1. Abstract
The sodium iodide scintillator detector is a very important type of inorganic scintillators. The main function of this scintillator is to detect gamma rays by interaction of ionizing radiation and the materials of that scintillator. In this experiment, we learned the applications of this detector and how the process of detecting happens.
2. Introduction
This lab experiment is designed to make the student learn about scintillation detectors and how do they detect gamma and beta ray such as the effect of a sodium iodide NaI(Tl) scintillator. This type of detector the scintillation materials interact with ionizing radiation to make an electrical excitation. This type of detectors operate when an incident radiation particle interact with the scintillator and deposit kinetic energy. This energy excites species in the scintillator, which make it emit some light as the return to their lower energy state. Then an incident photon would reflect until they leave or get absorbed by the scintillator where they convert to electrical charge. The best materials that work in scintillation are the ones with high atomic number and density, short decay time, and linear energy to light conversion. There are two types of scintillators, organic and inorganic. The best examples of organic scintillators areanthracene, plastics, and liquids.the best type of inorganic scintillator is NaI (Tl).The low scintillation efficiency can cause a very low energy resolution.
3. Experimental procedure and equipment
The equipments needed for this experiment are, a Hammamatsu H6533 1” PMT, a 1 inch sodium iodide crystal, an Optical coupling grease, a Preassembled Canberra/Ortec sodium iodide sensor (crystal, PMT), an Ortec 113 Preamplifier, Ortec 572A linear amplifier,Ortec 927 Amplifier, Ortec Maestro Acquisition Software, and a High voltage power supply with variable polarity, Oscilloscope. To assemble a detector we use an optical coupling compound between the crustal and the tube. To enclose the assembly we use an electrical tape and then we test for light at a voltage of 800 V. Next, we place a radioactive source such as Cs-137 close to the crystal of the detector and check for a large pulse that correspond to the photo peak and record it. Then we compare the rise/fall time of the signal out of the PMT and compare it to the shape seen in the radiation detection book by Knoll. Now we set the 113 preamplifier at a capacity of zero and record the output of the amplifier amplitude as a function of the voltage. we take ten data from the range of the voltage and check the amplifier and the preamplifier outputs for the rise of the voltage. next we plot the amplitude output on the amplifier vs. the applied voltage. From the ten data collected we check for the effect of the preamplifier capacitance on the pulse amplitude. We should also be able to determine the number of stages that your PM tube has. Next we check for the photopeak analysis. For that we change the voltage to about 900 Volts and adjust the amplifier so that the peaks of our radioactive source, Cs-137, is around 45% of the full scale on maestro. After checking the peaks we remove the source and take a background level spectrum. After recording the background we return the source about 2 inches away from the selector face and measure the amplitude average pulse, which is the same as the channel number, the energy resolution of the photopeak, the total number of the counts under the photopeak, and finally the net counts under the photopeak. We need to make graphs from MAESTRO. On the next part of the lab we are experimenting with the detector assembly by using the same system setting with the voltage, live time, and source geometry to use a 2 inches NaI(Tl) detector and record the pulse height from the scintillator. Then we adjust the amplifier gain until the photopeak is less than the MCA scale. We set the MCA display to show a logarithmic scale, and identify all the features that are important on the spectrum. Next we record the activity of our source and use an MCA to get the area under the photopeak. To determine the decay rate from the gamma ray emission rate, we use the decay scheme data obtained from the book. After that we remove all the sources, take a background level for gamma spectrum with the detector, and identify the peak on that spectrum. Finally, we change the voltage and the amplifier gain to a scale where the CS-137 photopeak at around 35% of the full scale and by using the same previous settings we expose the sensor to a low energy gamma ray such as Co-57 and a high energy gamma ray such as Na-22. To plot the energy of the photons vs. the voltage, we locate the peak for each source and make a linear polynomial fit.
4. Results
The first part of the lab requires us to assemble a detector. the 1 inch by 1 in NaI(Tl) crystal and the PM tube should be assembled together with an optical compound between them. When we place a Cs-137 source near the crystal, the amplitude of the pulse will be very large. To analyze the photopeak, the voltage should be around 900 volts and the gain of the amplifier is going to be around 45% of the full scale. The peak should be around 860, the FWHM around 0.85 FW[1/5]M:5.20, and the net area around 750±268.
The next part of the lab experiment requires a preassembled NaI(Tl) detector. The spectrum recovered from the software used is below.
Graph 1. Cs-137 spectrum in NaI(Tl) scintillator.
Now we can find the absolute activity of Cs-137 by putting the radioactive source about 10 cm away from the detector and get the following spectrum.
Graph 2.Cs-137 absolute activity.
The last part of the experiment is to plot the photon energy vs. the channel after setting the voltage gain setting to a value 35% of the full scale. We get the background radiation and plot for three radioactive sources Co-57, C0-60, and Cs-137 and plot their activity vs. the channel number The spectrum and the graphs are shown below.
Graph 3.Co-57 background activity vs. channel number.Graph 4.Co-60 background radiation activity vs. Channel number.
Graph 5.Cs-137 background activity vs. channel number.
The table below shows the channel numbers where the energy peaks accrue.
The photopeak graph is below:
Graph 5.channel numbers where the energy peaks occur.
5. Conclusion
This experiment was very important in making the student learn about the different types of scintillator detectors and how do they work. In addition, the experiment familiarizes the student with the most common type’s software for radiation detection such as MAESTRO. The one problem I had in this lab is missing the data for the first few parts and I had to investigate what to expect for some of them.
References
- G. F. Knoll, Radiation Detection and Measurement (John Wiley & Sons, Inc.,
New York, 2011)
- Smith, Hastings. “Gamma ray detector.” http://www.lanl.gov/orgs/n/n1/panda/00326398.pdf
- “ Temperature behavior of NaI scintillation detectors”. Arxiv. Web. 4 April 2014.http://arxiv.org/pdf/physics/0605248
- “NaI scintillation detector”, Horiba. Web 4 April 2014.http://www.horiba.com/uploads/media/RE09-18-098_03.pdf
- “Measurement of radiation and sodium iodide detector”,.Isu. Web 4 April.http://www.physics.isu.edu/radinf/naidetector.htm
- “Neutron detectors with scintillators”, john caunt. Web. 4 April 2014.http://www.johncaunt.com/detectors/scintillation-detectors/neutron-detection-with-scintillators/
- “ Custom scintillation and detectors”,. Berkelynucleoincs. Web. 4 April 2014.http://www.berkeleynucleonics.com/products/Custom_Scintillators_Probes.html
