DSpace at VNU: MEASUREMENT AT SATURNE, WITH A FREE POLARIZED NEUTRON BEAM, OF CROSS SECTIONS AND SPIN OBSERVABLES FOR THE NEUTRON-PROTON SYSTEM

Nuclear Physics A478 1988 533c-542c 533c North-Holland, Amsterdam MEASUREMENT AT SATURNE, WITH A FREE POLARIZED NEUTRON BEAM, OF CROSS SECTIONS AND SPIN OBSERVABLES FOR THE NEUTRON-PROTO

Nuclear Physics A478 (1988) 533c-542c 533c North-Holland, Amsterdam

MEASUREMENT AT SATURNE, WITH A FREE POLARIZED NEUTRON BEAM, OF CROSS SECTIONS AND SPIN OBSERVABLES FOR THE

NEUTRON-PROTON SYSTEM (Presented by Y Terrien) Section 1 : Y TERRIEN2, A.V DOBROVOLSKY`, A V KHANZADEEV`, G.A KOROLEV', J.C LUGOO, G.E PETROV, J SAUDINOS 5 , B.H SILVERMAN2, E.M SPIRIDENKOV',

A.A VOROBYOV% F WELLERS?

Section 2 : F LEHAR3, J BACHE, J BALLS, P CHAUMETTE° , J DEREGEL° ,

A DE LESQUEN3, M DE MALI 4, J FASRE, J.M FONTAINE=, G GAILLARDE,

V GHAZIKHANIAN', R HESSE, C.D LAC S , D LEGRAND2, F PERROT2,

R PESCHINA", E ROSSLE', PH SORMANI E, L VAN ROSSUMS, C.A WHITTEN' Section 3 : Y TERRIEN 2 , R BEURTEY 5 , B BONIN2, G BRUGE2, P COUVERT', J.C DUCHAZEAUBENEIX2, B FABRO 2 , J C FAIVRE2 , B MAYER?, M ROUGER?,

J SAUDINOSS , B.H SILVERMAN2, F WELLERS2, C WHITTEN2

Leningrad Nuclear Physics Institute, Gatchina, Leningrad 188350, USSR

2DPhN/ ME, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France

3SEPh-Dph/PE, CEN Sacalay, 91191 Gif-sur- Yvette Cedex, France

4STIPE-DPh/PE, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France

5LNS, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France

E DPNC, University ofGeneva, Geneva, Switzerland

University of California, Los Angeles, California, USA

s Albert-Ludwigs University, Freiburg, West Germany

Abstract: Experiments currently done at SATURNE (Saclay) with a free polarized neutron beam are described These are measurements for the np system of elastic and inelastic cross sections and analysing powers, and of ®trTand do,,

Avery large effort hasbeen devoted, theselast years, to thestudy of theelementary proton-proton interaction at intermediate energies, which can now be considered

as rather well known In particular, an extensive experimental program of measure-ments of pp spin observables has been achieved at SATURNE by the "Nucleon-Nucleon group", which has produced very interesting results for du,,, AQTand for single- and double-spin elastic scattering observables t)

For the neutron-proton interaction, the situation is much worse Data are scarce : the spin-averaged total cross section tr_ is known, but, for elastic scattering, the differential cross section and analysing power are poorly known (especially at small transfers) and only a few other spin cbservables have been measured Moreover, these experimentshave oftenbeen done with quasi-free scattering of neutrons bound inside a deuteron (beam or target) instead of using a free neutron beam I report here the results of two experiments done at the synchrotron SATURNE (Saclay) 0375-9474/88/$03.50 © Elsevier Science Publishers B V

(North-Holland Physics Publishing Division)

534c Y Terrienet aL / Measurement atSATURNE

with free polarized neutronbeams: (i) the measurementof the np elastic differential cross section and analysing power at small transfer (.ALA experiment: sect 1) ; (ii) themeasurementof the np total cross section differencesdorLandtlaT("NN group" experiment: sect 2) Some insights on the currently beginning studies of inelastic channels of the fip system arealso presented (ARCOLE experiment: sect 3) A few words about our polarized neutron beams The first was designed for the IKAR experiment, then asecond onewas made fortheNN groupbeam line with essentially thesame characteristics.Theprincipleis very simple : polarizeddeuteronsarebroken

up on a lighttarget, followed by amagnetic field which sweeps the charged particles (protons from the break-up or residual deuterons); then a several meter long iron collimator of a small diameter set at 0° selects the "spectator" neutrons which constitute the beam This hasbeen ;shown 2) to providearather monoenergetic beam (the energy width is mainly due to the Fermi momentum of the neutron in the deuteron), with good intensity (the yield of neutrons obtained per unit of incident deuteron beam intensity is a few 10 3 better than that obtained by a knock-out reaction with proton beams on adeuteriumtarget) Forincident deuterons polarized

