Comments on Beam-foil Spectroscopy

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Submitted on 1 Jan 1979

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Comments on Beam-foil Spectroscopy

S. Bashkin

To cite this version:

S. Bashkin. Comments on Beam-foil Spectroscopy. Journal de Physique Colloques, 1979, 40 (C1),

pp.C1-125-C1-138. �10.1051/jphyscol:1979123�. �jpa-00218403�

JOURNAL DE PHYSIQUE

Colloque

C1,

supplkment au n " 2, Tome 40, fkvrier

1979,

page

C1-125

Comments on Beam-fotl Spectroscopy

S. Bashkin*

Department o f Physics, The U n i v e r s i t y of Arizona, Tucson, Arizona 85721

A b s t r a c t

I' gPve a sununary o f t h e p r i n c i p a l advances i n beam-foil spectroscopy.

which have been made between t h e b i r t h o f t h i s technique and t h e present. I n a d d i t i o n , I pay p a r t i c u l a r a t t e n t i o n t o t h e i n t e r p r e t a t i o n o f data on t h e time- dependence of l i n e i n t e n s i t i e s , and argue t h a t t h e main cause o f c u r v a t u r e i n such data i s blends, r a t h e r than cascades.

.Resume

Je p&sente un somnaire des p r i n c i p a u x progres de l a Spectroscopie faisceau-lame depuis l a naissance de c e t t e technique. En o u t r e , j e p r e t e une a t t e n t i o n p a r t i c u l i e r e

3

l ' i n t e r p r e t a t i o n des donnees s u r l a depondance temporelle des i n t e n s i t e s des r a i e s , e t j'avance que l a cause l a p l u s importante de l a courbure de ces d e c l i n s r e s i d e dans l e m6lange de r a i e s p l u t 6 t que dans l e s cascades.

Some f i f t e e n years have passed s i n c e t h e modern b e a m f o i l method was i n t r o d u c e d 11-31 and t h e present conference o f f e r s a good o p p o r t u n i t y t o make some general remarks concerning t h e development i n t h i s f i e l d . I t w i l l be remembered t h a t t h e p r i n c i p a l , o r i g i n a l purpose o f b e a m f o i 1 spectroscopy was t o p e r m i t the measurement o f t h e m e a n l i v e s o f e x c i t e d e l e c t r o n i c s t a t e s 121. That purpose remains today as tRe s t n g l e most i m p o r t a n t a p p l i c a t i o n o f t h e beam- f o i l l i g h t source. I f t h e r e i s one t h i n g t h a t i s c l e a r from t h e p a s t f i f t e e n years, i t i s t h a t t h e r e i s a l o t o f d i s s a t i s f a c t i o n w i t h t h e l i f e t i m e deter- minations 14-71. Not o n l y can one f i n d numerous

*

Supported i n p a r t by NSF and ONR

c r i t i c i s m s of i n d i v i d u a l measurements, t h e very technique has been questioned as a way o f g e t t i n g a t these i n t e r e s t i n g numbers. That t h i s t o p i c i s s t i 11 o f major i n t e r e s t i s evidenced by t h e emphasis i n t h e program o f t h e present conference on ways o f i n - t e r p r e t i n g t h e l i f e t i m e data so as t o o b t a i n more be1 i e v a b l e numbers than a simp1 e a n a l y s i s can pro- vide. Those o f you who have read my own papers [8,9]

on t h e s u b j e c t know t h a t my p o i n t o f view i s some- what unorthodox; I s h a l l t r y t o e x p l a i n today why i t i s t h a t I adopt my p a r t i c u l a r p o s i t i o n .

To add substance t o t h e controversy concerning 1 if e t i m e measurements, I r e f e r t o Fig. 1, where I p l o t t h e o s c i l l a t o r s t r e n g t h as a f u n c t i o n o f r e - c i p r o c a l n u c l e a r charge f o r a t r a n s i t i o n i n t h e ZnI

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979123

C1-126 JOURNAL DE PHYSIQUE

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i s o e l e c t r o n i c sequence [lo-161. The t h r e e curves w h i c h a r e shown r e p r e s e n t v a r i o u s c a l c u l a t i o n s

[ l o -

121, w h i l e t h e p o i n t s t h a t l i e w e l l below t h o s e curves r e p r e s e n t d i f f e r e n t measurements [I 2- 153.

