ANALYZING POWER MEASUREMENTS FOR THE π+, π0 REACTION ON A POLARIZED 13C TARGET AT Tπ+ =163 MeV

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ANALYZING POWER MEASUREMENTS FOR THE π +, π0 REACTION ON A POLARIZED 13C TARGET

AT Tπ + =163 MeV

J. Görgen, J. Comfort, T. Averett, J. Dekorse, B. Franklin, B. Ritchie, J.

Tinsley, G. Kyle, B. Berman, G. Burleson, et al.

To cite this version:

J. Görgen, J. Comfort, T. Averett, J. Dekorse, B. Franklin, et al.. ANALYZING POWER MEA- SUREMENTS FOR THE π+, π0 REACTION ON A POLARIZED 13C TARGET AT Tπ+ =163 MeV. Journal de Physique Colloques, 1990, 51 (C6), pp.C6-583-C6-586. �10.1051/jphyscol:1990678�.

�jpa-00231093�

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ANALYZING POWER MEASUREMENTS FOR THE ( n ' , n o ) REACTION ON A POLARIZED

13c

TARGET AT Tx + = 163 MeV

J. GORGEN, J. COMFORT, T. AVERETT, J. DeKORSE, B. FRANKLIN, B. RITCHIE, J. TINSLEY~ G. KYLE*, B. BERMAN*, G.R. BURLESON*, K. CRANSTON*, A. KLEIN

,

J,A. FAUCETT*. , J. JARMER*.,

J.N. KNUDSON*', S. P E N T T I L A * ~ , N. TANAKA*., B. BRINKMOELLER**", D. DEHNHARD***, Y.F. Y E N * * * , H. B R E U E R * * * * , M.A. KHANDAKER****

D.L. NAPLES**** B.S. FLANDERS****, D. ZHANG**** M. BARLETT* * * * , G.W. HOFFMANN**'**

,

M. PURCELL***** and S. H$IBR~TEN* * * *

p i z o n a State University, U.S.A New Mexico State University, U. S. A.

'*LOS Alamos National Laboratory, U.S.A.

***university of M i m e s o t a r U.S.A.

* a * *

University of Maryland, U.S.A.

* X * * *

University of Texas, U.S.A.

. * * * * * University of Colorado, U.S.A.

A new approach to the study of the pion-nucleon isovector spin-dependent interaction in nuclei is being pursued in several experiments at the Clinton P. Anderson Meson Physics Facility (LAMPF) of the Los Alamos National Laboratory. The first of these experiments involves the measurement of the analyzing power of the (n+,nO) reaction on a transversely polarized 13C target at a beam energy of 163 MeV.

Parity and rotational invariance determine the basic pion-nucleon interaction to be of the form

tbo = f (B)

+

^ig(0)ii

^Z

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where ii =

(G

^XP ) / J g X l?]

(2

and

2

are the momenta of the incident and outgoing pions), and a' is the Pauli spinor for the nucleon. Thus, apart from an unobserv- able overall phase, there are three independent real-valued numbers that completely determine the properties of the transition operator t k , corresponding to three independent observables.

In the case of pion (spin-0) scattering from spin-l/2 targets, two important observables can be defined in terms of f (B) and g(B):

The cross-section for unpolarized scattering

and the analyzing power

where the arrow indicates the nucleon spin direction relative to ii, which is assumed to point up.

The spin independent amplitude f(0) and the spin dependent amplitude g(@) each have isoscalar and isovector parts. Elastic pion scattering is primarily sensitive to the isoscalar pieces, while single-charge-exchange (SCX) reactions are pro- portional to the isovector terms. There is currently very little information on the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990678

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COLLOQUE DE PHYSIQUE

isovector part of g(8) since SCX cross-sections a t forward angles are dominated by the isovector part of f(8). Any acceptable theory of pion-nucleus interactions should be able to correctly predict spin-insensitive as well as spin-sensitive observ- a b l e ~ . Inspection of eqn. 3 shows that the analyzing power A, is sensitive to g(8) as well as to the important phase relationship between f (B) and g(@). Our experiment provides information on the isovector part of that phase in the nuclear medium for the first time.

