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Mea surement Techniqu es, VoL 40, No. 10, 1 997

T he a t t en t i on o f t he r eaders i s drawn t o t he con ti nua ti on o f a se l ec t i on o f ar t i c le s on mat er i a l f ro m t he E l even t hIn t erna ti ona l Sem i nar on "A ccura te Mea surement s i n Nuc l ear Spec t roscopy " ( co mm enced i n I zmer i t e l ' nayaT ekhni ka , Nos . 6 and 8 , 1997) .

D E S I G N O F A P R I S M B E T A S P E C T R O M E T E R F O RO P E R A T I O N I N T H E P I K R E A C T O R

A . A . R o d i o n o v , Y u . L . K h a z o v , M . G . M a k s i m o v ,a n d Y u . L . K l e i m a n

U D C 5 3 9 .1 + 5 3 9 . 5

A descr i p t i on i s g i ven o f a p l anned pr i sm be t a spec t romet er i n t ended f or prec i se measu rement s o f conver s i one l ec t ron spec t ra bo t h f ro m rad i a ti ve cap t ure reac ti ons and rad i oac t ive sources , a nd i ts charac t er i s t ic s arepresen t ed .

Pow er fu l in s t rumen ta t ion i s be ing c rea ted fo r the PIK reac to r be ing bu i l t a t the S t . Pe te r s bu rg Nu c lea r Ph ys ic s Ins ti tu te(PIYaF) o f the R us s ian A cadem y o f Sc iences . T h i s reac to r w i l l be u s ed fo r inves t iga t ing the fundam en ta l p rope r t i e s o f ma t t e r ,in nuclear and e leme ntary -part ic le phys ics and in solid-s tate phys ics , an d a lso in order to use phys ica l me thod s in appl ied f ie ldso f k n o w l e dg e .

A s e t o f in s t rumen t s ha s bee n c rea ted fo r nuc lea r phys ic s re s ea rch , and the s e w i l l y i e ld mutua l ly com plem en ta ry re su l ts .One s uch in s t rumen t i s a p r i s m be ta s pec t rom e te r wh ich i s i n t ended fo r mak ing p rec i s e meas u rem en t s o f the s pec t ra o f e l ec t ronsemi t t ed bo th d i rec t ly in rad ia t ive neu t ron -cap tu re reac t ions and a l s o f ro m rad ioac t ive s ou rce s . A s we l l a s t r ad it iona ls pec t rome t r i c inves tiga t ions , t he magn e t i c mom en t s o f exc i t ed s t at e s o f nuc le i w i l l be s ys t ema t i ca lly de te rmin ed by meas u r ingthe hype r f 'me ene rgy s h i f t o f conve rs ion l ines , a s p ropos ed a t the PIY aF [ I -3 ] . B y employ ing the me tho ds o f be ta s pec t ros copyi t w i ll be pos s ib le to ob ta in in fo rm a t ion conce rn ing s o l id - s ta t e p rope r t i e s ( fo r example , s u r face phenom ena , chem ica l e f fec t s ) .M 6s s baue r s p ec t ros copy wi th conv e rs ion e lec t rons is a s ens i t ive me thod f o r inves tiga ting s u r face s and th in f i l l s .

When deve lop ing the de s ign , the ma in a t t en t ion was devo ted to ens u r ing a h igh accu racy fo r meas u r ing e l ec t ronene rg ie s (w i th a re l a t ive e r ro r o f the o rde r o f 10 -6 ) and to c rea t ing a s pec t rome te r hav ing ex tens ive exp e r imen ta l poss ib il it ie s.A t p re s en t , t he s pec t rome te r i s 60 -70% ready .