The d i s c r e p a n c y between t h e measured and c a l c u l a t e d v a l u e s i s u n s a t i ' s f a c t o r y . As l o n g as such d i s c r e p - a n c i e s p e r s i s t , i t i s no wonder t h a t use o f t h e beam f o i l s o u r c e f o r making t h e measurements i s suspect.

O f course, as o f t h e p r e s e n t time, t h e r e i s n o a l - t e r n a t i v e t o t h e b e a m f o i l method, so i t i s espec- i a l l y i m p o r t a n t t h a t we t r y t o u n d e r s t a n d t h e t r o u b l e s and t a k e s t e p s t o c o r r e c t them.

C l e a r l y t h e l i f e t i m e experiments cannot be done u n l e s s one has a l s o made a s p e c t r a l decomposition o f t h e l i g h t . Indeed, t h e s p e c t r o s c o p y remains as t h e fundamental o p e r a t i o n i n making use o f t h e beam-foi 1 l i g h t . Now i t was u n d e r s t o o d f r o m t h e f i r s t [2] t h a t the s p e c t r o s c o p y w o u l d s u f f e r f r o m t h e l i m i t a t i o n ' t h a t t h e b a p p l e r w i d t h o f t h e s p e c t r a l l i n e s w o u l d be s u b s t a n t i a l . I n f a c t , l i n e w i d t h s o f 10

A

o r more were n o t uncommon 1171. The f i r s t s u c c e s s f u l a t t a c k on t h e problem o f r e d u c i n g t h e l i n e w i d t h was made

+++I+

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f a c t i s t h a t such w i d t h s a r e s t i l l t o o g r e a t t o p e r m i t p r e c i s i o n s p e c t r o s c o p i c experiments. I t i s my view, on w h i c h I s h a l l expand i n a few moments, t h a t t h e l i f e t i m e d i f f i c u l t i e s o r i g i n a t e p r i m a r i l y i n t h e p o o r spectroscopy.

W h i l e i t seems i m p o s s i b l e t o i s o l a t e t r u l y narrow s p e c t r a l l i n e s , t h e r e have been a t t e m p t s t o over- come c e r t a i n aspects o f t h e l i n e w i d t h d i f f i c u l t i e s . One t h i n k s , f o r example, o f t h e quantum-beat exper- iments [25,26!. which e f f e c t i v e l y i s o l a t e p a i r s o f n e i g h b o r i n g s t a t e s , e n a b l i n g one t o f i n d t h e energy s e p a r a t i o n s o f f i n e - s t r u c t u r e and h y p e r f i n e - s t r u c - t u r e components [27-321. E q u a l l y i n t e r e s t i n g , such experiments have g i v e n some i n f o r m a t i o n on t h e n a t u r e o f t h e b e a m - f o i l i n t e r a c t i o n , a l t h o u g h t h a t i s s t i l l a m a t t e r o f c o n s i d e r a b l e c o n f u s i o n [33].

A n o t h e r approach, s t i l l n o t v e r y f r u i t f u l i n any g e n e r a l sense, i s t h e c r o s s i n g o f t h e i o n beam w i t h a l a s e r [34-421, f o r such a t e c h n i q u e o f f e r s t h e pos- s i b i l i t y o f g e n e r a t i n g narrow l i n e s o r a t l e a s t o f r e s t r i c t i n g t h e e x c i t a t i o n t o a s p e c i f i c l e v e l . The s m a l l d e n s i t y o f a p p r o p r i a t e t a r g e t l e v e l s when t h e i n t e r e s t i s i n a non-metastable e x c i t e d l e v e l makes experiments o f t h i s k i n d e x t r e m e l y d i f f i c u l t . The same t h i n g can be s a i d o f t h e a p p l i c a t i o n o f e x t e r - n a l f i e l d s t o t h e beams. W h i l e a number o f papers have appeared where s t a t i c o r o s c i l l a t i n g f i e l d s have been imposed on t h e r a d i a t i n g l e v e l s [29,42-791, t h e f a c t i s t h a t work o f t h a t n a t u r e has been q u i t e l i m i t e d .

