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A measurement of parity non-conserving neutron spin rotation in lead and tin
Volume 119B, number 4,5,6
PHYSICS LETTERS
23•30 December 1982
A MEASUREMENT OF PARITY NON-CONSERVING NEUTRON SPIN ROTATION IN LEAD AND TIN B. HECKEL and N.F. RAMSEY Harvard University, Cambridge, MA 02138, USA K. GREEN and G.L. GREENE 1 Rutherford-Appleton Laboratory, Chilton, Oxon 0 X l l OQX, UK R. Gti,HLER 2 and O. SCHAERPF lnstitut Laue-Langevin, 38042 Grenoble, France M. FORTE Physics Division, Joint Research Council, 1-21020 Ispra, ltaly W. DRESS and P.D. MILLER Oak Ridge National Laboratory, Oak Ridge, TAr 37830, USA R. GOLUB Technical University Munich, 8046 Garching, West Germany and
J. BYRNE and J.M. PENDLEBURY University o f Sussex, Brighton, BN1 9QH Sussex, UK Received 24 August 1982
The rotation, ~PNC, of a neutron beam polarization vector due to parity non-conserving forces is observed in natural Pb and Sn targets. The following values for ~PNC/I in units of 10 -6 rad/cm are found: Pb: +(2.24 ± 0.33), and Sn: -(3.19 ± 0.40). A positive sign corresponds to a right-handed rotation of the neutron spin about its momentum.
The experimental determination o f the nature and strength o f the weak interaction between nucleons has yet to be fully accomplished. The search for parity nonconserving (PNC) effects in n u c l e o n - n u c l e o n scattering has been limited to the p r o t o n - p r o t o n system and only recently has begun to show non-zero results [ 1 3]. A much richer testing ground is the study of PNC i Current address: Physics Dept., Yale University, New Haven, CN, USA. 2 Current address: Technical University Munich, 8046 Garching, West Germany. 298
effects in nuclear transitions, but such experiments are difficult to interpret because o f the incomplete understanding o f nuclear dynamics and the form o f the strong force at sub-femtometer distances [4]. The PNC neutron spin rotation measurement is a new technique to investigate the weak interaction between nucleons, providing information on both the n u c l e o n nucleon interaction [5] and nuclear structure. When a neutron with spin ~n and linear momentum Pn propagates through a material target, the weak inter. action (or any PNC force [6] ) between the neutron and atoms in the target can give the neutron coherent
Volume lI9B, number 4,5,6
PHYSICS LETTERS
forward scattering amplitude, f(0), a PNC component given by: fPNC (0)
= G'(on'Pn),
h = c = I,
sensitivity of less than 10 -6 rad/cm in the isotopes of Sn. In this letter we present the results of measurements of the magnitudes and signs of the PNC neutron spin rotation in natural Pb and Sn targets, performed with a more sensitive neutron polarirneter. The neutron polarimeter is shown schematically in fig. 1. It differs from the polarirneter of ref. [11] in the following important ways: (a) a neutron velocity selector is added to define the neutron beam energy spectrum; Co) "supermirror" neutron polarizers replace the F e - C o mirror polarizers; (c) two neutron beam monitors are employed; (d) the neutron beam cross-sectional area is 3 - 4 cm 2 and targets up to 50 cm long can be used; (e) two movable targets are necessary; and (f) the design of the solenoids producing the localized magnetic fields has been improved to increase the field homogeneity and reduce the external leakage magnetic fields from the solenoids. The operation of the polarimeter is briefly as follows (for more details see ref. [10] ). Cold neutrons from the Institut Laue-Langevin High Flux Reactor enter the neutron velocity selector from the left in fig. I. The "helical fin" velocity selector transmits a neutron wavelength spectrum that is ~1.5 A fwhm and falls off sharply at both ends of the spectrum [12]. The speed of rotation of the velocity selector is set so that all of the neutrons have wavelengths longer than the Bragg reflection cut-off wavelength of the target (~5 A for Pb and Sn). Monitor 1 records the neutron flux from the reactor while monitor 2 measures changes in the neutron flux due to variations of the velocity selector speed. The neutron beam then enters
(I)
