l'llL -q DEP IBTL ci T U S ngineer Office Ma ha t tan Dj rict P O Box 1539 Sant a FeJ tlew hie dc o Major General T F i'nrr ell Rm 5120 Nefl War Dept lildg Wa hington D C Receipt of Mater ial Origina l to be s i gned pe rs onally by t he recip nt LJ pli c te t o be r etained by th rec ipient Triplicate ret a ined by sender f ol ' su spe n s e ile and returned to the se ndt I have per sona lly r eceived fr Capta i lL_ 0 _-nes U S Engineer Off i _c P 0 Box 1539 Santa Fe Nov r xico t h- to r ial a identi f ied ba l m1 I as sume f ull r c-spons ibili ty f or the sa f e ha ndlir ora 5e an 1 t r an smi t t a l c l se 1 1her o of his r iato ria l jn a ccor clanc•J with existir r eguL -r ions The mat eria l i nc l udi ng i nc lozur o s and attac 1 rnents is idcntifie a s oP c -· rn I n ide nt i fying ni i terial avoid a ny r ef e r ence · hich mi ght cause the r ce ip f c o' 'm t o bec ome cl a ssif i e d Dat 3 of Document or Lett e r of Tr ans mittal De scription che ck or indic ate Lott e r _ Dr a wi nL _ R por t __Ot or _ _ Indicate No a nd t atur c of cop i e s 30_ _ __cc ____ PC_ _ _TC_ _ __ Othe r Number n turo Numbe r of i nclosur e anc a t tachmonts n cfe r enco or File No From Addr e s s ed To Dat o idl ed j I lett er uith I Major Geno -------- -- SO Signed origi nal CC Ca rbon copy PC tostat i c c opy TC Typ e d c r py -------- Signature DECLASSIFIED Authority _N ND 1oos P 0 BOX 1663 SA NTA F E N EW M E X ICO November 27 1945 L fhi§ f1BBUffl8Rt eBRlifotli rJf••• r ages 1 No '- copies series i Major General T F Farrell Room 5120 New War Department Building Washington D C Dear General Farrell I am enclosing a report on flash burn in Nagasaki and Hiroshima It isn't compl ete in that I don't have the photogra pha that should go with it If C • Matthews will send me a set of the photographs we had taken revise the report to include the appropria te ones the revision can wait until I come East again H Anderson and Sugarman of the laboratory here have promised to send you their report on the radioactivity of samples taken at Hiroshima and Nagasaki as soon as the measurements on these samples are completed Similarly Weisskopf will send you hi s analysis of the Japanese radioactivity measurements I am le aving for Berkeley tomorrow You will be able to reach me at the Radiation Laboratory there With best regards Robert Berber RS jsh SECRET DECLASSIFIED Authority NND 11 ooS • • Flash Burn at Nagasaki and Hiroshima This document consists oL-- --pages• e - R Serber A characteristic feature of the atomic bomb one which is quite foreign to ordinary explosives is that a very appreciable fraction of the total energy liberated goes into radiant heat and light Indeed for sufficiently large explosi1NIS the flash burn produced by this radiated energy_ may be expected to become the dominant cause of damage since the area of burn damage will increase in proportion to the energy release whereas the area of blast damage increases only·with the two-thirds power of the energy It was not anticipated that such a reversal of the mechanism of damage would be achieved by the Hiroshima and Nagasaki bombs and in fact it was not Effect of the flash wer however very evident and many casualties resulted from flash burn discussion of such casualties will be found in the Medical Report A In the present note we shall describe the other flash effects which were observed at Hiroshima and Nagasaki The duration of the flash from the bomb is so short just a few milliseconds that there is no time for the energy falling on a s face to be dissipated by thermal diffusion the flash burn is typically a surface effect For example the thermal diffusion coefficient of wood is about D 5 x 10-4 cm2 sec d Vfit Thus in two milliseconds heat will travel only a distance of the order 10-3 cm If the energy radiated is equivalent to 1000 tons of TNT 4 x 1019 ergs the energy flux at a distance of one mile is 3 0 calories cm2 • If this is absorbed in a layer of wood half a millimeter thick it will raise its temperature 300° C We saw in Nagasaki and Hiroshima many evidences of the burning of wood by this mechanism As soon as one went beyond the area of general fire it was a striking thing to see that all telephone poles trees and wooden posts in DECLASSIFIED Authorio IV ND l '1 oos i SECR l of o -----------------_copies series---- N • • 8f6RB -2- sight of the explosion were charred and blackened on the sides facing the detonation point · In fact in Nagasaki the most precise way we found of determining accurately the position of the explosion was to sight back from a number of such poles on different sides of the center To illustrate we may describe a number of e p l es found·moving northwards from the center at Nagasaki First one comes to -a row of fence posts at the north edge of the prison hill 03 miles from the explosion tops and upper parts of these posts were heavily charred The The charring on the front of the posts was sharply limited by the shadow of a wall This wall had however been completely demolished by the blast which of course arrived some time after the flash At the north edge of the torpedo plant l 05_miles from the explosion telephone poles were charred to a