Gamma-Ray Spectroscopy - Report Example

Gamma-Ray Spectroscopy This experiment was set up to achieve a number of objectives. Firstly, sodium iodide scintillation detector doped with thallium [NaI(Tl)] was used to test the performance of gamma-ray spectroscopy. Secondly, in an effort to determine the nature of interaction between the NaI(Tl) detector and the gamma radiation, four different radioactive sources, namely - 22Na, 60Co, 137Cs, and 152Eu, were used. This was done with a view that NaI(Tl) detector often provides a reliable interpretation of the ϒ-radiation spectra and clearly shows the most salient features such as photopeaks for all the sources used, Compton edge, characteristic x-ray peaks and backscatter, as well as the Compton continuum. Following different energies dissipated by different sources, calibration of the NaI(Tl) detector can easily be processed. As is seen, when a 60Co source is placed close to a NaI(Tl) detector, an additional peak spectrum is observed. Moreover, this procedure helps determine the efficiency as well as resolution of the NaI(Tl) detector. The present experiment sought to investigate the dependency of the photomultiplier tube of the NaI(Tl) detector alongside the high voltage supply that clearly showed a voltage requirement that needs to be stabilized and also demonstrated a strong dependency on the applied voltage. Basing on the radioactive sources, the resolution of the NaI(Tl) detector was found to range between 15.06% and 21.97%. However, this is not the appropriate value relative to the correct value obtained from Hyper Pure Germanium (HPGE). In this experiment, using 22Na, 60Co, 137Cs, and 152Eu as radioactive sources, the calculation for three efficiencies were done. These were intrinsic total efficiency (ɛi) found to be 22Na=50.49%, 60Co=44.8%, 137Cs=17.4% and 152Eu=45.66%, absolute total efficiency (ɛt) found to be 22Na=5.97, 60Co=5.3%, 137Cs=2.06%, and 152Eu=5.4%, the intrinsic photopeak efficiency (ɛp) is found to be 60Co=44.8% and 137Cs= 17.4%. These values indicate the efficiency of NaI(Tl) detector in detecting gamma radiation from radioactive material. The dependency of the system on voltage has also been demonstrated. 1. Introduction Of all the waves in the electromagnetic spectrum, gamma rays are found to have the smallest wavelength and the highest energy (NASA 2007). Gamma rays, like X-rays, are a type of electromagnetic radiation. These rays result from nuclear interaction (Gilmore & Hemingway, 1995). Gamma rays are released from radioactive atoms and also in nuclear explosions (NASA 2007). Figure 1 shows the gamma ray region of the electromagnetic spectrum in comparison with other waves. Figure 1: Electromagnetic spectrum showing rays of different wavelengths in comparison to one another (scienceminusdetails.com 2011) As described, gamma radiation is high energy and short wavelength radiation which can penetrate very deeply. Due to this reason, this radiation can lead to serious damage when it penetrates into living cells. A sodium iodide detector (NaI(Tl)) can be used to detect gamma radiation. The high value of atomic number (Z) of iodine present in NaI results in a high efficiency of gamma ray detection (Manus 2009). The addition of thallium through doping results in the activation of the NaI crystal. Radioactive materials often decay through the emission of a beta particle. These will then subsequently release gamma radiation in order to decay to the ground state. A number of detectors including the NaI(Tl) detector can be used not only to detect the resultant gamma rays but also to measure the energies of the resulting gamma rays. Interaction of gamma rays with NaI(Tl) detector results inelastic collisions, resulting in the elevation of secondary electrons to the conduction band. The movement of the secondary electrons to the conduction band leaves positive holes in the valence band of the NaI(Tl) crystal. The resultant positive holes drift towards the activator and ionize the activator atoms. Ionization of the activator results in the attraction of secondary electrons to the activator. The collision of electrons within the valence band of the activated inorganic crystal will result in excited energy states in the activator’s forbidden gap. Visible photons will result from the de-excitation of the activator (figure 2). Figure 2: Energy band structure of activated scintillator detector (Dyer 2004) These photons appear as fluorescent radiation which when passed through the photo multiplier tube (PMT) is converted to electrons with low energies of about 3 eV. NaI(Tl) is one of the few activated inorganic crystals that serve as scintillation detectors for the study of gamma radiation. The PMT has a photocathode within. The application of electrical potential in the PMT dynodes results in the acceleration of photoelectrons that in turn helps in the production of around 106 electrons for each photon. This results in the generation of an electric pulse that can be measured. 