NUCL 3000/5030 Laboratory 2 Fall 2013

Transcription

1 Lab #2: Passive Gamma Spec Measurements in Decoding Natural Radioactivity in SLC Area Objectives a. Learn basics of gamma spectroscopy b. Learn the equipment in Counting stations #4, #5 and #8 c. Apply learned basics of gamma spectroscopy d. Experimentally determine the level of natural radiation in soil samples from Salt lake City area e. Measure the gamma radiation intensity (passive counting) f. Identify the radioisotopes in soil samples Theory I. Gamma Spectroscopy and Apparatus Gamma spectroscopy is quantitative study of gamma-ray energy spectra produced in a gamma-ray spectrometer with its main application in identification and/or quantification of radionuclides. It is widely used technique applicable to various areas, including but not limited to environmental radioactivity monitoring, health physics personnel monitoring, nuclear materials safeguards and homeland security, nuclear forensics, and earth sciences. At UNEF, gamma spectroscopy is an important tool for passive counting and Neutron Activation Analysis (NAA). Gamma-rays are electromagnetic radiation of high frequency. They are produced by the decay of high energy states in nuclei and interactions of high energy sub-atomic particles. A gamma-ray spectrum is a histogram recording the energy of the gamma-rays released by the decay of nuclei or through nuclear interactions. Since the energy of each excited state in a nucleus is unique, the energy of the associated gamma ray is unique as well. Thus, a gamma-ray energy spectrum can be used as a fingerprint for nuclide identification. In most cases, in addition to energy, the amount of gamma radiation released is wellknown. For example, during beta decay of Co-60 nucleus, a gamma-ray will be released with energy of kev in % of the time. Another gamma-ray will be released with energy of kev in % of the time. Gamma spectroscopy system consists of several components, including gamma detector, associated electronics, and the system analysis software. Roughly speaking, spectroscopic gamma ray detectors and spectrometers are of two types: high resolution, based exclusively on high purity germanium (HPGe) 1

2 detectors or medium to low energy resolution, based on scintillators or room temperature semiconductors. For a particular application, the detector is normally selected based on the following major factors: including energy resolution, detection efficiency, timing requirement, cost, maintenance. Some of these factors are described as follows: Detector Electronics: The electronics of a gamma spectroscopy system include HV power supplies, preamplifier, shaping amplifiers, and multi-channel analyzer (MCA). A preamplifier is an electronic amplifier that prepares a small electrical signal for further processing. It is usually placed close to the detector to boost the signal and reduce the noise interference. The signal strength is increased to drive the cable without significantly degrading the signal-to-noise ratio. Shaping amplifiers are often used following the charge sensitive preamplifier stage and perform three functions. First, they provide an output pulse having a faster baseline restoration than the charge sensitive preamplifier output pulse. This is especially important at high-count rates, where pulses from consecutive events can pile up. Secondly, shaping amplifiers filter some of the noise from the preamplifier output signal. Finally, shaping amplifiers can also be used to provide extra gain to the signal, which may be very small (sub mv) at the preamplifier output. The MCA, in simplest form, analyzes a stream of voltage pulses and sorts them into a histogram or spectrum of the number of events versus pulse-height which may often relate to energy or time of arrival. The stored spectrum may then be displayed and analyzed. Genie 2000: The analysis software is the Genie 2000 developed by Canberra Industries. Genie 2000 spectroscopy software is a comprehensive data acquisition system inclusive of display and complete analysis of gamma spectrometry data. It provides independent support for multiple detectors, extensive networking capabilities, an intuitive and interactive human interface, and comprehensive batch procedure capabilities. In Genie 2000, acquisition and analysis capabilities are tightly integrated, providing an intuitive human interface and straightforward operation of various applications. Energy resolution: The energy resolution of a detector is defined as the Full Width Half Maximum (FWHM) divided by the location of the peak centroid. The energy resolution is a dimensionless quantity and expressed as a percentage. In general, alpha spectroscopy detector has less than 1% resolution and gamma ray spectrometer has the range of 5-10 % of its resolution. Detection efficiency: Uncharged radiations such as gamma rays and neutrons must undergo a significant interaction in the detector before they are detected. In general, the efficiency of a detector is less than 100% because of the travel distance of the radiations. Typically, counting efficiency can be divided into two different categories: absolute and intrinsic. The absolute efficiency is defined as e abs = number of pulses recorded number of radiation quanta emitted by source 2

3 The absolute efficiency depends on the detector types and counting geometry such as the distance from the detector. The intrinsic efficiency is defined as e int = number of pulses recorded number of radiation quanta incident on detector For isotropic source, the relationship between absolute and intrinsic efficiencies is where is the solid angle of the detector seen from the actual source position. High Purity Germanium Detectors: The High Purity Germanium detectors (HPGe) are manufactured from ultrapure germanium crystals. All HPGe detectors are just large, reversebiased diodes. The germanium crystal can be either n-type or p-type. The type depends on the concentration of donor or acceptor atoms in the crystal. In a semiconductor detector, radiation is measured by counting the number of charge carriers electrons and holes. Under the force of a transverse electric field, electrons and holes are collected by the electrodes. The result is a pulse signal that can be measured in an outer circuit. The amount of energy needed to create an electron-hole pair is a known value for a certain material and is independent of the energy of the incident radiation. Thus, the number of electron-hole pairs is proportional to the energy deposited in the active volume by the incident radiation. The energy need to produce an electron-hole pair inside a semiconductor is very low compared to the energy required for production of a charge carrier in other types of detector. As a result, the statistical variation of the pulse amplitude is much smaller in semiconductor detectors, which leads to a superior energy resolution. The density of a semiconductor detector is on par with a scintillation detector and much higher than that of a gaseous detector. So the detection efficiency of a semiconductor is higher than that of a gaseous detector in similar size. Calibration: Several factors such as source-detector geometry, and gamma ray energy may affect the quality of measurements made with HPGe or other types of detector. The accuracy of such measurements depends on the accuracy of the efficiency versus energy calibration curve and the accuracy of the decay data for the radionuclides from which calibration standard sources are used. Both half-life and radiation emission probabilities must be known to good accuracy. In general, NIST standard sources such as Co-60, Cs-137, and Na-24 are used for a detector calibration. 3