where the proportionality constant G' will be of order G, the weak-coupling constant [7,8]. In practice, a beam of cold ( ~ 10 - 2 eV)neutrons and a homogeneous target are used, for which coherent forward scattering can be the dominant scattering process. For a neutron beam with polarization vector perpendicular to Pn, it follows from eq. ( I ) that the two neutron helicity eigenstates will accumulate different phase changes upon passage through the target. The result is a rotation, ~PNC, of the polarization vector in the plane perpendicular to Pn given by: ~bpNC =
-4rtplRe(G'),
23/30 December 1982
(2)
where p is the atomic number density of the target and I is the target length. It can be shown [9,10] that the contribution to ~bpNC from the interaction between the neutron beam and the electrons in the target should be ~ 4 sin20 w - 1 (~0.04) times smaller than the bare nucleon-nucleon contribution to t~PNC, where 0w is the weak mixing angle in the standard weak-interaction model. Thus a measurement of the PNC neutron spin rotation, ~PNC, is a direct probe of the weak-interaction coupling between the neutron beam and the target nuclei. In an earlier report [11 ], we demonstrated that with a neutron polarimeter and a scheme to isolate PNC rotations of the neutron spin from parity conserving rotations, d~PNc/l could be measured with a
×
f~ •
Neutron
/.I //
Velocity
Guide Se,ector !tor~l ~ C
~ '
(
~
f Trim coil q
Lo + + i e d
t|
e+oo
T1 Coil W---
~
~ '"~'~\~ C..... 1 Sheetk" "
/ f
// //
~ t Samplel ,] ~
"~ ',Sample!,-'
/
\ \\ \ e~xlendlng FJeld shims -. . . .
Co~unte
nt
~"~~~____._______.J~
r
Fig. 1. The PNC neutron spin rotation polarimeter. The neutron beam polarization vector is along 2 and momentum along i. The targets in positions 1 and 2 are alternately moved into and out of the neutron beam via pneumatically driven rocker arms (not shown). 299
Volume 119B, number 4,5,6
PHYSICS LETTERS
a "supermirror" neutron polarizer that transmits 30% of the incident beam with 97%polarization [13]. The polarized neutrons enter the target region with spins parallel to the 2 axis of fig. 1 via non-adiabatic fast passage through the windings of the input solenoid. The target region is maintained at a magnetic field value of ~< 5 X 10 - 4 G by three concentric layers of magnetic shielding. In the target region are two similar targets, located at positions one and two, which are alternately moved into and out of the neutron beam. The central "n-coil" solenoid current is set to produce a magnetic field in the 2 direction that causes the neutron spins to precess by n rad. about the 2 axis upon passage through the coil. In this way any rotation of the neutron spin in the y z plane which is caused by the target is reversed in sign upon placing the target before or after the n-coil (positions 1 and 2 in fig. 1), whereas rotations due to residual magnetic fields in the target region are unchanged to first order when the targets are moved. Rex~ersal of the current in the output solenoid, which is coupled to the analysing supermirror polarizer via an adiabatic turning of the magnetic field by 17/2 rad, allows one to measure the projection of the neutron spin along the +~ and -)9 axes (equivalent to flipping the analyser of a crossed polarizer-analyser pair). Letting iV+ (N_) denote the number of neutrons registered by the counter for positive (negative) current in the output solenoid, then: sin ~b = (N+ - N _ )/PP(N+ + N _ ),
23/30 December 1982
pled to the n-coil, while the +2n and - 2 n measurements, like the n-coil-off measurements, should show the same neutron spin rotation for the two target positions (i.e. no PNC signal). One day of data accumulation was given to each of the measurements listed above, during which time the current in the output solenoid was reversed every second and the target position every minute. After such a 5 day data set, the targets would be removed from the polarimeter, interchanged between positions 1 and 2, and rotated in orientation before a new data set was begun. This procedure was used to look for systematic effects due to possible differences between the two targets. Data were taken with both natural Sn and natural Pb targets. The two Sn targets were cylinders 16 cm long with diameters of 2.5 cm and 99.999% quoted purity. There were six Pb targets, all of 2.5 × 2.5 cm 2 square cross section and 99.99% purity. Four of the Pb targets, two 20 cm long and two 10 cm long, were prepared by Nai'ura[ Sn
