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Ra da r a n d th e Ma n h a tta n P ro je c t
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8. 225 / ST S . 042, P h y s i c s i n t h e 20t h C e n t u r y Pr of e s s or Da v id K a is e r , 2 1 O c tobe r 2 0 2 0
1. “ W i n n i n g t h e W a r” : Ra da r
2. S t a r t i n g t h e M a n h a t t a n Pro j e c t
3. M a ki n g B o m b s
No t e : Th i s c l a s s s e s s i o n w i l l f o c u s o n s o m e c o n c e p t u a l c h a l l e n g e s a n d n e w i n s t i t u t i o n a l a r r a n g e m e n t s f o r p h y s i c i s t s — mo s t l y wi t h i n t h e U n i t e d S t a t e s — du r i n g t h e Se c o n d W o r l d W a r . T h e r e a r e ma n y mo r e a s p e c t s o f t h e w a r t i me p r o j e c t s t o c o n s i d e r , so m e o f w h i c h a r e i n t r o d u c e d i n t h e d o c u m e n t a r y f i l m Th e D a y A f t e r T r i n i t y . S t u d e n t s s h o u l d w a t c h t h a t f i l m o n t h e i r o w n . W e wi l l h a v e a n op t i on a l , i n f o r ma l d i s c u s s i o n o f t h e f i l m v i a Z o o m d u r i n g o u r o r d i n a r y c l a s s t i me o n M o n d a y , O c t o b e r 2 6 , 2 0 2 0 .
Ra da r
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R a d a r s y s t e ms w e r e fi r s t d e v e l ope d i n t he mi d - 1 9 3 0 s . T he i d e a w a s t o e mi t e l e c t r oma gne t i c w a v e s , l e t t he m r e f l e c t off of a n obj e c t , a nd d e t e c t t he “e c ho . ” B y u s i ng t he ( c ons t a nt ) s pe e d of l i ght c a nd me a s u r i ng t he t i me u nt i l a r r i v a l of t he r e t u r n s i gna l , t he d e v i c e s c ou l d d e t e r mi ne t he di s t an c e t o t he obj e c t . Mor e s ophi s t i c a t e d u ni t s me a s u r e d Do p p l e r s hi f t s t o a l s o d i s c e r n t he sp eed of t he t a r g e t obj e c t .
Both British and American military researchers had developed long-wavelength ( ~ 1 -10 m) systems before the start of the Second W orld W ar. Once the war began, intense efforts to improve the technology led to many physicists’ first direct involvement with military matters .
O ne of t he e a r l i e s t c ha l l e ng e s : d e s i gn r a d a r s y s t e ms t ha t u s e d s h o r t e r w a v e l e n gt h s i gna l s ( ~ 1 - 10 c m ) . R e me mbe r t ha t t he r e s o l v i n g p o w e r i s pr opor t i ona l t o w a v e l e n gt h ( x ~ ) . B e gi nni ng i n S e pt e mbe r 1 9 3 9 , Ge r ma n s u bma r i ne s ( “U- boa t s ”) be c a me a hu g e t hr e a t t o US a nd UK s hi ppi ng a nd na v a l ma ne u v e r s i n t he A t l a nt i c O c e a n. T he s u bma r i ne s c ou l d onl y be s pot t e d w he n t he i r t i ny pe r i s c o pe s s u r fa c e d a bo v e t he w a t e r .
Ra da r
Br i t i s h “ c a v i t y m a g n e t r o n , ” c a . 1 9 4 0
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B y 1 9 4 0 , B r i t i s h ph y s i c i s t s a nd e ngi ne e r s ha d d e v e l ope d a ne w “c a v i t y ma gne t r on”: a s ma l l d e v i c e t ha t c ou l d e mi t hi gh- po w e r e l e c t r oma gne t i c w a v e s w i t h ~ 1 c m. B y t he n, t he UK w a s u nd e r he a v y bomba r d me nt fr om Ge r ma n a i r c r a ft ; i t s e e me d i mpos s i bl e t o s c a l e u p t he r e s e a r c h a nd d e v e l opme nt of t he ne w s hor t - w a v e l e ngt h r a d a r s y s t e ms i n t he UK .
I n S e pt e mbe r 1 9 4 0 — mor e t ha n a y e a r be for e t he US e nt e r e d t he w a r !
