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Parity non-conservation in the capture of polarized thermal neutrons
621B
Volume 29B, number 9
PHYSICS
LETTERS
4 August 1969
.
PARITY
NON-CONSERVATION POLARIZED THERMAL
IN THE CAPTURE NEUTRONS
OF
Elisabeth WARMING A. E. X. Research
Establishment
Rid,
Denmark
Received 19 June 1969
The asymmetry in the intensity of y-radiation following the capture of polarized thermal neutrons in l13Cd has been measured with Ge(Li) detectors. The result, A = (-0.6 i 1.8) x 10-4, like that previously reported [l], gives no evidence for a non-zero effect.
There is still no definite determination of a parity non-conserving internucleon potential measured by the capture of polarized neutrons. A new experiment has recently been reported by Abov et al. [3], but an asymmetry as large as indicated by their results [2,3] is obtained neither in the present experiment nor in our previous [l] one (see table 2). The present determination of the asymmetry in the y-ray intensity of the capture of polarized thermal neutrons in 113Cd is essentially a repetition of our earlier experiment [l], with most of the equipment improved substantially. The most important new feature was the use of Ge(Li) detectors. The resolution was 50 keV at 9 MeV for the present measurements (of 2 days duration each). With a knowledge of the response function of the detectors [7] it is easier to distinguish between transitions than was previously possible with NaI(T1) crystals. The second escape peak is the most intense in the relevant energy region, and if it is used for the evaluation of the asymmetry of the 9.04 MeV transition, mixing of the 8.48 MeV transition cannot be avoided, but this mixing is as low as 5% (cf. table 2, where correction for the mixing is carried out). A 4096-channel analyser was used for the analysis of the spectrum, and the spectrum was cut at the energies 6.30, 6.80, 7.40, ‘7,99, 9.15 and 10.35 MeV. Starting from the highest energies (see fig. 1) contain a) background, b) photopeak, 1st and 2nd escapepeaks of the 9.04 MeV transition, plus photopeak of the 8.48 MeV transition, c) Compton background of the 9.04 MeV transition, plus different contributions from the 8.48, 7.83, 7.74 and 7.68 MeV transitions, d) and e) contributions from many different transitions. 564
The experimental equipment has been further improved by a new collimator system, with an increased neutron flux density of (4.0 f 0.4) x x 107 cm-%-l and a neutron polarization of 88 f f 3%. If the capture level of 114Cd contains a parity non-conserving potential it will cause a nonisotropic angular distribution for the 9.04 MeV ground-state transition. The expected distribution is of the form: W(e) = W,(l +A COE0) , where only the first terms of the power series have been taken into account; 0 is the angle between the neutron polarization and the direction of the emitted y-ray; A is given by [4]: 7MPv
L
6MeV
9Mev
400 Channel no.
Fig. 1. Gamma-ray spectrum of l14Cd corrected for background and with the energy intervals for the evaluation indicated.
Volume 29B, number 9
PHYSICS LETTERS
A = Pn A' = 2PnCFI(LL,.TfJ c) (p_o/(/mc) , r ~ 2
(pc)'+(pnc) where Pn is the degree of polarization of the neutrons, C is the orientation parameter of the capture state [4], FI(LL]fJ c) is the coefficient for the angular correlation [5], L is the multlpolarity of the T-ray, Jc and Jf are the spins of the compound and final levels, respectively, and for the matrix elements we have (pnc) << (Pc) giving the ratio (pc) (pnc) / ((Pc)2 + ~nc) 2) ~ ~nc) / (pc) , such that A is proportional to the admixture of the pnc potential. • W h e n W(O) is measured for 0 equal to 0 and ~, the asymmetry can be found from:
i l a r way not s e e n to give any o b s e r v a b l e change inA. To avoid the i n f l u en ce of changes in the n e u t r o n flux a 3He b e a m m o n i t o r was u s e d to c o n t r o l the r e v e r s a l of the n eu t r o n p o l a r i z a t i o n . Th e counting r a t e in the m o n i t o r w as 2000 s -1. Th e angle 0 w as 0 o r ~ + 0.1 r a d , due to the s i z e s of the t a r g e t , the d i m e n s i o n s of the lead c o l l i m a t o r s around the A l - f i l t e r s in f r o n t of the d e t e c t o r s and the s i z e of the d e t e c t o r s ; this g i v e s a c o r r e c t i o n of l e s s than 1%.