_21
(3) -6
where 4) is the projection of the neutron spin into the y z plane upon entry in the output solenoid and PP is the polarization product o f the polarizer-analyser pair. A PNC neutron spin rotation is seen as a change in ~ when the targets are alternated between positions 1 and 2. The testing for systematic effects (non-PNC signals) is an essential part o f the experiment. To verify that residual magnetic fields do not induce a target position-dependent neutron spin rotation, a null test is performed by running the experiment with the n-coil turned off. The total rotation of the neutron spin must be the same for the two target positions in the absence of a systematic effect. The n-coil is then energized to cause a Larmor precession about the 2 axis by +rr, - n , +217, and - 2 n rad in successive data runs. The +n and -Tr measurements should show the same PNC signal in the absence of systematic effects that are cou300
t
o,
I
I
I
I
I
I
1
2
3
4
5
mean
Na,ural
-2
I
I
I
I
I
I
I
2
3
t+
5
mean
Run
Number
Fig. 2. Summary of the data. The points marked by a circle are the results of the *r-coil-offruns (null test). The points marked by a cross are the average of the measurements made with +~r and -*r precession in the n-coil (4)PNC). The run numbers for the Sn data correspond to different orientations of the Sn targets. The run numbers for the Pb data are for different combinations of the Pb targets: (1) 20 cm cast Pb, (2) 20 cm cast Pb in a different orientation, (3) 10 cm cast Pb, (4) 30 (20 + 10) cm cast Pb, and (5) 20 cm rolled Pb.
Volume 119B, number 4,5,6
PHYSICS LETTERS
casting the Pb in a graphite mold. The remaining two Pb targets, 20 cm long, were cut from a sheet o f rolled
Pb. The experimental results of 7 weeks of data accumulation are shown in fig. 2. For clarity, only the data taken with the n-coil off (null test) and the average o f the +n and -Tr runs (~bPNC) are plotted in fig. 2. The results obtained are: CPNC//(natural Sn) = - ( 3 . 1 9 + 0.40) × 10 - 6 rad/cm, ~bPNCfl (natural Pb) = +(2.24 -+ 0.33) × 10 - 6 rad/cm, where a positive sign for SPNCcorresponds to a righthand rotation of the neutron spin about its momentum vector. Note that the sign ofSpN C (natural Sn) is opposite to that reported in ref. [11], due to a sign error in ref. [11]. The problems of signs and sign conventions are discussed further in footnote 1. The quoted errors are one standard deviation. To confirm our new determination of sign, a data run was performed with a 6.0 × 1.1 X 1.1 cm 3 target of ll7Sn ,2 in the polarimeter of ref. [11]. The result obtained was ~bPNC//(ll7Sn) = - ( 3 8 . 6 -+ 6.8) × 10 - 6 rad/cm, which when averaged with the original [11 ] PNC measurement in 117 Sn (with corrected sign as explained in footnote 1) gives: t~PNc/l (ll7Sn) = - ( 3 7 . 0 + 2.5) × 10 - 6 rad/cm. To within one standard deviation, the PNC neutron spin rotation in natural Sn can be fully explained by the 7.6% natural abundance of ll7Sn. It is therefore unlikely that another of the natural Sn isotopes will be found to possess a large PNC rotary power. ,1 There is considerable confusion in the literature about signs and sign conventions of PNC neutron spin rotation: refs. [7] and [11], and the present paper take ¢PNC to be positive for right-handed rotations of the spin about the linear momentum vector whereas ref. [8] uses the same symbol ~'PNC but the opposite sign convention. Furthermore, there are several sign errors in ref. [ 11 ] : the sign of A should be reversed; the right-hand side of eq. (4) should have a negative sign and the signs of all experimental results for qspNC in that paper should be reversed. Finally it should be noted that refs. [8] and [11] and the present paper useffor the scattering amplitude whereas ref. [7] uses the same symbol for the scattering length, which by convention has the opposite sign. ,2 The llTSn was obtained from Oak Ridge National Laboratory.