— a B r i t i s h d e l e g a t i on l e d b y Si r H e n r y T i z a rd c a me t o W a s hi ngt on, D . C . , t o a s k US c ol l e a gu e s t o t a k e o v e r r a d a r d e v e l opme nt . T he y ha nd e d o v e r a s i n g l e c a v i t y m a gn e t r o n , s o t ha t t he US g r ou ps c ou l d t r y t o r e v e r s e
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MI T ’ s Va n n e v a r B u s h , K a r l C o m p t o n a nd a fe w ot he r US c ol l e a gu e s me t w i t h t he s ma l l B r i t i s h g r ou p t o d i s c u s s pos s i bl e c ol l a bor a t i ons . A t one poi nt Compt on s t e ppe d ou t t o c a l l a n a s s i s t a nt a t MI T , t o s e e i f t he y c ou l d r e q u i s i t i on a c a mpu s pa r k i ng l ot t o c on v e r t i nt o t e mpor a r y r e s e a r c h s pa c e for t he ne w A l l i e d r a d a r e ffor t . T hi s be c a me t he “ Ra d i a t i o n L a b o r a t o r y , ” or “ Ra d L a b .”
MI T ’ s B u i l d i n g 2 0 : “ T h e P l y w o o d P a l a c e ”
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N e w In s t it u t io n s
T he r a d a r pr oj e c t be c a me one of t he fi r s t s pons or e d b y t he ne w Na t i o n a l D e f e n s e Re s e a r c h Co mmi t t e e ( NDR C) , or g a ni z e d a nd c ha i r e d b y V a nne v a r B u s h. T he NDR C w ou l d ope r a t e a s t he US g o v e r nme nt ’ s offi c i a l c ond u i t t o r e s e a r c h s c i e nt i s t s a nd e ngi ne e r s for t he w a r e ffor t . Mi l i t a r y offi c i a l s w ou l d d e s c r i be w ha t t e c hnol ogi e s t he y s ou ght , a nd t he NDR C w ou l d a r r a ng e c ont r a c t s w i t h l a bor a t or i e s ( bot h i nd u s t r i a l a nd a c a d e mi c ) t o c ond u c t t he ne c e s s a r y r e s e a r c h.
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B y Ma y 1 9 4 1 ( s t i l l be for e t he US ha d e nt e r e d t he w a r ! ) , B u s h c on v i nc e d US P r e s i d e nt F r a nk l i n R oos e v e l t t o ex p a n d t he s e e ffor t s i nt o a n e v e n mor e po w e r fu l s t r u c t u r e : t he Of f ic e of S c ien t if ic R esea r c h a n d D e v el op m en t ( O S R D) , w hi c h c ou l d a c t u a l l y o v e r s e e pr od u c t i on.
P a r t of B u s h ’ s s t r a t e g y : u s e co n t r a ct s ( r a t he r t ha n gi ft s or g r a nt s ) be t w e e n u ni v e r s i t i e s a nd t he fe d e r a l g o v e r nme nt , t o ma i nt a i n t he appe ar an c e of t w o e q u a l pa r t ne r s e nt e r i ng i nt o a bu s i ne s s a r r a ng e me nt . I n fa c t , mos t US u ni v e r s i t i e s w e r e st a r v ed for c a s h a ft e r ne a r l y a d e c a d e of t he Gr e a t De pr e s s i on. B u t a “c ont r a c t ” s y s t e m a ppe a r e d d i ffe r e nt t ha n a c e nt r a l i z e d “ba i l - ou t ” of hi ghe r e d u c a t i on b y t he fe d e r a l g o v e r nme nt .
Th e R a d L a b
MI T R a d L a b p e r s o n n e l i n t h e r o o f t o p fa c i l i t y o f B u i l d i n g 4 , 1 9 4 1 .
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T he MI T R a d L a b q u i c k l y be g a n r e c r u i t i ng pe r s onne l . I t be g a n w i t h 3 0 ph y s i c i s t s ( ma ny fr om ot he r u ni v e r s i t i e s ) , t hr e e s e c u r i t y gu a r d s , t w o s t oc k r oom c l e r k s , a nd a s e c r e t a r y . I t w a s l e d b y n u c l e a r ph y s i c i s t Le e D u B r i d ge , t hou gh i t q u i c k l y br ou ght i n e x pe r t s i n ma ny ot he r fi e l d s : e l e c t r i c a l e ngi ne e r i ng, me t e or ol og y , g e ol og y , ma t e r i a l s s c i e nc e , e v e n l i ngu i s t s .
