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PARITY VIOLATION IN 239U NEUTRON RESONANCES
S. Penttilä, C. Bowman, J. Bush, P. Delheij, C. Frankle, C. Gould, D. Haase, J. Knudson, G. Mitchell, H. Postma, et al.
To cite this version:
S. Penttilä, C. Bowman, J. Bush, P. Delheij, C. Frankle, et al.. PARITY VIOLATION IN 239U NEUTRON RESONANCES. Journal de Physique Colloques, 1990, 51 (C6), pp.C6-523-C6-526.
�10.1051/jphyscol:1990665�. �jpa-00230935�
COLLOQUE DE PHYSIQUE
Colloque C6, suppl6ment au n022, Tome 5 1 , 15 novembre 1 9 9 0
S. PENTTILA, C.D. BOWMAN, J;D. BOWMAN, J. E. BUSH* *
1
, P. P. J. DELHEIJ* * * C.M. F R A N K L E * , * * , C.R. GOULD*,**, D.G. HAASE*,** J.N. KNUDSON,G.E. MITCHELL*,**, H. POSTMA****, N.R.
ROB ER SON*'^*****
, S. J. SEESTROM, J.J. SZYMANSKI, V.W. WJAN and X. ZHU**,****'Fos Alamos National Laboraotory, LAMPF, Los Alamos, NM 87545, U.S.A.
North Carolina State University, Raleigh, NC 27695, U.S.A.
* *
Triangle Universities Nuclear Laboratory, Durham, NC 27706, U.S.A.
* X *
TRIUMF, Vancouver, B. C. V6T 2A3, Canada
* * X *
University of Technology, 2600 GA Delft, The Netherlands
* * . * * Duke University, Durham, NC 27706, U.S.A.
La non conservation de l a p a r i t e a
Qte
Btudibe dans l e 2 3 9 ~ en mesurant l a dbpendance de l a s e c t i o n e f f i c a c e , ri'sonante dans l'onde P, en fonction de l ' h 6 l i c i t i ' de neutrons i'pithermiques d i f f u s e s par 3 8 ~ . Une approche s t a t i s t i q u e dome l ' b c a r t type ~=0.58_:.3:me~ pour l'i'li'ment de matrice violant l a p a r i t e dans l e melange des'ondes S et P. Ceci correspond B y = 1 . 0 x 1 0 - ~ e ~ . On deduit de c e t t e largeur l a valeur c r p = 4 ~ 1 0 - ~ du rapport de l ' i n t e n s i t i ' ne conservant pas a c e l l e conservant l a p a r i t 6 dans l ' i n t e r a c t i o n e f f e c t i v e NN.Abstract - Parity nonconservation was studied in 239U by measuring the helicity dependence of the p-wave resonance cross section for epithermal neutrons scattered from 238U. A statistical approach was used to determine the root-mean-squared parity-violating matrix element for the mixing of p- wave and S-wave states to be M = 0.58&:::: meV. This corresponds to a parity-violating spreading width of
rPV
= 1.0. 10-? eV. This spreading width gives a value of 4 . 1 0 ~ ' for a p , the ratio of strengths of the parity-nonconserving and parity-conserving effective nucleon-nucleon interactions.1 - INTRODUCTION
A neutron captured by a nucleus forms highly excited states in the compound nuclear (CN) system at energies just above the neutron separation energy. The energy spacing of these complex states is small, a few electron volts. Weak, symmetry-breaking forces can modify the highly-excited CN states and in certain circumstances enhancements can occur and make parity violation easily observable. For instance, large parity-nonconserving (PNC) longitudinal asymmetries, at the ten percent level, have been measured in the 0.734-eV neutron p- wave resonance in I3'La /1,2,3,4,5/. This PNC effect is caused by the weak force which mixes states of the CN with the same angular momentum, JCN, but opposite parity. The angular momentum is given by
JcN =
f + i+
S, where1
is the ground-state angular momentum of the target nucleus, and S are the orbital angular momentum and spin of the neutron, respectively. In the low-energy regime, below 500-eV neutron kinetic energy, the S- and p-waves dominate the cross sections. The observed large enhancements in PNC effects are caused by 1) the first-order interference between the weak and strong interaction, 2) small CN level spacings, so that the small PNC force can produce a large mixing, and 3) large s-wave scattering amplitude - compared to pwave scattering amplitude; a small admixture of a S-wave state into a p-wave state produces a large longitudinal asymmetry. The total enhancement can be order of 105 in medium-weight and heavy nuclei.We have studied PNC in 239U by measuring the helicity ( S .