There a r e some ways, a l l r a t h e r obvious, r e a l l y , i n which t h e general beam-foil method has been ex- tended from t h e e a r l i e s t phases. These a r e s u b s t i - t u t i n g p h o t o - e l e c t r i c f o r photographic r e c o r d i n g 1801, e n l a r g i n g t h e range o f wavelengths s t u d i e d t o i n c l u d e e v e r y t h i n g from x-rays w i t h energies o f s e v e r a l keV 181-831 up t o l i n e s i n t h e near i n f r a r e d

[84], using p a r t i c l e energies as h i g h as 700 MeV [85], and g r e a t l y reducing t h e s h o r t e s t measurable l i f e t i m e s down t o a few picosec [86-901.

The v a r i a t i o n o f p a r t i c l e energy has perhaps been as u s e f u l as any o t h e r k i n d o f beam-foil develop- ment because i t has p e r m i t t e d t h r e e separate kinds o f experiments. One i s t h e o b s e r v a t i o n o f spectros- c o p i c behavior along .a

n

i s o e l e c t r o n i c sequence [91,92], t h e most extensive example being the move- ment o f t h e 2 P1/2,3/2 2 l e v e l s i n t h e L i I sequence and the 2 P l e v e l s i n the Be I 3 sequence a l l theway t o Kr, as Dr. D i e t r i c h w i l l describe l a t e r i n t h i s meeting [85]. Another i s the proper i d e n t i f i c a t i o n o f t h e i o n i z a t i o n stage t o which a s p e c t r a l 1 in e be-

longs. Although I have cautioned i n the p a s t [93]

t h a t t h e v a r i a t i o n o f l i n e i n t e n s i t y w i t h p a r t i c l e energy 1943 i s sometimes aninadequate t e s t o f t h e e m i t t e r ' s n e t charge, t h a t method does have diag- n o s t i c value. I'll r e t u r n t o t h i s i n j u s t a few moments. The t h i r d experiment a t h i g h energy takes advantage o f the f a c t t h a t h i g h p a r t i c l e energy gives r i s e t o ions o f h i g h n e t charge. For l e v e l s i n such ions, t h e decay r a t e s o f h i g h l y - forbidden op- t i c a l t r a n s i t i o n s o f t e n become r e a d i l y observable, and t h e l i t e r a t u r e a l r e a d y i n c l u d e s impressive data on magnetic d i p o l e , magnetic quadrupole, double e l e c t r i c - d i p o l e processes, and s p i n - f o r b i d d e n t r a n - s i ti o n s [i7,81,35-116:. S t i l l anot5cr o p p o r t u n i t y which e n e r g e t i c p a r t i c l e s p r o v i d e i s t h a t one can compare measured and c a l c u l a t e d values o f t h e Lamb S h i f t as a f u n c t i o n b o t h o f n u c l e a r charge and o f p r i n c i p a l quantum numbers. A t t h i s meeting we w i l l hear about such experiments from t h e Oxford, Argonne, and Berkeley Laboratories.

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c1-128 JOURNAL DE PHYSIQUE

L e t me g l v e an example o f t h e k i n d o f spectros- copy which has been made p o s s i b l e by t h e use o f p a r t i c l e energies i n r e g i o n o f hundreds of MeV.

F i g u r e 2 shows a spectrum of C r taken a t t h e Berke- l e y HILAC A c c e l e r a t o r 11161 w i t h a p a r t i c l e energy o f 464 MeV. T r a n s i t i o n s i n t h e L i - l i k e s t r u c t u r e are c l e a r l y present; what i s i n t e r e s t i n g i s t h a t these l i n e s had never b e f o r e been d e t e c t e d i n t h e labora- t o r y . U n f o r t u n a t e l y , i t i s probably a l s o c o r r e c t t o say t h a t they w i l l never be seen again, because these high-energy experiments a r e so expensive i n machine time t h a t t h e r e i s l i t t l e chance t o examine a p a r t i c u l a r system unless t h e r e i s a v e r y s p e c i a l i n t e r e s t associated w i t h i t . The Cr observations

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were made i n c i d e n t a l l y i n t h e course of o u r experi- ment on i r o n 11161 and we have no i n t e n t i o n o f try- i n g t o t a k e f u r t h e r data on C r .

