Ki'^'^v^^^^^''^^ MDDC - 1012 UNITED STATES ATOMIC ENERGY COMMISSION RADIATION HAZARDS OF BREMSSTRAHLUNG by H. M. Parker Clinton Laboratories This document consists of 8 pages. Date of Manuscript: April 1944 Date Declassified: May 28, 1947 This document is for official use. Its issuance does not constitute authority for declassification of classified copies of the same or similar content and title and by the same authors. Technical Information Division, Oak Ridge Directed Operations Printed in Uiuted States of America AEC, Oak Ridge, Tenn.-8-25-48-l,500 Price 10 cents PIADLA.TION HAZARDS OF BREMSSTRAHLUNG By H. M. Parker INTRODUCTION By and large, more damage his been caused to radiation workers by bremsstrahlung (continuous x-ray spectrum) than by any other type of radiation. This report is concerned not at all with hazards associated with general x radiation, but only with the bremsstrahlimg arising from the beta radiation of pile produced activities (either fission products or induced radioactivites). For convenience, bremsstrahlung will henceforth be angUcized to BS. It is evident, a priori, that the BS will rarely be the major factor in a radiation exposure and some apologia for this investigation is required; 1) This section is currently attempting to reconcile the observed gamma radiation from a large tank of liquid with the theoretical estimate. BS may be a correction factor comparable with the total uncertainty and its magnitude must be determined. 2) T. H. Davies recently pointed out that solutions in the new separations process show a re- markably high ratio of t3 to y activity. In one case, about 100 curies of j3 activity were associated with 1 curie of y activity. Special attention to the BS from a large mass of material may be required. 3) Brief mention of BS from tanks in the Project Literature refers to the radiation from the metal wall as the possible hazard. It seemed more likely that the radiation in the liquid would be greater. 4) In this laboratory there has been considerable misconception about the choice of container for a separated 3 emitter; whether a glass bottle containing such an emitter should be contained in an alu- minum lined shield, etc. This re- rt attempts to settle these four points. RADIATIVE ENERGY LOSS OF AN ELECTRON Nomenclature is that of Heitler's Quantum Theory of Radiation. Eg = electron energy, units of mc^ m = electron mass c = velocity of light rad = cross section for radiative energy loss, in units of 137 m^C N = number of atoms (molecules) per en??, Z nuclear charge NZ = total number of electrons per cm' NZ^^^ sum of Z^ of all nuclei per cm' 1 Mev will be considered equal to 2 mc^ MDDC - 1012 [ 1 2J MDDC - 1012 BS IN LEAD: TOTAL ENERGY Total energy loss per cm ( —J is given by Heltler on pages 221 and 222, The radiation loss — {— J = NE (p ^^^ . . . (equation 220) can be computed from page 143. The fractional energy loss by electrons of different energy in lead is given in Table 1. The energy converted into BS by the complete stopping of a 5 mc^ particle will be less than that obtained by the summation of the separate losses in Table 1. This upper limit will be taken as the total energy loss. BS IN LEAD: ENERGY DISTRIBUTION Heitler considers the continuous spectral distribution on pages 168 to 171. From these data the energy loss over the energy steps of Table 1 had been calculated by dividing the appropriate BS spec- trum into energy intervals of 0.4 mc^. Accuracy cannot be claimed for these spectra. Table 3 gives the spectral energy for successive energy losses of 1 mc^. The table is useful for calculations wherein an electron traverses a thin metal sheet and loses only part of its energy. Table 4 was derived from Table 3 by summation. These are the figures required in this report, corresponding to complete stop- page of the electron in the medium. BS m OTHER MEDIA The energy loss due to collisions