Study of the reaction n + d → 3He + π− from 400 to 580 MeV

Volume 93B, number 4

PHYSICS LETTERS

30 June 1980

STUDY OF THE REACTION n + d ~ 3He + 7r- FROM 400 TO 580 MeV ~ J. F R A N Z , H.P. GROTZ, M. KLEINSCHMIDT, L. LEHMANN, P. REICHMANN, E. R()SSLE and H. SCHMITT Fakulth't /fir Physik der Universith't Freiburg, D- 7800 Freiburg, Germany

Received 11 March 1980

Differential cross sections of the reaction n + d --, 3 He + 7r- have been measured for pion emission angles 100° < 0cmn < 180° in the energy range 400 < Tn < 580 MeV. Slightly increasing angular distributions towards backward emission have been found at all energies.

Coherent pion production on nuclei has recently been studied with increasing efforts with the aim of exploring nuclear structure at high momenta as well as the reaction mechanism. The basic process of this type of reaction is the pion production on deuterons. Particular emphasis has been devoted to the threshold region and to the medium energy range where one can expect that the intermediate A33-resonance will dominate the reaction mechanism. Most o f the experiments are for the (p, n) reactions in the angular range o f 0 ~ n ~< 150 ° for the emitted pion and no resuits are available for the neutron induced processes. Here the only information comes from the time reversed process [1] n - 3 H e ~ dn with a maximum CM angle of 150 ° . Our experiment is complementary to this and covers the backward angles 07r cm up to 180 °. It is in this angular region o f largest m o m e n t u m transfer where the theories are most sensitive to their input, particularly to the underlying assumptions on the reaction mechanism and the wavefunctions used. This is mainly relevant for the shape of the angular distribution. At forward angles the models reproduce the rather steep decrease whereas in backward direction the predictions on the shape differ quite substantially and this is for different reasons. If one pion Work supported by the German Bundesministetium ffir Forschung und Technologie. 384

exchange as the dominant reaction mechanism is assumed the resulting flat backward angular distribution reflects the corresponding shape of rid-elastic scattering cross section [ 2 - 4 , 6 ] . On the other hand, realistic wavefunctions which reproduce the experimentally found dip in the charge form factor at large momentum transfer will also strongly influence the angular distribution at least at comparable values of momentum transfer [5,6]. Thus it is not surprising that the differences between the proposed theories appear especially in the backward region. The absolute values of the cross sections are in most cases normalized rather arbitrarily and thus cannot be compared directly. The measurements have been performed at the neutron beam at SIN with its continuous energy spectrum. The set up, sketched in fig. 1, is described in detail elsewhere [7]. The energy determination of the incident neutrons is performed by measuring the time of flight on a 60 m path. With a time resolution of 0.9 ns FWHM an energy resolution of better than 1.5% is obtained. The target consisted of a cylindrical cell (8 × 8 cm) with 125/am capton walls, filled with liquid deuterium. The reaction products have been analyzed by a magnetic spectrometer of large angular and m o m e n t u m acceptance equipped with drift chambers before and behind the magnet, as shown in the exploded view of fig. 1. Two drift chambers with

Volume 93B, number 4

PItYSICS LETTERS

30 June 1980

Experimental hail

6

s n E 1-neutron-beam-facility 1 n-production-target (Target E) 2 collimator 3 "f- absorber,cleaning-magnet,beam-stopper 4 cleaning-magnet 5 neutron experimental area 6 n-beam clump 7 magnetic spectrometer T:target, Di:drift-chamber, Si=scintillator

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ec" Fig. 3g. Fig. 3. Differential cross section of nd ~ 3Herr- at different energies. Results of other experiments [1,8 12] at nearby energies are included after conversion to this process. The lines represent theoretical calculations. Solid line: Locher and Weber [6], dashed line: Fearing [5], dotted line: Bhasin and Duck [2]. the errors and have been combined. The absolute normalization was obtained by comparison with the neutron proton charge exchange cross section which was measured under the same conditions. Due to the energy loss in the target only the 3He particles emitted forward in the center of mass system could be analyzed. The experimental data were binned in primary neutron energy intervals of 30 MeV. The experimental results are shown in fig. 3. For comparison with other experiments our differential cross sections are plotted versus the corresponding pion emission angle. A systematic uncertainty of about 5% arising from the absolute values of the reference cross sections is not contained in the indicated errors. The main feature of our results, given in fig. 3, is a rather flat or a slightly increasing angular distribution in the backward hemisphere for all energies. The very similar shapes of the differential cross sections result in excitation functions at a given angle of nearly the same small slopes. We have included in fig. 3 the results o f other experiments on the related reactions 7r-3He -+ nd [1], pd ~ t~ + [ 8 , 1 0 - 1 2 ] and pd ~ 3HeTr0 [ 9 , 1 1 - 1 3 ] after due transformation to our process, assuming detailed balance or isospin invariance to be valid. Good