T he des ign makes p rov i s ion fo r :chang ing the s pec t rom e te r pa rame te r s ( the d is pe rs ion , s o l id ang le ) du r ing ope ra t ion by in t e rchang ing s ec t ions o f the

lens windings ;- - the poss ibi l i ty of applying to a ta rget cons is t ing of 20 s t r ips a b ias ing potent ia l which can be dis t r ibuted betweenthe s t r ip s , s o inc rea s ing the e f fec t ive a rea o f the t a rge t by a fac to r o f 20 ;- - u s ing a m u l t ip le de tec to r mak ing i t poss ib le to reco rd 20 e l ec t ron s pec t ra s imu l t aneous ly ;- - m e a s u r i n g e - 3 " a n d e - X coincidences ;- - u s ing an in t e rmi t t en t beam o f neu trons when ope ra t ing on - l ine w i th the reac to r .Va r ious types o f p r i s m s pec t rom e te r we re cons ide red in de ta i l i n pape rs p re s en ted a t a s emina r on " Pr i s m Spec t rome te r s

and T he i r A pp l i ca tions " [4 ] .A s chema t i c d i ag ram o f the p r i s m s pec t rome te r fo r the PIK reac to r i s s hown in F ig . 1 . T he m agne t i c p r i s m i s the mos t

demand ing pa r t o f the s pec t rome te r . T he d i s pe rs ion and the max imum ape r tu re depend on the s hape o f the magne t i c f i e ld i tp roduces in the gap . T he de s ign o f the magne t i c p r is m i s g iven s chema t i ca lly in F ig . 2 , wh ich s hows on ly the fe r romagne t i c

T rans la t ed f rom Izm er i t e l 'naya T ekhn ika , No . 10 , pp . 54 -57 , Oc tob e r , 1997.0543-1972 /97 /4010-10 03518 . 00 9 P lenum Pub l is h ing Corp ora t ion 1003

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JFig. 1 . La you t of the pr ism beta spect rometer for the PIK reactor : 1)ma gne tic prism ; 2) coll imator lens; 3) focu sing lens; 4) detecto r; 5) refer enc eNM R m agnet ; 6) rota ting coil s ; 7) ta r ge t -s ou rce ; 8) neut ron guide; 9)reactor biologica l screen; 10) ac t ive-zone heavy -water ref lec tor .

119NMRMCO

Y

Fig. 2 . Sch emat ic diagram of the magne t ic pr ism sp ect rometer .par ts and the energiz ing coi l s . The pr ism has plane pole pieces of area 1700 x 350 mm 2 and manufac tu red f rom 79NM a l loy .The w ork ing gap be tween the po l e p i eces i s 200 m m and the yoke i s o f ARM CO sof t iron . The magne t i c i nduc t ion a t thedist r ibut ion maxim um is var ied in the range 20-400 G. The requ i red two-dimension al charac ter is t ics of the mag net ic f ie ld inthe working region ( the re la t ive var ia t ion in the magnet ic f lux through u ni t a rea a long the t ra jec tory must be sma l ler than 10 -5along the x axis of the pr ism) i s obta ined by dividing each pole piece into two para l le l p la tes wi th an auxi l ia ry 50-ram gap.Provision i s ma de fo r a l igning the pole pieces.

The col l imat ing and focusing magnet ic lenses (see Fig. 1) a re 2500-mm long and have 220-mm d iam eter distr ibutedsect ioned windings. By sw i tching in di f ferent se ts o f windings, o ne can change the shape o f the f ie ld in the lenses, and thismak es i t possible to var y the focal length in the range 450-1500 ram. This provides o pera t ional cont rol of the dispersio n D andthe solid angle fl :1004

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D = 2 f r t g c t, ~ a x = R 2 1 ( 4 f 2 ) ,where j~ i s the foca l l eng th o f the focus ing l ens; = i s the ang le o f inc idence o f a pa ra l l e l e l ec t ron beam on the p r i s m, wh ichdepends on the shape of the pr ism f ie ld; R is the in ternal radius of the lens tube; f c i s the foca l l eng th o f the co l l ima to r l ens.T he l ens e s a re s h ie lded by a m u l t i l aye r magne t i c s c reen made o f 79NM a l loy in o rde r to e l imina te the in f luence o f ex te rna lmagne t i c f i e ld s on the e l ec t ron t ra j ec to r i e s.