L)
dependence of the total cross section for polarized low-energy neutrons incident on 238U( L
is the neutron momentum) /6/. The coherent forward- scattering amplitude can be expressed asf
(0") = fpc+
fkNc, where fPC is the parity-conserving componenthis
work was supported in part by the U.S. Department of Energy, Office of High Energy and Nuclear Physics, Contract Nos. DE-AC05-'76ER01067 and DE-FG05-88ER40441.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990665
C6-524 COLLOQUE DE PHYSIQUE
and the parity-nonconserving component fPNC depends on B -
L.
The imaginary part of fPNc gives then the helicity dependence of the total cross section according to the optical theorem.2 - EXPERIMENT
This experiment took advantage of the intense pulsed epithermal neutron flux produced at the Los Alamos Neutron Scattering Center (LANSCE). The 800-MeV proton beam pulses from the Los Alamos Meson Physics Facility linac are accumulated in the storage ring and ejected as short 250-11s pulses. The extracted protons strike a tungsten target and the resulting spallation neutrons are moderated and collimated to a low-energy beam. The neutrons are polarized by transmission through a spin filter consisting of longitudinally polarized protons 171. The proton polarization is monitored by nuclear magnetic resonance techniques and calibrated by measurements of the neutron transmission through the spin filter. The neutron polarization was about 40% with an uncertainty of 5%. The helicity state of the neutrons is changed by a system of magnetic fields.
Neutron energy is measured by time-of-flight over a distance of 56 m. The neutrons traverse a metallic sample of 238U (36.0 g/cm2), and are detected with an array of 'Li-glass scintillators, each 1.0 cm thick and 13.3 cm in diameter and viewed by high current photomultiplier tubes /8,9/. The instantaneous neutron rates in the detector can be as high as loiO neutrons/s. A neutron spectrum was measured by periodically sampling the phototube currents with a transient digitizer. The linear and quadratic time-dependent drifts are cancelled by a data run consisting of a thirty-minute sequence of programmed spin reversals. All resonances in the energy interval from a few to several hundred eV could be measured for each beam burst. A neutron transmission spectrum is shown as a function of neutron kinetic energy in Fig. 1.
50 100 150 200 250
Neutron Energy (eV)
Fig. 1 - The experimental asymmetry E = (N+ - N-)/(N+
+
N-), where Nf and N- are the counts for the two helicity states. The lower part shows a neutron transmission spectrum as a function of kinetic energy.The large dips belong to S-wave resonances. Several weak p-wave resonances can also be seen.
3 - PNC MIXING MATRIX ELEMENT FOR 239U
Seventeen pwave resonances in 239U up to E, w 300 eV were analyzed. The transmitted neutron yield in the neigl~borl~ood of these p-wave resonances was fitted and the parity-violating longitudinal asymmetry of the
resonance cross section Pi = (a+ - a-)/(a+
+
a-) was extracted /6/. The mean value of Pi after averaging over 200 data runs is listed in Table I. The uncertainty in each mean was determined from the fluctuations in the individual values of Pi. There are five resonances showing greater than two standard deviation PNC effects (only 0.7 such effects are expected from random fluctuations).Table I. Parity-violating longitudinal asymmetries for neutron resonances in 238U.