23/30 December 1982
It will be recalled from eq. ( 1 ) t h a t Im(G') * n ' P n will cause a different total scattering cross section for + and - helicity state neutrons: A n = (ti+ - o )/(tl+ + o _ ) ¢c Im(G'), where o± are the neutron total cross sections for + helicity state neutrons. For thermal neutrons scattering from 117Sn, one h a s A n = +(6.2 -+-0.7) X 10 - 6 [14]. The relative sign difference between $PNc(tl7sn) a n d A n ( l l 7 S n ) [i.e. between Re(G') and Im(G')] is then consistent with the model that the PNC effects found in this isotope result from scattering enhanced by a neutron p-wave resonance at 1.3 eV [ 1 5 - 1 7 ] . The large PNC neutron spin rotation in Pb is surprising. The measured value is roughly a factor o f 20 larger than that predicted [10]. Unlike for 117Sn, there are no known low-energy neutron p-wave resonances in the Pb isotopes to explain the enhancement of the PNC rotation. The next step toward understanding the source of¢PNc(Pb ) will be to isolate the isotope(s) responsible. In conclusion we have observed large PNC spin rotations with neutrons passing through isotopically natural samples of lead and tin. These rotations are more than an order of magnitude larger than expected on the basis of neutron weak interactions with nuclei viewed as an assembly of non,interacting nucleons. The effects probably arise as a result of nuclear structure complexities, in particular the presence of low energy p-wave resonances [17]. This hypothesis is supported in the case of tin, where the isotope ll7Sn has been identified as the major contribution to the effect and where a p-wave resonance at 1.3 eV is already known [16]. The situation with respect to lead and its resonance structure is less well known, and merits further investigation. We would like to thank the members of the Institut Laue-Langevin for their support, S. Lloyd and A.G.D. Payne for their technical assistance, and Prof. E. Purcell for his timely advice. This work was supported in part by the National Science Foundation under Grants numbered PHY-78-08561 and INT-8021912, and by the Science and Engineering Research Council (UK) through the Rutherford Appleton Laboratory.
301
Volume I19B, number 4,5,6
PHYSICS LETTERS
References [1] N. Lockyer et al., Phys. Rev. Lett. 45 (1980) 1821. [2] D.E. Nagle et al., in: High energy physics with polaxized beams and targets, ed. G.H. Thomas, AIP Conf. Proc. No. 51 (American Institute of Physics, New York, 1978) p. 224. [3] R. Balzer et al., Phys. Rev. Lett. 44 (1980) 699. [4] D. Tadic, Rep. Prog. Phys. 43 (1980) 67. [5] A. Serebrov, in: Proc. 14th LIYaF Winter School (USSR Academy of Sciences, Leningrad, 1979) p. 28. [6] L. Stodolsky, Phys. Lett. 96B (1980) 127. [7] F.C. Michel, Phys. Rev. 133B (1964) 329. [8] L. Stodolsky, Phys. Lett. 50B (1974) 352. [9] A. Barioso and D. Tadic, Nucl. Phys. A294 (1978) 376.
302
23/30 December 1982
[10] B. Heckel, Ph.D. Thesis, Harvard Univ. and Insfitut Laue-Langevin Report No. D6306. [11] M. Forte et al., Phys. Rev. Lett. 45 (1980) 2088. [i2] H. Tieben and W. Wendt, PTB-Bericht FMRB-61, Physikalisch-Technische Bundesanstalt, Braunschweig, FRG. [13] O. Schaerpf, in: Symp. of Neutron scattering (Argonne Laboratory, 1981), AIP Conf. Proc., to be published. [14] E. Kolomensky et al., LINP preprint No. 662 (1981). [15] O.P. Shuskov and V.V. Flambaum, Proc. 16th LIYaF Winter School (USSR Academy of Sciences, Leningiad, 1981) p. 200. [16] V.P. Alfimenkov et al., JINR preprint No. D3-81-480 (1981). [17] V. Bunakov and V. Gudkov, Z. Phys. A303 (1981) 285.
