SILVER-111 BETA-RAY SOURCES W. WAYNE MEIIIE D. N. SIfDERMAN IP-122 Prepaper for Limited Distribution by the College of Engineering Industry Program August, 1955

We wish to express appreciation to the authors for permission to give this prepaper a limited distribution under the College of Engineering Industry Program. ii

SILVER-lll BETA-RAY SOURCES W, Wayne Meinke and Do N. Sunderman Department of Chemistry, University of Michigan Ann Arbor, Michigan Mul>i-millicurie sources of bleta emitting materials haver found many uses during the past decade. Biological systems have most often been subjected to these radiations (1-5), although investigators have also used the sources with chemical (6), and physical (7,8) systems investigators repgort using source of phosphorous-Bakelite (9,10), Sr9oY90 (5,u), Ru109-Rh1-09 (4,12), T12 (13), and Pd109 (8). Develo1pment in this laboratory (14,15) of a new, rapid, high decontamination method for the separation1 of silver by isotopic exchange makes possible the routine production of large beta-ray sources of Agll from neutron bombarded palladim to supplement this list of sources. Source Preparation: Palladium metal, when irradiated in a nuclear reactor forms 13.6-hr Pdl09 26-min Pd111, and a small amount of 17-day Pd13. The Pd11 decays to 7.6-day Agll. in processing, these Ag+l atcons must be separated from grams of palladium-Lan element which is quite similar chemically to silver. Previous methods of separation proved tedious and not readily adaptable to high level processing. The separation of silver by isotopic exchange, however, proves to be a very specific type of reaction which suffers little interference from macro amounts of palladium, Silver iodide is the compound used, since it suffers less interference from palladium than the silver chloride described prevaiously A "silver iodide" electrode is prepared and the electrode immersed in the radioactive solution to allow the process of isotopic exchange to occur and thus separate the radioactive silver from impurities, The silver iodide is then dissOlved from the electrode and silver electrolysed frm this solution as the metal on a platinum electrode of desired shape to form the source. For this final step, the silver may be plated on a variety of basic metals such as copper, brass, steel, nickel, and many ferrous and non-ferrous alloys, as well as platinum, instructions for such electroplating processes may be f ound in a book on electroplating by Gray (16). To prepare the electrode, a platinum gauze about l-l/4" high and 3/8" in diameter is coated with metallic sil~ver by plating fron 10 ml of solution

-2 -containing 10 mg of silver as the nitrate in 3 M NaCN and 0.5 M NaOH. The plating process requires about 4 minutes at 3 volts and results in the deposition of about 5 mg of silver. The electrode is washed with water for 30 seconds and immersed in 10 ml of a solution 0.05 M in HI and 0.05 M in NaHSO3, The current is then reversed, the silver surface used as the anode, and electrolysed at 1-1/2 volts for about 5 minutes changing the silver to silver iodide. The finished electrode is washed and can be stored in 1-1 HNm3. To separate radioactive silver from other activities (except halides) the "tsilver iodide electrode" is immersed for 5 minutes at room temperature in about 10 ml of an acid solution containing the active material. The solution may be stirred magnetically. The electrode is then removed and washed with a stream of 1-1 HN03 for one minute, the thorough wash being essential when high decontaminations are required. To form the Ag 1 source, the electrode from above is again immersed in the alkaline cyanide bath and the silver iodide coat dissolved and the silver plated as metal on a source holder of platinum or other metal. Sources prepared in this way give an adherent coat of metallic silver. Contamination by rub-off may be lessened by plating a —thin coat of inactive silver over the active silvrer. When silver is used as the support for the silver halide surface, much poorer deposits are obtained Iti is also possible to use the silver iodide electrode directly and eliminate an extra step in the procedure. The silver iodide tends to be somewhat chalky and brittle, however, and would require a heavier coating to prevent it from contaminating materials it touches than does the metal source. These sources can be prepared in any of a number of shapes and forms from a point to a line or a cube or a gauze, or an irregularly shaped object merely by changing the shape of the cathode in the final plating step. Yields of the radioactive silver vary with the type of electrode used in the exchange step. For the platinum gauze used in these experiments, greater than 98% yield was obtained in 10 minutes at room temperature. Increasing the temperature decreases the time required to obtain this yield. Smaller amounts of inactive silver may be used on the gauze if a source of very high specific activity is required. When submilligram. amounts of silver are used, near quantitative yields are not obtained because of chalking of the silver iodide. Concentrated solutions of palladium inhibit this exchange process only at concentrations greater than 0.5 M, Similarly, other foreign ions have anl inhibiting effect at concentrations of about 2 X and above. Results of a number of experiments on yields, decontaminations and interferences using this