2. Theory Attenuation of gamma rays has three key processes. These include pair production, photoelectric effect, and Compton scattering. The photoelectric effect happens with photons with energy obtained from electron volts to over 1 MeV. In a photon energy that is considered to be high in comparison to the rest energy of electron of about 511 keV. On the other hand, Compton scattering happens whenever a pair production takes place at an energy that is higher than 1.022 MeV. The resolution of energy (R) for a Nal(Ti) detector could be defined and calculated through the relation: (1) Where R represents resolution, H0 represents the number of the channels that corresponds to the peak centroid. For the calculation of the current activity, the equation to be used includes: (2), where (3) Source Half Life (Years) Activity (kBq) Current Activity (kBq) 22Na 2.6 406 (1/12/88) 101.93 60Co 5.27 420 (1/12/88) 183.22 137Cs 30.17 412 (1/12/88) 237.92 152Eu 13.2 344 (5/8/94) 137.72 Table 1: Current activity of 22Na, 60Co, 137Cs, and 152Eu The Absolute total efficiency) is expressed as: (4) Ct: Total count number. (5) Where Ds represents the rate of disintegration of the source through the use of the equation label (2) and (3). In this respect, = The gamma fractional number emitted per disintegration. In 60Co =2 and in 137 Cs = 0.85. Total efficiency) is calculated as (6) Where: represents the total gamma-rays number that are incident on the detector and is calculated using the relation: (7) Ω represents the detector crystal solid subtended angle at the point source. Ω=2П (8) Where d represents distance from detector which is given as 6.5 cm, and a represents the detector radius which is given as 5.5 cm. The expression of the intrinsic photopeak efficiency ( ) is given by the relation: (9) In this case, Cp will represent the net amount of counts in a photopeak that corresponds to the energy . 3. Experimental Methodology This experiment aimed at interpreting the gamma spectra for four radioactive isotopes in terms of the interactions that occur upon the interaction of their gamma radiation with NaI(Tl) activated crystal. Five interactions have been characterised, namely – photopeak, Compton edge, Compton continuum, Backscatter peak, Characteristic X-ray peaks. A secondary goal was to perform energy calibration through which the energies of radiation from other radioactive isotopes could be measured after detecting their radioactivity. The energy resolution and energy efficiency also had to be calculated along with estimating the dependency of the system on voltage. To achieve these aims, the experiments were carefully performed. The radiation laboratory of the physics department at University of Surrey was utilized for this experiment. NaI(Tl) detector was setup along with the other equipment including a high voltage power supply, PMT, amplifier, computer monitor and multi-channel analyser (MCA). The experimental setup is shown in figure 3. Measurements were taken for the four radioactive isotopes using the NaI(Tl) detector. The 137Cs was placed at a distance of around five centimetres from the detector window and the spectrum was obtained by adjusting the amplifier controls. Figure 3: Electronic block diagram of the experimental setup (modified from Manus 2009) The spectrum for the radioactive isotope is collected by adjusting the coarse and fine gains of the amplifier such that the FWHM (full width at half maximum) at 662 KeV peak encompasses 5 channels. The counting time for the MCA was set as 300 seconds. The 137Cs is replaced with 60Co and the spectrum is recorded and matched. Additional peak is observed at around 2.5 MeV of energy for 60Co. All peaks within the ROI (region of interest) are marked for the two radioactive isotopes in a software called Maestro. The energy spectrum for all the radioactive isotopes is studied and their energy values obtained in the spectrum are added after obtaining the ROI for particular peaks in the spectrum. The detector efficiency and resolution are also calculated 4. Results and Discussion 4.1. Interpretation of Spectra for Gamma Rays The spectra for the radioisotopes were recorded at a shaping time of 0. 