PHYSICS LETTERS
23•30 December 1982
A MEASUREMENT OF PARITY NON-CONSERVING NEUTRON SPIN ROTATION IN LEAD AND TIN B. HECKEL and N.F. RAMSEY Harvard University, Cambridge, MA 02138, USA K. GREEN and G.L. GREENE 1 Rutherford-Appleton Laboratory, Chilton, Oxon 0 X l l OQX, UK R. Gti,HLER 2 and O. SCHAERPF lnstitut Laue-Langevin, 38042 Grenoble, France M. FORTE Physics Division, Joint Research Council, 1-21020 Ispra, ltaly W. DRESS and P.D. MILLER Oak Ridge National Laboratory, Oak Ridge, TAr 37830, USA R. GOLUB Technical University Munich, 8046 Garching, West Germany and
J. BYRNE and J.M. PENDLEBURY University o f Sussex, Brighton, BN1 9QH Sussex, UK Received 24 August 1982
The rotation, ~PNC, of a neutron beam polarization vector due to parity non-conserving forces is observed in natural Pb and Sn targets. The following values for ~PNC/I in units of 10 -6 rad/cm are found: Pb: +(2.24 ± 0.33), and Sn: -(3.19 ± 0.40). A positive sign corresponds to a right-handed rotation of the neutron spin about its momentum.
The experimental determination o f the nature and strength o f the weak interaction between nucleons has yet to be fully accomplished. The search for parity nonconserving (PNC) effects in n u c l e o n - n u c l e o n scattering has been limited to the p r o t o n - p r o t o n system and only recently has begun to show non-zero results [ 1 3]. A much richer testing ground is the study of PNC i Current address: Physics Dept., Yale University, New Haven, CN, USA. 2 Current address: Technical University Munich, 8046 Garching, West Germany. 298
effects in nuclear transitions, but such experiments are difficult to interpret because o f the incomplete understanding o f nuclear dynamics and the form o f the strong force at sub-femtometer distances [4]. The PNC neutron spin rotation measurement is a new technique to investigate the weak interaction between nucleons, providing information on both the n u c l e o n nucleon interaction [5] and nuclear structure. When a neutron with spin ~n and linear momentum Pn propagates through a material target, the weak inter. action (or any PNC force [6] ) between the neutron and atoms in the target can give the neutron coherent
Volume lI9B, number 4,5,6
PHYSICS LETTERS
forward scattering amplitude, f(0), a PNC component given by: fPNC (0)
= G'(on'Pn),
h = c = I,
sensitivity of less than 10 -6 rad/cm in the isotopes of Sn. In this letter we present the results of measurements of the magnitudes and signs of the PNC neutron spin rotation in natural Pb and Sn targets, performed with a more sensitive neutron polarirneter. The neutron polarimeter is shown schematically in fig. 1. It differs from the polarirneter of ref. [11] in the following important ways: (a) a neutron velocity selector is added to define the neutron beam energy spectrum; Co) "supermirror" neutron polarizers replace the F e - C o mirror polarizers; (c) two neutron beam monitors are employed; (d) the neutron beam cross-sectional area is 3 - 4 cm 2 and targets up to 50 cm long can be used; (e) two movable targets are necessary; and (f) the design of the solenoids producing the localized magnetic fields has been improved to increase the field homogeneity and reduce the external leakage magnetic fields from the solenoids. The operation of the polarimeter is briefly as follows (for more details see ref. [10] ). Cold neutrons from the Institut Laue-Langevin High Flux Reactor enter the neutron velocity selector from the left in fig. I. The "helical fin" velocity selector transmits a neutron wavelength spectrum that is ~1.5 A fwhm and falls off sharply at both ends of the spectrum [12]. The speed of rotation of the velocity selector is set so that all of the neutrons have wavelengths longer than the Bragg reflection cut-off wavelength of the target (~5 A for Pb and Sn). Monitor 1 records the neutron flux from the reactor while monitor 2 measures changes in the neutron flux due to variations of the velocity selector speed. The neutron beam then enters
(I)