B y 1 9 4 2 , t he R a d L a b s t a ff n u mbe r e d 2 0 0 0 ; b y 1 9 4 5 , i t ha d g r o w n t o 4 0 0 0 , i nc l u d i ng 5 0 0 a c a d e mi c ph y s i c i s t s . T he L a b s pe nt $1 m pe r m o n t h ( ~ $1 5 m i n 2 0 2 0 $) . T hr ou ghou t t he w a r , MI T s e c u r e d O S R D d e fe ns e c ont r a c t s w or t h $1 0 0 m ( ~ $1 . 5 b i n 2 0 2 0 $) , s u bs t a nt i a l l y mor e t ha n a ny
ot he r u ni v e r s i t y a nd t hr e e t i me s mor e t ha n A T & T , Ge ne r a l E l e c t r i c , R CA ,
Du P ont , a nd W e s t i nghou s e co m b i n e d !
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R a d L a b s t a ff d e s i gne d a nd pr od u c e d do z e n s of d i s t i nc t r a d a r s y s t e ms ( g r ou nd - to - ai r , ai r - to - se a , e t c . ) i n t he ~ 1 c m r a ng e , a nd c ond u c t e d t e s t s fr om MI T r ooft ops t o d e t e c t a i r c r a ft fr om ne a r b y a i r por t s ( no w H a ns om A i r F or c e B a s e a nd L og a n A i r por t ) . T he y a l s o t r a i ne d ne a r l y 10, 000 ac t i v e - du t y s e r v i c e me mbe r s on c a mpu s ho w t o u s e t he ne w r a d a r s y s t e ms .
Th e R a d L a b
Ju l i a n S c h w i n g e r a t t h e M I T R a d L a b , 1 9 4 0 s
A t fi r s t , ma ny of t he t he or e t i c a l ph y s i c i s t s a t t he R a d L a b w e r e pr e t t y a r r og a nt ; a ft e r a l l , r a d a r w a s “me r e l y ” a pr obl e m i n c l a s s i c a l e l e c t r oma gne t i s m, not fa nc y q u a nt u m t he or y or n u c l e a r ph y s i c s . B u t a s t he y q u i c k l y l e a r ne d , c a l c u l a t i ng E a nd B fi e l d c onfi gu r a t i ons for re a l i s t i c d e v i c e s , r a t he r t ha n s i mpl e t o y pr obl e ms , w a s “nont r i v i a l . ”
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T he ph y s i c i s t s w e r e v e r y s k i l l e d a t e x pl oi t i ng
sy m m et r i es t o s i mpl i fy c a l c u l a t i ons . . .
Th e R a d L a b
A t fi r s t , ma ny of t he t he or e t i c a l ph y s i c i s t s a t t he R a d L a b w e r e pr e t t y a r r og a nt ; a ft e r a l l , r a d a r w a s “me r e l y ” a pr obl e m i n c l a s s i c a l e l e c t r oma gne t i s m, not fa nc y q u a nt u m t he or y or n u c l e a r ph y s i c s . B u t a s t he y q u i c k l y l e a r ne d , c a l c u l a t i ng E a nd B fi e l d c onfi gu r a t i ons for re a l i s t i c d e v i c e s , r a t he r t ha n s i mpl e t o y pr obl e ms , w a s “nont r i v i a l . ”
A s c h e m a t i c o f j u s t o n e por t i on of a w a v e g u i d e an t e n n a f e e d
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So m e o f t h e w a v e g u i d e c o m p o n e n t s i n r e g u l a r us e b y t h e m i d - 1940s
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Ju l i a n S c h w i n g e r a t t h e M I T R a d L a b , 1 9 4 0 s
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Th e R a d L a b
Ju l i a n S c h w i n g e r a t t h e M I T R a d L a b , 1 9 4 0 s
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Com pl i c a t e d c i r c u i t s c ou l d be si m p l i f i e d b y c o n st r uc t i n g ef f ec t i v e ci r cui t s : m e a s u r e t h e c u r r e n t f l o w i n g i n on one s i d e a nd f l o w i ng ou t of t he o t h e r , a n d i n f e r a n o ve r a ll re s i s t a n c e .
