ELSEVIER

29 September 1994

Physics Le~ers B 336 (1994) 303-307

PHYSICS LETTERS B

Search for parity nonconservation in the compound nucleus reaction via an isobaric analog resonance using a thick target K. Kimura a, S. Kouda b, H. Nakamura b, T. Nakashima b, H. Ochi-ishi b

a Nagasaki Institute of Applied Science, Aba-machi, Nagasaki 851-01, Japan b Department of Physics, Kyushu University, Fukuoka 812 Japan

Received 3 January 1994; revised manuscript received 20 June 1994 Editor: R.H. Siemssen

Abstract

Parity nonconservation in the compound nucleus reaction via the isobaric analog resonance in 9°Zr+p (En = 5.92 MeV, .U = ½+) was searched for by measuring the longitudinal analyzing power Az of the elastic scattering protons. Using a thick target the excitation function of Az from Ep = 5.60 to 6.05 MeV was simultaneously measured from the difference of the spectral shapes of back scattered protons for positive and negative beam helicities. No explicit parity violation was observed down to Az ~ 3 × 10 -4.

Large parity violating asymmetries, reaching the or- der of 10 -1 , were observed in low-energy neutron res- onance experiments [ 1 ] on heavy nuclei. Prior to these experiments, such a large enhancement of parity non- conservation (PNC) in compound nucleus reactions, especially as seen in p-wave neutron resonances, was predicted to occur [2] due to the following two rea- sons: ( 1 ) small level spacings in highly excited com- pound nuclear states bring about an accidental large parity mixing between two close lying Pl/2- and sv2- neutron resonances and (2) even the small admixture of an s-wave component into the p-wave resonance can bring about a large parity violating asymmetry be- cause s-wave neutrons have much larger decay ampli- tudes than p-wave neutrons at low energies.

Recently a new effect was observed by the TRIPLE Collaboration [3] which showed that the PNC ana- lyzing powers of various p-wave neutron resonances on 232Th have definite sign correlations. This result shows that the statistical arguments given above are

not sufficient to explain the origin of the large PNC effect. Led by this new finding, one-body couplings with distant states, like spin dipole states [4] or sin- gle particle resonances [5] , were proposed to be im- portant. Recent theoretical discussions [ 6 ] are mainly focused on this so-called valence mechanism [7].

I f large PNC analyzing powers arise from such non-statistical origins, they should persist in the over- lapping resonance regions. Also parity mixing due to the valence mechanism would be more important for resonances with simpler configurations, like iso- baric analog resonances ( IARs) , than ordinary com- pound nuclear resonances. Isobaric analog resonances in heavy nuclei are not single resonances but are frag- mented into the surrounding dense compound nuclear states through the isospin mixing Coulomb interac- tion [ 8 ] and they exhibit micro giant-resonance struc- tures. Among these fragmented structures parity vio- lating asymmetries are expected to show sign corre- lations if the one-body coupling with the simple con-

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304 K. Kimura et al. / Physics Letters B 336 (1994) 303-307

figurations is important for parity mixing. We, there- fore, tried to investigate parity mixing in the IAR via the measurement of the longitudinal analyzing powers Az of the elastic scattering of protons on 9°Zr around

1 + the J~ = ~ IAR (ER = 5.92 MeV), the first excited 1+ state analog of 91Zr. The J~ = 7 resonance was se-

lected because near A = 90 the Pl/2 single particle strength becomes maximum, favorable for the valence mechanism, whereas the sl/2-strength becomes mini- mum and thus a maximum parity mixing effect is ex- pected [6].

Since the IAR in 9°Zr+p is fragmented into many fine structure states with widths of several keV [9], beam energy fluctuations bring about large system- atic errors in the relative cross section measurements for positive and negative beam helicities using a thin target. The measurement of sign correlations of A z among the fine structures relevant to the present study might be feasible by observing A z averaged over en- ergy. For studying them, we used the thick target method, which enabled us to measure the excitation function of Az over a wide energy range in one step and to get rid of the errors due to charge collection as explained below.