The experiment on i r o n a t h i g h energy a l s o con- t a i n e d an unexpected r e s u l t . F i g u r e 3 shows p a r t of t h e spectrum taken w f t h i r o n a t an energy o f 491 MeV.

The L i - l i k e f e a t u r e a t 225 i s w e l l i s o l a t e d from i t s n e a r e s t neighbor, t h e He-1 i k e component a t 271

1.

I might say t h a t w h i l e t h e L i - l i k e s t r u c t u r e had been seen i n t h e s o l a r corona, the H e - l i k e f e a t u r e had n o t been detected p r e v i o u s l y . L e t me c a l l a t t e n - t i o n t o t h e f a c t t h a t t h e s p e c t r a l r e g i o n between those two l i n e s i s devoid o f any f e a t u r e o f i n t e r e s t . On t h e o t h e r hand, i f we l o o k a t F i g . 4, wesee t h a t

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same s p e c t r a l r e g i o n observed w i t h t h e same p a r t i c l e c u l a r i n t e r p r e t a t i o n . U n f o r t u n a t e l y the l i t e r a t u r e energy, b u t we now f i n d t h a t t h e r e i s a new l i n e i n - contains a measurement o f t h e decay time o f t h i s t e r p o l a t e d between t h e H e - l i k e and L i - l i k e features. f e a t u r e , and t h e l i f e t i m e which i s deduced from The new l i n e o r i g i n a t e s i n a t r a n s i t i o n i n B e - l i k e those data i s a s c r i b e d t o a l e v e l i n F 111 [1181.

i r o n . The d i f f e r e n c e between Figs. 3 and 4 i s t h a t t h e f o i l used f o r t h e second spectrum had a t h i c k - ness o f o n l y 40 micrograms/cmL whereas t h e o r i g i n a l f i g u r e was o b t a i n e d w i t h a f o i l o f 206 micrograms/

cm2. By reducing t h e f o i l thickness, i t was p o s s i b l e t o emphasize l o w e r charge components i n t h e r a d i a t - i n g beam and i t was t h i s which p e r m i t t e d us t o see t h e B e - l i k e t r a n s i t i o n . However, what t h i s a l s o shows i s t h a t t h e b e a m - f o i l i n t e r a c t i o n cannot be restp1,cted t o a s u r f a c e phenomenon. C l e a r l y , t h e r e i s a t l e a s t some i n f l u e n c e o f t h e i n t e r n a l volume o f t h e f o i l , and t h i s behavior, which i s e s p e c i a l l y easy t o see i n t h e high-energy experiments whereone can go from equi 1 i b r i um t o non-equi 1 i b r i um charge d i s t r i b u t i o n s , must be taken i n t o c o n s i d e r a t i o n i n any model o f t h e b e a m f o i l process.

L e t me t u r n my a t t e n t i o n now t o t h e l i f e t i m e problem, where t h e r e a r e a t l e a s t t h r e e separate matters t o be considered. I s a i d a few moments ago t h a t I t h i n k a major source o f d i f f i c u l t y i n t h e

l i f e t i m e work l i e s i n t h e poor spectroscopy. L e t me g i v e an example. F i g u r e 5 shows two s p e c t r a r e c e n t l y obtained a t The U n i v e r s i t y o f Lund [117]. These are o f f l u o r i n e as seen a t 2MeV and 3 MeV, r e s p e c t i v e l y . What i s o f i n t e r e s t i s t h e s e t o f l i n e s , i n c l u d i n g t h e l i n e a t 3042

A ,

a t t r i b u t e d t o F 111. Now t h e energy v a r i a t i o n o f l i n e i n t e n s i t y makes i t q u i t e c l e a r t h a t t h e l i n e a t 3042

A

i s d i f f e r e n t from t h e o t h e r l i n e s . a t t r i b u t e d t o F 111. I n f a c t , comparison o f t h e data a t 2 MeV and 3 MeV s t r o n g l y suggests t h a t t h i s p a r t i c u l a r l i n e o r i g i n a t e s i n a t r a n s i - t i o n i n F I V . C o n s u l t a t i o n w i t h Palenius, who has a l o t o f unpublished data on F, supports t h i s p a r t i -