A t fi r s t , ma ny of t he t he or e t i c a l ph y s i c i s t s a t t he R a d L a b w e r e pr e t t y a r r og a nt ; a ft e r a l l , r a d a r w a s “me r e l y ” a pr obl e m i n c l a s s i c a l e l e c t r oma gne t i s m, not fa nc y q u a nt u m t he or y or n u c l e a r ph y s i c s ! B u t a s t he y q u i c k l y l e a r ne d , c a l c u l a t i ng E a nd B fi e l d c onfi gu r a t i ons for re a l i s t i c d e v i c e s , r a t he r t ha n s i mpl e t o y pr obl e ms , w a s “nont r i v i a l . ”
T he ph y s i c i s t s q u i c k l y l e a r ne d fr om e ngi ne e r s a “bl a c k- bo x ” a ppr oa c h t o c a l c u l a t i ng: w or k i n t e r ms of ef f ec t i v e c i r c u i t s r a t he r t ha n i nd i v i d u a l , c ons t i t u e nt pa r t s ; foc u s on i npu t - ou t pu t r e l a t i ons hi ps .
Th e R a d L a b
A t fi r s t , ma ny of t he t he or e t i c a l ph y s i c i s t s a t t he pr e t t y a r r og a nt ; a ft e r l l d
e l
w e r e s s i c a l i c s ! B u t
gu ons for
mpl e t o y pr obl e ms , w a s “nont r i v i a l . ”
T he ph y s i c i s t s q u i c k l y l e a r ne d fr om e ngi ne e r s a “bl a c k- bo x ” a ppr oa c h t o c a l c u l a t i ng: w or k i n t e r ms of ef f ec t i v e c i r c u i t s r a t he r t ha n i nd i v i d u a l , c ons t i t u e nt pa r t s ; foc u s on i npu t - ou t pu t r e l a t i ons hi ps .
Ju l i a n S c h w i n g e r a t t h e M I T R a d L a b , 1 9 4 0 s
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Com pl i c a t e d c i r c u i t s c ou l d be si m p l i f i e d b y c o n st r uc t i n g ef f ec t i v e ci r cui t s : m e a s u r e t h e c u r r e n t f l o w i n g i n on one s i d e a nd f l o w i ng ou t of t he o t h e r , a n d i n f e r a n o ve r a ll re s i s t a n c e .
Ra da r
B y 1 9 4 3 , i mpr o v e d mi c r o w a v e - r a d a r u ni t s ha d be e n d e v e l ope d a nd i ns t a l l e d not j u s t for g r ou nd - ba s e d s c a nni ng s t a t i ons , bu t a l s o on- boa r d a i r c r a ft a nd na v a l v e s s e l s . T he y he l pe d r e i n i n t he ( pr e v i ou s l y u nc he c k e d ) Ge r ma n U- boa t c a mpa i gns , a s w e l l a s Ge r ma n a i r - r a i d s o v e r t he UK .
R a d a r s y s t e ms w e r e not onl y d e pl o y e d d e fe ns i v e l y . A t t he O S R D- s pons or e d Ap p l i e d Ph ys i c s L ab a t J ohns H opk i ns Uni v e r s i t y , r e s e a r c he r s d e v e l ope d t he pr o x i m i t y fu z e , w hi c h u s e d t i ny r a d a r s y s t e ms e mbe d d e d w i t hi n a r t i l l e r y s he l l s t o t r i g g e r t he e x pl os i v e onc e
t he s he l l r e a c he d a pr e - s e t d i s t a nc e fr om i t s t a r g e t .
W he r e a s pr e v i ou s a nt i - a i r c r a ft e ffor t s ha d r e q u i r e d hu n d r ed s of r ou nd s ( on a v e r a g e ) t o hi t a fa s t - mo v i ng a i r c r a ft , w i t h pr o x i mi t y fu z e s t he y onl y ne e d e d ( on av e r a g e ) tw o .
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A ft e r t he w a r i t be c a me c ommon for v e t e r a ns of t he R a d L a b t o s a y t ha t n u c l e a r w e a pons mi ght ha v e en d ed t he w a r , bu t r a d a r wo n t he w a r .
An t i - ai r c r af t s h e l l s wi t h p r o x i m i t y f u z e s du r i n g t h e B a t t l e o f G u a da l c a n a l , 1 9 4 3
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Q u e s tion s ?
St a r t in g t h e Ma n h a t t a n P ro je c t
T he ot he r ma j or O S R D pr oj e c t d u r i ng t he w a r w a s t he M a n h a t t a n E n gi n e e r Di s t r i c t ( ME D) , or Ma n h a t t a n P r o j e c t , pu r s u e d j oi nt l y w i t h t he US A r m y Cor ps of E ngi ne e r s . P l a nni ng be g a n i n J u ne 1 9 4 2 i n a n NY C offi c e of t he Cor ps , a nd ( a t t he t i me ) s e v e r a l ph y s i c i s t - me mbe r s of t he pr e l i mi na r y “ A d v i s or y Commi t t e e on Ur a ni u m” ( i nc l u d i ng Enr ic o F e r m i ) w e r e a t ne a r b y Col u mbi a Uni v e r s i t y .
