Miles Pickaring
Reed College Portland, Oregon 97202
A Freshman Experiment In Neutron Activation Analysis
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1he grou~ingimportance of neutron activation analysis in environmental, forensic and industrial applications hardly needs to be emphasized. This experiment was designed to be an introduction to neutron activation analysis, as well as to begin the teaching or radiochemical techniques in the freshman year. A great deal of streamlining has been done to make it "goof proof" even for freshmen. To this end a simplified tracer chemistry was developed and a much quicker (and cruder) method of mounting samples is used. The specific application of neutron activation analysis to be studied is the determination of &In in "pure" iron wire. Our irradiations were done in the Reed Reactor Facility, but since not everyone has a reactor downstairs, an appendix is attached giving information about irradiation times if delays occur between activation and the class period. A sample of wire is exposed to a neutron flux in a reactor. The trace amounts of 55Mn present will capture a neutron to become S 6 A h , which is radioactive, a 8- emitter with a half-life of 2.55 hr. By measuring the amount of radioactivity produced in the wire and comparing it to a standard, designed to have a known concentration of Mn, the concentration of Mn can be calculated directly. The sample and the standard solution of manganese will be exposed to an ideutical flux for an identical time, so even if the flux or time is not known precisely, it will not affect the result. Corrections will also not have to be made for the number of 8 particles not entering the GM tube (the geometry factor) since the samples will he mounted and counted identically. The Fe itself does not become very radioactive. This is due to the fact that most of the Fe is SBFewhich will capture to become 5'Fe which is stable. The only iron isotope which is a problem is We, which is present in low abundance, and has a low capture cross section (probability of picking up a neutron). "Fe is also long-lived so that even if some is produced, the rate of decay will be small. There udl be traces of other radioactivities present, however, and some purification of the Mn will be necessary. Theconcentrationof theS6nhisso small that ordinary chemical manipulation would be very difficult. Precipitation would be impossible since the solubility product would not be exceeded, and adsorption of Mn on the container walls would be a problem. Therefore, after the irradiation is over, a larger quantity of inert i\ln (10 mg) will be added. Since there is no chemical way of distinguishing between "cold" 6SAIn and 56Mn, all of the radioactive manganese will be carried through the separation with the "cold" 5SMn. 430 / lournol of Chemicol Education
The "cold" Mn is called a carrier, the 5eMnis called a tracer. This experiment is quite safe and there will be no significant exposure of students to radiation. However, the student must treat the radioactive material with some respect, to avoid spreading it into the counter and causing increased backgrounds, or ingesting it. Disposable gloves and safety glasses should be worn. Students should also he required to wear shoes (not sandals). Absorbent paper shauld be spread on benchtops and a mat placed a t the door to the laboratory, so that in the improbable event that some active material is spilled, it is not tracked all over the building. Students should be cautioned not to scratch their faces with their gloves on, and to leave their gloves behind when having samples counted. Most of these precautions are not life and death matters, but the development of good radiochemical technique early saves trouble later. All the species present have relatively short halflives except "Fe, and this is produced in very small quantities. Hence, there is no danger of permanent contamination of laboratory areas. We had about eight spills with seventy students. None soaked through the absorbent paper, presented any difficult cleaning problem, or created a radiation hazard. Counting was done by a "counting elf," a student from Reed's nuclear chemistry course. One Geiger Muller counter was quite adequate. The "elf" also does such things as explain the counter operation and answer questions. The equipment is situated in another room to lower the probability of contamination of the counter, and to keep the background down as low as possible. Sample Preparation
The iron wire used was reagent grade iron wire (Baker's analyzed reagent) containing O.lyo Mn according to the manufacturer. We used samples 2 4 om long, packed into a small plastic vial for irradiation. The standard was manganese acetate solution a t a concentration of 20 ~ g / m so l that 100 X would contain exactly 2 pg of AIn. The acetate is used so that no radioactivity due to the anion would be present. Care should be taken that the irradiation of the standard and sample are as nearly identical as possible. The samples should not be handled without gloves before irradiation, since fingerprints will become intensely radioactive, due to the presence of 24Nafrom the salt. The total activity of the whole vial reached 60 mr/hr at contact after irradiation, but most of this seemed to be short range 8.