In i ts deve loped ve rs ion , t he de tec to r t akes the fo rm o f 20 p rop or t iona l chambers . E ach ch amb er ha s a 1 x 60 - ramen t ry w indow loca ted in the foca l p l ane o f the focus ing l ens , t he foca l p l ane be ing pos i t ioned ou t s ide the l ens w ind ings , s op rov id ing f ree acces s to the d e tec to r . T he co l l ec t ion s ys t em enab le s 20 conv e rs ion e l ec t ron s pec t ra to b e reco rd eds imul t aneous ly . T he t a rg e t - s o u r ce cham ber is a l so pos it ioned ou t side the magne t i c l ens and i s r ead i ly acces s ib le . T he s ou rcetakes the fo rm o f a th in 1 60 -m m s t r ip ( in the ca se when a b ia sing po ten t i a l is app l i ed , 20 s t r ip s loca ted in the foca l p laneof the co ll ima to r l ens ) . Wh en wo rk ing w i th an ex t rac ted neu t ron beam, the s t r ip i s l oca ted at t he max im um of the f lux o fthe rma l neu t rons focus ed by a neu t ron gu ide . A ve rs ion o f the s ou rce ha s been deve loped wh ich p rov ides fo r meas u r ingco inc idences be tw een conve rs ion e l ec t rons and x - o r 3 , -rays.

T he neu t ron gu ide cons i s t s o f two mi r ro rs , s pu t t e red w i th 58Ni , wh ich a re cu rved a long an invo lu te and focus thethe rma l neu t rons f rom the reac to r re f l ec to r on to the s ou rce . T he neu t ron gu ide i s pos it ioned in an inc l ined reac to r channe l andthe neu t ron -gu ide chamb er i s evacua ted . T he s h ie ld o f the neu t ron gu ide ens u re s tha t r ad iat ions f rom the re ac to r a re abs o rbed .

T he power s upp ly fo r the p r i s m and l ens e s p rov ides p rog rammed va r i a t ion o f the cu r ren t in the range 0 . 5 -30 A .Inc luded in i t i s a fundamen ta l ly impor tan t pa r t , a s ys t em fo r s t ab i l i z ing the p r i s m magne t i c f i e ld wh ich i s ba s ed on thew e l l - k n o w n m e t h o d o f r o t a t i n g c o i l s ( s e e , f o r e x ~ m p l e , [ 5 ] ) . T h e m e t h o d h a s b e e n h i g h l y r e c o m m e n d e d f o r m a n y b e t as pec t rome te r s and p rov ides a re l a t ive magne t i c f i e ld in st ab il it y o f a ro und 10 -5 . T he fo l lowing bas ic fac to r s p re ven t the s t ab il it yf rom b e ing inc rea s ed : ag ing and t empe ra tu re d r i f t o f the re fe ren ce magne t ; no i s e in the bea r ings and s l id ing con tac t s ; ag ing ,t empe ra tu re d r i f t , and d i f fe ren t i a l non l inea r i ty o f the meas u r ing po ten t iome te r . Spec ia l meas u re s w e re t ake n wh en des ign ingthe s t ab il i za t ion s ys t em in o rd e r to reduce the in f luence o f the s e fac to r s .

T hus , i n o rde r to reduce the in f luence o f ag ing and t empe ra tu re d r i f t , t he magne t i c f i e ld in the re fe ren ce e l ec tromagn e twas s t ab i l iz ed by h igh - re s o lu t ion nuc lea r magne t i c re s onance (NM R) wh ich ens u red a re l a t ive in s t abi l it y o f the f i e ld in the gapof 10 -7 . Such s t ab i li t y was ach ieved by fo rm ing the f requency app l i ed to the NM R s ens o r in a Ch6-31 f requ ency s yn thes ize rwh ich gu a ran teed a long- t e rm re l a t ive f requency in s tab il it y o f 10 - s . T h e range o f f requency va r i a t ion is 7 -23 M Hz w hichcor re s ponds to a va r i a t ion o f the magne t i c induc t ion in the re fe rence magne t o f 1650-5420 G . T he s igna l - to -no i s e ra t io in theNM R d i s pe rs ion s igna l , fo r a l i ne w id th o f abou t 50 Hz , ha s a va lue n ea r 300 a t the ou tpu t o f the s t ab i li z a t ion s ys t em. T h i smakes i t pos sib le to ens u re a re l a t ive ins tab il it y o f the signa l f req uency o f the o rde r o f 10 - s on av e rage o ve r the range . T hef requenc y s yn thes ize r con t ro l s a com pute r wh ich ope ra te s in acco rdance w i th a s pec i f ied p rog ram o f meas u rem en t s .