-3 -procedure will be reported elsewhere. Characteristics of the Source: The only silver isotopes present in a freshly bombarded sample of pure palladium would be the short-lived isomers of 44-sec A9g07 and 39-sec Ag209 in addition to the 7.6-day Agll. Unfortunately, "high-purity" palladium often contains same silver metal impurity [see typical analysis (8)3 which would give small amounts of 270-day Agllo by neutron capture. A typical impurity of 0,002% silver would give only about 0.002% as much activity of this long-lived isotope as of the 7.6-day activity in a one week's bombardment, and would be negligible. Thus the primary silver activity present in these sources will be Agllo A source of pure Ag-ll would weigh only 6.5 x 10-9 ga/mc, but since several mg of silver carrier are required to utilize this exchange method, the silver will not be carrier-free. The Aglll emits several beta rays of about 1 Mev and a small percentage (10%) of gamma rays of about 0.3 Mev. The surface density of silver metal is about 26 mg/cm2/mil. Thus, the "half-thickness" for the silver beta rays is about 2-1/4 mils of silver. (This term "half-thickness" is useful for our discussion, although it admittedly is not very exact. ) Since in this procedure 10 mg or less of silver carrier is used, at least half of all the beta rays emitted will not be absorbed in most source arrangements. Hence, much less activity is lost by absorption in these silver sources than in the p32 plaques. The most active source made to date in this laboratory has been several millicuries, but the same procedure is suitable for many curies with the same amount of carrier, The graph in Fig. 1 shows the amount of Ag1ll per gram of Pd which may be obtained in a I-week irradiation at various reactors. Reactors such as the NRX at Chalk River and the 1MR in Idaho can accept up to 50 or more grams of palladium in their high flux -position. Thus, multicurie sources could be made by this exchange procedure. Regardless of the activity level of the Agl2l, the source can still be kept in an easily handled form. Comparison of Sources: Characteristics of a number of kinds of beta-ray sources are listed in Table I. Sr90 suffers the major disadvantage of difficulty of mounting and of high personnel hazard. p32 has been used extensively in Bakelite plaques for many types of irradiations- -especially in the biological and medical field, but is inherently bulky and prone to contamination. Sources of Ag12- compare favorably with the other sources given in the table. The 7.5-day half life is some disadvantage, but the ease of preparation should offset this. In its favor is the possibility of preparing multicurie

sources in any of a multitude of shapes which best fit the Material or object being irradiated. Since silver is relatively non-toxic (20) in small concentrations, it does not present the problems of Sr90. The gamma ray associated with the silver beta ray is of low energy and low abundance and should have a negligible effect on the handling problems. Acknowledgements This work was supported in part by the U. S. Atomic Energy Commission. The radioactive palladium used was obtained through the Isotope Division of the U. S. Atomic Energy Commission, Oak Ridge, Tennessee,

TABLE I. Comparison of beta-ray sources. Half-Life Radiations (Mev)(lt) Form Specific Activity Personal Combust- How obtained (17 ) - used or available hazard(18,19) ible? (curies/gm) pd109-Ag109m 13.6 h1r Pd 0.961 none Metal foil X-10 - MTR low no Irradiation of 0.2 - 100 Pd metal in Ag e' 0.087 1 _ _ _ _ neutron flux Ag1 7.60day 1.04(91%) 0o340(8%) Ag metal elec- 1 - 1000 low no Formed carriero0,8 (1%) 0.243(1%) troplated on free from 26-min 0 70 (8%) metal cathode. Pd111 in pallaThin layer of dium irradiated inactive Ag with neutrons _ gives protection -p32 14.30day 1.701 none Red phosphorus X-10 - 0o0058 low yes Irradiation of mixed with plaque in neutron Bakelite flux Rul- Rh10 6 1.O yr Ru 0.0392 none Ru metal elec- | — 20 Inter- no Fission troplated on Cu, mediate products Rh Pt, Au or Ni 3.53(68%) 0.513(23%) cathode and. 3.1 (11%) 0.624(12%) covered with thin 2.44(12%) others(3%) layer of silver 2.0 (3%) for protection others (6) T1204 l 4.1 yr o765(980) none Thallium salt or 0.02 low no Irradiation of EC (2%) metal thallium in......................... -.,_.... I..._ neutron flux Sr90 y90 19.9 yr Sr 0.61 none SrC12 incorpor- 1200 high no —but Fission products ated in vials (carrier free) easily Y 2.18 none or sealed in ruptured plastic or metal with thin window -5 -

Fig. 1. Agll activity attainable in one week's bombardment at different neutron flux levels. MATERIALS TESTING REACTOR 1.0 CHALK RIVER, CANADA a.. BROOKHAVEN C,) w cNATONAL LAB. ' 0 OAK RIDGE NATIONAL -6 -

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