where the proportionality constant G' will be of order G, the weak-coupling constant [7,8]. In practice, a beam of cold ( ~ 10 - 2 eV)neutrons and a homogeneous target are used, for which coherent forward scattering can be the dominant scattering process. For a neutron beam with polarization vector perpendicular to Pn, it follows from eq. ( I ) that the two neutron helicity eigenstates will accumulate different phase changes upon passage through the target. The result is a rotation, ~PNC, of the polarization vector in the plane perpendicular to Pn given by: ~bpNC =
-4rtplRe(G'),
23/30 December 1982
(2)
where p is the atomic number density of the target and I is the target length. It can be shown [9,10] that the contribution to ~bpNC from the interaction between the neutron beam and the electrons in the target should be ~ 4 sin20 w - 1 (~0.04) times smaller than the bare nucleon-nucleon contribution to t~PNC, where 0w is the weak mixing angle in the standard weak-interaction model. Thus a measurement of the PNC neutron spin rotation, ~PNC, is a direct probe of the weak-interaction coupling between the neutron beam and the target nuclei. In an earlier report [11 ], we demonstrated that with a neutron polarimeter and a scheme to isolate PNC rotations of the neutron spin from parity conserving rotations, d~PNc/l could be measured with a
×
f~ •
Neutron
/.I //
Velocity
Guide Se,ector !tor~l ~ C
~ '
(
~
f Trim coil q
Lo + + i e d
t|
e+oo
T1 Coil W---
~
~ '"~'~\~ C..... 1 Sheetk" "
/ f
// //
~ t Samplel ,] ~
"~ ',Sample!,-'
/
\ \\ \ e~xlendlng FJeld shims -. . . .
Co~unte
nt
~"~~~____._______.J~
r
Fig. 1. The PNC neutron spin rotation polarimeter. The neutron beam polarization vector is along 2 and momentum along i. The targets in positions 1 and 2 are alternately moved into and out of the neutron beam via pneumatically driven rocker arms (not shown). 299
Volume 119B, number 4,5,6
PHYSICS LETTERS
a "supermirror" neutron polarizer that transmits 30% of the incident beam with 97%polarization [13]. The polarized neutrons enter the target region with spins parallel to the 2 axis of fig. 1 via non-adiabatic fast passage through the windings of the input solenoid. The target region is maintained at a magnetic field value of ~< 5 X 10 - 4 G by three concentric layers of magnetic shielding. In the target region are two similar targets, located at positions one and two, which are alternately moved into and out of the neutron beam. The central "n-coil" solenoid current is set to produce a magnetic field in the 2 direction that causes the neutron spins to precess by n rad. about the 2 axis upon passage through the coil. In this way any rotation of the neutron spin in the y z plane which is caused by the target is reversed in sign upon placing the target before or after the n-coil (positions 1 and 2 in fig. 1), whereas rotations due to residual magnetic fields in the target region are unchanged to first order when the targets are moved. Rex~ersal of the current in the output solenoid, which is coupled to the analysing supermirror polarizer via an adiabatic turning of the magnetic field by 17/2 rad, allows one to measure the projection of the neutron spin along the +~ and -)9 axes (equivalent to flipping the analyser of a crossed polarizer-analyser pair). Letting iV+ (N_) denote the number of neutrons registered by the counter for positive (negative) current in the output solenoid, then: sin ~b = (N+ - N _ )/PP(N+ + N _ ),
23/30 December 1982
pled to the n-coil, while the +2n and - 2 n measurements, like the n-coil-off measurements, should show the same neutron spin rotation for the two target positions (i.e. no PNC signal). One day of data accumulation was given to each of the measurements listed above, during which time the current in the output solenoid was reversed every second and the target position every minute. After such a 5 day data set, the targets would be removed from the polarimeter, interchanged between positions 1 and 2, and rotated in orientation before a new data set was begun. This procedure was used to look for systematic effects due to possible differences between the two targets. Data were taken with both natural Sn and natural Pb targets. The two Sn targets were cylinders 16 cm long with diameters of 2.5 cm and 99.999% quoted purity. There were six Pb targets, all of 2.5 × 2.5 cm 2 square cross section and 99.99% purity. Four of the Pb targets, two 20 cm long and two 10 cm long, were prepared by Nai'ura[ Sn
