
2.952
From Table (3-4)
we note that increase amplification leads to
increased the total area of the spectrum as shown in figure (3-8), while energy resolution decreased as well as being at the first photo peak the larger than second photo peak , as shown in Figures (3-9) &(3-10) .
44
3-3 Conclusions
1- The large size of the crystal detector leads to increased capacity pulse
emerging from detector (2"x2") which appear with a capacity greater than the detector (1"x1.5") leading to an offset for their peaks where increasing both (T.A) and (Sc.A). 2- present study showed that the energy resolution less to increase the number of channels and voltage relationship exponential decreasing they are in detector (1"x1.5") less than it is in detector (2"x2").while increasing photo peak Location to increase the number of channels and the voltage, linear relationship. 3- At increase the size of crystal, the Photo peak increases too, because the number of photons entering the crystal detector (2"x2") be the largest and photo peak higher than at detector (1"x1.5"). 4- Increase the amplification means increasing the number of pulses generated inside the detector and thus increasing (T.A), (Sc.A) and (Ph.P) due to increased capacity pulse that led to the widening spectrum and creep photo peak position and thus exit the peak from the axis of the channel, while increasing amplification lead to increased (FWHM) in other words increase width photo peak that leads to decreases portability energy analysis (E.R).
47
5HIHUHQFHV
48
References >@ &DPEHU + ³Introduction to Health Physics´ nd ed., Pergamon Press, New York, 1987.
[2] U.S.Environmental Protection AgHQF\ ³8QGHUVWDQGLQJ 5DGLDWLRQ +HDOWK 3K\VLFV´ http://www.epa.gov/radiation/understand/health_effects.htm --> [3] 7VRXOIDQLGLV 1 ³Measurement and Detection of Radiation´ +HPLVSKHUH Publishing Corporation, Washington, 1983. [4] 3ULFH:-³Nuclear Radiation Detection´3HUJDPRQ3UHVV86$) [5] -RQHV55 56RXWKZRRG³Radiation and Health: the Biological Effect of Low-level Exposure to Ionizing Radiation´-RKQ:Lley & Sons, Chaichester, 1987. [6] 7D\DO'&³Nuclear Physics´+LPDOD\D3XEOLVKLQJ+RXVH/RQGRQ [7] '\VRQ 1 $ ³An Introduction to Nuclear Physics with Application in Medicine and Biology´(OOLV+RUZRRG/LPLWHG&KLFKHVWHU [8] 6LHJEDKQ . ³Alpha-, Beta- and Gamma-Ray Spectroscopy´ 1RUWK-Holland Publishing Company, Amsterdam, 1974. [9] .UDQH . 6 ³Introductory Nuclear Physics´ nd ed., John Wiley & Sons, New York, 1988. [10] )HGHUDO(PHUJHQF\$JHQF\³)XQGDPHQWDO&RXUVHIRU5DGLRORJLFDO0RQLWRU´ U.S.A, 1984. [11] +LQH*³Instrumentation in Nuclear Medicine´$FDGHPLF3UHVV1HZ