The 13C spin-parity of 112- keeps the number of spin-dependent observables to a minimum and therefore represents a sound basis for exploring pion-nucleus spin physics. The isobaric-analogue-state (IAS) transition leads to the ground state of 13N, which due t o a favorable reaction Q-value minimizes backgrounds, and the 13C(rf, r O ) cross sections have been measured at 165 MeV.' Analyzing powers were predicted from two separate reaction models prior to the e ~ ~ e r i m e n t . ~ ' ~

Measurements were made a t the Low Energy Pion channel with the LAMPF no

~ ~ e c t r o m e t e r . ~ The kpectrometer was positioned at scattering angles of 25", 38O, and 55". The angular resolution of the spectrometer was about 6O, and two or three separate angles were binned from the data for each of the spectrometer settings.

The target consisted of frozen beads of ethylene glycol [OH-(CH2)2-OH], approx- imately 1-2 mm in diameter, that were placed in a 3He bath inside a target cavity within a 3He refrigerator. The target materials were placed in a uniform magnetic field of about 2.5 Tesla, and microwaves of frequencies near 70 GHz were applied to the cavity. Hyperfine interactions between the electrons and the nuclei induced 13C polarizations of about 26%. The polarization direction was perpendicular t o the scattering plane, i.e. ii

.

^a'⁼^f1.

The spectra were analysed by fitting one or more peaks, along with background functions, to the low-excitation region. The peak shapes were determined from a Monte Carlo simulation of the entire experimental arrangement which correctly reproduces the signal shape observed in the basic n-p -+ non reaction. The as- sumption of a single peak did not result in a good representation of the spectra, and the extracted cross sections were typically less than those reported in an earlier experiment.' However, a good description was consistently obtained by including peaks for two excited states as well. The positions of the three peaks closely followed the kinematics of the ground state and states near 3.5- and 7.5-MeV excitation. The summed cross sections of the ground-state and 3.5-MeV state are in good agree- ment with the earlier resu1ts.l The features of the analysing powers are stable to changes in details of the fitting procedure. We were not able to resolve the 1/2+

state at 2.4 MeV. The analogue of this state in 13C is only weakly excited in inela- sic pion scattering.' We thus assume that the IAS transition is dominant and the 1/2+ transition can be neglected. The observed first-excited state corresponds to the collectively enhanced 312-, 5/2+ doublet separated by about 3.5 MeV from the IAS. The peak at the highest excitation involves transitions to several states.

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@c.m. @c.m.

Fig. l Cross-sections and analyzing powers for the 138(7rf , X ' ) reaction a t 163 MeV. a ) IAS transition, b) 3/2-, 5/2+ doublet a t 3.5 MeV. The solid curves result from coordinate-space DWIA calculations by Siegel and Gibbs, the dashed lines represent momentum-space DWIA calculations by Chakravarti.

Siegel and Gibbs' coordinate-space DWIA calculation reproduces the observed IAS cross-sections very well and follows the trend of the analyzing-power data at forward angles, but disagrees at large angles. Chakravarti's momentum-space DWIA calculations for the IAS cross-sections are low by a factor of about 2.5 and the corresponding analyzing powers disagree with our data over most of the angular range. Both calculations have difficulties in describing the excited-state

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COLLOQUE DE PHYSIQUE

cross sections and analyzing powers, although the 5/2+ transition has not yet been included.

The principal difference in the two DWIA calculations at present is the treatment of the medium modifications to the scattered pion waves. Chakravarti's calculations a t present do not include such modifications. Although the modifications have a large effect on cross sections, the analyzing powers appear to be much less sensitive to them. On the other hand, since the calculations of Siegel and Gibbs appear to have only a weak sensitivity to different models of nuclear structure:

the discrepancies observed in describing the analyzing-power data suggest that the reaction mechanism is still not well described. Additional theoretical work on the modifications of the elementary pion-nucleon interaction by the nuclear medium, especially its spin-dependent features, appears to be necessary.

References

A. Doron e t al., Phys. Rev. C 26,189 (1982).

D. Ernst in Workshop on Physics with Polarized Nuclear Targets, ed.

by J. J. Jarmer, Los Alamos National Laboratory Report LA-10550-C, 1986.

P. Siegel, ibid.

H. W. Baer e t al., Nucl. Instrum. Methods 180, 445 (1981).

D. Dehnhard e t al., Phys. Rev. Lett. 43, 1091 (1979) P. Siegel and W. R. Gibbs, private communication.

S. Chakravarti, private communication.