I OFT ORNL P 1928 . . :25 |L4 MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STANDARDS -1963 ORNLP - 1928 cokf-660207-2 V · LOL! LIERGY GA.MA AND X-RAY FROM RADIQACTIVE SOURCES * R. S. Pressly Isotopes Development Center Oeu: Ridge National Laboratory Oak Ridge, Tennessee RIMASE POR ANNOUNCEMENT II HUCLIMR SCIENCE ABSTRACTS ABSTRACT The methods used in the fabrication of low-energy-gamma, -. bremsstrahlune and x-ray sources are described. The spectrums .-. of the radiation from several sources are given with efficiency measurements. The newer radioisotopes which are being investi- gated and their availability are discussed. .-.***** ***---- INTRODUCTION - ** Many radioactive sources have been used in instruments for the mee- - surement of thickness and density of liquids, solids, and slurries. Padio- graphs are made with sources to show voids, cracks, or inclusions in welds - - - and imperfections in large metal castings. Low-energy-gamma sources are particularly useful for the measurement of the thicknesses of thia metal coatings or densities or materials which are composed of the elements of low atomic number. Gamma-backscatter instruments are particularly sensi- tive because they make use of low-energy photons which are easily scattered. Much use has been made of radioisotopes in analysis of trace elements such as the use of tritium bremsstrahlung sources to detect the presence Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. WA of trace amounts of sulfur in petroleum. By the excitation of elements to emit characteristic X-reys, quals.- tative and quantitative analyses can be made with samples of ores to - . . determine their mineral content and value. Such analyses can be effected by the use of specific energy sources such as entimony KQ X-ray excited by the 147 Pra beta ray to measure silver content or the use of the 183xe gamma to analyze for gold. Bremsstrahlung sources emit radiation with a wide . . sayg'ati. range of cnergies. Beceuse of this, such sources can be used to produce characteristic energy peaks from many different elements. Those energy peaks can often be resolved and measured. To aid in designing radioactive sources which yield x-ray or low-energy-gamma radiation, or perhaps entirely bremsstrahlung, one should become aware of the physical interactions of beta and gamma radiation with target material. As a beta particle comes from the nucleus, internal bremsstrahlung is produced. This is the result of the sudden change of charge in the nuclear emitting field. It is present in all beta radiation to the extent of -0.79 of the beta emissions. As beta impinges externally on target material (which also includes the mass of the radioactive substance) it may come close to the nucleus so that its direction is changed or it is stopped. Under these conditions external breinsstrahlung is produced. Compton and Allison' give an empirical formula for the energy fraction o1 incident monoenergetic electrons converted to bremsstrahlung in a thick target: LEGAL NOTICE ......... This report was prepared u an account of Gorenmont sponsored work, Walther the Onited States, nor the Commission, nor ww pornou moins au bobol of the Commutatan: A. Yakes any warranty or representation, aproaped or implied, wide rompect to the noor racy, completansou, or wafulnus of the taformation contained to the report, or that he was of any information, apparatus, method, or process declowed tu dhe report may not to tayo primately owned redata; or A. AMB nay labtudes with respect to the wo of, or bor doimingue non ten Brom the one of my taformation, apparatue, method, or prono disclosed to the reports ; As wood a the abova, "pornon voting on behalf of the Countdom" nobudo M- ploys or contractor of the Commission, or play of mal contractor, to the one that much employee or contractor of the Commandan, of omployees of work contractor puren. diavominates, or provides noorus te, nwy wormation par to vu wployment or contrnet with the Coranlaaton, or Ms employment with no contractor. -X (ZE) where 2 - atomic puraber of absorber 1o - Incident electron energy in Mev Kw constant Evans has approximated the values for K to be (0.7 + 0.2) 10-3 ijev-. For a continuous beta-ray spectrum he has derived an approximate equation for the fraction of the beta-ray energy which is converted to external bremsstrahlung: I = Ko(ZE) 2 = atomic number Eo = maximum beta energy Ko is estimated to be approximately 0.33 x 10-9 Mevod. The probability of bremsstrahlung emission increased with ze and decreases with the square of the mass of the incident particle. Because of the difference in the masses of an electron and an alpha particle, alpha particles impinging on target materials produce a small quantity of brezs- strahlung even though the kinetic energy is high (4 to 5 Mev). Alpha particles ionize and produce x-rays in the same manner as do beta particles. The spectrum from alpha x-ray sources contains less bremsstrahlung or "white" radiation. If the incident beta particle has energy greater than the binding energy of the electrons of the target material, it can remove electrons from the K, L, and M energy shells. As the vacancies are filled from some other energy level, x-rays are produced. As low-energy betas and low-energy photons are attenuated by the materials of low atomic number, much of the energy loss is by lonization . . . ! . . .. 