is very nearly proportional to (NZ). The radiative energy loss is proportional to (NZ^). Hence, the radiative loss in media other than lead can be written down. It can be assumed that the spectral distribution will be the same in all cases. Table 5 gives a few values of energy loss in convenient media for 2 Mev particles, values for any two substances wiU be the same at other energies. The ratio of Table 1. Energy loss of an electron in lead. Eo - mc^ mc^ Mav d rad/<^ f-de) rad /'-de\ tot Fractional loss by BS .5 .25 5.35 .34 mc^ 26 mc2 .013 1 .5 5.55 .704 23 .030 2 1 6.5 1.66 24.7 .067 3 1.5 7.48 2.87 26.8 .107 4 2 8.23 4.20 28.7 .147 5 2.5 8.80 5.61 30.6 .183 Table 2. Total BS by an electron in lead. Mev .25 .5 1 1.5 2 2.5 BS energy (Mev) .007 .011 .045 .10 .17 .26 MDDC - 1012 [3 Table 3. Spectral distribution of BS (in Pb) over different energy intervals. Electron energy mc^ 5—^4 4-^3 3^2 2-^1 1— »0 BS energy mc^ .183 .147 .107 .067 ^.02 BS energy intei •val 2 BS spectral energy mc? x 10^ 0- .4 mc ' 32.3 32.4 29.9 26.9 12.6 ^ .4- .8 27.9 26.5 23.4 17.5 5.9 .8-1.2 23.8 21.4 17.9 11.8 1.5 1.2-1.6 20.1 17.5 13.6 7.5 - 1.6-2.0 16.9 14.1 10.0 3.2 2 -2.4 14.5 11.5 7.1 - 2.4-2.8 12.3 9.0 4.7 2.8-3.2 10.2 7.1 0.9 3.2-3.6 8.5 5.3 - 3.6-4.0 6.9 2.5 4.0-4.4 5.5 - 4.4-4.8 3.8 4.8-5.0 0.7 1 Table 4. Total spectral distribution of BS (in Pb) for given primary electron energy. ^""^'^Mev energy » 1 1.5 2 2.5 BS energy ^^^ interval i BS energy in Kev - .1 19.8 34.7 50,9 67.0 .3 11.7 23.4 36.7 50.6 .5 6.7 15.6 26.3 38.2 .7 3.8 10.6 19.3 29.4 .9 1.6 6.6 13.7 22.1 1.1 - 3.6 9.3 16.6 1.3 2.4 6.9 13.0 1.5 0.45 4.0 9.1 1.7 - 2.7 6.9 1.9 1.3 4.7 2.1 - 2.8 2.3 1.9 2.5 0.35 Total 43.6 97.4 172.0 262.7 Table 5. Fractional energy loss of 2 Mev particle as BS. Material Fractional energy loss Material Fractional energy loss H,0 .0136 20% UNH .0291 Pyrex glass .0214 Fe .0512 Common glass .0232 Pb .147 Al .0272 U .162/ 4] MDDC - 1012 BS FROM TANKS OF RADIOACTIVE LIQUID For simplicity, cylindrical tanks will be considered with the observer on the axis produced. The absorbing wall of the tank is then a flat plate. BS FROM AN INFINITELY LARGE TANK OF LIQUID If Em = energy of BS in the interval of average energy in Mev Jim = linear absorption coefficient of liquid for m Mev radiation (CT3^)m = scattering absorption coefficient of m Mev radiation in air.* The ionizatiun outside a large tank of liquid containing a j3 emitter only is I = const X L ^^-^ (equation 31C) taken over all values of m. /im The magnitude of the BS is conveniently shown by comparing the effect of 1 curie of 2 Mev p activity per cc with 1 curie of 2 Mev y activity. Some typical cases are given in Table 6. The BS considered here is from the liquid only. The fil- tration of the postulr.ted tank walls is computed by applying factors F,(/im d), since the absorption oc- curs through an indefinitely extended sheet, thickness d. It is clear that the BS does not vary radically with the composition of the liquid. For all practical purposes, figures for 20% UNH solution will apply well enough to any solution in the separation process. With a normal tank wall of about 3 mm Fe, /s^lSO curies of 2 Mev /3 will give total gamma activity equivalent to 1 curie of 2 Mev y. This answers 1(1) and indicates that 1(2) may just become of some significance. Table 6, BS from large tanks. Liquid HP equiv. 