In order to extract the basic nuclear property M , the root-mean-squared PNC mixing matrix element, from the set of experimental values of
Pi,
we treated individual matrix elements as random variables having a common Gaussian distribution with mean zero and variance M2. This assumption is based on experience with other observables and models of symmetry breaking in chaotic quantum systems /10,11/. For mixing of several S-states with the ith pwave state the relationship between the longitudinal asymmetry Pi and the mixing matrix elementsxi
can be written /6/where the sum over j gives contributions from several S-wave resonances with same spin but opposite parity.
The coefficients Aij are functions of the known resonance parameters 1121. The Pi's are normalized by defining a new'variable
Q i
E P i / ( C j A:j)1/2. The sum of independently Gaussian-distributed random variables with mean zero is also a Gaussian random variable with mean zero 1131. Therefore,Q i
has the same distribution as each of theKj,
a Gaussian distribution of mean zero and variance M 2 , which can be determined directly from the set of experimental values of Pi without determining the individual for each Pi. Detailed analysis could be performed for 17 pwave resonances. Because the ground-state angular momentum of 238U is zero, the possible values of JCN are then 112 for S-waves and 112 or 312 for pwaves;only the spin-112 p-wave resonances can mix with spin-112 S-wave resonances and exhibit parity violation.
Assuming the spin multiplicity 2JcN
+
1 dependence for the level density, there are twice as many spin-312 resonances as spin-l/2 ones 1141. A maximum likelihood analysis gives the most probable value of M and the most compact 68% confidence interval as M = 0.58200::: meV.4 - DISCUSSION
The spreading width
rPV
for the parity-violating interaction can be related to M byrPV
= 2nM2/D. Here D is a level spacing. Using the methods of statistical nuclear physics, French et al. /15/ have related the time- nonconserving spreading widthrTV
to the ratio of the strengths of the T-nonconserving to T-conserving interaction, a ~ , such thatrTV
X 2n1o5a; eV. Assuming this relationship applies to parity, we can also deduce the ratio of the strengths of the P-nonconserving to P-conserving interactions, a; = rPv/2n105 eV.Taking the spin-112 S-wave level spacing to be equal to the spin-112 p-wave level spacing gives D=21.0 eV and ap x 4 . 1 0 - ~ , which is in qualitative agreement with the ratio of the weak- t o strong-coupling constants, GFk$/Gs W 2 . IO-~, where kF is taken as a typical momentum transfer in NN interactions in a nucleus.
These results indicate that the manifestation of the weak, parity-violating force in the CN is qualitatively understood. By studying parity violation in other nuclei we hope to learn the mass number and energy dependence of the parity-violating spreading width. At present, we have preliminary data for parity-violating longitudinal asymmetries in 233Th (I = 0) and '161n (I = 912).
C6-5 26 COLLOQUE DE PHYSIQUE
The fact that M agrees with the expectation is encouraging for the use of the CN to study time-reversal invariance (TRI). Violation of time-reversal symmetry is also expected to be greatly enhanced in the CN.
We plan to test TRI via the P-even T-odd term ( I - &)(S-
I
Xi)
using a rotating aligned 165Ho target /16/.This term has a maximum when the target alignment axis is at_45' to the beam direction and the neutron polarization is transverse, perpendicular to the plane defined by k and
1
/17,18/. With this target we expect t o set a bound on a ~ , which is a few orders of magnitude lower than the current limit ~ 1 0 - ~ - 1 0 - ~ /19/ in a reasonable run time.The authors wish to thank V.E. Bunakov, E.D. Davis, G.T. Garvey, and H.A. Weidenmiiller for valuable discussions. We thank R. Mortensen for his technical assistance and dedication to this experiment. We also appreciate valuable contributions from the technical staff of LAMPF and LANSCE.
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/12/ ENDFIB-V1 data file for 238U (National Nuclear Data Center Brookhaven National Laboratory, Upton, NY, July 1989), resonance evaluation by Moxon, M.
/13/ Eadie, W.T., et al., Statistical Methods in Experimental Physics (North Holland, Amsterdam, 1971), p. 59.
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/17/ Bunakov, V.E., Phys. Rev. Lett. &(1988) 2250. l /18/ Gould, C.R., et al., Int. J. Mod. Phys. A5 (1990) 2194.
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