• • I I -s Cf RELATIONS WITH JAPAN EFFECTS OF THE BOMBING OF JAPAN AND OTHER RELEVANT MATTERS 1 e Proposed LUseion to Japan Destruction of cyclotrons etc N B The c ontents of this file read forward ae in a book • •c- t M 1 u • THE BRITISH SUPPLY COUNCIL IN NORTH AMERICA TOP SEC t Oe r 1r John • ¥ fl' Box OBO B JAM i'I F IIANKI IN STATION WAIHI NGTOH 4 0 C Jul y 'l r 1945 • I You will have received a brief message to tell you that the test or tbe 49 bomb has been completely successf'ul but I think you will be interested to hear some details and impressions l enclose two accounts one by Frisch whlch I think is probably the best single descr11ition of the event and one by Tuck wllicn is accompanitd by sketc11es I also enclose a photo raph wnich was taken from the same spot as these observations I visited the ite of the test on Thursday July 1 to see the t eneral lay-out and to inspect the numerous experiments which were set ip or in process of being set up for measure -Ent and observation Unly the simplest of these were actually ready and much remained to be tlone however in spite of the difficulties of workin in the desert practically everything was got ready in time I had hoped to see the parts or the gadget but these had not yet arrived I was only able to see the prepurations for the finul tests on the 49 sphere and the initia or You already know the main details of the assembly I will only repeat that the weight of 49 was just over kg This is 97 percent of the critical mass hen the 49 is in the delta phase uensity of rather less than 16 compared with normal density of about 19 'his was just a little too mucti and conse4 uently the chance of pre-detonation was somewhat higher than we likea aoreover there i1ad been a li t tle trouble in the manufacture ot' the two h lves or the sphere which necessitated the mo it careful assembly at base These points with the many other factors on which success depended gca ve cause for a great deal or anxiety 6 The test was scneduled for 4 a m on the morning of July 16 I oecideu not to go to the 1 Sase l a np where Jroves Oppenheimer etc ere located but to go ith the Coordinating Council to a point in the hills about w miles or so from the gadget whence a better vie was possible 1 tl f TOP SECRET _ • ar John H Qderson • • 2• The weather was unfavourable and it seemed for soi i e time that the test might have to be postponed but it was finally decided to ta Ce a chance and at 5 15 we were told that the zero bour as 5 30 The first grey light of dawn was appearing as we lay or sat on the ground l xcept for the J'aint twitterings of a few eurly birds there was complete silence l'hen a great blindinj light lit up the sky and earth as 1f God himself had appeared among us After u second or so I peeped round the dark glass with which I was shielding my eyes but the light was still so intense that l was almost blinded Then l saw some of the phenomena but not all whici F-r·i ch describes the ball of flame the blue and purple lumino ilties etc fter aboat l Ju seco 1cts t 11 re came the report of the explosion sudden and sharp - s if the skies had era eked Then the hills themselves took alarm and ut tared rumblin protests for what seemed several minutes Meanwhile the column of gas had risen anJ it contln ied to ri 1e in a fairly definite cylinder with a mushroom top It rose vtith surprisine speed There was an inversion at abo 1t 17 uuo feet and here the cloud spread out to some extent but the central portion pushed througn this layer and continL1ed to rise After so ne minutes the column had reached a height Mlich I estimated by rou h measurement to be about 40 UuO f et llty anxieties over the unct rtaint1es of the test had made me somewhat nervous and the awe-inspirlnJ nature of the outcome quite overwnelmed me JJ lthoJ gh ever1tt1 i ng happened almo 1t exactly as I had imagined it the renlity was shattering LVen now a week later I am filled vith awe when l loott bacK on this moment It was a vision from the Book of kevelation ovt as to consequences and results Althollgh the explosion was observed over a very wide areu the explanation that it was due to an am lunltion magazine seems to have been accepted and there has been no undue publ1c1 ty or awkward en4 uiry ' 'his has been helpad by the fact that there were few habitations in the neighbourhood and that no evacuation was found neces ary The bulk of the fission pro iu ts and the reraaininY 4Y appeur to have ascende l into the up er tmosphere as I anticipated 'l'here ls of course strong contamination in the i·ninediate vicinity of the gadget extending to some hund rl3ds of yards The steel tower 100 f't high on which the gadget was placed has completely disappeured probably vapourised in the greut heat ' 'here 13 a crater of about 140 feet in diameter sur r ounded by a green area of some 1100 feet diameter This is probably a vitreous surface formed by the action of the heat on the sand a kind of bottle glass TOP SECRET • ·- iir John Anderson 3• Such measurements as were available when I left Y last Thursday July 19 indicated that the blast of the explosion was equivalent to the explosion of about lU to 12 0CJO tons of T N T Many of the instruments cannot yet be recovered owing to radioactive contamination and it will be several days before any accurate results are available It seems very likely that the pressure - distance curve wi 11 be rather different from that due to a r N T chc 1 rge since our explosion t akes place much more quickly 'l'he effects are likely to die off more quickly with distance Some estimates have been made of the amount of energy released but they are still very rough I think it is reasonably certain that the amount of energy is equal to that released in the explosion of at least LO OUO tons it may be considerably more Je have always expected that the shock wave would be considerably le s than would cor1·espond to tne energy release If all the 49 in the gadget had been burned the energy release would have been 100 OCJO tons of T N T The explosion has therefore been more erficient than calculation led us to expect This is partly due to the fact that the calculations were deliberately Kept on the conservative side partly because of the effect of radiation which would increase the efficiency There seems to be little doubt that the radiation from the gadget is quite strong The figures I have given you are rough but I do not think they can be far wrong I hope to get better data by the end of this week but it will be some time before the final results are available More accurate figures ca a do no more than confirm the main facts wnich are already perfectly clear The nuclear reaction worKs and as far as can be judged from the first analysis of the observations exactly as predicted The implosion method of assembly even in this first form is successful The efficiency of the nuclear reaction is high enough to give a weapon of military significance I wish to add here for I think you will be glad to know it that a substantial part of the success of t his operation has b een due to the contributions and efforts of our British group small as this group is 1 e can be quite satisfied that we have borne our share iithin the limitations imposed by present circumstances I should like now to be able to say 11 Nunc dimitt1s 11 but the end is not yet 'l'he gadget used in the test was not exactly in a form which can be used in combat ' good deal of TOP SECRET 4 ir John xnderson prepcru tion towurds combat use has been made alrt ady but much still remains to be done -Lhis is recognised by some of our U i J colleagues but not by all I think it 1s possible to get all the tests carr ied out and still to me t the expected schedule but there is no time to spare Yours si nee rely l'he hight Honourable uir John llnderson M l' rnr Cabinet uffices Great Georeg ut Lonaon 1 1 1t