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Longitudinal polarization of beta-particles from 60Co, 32P and 170Tm
De "Waard, H. Poppema, O.J. 1957
Physica XXIII 597-598
L E T T E R TO T H E E D I T O R
L o n g i t u d i n a l p o l a r i z a t i o n of b e t a - p a r t i c l e s f r o m 6°Co, 32p and ~7°Tm N o n - c o n s e r v a t i o n of p a r i t y in b e t a - d e c a y m a y b e e v i d e n c e d b y a l o n g i t u d i n a l p o l a r i z a t i o n of b e t a - p a r t i c l e s l) 2). I t is c o n c e i v a b l e t h a t t h e d e g r e e . o f p o l a r i z a t i o n d e p e n d s on t h e t y p e of i n t e r a c t i o n i n v o l v e d in t h e decay. B e t a - p o l a r i z a t i o n m e a s u r e m e n t s m a y , t h e r e f o r e , a p a r t f r o m d e m o n s t r a t i n g p a r i t y n o n - c o n s e r v a t i o n , yield i n f o r m a t i o n a b o u t i n t e r a c t i o n in b e t a - d e c a y . W i t h t h i s a i m in m i n d , a series of m e a s u r e m e n t s of t h e l o n g i t u d i n a l p o l a r i z a t i o n of b e t a - p a r t i c l e s h a s b e e n s t a r t e d a t t h i s I n s t i t u t e , t h e first resuIts of w h i c h will b e rep o r t e d in t h i s letter. *) T h e e x p e r i m e n t a l m e t h o d consists of first c o n v e r t i n g t h e l o n g i t u d i n a l p o l a r i z a t i o n of t h e b e t a - p a r t i c l e s i n t o a t r a n s v e r s e one b y d e f l e c t i n g t h e m o v e r a b o u t 90 ° in a n e l e c t r o s t a t i c field, w h i c h also selects a n e n e r g y i n t e r v a l . T h e m e a n r a d i u s of c u r v a t u r e of t h e d e f l e c t o r is 12 cm. T h e deflected e l e c t r o n s are t h e n s c a t t e r e d o v e r large angles in a gold foil, t h e p l a n e of t h e foil b e i n g p e r p e n d i c u l a r to t h e i n c i d e n t b e a m . T h e leftr i g h t a s y m m e t r y of t h e s c a t t e r e d e l e c t r o n s **), w h i c h p r o v i d e s a m e a s u r e for t h e p o l a r i z a t i o n of t h e b e a m , is d e t e c t e d b y t w o s c i n t i l l a t i o n c o u n t e r s p l a c e d d i a m e t r i c a l l y w i t h r e s p e c t to t h e b e a m a n d s u b t e n d i n g s c a t t e r i n g angles f r o m 50 to 87 degrees. Differences d u e to c o u n t e r efficiency a n d g e o m e t r y are e l i m i n a t e d b y c o m p a r i n g t h e a s y m m e t r y w i t h t h a t o b t a i n e d a f t e r r e p l a c i n g t h e gold foil b y a m i c a foil of c o m p a r a b l e s c a t t e r i n g power. F o r t h i s l a t t e r foil t h e a s y m m e t r y due to t h e p o l a r i z a t i o n of t h e i n c i d e n t p a r t i c l e s s h o u l d be small. Moreover, t h e c o u n t e r s c a n be r o t a t e d a r o u n d t h e b e a m , w h i c h e n a b l e d us t o c h e c k t h a t t h e r e is no " f r o n t to b a c k " a s y m m e t r y . I n o r d e r to k e e p m u l t i p l e a n d p l u r a l s c a t t e r i n g effects, w h i c h t e n d to r e d u c e t h e p o l a r i z a t i o n a s y m m e t r y 5), as low as possible t h e gold s c a t t e r i n g foils are m a d e v e r y t h i n (less t h a n 0.1 micron). T h i s implies t h a t o n l y a v e r y s m a l l f r a c t i o n of t h e b e a m is s c a t t e r e d i n t o t h e c o u n t e r s ( a b o u t one p e r t h o u s a n d i n c i d e n t electrons). M u c h care m u s t t h e r e f o r e b e t a k e n t o r e d u c e s p u r i o u s s c a t t e r i n g f r o m baffles a n d t o p r o v i d e effective s h i e l d i n g f r o m g a m m a - r a y s . T y p i c a l l e f t - r i g h t a s y m m e t r i e s m e a s u r e d for 60Co, 32p a n d lV0Tm are g i v e n in t h e Table. B e t a - e n e r g i e s n e a r 168 keV (v/c = 0.66) were selected in e a c h case. T h e a v e r a g e f r o n t to b a c k a s y m m e t r y was f o u n d t o be 0.99 ± 0.03. TABLE L/R (typical run)
Pexp (averaged)
I Ptheor ] (ace. to 2-component ] neutrino theory) ± 0.66 ± 0.66 ] -1
6°Co 32p tT°Tm
1.125 ± 0.03 1.11 ± 0.025 1.10 ± 0.036
- - 0 . 4 9 ± 0.11 - - 0 . 5 0 -± 0.11 - - 0 . 3 5 ~ 0.11
*) Apparently such experiments are being started in several laboratories. While our measurements were in progress, the first positive results were reported by F r a u e n f e l d e r et al. a). Also Wolfens t e i n and Page 4) announce that they are preparing a similar experiment. **) To avoid coufusion we adopt Frauenfelders definition for "left" viz. p3(pl × p2) > O, p t , po andp3 being, respectively, the electron momenta immediately after emission from the source, before scattering and after scattering from the foil.
