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Neutron Activation Analysis

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Neutron Activation Analysis NEUTRON ACTIVATION ANALYSIS This experiment has a two-fold objective. One is to determine the percentage copper in a brass sample from the amount of radioactivity that has been produced by bombardment with a beam of neutrons, and the other is to 62 Cu determine the half-life of the 29 isotope from the rate at which its neutron-induced radioactivity decays. PRINCIPLES Certain elements, when bombarded with a beam of neutrons, are made radioactive by a reaction that occurs in the nucleus. In this experiment, you will expose copper and brass samples to a beam of "slow" neutrons and the radioactivity that is produced will be used to determine the percentage of copper in the brass sample. The neutrons are produced by a sequence of nuclear reactions occurring in a small neutron source encapsulated in stainless steel and immersed in a large tank of water (a neutron howitzer) to protect the user from the neutrons. The source contains about 80 239 Pu grams of plutonium-239 94 which is a radioactive isotope that decays spontaneously according to the following reaction: 239 235 4 Pu U + He (an alpha particle) 94 92 2 The alpha particle is then absorbed by the beryllium that is mixed with the plutonium, leading to the following reaction: 9 4 1 12 Be + He n + 6C 4 2 0 The neutrons that are produced have a very high energy (several million electron volts). These neutrons are slowed down by collisions with hydrogen atoms in the water and the plastic material surrounding the source. They are therefore "slow" by the time they reach the samples being irradiated. The neutrons which are not absorbed by a sample are ultimately absorbed by the water (via a nuclear reaction with hydrogen atoms) before they have a chance to escape from 1 the howitzer. Approximately 107 neutrons per second are produced by the 5 curie source in our laboratory. Although the neutron source produces radiation at a level that can be harmful, a consultant has confirmed with exhaustive testing that you will not be exposed to significant levels of radiation above background. The water bath in which the source is immersed provides very effective shielding. As an additional precaution, the neutron source is stored in its own room separated from the counting room by a wall. 63 65 Cu Cu Natural copper is a mixture of two isotopes, 29 and 29 , which interact with these neutrons according to the equations: 63 1 64 Cu + n Cu + γ 29 0 29 65 1 66 Cu + n Cu + γ 29 0 29 The newly formed copper isotopes are unstable and undergo several types of 64 66 Cu Cu decay. The half-life of 29 is about 150 times longer than the half-life of 29 , and it is only the latter isotope that we are concerned with. The decay equation is: 66 Cu 0 66 –- e + Zn (stable) 29 1 30 The electrons that are produced in this decay process are detected by a radiation 66 Cu counter and their rate of production is a measure of the number of 29 atoms that are present at any given instant in the copper or brass sample. The samples used in this experiment are so thick that any additional thickness would result in no increase in the counting rate; the additional decay products would be self-absorbed by the sample before reaching the Geiger-Müeller tube. The thickness or weight of the samples is thus immaterial as long as the copper and brass samples have the same cross sectional area and are counted in identical positions relative to the window of the Geiger-Müeller tube. For analytical purposes it is necessary to find the number of counts per minute (cpm) corresponding to the instant the samples were removed from the howitzer (i.e., corresponding to t = 0). Only this value of cpm will be proportional to the amount of stable non-radioactive copper in the sample; any later value of cpm would be low, soon vanishing to zero. Because of the statistical, fluctuating nature of radioactivity, this value of cpm for t = O can be obtained reliably only by doing a least-squares plot of the data and then calculating the desired value from the equation of the best-fit line for t = 0. 2 You will be able to observe the samples while they are being irradiated but the instructor will transfer the samples into and out of the howitzer. Since the half-life of 66 Cu 29 is so short, you must be prepared to begin counting soon after you receive your radioactive sample. One of the major constituents of brass is zinc, and with neutrons of the energy used in the activation of your samples, there will be a small amount of activity resulting 65 64 from the radioactive decay of Zn produced by the bombardment reaction Zn (n, γ) 30 30 65 Zn 30 . However, the amount of activity from this source will be so small that it may be neglected in your experiment. The theory behind the relationship between half-life of an isotope and its rate of decay is given in your text. EXPERIMENTAL PROCEDURE 1. Determining the Background. The Geiger counter should have been switched ON in advance of the experiment and permitted to warm up so as to give stable results. (The POWER should be turned on for one minute before switching ON the HIGH VOLTAGE and setting it to the voltage indicated on the Geiger-Müller tube.) The counting time in minutes is set by a combination of the setting of a two-digit rotary dial and an attenuation switch with settings of 0.1, 1, and 10. For example, if the rotary dial is set at 47 and the switch at 1, a counting period of 47 x 1 = 47 minutes is established. You will want to measure the background for 15 minutes so set the dial at 47 and the switch at 1. Initiate the measurement by pressing the small red COUNT button. The red light above the COUNT button will be illuminated during the measurement and will go out once the counting period is over. You can halt the process by pressing the small red HOLD button but if you do, you lose all information on time. Once the measurement period is over, read the accumulated counts on the LED display. From the number of counts and the elapsed time, calculate the average number of background counts per minute (cpm). Once you have completed the measurement of the background, set the time for 0.8 minutes (rotary dial at 08 and the attenuation switch at 0.1) in preparation for the next step. 2. Positioning the Samples for Counting. Two counting sessions are involved, the first with pure copper and the second with brass. Both are handled in the 3 same way. When the neutron-bombarded sample is taken out of the howitzer, pick it up with tweezers, transport it to the Geiger counter with a small tray (provided), and finally position it in the depression machined for this purpose in the sample holder. With both samples, the paper label must be on the bottom. Be sure to record the number on the brass sample label. The lab assistant will have a stopwatch and will count exactly 2 minutes from the time irradiation is stopped. Within this two minute period you must have reset the timer, have the sample positioned under the window of the Geiger-Müller tube at its highest position, and be prepared to press the COUNT button on your counter (see next paragraph). 3. Counting the Samples. When the two-minute period is up, the lab assistant will start the lab timer (on the wall) and, simultaneously, you will press the COUNT button on your counter to initiate counting for 0.8 min. As soon as the counting period has elapsed, record the total counts along with the corresponding counting interval (0.8 min.) next to t = 2 min in your notebook. Note that when you press the COUNT button, you reset the LED display to zero. When the minute hand on the lab timer sweeps past "0" again (corresponding to t = 3 min after completion of irradiation), start another 0.8 min. count by pressing the COUNT button. This process is repeated until t = 25 min after completion of irradiation. The lab assistant will irradiate the brass samples while you are measuring the rate of decay of the pure copper sample, using an irradiation time which is identical to that of the copper. Count the brass sample in exactly the same way as you did the copper sample. The same trap position (top tray) is used and the sample is inserted so that any label is pointing down. When you have finished counting each sample, use your tweezers to place it in the container marked for this purpose near the counters. If you touch the sample, don't panic as copper and brass are not water soluble. However, as a precaution, immediately wash your hands. A small sink is located in the room. DATA ANALYSIS 1. For general discussion and examples, see the section "Curve Fitting and the Method of Least Squares". 2. Because the radioactive decay observed in this experiment is first order, your data of counts per minute (CPM) versus time (T) will fit an exponential curve. 4 Therefore, a plot of the ln(CPM-B), where B is the background counts per minute, versus T will yield a straight line. From the slope of this line we can determine the "half-life" of Cu and from the intercept we can determine the initial CPM.
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