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MDDC  1012 UNITED STATE S ATOM ENERGY COMM MISSION RADIATION OF by H. M. Parker Clinton Laboratories This document consists of 8 pages. HAZARDS Date of Manuscript: Date Declassified: April 1944 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 United States of America AEC, Oak Ridge, Tenn.825481,500 Price 10 cents BREMSSTRAHLUNG C;; ?Ik/ll II WI RADIATION HAZARDS OF BREMSSTRAHLUNG By H. M. Parker INTRODUCTION ":E been caused to radiation workers by bremsstrahlung (continuous Xray 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 bremsstrahlung arising from the beta radiation of pile produced activities (either fission products or induced radioactivites). For convenience, bremsstrahlung will henceforth be anglicized 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 secUtion 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 solution in the new separations process show a re markably high ratio of 3 to , activity. In one case. about 100 curies of 3 activity were associated with I curie of > activity. Special attention to the BS from a large mass of material may be required. 2) 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. = electron energy, units of mec' = electron mass c = velocity of light ~ rad = cross section for radiative energy loss, in units 137 m"c4 = number of atoms (molecules) per cmn, Z nuclear charge = total number of electrons per cm'  sum of Z2 of all nuclei per cm3 1 Mev will be considered equal to 2 mce By and large, more damage has MDDC BS IN LEAD: TOTAL ENERGY de Total energy loss per cm ( tot dea The radiation loss drd = NE vdx rad I  1012 is given by Heitler on pages 221 and 222. rad .. (equation 220) can be computed from page 143. * .u 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 mc2 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 me2 . Accuracy cannot be claimed for these spectra. Table 3 gives the spectral energy for successive energy losses of 1 mc2. 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 IN OTHER MEDIA The energy loss due to collisions is very nearly proportional to (NZ is proportional to (NZ2). 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. The radiative energy loss Table 5 gives a few values of energy loss in convenient media for 2 Mev particles. The ratio of values for any two substances will be the same at other energies. Table 1. Energy loss of an electron in lead.  me Mev d QZrad (de\ dr~/tot Fractional loss by BS mc2 .704 1.66 2.87 4.20 5.61 Table 2. Total BS by an electron in lead. Mev .25 .5 1 1.5 2 2.5 :i ~ rad~ MDDC  1012 Table 3. Spectral distribution of BS (in Pb) over different energy intervals. Electron energy mc2 BS energy me2 BS energy interval 10 .02 BS spectral energy mc2 x 10O .4 mc2 .8 1.2 1.21.6 1.62.0 2.4 Table 4. Total spectral distribution of BS (in Pb) for given primary electron energy. Primary Mev Mev energy  BS energy Mev Interval Mev interval BS energy in Key Total 43.6 97.4 172.0 262.7 Table 5. Fractional energy loss of 2 Mev particle as BS. Material Fractional Material Fractional energy loss energy loss HO .0136 20% UNH .0291 Pyrex glass .0214 Fe .0512 Common glass .0232 Pb .147 IEEE: 2.42.8 2.83.2 3.23.6 3.64.0 4.04.4 4.44.8 4.85.0 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 gm = linear absorption coefficient of liquid for m Mev radiation (aa)m = scattering absorption coefficient of m Mev radiation in air.* The ionizatiun outside a large tank of liquid containing a P emitter only I= const: The magnitude of the BS per cc with 1 curie of 2 y,("as)m Em .or  x S (aa) Em (equation 310) taken over all values of m. cm conveniently shown by comparing the effect of 1 curie of 2 Mev 3 activity Mev v activity. Some typical cases are given in Table 6. The BS considered here is from the liquid only. The fil tration of the postulated tank walls is computed by applying factors F,(gm 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, n150 curies of Mev p will give total gamma activity equivalent to 1 curie of 2 Mev y. This answers I(1) and indicates that 1(2) may just become of some significance. Table 6. BS from large tanks. Liquid HzO equiv. 20% UNH Pb Tank wall 0 5 mm Pb '0 3 mm Fe 5 mm Pb 10 mm Fe 0 5 mm Pb Energy M I x 107 I x 107 I I I I I x 107 I x 107 .