Incident protons lose energy in the target and hence the scattering energy is dependent on the depth in the target where the nuclear scattering occurs. In case of extreme backward scattering from heavy nuclei, the scattered protons again lose almost the same amount of energy as the incident protons on their way back from the scattering point to the target surface. Their spectral shape thus represents the excitation function of the nuclear scattering expanded twice in scattering energy if the energy loss of protons in the target is much larger than the energy resolution of the detec- tors. But the target should not be so thick that multiple nuclear scatterings cannot be neglected. This excita- tion function is inherently averaged over energy due to the energy straggling in the target and also due to the finite energy resolution of the detectors. Thus the en- ergy averaged Az can simply be determined from the difference of the spectral shapes of the back scattered protons from such a thick target for positive and neg- ative beam helicities and the error in the beam charge collection is reflected as a constant offset of Az.

The experiment was performed at the tandem accel- erator facility of Kyushu University. Polarized protons

800000

600000

40o~o

Olab=160*

• °

160(p,P) 2000OO

1 2 C ( I ' P ) ~ i

o ~' " , , ; 3.5 4.0 4.5 5.0 5.5 6.0

Energy (MeV)

Fig. 1. Energy spectra of protons scattered in the backward angle 0lab = 160 ° from a thick (20/~m) natural Zr target at Ep = 6.05 MeV. The broad elastic peak shape represents the excitation func- tion of the elastic scattering of protons on 9°Zr around the isobaric analog resonance (ER = 5.92 MeV).

were supplied from the Lamb-shift type ion source. Beam polarizations were 0.81 and 0.57 for positive and negative beam helicities, respectively. A natural zirconium metallic foil (abundance of 9°Zr: 51.45%) of 20/zm thickness and having a polished surface was used as a target. The energy loss of the protons in the target equals about 450 keV, which is much larger than the total width of the IAR, 77 keV [9], and thus the whole resonance structure can be covered in one mea- surement. Energy spectra of the scattered protons were measured by four solid state silicon detectors (energy resolution 25 keV) located symmetrically around the beam axis at the backward angle 0lab = 160 °.

In the Az-measurement beam helicities were alter- nately switched between positive and negative values at every 100 nC of beam charge, which corresponded to a time interval of about 10 seconds, and energy spec- tra of the scattered particles were accumulated in the histogram memories separately for positive and neg- ative beam helicities. The energy spectrum thus ob- tained at the incident beam energy Ep = 6.05 MeV is shown in Fig. 1. The elastic scattering protons exhibit a broad peak, 900 keV wide, twice as large as the en- ergy loss of the protons in the target. This peak shape looks just like the energy averaged excitation func- tion of the elastic scattering of protons on 9°Zr around

K. Kimura et al. /Physics Letters B 336 (1994) 303-307 305

the IAR (ER = 5.92 MeV) studied in Ref. [9] . Fine structures of the IAR observed in this reference are washed out in our spectrum because of energy strag- gling in the target and hence a clear single resonance pattern is revealed. Nearly 40% of the elastic scatter- ing yield comes from other Zr isotopes contained in the natural target. A slight bump around the proton energy of 5.7 MeV is probably due to the IAR in 9 2 Z r

(abundance 17.15%) + p . All other isotopes contribute only nonresonant scattering yields. Inelastic scatter- ings from all Zr isotopes do not overlap with the elas- tic peak because all the first excited states are higher than Ex = 900 keV. The spectral shape was well fitted by a simplified excitation function assuming Coulomb scattering plus a single resonance having a total width FT = 98 keV, a partial width F0 = 47 keV and a reso- nance energy ER = 5.905 MeV. The width parameters are larger than those given in Ref. [9] . This differ- ence can be ascribed to the energy averaging of our excitation function.