-

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c1-130 JOURNAL DE PHYSIQUE

Obviously, if t h e l i n e does indeed come from F I V , t h e r e i s no way i n which a comparison o f t h e theo- r e t i c a l and experimental values c o u l d be p h y s i c a l l y s i g n i f i c a n t f o r F 111. We see from t h i s example t h a t an a b s o l u t e l y e s s e n t i a l requirement o f l i f e t i m e de- t e r m i n a t i o n s i s t h a t one c a r r y o u t t h e spectroscopy over a wide enough energy range t h a t t h e i d e n t i f i - c a t i o n o f the t r a n s i t i o n be unambiguously estab- l i s h e d . Sometimes, o f course a p a r t i c u l a r experimen- t a l group faces l i m i t a t i o n s o f apparatus which make i t impossible f o r such observations t o be c a r r i e d o u t . I n such cases, i t seems t o me, the decay must be h e l d i n abeyance u n t i l , perhaps through c o l l a b o r a t i o n w i t h people i n another l a b o r - a t o r y , t h e l e v e l i d e n t i f i c a t i o n can be placed on a f i r m foundation. I n t h e present instance, i t m i g h t t u r n o u t t h a t t h e r e i s an a c c i d e n t a l b l e n d o f l i n e s from F I 1 1 and F I V a t t h e wavelength i n question.

That, however, would n e c e s s a r i l y g i v e a curved s e t o f decay data, and f u r t h e r i l l u s t r a t e s t h e need f o r improved spectroscopy i n making l i f e t i m e determina- t i o n s .

A second problem w i t h t h e l i f e t i m e measurements i s t h a t s i g n a l s a r e o f t e n weak and, e s p e c i a l l y i f a l i n e looks r e l a t i v e l y i s o l a t e d from any o t h e r l i n e s , i t i s common p r a c t i c e t o open t h e s l i t s o f t h e spec- trometer. A t t h e same time, t h i s increases t h e l e n g t h o f the beam which i s under observation.

Again, i t i s my f e e l i n g t h a t t h i s i s a poor proced- u r e and gives r i s e t o i n t r i n s i c a l l y poor r e s u l t s . The reason i s two-fold. I n t h e f i r s t place, t h e con- t r i b u t i o n o f background becomes r e l a t i v e l y more im- p o r t a n t as t h e s l i t s are opened. Moreover, one r a r e l y mpkes measurements on t h e background o r o f t h e spectrum as a f u n c t i o n o f downstream d i s t a n c e so t h a t t h e r e j a t i v e p r o p o r t i o n s o f signal-to-back- ground may change i n an unknown fashion as one looks a t l a t e r and l a t e r times. There i s , thus, no way i n

which measurements w i t h wide s l i t s can produce quan- t a t i v e l y s a t i s f a c t o r y data on l i f e t i m e s .

The second reason i s n i c e l y i l l u s t r a t e d by Fig. 6 which was k i n d l y provided by a group a t Stockholm [119]. F i g u r e 6 shows t h e 1 ig h t from n = 4 i n hydro- gen e x c i t e d by a s i n g l e f o i l , t h e t o p curve being obtained w i t h protons, t h e middle w i t h diatomic hydrogen ions, and the bottom w i t h t r i a t o m i c hydro- gen ions, a l l w i t h t h e same p a r t i c l e v e l o c i t y . The f a m i l i a r quantum-beat s t r u c t u r e i s apparent, espec- i a l l y i n t h e topmost curve. Two o t h e r i n t e r e s t i n g c h a r a c t e r i s t i c s appear. The f i r s t i s t h a t t h e r e i s no simple monotonic decay o f the l i g h t from n = 4, nor, of course, would t h e r e be from any o t h e r l e v e l i n hydrogen except p o s s i b l y n = 2 . Consequently, t h e a n a l y s i s o f t h i s decay curve i n terms o f a l i f e t i m e c l e a r l y i s n o t s u s c e p t i b l e t o a simple treatment i n terms o f an exponential o r a sum o f exponentials.

The p h y s i c a l phenomenon which i s i n v o l v e d here i s much more complicated and, as we a l l know, o r i g i - nates i n a r a t h e r s p e c i a l k i n d o f b l e n d i n g o f t r a n -

s i t i o n s from degenerate s t a t e s .

