Lui Chi Kong
Department of Physics, The Hong Kong University of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong
27th May, 2003
Abstract
This paper aims to focus on the study of Alpha, Beta and Gamma Spectroscopy. In Alpha Spectroscopy, the spectra of Po-210 and Am-241 sources were studied and the calibration curves for several energy ranges were constructed. The resolution of the detector and the source activity of Po-210 source were also investigated. In Beta Spectroscopy, the spectra of Sr-90 and Tl-204 source were studied and their total decay energies were determined. In Gamma Spectroscopy, the single- and multi-channel scintillation systems were used to study the spectra of Cs-147 and Co-60 sources. The resolutions of the two systems were also compared. An unknown source was inferred by obtaining and studying its spectrum.
Introduction
The term ‘spectroscopy’ can be treated as the fine structure of the radiation energy distribution from a given nuclide. Since the alpha and beta particles have different ionization power whereas gamma ray does not have direct ionization, the detector and the environment for detection are different.
In the experiment, the Model 5030 Alpha/Beta Spectrometer was used to study the alpha/beta spectroscopy, while the Na(Tl) detector and a multi-channel scintillation analyzer were used for studying the Gamma spectroscopy.
Theory
In alpha decay, when an alpha particle is ejected from a radioactive nucleus, the nucleus loses two protons and two neutrons. The decay process is represented in the form of a nuclear equation as
where X and Y are the initial and final nuclear species. For each distinct transition between the initial and final nucleus, a fixed energy difference characterizes the decay. This energy is shared between the alpha particle and the recoil nucleus in a unique way, so that each alpha particle appears with the same energy. Therefore, the alpha particles are mono-energetic and there is only one peak appear in the alpha spectrum.
While in beta decay, a negatron or positron is emitted from a nucleus. The nuclear equations for negatron and positron emission are
where X and Y are initial and final nuclear species. The symbols υ’ and υ represent antineutrino and neutrino. The total energy given up by the nucleus of the decaying nuclide in beta decay is shared by the ejected beta particle, the neutrino and the recoiling daughter nucleus. Each of the parts can have any combination of the total decay energy.
For gamma radiation, a gamma-ray photon is unchanged and creates no direct ionization or excitation of the material through which it passes. The detection of gamma rays is therefore to cause the gamma-ray photon to undergo an interaction that transfers all or part of the photon energy to an electron in the absorbing material. Therefore, a gamma-ray spectrometer acts as a conversion medium in which gamma rays have a reasonable probability of interacting to yield one or more fast electrons and function as a conventional detector for these secondary electrons.
The use of scintillation mechanism is one of the oldest techniques used for detecting radiation. Several stages in a scintillation detector involve during which the energy carried by the radiation is transformed before being in the form of signal suitable for data processing. There are two basic types of scintillator material: organic and inorganic. An inorganic scintillator of sodium iodide (NaI) was used in the experiment. Electrons in a crystalline solid occupy levels which are grouped into bands separated by energy band gap. The valence band is full of electrons that are weak bound to their individual atoms, and the conduction band is generally empty. Incoming radiation causes many electrons to be excited into the conduction band, leaving holes in the valence band. When energy is deposited in the crystal, a fraction of the excited electrons and holes migrate to the activator excited states. Since the energy difference between the activator is less than the band gap, then energy of photons emitted when these states lose their energy is below that need to re-excite the main crystal and the scintillator is transparent to them. [1]
Experimental Apparatus and Procedures
For Alpha/Beta Spectroscopy, the Model 5030 Alpha/Beta Spectrometer connected with Quantum 8 Multichannel Analyzer (MCA) was used.
In Alpha Spectroscopy, the spectra of the sources Po-210 and Am-241 for energy ranges 5-7, 4-6 and 3-8 MeV were obtained. The energy calibration curves for the energy ranges and the resolution of the detector were then determined from the spectra of the sources. The source activities of Po-210 in different shelf of the spectrometer were determined from the values of net counts, source-detector distance and the active area of the detector.
In Beta Spectroscopy, the energy calibration curve was determined from the peak channels of the spectra of different pulser energies. The spectra of the source Sr-90 and Tl-204 were then obtained and their total decay energies were determined.
In Gamma Spectroscopy, the single- and multi-channel scintillation systems were used. The single-channel scintillation system consists of Model 575 Scaler/Timer, Model 2010 Amplifier/Analyzer and Model P-2000 NaI(Tl) Probe. The spectra of the sources Cs-137 and Co-60 were obtained by conducting one-minute counts with baseline E-dial settings. An energy calibration curve for the NaI(Tl) detector was constructed using the spectra of the gamma sources. The resolution of the detector was determined by calculating the ratio of the width at half maximum count rate (FWHM) and the peak center point.
The multi-channel scintillation system consists of the Quantum 8 MCA and the NaI(Tl)scintillation detector. The Quantum 8 MCA was calibrated as 1keV/channel. The spectra of the known source Cs-137 and an unknown source were obtained using the system. The unknown source was then deduced from its spectrum.
Data Analysis and Results
For Alpha Spectroscopy, the energy calibration curves for the energy ranges are obtained from the centriod channels of the peaks of Po-210 and Am-241 spectra.
For energy range 5-7 MeV, the energy calibration curve is
y = 0.00226x + 5.08333 (4)
For energy range 4-6 MeV, the energy calibration curve is
y = 0.00243x + 3.66343 (5)
For energy range 3-8 MeV, the energy calibration curve is
y = 0.00548x + 2.60097 (6)
where x is the channel number and y is the corresponding energy in MeV.