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 14.3 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 + 7) is used for low energy where the photoelectric absorption is significant. MDDC BS FROM A TANK WALL  1012 Consider the number of particles incident on a plane wall from a liquid emitting n particles/cmr/sec. Number Incident on unit area from volume element dV is dN= nd cos 4 (410) 4xr2 where t = angle between normal and direction of element dV. = distance to element dV = azimuthal angle Figure 1. n r2 dr ded where R is the range. Equation 410 agrees with the convenient approximation that of all electrons emitted within the range thickness R, onehalf approach the wall, and of them onehalf again will fail to reach the wall because of obliquity. It has been shown previously (CH930) that the average energy of p particles reaching a surface Hinder 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 impinging 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 = Const x Z (oa) m Em Fo (pm t) It is assumed provisionally that the energy Em is radiated uniformly in all 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 than 1.5 Mev has 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 10 mm Fe wall Energy m aoE x 18 Ix 108 Ix 10 Total 25.09 2 Mev y in liquid Curies of 8 equivalent to 1 curie y in liquid 67.3 x 10' 2880 4.4 x 104 6200 i ::" 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 wall, 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 liquid I wall where g and g liquid e d UaE wall eL.L'd are the absorption coefficients in liquid and wall. For 3 mm Fe I wall/I liquid 10 mm Fe I wall/I liquid = 0.3 and 139 curies 3: = .024 and 178 curies p 20% UNH and a steel wall = 1 curie = 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 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 a "deep tank" and the shallow tank case need not be considered. DIRECTIONAL FEATURE OF WALL RADIATION High energy BS is confined to the forward direction within a solid angle of 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 resultant 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 "y Uniform E for t = 0w I = KE Fo( t) Cos 4E cos, <, = 0 */2 It = 4KE F (p t) 0, < = 7/2 w Cos2 6E cos <, z= 0w/2 I1= 6KE F (g t) 0, > /2 Fermi distribution 1.857 E(cos 4 + V3 cos2P) IF = 1.857 Fi 3.217 F2 0, >4/2 Cos' 2(n + 1)E cos" n1 = 2(n1)KE Fn n t) Lm~/E~. MDDC  1012 Some values of these functions are given in Table 9. For a steel tank 3 mm thick (p t) ranges from 0.1 to 1.0. If one considers the combination of varying 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, 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 Th Kg it> ~ >0 ~L <4 p Table 9. jt I/I i/ IF/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 of curries of 2 Mev p activity which gives rise to BS equal in ionizing power to 1 curie of 2 Mev v ac tivity under the same conditions. , To convert BS values to roentgens a series of alleged values for 2 Mev v cases is given in Table 10. The tanks are supposed infinitely deep. FROM SHIPPING CONTAINERS The problem here is to decide whether 1) it is necessary to line a container with material of Jo1 atomic number or (2) whether additional lead shielding should be allowed in either case to cut mown the BS component in a mixed 3 and y source. It will be supposed that all particles of primary energy 2 Mev strike the wall with the full energy. hn some cases this will mean by the argument given in the section, BS from a Tank Wall, that the ef feet quoted will be that due to3 Mev particles. COMPACT SOURCE IN LEAD OR ALUMINUM Co insider 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 me of y activity (2 Mev) per curie p activity. For lead containers there is a pos sible radiation hazard. Since a container of less than 1inch wall is hardly worth consideration, a Table 10. Gamma radiation from sundry tanks containing 1 curie of 2 Mev v activity per Ce. Tank wall 0 3mm Fe 10 mm Fe 5 mm Pb bist. from Tank ^ tank diameter Dosage rate in kr/hr 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 S2326 1731 1062 1275 MDDC 0.3 0.4 0.5  1012 \4 / 1oo \ ~/ II 14 _61 0 0 0 500    4 100 1.0 a 3 4 5 67 10 CM Pb SHIELD Figure convenient rule is that the BS hazard is about 1 me y equivalent per curie 3. If an aluminum liner is used, the radiation is reduced by a factor of 5 or 6. EXTENDED SOURCE IN LEAD, ALUMINUM, OR GLASS In general, a strong source will consist either of a pellet of perhaps 1inch diameter or of a solu tion. In the first case, about onefifth or less of the 8 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 BS is then independent of the outer container uid or glass. as the B radiation is essentially all absorbed in the liq With no outer shield there will be about 1 me y equivalent per curie B, with shield of 1inch Pb or more, 0.2 me 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. """" :Pi UNIVERSITY OF FLORIDA IllllUlU 1111111111II111111111111I11Ulll 3 1262089097348 * 4^ I S i 11 I I i ".?. *:. 