The second f e a t u r e i s t h a t t h e general behavior o f t h e decay i s q u i t e d i f f e r e n t depending on t h e i o n which i s used f o r t h e e x c i t a t i o n . Now i t i s ob- vious t h a t these various f e a t u r e s become washed o u t i f one looks a t a f i n i t e l e n g t h o f beam, and t h i s , I b e l i e v e , adds substance t o my c l a i m t h a t one can n o t g e t good l i f e t i m e determinations unless one ex- amines an almost i n f i n i t e s i m a l beam l e n g t h . I t i s simply n o t so t h a t t h e a l g e b r a i c expression which says t h a t t h e beam l e n g t h c o n t r i b u t e s o n l y a con- s t a n t h y p e r b o l i c m u l t i p l i c a t i v e term t o the expon- e n t i a l decay i s t h e way i n which n a t u r e behaves.

While everyone i s now w i l l i n g t o concede a f t e r t h e f a c t t h a t t h e case o f hydrogen i s s p e c i a l , i t i s my c o n t e n t i o n t h a t every o t h e r system demands t h a t i t be s t u d i e d i n i n f i n i t e s i m a l l e n g t h s o f beam. Other-

DETECTOR POSITION ( m m ) Fig. 6.

IntennME de

H6

%

pobXiofl de La Lame.

I&e.n&.dy 0 6 H6

%

bod? p06&0fl.

w i s e , t h e decay d a t a may conceal p h y s i c a l l y s i g n i - f i c a n t q u a n t l t i e s which would make t h e a n a l y s i s o f t h e decay c u r v e i n terms o f a l i f e t i m e v i r t u a l l y meaningless.

L e t me now g i v e some s p e c i f i c examples o f o t h e r problems w h i c h r i s e in i n t e r p r e t i n g decay curves.

F i g u r e 7 shows some decay d a t a t a k e n by L e a v i t t and m y s e l f f o r a t r a n s i t i o n i n B r

V I I

[120]. These d a t a a r e p r o b a b l y as good as any e v e r t a k e n on such a s u b j e c t . The r a n g e o f i n t e n s i t i e s i s a p p r o x i m a t e l y a f a c t o r o f 200, s o t h e decay was f o l l o w i d f o r n e a r l y f o u r mean 1 iv e s . One sees t h a t t h e t y p i c a l a n a l y s i s has been c a r r i e d o u t and assignments made t o t h e t h r e e d i f f e r e n t e x p o n e n t i a l s w h i c h a r e s a i d t o c o n t r i b u t e t o t h e o v e r a l l curve. F i g u r e 8 shows o s c i l l a t o r s t r e n g t h s as a f u n c t i o n o f r e c i p r o c a l

n u c l e a r charge f o r t h e t r a n s i t i o n i n q u e s t i o n . There a r e v a r i o u s c a l c u l a t i o n s [ I 2 1

1,

below w h i c h a r e e x p e r i m e n t a l p o i n t s from d i f f e r e n t sources [12- 15,122-1241. The p r e s e n t d a t a g i v e r i s e t o t h e p o i n t l a b e l l e d B and one sees t h a t t h i s number i s much c l o s e r t o t h e t h e o r e t i c a l c u r v e s than i t s p r e - decessor i n t h e l i t e r a t u r e . From t h i s and a l s o f r o m t h e q u a l i t y o f d a t a one m i g h t c o n c l u d e t h a t t h e p r e - s e n t measurement i s i n d e e d s a t i s f a c t o r y . On t h e o t h e r hand, t h e r e i s t h e p o i n t B ' , which i s deduced f r o m p r e c i s e l y t h e same d a t a as p o i n t B. The d i f f e r e n c e between B and B ' i s t h a t t h e l a t t e r i s based on a d i f f e r e n t k i n d o f a n a l y s i s . I n p a r t i c u l a r , t h e d a t a were d i f f e r e n t i a t e d . I n p r i n c i p l e , such d i f f e r e n t i a -

t i o n adds w e i g h t t o t h e t r a n s i t i o n p r o b a b i l i t y , b u t one a l s o knows t h a t d i f f e r e n t i a t i n g d a t a i t s e l f i n -

JOURNAL DE PHYSIQUE

DISTANCE DOWNSTREAM

Fig.7. Cornbe de dEchoibbance de La UranbLtLon 44 2 ~ 7 / 2 - 4 p 2 P o 3 / 2 danb L a hi?guence Cu 7 . La f d e a p p l h t i e n t 6 R'L v77.