Table 1 The measured asymmetries. 6.30- 6.80 6.80- 7.40 7.40- 7.99 7.99- 9.15 9.15-10.35
A' = (W(0) - W(~))/(W(O) + W(~)). The following procedure was used: first, W(0) and W(~) were measured while the direction of the neutron polarization was reversed about every 20 s. To allow for possible asymmetry of the equipment, the b e a m was then depolarized and W(0) and W(~) remeasured. The value, Ao, obtained in this way was used as zero, giving the parity asymmetry A = A ' - A o . The asymmetry of the background was measured frequently by the same procedure as for the parity asymmetry, but with the Cd sample removed. Background due to neutrons scattered in the sample is not taken into account, since the scattering cross section is only about 0.3% of the absorption cross section. The background correction was carried out separately for the measurements with polarized and depolarized beams as follows: A" = A'(1
-B'b/A')/(1
4 August 1969
MeV: MeV: MeV: MeV: MeV:
(0.5:e0.9) (-1.9~1.0) (2.1-1.5) (-0.6~1.7) (-2.8,4.9)
x10 -4 × 10-4: × 10-4 , x 10-4 , x 10-4 .
F o r c o m p a r i s o n with our p r e v i o u s r e s u l t [1] and t h o se of Abov et al. [2, 3] it i s n e c e s s a r y to c a r r y out the c o r r e c t i o n f o r the influence of t h e 8.48 MeV t r a n s i t i o n on the data f o r the 9.04 MeV t r a n s i t i o n . In t ab l e 2 a r e l i s t e d the four r e s u l t s . The r e l a t i v e intensity, b, of the 8.48 MeV t r a n s i tion is e s t i m a t e d f r o m the r e s p o n s e functions f o r the NaI(T1) [6] and Ge(Li) [7] d e t e c t o r s . The t ab l e also contains the r e s u l t s c o r r e c t e d f o r the 8.48 MeV ' b a c k g r o u n d ' , f o r which B is t h e o r e t i c a l l y z e r o (the 8.48 MeV t r a n s i t i o n is an M 1 / E 2 m i x e d transition)~ and in the l a s t column, the likelihood (by a x 2 - t e s t ) f o r the r e s u l t b ei n g c o n s i s t e n t with z e r o .
-b) ,
w h e r e A ' i s the d e t e r m i n e d a s y m m e t r y , B' is t he c o r r e s p o n d i n g a s y m m e t r y in the background and b i s the b a c k g r o u n d i n t e n s it y r e l a t i v e to the spectrum. The n e u t r o n p o l a r i z a t i o n was found to be 0.88 ± 0.03 by m e a n s of a m e a s u r e m e n t of the c i r c u l a r p o l a r i z a t i o n of the 5.43 MeV t r a n s i t i o n in 33S. The p o l a r i z a t i o n of the d e p o l a r i z e d b e a m w a s in the s a m e way m e a s u r e d to b e 0.00 ± 0.03. T h e g e o m e t r y is not c o m p l e t e l y th e s a m e f o r the two n eu t ro n p o l a r i z a t i o n s , b e c a u s e of the f i e l d f l i p p e r , which might give r i s e to spin f l i p ping. In o r d e r to d e t e r m i n e the i n f l u e n c e of a p o s s i b l e d i f f e r e n c e in the two n e u tr o n p o l a r i z a ti ons (for 0 = 0 and ~), the c a l c u I a t i o n s w e r e c a r r i e d through with P(0) = 0.88 and P(y) = P(0) + 0.03. T h e v a r i a t i o n found f o r A was l e s s than 2%. T h e c o r r e s p o n d i n g d i f f e r e n c e in the p o l a r i z a t i o n of the d e p o l a r i z e d b e a m w a s in a s i r e -
Table 2 Comparison of previous and present results. Asymmetry Reference determined (× 10-4) [2]
[3] [1] Present result
b*)
Asymmetry Likelihood corrected (× 10-4) for zero
-3.7 + 0.9 -3.5 ~- 1.2 -2.5 ± 2.2
0.40 0.20 0.00
-6.1 :e 1.5 -4.4 • 1.5 -2.5 ~- 2.2
< 0.1% 0.2% 25%
-0.6 :e 1.7
0.05
-0.6 ± 1.8
75%
*) See text. F r o m t ab l e 2 it i s s e e n that only the f i r s t m e a s u r e m e n t of Abov et al. [2] i n d i c a t e s a n o n z e r o effect within the 0.1% confidence l i m i t . The m e a s u r e m e n t s t o g e t h e r do not i n d i c a t e an a s y m m e t r y as d i s t i n c t as that shown by the f i r s t m e a s u r e m e n t of Abov et al.