The FWHM and the detector resolution for energy range 5-7 MeV are 12.5 channels and 33.33 keV respectively. The FWHM and the detector resolution for energy range for energy range 4-6 MeV are 11.0 channels and 30.39 keV respectively. FWHM and the detector resolution for energy range 3-8 MeV are 4 channels and 24.52 keV respectively. The manufacturer’s specification on the detector resolution is 36 keV. Hence the percentage discrepancy is 7.42% for energy range 5-8 MeV, 15.58% for energy range 4-6 MeV, and 31.89% for energy range 3-8 MeV.
The source activity A of Po-210 at shelf 3 of the spectrometer is given by the equation
where C is total counts, T is total counting time, G3 is the geometry factor at shelf 3 and fa is the abundance of Po-210. Therefore, the source activity of Po-210 at shelf 3 is 466.594 Bq, or in terms of decay per minutes (dpm), 27995.64 dpm.
For Beta Spectroscopy, the energy calibration curve is
y = 0.00102x + 0.03697 (8)
where x is the channel number and y is the corresponding energy in MeV.
The spectrum of Sr-90 source shows only one peak, whereas the spectrum of Tl-204 source shows a much lower peak and there is a non-zero continuous spectrum after the peak. The experimental value of total decay energy is 0.303 MeV for Sr-90 and 0.466 MeV for Tl-204.
For Gamma Spectroscopy, the energy calibration curve for NaI(Tl) detector is
y = 0.00189x + 0.04133 (9)
where x is the E-dial setting and y is the corresponding energy in MeV.
Using the single-channel scintillation system, the spectra of Cs-137 and Co-60 sources were obtained. The position of backscatter peak, Compton region and the photo-peak of Cs-137 can be observed in the spectrum of Cs-137, but the 137Ba X-ray peak cannot be seen clearly. The Co-60 sum peak cannot be seen in the spectrum of Co-60. One possible reason is that the detector cannot see both gammas together since the two gamma rays did not have short interaction and light production time. From the spectrum of Cs-137, the resolution of the NaI(Tl) detector is 8.82%.
Using the multi-channel scintillation system, the spectrum of Cs-137 was obtained. The 137Ba X-ray peak can be seen clearly this time. The resolution of the detector is 6.05%; therefore the detector has higher resolution than the one in the single-channel system. The spectrum of the unknown source is obtained, and the source was inferred to be 57Co.
Discussion
In the experiment, alpha/beta spectroscopy has to be operated in an environment different from that for gamma spectroscopy, because of their different nature of radiation and ionization. Alpha-decay produces fast moving helium nucleus while beta-decay generates high speed electrons. Electromagnetic radiation is produced in gamma radiation. Also, the ionization power of alpha particles is stronger than that of beta and gamma decay. Therefore, the alpha/beta spectrometer was designed to be a single stand-alone instrument that contains all necessary electronic and mechanical components to permit alpha- or beta- high-resolution spectroscopy utilizing scilicon charged particle detectors, while gamma-ray spectrometer has to act as a conversion medium in which incident gamma rays have a reasonable probability of interacting to yield more fast electrons and function as a conversion detector for those secondary electrons.
In beta spectroscopy, beta decay does not always give a discrete energy spectrum. The energy peak of the source Tl-204 is not discrete because each of the ejected beta particles, the neutrino and the recoiling daughter nucleus can have any combination of the total decay energy. However, Sr-90 does have a discrete energy peak in the spectrum because the decay is an internal conversion, which produces conversion electrons that are generally mono-energetic.
In gamma spectroscopy, for Cs-137 the final peak at the low end of the pulse height scale is the K X-ray from Ba-137. This X-ray occurs when internal conversion takes place as an alternate mode. The Cs-137 nucleus emits a beta and goes to an excited state of Ba-137. The 0.662 MeV gamma-ray result when this excited Ba-137 nucleus de-excites or goes to the ground state by ejecting a K-shell electron. When this internal conversion occurs, a shell vacancy occurs and another shell electron tumbles down to fill the vacancy, thereby generating a K x-ray. In the case of Co-60, gamma-rays of 1.17 MeV and 1.33 MeV are emitted of one another for each disintegration.
The single channel scintillation analyzer (SCA) system serves to record the pulser spectrum from a steady-state source, while the operation of the multi-channel scintillation analyzer (MCA) is based on the principle of converting analog signal to equivalent digital number. [2] Compared to MCA, SCA system has several disadvantages, including long measurement time required, waste of information and smaller resolution power. The MCA system is therefore more convenient and effective in studying gamma spectroscopy. However, one defect of the MCA system is the spectrum stabilization and the spectrum stabilizer inside the system is not fully effective enough.
Conclusion
To summarize, the spectra of several radioactive sources and the principle of their spectroscopy were studied. An unknown source was inferred to be 57Co.
The resolution of the Model 5030 Alpha/Beta Spectrometer is found to be around 24.52-33.33 keV. The source activity of Po-210 is determined to be 466.594 Bq.
The resolution of the NaI(Tl) detectors in SCA and MCA are found to be 8.82% and 6.05% respectively. MCA system is better than SCA system in gamma spectroscopy study, in terms of shorter time of measurement and higher resolution power. However, the spectrum stabilization is not effective enough, which remains to be improved.
References
[1] Lilley, J., Nuclear Physics: Principles and Applications, 1st edition, West Sussex: John Willey & Sons Ltd., 2001, pp.156-158
[2] Knoll, G. F., Radiation Detection and Measurement, 2nd edition, New York: Oxford University Press, 1988, pp.288-289
Figure Captions
Figure 1: System Configuration of Model 5030 Alpha/Beta Spectrometer connected with Quantum 8 Multi-channel Analyzer
Figure 2: System Configuration of Nucleus AA-210 Single Channel Scintillation Spectroscopy system, showing the component parts.
Figure 3: Spectra of (a) Sr-90, (b) Tl-204, (c) Cs-137 and (d) Co-60, showing the plots of counts against channel number / E-dial setting