depth of bout half a millimeter A light piece of wood similar to the slats in an orange crate was found leaning against one of the telephone poles Its front surface was charred in the same way as the pole but it was evident that it had actually een ignited The wood was blackened through a couple of cracks and nailholes and around the edges onto the back surf ace • It seemed likely that this piece of ··l wood _had flamed up under the flash for a few seconds before the flame was blown out by the wind of the blast Still further between 1 0 and 1 5 miles ' from the explosion along the road running north-east from the torpedo plant were many trees and poles showing blackening near the top Some of the poles had platforms The shadows cast by the platforms were clearly visible and showed that the bomb_had detonated at a considerable height somewhere in the neighborhood of the 1650' for which it was set turned north and crossed the mountain ridge all the way to the top of the ridge DECLASSIFIED Authorio v ND1'f oos 1 Finally the row of poles The flash burn was plainly visible The furthest burn observed was at 2 0 • miles from the explosion • 8ECREl -3- Because of the rough topography it was difficult to put a good upper limit on the blackening distance H wever I doubt that there would be any noticeable blackening beyond 2 25 miles v f Another striking eff act of the flash burn was the Autumnal appearance of the bowl formed by the hills on three sides of the explosion point ridges are about 1 5 miles from the center 1 The Throughout this bowl the foliage -- turned yellow although on the far sides of the ridges the countryside was quite green At Hiroshima poles were heavily blackened at a point 1 65 miles south of the explosion This is just north of the airfield poles at 1 9 miles were unmarked On the airfield To the east of the explosion it was possible to follow the blackening of telephone poles to 1 8 miles Beyond this no blackening was visible Another interesting example of flash burn was found in a room on the fourth floor of the Hiroshima post office building which was 1 02 miles from the explosion The way in which the walls were burned through the glass windows is shown in the photograph In spite of the oblique angle at which the radiation struck the surface of the fibre board which formed the upper wall was charred and the varnish on the wooden lower wall was burnt and blistered Since the direction and distance to the point of explosion we e known it was possible to determine from the angle of the shadows cast by the window frames the height of detonation of the bomb This turned out to be 1880' r501 the probable error being estimated from the agreement between measurements made on different shadows bomb was supposed to detonate at an altj_tude of 1850' The It will also be noticed in the photograph that the shadows of the window frames widen as they cross the walL By measuring the angle of this penumbra it SfCR f T was possible to estimate the DECLASSIFIED I Authority ND 1100s· • diameter ·or the ball of fire at the time the flash burn was made came out to be 300 1 • The diameter According to Hirschfelder LAMS-221 the ball of fire reaches· this radius-in 3 milliseconds It may be interesting to remark that the temperature of a black body radiator of JOO• diameter which would emit the energy equivalent of 1000 tons of TNT in this time would be 30000° C The energy flux is 5000 times that reaching us from the sun It was difficult to come to any definite conclusion on the effic y of the flash in setting fires either directly or conceivably through the spreading of burning debris by the blast The difficulty of course lies in the presence in Japanese houses of other fire making agencies such as stoves Matt'J ey and Reines LA-364 have concluded on the basis of tests made at Trinity that the flash would not be effectiye in starting fires at distances of half a mile or more Eyewitness accounts such as that of the Japanese doctor in Nagasaki who stated that wooden buildings at O fmile caught fire immediately do not seem very conclusive Perhaps the best ind tcation we have that fires were set by flash was the observation that in the park of the military reservation at Hiroshima the top of a large tree was burnt off aM- at a distance of- o '2 miles from the explosion This tree was on the far side of a moat and there was no other fire within 100 yards At both Nagasaki and Hiroshima the radius of nearly complete demolition of Japanese housing by blast was larger than the radius of complete coverage by fire The completely burnt-over area at Hiroshima reached over a wide arc on the southern windward side of_town a surprisingly well-defined radius of 1 2 miles from the explosion The radius of blast destruction was 1 6 miles _ In the band between 1 2 and 1 6 miles there were many burnt ·patches but no general fire Nagasaki the corresponding radii were 1 8 and 2 2 miles I DEcu sslF1ED Authorio I At The erlstance of such a band demol ished but onl y partial ly burnt suggests the possibility that the totally burnt region marks the l imit of f l ash fires can hardly be regarded as defini tive DECLASSIFIED - Authorit y However this argument