- - 597
~98 '
LONGITUDINAL POLARIZATION OF BETA-PARTICLES •
T h e degree of p o l a r i z a t i o n Pexp h a s b e e n d e d u c e d f r o m t h e a s y m m e t r i e s u s i n g t h e expression P ~ (1/a)[L/R1 ) / ( L / R + 1)]sec ~0, w h e r e ~ is t h e a v e r a g e p o l a r i z a t i o n a s y m m e t r y f a c t o r for t h e g e o m e t r y used 6) ( m e a s u r e m e n t s were m a d e p a r t l y w i t h = 0.12, p a r t l y w i t h ~ = 0.152) a n d T is t h e a n g l e b e t w e e n e l e c t r o n spill a n d m o m e n t u m a f t e r deflection (in o u r case 9 = 220) • F r o m t h e s e results, t h e following c o n c l u s i o n s c a n b e d r a w n : 1) T h e a b s o l u t e v a l u e of t h e degree of p o l a r i z a t i o n for 60Co a n d 32p a p p e a r s t o b e s o m e w h a t s m a l l e r t h a n Iv~el, t h e v a l u e e x p e c t e d o n t h e basis of Y a n g a n d L e e ' s t w o c o m p o n e n t t h e o r y of t h e n e u t r i n o for allowed t r a n s i t i o n s like t h o s e i n v o l v e d here. T h i s m a y b e d u e t o d e p o l a r i z a t i o n effects in t h e s c a t t e r i n g foil. 2) T h e n e g a t i v e sign of P shows t h a t spin a n d m o m e n t u m d i r e c t i o n s of t h e e m i t t e d e l e c t r o n s are opppSite b o t h for t h e allowed d e c a y s of 60Co a n d 3~p a n d for t h e first o r d e r f o r b i d d e n "~eca/ of 17°Tin. 3) I t c a n n o t b e e:,.cluded t h a t t h e effect for 170Tm h a s t h e s a m e m a g n i t u d e as for t h e allowed t r a n s i t i o n s ; t h e s o m e w h a t s m a l l e r v a l u e of Pexp for t h i s case m a y well b e c a u s e d b y t h e f a c t t h a t a r a t h e r t h i c k source ( a b o u t 1 m g / c m 2) h a d to b e used here, in w h i c h m o r e d e p o l a r i z a t i o n m a y h a v e o c c u r r e d t h a n in t h e v e r y t h i n sources of 32p a n d 60Co t h a t were available. A good e v a l u a t i o n of d e p o l a r i z a t i o n effects is e s s e n t i a l for m a k i n g t h e m e t h o d d e s c r i b e d a b o v e a useful tool for i n v e s t i g a t i n g i n t e r a c t i o n in b e t a - d e c a y . T h i s is n o w t h e first o b j e c t of o u r f u r t h e r i n v e s t i g a t i o n s . T h e a u t h o r s are m u c h i n d e b t e d to Mr. J. v a n K l i n k e n for his u n f a i l i n g a s s i s t a n c e in t h e course of t h e s e e x p e r i m e n t s . O u r sincere t h a n k s are d u e to Prof. FI. B r i n k m a n, D i r e c t o r of t h i s I n s t i t u t e , for his s t i m u l a t i n g i n t e r e s t . O n e of us (O.J.P.) is i n d e b t e d to t h e m a n a g e m e n t of t h e K o n i n k l i j k e / S h e l l L a b o r a t o r i u m a t A m s t e r d a m for g r a n t i n g p e r m i s s i o n to t a k e p a r t in t h i s work. T h i s i n v e s t i g a t i o n is in p a r t s u p p o r t e d b y t h e N e t h e r l a n d s O x g a n i z a t i o n for P u r e R e s e a r c h (Z.'W.O.) t h r o u g h t h e " S t i c h t i n g v o o r F u n d a m e n t e e l O n d e r z o e k d e r M a t e r i e " (F.O.M.). H. DE "VVAAI~I) O. J. POPPEMA ReceivEd 13-5-57.
