Measurement of longitudinal asymmetry in neutron radiative capture reactions

Nuclear Physics A479 (1988) 737c-741c North-Holland, Amsterdam

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MEASUREMENT OF LONGITUDINAL ASYMMETRY IN NEUTRON RADIATIVE CAPTURE REACTIONS Y. MASUDA, T. ADACHI, S. ISHIMOTO, E. KIKUTANI, M. KOHGI', H . KOISO, A. MASAIKEZ and K. MORIMOTO National Laboratoryfor High Energy Physics, Oho-machi, Tsukuba-gun, Ibaraki-ken, 305 Japan Abstract: The helicity dependence was measured in neutron radiative capture reactions for various nuclei using a polarized low-energy neutron beam. The neutron spin was polarized upon passing through a dynamically polarized proton filter. The neutron polarization was around 72% . The value of the asymmetry A,, in the p-wave resonance of '39La at 0.734 eV was (9.4 :k 0.4)% . Measurements were also carried out for ""La, 9s Mo,'°oPd and 'x9Xe . For the resonance of "sLa at 2.99eV, A L = (0.37 -*0.22)% . For the p-wave resonance of'°s Pd at 2.96 eV, A L =-(0.3010.18)% . For the p-wave resonance of 9s Mo at 12.1 eV and the resonance of '29Xe at 9.5 eV, preliminary values of the asymmetry A,, were smaller than 0.4% and 1%, respectively.

1. Introduction Recently, new kinds of observables concerning a parity-nonconserving (PNC) effect in nuclei have been measured using polarized low-energy neutron beams. One of them is the neutron spin rotation during transmission through a medium') . Another is the longitudinal asymmetry in neutron absorption at a p-wave resonance''). The observed asymmetries in the latter experiments have been very large andbeyond theusual understandingof PNC effectsin nuclei . This is very interesting notonly fornuclear physics, but also forparticle physics.The results also encourage us to conduct experiments concerning T-violation in nuclei °) . In thepresent paper, we describe thehelicity dependence of the neutron radiative capture reaction, which was measured using a polarized low-energy neutron beam. 2. Experimental method The experiment was carried out using a pulsed neutron beam from the spallationneutron source at KEK (KENS) . Aschematicview of the e~perimental arrangement is shown in fig . 1. The neutron spin was polarized after being passed through a polarized proton filter which was installed in a neutron beam line s ). The filter consisted of ethylene glycol containing a few percent of Crv complexes. Protons in ' Department of Physics, îohoku University, Sendai, 980 Japan. 2 Department of Physics, Kyoto Univershy, Kyoto, 606 Japan. 0375-9474/88/$03 .50 © Elsevier Science Publishers B .V. (North-Holland Physics Publishing Division)

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Y. Masudu et al / Neutron radiative capture

polarized proton î71 "n . A c

-neutron source A

_

I

A

-

v

\ neutron collimator microwave 70 GHz 1 plastic counter spin

neutron beam

1

-

I' . -. T1,-0 1 / 1 ~~ 1V I sole- __T_ _-_ .r'

25T i

52/ .m magnet stlperconducting magnet /

6.47m

target

i

lom

neutron shield (B4C)

Fig. 1 . Experimental arrangement. The incident neutron beam was collimated to a 20 mm diameter circle in cross section and transmitted through a polarized proton filter of 24 mm width, 30 mm height and 10 mm thickness. The transmitted beam was also collimated to a 20 mm diameter circle. Since the transmitted neutron polarization was transverse to the beam, the nuetron spin was gradually rotated to the longitudinal direction by a 1 m long solenoid. The inner surface of the solenoid was covered with B4C in order to shield the y-ray counter encircling the target.

the filter material were dynamically polarized by means of 70 GHz microwaves at a magnetic field of 2.5 T in a 3He cryostat . Thetypical proton polarization was 80%. Since the filter was placed in a vertical magnetic field, the spin of the neutron was vertically polarized.Theneutron spin wasrotated to directions parallel or antiparallel to thebeam using a 150 G solenoid installed downstream of the filter. The magnetic field was designed so as to satisfy the condition for the adiabatic passage. A target was placed in the solenoid so as to be irradiated with a longitudinally polarizedneutron beam . The y-rays emitted from thetarget during radiative capture reactions were detected by an annular plastic-scintillation counter encircling the target. The counter was shielded by lead blocks against background y-rays emitted from surrounding materials. Scattered neutrons from the target were absorbed by a B4C powder inserted into the solenoid . The neutron energy was measured by the time-o :flight method (TOF) using the neutron production time and the -/-ray detection time as the start and stop timing, respectively. The neutron polarization direction was reversed every 2.5 sec. The neutron beam intensity wasmonitored before transmission through the filter andaftertransmission through the target by a U-fission chamber and a 'He counter placed in the neutron beam line, respectively. The neutron polarization was determined from the neutron transmission through the polarized proton filter using the formula P = i [refs . ea)] . Here, T and To are the neutron transmissions with and without proton polarization in the filter, respectively. A typical value of the neutron polarization was (72:±:2)% at 0.734 eV.