3:21 TA and consequently the ratio between the amount of x-radiation produced and the quantity of bremsstrahlung produced is increased by the use of materials of low atomic number. Beta and gamma scattering, ionization, and the conversion of low- enerey gamina.s produce secondary and tertiary results which modify the spectrum from a source. EMISSION-RATE AND ENERGY MEASUREMENTS The energies and emission rates from gamma, x-ray, and bremsstrahlung sources were measured by a 400-channel gammal spectrometer (KIDL transistorized inodel 34-8), which uses a standard 3- by 3-in. NaI(11) crystal. The spectro- ineter was standardized against radioactivities that contained photon energies as near as possible to those of the expected radiation. Either the measure- ments were made at a convenient distance or the radiation was collimated so as to measure a known fraction of the radiation. The production of a gaussian radiation peak, as with secondary x-ray production by beta radiation, per- mitted determination of the source output by summing in the desired peak area and correcting for distance geometry. No correction was made for self- absorption or window thickness. Source efficiencies were calculated by comparing the output of the source 1.0.cm away with the quantity of radiation actually in the source. All measurements were made with the detector crystal covered with 1.23 g/cme of beryllium. The ratio of the Ka or gamma peak to the total photon output of the source was determined by extending the x-ray peak downward so as to enclose certain onerries and sumraing under th18 fraction of the graphs. Total photon emission was determined by summing under the entire curves. SOURCE DESIGN A radioactive source is designed for a particular application with adequate consideration for shielding and the integrity of the source capsule in the environment in which it will be used. With low-energy-X-ray and gaxama sources the materials of construction are aluminum, beryllium, and stainless steel. For geseous sources copper, glass, and stainless steel are widely used. It is necessary to bond the radioactivity to some metal or to fuse a radioactive mixture to a ceramic so in order to receive consistent radiation readings from the source. Promethium-147, posr, and 57co can be con exchanged. for the sodium ion in a molecular sieve. The radioisotope-labeled molecular sieve is heated to 1100°C to form a ceramic. Another method for fixing the radioactivity involves absorbing a solution of it into porous graphite. Upon heating, the solvent is evaporated, leaving the radioactive material within the pores of the graphite. Homogeneous sources are made containing 144ce203 and 241 Amzog. (Fig. 1) If a "pure" beta emitter is used in the fabrication of a tremsstrahlung source, the contribution to the photon spectrum is created by the absorption of beta radiations in the materials of construction. If the maximum beta encrgy is below that energy necessary to remove the K or L electron of the material of construction, and/or the fluorescent x ray does not interfere, 2. . MAA 2 then one has a useful bremsstrahlung source such as is shown in Figures 10, 12, xud 17, With a basic bremsstrahlung source, the lower energies can 'e absorbed and the simulated "peak" shifted to a higher energy. Radioisotopes which decay with appreciable hali-lives (six months to three years) to produce low-energy photons are not abundant. The design problems associated with these quclides are: 1) making a safe source; 2) getting the unattenuated beam of activity outside of the source capsule; and 3) making the source of radiation as near a point source as 18 possible. The source wiadok can be made of beryllium or aluminum to reduce attenuation, and reflectors composed of high-Z metals cen be placed behind the l'adioactivity to increase the Rayleigh or coherent scatterie : Nini).... As with 109cd two gamma energies are present. The lower energy gamma can be absorbed and the higher energy one used for measurements. (Fig. 3) More efficient sources can be made with gamma emitters than can be made by secondary produced x-ray sources and the spectrum of the radiation from the sources will show less bremsstrahlung. (Figures 3, 4, 5, 6, and 7) Sources which produce x radiation can be fabricated by allowing beta. radiation to impinge upon target material. Sometimes the target material is the radioactive material itself, as with etc and 147Pmzog. A reflective type