Natuu. l~undig Laboratorium, Rijksuniversiteit, Groningen, Nederland
N o t e added i'n proof: R e c e n t l y we also m e a s u r e d a degree of p o l a r i z a t i o n _P = -- 0.16 :t: 0.03 for 168 keV b e t a - p a r t i c l e s f r o m a t h i n source of 19SAu. T h e m u c h s m a l l e r a b s o l u t e v a l u e of t h e d e g r e e of p o l a r i z a t i o n in t h i s case, w h i c h c a n h a r d l y b e reconciled w i t h t h e c a l c u l a t i o n s of Alder, S t e c h a n d W i n t h e r (to b e p u b l i s h e d ) for f i r s t f o r b i d d e n t r a n s i t i o n s , seems t o i n d i c a t e t h e p r e s , n c e of V- or A - i n t e r a c t i o n . REFERENCES 1) 2} 3)" 4) 5) 6)
Lee, T. D. and Yang, C. N'., Phys. Rex,. 105 (1957) 1671. J a c k s o n , J. D., T r e i m a n , S. B. and \ ¥ y l d , H. W. to be published. F r a u e n f e l ' d e r et al., to be published in the Phys. Rev, W o l f e n s t e i n , L. and Page, L., Bull. Am. phys. Soc. 2 (1957) 190. ef. T o l h o e k , H. A., Revs. mod. Phys. 28 (1956) 277. polarization asymmetry factors have been taken from the work of S h e r m a n , N., Phys. Rev. 103 (1956) 1601.
Physica XXIII 597-598
L E T T E R TO T H E E D I T O R
L o n g i t u d i n a l p o l a r i z a t i o n of b e t a - p a r t i c l e s f r o m 6°Co, 32p and ~7°Tm N o n - c o n s e r v a t i o n of p a r i t y in b e t a - d e c a y m a y b e e v i d e n c e d b y a l o n g i t u d i n a l p o l a r i z a t i o n of b e t a - p a r t i c l e s l) 2). I t is c o n c e i v a b l e t h a t t h e d e g r e e . o f p o l a r i z a t i o n d e p e n d s on t h e t y p e of i n t e r a c t i o n i n v o l v e d in t h e decay. B e t a - p o l a r i z a t i o n m e a s u r e m e n t s m a y , t h e r e f o r e , a p a r t f r o m d e m o n s t r a t i n g p a r i t y n o n - c o n s e r v a t i o n , yield i n f o r m a t i o n a b o u t i n t e r a c t i o n in b e t a - d e c a y . W i t h t h i s a i m in m i n d , a series of m e a s u r e m e n t s of t h e l o n g i t u d i n a l p o l a r i z a t i o n of b e t a - p a r t i c l e s h a s b e e n s t a r t e d a t t h i s I n s t i t u t e , t h e first resuIts of w h i c h will b e rep o r t e d in t h i s letter. *) T h e e x p e r i m e n t a l m e t h o d consists of first c o n v e r t i n g t h e l o n g i t u d i n a l p o l a r i z a t i o n of t h e b e t a - p a r t i c l e s i n t o a t r a n s v e r s e one b y d e f l e c t i n g t h e m o v e r a b o u t 90 ° in a n e l e c t r o s t a t i c field, w h i c h also selects a n e n e r g y i n t e r v a l . T h e m e a n r a d i u s of c u r v a t u r e of t h e d e f l e c t o r is 12 cm. T h e deflected e l e c t r o n s are t h e n s c a t t e r e d o v e r large angles in a gold foil, t h e p l a n e of t h e foil b e i n g p e r p e