565
Votume 29B, number 9
PHYSICS
LETTERS
I w i s h to thank D r . J i ~ i K o p e c k y f o r m a n y useful discussions.
2. Y.G. Abov, P.A. Krupchitsky and Yu. A. Oretovsky, Phys. Letters 12 (1964) 25. 3. Y.G. Abov et al., Phys. Letters 27B (1968) 16. 4. R . J . B[in-Stoyle, Phys. Rev. 120 (1960) 131. 5. A.H. Wapstra, G . J . Nijgh and R. Van Lieshout, Nuc l e a r Spectroscopy Tables (North-Holland, A m s t e r dam, 1959). 6. L . J a r c z y k et al., Nucl. Instr. Meth. 17 (1962) 310. 7. J. Kopeck3~, W. P~tynski and E. W~rming, Nucl. ~nstr. Meth. 50 (1967) 333,
R e f e ~'enc e s
1. E. Warming, F. Stecher-Rasmussen, W. Ratynski and J.Kopeck~, Phys. Letters 25B (1967) 200, * * * * *
566
4 August 1969
Volume 29B, number 9
PHYSICS
LETTERS
4 August 1969
.
PARITY
NON-CONSERVATION POLARIZED THERMAL
IN THE CAPTURE NEUTRONS
OF
Elisabeth WARMING A. E. X. Research
Establishment
Rid,
Denmark
Received 19 June 1969
The asymmetry in the intensity of y-radiation following the capture of polarized thermal neutrons in l13Cd has been measured with Ge(Li) detectors. The result, A = (-0.6 i 1.8) x 10-4, like that previously reported [l], gives no evidence for a non-zero effect.
There is still no definite determination of a parity non-conserving internucleon potential measured by the capture of polarized neutrons. A new experiment has recently been reported by Abov et al. [3], but an asymmetry as large as indicated by their results [2,3] is obtained neither in the present experiment nor in our previous [l] one (see table 2). The present determination of the asymmetry in the y-ray intensity of the capture of polarized thermal neutrons in 113Cd is essentially a repetition of our earlier experiment [l], with most of the equipment improved substantially. The most important new feature was the use of Ge(Li) detectors. The resolution was 50 keV at 9 MeV for the present measurements (of 2 days duration each). With a knowledge of the response function of the detectors [7] it is easier to distinguish between transitions than was previously possible with NaI(T1) crystals. The second escape peak is the most intense in the relevant energy region, and if it is used for the evaluation of the asymmetry of the 9.04 MeV transition, mixing of the 8.48 MeV transition cannot be avoided, but this mixing is as low as 5% (cf. table 2, where correction for the mixing is carried out). A 4096-channel analyser was used for the analysis of the spectrum, and the spectrum was cut at the energies 6.30, 6.80, 7.40, ‘7,99, 9.15 and 10.35 MeV. Starting from the highest energies (see fig. 1) contain a) background, b) photopeak, 1st and 2nd escapepeaks of the 9.04 MeV transition, plus photopeak of the 8.48 MeV transition, c) Compton background of the 9.04 MeV transition, plus different contributions from the 8.48, 7.83, 7.74 and 7.68 MeV transitions, d) and e) contributions from many different transitions. 564
The experimental equipment has been further improved by a new collimator system, with an increased neutron flux density of (4.0 f 0.4) x x 107 cm-%-l and a neutron polarization of 88 f f 3%. If the capture level of 114Cd contains a parity non-conserving potential it will cause a nonisotropic angular distribution for the 9.04 MeV ground-state transition. The expected distribution is of the form: W(e) = W,(l +A COE0) , where only the first terms of the power series have been taken into account; 0 is the angle between the neutron polarization and the direction of the emitted y-ray; A is given by [4]: 7MPv