2 2

Decay c w v e 06 Xhe Rhav~sLCian 46 S 7 1 2 - 4 p p o 3 1 2 i n t h e Cul bequence. The h e bdangb a21 & V11.

troduces a d d i t i o n a l u n c e r t a i n t y . I t i s f a r from a t e d n o t o n l y t h e l o w e s t e x c i t e d s t a t e t h e decay o f c l e a r t h a t d i f f e r e n t i a t i o n o f t h e data produces a which i s e x h i b i t e d i n Fig. 9 b u t a l s o o t h e r e x c i t e d m r e s a t i s f a c t o r y number than t h e o r d i n a r y treatment. s t a t e s a l l t h e way up t o t h e i o n i z a t i o n l i m i t . What I mean t o say i s t h a t when L e a v i t t and I cannot

come t o agreement between ourselves about o u r own data, i t i s h a r d l y s u r p r i s i n g t h a t people making separate measurements i n separate 1 a b o r a t o r i e s pub-

Cu lsoslectronic

1 is h numbers which disagree sometimes by 1 arge fac-

..

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t o r s . This s i t u a t i o n leads me t o t h e conclusion t h a t

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i t i s n o t p o s s i b l e t o deduce r e l i a b l e l i f e t i m e n u m bers when t h e r e i s s u b s t a n t i a l c u r v a t u r e i n t h e

decay data, /

F i g u r e 9 shows a d i f f e r e n t s i t u a t i o n . This comes z ^/-

,'

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'

0

5 n i i a

from t h e work c a r r i e d o u t a t Berkeley [ I 1 6 1 on

0 .01 02 D3 .04 .C6

t r a n s i t i o n s i n L i - 1 i k e i r o n . I p r e v i o u s l y showed I/=

t h e spectrum from which t h i s decay curve was obtain-

FQ.

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2 2

ed; t h a t spestrum showed t h a t H e - l i k e i n n was a l s o

6 Vb

^pow

ⁱⁿ

.Ownbition lb S ~ / ~ - 4 p P 0 3 ~ -f.a bEquence AoUe&onique CuI.

produced i n t h i s work. Consequently, i t would n o t be 2

6 -

v b do& .the . k m h % n 46

s ~ ~

ⁱⁿ

t h e ~ - ~ ~ ~ P ~ ~ ~ ~

s u r p r i s i n g t o f i n d t h a t i n L i - l i k e i r o n , we gener- CUI O o d e c t h o n i c

bequence.

account f o r t h e absence o f c u r v a t u r e as w e l l as f o r

DISTANCE (em) 491 MeV ~ e + " m q g / c m 2 C o r k . SOOIL slits

Fig.

9 .

Cowtbe de dEorrohbance de l a .txanbfion

2 2

24 S J / ~ - 2 p p 0 1 / 2

poun Fe avec

3

Uecthorts.

Decay ewve

06

the tuwaition

Zb 2

S ~ / ~ - Z P % ~ J / ~ doh

3

decfrton Fe.

Therefore i t would again n o t be s u r p r i s i n g t o f i n d c u r v a t u r e f o r t h i s decay because t h e l o w e s t e x c i t e d s t a t e i s i n v o l v e d , h i g h e r e x c i t e d s t a t e s a r e almost c e r t a i n l y generated, and those h i g h e r s t a t e s must decay f o r t h e most p a r t through t h e 2 P system o f 2 i n t e r e s t . Nonetheless, t h e d a t a are as exponential as one c o u l d ever hope t o f i n d , over an i n t e n s i t y range o f a f a c t o r o f 25. One must t h e r e f i r e ask why i t i s t h a t t h i s l o w - l y i n g s t a t e , and t h i s i s char- a c t e r i s t i c o f many decay measurements, should ex- h i b i t an exponential decay w h i l e h i g h - l y i n g s t a t e s almost w i t h o u t exception show curvature. The propon- ents o f cascade c b n t r i b u t i o n s t o decay curves must