4 II. Passive Counting NUCL 3000/5030 Laboratory 2 Fall 2013 Passive counting does not require any irradiation of a sample. Any raw sample from environment naturally contains some small amounts of radioisotopes, such as but not limited to U-235, Pu-239, Rn-226, Rn-222n, and Bi-214. Most of these isotopes are decay products of U-235, U-238, and Th-232. In general, passive counting requires long counting times, at least 24 hours if not more, because of low activities. In Lab #2.2, we will use soil samples from the Salt Lake City area. These soil samples you will be preparing in the UNEF radiochemistry lab. Each soil sample will have a mass of 150 grams to reduce the total counting time. These soil samples will be counted for one-hour per each sample. Figure 1. A 200 ml plastic sample container, and Counting stations #4 and #8 You will need to practice filling out a sample log: Normally this is accomplished by opening the red laptop, located in MEB 1205, on the table near the entrance, at the dressing out station (where booties and lab coats are put on). Due to the large number of students and time constraints, you will fill this out by hand or electronically duplicate it and submit it as part of your lab report. It is important that you know how to fill out the sample log, as it is a requirement if you should ever need to bring a sample into the facility. A completed example is shown as follows: 4

5 Lab Procedure Important information We will measure the gamma rays that are going to be emitted from the soil samples using the HPGe detector. The obtained spectra will be automatically analyzed by Genie 2000 software. One of the three Counting stations, #4, #5 and #8, will be used for the passive counting of each the soil samples. Since the soil samples emit very weak radiation signals, each soil sample will be counted for one day. Overview: In groups of 5 to 6 you will perform passive gamma spectroscopy on the provided soil sample. Due to the size of the class, you will only stay with your soil sample for 3-5 minutes once scanning has started. Then you will exit the facility, and use the 24 hour scan data provided on the overhead to complete the lab. 5

6 Steps 1. Dress out for the lab by Donning / putting on Booties and Lab Coat 2. You will be provided with one soil sample by the Lab Instructor. 3. Go to your assigned counting station. Assist the Lab instructor to set up the Counting stations #4, #5 or #8 if necessary. 4. When instructed, and with help from the lab instructor, turn on the counting station equipment; place your soil sample on the top of gamma detector (the lab instructor will help with this). 5. Open Genie 2000 software on the host computer; open the data source for the HPGe detector, Set the counting time for 24 hours and begin to acquire the sample data. 6. Once you started counting your sample, and feel comfortable with the steps necessary to set up and count a sample you are welcome to exit the white wing. 7. Exit procedure: In Genie-2000 secure detector high voltage. Close Genie-2000 Fold labcoats properly, and give them to your Lab Instructor prior to exiting. (No improperly folded labcoats will NOT be accepted, you will have to fold them properly before exiting the lab) Remove booties Exit the white wing. 8. Using the data provided to you on the overhead projector in room 1206 record the data that was obtained from the 24 hour scan previously performed on the sample. 9. Record each gamma peak with its energy and activity writing in the table as follows: Utah Soil Analysis Energy (kev) Isotope Activity (Ci) Counting time Detection efficiency 6

7 Lab #2 Report: Your lab report should be 2-3 pages, type written report, in paragraph format. The report should cover all of the following information/ talking points, although not necessarily in order. Additionally you will attach an appendix that includes a simulated Log Data. Use good technical writing skills to create a professional looking report on your lab experience. 1. What is a gamma ray? Where will you expect to find the gamma rays? 2. What is spectroscopy? Where is gamma spectroscopy used? 3. Describe the main scientific principles of gamma spectroscopy. 4. What is natural radioactivity? How many natural radioactive chains do exist currently on the planet? 5. Describe what is passive counting and where it is used and why. 6. Describe the measurement process you used at the UNEP facility, including how a sample is logged into the facility for tracking. 7. Discuss the data collected on Table (Pg. 6), what does this information tell you about the sample? What Isotopes did you find? a. Note: You may want duplicate the table and insert it into your lab report for reference 8. Correlate the table data to any reference you may find regarding the background radiation in Salt Lake City area. Discuss it! 9. Conclusion Appendix: 1. Fill in the sample log below for your sample using the following information, and referring to the sample log and key located on the previous page. (Note: The log can be done by hand and attached to your report, or the table can be recreated electronically and inserted into you lab report.) Information: The sample arrived on Sep 17 of this year, but no one Sent it (N/A). It s a soil sample from outside the MEB building, and you re the custodian. s will be counted during Lab 2.2, and then stored in the facility storage cabinet one after the counts Arrival (Day/Mon/Year) Sender Identifier Description Custodian Objective Counting Date Fate 7