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Nuclear Physics A478 (1988) 533c-542c
North-Holland, Amsterdam
533c
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.
2 DPhN/ ME, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France.
3 SEPh-Dph/PE, CEN Sacalay, 91191 Gif-sur- Yvette Cedex, France.
4 STIPE-DPh/PE, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France.
5 LNS, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France.
E DPNC, University of Geneva, 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 ®trT and 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 measurements of pp spin observables has been achieved at SATURNE by the "NucleonNucleon 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 experiments have 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)
Y. Terrien et aL / Measurement at SATURNE
534c
with free polarized neutron beams: (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 differences dorL and tlaT ("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.Theprinciple is very simple : polarizeddeuteronsarebroken
up on a light target, 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 provide arather 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 adeuterium target). 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
neutron beam 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 neutron beam made at SATURNE (IKAR) .
1. ip elastic scattering at small transfer
This sectiondeals with themeasurements of differential crosssection and analysing
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, for this 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 between the inner anode 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 (I KAR) .
Y. Terrien et aL / Measurement at SATURNE
536c
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) .
®ARNOT
P.S.A . ---- BYSTRICKY et al .
FI
378MdV
(" -THIS EXPERIMENT
EXP.{ ® -BERSBACH et aL(tM)
l~ -CARL.NI et CIL W191
~°
°
w
°
®
c
°
c
"
481MeV
582MeV
784MeV
884
e1085 Nkv
0 0
Q01
.02
0
003
.04
0
0.05
0.
0.07
-t . (GeV/c) 2
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 differential 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 of small 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 causes of error, the main being background
subtraction and monitor calibration, 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
trtou which is well-known . The agreement of our data with the PSA predictions
shows a good experimental consistency between our normalization andthose of the
experiments giving trtat . 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 differences were measured forthe first time in
a neutron-proton transmission experiment. Here are presentedtheresults of der;(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 transmission 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. Terrien et al. / Measurement ai SATURNE
CH2
S3A
CH
T3A
S
Fig. 4 . Scheme of the experimental set-up used to measure Air., 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 concludes 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. For AoT the 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 the results of the
AQT.L
experiment
np
Tkin
GeV)
I= 0
=WoT
-AQL
value
(mb)
statistical
error
systematic
error
0.63
-4 .45
*2 .28
0.88
-2 .99
*2.05
0.98
-4.51
1 .08
-1 .33
1
2
value
(mb)
statistical
error
systematic
error
Im c
lm d
*0 .31
5.62
±1.27
±0.54
*0 .21
12 .51
±3 .53
±0.73
*1 .45
±0.33
10 .41
*2 .62
*0.62
±1 .47
±0.20
5.60
*2 .60
±0.35
0.376
±0.294
0.088
*0.396
-0.243
*0.370
-0.210
*0.392
-0.320
±0.294
-0.609
±0 .396
-0 .876
*0.370
-0 .295
±0.392
3
4
5
6
7
8
9
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 experiment had measured ®trL(pd) and 4aL(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 calculated by Kroll. Our results above 0.8 GeV seem to disagree with an extrapolation
of those obtained by the pd-pp subtraction method .
Fig. 5 shows also the PSA predictions . The analysis of Arndt et al.') (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 of input will also modify the Saclay-Geneva predictions for trt_
Twoofthethree imaginaryparts of invariant amplitudes forNN forward 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 of 0.9 to I GeV.
3. Study of inelasticchannels for the iip system
The resonance-like effects observed in the 3 F3 and '132 partial waves in pp elastic
scattering have been shown to be strongly coupled to inelastic channels, which has
5440cc
Y. Terrien et al. / Measurement at
10-
SATURNE
61tot =- AUT/ 2
8-
n-p
6-
420 [---- ----------------2
1
-4
E
60
-A6L n-p
40
20
-------------
0.2
04
0.6
0.8
Tkin (GeV)
1 .0
1.2
Fig. 5. Resultsof thepresentexperiment (solid dots)and values deduced from theZGS experiment ")
(open circles). Da3h-dotted lines are PSAofArndtet al ") andsolid lines are Saclay-Geneva PSA[ref. s)].
renewed, these last years, the interest in studying inelastic channels of a nucleonnucleon 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
Y. Terrien et ai. / Measurementat SATURNE
MWPC
MWPC
X
541c
3,iIn
Liquid
Target
Neutron
Beam
Fig. 6. Scheme of the experimental set-up used to Study
np inelastic channels (ARCOLE) .
N'
64
CORR
Fig. 7. Histogram of theangular correlation 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
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3)
4)
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F. Lehar, J. Phys . Soc. Jap. 55 (1986) suppl . 284
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Y . Terrien and F. Wellers, J. de Phys. 46 (1985) 1873
G.A. Korolev et al, Phys. Leu . 165B (1985) 262
A .V. Dobrovolsky et al, Nucl. Phys. B214 (1983) 1
G.N . Velichko et al, Sov. J. Nucl . Phys . 42 (1985) 837
R.A. Arndt, Phys. Rev. D28 (1983) 97 ;
Y. Higuchi et al, "l983 INS Symposium", Tokyo (1983)
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
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