i t s presence, p a r t i c u l a r l y when t h e absence i s sur- p r i s i n g , as i n thSs instance. It i s again my view t h a t t h e reason t h i s decay i s exponential w h i l e those from h i g h e r s t a t e s are n o t comes s i m p l y from the f a c t t h a t t h e y i e l d o f t h i s r a d i a t i o n i s over- whelming, whereas when one l o o k s a t t h e l o w e r y i e l d s

from t h e h i g h e r e x c i t e d s t a t e s , i t i s i n e v i t a b l e t h a t t h e background o r b l e n d i n g which i s i n t r o d u c e d

?!hen t h e s l i t s a r e opened i n t r o d u c e s non-physical c o n t r i b u t i o n s t o t h e decay curve and t h e r e f o r e one has curvature. Thus, I repeat t h a t t h e source o f c u r v a t u r e i s almost c e r t a i n l y n o t cascades b u t i s blending. Others have a l s o o c c a s i o n a l l y blamed b l e n d i n g f o r disputes over l i f e t i m e determinations [125,126].

714 Mev Kr on C Foil 600

I k

Q

51

t

l psec

H

I00 10 20 30

DISTANCE (mm)

F i g .

10. Cowbes de dEchoOdance des

Z u n h i t i a r t s

2 b 2 s 1 / ~ - 2 ~ b 0 ~ / ~ , 3 / 2 pow

Ktc

avec

3 2

Uecthorts.

2

Decay c a v e s

06

f i e

;Dzan.b&v~ 20

S

J 1 2 - 2 p

doh

3

e.tec.&on

DL. 312

C1-134 JOURNAL DE PHYSIQUE

As a f i n a l n o t e on t h i s s u b j e c t , I show F i g . 10 where we see decay curves f r o m t h e Pj12, 2 2

P1 /2 ' n = 2 l e v e l s i n L i - l i k e Kr. F i g u r e 10 comes from work conducted by D i e t r i c h and c o l l a b o r a t o r s and w i 11 be d i s c u s s e d by O i e t r i c h [85] a t t h i s conference. A l l I w i s h t o p o i n t o u t i s t h a t t h e two curves a r e q u i t e e x p o n e n t i a l e x c e p t t h a t t h e decay o f t h e 'plI2 l e v e l shows an anomalous b e h a v i o r n e a r t h e f o i l . I f one wishes t o a s c r i b e t h i s c u r v a t u r e t o t h e e f f e c t s o f cascades one must t h e n a l s o a c c o u n t f o r t h e f a c t t h a t t h e decay o f t h e companion d o u b l e t component shows no such i n f l u e n c e . I b e l i e v e t h a t i t i s n o t p o s s i b l e t o f i n d a l e v e l scheme w h i c h w o u l d p e r m i t cascades t o one component o f t h i s d o u b l e t system and

n o t t o t h e o t h e r . I t seems t o me t h a t t h e most

p l a u s i b l e e x p l a n a t i o n o f t h i s e f f e c t i s t h a t t h e one decay was more a f f e c t e d by b l e n d s f r o m background and perhaps a n o t h e r chance l i n e i n t h e neighborhood t h a n t h e o t h e r .

I t would, I t h i n k , be h e l p f u l t o t h e s u b j e c t o f l i f e t i m e d e t e r m i n a t i o n s if t h o s e o f my c o l l e a g u e s , many o f them i n t h i s room, who s u p p o r t t h e cascade i n t e r p r e t a t i o n o f c u r v a t u r e i n decay d a t a were t o address themselves t o t h e examples I have g i v e n and show how t h e cascade c o n t r i b u t i o n s c o u l d g i v e r i s e t o t h e o b s e r v a t i o n s . F a i l i n g t h a t , I b e l i e v e i t i s f a i r t o c o n c l u d e t h a t o u r e n e r g i e s s h o u l d be d e v o t e d n o t t o t h e a l g e b r a i c a n a l y s i s o f cascades b u t t o im- provements i n t h e s p e c t r o s c o p y i f we a r e t o o b t a i n r e l i a b l e 1 if e t i m e numbers.

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