So m e o f t h e e a r l i e s t M E D o f f i c e s we r e i n t h e W o o l w o r t h B u i l d i n g o n Br o a d w a y i n N Y C
T he pr oj e c t e x pa nd e d t o mor e t ha n 30 s i t e s a c r os s t he US a nd Ca na d a , e mpl o y i ng mor e t ha n 1 2 5 , 0 0 0 pe opl e — mos t of w hom k ne w v e r y l i t t l e a bou t t he na t u r e of t he t op- s e c r e t pr oj e c t .
Ma j or s i t e s i nc l u d e d Chi c a g o , I L ; Oa k R i d g e , T N ; Ha n f o r d , W A ; a nd Lo s A l a m o s , N M .
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C h a in R e a c t io n s
© University of Chicago.
Ch i c ag o P i l e - 1 u n d e r c o n s t r u c t i o n , 1942
Th e f i r s t sel f - sust ai ni ng ch ai n r eact i on we n t “ c r i t i c a l ” o n D e c e m b e r 2 , 1 9 4 2 .
© Argonne National Laboratory.
A t e a m a t t he Chi c a g o “Me t a l l u r gi c a l L a bor a t or y , ” l e d b y Enr ic o F e r m i , s t u d i e d fi s s i on r e a c t i ons . T he y bu i l t Chi c a g o P i l e - 1 , t he fi r s t w or k i ng n u c l e a r r e a c t or , i n a s q u a s h c ou r t u nd e r t he bl e a c he r s e a t s of t he S t a g g F i e l d s t a d i u m on c a mpu s .
E a c h t i me a u r a ni u m n u c l e u s fi s s i one d , 2 - 3 a d d i t i ona l ne u t r ons w e r e r e l e a s e d . T hos e ne w ne u t r ons c ou l d fi s s i on ne i ghbor i ng n u c l e i , a nd s o on: a ch a i n r e a ct i o n .
T he “pi l e ” c ons i s t e d of 5 7 l a y e r s of
c l os e l y pa c k e d g r a phi t e br i c k s ( mo d e ra t o rs t o s l o w d o w n ne u t r ons ) , i nt e r l e a v e d w i t h
s ma l l c hu nk s of u r a ni u m me t a l . T he pi l e a l s o i nc l u d e d s e v e r a l 1 4 - foot c a d mi u m “c ont r ol r od s ” t ha t c ou l d be pu s he d i n
or ou t t o abs o r b ne u t r ons a nd pr e v e nt a
ru n - aw ay c ha i n r e a c t i on.
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faq-fair- use/. 14
N e w F is s ile Ma t e ria ls
During 1940-41, Berkeley nuclear chemist Glenn Seaborg led a team that successfully produced genuine transuranic nuclei. They bombarded uranium with neutrons; sometimes the neutrons were captured and then underwent beta deca y . In F ebruary 1941, the team first produced plutonium via double beta- de c ay:
ne u t r on c a pt u r e do u b l e b e t a - de c a y
T he ne x t mont h, S e a bor g a nd F e r mi ’ s for me r c ol l e a gu e , E m i l i o S e gr è , d e mons t r a t e d t ha t pl u t oni u m c ou l d u nd e r g o fi s s i o n . W i t hi n a fe w mont hs , S e a bor g mo v e d t o t he Chi c a g o Me t L a b t o j oi n F e r mi a nd c ont i n u e hi s e x pe r i me nt s on pl u t oni u m, i nc l u d i ng me a s u r e me nt s of i t s fi s s i on r a t e u nd e r v a r i ou s c ond i t i ons .
L o s A la m o s
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U . S . B r i g a d i e r Ge ne r a l Le s l i e G r o v e s w a s pu t i n c ha r g e of t he ME D , o v e r hi s i ni t i a l obj e c t i ons . ( H e w a s e a g e r t o ha v e a n a c t i v e c omba t r ol e i n t he w a r , a nd d i d not t hi nk t ha t t he a bs t r a c t - s ou nd i ng w e a pons pr oj e c t w ou l d pl a y m u c h r ol e i n t he pr e s e nt c onf l i c t . ) H e t a ppe d B e r k e l e y t he or e t i c a l ph y s i c i s t J. R o b e r t Op p en h eim er t o s e r v e a s s c i e nt i fi c d i r e c t or of t he L os A l a mos L a bor a t or y .