From the measured energy spectra we determined the longitudinal analyzing power Az corresponding to each energy channel i using the equation,

2 N+(i ) - N _ ( i ) Az ( i ) - - - , (1)

p+ + p_ N+(i ) + N _ ( i )

where p ± denote the beam polarizations for i helic- ities and N+ ( i ) denote the number of counts of the elastic scattering protons in channel i for ± beam he- licities. The Az-values determined separately by the four detectors were averaged. The result is shown in Fig. 2. Error bars include only the statistical error. Residual transverse polarizations Px and py of the beam, which were measured to be less than 1% using the 160 target, bring about false asymmetries to the A z which depend on the square of pxAx and /o r pyAy, where Ax and Ay are the transverse analyzing pow- ers. Since the the transverse analyzing powers of the

I + elastic scattering of protons around the J'~ = ~ reso- nances are generally very small, in our case measured to be about 2% at the maximum value, they give rise to false asymmetries that are negligibly small compared to the present statistical error, 1 0 - 3 . The energy bins of the data in Fig. 2 are much smaller than the en- ergy straggling and the detector resolution, and hence channel-to-channel fluctuations of the data mainly re- flect the statistics. In order to obtain better statistics

<~

5.0 5.2 5.4 5.6 5.8 Detected Proton Energy (MeV)

Fig. 2. Longitudinal analyzing powers Az of the elastic scattering of protons from a thick Zr-target in the scattering angle 0lab = 160 ° . They were obtained from the difference of spectral shapes of the scattered protons for positive and negative beam helicities.

1.0

0.5

I I I I

,~ 0.0

<~

-0.5

-1.0 5.6 5.7 5.8 5.9 6.0

Ep (MeV)

Fig. 3. Moving average of the Az -values in Fig. 2. The averaging interval was varied from 103 keV to 25 keV from left to right in the energy scale of Fig. 2 corresponding to energy straggling of protons in the target. Horizontal scale represents the scattering energy of protons in the target. The solid curve is a resonance fit using the Eq. (6) given in the text with a constant offset.

we averaged them over the energy corresponding to the squared sum of the energy straggling and the de- tector resolution. Energy stragglings were estimated by Bohr 's formula. The averaging intervals were thus varied from 103 keV, in the lowest energy region of Fig. 2, to 25 keV, in the highest energy region depend- ing on the energy loss. They are smaller than the to- tal width of the IAR, 77keV ( twice this value in the energy scale of Fig. 2) . The moving averaged Az are shown in Fig. 3. The horizontal scale of this figure

306 K. Kimura et al. / Physics Letters B 336 (1994) 303-307

is converted into the incident proton energy at which the nuclear scattering occurred. The data points are seen to oscillate around a constant offset of (A z)o -1 .5 × 10 -4, which we presumed to come from the error of the charge measurement but not from parity violation and which should be subtracted. Deviation of Az from the offset should represent the actual par- ity violation. Whether the oscillatory structures of Az seen in Fig. 3 are correlated with the substructures of the IAR (several keV wide [9], much wider than the

1+ average level spacing, 0.1 keV, of 7 compound nu- clear states) is of great interest. Such comparison be- comes meaningful only above a proton energy of 5.9 MeV in Fig. 3 where the energy straggling becomes small and comparable to the width of the substructures. But an explicit correlation between the substructures of the IAR and our Az could not be seen. Also the root mean square deviation of Az from the offset was equal to the standard deviation, 2 × 10 -4. Thus the oscillation in our data seems to be due to the statistics.

Here we discuss the limit of Az correlated with the gross structure of the IAR inferred from the present data. In order to estimate this we tried to obtain a simplified expression for Az in terms of resonance parameters. Since Az measures the difference of cross sections for positive (o-+) and negative (~r_) beam helicities, it is given as

Az - o'+ - or_. (2) o-+ + o ' _

because lal >> Ibl for low-energy scatterings and also I cos01 > I sin01 at the backward angle of this exper- iment.