n d i c u l a r to t h e i n c i d e n t b e a m . T h e leftr i g h t a s y m m e t r y of t h e s c a t t e r e d e l e c t r o n s **), w h i c h p r o v i d e s a m e a s u r e for t h e p o l a r i z a t i o n of t h e b e a m , is d e t e c t e d b y t w o s c i n t i l l a t i o n c o u n t e r s p l a c e d d i a m e t r i c a l l y w i t h r e s p e c t to t h e b e a m a n d s u b t e n d i n g s c a t t e r i n g angles f r o m 50 to 87 degrees. Differences d u e to c o u n t e r efficiency a n d g e o m e t r y are e l i m i n a t e d b y c o m p a r i n g t h e a s y m m e t r y w i t h t h a t o b t a i n e d a f t e r r e p l a c i n g t h e gold foil b y a m i c a foil of c o m p a r a b l e s c a t t e r i n g power. F o r t h i s l a t t e r foil t h e a s y m m e t r y due to t h e p o l a r i z a t i o n of t h e i n c i d e n t p a r t i c l e s s h o u l d be small. Moreover, t h e c o u n t e r s c a n be r o t a t e d a r o u n d t h e b e a m , w h i c h e n a b l e d us t o c h e c k t h a t t h e r e is no " f r o n t to b a c k " a s y m m e t r y . I n o r d e r to k e e p m u l t i p l e a n d p l u r a l s c a t t e r i n g effects, w h i c h t e n d to r e d u c e t h e p o l a r i z a t i o n a s y m m e t r y 5), as low as possible t h e gold s c a t t e r i n g foils are m a d e v e r y t h i n (less t h a n 0.1 micron). T h i s implies t h a t o n l y a v e r y s m a l l f r a c t i o n of t h e b e a m is s c a t t e r e d i n t o t h e c o u n t e r s ( a b o u t one p e r t h o u s a n d i n c i d e n t electrons). M u c h care m u s t t h e r e f o r e b e t a k e n t o r e d u c e s p u r i o u s s c a t t e r i n g f r o m baffles a n d t o p r o v i d e effective s h i e l d i n g f r o m g a m m a - r a y s . T y p i c a l l e f t - r i g h t a s y m m e t r i e s m e a s u r e d for 60Co, 32p a n d lV0Tm are g i v e n in t h e Table. B e t a - e n e r g i e s n e a r 168 keV (v/c = 0.66) were selected in e a c h case. T h e a v e r a g e f r o n t to b a c k a s y m m e t r y was f o u n d t o be 0.99 ± 0.03. TABLE L/R (typical run)
Pexp (averaged)
I Ptheor ] (ace. to 2-component ] neutrino theory) ± 0.66 ± 0.66 ] -1
6°Co 32p tT°Tm
1.125 ± 0.03 1.11 ± 0.025 1.10 ± 0.036
- - 0 . 4 9 ± 0.11 - - 0 . 5 0 -± 0.11 - - 0 . 3 5 ~ 0.11
*) Apparently such experiments are being started in several laboratories. While our measurements were in progress, the first positive results were reported by F r a u e n f e l d e r et al. a). Also Wolfens t e i n and Page 4) announce that they are preparing a similar experiment. **) To avoid coufusion we adopt Frauenfelders definition for "left" viz. p3(pl × p2) > O, p t , po andp3 being, respectively, the electron momenta immediately after emission from the source, before scattering and after scattering from the foil.