_21
(3) -6
where 4) is the projection of the neutron spin into the y z plane upon entry in the output solenoid and PP is the polarization product o f the polarizer-analyser pair. A PNC neutron spin rotation is seen as a change in ~ when the targets are alternated between positions 1 and 2. The testing for systematic effects (non-PNC signals) is an essential part o f the experiment. To verify that residual magnetic fields do not induce a target position-dependent neutron spin rotation, a null test is performed by running the experiment with the n-coil turned off. The total rotation of the neutron spin must be the same for the two target positions in the absence of a systematic effect. The n-coil is then energized to cause a Larmor precession about the 2 axis by +rr, - n , +217, and - 2 n rad in successive data runs. The +n and -Tr measurements should show the same PNC signal in the absence of systematic effects that are cou300
t
o,
I
I
I
I
I
I
1
2
3
4
5
mean
Na,ural
-2
I
I
I
I
I
I
I
2
3
t+
5
mean
Run
Number
Fig. 2. Summary of the data. The points marked by a circle are the results of the *r-coil-offruns (null test). The points marked by a cross are the average of the measurements made with +~r and -*r precession in the n-coil (4)PNC). The run numbers for the Sn data correspond to different orientations of the Sn targets. The run numbers for the Pb data are for different combinations of the Pb targets: (1) 20 cm cast Pb, (2) 20 cm cast Pb in a different orientation, (3) 10 cm cast Pb, (4) 30 (20 + 10) cm cast Pb, and (5) 20 cm rolled Pb.
Volume 119B, number 4,5,6
PHYSICS LETTERS
casting the Pb in a graphite mold. The remaining two Pb targets, 20 cm long, were cut from a sheet o f rolled
Pb. The experimental results of 7 weeks of data accumulation are shown in fig. 2. For clarity, only the data taken with the n-coil off (null test) and the average o f the +n and -Tr runs (~bPNC) are plotted in fig. 2. The results obtained are: CPNC//(natural Sn) = - ( 3 . 1 9 + 0.40) × 10 - 6 rad/cm, ~bPNCfl (natural Pb) = +(2.24 -+ 0.33) × 10 - 6 rad/cm, where a positive sign for SPNCcorresponds to a righthand rotation of the neutron spin about its momentum vector. Note that the sign ofSpN C (natural Sn) is opposite to that reported in ref. [11], due to a sign error in ref. [11]. The problems of signs and sign conventions are discussed further in footnote 1. The quoted errors are one standard deviation. To confirm our new determination of sign, a data run was performed with a 6.0 × 1.1 X 1.1 cm 3 target of ll7Sn ,2 in the polarimeter of ref. [11]. The result obtained was ~bPNC//(ll7Sn) = - ( 3 8 . 6 -+ 6.8) × 10 - 6 rad/cm, which when averaged with the original [11 ] PNC measurement in 117 Sn (with corrected sign as explained in footnote 1) gives: t~PNc/l (ll7Sn) = - ( 3 7 . 0 + 2.5) × 10 - 6 rad/cm. To within one standard deviation, the PNC neutron spin rotation in natural Sn can be fully explained by the 7.6% natural abundance of ll7Sn. It is therefore unlikely that another of the natural Sn isotopes will be found to possess a large PNC rotary power. ,1 There is considerable confusion in the literature about signs and sign conventions of PNC neutron spin rotation: refs. [7] and [11], and the present paper take ¢PNC to be positive for right-handed rotations of the spin about the linear momentum vector whereas ref. [8] uses the same symbol ~'PNC but the opposite sign convention. Furthermore, there are several sign errors in ref. [ 11 ] : the sign of A should be reversed; the right-hand side of eq. (4) should have a negative sign and the signs of all experimental results for qspNC in that paper should be reversed. Finally it should be noted that refs. [8] and [11] and the present paper useffor the scattering amplitude whereas ref. [7] uses the same symbol for the scattering length, which by convention has the opposite sign. ,2 The llTSn was obtained from Oak Ridge National Laboratory.