20% UNH Pb Tank wall ' 5 mm Pb' ' 3 mm Fe : 5 mm Pb 10 mm Fe"' ' 5 mm Pb ' Energy M Ix 10' Ix 10' I I I I Ix 10' Ix 10' .1 Mev 9.0 _ 12.5 2.0 _ .1 .3 _ .3 10.5 .2 11.2 5.6- .2 1.9 2.6 .05 .5 9.5 1.4 14.6 8.2 2.2 3.2 4.8 .7 .7 7.9 2.3 1.4.3 8.6 4.2 3.9 6.2 1.8 .9 6.3 2.2 11.2 7.1 4.0 3.4 5,5 2.0 1.1 4.5 1.7 7.9 5.1 3.2 2.6 4.3 1.7 1.3 3.5 1.4 6.3 4.2 2.7 2.3 3,4 1.5 1.5 2.0 1.0 3.8 2,7 1.7 1.5 2,0 .9 1.7 1.4 .7 2.6 1.8 1.2 1.0 1.4 .7 1.9 .7 .4 1.3 .9 .7 .6 ,7 .4 Total 55.3 11.3 85.7 46.2 20.1 20.5 26.2 9.8 2 Mev y 12100 6600 10200 7300 5600 4400 1100 600 Curies /3 equiv. to 219 583 119 157 278 214 42 61 1 curie y ("a + '') is used for low energy where the photoelectric absorption is significant. MDDC - 1012 [5 BS FROM A TANK WALL Consider the number of particles incident on a plane wall from a liquid emitting n particles/cm^/sec. Number incident on unit area from volume element dV is ndV dN^ cos lj> (410) 4 X r' where ^ = angle between normal and direction of element dV. r = distance to element dV = azimuthal angle Figure 1. R, 277 77/2 N =J J J - — 5 sin cos ^ dr d d nR 4 (411) where R is the range. Equation 410 agrees with the convenient approximation that of all electrons emitted within the range thickness R, one-half approach the wall, and of them one-half again will fail to reach the wall because of obUquity. It has been showi previously (CH930) that the average energy of ;3 particles reaching a surface under these conditions is 0.6 x primary energy. As the higher energies are emphasized in the BS one can reasonably take 1.5 Mev as the effective energy of electrons impingii^ on the wall from 2 Mev particles in the liquid. BS FROM AN INFINITE STEEL WALL THICKNESS t The radiation from an indefinitely large sheet of thickness t, when the far side is a BS emitter due to electrons impinging on it is I^ = Const X L (cTa) m Em F^ (jumt) (420) It is assumed provisionally that the energy Em is radiated uniformly in aU directions. Table 7 gives values for the BS in steel walls with 20% UNH solution. This method of calculation will give poor values for great wall thicknesses because no BS harder tlian 1.5 Mev lias been allowed. Calculations with the full 2 Mev energy and the present thickness give results about twice as great. Table 7. BS from steel tank wall. 20% UNH solution. 3 mm Fe wall A 10 mm Fe wall Energy m r a^Ex 108 I x 10° ' Ix 10« .1 9.3 2.32 .012 .3 7.2 6.82 1.80 .5 5.0 6.00 1.65 .7 3.34 4.35 1.52 .9 2.02 2.82 1.04 1.1 1.03 1.51 .60 1.3 .67 1.07 .44 1.5 .12 .20 .08 ToUl 25.09 7.14 2 Mev y in liquid 67.3 X 10< 4.4 X 10* Curies of j3 equivalent to 1 curie y in liquid 2880 6200 6] MDDC - 1012 In any case it is certain that the wall effect is much less than that from the liquid. For 3 mm Fe wall, I wall/I liquid = .054 For 10 mm Fe waU, I wall/I liquid = .035 SMALL TANKS The other extreme case is that of a deep tank of very small surface area. In common units I Uquid =£ (-^^ Uquid e" '^'^ I wall ^Z (<^aE wall e" P-'^ where /i and ii ' are the absorption coefficients in liquid and wall. For 20% UNH and a steel wall 3 mm Fe I wall/I liquid = 0.3 and 139 curies ^ = 1 curie y 10 mm Fe I wall/I liquid = .024 and 178 curies (3 = 1 curie y Neither the ratio of I wall/I liquid nor the curie equivalents have changed much. All other values for deep tanks will be between these limits. Shallow tanks could obviously give values of I wall/I liquid greater than these. But for all practi- cal purposes a tank 15 cm deep is a "deep tank", and the shallow tank case need not be considered. DIRECTIONAL FEATUHE OF WALL RADIATION High energy BS is confined to the forward direction within a solid angle of -^mc^/E^. The BS from the bombarded wall will, therefore, be almost wholly confined to the forward hemisphere. As a rough correction the wall radiation in 4.2 and 4.3 should be approximately doubled. However, relation (equa- tion 410) indicates that the incident radiation obeys a cosine law. Therefore, the resiUtant BS will also be directional. To maintain the total energy emission constant, the relations shown in Table 8 hold for postulated directional effects. Table 8. Emission law Energy emission at angle Bs from a large plate Uniform Cos E f or (^ = -•TT 4E cos, , = 0-»7r/2 0, (#> = 7r/2 -» TT I = KE F„ (/i t) I,= 4KE F,(Mt) Cos= 6E cos(^,0= 0— >jr/2 1^= 6KE F^{ii t) 0,> Tr/2 Fermi distribuUon 1.857 E(cos <^ + Vs cos^(t>) Ip = 1.857 Fi — 3.217 F^ 0,<^ >tt/2 CosH 2(n + 1) E cosn (/> Ij^ = 2(n - 