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3. Results and discussion In fig. 2, typicalTOFspectra observed fora La target around thep-wave resonance of "'La at 0.734 eV areshown. Thecrossesanddots represent thespectra forpositive and negative neutron helicity states, respectively . The background in the resonance region was estimatedby an interpolation of the background spectra near the peak. The asymmetry A was calculated using the equation P__N( +) -N(-) A N( +) +N(-) . Here, N(+) and N(-) are the counting rates of y-rays in the resonance region after subtracting the background for positive and negative helicity states, respectively, and P is the neutron polarization.The results of each run areshownin table 1. The asymmetry A was corrected for an effect due to a decrease in the incident neutron intensity by scattering andabsorption in thetarget.Thecorrectedasymmetry is defined as AL. The average value of the asymmetry A,, for the two runs is (9 .410.4)% . The asymmetry A,, is also plotted against the neutron. energy (fig . 4). TOFspectra around the 2.99 eV resonance of "8I .a were simultaneously observed, since ' 38 La is contained in thetarget with a natural abundanceof 0.09% .The spectra are shown in fig. 3. The asymmetry was determined to be A,,=(0.37±0.22)% . The asymmetry was also measured for"Mo,'OB Pd and 129 ,11e. For the p-wave resonance of 'a8 Pd at 2.96eV, A,,=-(0.3010.18)%. For the p-wave resonance of 98Mo at 12 .1 eV and the resonance of ' 29Xe at 9.5 eV, preliminary values of the asymmetry A,, were smaller than 0.4 and 1%, respectively. For the resonance of '39La at 0.734 eV, the helicity dependence was observed at Dubna by the transmission method 2). The asymmetry was determined to be AL = (7.3±0 .5)% . The origin of thedifference between Dubna's result and our result has

""its

c

01 .0

0.9

0.8

+ positive helicity . negative helicity "***. ..,... .. . . ... ... .»..

0.7

0.6

neutron energy (eV) Fig. 2. TOFspectraaround the0.734eV resonance of '"La . The crosses and dots refer to the spectra for positive and negative neutron helicity states, respectively.

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Y. Masuda et al. / Neutron radiative capture TABLE I

Values of asymmetry

'"La 0.734eV run 1 run2 average previous value (result at Dubna) -La 2.99 eV run 1 run2 average

PA(%)

P.(%)

7.110.4 6.910.4

7812 7212

A(%) 9.110.5 9.610 .6 9.3 :0 .4 9.5 :1 .2 `)

7412 70 :2

0.47:0.31 0.26:0.30 0.36 :0.22

-0.19:0.12

73 :2

-0.26 :0.16

"Pd 2.96eV

0.35* 0.23 0.18 :0.21

A,(%) ° ) 9.2 :0.5 1) 9.710.6 °) 9.4 :0.4 7.3 :0.5 °) 0.48 :0.32 b) 0.26 :0.31 b) 0.37 :0.22 -0.30:0.18

°) Theasymmetry wascorrected for an effect dueto adecrease in the incident neutron intensity by scattering and absorption in the target . b) Results for a25 mm diameter and 5mm thick La disk target. `) Results for a 20 mm diameter and 40 mm thick Laz03 powder target 3). °) Ref.z). 3

+ positive helicity " negative helicity

0

c0

2

a 0

0

4.0

3.5

30

2.0

neutron energy (eV)

Fig. 3. TOF spectra around the 2.99eV resonance of r3sLa. The two spectra for positive and negative neutron helicity states almost coincided with each other in this resonance. not yet been clearly explained. Regarding the detection method,the y-ray measur,,ment is useful for identifying the radiative capture reaction, while the transmission measurementalso includes an effect due to neutron scattering around the resonance. According to the theory of Sushkov and Flambaum, the large enhancement in 139La can be explained as follows'). First, the s-wave

the p-wave resonance of

admixture amplitude, which contributes to the PNC effect, is larger by a factor of

1/kR than the primaryp-save amplitude, where k is the neutron wave number and R is the nuclear radius . Secondly, tl:e narrow level spacing between s- and p-states

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a

T

N E E T N O

0.85

0.8

0 .75

0.7

0.65

neutron energy (eV) Fig . 4. Energy dependence of the asymmetry A,, around the p-wave resonance of "'La at 0 .734 eV.

contributes to a large enhancement in the parity mixing matrix element. Following the above explanation, the asymmetry has a Ilk dependence. In fig. 4, the Ilk dependence of the asymmetry.is indicated by the dotted line. The authors would like to thank Prof. H. Sasaki, Prof. N. Watanabe and Prof. H. Sugawara of KEK for their kind encouragt;ment and also Prof. J. Arafune of Tokyo University for his helpful discussions. The authors would also like to thank Mr. H. Fujimoto of Tohoku University and A. Konaka of Kyoto University fortheir kind help in the operation of the equipment during the experiment. References 1) 2) 3) 4) 5) 6) 7) 8)

M. Forte et aL, Phys. Rev. Lett. 45 (1980) 2088 V.P. Atfimenkov et aL, Nucl . Phys . A399 (1983) 93 Y. Masuda et al., Hyperfine Interactions 34 (1987) 143 E.G. Adelberger, Proc. 1984 INS-RIKEN Int. Symp. Heavy ion physics, Tokyo, J. Phys . Soc. Jpn. 54 (1985) Suppl. 16 S. Ishimoto et aL, Jpm . J. Appl . Phys. 25 (1986) L246 M . Ishlda et al., in preparation Y. Masuda er aL, ~Vuct. Instr. Meth., submitted V.I. Lushchikov et aL, Sov. J. Nucl . Phys . 10 (1970) 669 S. Hiramatsu et aL, J. Phys. Soc . Jpm . 45 (1978) 949 O.P. Sushkov and V.V. Flambaum, Sov . Phys. Usp . 25 (1982) 1