source, where the beta source is shielded from the detector and the produced x-ray is directed toward the detector, is another type of design. - ..: Mixed sources incorporated the radioactive material in close contact iii with the target material, as with 147 Pře Og mixed with powdered antimony or . . mb. ...c.no wwwvorietaiso vintrings in tin and pressed into a pellet. With a transmission or opposition type, the radioactivity is placed between thin sheets of target material. One sheet 16 thick enough to produce and scatter the x radiation toward the face on the source. The thin sheet will produce and allow the transmission of x radiation. For a mixed source, the energy of the desired x radiation determined pellet thickness, which should equal ~2 relaxation lengths of the x-radiation energy. If, however, the range of the beta is not the thickness of the pellet, nothing is gained by exceeding the range. If the beta radiation penetrates the pellet and the window, a beta shield composed of lucite or K film or low-2 metal should be over the face of the source. A reflector composed of high-2 metal is placed behind the source pellet to backscatter the beta and the x radiation. The design of the opposition source has similar problems in that the . - 1 source material and target material toward the face of the source should not be greater in thickness than two relaxation lengths of the x radiation. The reflector may exceed the range of the x radiation or the beta backscatter saturation thickness. The most efficient opposition source can be fabricated by making the source capsule from target material and using low-energy-beta . V radiation. In the beta-excited x-ray sources there are three methods which produce the desired peak: 1) ionization of the target by beta radiation; 2) ioniza- tion of the target by higher energy bremsstrahlung; and 3) the degrading or loss of energy of the slightly higher bremsstrahlung to shift the energy with- in the desired peak. I . . LA ** SOURCE FABRICATION Samariun-151 Samarlum-251 18 formed in uranium fission and 18 separated, along with Inactive samarium, in the ion exchange purification o1 147pm. The 1515m varies from 1 to 4% of the total samarium, depending on the age of the rare earth fraction before separation. Samarium-151 decays by a 0.075-lev beta and a 0.021-idev gamma and has a half-lifc of 93 years. A veighed amount of f1ssion samarium oxide, which contained 446 151,5m dy mass spectrographic analysis, was placed in an aluminum source holder that had a 15-mil aluminum window. The radioactive material (2.4 mg) was in a cylindrical space 7.0 mm dia and 2.0 mm deep but did not fill it. The edd opposite the active material was Heliarc-welded and leak-tested. The spectrum (Fig. 7) shows a ~21-kev gamina peak. The output of this source was calculated to be 9.7 x 108 photons/min at 1.0 cm from the source face, an efficiency of 0.14%. The ratio of the 21-kev gamma to the total radiation from the source is 0.815. Iron-35 Iron-55 decays by K capture to yield the characteristic 5.9-kev K x ray of manganese. Because of this low-energy radiation, it is difficult to fabricato a source vită a predetermined output. The most successful- technique is evaporation of 55Fe(NO3), solution onto a platinum strip and decomposition of the nitrate to Fe2O3. The platinum strip and oxide are heated by electrical resistance in a hydrogen atmosphere to reduce the oxide to the metal. A thin film of aluminum evaporated over the surface 1 ... i .. d ili. MiniIX to Pin man . 12.01.03 to prevent oxidation. À 3-mil beryllium window is sealed over the acti:e source arco. (746. 8) he output of such a source is limited by seli-ahsorption, such sources can be used to radiograph very thin sections. Watermarks on stamps and details of flowers (Fig. 9) can be shown on ilin by using 'prolonged exposures. Tritium reinsstrahlung Source bremsstrahlung source was fabricated by adsorbing tritium gas onto a thin Leyer oỉ titaniun. The titanium metal layer is vacuur evaporated onto & disk of platinu:n. When the titanium is heated to ~370°C in a tritiui atmosphere, the tritium is adsorbed on the titanium. The beta enercy (~18 kov muximun) is sufficient to excite the characteristic x zmy from titaniu!. · A resolution of the enerøy peaks by proportional counting sho:s the characteristic x ray and the higher energy bremsstrahlung pauk. Combined together and measured by a 3 in. x 3 in. sodium iodide crystal. The radiation appears as bremsstrahlung radiation at 12.5-kev enersy (Fig. 10) Technetiuin-99 Technetium-99 emits a 290 kev beta and has a half-life of 2.12 x 105 years. A nillicurie of pure technietium metal weighs 280 ing. The beta purčicles produce K x rays of ~18 kev. Powdered metal of 91% purity, 260 me, vas compressed into a 7.0 by 2.0-mm cylindrical pellet, which vas encapsulated in 15-mil aluminum. This source contained 2.85 inc of 98TC, and its output at 