The T-matrix was derived by summing the reso- nance approximation formula given in Ref. [ 11 ] over many underlying p-resonances. Following the discus- sions given by Bowman et al. [5], we assume that the non-fluctuating part of the T-matrix arises from cou- pling of the IAR with the Pl/2-single particle state. Then the energy averaged T-matrix can be expressed as

Tl-'~=exp{i(~o+~l)} F~

V/~-o AR (IARIVpNcIP,/2) x (5) i E - ER + 7Fr

where IPl/2} denotes the wave function for the target ground state plus a valence proton in the pu2-orbit. The quantities ~0 and (1 are potential scattering phase- shifts for s- and p-waves, respectively. F IAR is the

ground state (s-wave) proton width of the IAR, F~ is an average ground state proton width of background p-wave resonances ,~ and FI is the p-wave single par- ticle width. Energy averaging is taken over an inter- val smaller than the total width of the IAR but wider than the average level spacing d of the background p-resonances. Now let us assume that the scattering amplitude a(O) equals the pure Coulomb scattering amplitude. Then the energy averaged Az is given by

For the elastic scattering of protons from a spin-zero target, the cross sections are given by

o-a: = La :~ a'[ 2 + Ib + b'[ 2, (3)

where a and b are parity conserving scattering ampli- tudes of nonhelicity-flip and helicity-flip parts [ 10], respectively, and a ~ and b ~ are parity violating scatter- ing amplitudes of the respective parts. In case of the s- and p-wave mixing, parity violating amplitudes are given by a ' = ~Tlo cos 0 and b' = ~kTl0 sin 0 using a parity mixing T-matrix, Tlo, where k is the wave num- ber. Then Az becomes

2Re{a*a' + b*b'} Az = la[Z + [a,i 2 + ib12 + ib,[ 2

_ Ira{a'T10 cos 0} (4) - k L a L 2 '

A--f ~_ 2r/ (sin c°s 0 sin((° +sol + ~ ) 7r~-'a ~V/~° d ' x/-~l

E - E R - A × (IARIVpNclPl/Z} (E - ER) 2 + ¼(Fr) z ' (6)

where r/is the Sommerfeld parameter, a = 2r/ln(sin o )

and A = ½Fr cot(s% + ~:1 + a ) . The solid curve in Fig. 3 shows the fitted result using

this equation. From this fit we obtained the upper limit of the PNC matrix element as

I(IARIVpNcIP~/2)] < 320 (eW). (7)

2~r rA Here we used that -2--1 equals the p-wave proton transmission coefficient and FI = 3 MeV. A con- stant offset of Az and also elastic scattering contri- butions from other isotopes were taken into account.

K. Kimura et al. / Physics Letters B 336 (1994) 303-307 307

This upper limit is still much larger than the estimate of the single particle matrix element, N50 eV [5], if the isovector PNC coupling constant is equal to the isoscalar part. Furthermore, since the wave function

of the IAR contains the single particle configuration, i.e. the target ground state plus sl/2-proton component,

with an amplitude of only 1 / ~ 1 [8], where T

is the isospin of the target nucleus, it might be nec- essary to lower the above upper limit further down to

50 eV/x /2T + 1 ,-~15 eV for observing any average PNC effect around the IAR.

Parity conservation in a compound nucleus reaction

proceeding through an isobaric analog resonance in heavy nuclei was tested for the first time. Using a thick target the longitudinal analyzing powers of the elastic scattering of protons on 9°Zr from Ep = 5.60 to 6.05 MeV covering the whole structure of the IAR were measured simultaneously. Although the present exper- iment did not give us conclusive evidence on parity violation, it would be very interesting to determine the magnitudes of parity mixing in isobaric analog reso- nances because this allows one to study the isovector

type of the parity nonconserving nuclear interaction.

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