L os A l a mos be g a n ope r a t i ons i n A pr i l 1 9 4 3 , t a k i ng o v e r a bo y s ’ s c hool i n r u r a l Ne w Me x i c o . O ppe nhe i me r r e c omme nd e d t he l oc a t i on, ba s e d on pr e v i ou s t r i ps t o t he r e gi on.
O ppe nhe i me r ( bor n i n 1 9 0 4 ) ha d s t u d i e d i n Ca mbr i d g e a nd Göt t i ng e n i n t he mi d - 1 9 2 0 s , j u s t a s t he ne w w or k on q u a nt u m me c ha ni c s w a s e me r gi ng . H e w a s hi r e d a t B e r k e l e y an d Ca l t e c h i n 1 9 2 9 t o t e a c h t he or e t i c a l ph y s i c s a nd t o he l p bu i l d u p US s t r e ngt h i n t he fi e l d . B e for e t he w a r , he ha d v i r t u a l l y no e x pe r i e nc e w i t h e i t he r e x pe r i me nt s or w i t h l a r g e - s c a l e or g a ni z a t i on.
L o s A la m o s
A s s oon a s ne w r e c r u i t s be g a n t o a r r i v e a t L os A l a mos , O ppe nhe i me r ’ s for me r pos t d oc , Ro b e r t Se rb e r , g a v e a s e r i e s of 5 l e c t u r e s a s a n “i nd oc t r i na t i on c ou r s e . ” A not he r ph y s i c i s t , Ed w a r d C ond on , t ook not e s . T he 2 4 - pa g e d oc u me nt be c a me LA - 1 , t he fi r s t ( t op- s e c r e t ) t e c hni c a l r e por t of t he l a b . I nfor ma l l y , i t be c a me k no w n a s t he “L os A l a mos P r i me r . ”
z
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en erg y rel ea s e f ro m co n v en t i o n a l ex p l o s i v es l i k e T N T :
co d e :
Ge t t i ng j u s t on e k g of U 235 t o u nd e r g o fi s s i on w ou l d ha v e t he e x pl os i v e i mpa c t of 20, 000 t o n s of T N T .
W h ic h F u e l t o Us e ?
F i s s i ona bl e ma t e r i a l s :
T he l a r ge s t r e a c t i on r a t e s w e r e for U 235 w i t h sl ow ne u t r ons . B u t t he ne u t r ons
re l e a s e d fr om fi s s i on w e r e fas t ( E ~ 1 Me V ) . F or ne u t r ons a t t hos e e ne r gi e s , t he l a r g e s t r e a c t i on r a t e w a s for plu t o n i u m .
Cha l l e ng e s : i s ol a t e U 235 fr om U 238 , a nd / or s c a l e u p pr od u c t i on of P u 239 a bi lli o n fo ld , fr om µ g t o k g!
mos t l y s t a bl e
hi ghl y fi s s i ona bl e , bu t onl y 0 . 7 % of na t u r a l l y oc c u r r i ng or e
hi ghl y fi s s i ona bl e , bu t a t t he t i me i t onl y e x i s t e d i n m i c r o gr a m s
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O a k R idg e : Ura n iu m Is o t o p e Se p a ra t io n
T he O a k R i d g e ME D fa c i l i t y foc u s e d on s e pa r a t i ng U 235 fr om U 238 . B ot h i s ot ope s be ha v e d i d e nt i c a l l y ch e m i ca l l y , s o t he y ne e d e d t o e x pl oi t t he ti n y ma s s d i ffe r e nc e ( ~ 1 . 3 % ) .
Th e m i l e - lo n g U - sh a p e d K - 25 g as e o u s d i f f u s i o n pl a nt a t O a k R i d g e : a t t he t i m e , t he la r g e s t fa c t o r y
ev e r b u i l t u n d e r o n e r o o f
Se e Al e x W e l l e rs t e i n , “I n s i d e K - 25, ” ht t p: / / bl og . n u c l e a r s e c r e c y . c om /2 0 1 3 /
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Ga s e o u s D i f f u s i o n : he a t u r a ni u m he x a f l ou r i d e g a s ( UF 6 ) . I n e q u i l i br i u m,
s o v 235 > v 238 . F or c e t he g a s i nt o a c ha mbe r w i t h pe r me a bl e me mbr a ne s . T he fa s t e r - mo v i ng mol e c u l e s w i t h U 235 w i l l d i ffu s e mor e q u i c k l y . E nr i c hme nt fr om one c y c l e : 1.0 0 4 3 ! S o r e pe a t . . . a t h o u s an d t i m e s .