1.0 on was calculated to be 2.4 x 107 characteristic K x rays/min. This is 0.38% of the totel beta disintegrations per minute in the source holder. Note (Fig. 11) the . . .... . ----- - -- ----- ---------- - - presence of ino high-cncicy bremostrahlunc radiation with the characteristic * ray. Ine ručio of the output of the characteristic x ray to the total. -- bremostrcillung radiation is 0.344. - - Promuthium-147 with Molybdenum Holybdenum K x radiation was produced by 247 Pin betas in a so.:200 foried I'l'oi! 20 mc or 147 PngOg between two 1.2 mil thick molybdenum foils. This molybdenum foil thickness was sufficient to absorb >981 of the beta radiation. The source vos encapsulated in alurinum and which had a 10-nil window; the efficiency vas calculated to be 0.15 at 1.0 cm from the face. A more - - - - - officient source 0.6; wes fabricated by reducing the thicknese or the * - - - molybdenum foil to 0.79 mils to decrease the attenuation of the secondary Susanne * roys. The radiation spectrum is shown iu Fig. 13. Promethium-.247 with Tin Ten millicuries of 147Pm was placed between 2-mil foils of cin metal and encapsulated in 10-121 aluminum. The source was placed 40 cm from the detector to obtain a spectrum (Fig. 14) at 1.0cm, the radiation from the characteristic K x ray of tin is calculated to be 2.08 x 108 photons/min, an exficiency of nearly 0.95%. This characteristic x-ray peak was -59%oi 2 the intal bremsstrahlung prcc:cni. Technctiun-22. Tungsten Although the specific activity of 98rc is low, the beta energy (290 kev) is sufficient to produce the characteristic x radiation from tungsten. l'owdered 99Tc metal was placed on strips of tungsten which were then heated by electrical resistance in an inert atmosphere or vacuum. An alloy . .. - is forincd between tungsten and technetium metal to produce a mixed source. 'Tier tungsten x ray of w60 kev is shown in the radiation spectrum from the source. (Fig. 15) There are several lesser known radioisotopes that may prove to have many useful applications. If half-life and energy of radiation are not suitable for one purpose, they may be exactly what are desired in another. Xenon-133 (Fig.5) has a half-life of 5.3 days. This gaseous radioisotope is used l'or leak testing of tanks, coils, small volumes, and other intricate equip- went that cannot be inspected without destructive testing. Its 81-key gamma is easily measured. The gas can be frozen out and recovered, or admitted to the atmosphere to decay in a short time. ihe short half-life and energy of radiation of 193%e has proved to be an advantage in redical application. With 133xe in the blood strcan, ratos of Tlou, volume oỉ blood pumped by the neart, and other measurements can be made. Krypton-79, if available, may be used simlarly. Its shorter heli- life may prove to be an advantage. Com:It-57 is produced in the cyclotron by a (p, 2p) reaction on 581. Cobalt-58 is also produced at the same time by a (p,a) reaction on 61N1. Cob.at-58 decays with a shorter half-life (58 a) than does 57co (120 à). 1116 principal comune cctivity from 57Co has an energy of 120 kev. (Fig. 6) Promethiui- 1447 decays ty bcta emission and also by gouna emission 01 hlouzh : 1244 kez choroy (Fig. 16) Illi üdic covelopmenü of more sensitive instruments for the resolving en Oldur'cuent or low-cncrcy photon radiučions, less radioactivity would be . 1, .. ce . required in a. source. Then one considered such ?.ow-energy sources as the characteristic bariun x ray produced by 14c in barium carbonate sources, 38Ar bremsstrahlung sources, and 63N1 (67 kev) secondary x-ray sources. - REFERENCES 1. Compton, i. l., ond Allison, S. K., X-rays in theory and experirent. 2. Evans, Robley, D., the atomic nucleus, chap. 21, p. 616. . yang . APPENDIX . . -- - - Radiograph showing the CeO2 within graphite matrix. Fig. I I'Ic. 2 Absorption by stainless steel of the 22 kev and 88 -- - -- . -.. 70 . . .. . i'ic. 3 Fig. 4 Fig. 5 .. kev fronn 209cd. ' Tačiation froin 209ca sou::ce. Radiation from 241Am source. Radiation from 199%e source. hadiation from 57co source. Radiation from 1515m source. .. .... no Fic. 6 Iron-55 source. Ridiograph clematus flowcr'. Fig. 7 Tis0 Fig. 9 ris. 10 lic. 1.2 Fig. 12 ris. 13 hadiation iron 11-Ti Gource. Radiation from 98Tc source. Tiadiation from 147Pm-Al source. radiation from 147Pm-Mo source. Fic. Fig. 1.6 1'16. 17 Rodiation from 147 Pin-Sn source. liadiation from 98TC-W source. Gamna radiation from 2473X. radiation from 147Pm-U20g source. incitation of K alpha x ray of various targets by the radiation iron 147 Pun-U30g source. li'ie. 10 , . عر: ۰۰ . '. ' : - T | , ط . به . چی = = = ." - تایید" ؟ FIGURE / RADIOGRAPH SHOWING 7HE PRESENCE IV " في " عمر / /" پر دم 4. ق هه ل نهم7 5/L . و با زه) ماه م . ر مو - مم. ... : X۰۲ : : : : : . . . . . . (ان) * " . د سیمه کی دت) : ۲۰ DANHMADMITIIMIHHIKA WRTHUHUH U Hill Hi!!! 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