1) KE F^ (ix t) Mt lyi \n Ip/I .01 0.941 0.729 0.827 .05 1.342 1.106 1.216 .10 1.586 1.371 1.471 .20 1.879 1.727 1.798 .50 2.334 2.375 2.357 1.0 2.709 3.001 2.866 2.0 3.071 3.698 3.408 5.0 3.478 4.592 4.075 MDDC - 1012 [ 7 Some values of these functions are given in Table 9. Table 9. For a steel tank 3 mm thick (m t) ranges from 0.1 to 1.0. If one considers the combination of varsring emission law at different energies with changes in the functions, it can be seen that a universal factor of 2 would be a good ap- proximation. The BS from the wall is, therefore, not greater than about 10% of that from the tank in the usual cases. MAGNITUDE OF THE EFFECTS IN ROENTGENS It has seemed convenient to use the curie equivalent method of presentation, namely, to write down the number of curies of 2 Mev activity which gives rise to BS equal in ionizing power to 1 curie of 2 Mev y ac- tivity under the same conditions. To convert BS values to roentgens a series of alleged values for 2 Mev y cases is given in Table 10. The tanks are supposed infinitely deep. BS FROM SHIPPING CONTAINERS The problem here is to decide whether (1) it is necessary to line a container with material of low atomic number or (2) whether additional lead shielding should be allowed in either case to cut down the BS component in a mixed j3 and y source. It will be supposed that all particles of primary energy 2 Mev strike the wall with the full energy. In some cases this will mean by the argument given in the section, BS from a Tank Wall, that the ef- fect quoted will be that due 10-^3 Mev particles. COMPACT SOURCE IN LEAD OR ALUMINUM Consider that all the electrons emitted immediately enter the metal. The resulting BS is shown in Figure 2 for thickness of lead shield to 10 cm expressed as curie equivalents as before, and also more conveniently here as mc of y activity (2 Mev) per curie /3 activity. For lead containers there is a pos- sible radiation hazard. Since a container of less than 1-inch wall is hardly worth consideration, a Table 10. Gamma radiation from sundry tanks containing 1 curie of 2 Mev y activity per Ce. Tank wall Dist. from tank . 3mm Fe 10 mm Fe 5 mm Pb Tank diameter ( Dosage rate in kr/hr - ■ s 1 : cm 20 cm 958 751 500 623 1 50 1704 1363 852 1041 1 100 2161 1640 1019 1240 10 20 378 227 166 247 10 50 951 783 575 692 10 100 1328 1221 906 1075 25 20 157 .81 50 104 25 50 475 356 261 350 25 100 769 869 635 756 oo 2326 1731 1062 1275 8] MDDC - 1012 oa. < 0.5 tr I 0.4 'H 0.3 0.2 0.1 ~~^ -^ MC Y PER CURIE fi ^ \ / \ \ / / / / / >< \ V CURIES ;8 EQUIVALENT ■^ v^ TO 1 CURIE y iX V -^ 0.1 1000 5 CQ. 500 0.2 0.3 0.4 0,5 1.0 CM Pb SHIELD 4 5 6 7 10 100 Figure 2. convenient rule is that the BS hazard is about 1 mc y equivalent per curie ^. If an aluminum liner is used, the radiation is reduced by a factor of 5 or 6. EXTENDED SOURCE IN LEAD, ALUMINTJM, OR GLASS In general, a strong source will consist either of a pellet of perhaps 1-inch diameter or of a solu- tion. In the first case, about one-fifth or less of the /3 particles will expend themselves in the sur- rounding metal and the remainder in the pellet itself. The total BS would be about twice as great in a lead container as in an aluminum one and the choice of metal container is not likely to be critical. The second case is that which arises frequently in the laboratory. A solution is to be shipped in a glass bottle. A typical bottle is 3.2 cm diameter, 2.65 mm wall thickness, filled to 3 or 4 cm. The 33 is then independent of the outer container as the /3 radiation is essentially all absorbed in the liq- uid or glass. With no outer shield there will be about 1 mc > equivalent per curie p, with shield of 1-inch Pb or more, < 0.2 mc per curie. Containers for such bottles should not have aluminum liners as it is uneconomical not to introduce the lead shielding at the smallest possible radius. UNIVERSITY OF FLORIDA 3 1262 08909 7348