23/30 December 1982
It will be recalled from eq. ( 1 ) t h a t Im(G') * n ' P n will cause a different total scattering cross section for + and - helicity state neutrons: A n = (ti+ - o )/(tl+ + o _ ) ¢c Im(G'), where o± are the neutron total cross sections for + helicity state neutrons. For thermal neutrons scattering from 117Sn, one h a s A n = +(6.2 -+-0.7) X 10 - 6 [14]. The relative sign difference between $PNc(tl7sn) a n d A n ( l l 7 S n ) [i.e. between Re(G') and Im(G')] is then consistent with the model that the PNC effects found in this isotope result from scattering enhanced by a neutron p-wave resonance at 1.3 eV [ 1 5 - 1 7 ] . The large PNC neutron spin rotation in Pb is surprising. The measured value is roughly a factor o f 20 larger than that predicted [10]. Unlike for 117Sn, there are no known low-energy neutron p-wave resonances in the Pb isotopes to explain the enhancement of the PNC rotation. The next step toward understanding the source of¢PNc(Pb ) will be to isolate the isotope(s) responsible. In conclusion we have observed large PNC spin rotations with neutrons passing through isotopically natural samples of lead and tin. These rotations are more than an order of magnitude larger than expected on the basis of neutron weak interactions with nuclei viewed as an assembly of non,interacting nucleons. The effects probably arise as a result of nuclear structure complexities, in particular the presence of low energy p-wave resonances [17]. This hypothesis is supported in the case of tin, where the isotope ll7Sn has been identified as the major contribution to the effect and where a p-wave resonance at 1.3 eV is already known [16]. The situation with respect to lead and its resonance structure is less well known, and merits further investigation. We would like to thank the members of the Institut Laue-Langevin for their support, S. Lloyd and A.G.D. Payne for their technical assistance, and Prof. E. Purcell for his timely advice. This work was supported in part by the National Science Foundation under Grants numbered PHY-78-08561 and INT-8021912, and by the Science and Engineering Research Council (UK) through the Rutherford Appleton Laboratory.
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References [1] N. Lockyer et al., Phys. Rev. Lett. 45 (1980) 1821. [2] D.E. Nagle et al., in: High energy physics with polaxized beams and targets, ed. G.H. Thomas, AIP Conf. Proc. No. 51 (American Institute of Physics, New York, 1978) p. 224. [3] R. Balzer et al., Phys. Rev. Lett. 44 (1980) 699. [4] D. Tadic, Rep. Prog. Phys. 43 (1980) 67. [5] A. Serebrov, in: Proc. 14th LIYaF Winter School (USSR Academy of Sciences, Leningrad, 1979) p. 28. [6] L. Stodolsky, Phys. Lett. 96B (1980) 127. [7] F.C. Michel, Phys. Rev. 133B (1964) 329. [8] L. Stodolsky, Phys. Lett. 50B (1974) 352. [9] A. Barioso and D. Tadic, Nucl. Phys. A294 (1978) 376.
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[10] B. Heckel, Ph.D. Thesis, Harvard Univ. and Insfitut Laue-Langevin Report No. D6306. [11] M. Forte et al., Phys. Rev. Lett. 45 (1980) 2088. [i2] H. Tieben and W. Wendt, PTB-Bericht FMRB-61, Physikalisch-Technische Bundesanstalt, Braunschweig, FRG. [13] O. Schaerpf, in: Symp. of Neutron scattering (Argonne Laboratory, 1981), AIP Conf. Proc., to be published. [14] E. Kolomensky et al., LINP preprint No. 662 (1981). [15] O.P. Shuskov and V.V. Flambaum, Proc. 16th LIYaF Winter School (USSR Academy of Sciences, Leningiad, 1981) p. 200. [16] V.P. Alfimenkov et al., JINR preprint No. D3-81-480 (1981). [17] V. Bunakov and V. Gudkov, Z. Phys. A303 (1981) 285.