- - 597
~98 '
LONGITUDINAL POLARIZATION OF BETA-PARTICLES •
T h e degree of p o l a r i z a t i o n Pexp h a s b e e n d e d u c e d f r o m t h e a s y m m e t r i e s u s i n g t h e expression P ~ (1/a)[L/R1 ) / ( L / R + 1)]sec ~0, w h e r e ~ is t h e a v e r a g e p o l a r i z a t i o n a s y m m e t r y f a c t o r for t h e g e o m e t r y used 6) ( m e a s u r e m e n t s were m a d e p a r t l y w i t h = 0.12, p a r t l y w i t h ~ = 0.152) a n d T is t h e a n g l e b e t w e e n e l e c t r o n spill a n d m o m e n t u m a f t e r deflection (in o u r case 9 = 220) • F r o m t h e s e results, t h e following c o n c l u s i o n s c a n b e d r a w n : 1) T h e a b s o l u t e v a l u e of t h e degree of p o l a r i z a t i o n for 60Co a n d 32p a p p e a r s t o b e s o m e w h a t s m a l l e r t h a n Iv~el, t h e v a l u e e x p e c t e d o n t h e basis of Y a n g a n d L e e ' s t w o c o m p o n e n t t h e o r y of t h e n e u t r i n o for allowed t r a n s i t i o n s like t h o s e i n v o l v e d here. T h i s m a y b e d u e t o d e p o l a r i z a t i o n effects in t h e s c a t t e r i n g foil. 2) T h e n e g a t i v e sign of P shows t h a t spin a n d m o m e n t u m d i r e c t i o n s of t h e e m i t t e d e l e c t r o n s are opppSite b o t h for t h e allowed d e c a y s of 60Co a n d 3~p a n d for t h e first o r d e r f o r b i d d e n "~eca/ of 17°Tin. 3) I t c a n n o t b e e:,.cluded t h a t t h e effect for 170Tm h a s t h e s a m e m a g n i t u d e as for t h e allowed t r a n s i t i o n s ; t h e s o m e w h a t s m a l l e r v a l u e of Pexp for t h i s case m a y well b e c a u s e d b y t h e f a c t t h a t a r a t h e r t h i c k source ( a b o u t 1 m g / c m 2) h a d to b e used here, in w h i c h m o r e d e p o l a r i z a t i o n m a y h a v e o c c u r r e d t h a n in t h e v e r y t h i n sources of 32p a n d 60Co t h a t were available. A good e v a l u a t i o n of d e p o l a r i z a t i o n effects is e s s e n t i a l for m a k i n g t h e m e t h o d d e s c r i b e d a b o v e a useful tool for i n v e s t i g a t i n g i n t e r a c t i o n in b e t a - d e c a y . T h i s is n o w t h e first o b j e c t of o u r f u r t h e r i n v e s t i g a t i o n s . T h e a u t h o r s are m u c h i n d e b t e d to Mr. J. v a n K l i n k e n for his u n f a i l i n g a s s i s t a n c e in t h e course of t h e s e e x p e r i m e n t s . O u r sincere t h a n k s are d u e to Prof. FI. B r i n k m a n, D i r e c t o r of t h i s I n s t i t u t e , for his s t i m u l a t i n g i n t e r e s t . O n e of us (O.J.P.) is i n d e b t e d to t h e m a n a g e m e n t of t h e K o n i n k l i j k e / S h e l l L a b o r a t o r i u m a t A m s t e r d a m for g r a n t i n g p e r m i s s i o n to t a k e p a r t in t h i s work. T h i s i n v e s t i g a t i o n is in p a r t s u p p o r t e d b y t h e N e t h e r l a n d s O x g a n i z a t i o n for P u r e R e s e a r c h (Z.'W.O.) t h r o u g h t h e " S t i c h t i n g v o o r F u n d a m e n t e e l O n d e r z o e k d e r M a t e r i e " (F.O.M.). H. DE "VVAAI~I) O. J. POPPEMA ReceivEd 13-5-57.
Natuu. l~undig Laboratorium, Rijksuniversiteit, Groningen, Nederland
N o t e added i'n proof: R e c e n t l y we also m e a s u r e d a degree of p o l a r i z a t i o n _P = -- 0.16 :t: 0.03 for 168 keV b e t a - p a r t i c l e s f r o m a t h i n source of 19SAu. T h e m u c h s m a l l e r a b s o l u t e v a l u e of t h e d e g r e e of p o l a r i z a t i o n in t h i s case, w h i c h c a n h a r d l y b e reconciled w i t h t h e c a l c u l a t i o n s of Alder, S t e c h a n d W i n t h e r (to b e p u b l i s h e d ) for f i r s t f o r b i d d e n t r a n s i t i o n s , seems t o i n d i c a t e t h e p r e s , n c e of V- or A - i n t e r a c t i o n . REFERENCES 1) 2} 3)" 4) 5) 6)
Lee, T. D. and Yang, C. N'., Phys. Rex,. 105 (1957) 1671. J a c k s o n , J. D., T r e i m a n , S. B. and \ ¥ y l d , H. W. to be published. F r a u e n f e l ' d e r et al., to be published in the Phys. Rev, W o l f e n s t e i n , L. and Page, L., Bull. Am. phys. Soc. 2 (1957) 190. ef. T o l h o e k , H. A., Revs. mod. Phys. 28 (1956) 277. polarization asymmetry factors have been taken from the work of S h e r m a n , N., Phys. Rev. 103 (1956) 1601.