L
6MeV
9Mev
400 Channel no.
Fig. 1. Gamma-ray spectrum of l14Cd corrected for background and with the energy intervals for the evaluation indicated.
Volume 29B, number 9
PHYSICS LETTERS
A = Pn A' = 2PnCFI(LL,.TfJ c) (p_o/(/mc) , r ~ 2
(pc)'+(pnc) where Pn is the degree of polarization of the neutrons, C is the orientation parameter of the capture state [4], FI(LL]fJ c) is the coefficient for the angular correlation [5], L is the multlpolarity of the T-ray, Jc and Jf are the spins of the compound and final levels, respectively, and for the matrix elements we have (pnc) << (Pc) giving the ratio (pc) (pnc) / ((Pc)2 + ~nc) 2) ~ ~nc) / (pc) , such that A is proportional to the admixture of the pnc potential. • W h e n W(O) is measured for 0 equal to 0 and ~, the asymmetry can be found from:
i l a r way not s e e n to give any o b s e r v a b l e change inA. To avoid the i n f l u en ce of changes in the n e u t r o n flux a 3He b e a m m o n i t o r was u s e d to c o n t r o l the r e v e r s a l of the n eu t r o n p o l a r i z a t i o n . Th e counting r a t e in the m o n i t o r w as 2000 s -1. Th e angle 0 w as 0 o r ~ + 0.1 r a d , due to the s i z e s of the t a r g e t , the d i m e n s i o n s of the lead c o l l i m a t o r s around the A l - f i l t e r s in f r o n t of the d e t e c t o r s and the s i z e of the d e t e c t o r s ; this g i v e s a c o r r e c t i o n of l e s s than 1%.
Table 1 The measured asymmetries. 6.30- 6.80 6.80- 7.40 7.40- 7.99 7.99- 9.15 9.15-10.35
A' = (W(0) - W(~))/(W(O) + W(~)). The following procedure was used: first, W(0) and W(~) were measured while the direction of the neutron polarization was reversed about every 20 s. To allow for possible asymmetry of the equipment, the b e a m was then depolarized and W(0) and W(~) remeasured. The value, Ao, obtained in this way was used as zero, giving the parity asymmetry A = A ' - A o . The asymmetry of the background was measured frequently by the same procedure as for the parity asymmetry, but with the Cd sample removed. Background due to neutrons scattered in the sample is not taken into account, since the scattering cross section is only about 0.3% of the absorption cross section. The background correction was carried out separately for the measurements with polarized and depolarized beams as follows: A" = A'(1
-B'b/A')/(1
4 August 1969
MeV: MeV: MeV: MeV: MeV:
(0.5:e0.9) (-1.9~1.0) (2.1-1.5) (-0.6~1.7) (-2.8,4.9)
x10 -4 × 10-4: × 10-4 , x 10-4 , x 10-4 .
F o r c o m p a r i s o n with our p r e v i o u s r e s u l t [1] and t h o se of Abov et al. [2, 3] it i s n e c e s s a r y to c a r r y out the c o r r e c t i o n f o r the influence of t h e 8.48 MeV t r a n s i t i o n on the data f o r the 9.04 MeV t r a n s i t i o n . In t ab l e 2 a r e l i s t e d the four r e s u l t s . The r e l a t i v e intensity, b, of the 8.48 MeV t r a n s i tion is e s t i m a t e d f r o m the r e s p o n s e functions f o r the NaI(T1) [6] and Ge(Li) [7] d e t e c t o r s . The t ab l e also contains the r e s u l t s c o r r e c t e d f o r the 8.48 MeV ' b a c k g r o u n d ' , f o r which B is t h e o r e t i c a l l y z e r o (the 8.48 MeV t r a n s i t i o n is an M 1 / E 2 m i x e d transition)~ and in the l a s t column, the likelihood (by a x 2 - t e s t ) f o r the r e s u l t b ei n g c o n s i s t e n t with z e r o .