30 June 1980

agreement is found with the results o f K~illne et al. [1] in the overlapping angular region of the inverse reaction at TTr- = 200 MeV, corresponding to a neutron incident energy o f 504 MeV. This is in contrast to the rather strong increase found by Dollhopf et al. [8] at 470 MeV for the reaction pd -+ tTr+. It can also be noticed that we do not reproduce the decreasing slope found by the same authors [8] at 590 MeV incident energy. But around 07r = 110 ° our cm results agree well in magnitude with that o f Dollhopf et al. [8] and Carroll et al. [9]. It sould be mentioned that no previous experiment covered the extreme backward angles at 0 ~r > 160 °. cm For comparison with theories we have included predictions from three authors [2,5,6]. The curves have been taken from the figures and transformed to our reaction channel if necessary. The shapes of our angular distributions are best described by the models [ 2 - 4 ] which relate the reaction to pion rescattering or one pion exchange, respectively. In these models the process is related to 7rd scattering as a subprocess and the observed flat angular distributions reflect the corresponding shape of the elastic 7rd cross sections. This is supported by the excitation functions of the very backward angles which show rather little energy dependence. The absolute values of the cross sections in ref. [2] are too large however even by treating the triton form factor as a free parameter to be adjusted to the experimental data in the forward direction~ The discrepancy can be attributed to ~d scattering data used in this case. With recent experimental results on 7rd-scattering as input Gibbs and Hess [3] get reasonable agreement with experiment at backward angles. Calculations on the basis of a two nucleon model have been performed by several authors [ 4 - 6 ] . In this model the coherent pion production on deuterons is described in terms of the relevant NN ~ dTr cross section. The results are in fair agreement with experiment below and around 470 MeV as far as the shape is concerned. The results of Fearing [5] using realistic wavefunctions in a DWIA calculation do not reproduce the observed flat distribution at higher energies. Better agreement is obtained by Locher and Weber [6] by taking into account b o t h the pion exchange amplitude and the two nucleon contribution. For the comparison (fig. 3) we took the results of the coherent difference o f these two amplitudes. 387

Volume 93B, number 4

PHYSICS LETTERS

On the basis o f a microscopic description Green and Maqueda [14] c o n c l u d e that m o r e refined m o d els are needed to a c c o u n t for the observed cross sections in the energy region where the A33-resonance plays a d o m i n a n t role.

References [1] J. l~ffllne et al., Phys. Rev. Lett. 40 (1978) 378. [2] V.S. Bhasin and I.M. Duck, Phys. Lett. 46B (1973) 309. [3] W.R. Gibbs and A.T. Hess, Phys. Lett. 68B (1977) 205.

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[4] G.W. Barry, Phys. Rev. D7 (1973) 1441. [5] II.W. Fearing, Phys. Rev. C l l (1975) 1210; Phys. Rev. C16 (1977) 313. [6] M.P. Locher and H.J. Weber, Nucl. Phys. B76 (1974) 400. [7] Th. Fischer et al., Nucl. Instr. Meth. 156 (1978) 199. [8] W. Dollhopf et al., Nucl. Phys. A217 (1973) 381. [9] J. Carroll et al., Nucl. Phys. A305 (1978) 502. [10] E. Aslanides et al., Phys. Rev. kett. 39 (1977) 1654. [11] D. Hatting et al., Phys. Rev. 119 (1960) 1716. [12] A.V. Crewe et al.,Phys. Rev. 118 (1960) 1091. [13] N.E. Booth, Phys. Rev. 132 (1963) 2305. [14] A.M. Green and E. Maqueda, Nucl. Phys. A316 (1979) 215.