5µs and a counting time of 300 seconds. For 137Cs, the photopeak was observed at 662.3 KeV, the characteristic X-ray peaks were recorded at 74.2 KeV while the Compton continuum, Compton edge and Backscatter were observed at 30.6 KeV. In case of 60Co at the same counting rate and shaping time, two photopeaks were recorded at energies of 1132.8 and 1172 KeV. The values for all four parameters for both 137Cs and 60Co are shown in appendix 1 and 2. The details of peak and FWHM for both 137Cs and 60Co at 0.5µs shaping time and a distance of five centimetre from the detector are shown in table 2. Radiation Source Peak FWHM Net Area Count 60Co Peak 1 13450 61.95 769302 Peak 2 13843 72.45 824335 137Cs 376791 48.23 1468060 Table 2: Peak, FWHM and net area counts for 137Cs and 60Co at 5cm distance from detector and at 0.5µs shaping time An additional peak is observed upon moving 60Co closer to the detector. This peak is observed at 2 MeV. 4.2. Energy Calibration The NaI(Tl) detector was calibrated using 137Cs, 60Co, and 22Na (Appendix 3). This was done by adding all the peaks from the radioactive sources to the calibration data in the software, displaying the respective energies. 4.3. Energy Resolution Following the calibration of the detector, the resolution is determined using the energy values as per the equation 1 (table 3). If a log-log graph is plotted, a linear relation can be observed between log of Energy vs. log of Resolution. This is shown in figure 4. The error is taken as +/-0.08. Radiation Source Channel Energy (KeV) FWHM (KV) Resolution (%) 22Na 190.6 512 41.87 21.97% 460 1272 71.79 15.61% 60Co 425 1172 61.95 14.58% 481 1332.8 72.45 15.06% 137Cs 245 662.3 48.23 19.69% Table 3: Resolution for all radioactive sources Figure 4: Linear relation for log Resolution versus log Energy for detector 4.4. Efficiency of Detection The absolute total efficiency (ɛt), intrinsic total efficiency (ɛi) and intrinsic photopeak efficiency (ɛp) can be calculated using equations 2-9, as shown in table 4. Radiation Source Energy (KeV) Ct (Counts.300sec) Emitted Gamma x300 (Bq) Current Activity x300 (Bq) Absolute Peak Efficiency ɛt (%) ɛi (%) ɛp (%) 137Cs 662 1.47E+06 376791 60741655 0.62 2.06 17.39 5.24 60Co 1173 291235 13450 5495015 0.24 5.30 44.79 2.07 1332 291235 13843 5495564 0.25 5.30 44.79 2.13 22Na 511 182640 43274 5534541 0.78 5.97 50.49 6.61 1274 182640 8543 3056045 0.28 5.97 50.49 2.36 152Eu 121 1.29E+06 94736 6836728 1.39 5.40 45.66 11.71 344 1.29E+06 63702 6363085 1.00 5.40 45.66 8.46 778.9 1.29E+06 10482 3109778 0.34 5.40 45.66 2.85 964.13 1.29E+06 7594 3504481 0.22 5.40 45.66 1.83 1112.12 1.29E+06 17287 6171714 0.28 5.40 45.66 2.37 1408.01 1.29E+06 10029 5040233 0.20 5.40 45.66 1.68 Table 4: Detection efficiencies for radioactive sources 4.5. Photomultiplier Tube Gain By increasing the voltage by 10 V and recording the peak channel thus obtained, a log-log plot can be obtained taking log of channel and log of voltage. A linear relation is obtained as seen in figure 5. Upon further observation, it is seen that there is a high voltage dependence. Minor fluctuations in voltage are thus harmful for the experiment. A stable source of power is required for accurate experimentation. The voltage could thus serve as a major source of error (+/- 0.01 kV, with 0.01/0.4 = 2.5% fractional error). The error due to channels could be taken as +/-0.05 with 0.05/13.77 = 0.4% fractional error. Figure 5: Log V vs. log channel for detector, showing linear relationship and voltage dependence 5. Conclusion This experiment has demonstrated the efficiency of NaI(Tl) detector in detecting and measuring gamma radiation from radioactive sources. The parameters of the energy spectrums that were studied include photopeaks, Compton continuum, Compton edge, backscatter and characteristic x-ray peaks. Calibration of the detector was performed and the photopeaks, resolution and detection efficiency were calculated. The resolution was found to range from 15.06% to 21.97%, which does not compare well with Hyper Pure Germanium (HPGE) detectors. The NaI(Tl) detector is found to be a good detector of gamma radiation from radioactive isotopes, and it is also seen that the detector is highly affected by the voltage supply. Intrinsic total efficiency (ɛi) was found to be 22Na=50.49%, 60Co=44.8%, 137Cs=17.4% and 152Eu=45.66%, absolute total efficiency (ɛt) found to be 22Na=5.97, 60Co=5.3%, 137Cs=2.06%, and 152Eu=5.4%, the intrinsic photopeak efficiency (ɛp) is found to be 60Co=44.8% and 137Cs= 17.4%. References Dyer, SA 2004, Wiley Survey of Instrumentation and Measurement, John Wiley & Sons, New York. Gilmore, G & Hemingway, J 1995, Practical Gamma-Ray Spectrometry, John Wiley & Sons, Chichester. Manus 2009, Laboratory Practical Course: Scintillation Detectors, viewed 2 January 2012, < https://indico.cern.ch/getFile.py/access?contribId=67&sessionId=11&resId=1&materialId=slides&confId=78565>. NASA 2007, The electromagnetic spectrum, viewed 2 January 2012, < http://science.hq.nasa.gov/kids/imagers/ems/gamma.html>. Scienceminusdetails.com 2011, What is Nuclear Radiation and How Can It Hurt Me, viewed 2 January 2012, < http://www.scienceminusdetails.com/2011/03/what-is-nuclear-radiation-and-how-can.html>. Read More