-b) ,
w h e r e A ' i s the d e t e r m i n e d a s y m m e t r y , B' is t he c o r r e s p o n d i n g a s y m m e t r y in the background and b i s the b a c k g r o u n d i n t e n s it y r e l a t i v e to the spectrum. The n e u t r o n p o l a r i z a t i o n was found to be 0.88 ± 0.03 by m e a n s of a m e a s u r e m e n t of the c i r c u l a r p o l a r i z a t i o n of the 5.43 MeV t r a n s i t i o n in 33S. The p o l a r i z a t i o n of the d e p o l a r i z e d b e a m w a s in the s a m e way m e a s u r e d to b e 0.00 ± 0.03. T h e g e o m e t r y is not c o m p l e t e l y th e s a m e f o r the two n eu t ro n p o l a r i z a t i o n s , b e c a u s e of the f i e l d f l i p p e r , which might give r i s e to spin f l i p ping. In o r d e r to d e t e r m i n e the i n f l u e n c e of a p o s s i b l e d i f f e r e n c e in the two n e u tr o n p o l a r i z a ti ons (for 0 = 0 and ~), the c a l c u I a t i o n s w e r e c a r r i e d through with P(0) = 0.88 and P(y) = P(0) + 0.03. T h e v a r i a t i o n found f o r A was l e s s than 2%. T h e c o r r e s p o n d i n g d i f f e r e n c e in the p o l a r i z a t i o n of the d e p o l a r i z e d b e a m w a s in a s i r e -
Table 2 Comparison of previous and present results. Asymmetry Reference determined (× 10-4) [2]
[3] [1] Present result
b*)
Asymmetry Likelihood corrected (× 10-4) for zero
-3.7 + 0.9 -3.5 ~- 1.2 -2.5 ± 2.2
0.40 0.20 0.00
-6.1 :e 1.5 -4.4 • 1.5 -2.5 ~- 2.2
< 0.1% 0.2% 25%
-0.6 :e 1.7
0.05
-0.6 ± 1.8
75%
*) See text. F r o m t ab l e 2 it i s s e e n that only the f i r s t m e a s u r e m e n t of Abov et al. [2] i n d i c a t e s a n o n z e r o effect within the 0.1% confidence l i m i t . The m e a s u r e m e n t s t o g e t h e r do not i n d i c a t e an a s y m m e t r y as d i s t i n c t as that shown by the f i r s t m e a s u r e m e n t of Abov et al.
565
Votume 29B, number 9
PHYSICS
LETTERS
I w i s h to thank D r . J i ~ i K o p e c k y f o r m a n y useful discussions.
2. Y.G. Abov, P.A. Krupchitsky and Yu. A. Oretovsky, Phys. Letters 12 (1964) 25. 3. Y.G. Abov et al., Phys. Letters 27B (1968) 16. 4. R . J . B[in-Stoyle, Phys. Rev. 120 (1960) 131. 5. A.H. Wapstra, G . J . Nijgh and R. Van Lieshout, Nuc l e a r Spectroscopy Tables (North-Holland, A m s t e r dam, 1959). 6. L . J a r c z y k et al., Nucl. Instr. Meth. 17 (1962) 310. 7. J. Kopeck3~, W. P~tynski and E. W~rming, Nucl. ~nstr. Meth. 50 (1967) 333,
R e f e ~'enc e s
1. E. Warming, F. Stecher-Rasmussen, W. Ratynski and J.Kopeck~, Phys. Letters 25B (1967) 200, * * * * *
566
4 August 1969