at P percent in any direction, the neutrons at 0° remain polarized with practically the same percentage in the same direction Fig 1 shows a schematic picture of the neutronbeam made for the IKAR experiment, where, with the diameter of the iron collimator restricted to 1.5 cm to get a well defined beam, we could obtain 106 neutrons for the 3x 10'° deuterons available at SATURNE before the current installation of the new pre-injector MIMAS An original method calibration of the neutron beam ïmensity based on the comparison between the n-°He and p.4He elasticdifferential crosssections 3)hasbeen used to obtain theabsolute normalization

of the cross sections presented in sect 1

d Beam

Polarized neutron beam

Fig 1 Schematic picture of the'polarized neutronbeam made at SATURNE (IKAR)

1 ip elastic scattering at small transfer

This sectiondeals with themeasurements ofdifferential crosssectionandanalysing powerfor np elastic scattering Partial results concerning the analysing power have already been published 4) 1 present here the results of measurements of absolute differential cross section made with essentially the same apparatus, for the elastic

Y Terrien et aL / Measurement at SATURVE 535c scattering of a free neutron beam by protons, at T= 378, 481, 582, 683, 784, 884 and 1085 MeV The experimental set-up is shown in fig 2 The chamber IKAR is filled with CH4 gas at 14.2 atm, and serves as both target (hydrogen) and detector

It has been used many times associated with an external trigger [see, for example, ref.'): measurement of pp elastic differential cross section] It is an ionization chamber with a cathode, a grid and, forthis experiment, five concentric anodes set perpendicular to the beam direction An the volume between the cathode and the grid, electrons created along the track of the recoiling proton, which is emitted at slightly less than 90°, migrate towards the grill tinder the action of an electric field parallel to the beam direction and are collected by the anodes The cathode signal

is used as a trigger, this auto-triggering mode of operation of MAR being used for the first time The time difference betweenthe inneranode andcathode signals (8 ws maximum) gives the position of the interaction vertex, while the small differences between the times of the anodes are related to the recoil angle of the proton For recoil protons which stop inside the volume seen by the anodes, the sum of the amplitudes of all anode signals is proportional to the recoil energy T, Protons of energy greater than 15 MeV leave the chamber, so we compute their total energy

Tr from their energy loss in the gas Correlations between the amplitudes of the

Rl R2 R3 Fig 2 Experimental set-up of the np elastic scattering experiment (IKAR)

536c Y Terrien et aL / Measurement at SATURNE

anode signals allow us to identify the recoiling particle as aproton, and correlations between amplitudes and times (i.e between T, and e,) allow separation of free elastic scattering events from the quasi-free scattering of protons in CH, For the measurement of analysing power"), since IKAR does not provide any information

on theazimuth of the recoil proton, we detected the forward scattered neutron with left and right high efficiency (=70%) neutron counters made of large plastic scintillators blocks (fig 2)

P.S.A ®ARNOT BYSTRICKY et al (" -THIS EXPERIMENT 378MdV EXP.{ ® -BERSBACH et aL(tM)

l~ -CARL.NI etCIL W191

~° ° w ° ® c ° c " °

481MeV 582MeV

784MeV 884

FI

e1085 Nkv

0 0

0.02 003 0.04 0.05 0 0.07 -t (GeV/c) 2

Q01

Fig 3.Absolute differential cross section for np elastic scattering

Y Terrien et at, Measurement at SATURNE 537c Having obtained, for each "good elastic scattering event", the transfer -t by the formula (valid forelastic scattering) -t =2mTr, we canextract the absolute differen-tial cross section from the experiment provided that we have calibrated the monitor viewing the neutron beam This monitor is made of 2 plastic scintillator telescopes locatedleft and right at 6° away from the neutron beam Nothaving the possibility, with our apparatus, to measure the charge exchange reaction p(n, p)n which is one usual way to calibrate neutron beam monitors, we have used an original method Having demonstrated that nuclear differential cross sections for n4He and p-4He can be considered as equal within 1-2% in the range ofsmall transfers 3), we have filled IKAR with He and measured relative cross sections with our uncalibrated monitor Comparison with the previously measured absolute p-'He cross sections ~) gives an absolute calibration of the monitors