T he c omb ined po we r s upp ly and m agne t i c f i e ld s tab i li z a t ion s ys t em o f the re fe rence e l ec t roma gne t func t ions in thefo l lowing way ( s ee F ig . 3 ) . T he chos en f reque ncy i s app li ed to the NM R s ens o r loca ted in the mag ne t gap wh e re the magne t i cinduc t ion i s e s t ab l i s hed . An au toma t i c s cann ing s ys tem ra i s e s the f i e ld to the re s onance cond i t ions . T he nuc lea r magne t i cre s onance s igna l appea r ing in the s ens o r i s ampl i f i ed by r f and I f ampl i f i e rs . A f te r pa ss ing th rough a ph as e s ens i t ive de tec to r ,the NM R s ignal s w i t ches on the magne t i c f i e ld s t ab i l iz a t ion un i t and s imu l t aneously d i s connec ts the au tom a t i c s cann ing s ys tem.Dur ing the t ime w hen the f reque ncy i s be ing s wep t , a c i rcu i t tun ing s ys tem con t inuous ly mon i to r s the r f c i rcu i t o f the s enso rs o a s to ens u re tha t i t i s t uned to the f requency o f the s yn thes ize r wh i l e meas u remen t s a re be ing m ade .T he magne t i c f i e ld g rad ien t nea r the s ens o r i s a func t ion o f the magne t induc t ion f i e ld . T h i s dep enden ce i s compens a tedby au toma t i ca l ly ad jus ting the cu r ren t s in s h imming co i l s .

A s pec ial dev ice was deve loped in o rd e r to reduce the in f luence o f no i se in the bea r ings and s l id ing con tac t s on thes tabi l ity of the pr ism ma gnet ic f ie ld (see Fig . 1) . I t consists of a double can t i lever shaft (spindle) on gas bearin gs , d i fferent ia l lys wi tched meas u r ing co i l s pos i t ioned in the gaps o f the magne t i c p r i s m and the re fe rence magne t , and a ro ta t ing t rans fo rmer .T he l a t t e r is l oca ted be tween the m agne t s , i ts p r imary w ind ing , mou n ted on the s ha f t, be ing s wi tched in to the meas u r ing co i lc i rcu i t and i t s s econd a ry ( s t a t iona ry ) w ind ing s e rv ing to coup le ou t the d i f fe ren ce signa l. T he de s ign fea tu re s o f the dev ice a rede te rmined by the s t r ingen t r equ i remen t s im pos ed on the pe rmis s ib le va lue s o f the rad ia l wobb le and ax ia l d i s p lacemen t o f thes p ind le (0 . 001 ram ) and a l s o by the long du ra t ion o f con t inuous ope ra t ion and cons ide rab le s e rv ice l i f e . T he can t i l eve r pa r t so f the s p ind le , whe re the m eas u r ing co i l s a re located , a re made in the fo rm o f ho l low a lumina ce ram ic tubes . T he meas u r ingco i ls c an be p rev ious ly s e t s o tha t t he i r c en te r l i nes o f s ym met ry co inc ide . T he ga s bea r ings a re p re s s u r i zed wh en s t a r t ing up

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Field (current) presett ing sweep6 k Hz

6 kHz I foscillator

Powersupply

Electromagnetrack with stabil-ization-currentdc amplifierand rectifiers

Magnetic f ield stabil izer m onitorI

Refere nce voltage .p [ 'FDI (resolution) i M agn etic f ield NM R Outputs tabi lizat ion un i t ~ am pl i f ie rReference voltage _ signalFD2 (stabilization) - I

!

i r f - tun ing and swi tching sweepI s i g n a l

Band-pass [amplifier Sensor circuit Shim current and7-14 MH z, tuning unit offset current14-23.5 MH z control unit

circuit tuning NMR s ignal " -Mo dulation current I "~ ~!