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H a n f o rd: P lu t o n iu m F ro m R e a c t o rs
A t t he H a nfor d ME D s i t e , s t a ff foc u s e d on s c a l i ng u p F e r mi ’ s Chi c a g o P i l e - 1 r e a c t or t o i nd u s t r i a l s c a l e . T he i r j ob w a s t o pr od u c e k i l og r a ms of plu t o n i u m b y ne u t r on bomba r d me nt of U 238 .
F R e a c t o r c o m p l e x a t H a n f o r d u n de r c o n st r u c t i o n , c a . 1 9 4 4 .
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T he r e a c t or c ompl e x e s a t H a nfor d r e q u i r e d mor e t ha n on e b il l ion c u b ic m et er s of c on c r et e .
In s i d e H a n f o r d ’ s B r ea ct o r
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Q u e s tion s ?
C rit ic a l Ma s s
E a c h t i me a n u c l e u s fi s s i one d , 2 - 3 a d d i t i ona l ne u t r ons w e r e r e l e a s e d . I f t he v ol u me of fi s s i ona bl e ma t e r i a l w e r e to o s m a l l , t oo ma ny ne u t r ons ( on a v e r a g e ) w ou l d esc a p e t he ma t e r i a l be for e c a u s i ng a d d i t i ona l fi s s i ons : fi z z le .
W i t h t e a ms of ( mos t l y ) w ome n “c ompu t e r s ” u s i ng me c ha ni c a l Ma r c ha nt c a l c u l a t or s , r e s e a r c he r s a t L os A l a mos c a l c u l a t e d t he cr i t i ca l si z e of U 235 : R c ~ 9 c m , s o
Sa m u e l G l a s s t o n e , Ef f e c t s o f N uc l e a r W e a p o ns (1 9 5 7 )
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t a mpe r , c ou l d e x pe c t R c ~ 3 c m , s o M c ~ 2 k g, not 50 kg .
G u n A s s e m b ly
H o w t o i ni t i a t e a c ha i n r e a c t i on? S e r be r e x pl a i ne d one me t hod : pr e pa r e t w o su b c r i t i c a l pi e c e s of U 235 a nd s hoot t he m t og e t he r w i t hi n a bou t 1 0 - 5 s e c ond s ; s u c h s pe e d s w e r e c ons i s t e nt w i t h m u z z l e s pe e d s fr om US A r m y gu ns .
T he r e w ou l d a l s o ne e d t o be a ne ut r o n s o ur c e t o i ni t i a t e t he c ha i n r e a c t i on. S e r be r s u g g e s t e d a t t a c hi ng a n - e mi t t e r ( l i k e r a d i u m or pol oni u m) t o one s u bc r i t i c a l pi e c e a nd be r y l l i u m t o t he ot he r , s o t ha t w he n t he t w o pi e c e s c a me t og e t he r t he y w ou l d ( i n e ffe c t ) r e pr od u c e Cha d w i c k ’ s 1 9 3 2 e x pe r i me nt a nd pr od u c e ne u t r ons .
T he g r ou p w a s s o c onfi d e nt of t hi s me t hod t ha t t he ve r y fi r s t de t o n at i o n of t hi s t y pe of bomb w a s o v e r t he J a pa ne s e c i t y of H i r os hi ma on Au g u s t 6 , 1 9 4 5 .
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Ru i n s n e a r t h e H i r o s h i m a P r e f e c t u r a l In d u s t r i a l P r o m o t i o n H a l l .
Im p lo s io n
Ma j or c ha l l e ng e a t L os A l a mos , A u gu s t 1 9 4 4 : T he gu n a s s e mbl y me t hod d i d not w or k for plu t o n i u m bombs , be c a u s e t he sp on t a n eou s fi s s i o n r at e w a s t oo hi gh; e a c h s u bc r i t i c a l ma s s t e nd e d t o fa l l a pa r t be for e a c r i t i c a l ma s s c ou l d be a s s e mbl e d .