Our results are presented in fig 3, where the (small) errors bars are statistical only Taking into account all other causesoferror, the main being background subtraction and monitorcalibration, we estimate the overall normalization error of the cross sections presented in fig 3 to be 4-7% depending on the energy In fig 3 are also presented the results of phase-shift analyses (PSA) made by Arndt (solid line, ref.')) and Bystricky et al (dotted line, ref s)) before the inclusion of our normalized data For Bystricky et aL, a preliminary unnormalized version of our data was included in the ft, which constrains the slope In these analyses, tire normalization of d(r/dt is mainly governed by the size of the np total cross section trtouwhich is well-known The agreement of our data with the PSA predictions shows a good experimental consistencybetween our normalization andthose of the experiments givingtrtat.It must be noted, however, that ourresultsat 378 MeV are higher than those predicted, by more than the experimental error

2 Measurement of Aar(op) and Aa&p) from 0.63 to 1.08 GeV Spin-dependent total cross-section differenceswere measured forthe first time in aneutron-proton transmission experiment Here are presentedtheresults ofder;(np) and daL(np) at 0.63, 0.88, 0.98 and 1.08GeV These results on the isospin mixed

I =0, 1 np system together with the corresponding pp (pure I =1) data yield two

of the three spin-dependent forward scattering amplitudes for isospin I =0 The free neutron beam, with a polarization oriented either along ii (vertical) or

k (horizontal) directions, was transmitted through the 35 mm thick, 40 mm wide and 49 mm high polarized proton target') The target material was pentanol, with

a typical proton polarization of 85% The relaxation time of the target was -30 days at 38 mK in a holding field of 0.33 T

The experimental set-up is shown in fig 4 The beam monitor S and the trans-mission detector T were of similar design The 40 mm thick CHZ convertors are placed immediately after large veto scintillators, and charged particles emitted forward are detected by two counters in coincidence The radiator of the beam

53ßc Y Terrienet al / Measurement ai SATURNE

T3A S3A

S

Fig 4 Scheme of the experimental set-up used to measureAir.,and dog for the np system monitor (and of the transmission detector) is followed by two scintillators S1 (TI) and Ç2 (T2) of the same diameter Compared to the active-target method this array

is less efficient but provides better stability of detection efficiency since the pulse height distribution is peaked far above the discriminator threshold Note that only stability is important The results depend neither on the absolute efficiencies of S and T nor on their ratio On the other hand the detection efficiency (1 3% for S and 1 8% for T) must be independent of the neutron polarization The solid angle subtended by the transmission detector at the center of the target could vary between

17 = 0.6 and d = 2 5 mss Most of the data were taken with 17 = 2.5 msr (0 =1 6' lab) From a comparison of pp and np nuclear scattering amplitudes up to 0 =1 6° lab, one expects practically no difference for the slopes of the transmission functions

Aa=f(D) From the measured slopes in pp transmission experiments to) one con-cludes that the difference between the measured values ®trL (B =1 6°) and the extrapolated value ®o-L(f)=0°) is less than 0.1 mb This is much smaller than the experimental crcors in the present experiment ForAoTthe expected slope is of the same order as for ®o-,, The probability of missing a count in the transmission detector because of quasi simultaneous beam particles is 1 5 x 10 -6

The efficiency of the detectors S and T will depend on the transverse beam polarization if the detector components or the entire detectors are misaligned perpendicularly to the beam polarization The resulting effects are independent of the target polarization All these effects cancel out when taking the simple average between measurements with opposite target polarizations This was verified at two energies, with a precision of about 1 mb (statistical error)

The total number of counts in the transmission counter was 0.5 x 10" for Ao-T at 0.63 GeV and ^-1 6 x lOs for the other measurements

The results calculated using the formulae of ref '°) are given h table 1 The errors given in columns 3 and 6 represent statistics and the random-like fluctuations The systematic errors in columns 4 and 7 take into account the uncertainties in beam and target polarizations, in ' ydrogen content in the target, and an estimate of the residual error due to misalignments The sum of the two errors is an upper 1?mit of the total error No correction was made for the finite solid angle

Y Terrien et al / Measurementat SATURNE 539c

TABLE 1

In columns 2to 7 are given theresults of the AQT.L experiment

np

Errors are defined in the text Columns 8 and 9 show the results for two of the forward scattering amplitudes in units of o mbr