!ElecttQ- E le ct ro - Sens_o_rs Picku p andmagnet ] magnet 7-14 MH z. feedb ack(main 14-23.5 MH z coilsc u r r e n t w in d i n g s )

I , = = JShim current control

Unit for fieldand frequencysweep, automaticscanning and stabil-ization switching-- Computer

iCh6-31 ~rcq~.e~cylyntheszier Ch6-54

I i V5-28 It

Current (f ield) m onitoring [F ig . 3 . B lo ck d i ag ram o f t h e NM R s t ab i li ze r fo r t h e f i e l d o f t he r e f e r en ce m ag n e t .

an d ceas in g o p era t i o n , b u t i n t h e o p era t i n g r eg ime a t a ro t a t io n a l f r eq u en cy o f 1 33 rp s t h e p res su r i za t i o n i s swi t ch ed o f f an dg asd y n am ic su sp en s io n co m es i n to fo rce . T h e su sp en s io n co mp o n en t s a r e m ad e o f a l u min a ce ram ic wh ich i s ex cep t io n a l l y h a rdan d r es i s t an t t o wear .

Th e u s e o f N M R s t ab i li za t i o n en su res a r e la t i v e i n s tab i li t y o f 1 0 -7 fo r t h e r e f e r en ce m ag n e t f i e l d , an d t h i s i s twoo rd er s o f mag n i tu d e b e t t e r t h an t h a t ach i ev ed i n p rev io u s ly u sed d e m ag n e t s . A i r b ea r i n g s a r e em p lo y e d i n th e mech a n i ca lsy s t em fo r ro ta t i n g t h e measu r in g co i l s . M o v ab l e co n t ac ts , wh ich w ere an i n su p erab l e so u rce o f n o i se , h av e b e en e l imin a ted .Est ima tes ind icate that the re lat ive ins tab il i ty o f the magne t ic f ieM in the p r ism is lowe r than 10 -6 .

Le t u s g iv e so me p ara me te r s o f th e b e t a sp ec t ro mete r . D i sp er s io n s o f 5 2 0 , 3 0 0 , an d 1 6 0 cm c o r r esp o n d t o so l id an g leso f 0 .1 , 0 .3 , an d 1 % o f 4 ~r an d r eso lv in g p o we r s , t ak in g ab er r a t i o n s in to acco u n t , o f 3 .4 x 1 0 -4 , 6 x 1 0 -4 , an d 2 x 1 0 -3 .Th e so u rce i s a 1 x 6 0 -m m s t r i p . Th e r e l a t iv e en erg y e r ro r i s ( 2 -5 ) x 1 0 -6 . Th e e n erg y r an g e o f t h e sp ec t ro m ete r is0 -2 .5 M e V. Th e n eu t ro n f l u x o n th e so u rce is 3 x 1 09 n eu t r o n s -c m- 2 - sec -1 .

REFERENCES

.

2 .A. I . Eg o ro v , A . A . Ro d io n o v , A . S . Ry l ' n ik o v , e t a l . , P i s 'ma Zh . E k sp . Teo r . F i z . , 2 7 , 5 1 4 (1 9 7 8) .A . A . Ro d io n o v an d L . P . Kab in a , "The effect o f hyperfine displacement o f K conversion lines. M easurement o f energydifferences o f K and L conversion lines in spectra o f l 6 9 y b a n d 1 92 11 , " P rep r in t No . 2 1 2 8 [ i n Ru ss i an ] , S t . P e t e r sb u rgIn s t i t u t e o f Nu c l ea r P h y s i cs , Ga t ch in a (1 9 9 6 ) .

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.

4 .5 .

A. A . Rod iono v and L . P. Kabina , Izv. Ross isk . Akad. N auk Ser . Fiz . , 60, No. 5 , 126 (1996).Prism Beta Spectrometers, Pyargale , Vi lnius , L i thuania (1971).W . M ampe , K . Sc h reckenb ach , P . J euch , e t a l . , Nue l . ln s t rum. M e thods , 154 , 127 (1978) .

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