Experimental
The following chemistry will remove stray radioactivity from the 56Mnbefore counting. All operations must be done over absorbent paper and wearing gloves. (1) Each student is given a length of irradiated Fe wire by the instructor who will measure the Length of this to the nearest millimeter. The weight of the wire per meter will be posted. (2) Exactly 1.0 ml of Mn carrier is added t o an Erlenmeyer flask containing the wire. This solution is made up to contain 10 mg of Mn'+ carrier/ml. The wire will dissolve in a few milliliters of cone. HCI. (The flask must be held with tongs since the gloves are flammable.) When the wire appears to have dissolved, i t should be boiled for .5 min, adding H-O if the solution threatens to boil dry. Let the solution cool. (3) At this point the instructor gives each student 0.1 ml of iMn standard. One ml of Mn carrier, and about one ml of Fe carrier (10 mg/ml of Fe3+ in 4 M HCI) is added and both tubes are carried through bhe chemistry below. (4) The solutions ale poured into centrifuge tubes and 10 ml of salurated NH,N03 solution is added. Thiv will form a buffer with the NHIOH to he added on the next step. (5) The solutions are neulrdized with NH40H (conc.), added dropwise a t first. When neutralization has occurred, a brownish precipitate will form. If a stirring rod is used, the stirring rod will be contaminated afterward and must be washed off into the "Radioactive Waste" bottle. The brownish precipitate is Fe(OH)*. The l\ln(OH), will not precipitate a t this pH, and remains in the solution. Centrifuge. Occasionally precipitation fails to occur. If this happens, the solution should be acidified, AIS+ carried (10 mglml) added and both AI(OH), and Fe(OH)s precipitated with NH40H again. Aluminum hydroxide tends t o form a precipitate which is easier to handle. (6) The clear solution is decanted into a flask. The gel a t the bottom of the centrifuge tube adsorbs all the stray ions in the solution a t trace concentrations and thus is quite radioactive. It should he put into the radioactive waste container and the Lube rinsed out once again into the radioactive waste container. Then it can be washed a t the sink as usual. Deposit gloves in the cardboard box marked "Solid nadioaetive Waste." (7) To the clear liquid from step T,, a few drops of 6% H20z is added. Evolution of oxygen will occur. The Mn will he oxidized to MnO*which will form a brown precipitate an standing. (8) The MnOl is collected in weighed Gooch crucibles. If the sample is n o t evenly distributed on the bottomof the crucible, i t should be spread out for better counting and drying. After rinsing with a few ml of H?O, followed by acetone, the sample is dried with suction. The MnOz will be a tan color when dry, and dark brown when wet. (9) The crucible is covered with weighed parafilm, cleaned on the outside with tissue and weighed. At this point, there should be no more Mn a t the end than there was a t the beginning. If the weight is grossly excessive, the sample must he dried again and reweighed. (10) The samples are counted on a Geiger-Muller counter for 1 mi". The "axe" of the samule (time e l a ~ s e dsince bombardment) most be'iecorded. ~ a e hsample should be counted a t least twice, an hour or so apart. (11) Clean up. Glassware used in steps 5-10 can be washed in the sink the next day, since the Mn will have decayed away. Glassware used before step 5 should he rinsed into radioactive waste with a minimum volume of water, then washed in the sink.
able guesses as to the amounts of Mn that came through the procedure, and this has not drastically affected the final results. While more accuracy can be obtained by using the filter chimney mounting used in radiochemical laboratories, the Gooch crucible mounting is faster, and more foolproof.) (2) The counts from the counting elf are plotted on semi log paper with activity on the y axis, and time elapsed since the end of bombardment on the x axis. A line is drawn through the points with a slope such that the activity coordinate is reduced by a factor of 2 every 2.56 hr. This line is extrapolated back to to (zero elapsed time). The best estiniate of the error (standard deviation) will be * d N where N is the number of counts in each point. Separate plots are drawn for both standard and sample. (3) The activities of the standard and sample obtained in Step 2 are divided by the chemical yield factor. This will give the activity if no i \ h has been lost. (4) The ratio of corrected activities will be the ratio of grams of ;\'In in the standard to grams of i\ln in the sample. From this fact it is easy to calculate the numbcr of grams of hln in the sample. Division of this number by the weight of the wire (gotten by multiplying its length times its weight/unit length) will given the % Mn in "pure" iron wire. Results
The results of the class indicate that the value falls between 0.02-0.0670 hn'ln. Some results were so far away that one suspects the students' calculations. Almost nobody failed to understand the point of the experiment, and was unable to do the calculations. The presence of radioactivity seemed to stimulate considerable interest. The apprehension generated a t the beginning of the experiment subsided as the students gained competence and became familiar with the operations. While it will be some time before many of these people regard radioactivity as an old friend, their interest was sufficiently stimulated to cause several of them to think about further training in nuclear chemistry. Appendix on Irradiation Irradiations can be arranged by outside users a t most nuclear facilities. I n general, a fee is charged, mostly for the packaging to meet trmsportation requirements. All facilities have Radiation safety officers who can assist with the problem of compliance with laws regarding transportation of radioactive material. The duration of irradiation of the samples can be calculated as follows
Time (min) =
Calculalions
(1) A chemical yield factor is calculated for both the standard and sample. Some of the Mn will be lost in the separation, but we can correct for this. The carrier will be lost in exactly the same ratio as the 56Mn. Hence the chemical yield factor will be y =
$m factor exp (-.278(t 3))
+
where t is the time in hours from end of irradiation to the beginning of class. ThefEuz factor can be obtained below. flux (n/cma see)
flux factor
wt of MnO. afterward X fraction of Mn in MnOx wt of Mn in carrier added
(In some cases students have measured grossly unreasonable yield factors (y > 1) but have used reasonVolume 49, Number 6, June 1972
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431
The above formula is derived on the assumption that the irradiation time is short compared to the half-life. Also it is assumed that the sample is %3 em long. Longer samples need ~rooortionatelvless irradiation. I t will show serious deviations far calculated irradiation times longer than 3 hr. The experiment should not be attempted with longer irradiation times, since a saturation effect occurs, in which "Mn is being produced
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Journal o f Chemical Education
ahout as fast as it is decaying, and radioactivity from other impurities becomes a problem.
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Acknowledament
The assistance of the Supervisor and operators of the Reed Reactor Facility is gratefully acknowledged.