Fig 5 showsthe results of the present experiment (solid dots) Fig 5b shows also other results calculated from data obtained at ANL ZGS [ref.'s)].The ZGS experi-ment had measured ®trL(pd) and4aL(pp)by transmission of polarized protons through a partially deuterated polarized target Determination of dt7L(np) from these data requires corrections for 3-body final state interactions which were calcu-lated by Kroll Our results above 0.8GeV seem to disagree with an extrapolation

of those obtained by the pd-pp subtraction method

Fig 5 shows also the PSA predictions The analysis ofArndt etal.') (dash-dotted line) includes none of the data points shown The Saclay-Geneva PSA [ref.")] (solid line) includes the ZGS data (open circles) Replacing the ZGS points by the newdata will result in an energy dependence of dtrL closer to the one predicted by Arndt This change ofinput will also modifythe Saclay-Geneva predictions for trt_ Twoofthethree imaginaryparts of invariantamplitudes forNNforward scattering

in the isospin I = 0 state were determined using the relations from ref.12).Im c and

Im d for I=0 were calculated using for pp the PSA fit to the data s) and for np the values given in columns 2 and 5 The magnitude of (Im c) is small and the sign

is not well determined at the present level of experimental precision The value of (Imd) seems to be larger and negative, at least in the region of0.9 to I GeV

3 Study of inelasticchannels for the iip system

The resonance-like effects observed in the3F3 and '132 partial waves in pp elastic scattering have been shown to be strongly coupled to inelastic channels, which has

GeV)

value

(mb)

=WoT

statistical

error systematicerror value(mb)

- AQL statistical error systematicerror

I=

Im c

0

lm d

±0.294 ±0.294

*0.396 ±0 396

*0.370 *0.370

*0.392 ±0.392

5440cc Y Terrien et al / Measurement atSATURNE

E

-0.2 04 0.6 0.8 1 0 1.2

Tkin (GeV)

Fig 5 Resultsof thepresentexperiment (solid dots)and values deducedfrom theZGS experiment ") (opencircles) Da3h-dottedlinesare PSAofArndtet al ") andsolidlinesare Saclay-Geneva PSA[ref.s)] renewed, these last years, the interest in studying inelastic channels of a nucleon-nucleon system While much data exists for the pp (I=1) system, very little has been measured for the isospin mixed I=0, 1 np system Using our free polarized neutron beam, we want to investigate the fip->ppir and fip-dw'lr reactions The first is one of the major inelastic channels fur the np system In both cases, the physical purposes are, mainly, to estimate the contributions of the intermediate isobaric states QN and ®®, and to try to determine whether, for the reaction fip->ppor-, there exists a contribution of the I=0 amplitude, which is forbidden

to proceed via the AN intermediate state (isospin conservation) We will also be able to search for narrow states in histograms of invariant mass of 2 protons (in fip-ppir-) and of 2 pions (in fip->dir'lr-)

Theexperimental method that we useis based on kinematical considerations We have performed a first experimental test with a system (ARCOLE) composed of lateral andforward multiwire chambers and a trigger of plastic sc'tntillators (see fig 6) This set-up permits measurement of the 6 angles defining the directions of the

3 outgoing charged particles Since only 5 independent quantities are needed to

6-

4-

2- -2

-4

0

[ 1

60

-A6L n-p 40

20

64

Y Terrien et ai / Measurementat SATURNE 541c

MWPC

Neutron Beam

MWPC

Liquid Target

3,iIn

X

Fig 6 Scheme of the experimental set-up used to Studynpinelasticchannels (ARCOLE)

CORR

Fig 7 Histogram of theangularcorrelation function for the reaction Hp-.ppv.

542c Y Terrier et aL / Measurement at SATURNE

define the kinematics of an event, these 6 angles must verify a correlation function, different for each reaction Indeed, one can see in fig 7, where we present the histogram of the values of this function for all events assumed to come from

np-pprr -, that the events which come from this reaction (channel 100) and those which do not are fairly well separated

References 1) F Lehar, J Phys Soc Jap 55 (1986) suppl 284

2) G Bizard et al., Nucl Instr Meth 111 (1973) 451

3) Y Terrien and F Wellers, J de Phys 46 (1985) 1873

4) G.A Korolev et al, Phys Leu 165B (1985) 262

5) A V Dobrovolsky et al, Nucl Phys B214 (1983) 1

6) G.N Velichko et al, Sov J Nucl Phys 42 (1985) 837

7) R.A Arndt, Phys Rev D28 (1983) 97 ;

Y Higuchi et al, "l983 INS Symposium", Tokyo (1983)

8) J Bystricky, C Lechanoine-Leluc and F Lehar, J de Phys 48 (1987) 199 and preprint DPhPE 86-28, Saclay 1986, J de Phys., submitted

9) R Bernard et al., Nucl Instr Meth A249 (1986) 176

10) F Perrot et al, Nue: Phys 8278 (1986) 881

11) I P Auer et al, Phys Rev Leu 46 (1981) 1177;

P Kroll, private communication (1982)

12) J Bystricky, F Lehar and P Winternitz, J de Phys 39 (1978) 1