Technical noteNote on the comparison of experimental and simulated gamma energy spectra for NaI with 137Cs, 60Co, and 241Am
Introduction
The interaction of radiation with matter can be simulated using the well-known Monte Carlo (MC) method. MC simulations require relevant data such as geometry details and properties of the radiation source of interest (particle type, energy, radiation direction, etc.), description of the detector (target) of interest and its surrounding, and physics models and data libraries to be used (De Lima, 2011).
In nuclear medicine, the development of new imaging devices, reconstruction algorithms, correction and optimization techniques, acquisition protocols, and description of time variable phenomena, such as detector or source movements, are frequently based on appropriate simulations, particularly using MC techniques similar to those used in particle physics (De Lima, 2011). One of the commercially available MC programs is FLUKA (Ferrari et al., 2005, Battistoni et al., 2007). The main reason for developing FLUKA in the past was to realistically simulate radiation interactions in matter using accurate physics models (Ferrari et al., 2005, Battistoni et al., 2007).
ROOT is an object-oriented framework aimed at solving the data analysis challenges of high-energy physics. It is based on C++ language. It is additionally used for advanced data analysis such as MC simulations in the field of subjects. Further information can be found in its user guide (ROOT, 2009).
Many inorganic scintillators are crystals of the alkali metals, in particular alkali iodides, that contain a small concentration of an impurity for example NaI(Tl). The element in parentheses is the impurity or activator (Tsoulfanidis, 1995). The most notable property of NaI(Tl) is its excellent light yield. It has come to be accepted as the standard scintillation material for routine gamma-ray spectroscopy (Knoll, 2000).
In many applications of radiation detectors, the object is to measure the energy distribution of the incident radiation. These efforts are classified under the general term radiation spectroscopy (Knoll, 2000). For detectors designed to measure the energy of the incident radiation, the most important factor is the energy resolution. The energy resolution (R) is usually given in terms of the full width at half maximum (FWHM) and the energy (or channel) corresponding to the maximum of a peak (Leo, 1994).
Uncharged radiations such as gamma-rays or neutrons must first undergo a significant interaction in the detector before detection is possible. Because these radiations can travel large distances between interactions, detectors are often significantly less than 100% efficient. It then becomes necessary to have precise figure for the detector efficiency in order to relate the number of pulses counted to the number of neutrons or photons incident on the detector (Knoll, 2000). Hence, the absolute counting efficiency (εabs) is defined as
The peak to total ratio (PT) is determined from the number of photopeak counts divided by the total area under the photopeak (Knoll, 2000).
Peak to Compton ratio (PC) is the ratio of the counts in the highest photopeak channel to the counts in a typical channel of the Compton continuum associated with that peak (Knoll, 2000).
Peak to valley ratio (PV) is also defined as the ratio of the highest peak counts to the valley counts (Abbene et al., 2007).Another important parameter for the measurement is the absolute photopeak counting efficiency (PCEabs). This quantity is determined from Eq. (5) in parallel with Eq. (1).
Chen and Wei (2008) studied characteristics of cadmium zinc telluride (CZT) and NaI spectrometers by using an MC code. Gamma-ray response functions of an NaI(Tl) detector were determined by means of MC simulations by Shi et al. (2002). Orion and Wielopolski (2000) carried out a simulation study of response functions of bismuth germanate (BGO) and NaI(Tl) detectors at various energies. FLUKA simulations were used to determine the response function of a neutron detector by Borio di Tigliole et al. (2001). Ashrafi et al. (2006) modeled an NaI(Tl) detector by using GEANT. Coincidence measurements were carried out by using a spectrometer which consisted of NaI(Tl) detectors by Britton et al. (2013), and the obtained experimental results were compared to the results acquired with GEANT4. Janus and Wojtowicz (2009) obtained 137Cs energy spectrum by using BaF2:Ce scintillator.
The experimental and theoretical 137Cs, 241Am, and 60Co energy spectra were obtained in this study. Their experimental energy spectra were acquired by using an NaI(Tl) detector. Energy spectra of these gamma sources, FLUKA MC simulations and ROOT program were used for direct comparison.
Section snippets
Experimental and simulation configurations
The experimental energy spectra of the aforementioned gamma sources were acquired by using an NaI(Tl) scintillation detector.
Solid 137Cs, 241Am, and 60Co radioactive standard point sources were used in this work. The activities of the sources were 5 (±20%), 10 (±5%), and 0.1706 (±1.9%) μCi, respectively. 137Cs, 241Am, and 60Co isotopes emit 661.60 keV, 59.50 keV, 1.17 and 1.33 MeV-energy gammas, respectively. The NaI(Tl) scintillation detector manufactured by REXON Inc. (Beachwood, OH, USA) has a
Results
The experimental energy spectra of the isotopes are shown in Fig. 2, Fig. 3, Fig. 4. These figures are the screen captured view of the MCA.
The gross count rates in the photopeaks for 137Cs, 241Am, and 60Co (for 1.17 MeV and 1.33 MeV peaks) from the experimental results were calculated as 17,144, 26,275.541, 545.912, and 449.416 s−1, respectively. The total count rates of gamma sources of interest were also determined. These values for 137Cs, 241Am, and 60Co were calculated as 17,150, 2627, and
Conclusions and discussion
The energy spectra of standard gamma sources (137Cs, 241Am and 60Co) were obtained by using experimental and theoretical methods. The experimental energy spectra of the isotopes were acquired with an NaI(Tl) spectrometer. FLUKA and ROOT programs were used to obtain theoretical spectra and compare them with experiments. Authors’ comprehensive literature survey has indicated that no study published in the past included the simulated and measured energy spectra acquired for the aforementioned
Acknowledgements
This work was supported by TUBITAK, the Scientific and Technological Research Council of TURKEY under Projects No. 197T087 and 111T571, and by EBILTEM, Center of Science and Technology, Ege University under Project No. 99 BIL 001, and Scientific Research Foundation of Ege University under Project No. 11 FEN 085.
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