A l t e r na t e me t hod : im p l os ion . S e t u p a s phe r i c a l l y s y mme t r i c a r r a ng e me nt of s u b- c r i t i c a l P u pi e c e s . S u r r ou nd t he m w i t h sha p ed c h a r ge s of c on v e nt i ona l e x pl os i v e s . Gi v e n t he i r s ha pe s a nd d i s t i nc t bu r n- r a t e s , t he y w ou l d c r e a t e a s y nc hr oni z e d , in - go i n g pr e s s u r e w a v e t o r a pi d l y c ompr e s s t he P u pi e c e s i nt o a c r i t i c a l ma s s .
T hi s c r e a t e d ma j or c ha l l e ng e s , bot h t he or e t i c a l a nd e x pe r i me nt a l : ho w t o s ha pe t he c on v e nt i ona l - e x pl os i v e c ha r g e s i nt o a k i nd of le n s ; ho w t o pr od u c e c on v e nt i ona l c ha r g e s w i t h t he r e q u i s i t e
pu r i t y ; ho w t o c oor d i na t e t he e l e c t r oni c s s o t he r e s u l t i ng s hoc k w a v e r e t a i ne d s phe r i c a l s y mme t r y , . . .
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T rin it y T e s t
No r r i s B r a d b u r y w a s r e s p o n s i b l e f o r t h e fi n a l a s s e m b l y o f t h e “ g a d g e t ” b e fo r e t h e
Tr i n i t y t e s t o n J u l y 1 6 , 1 9 4 5
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P r oj e c t l e a d e r s w e r e muc h l e s s c onfi d e nt a bou t t he c ompl i c a t e d i mpl os i on d e s i gn t ha n t he gu n- a s s e mbl y me t hod . T he y a r r a ng e d for a t e s t i n t he d e s e r t ou t s i d e A l bu q u e r q u e , NM, w hi c h O ppe nhe i me r d u bbe d t he “T r i ni t y ” t e s t .
P l u t oni u m r e ma i ne d s o r a r e t ha t l e a d e r s fi r s t pl a nne d t o e nc a s e t he bomb i n a hu g e s t e e l c ont a i ne r : i f t he bomb w e r e a d u d , t he y c ou l d s c r a pe off t he pl u t oni u m fr om t he i ns i d e of “J u mbo . ” I n t he e nd , t he y fe a r e d t ha t “J u mbo” w ou l d i nt e r fe r e w i t h i ns t r u me nt a t i on, s o t he y d i d n ’ t u s e i t .
Th e “ J u m b o ” c o n t a i n e r a r r i v i n g i n N M
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T he bl a s t w a s s o i nt e ns e t ha t i t fu s e d s a nd i nt o gl a s s : r a d i oa c t i v e “t r i ni t i t e . ”
Courtesy of Shaddack on Wikipedia. Used under CC BY.
25
“F a t M a n”
Th e “ F a t M a n ” P u - im p lo s io n b o m b b e in g p r e p a r e d on t he i s l a nd of T i ni a n, ne a r J a pa n, A u g u s t 1 9 4 5
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T hr e e w e e k s a ft e r t he T r i ni t y t e s t — a nd j u s t t hr e e d a y s a ft e r t he bombi ng of H i r os hi ma — a pl u t oni u m i mpl os i on bomb w a s d r oppe d on t he c i t y of Na g a s a k i , on A u gu s t 9 , 1 9 4 5 .
Su r v i v o r s r e - en t er t h e ci t y a f t er t h e b o m b i n g
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Mu s h r o o m c l o u d r i s i n g o v e r N a g a s a k i
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Th e Da y A f te r T r in ity
Ma n y mor e q u e s t i ons t o t hi nk a bou t .
W a t c h t he fi l m on y ou r o w n. W e w i l l t he n ha v e a n op t ion a l , i nfor ma l d i s c u s s i on d u r i ng c l a s s t i me on Mond a y , 2 6 O c t obe r . S ome q u e s t i ons t o t hi nk a bou t :
• W ha t mot i v a t e d s c i e nt i s t s a nd e ngi ne e r s t o w or k on t he Ma nha t t a n P r oj e c t ? Di d t he i r mot i v a t i ons c ha ng e o v e r t i me ?
• H o w w a s t he d e c i s i on ma d e t o u s e t he bombs ?
• W ha t i mpa c t d i d t he y ha v e on t he c ou r s e of t he w a r ?
• W ha t r e a c t i ons d i d s c i e nt i s t s a nd ot he r s ha v e t o t he u s e of t he ne w w e a pons ?
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STS.042J / 8.225J Einstein, Oppenheimer, Feynman: Physics in the 20th Century
Fall 202 0
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