Spin observables in neutron-proton elastic scattering

Article

Reference

Spin observables in neutron-proton elastic scattering

AHMIDOUCH, Abdellah, et al.

AHMIDOUCH, Abdellah, et al . Spin observables in neutron-proton elastic scattering. PSI

annual report. Annex 1, Nuclear and particle physics newsletter, Muons in solid-state physics and chemistry , 1992, p. 33-36

Available at:

http://archive-ouverte.unige.ch/unige:113770

Disclaimer: layout of this document may differ from the published version.

The knowledge

of

the hadronic forces

is

one

of

the cen-

tal

^questions

for our

understanding

of the

confined and stable matter.

A

microscopic and non perturbative theory will hopefully derive these interactions which are,

in

terms of quarks and gluons, analogous

to

the valence forces

of

Van der Waals. But basic problems remain almost

fully

un- solved: the

validity of

the constituent quark model, the ex- istence of

3-q

forces, the relations between the chiral sym- metry, the confinement and the spin-parity

of

the hadrons, the virtual existence

of

a strange quark

in

the proton, etc.

An older, alternative approach explains these forces in term of potentials based

on

the meson exchanges between two nucleons. The main weakness

of

this point

of

view is that

these exchanges take place over distances which are similar to or smaller than the overlap dimension

of

the interacting hadrons.

An

excellent survey and criticism

of

^the present theoretical ideas has been presented by D. Wilkinson _[1].

SPIN OBSERVABLES IN NEUTRON.PROTON ELASTIC SCATTERING

R-87-L2, FREIBURGÆR

-

GENEVA

-

^pSI

-

PRAGUE

-

SACLAY

A. Ahmidouch*, J.

Arnoldt, B.

van den Brandtt,

M.

Dauml, Ph. Demie-rre*, R. Drevenak$,

M.

Finger$, M.J. Finger$, J' Franzt,,N. Goujon*,

N.

Hamann**, E. Heer*, R. Hess*s, Z.E. Janout$, J.A.

Konterl,

C. Lechanoine-Leluc*, F. Lehartt, S. Mangot, Ch. Mascarini*, Ch. Pletzl , D. Rapin*, E. ^Rôsslet, R. Schirmaiert,

p.A.

Schmelzbachl,

tt. s.h*ittis, ^--

M. Slunecka$,

B.

Vuaridel*

*

DPNC, University

of

Geneva,

CH-Izll

Geneva, Switzerland

f

^Fakultât

^fùr

Physik der Universitiit Freiburg, D-7800 Freiburg, Germany

*

^{PSI, Paul Scherrer Institut, CH-5232}

Villigen-pSl,

Switzerland

$

^Charles University, Prague, Czechia

T ^DAPNIA,

CEN-Saclay, F-91191 Gif-sur-Yveue, France

**

GERN, on leave from Fakultât

fiir

Physik der universitât Freiburg, Germany

Introduction

sis on experiments testing the fundamental symmetries, like charge symmetry and parity conservation. Thus

in

contrast

to the

situation

for

the

pp

spin data,

a

paradoxical lack

of

data exists below 500

MeV on

neutron-proton scatter-

ing

and the

I=0

phase shift analysis remains unprecise [6].

The largest possible set

of

observables and a strong cross checking

of

the measurements by different laboratories are necessary conditions to control the systematic enors. In the future, SATURNE

will

extend the

fri

measurements up to 3 GeV by using a 6LiD polarized target. LAMPF

will

study the

df

^{inelastic reactions.}

In

1986 a polarized neutron beam was successfully set up

in

the

nEl

beam

line.

This beam was used for the mea- surement

of

the spin correlation parameters Aoonn, Aoo"r,

Aq6s

and Aook,,

[8]

^and

of

the spin dependent total cross section differences

Aa;(np)

and

Aa7(np)

[9].

NA2 Beam Line Experimental Situation

"Nucleon-nucleon elastic scattering and total cross sections"

have been reviewed recently

[2].

^The

N-N

elastic scatter- ing is one of the most basic reactions involving the strong interaction.

In

general, there are

five I _{= 0}

^and five

I ₌ I

scattering amplitudes at each angle and energy. Therefore,

at least ten spin observables must be measured for pp scat- tering and for np scattering to obtain unique values for both

the real and imaginary parts

of

these amplitudes.

A

^wealth

ofnew data on proton-proton spin observables has permit-

ted a reasonable determination

of

^the

I ₌

1 amplitudes:

in

terms of phase shifts and inelasticities

up to

1800 MeV,

and also in terms of a model-independent fashion over part of the scattering angle range up to 2800

MeV

and at 5140 MeV.

The neutron-proton elastic scattering

is

investigated at all the accelerators producing _{a beam} ofpolarized neutrons, i.e. TRIUMF between 200 and 520

MeV, LAMPF 500-

800 MeV, SATURNE

II

80È1100 MeV, and PSI 200-560 MeV. Looking at the spin observables, one finds today that LAMPF [3] ^and

SATURM [4],

^have produced many pre-

cise data of different kinds, over a large angular acceptance and at different energies.

TRIUMF

has

put

more empha-

In

December 1991, the new

NA2

beam line came into op- eration. This very intense polarized n-beam, created by the charge exchange reaction

C(F,flX

at 0o, has unique fea-

tures:

1) continuous energy between

-250 ^MeV .*

₅₈₀

MeV,

2) average polarization -35Vo

-

45Vo with all ^possi- ble orientations (.î,

â,

Â;, 3; intensity

-

5.106f/s .cm2 at 12

m from

the production target using the polarized

fbeam of

very high intensity

of

PSI (10

pA)

(see

ref. t10l).

^Fol-

lowing the

same approach as the one used

in pp

elastic scattering

[7],

^{we expect}

to

measure the spin observables shown

in

table

I in df

elastic scattering between 300 and 580

MeV

and between 90o and 1600 c.m. This

will

enable us to reconstruct the 5 complex amplitudes

of

the nucleon- nucleon system with isospin I ₌

0.

The experimental layout has

two

target stations

working

simultaneously. The first experiment

is

equipped

with

the polarized target. The sec- ond experiment has an

LH2

target and

will

observe new spin parameters where the longitudinal polarization

of

re- coil protons is analyzed. Both set-ups are

fully

operatonal.

Three periods

of

polarized beam

time

were attributed in 1992

to

this

program:

the March period was affected by the setting-up

of

this new beam

line

and

only 5

effective days were made available. During the period

of

July, the beam was given to the medical operation during the day and

7

4000

3000

2000

1000 a

k Hd

H

z

o

Figure

1:

Energy spectrum

of

the 0o neutron beam in the

NA2 area. As

compared

to the 'old'

3.40 spectra from the

nEl

beam, the quasielastic peak is reduced due to Pauli blocking.

this mode

of

running gave unexpected

difficulties; only

1

week was

effectively

available

for the np-program.

The period

of

October was very successful and a beam of more thanT

pA

of polarized protons was delivered at the neutron production target routinely during 2 weeks.

Neutron Spectrum in NA2

Neutrons produced

by

590

MeV

^protons

on

carbon have a continuous energy spectrum, consisting

of

a quasielastic peak around 540

MeV,

and

a

broad continuum

at

lower energies. This spectrum

is well

known

from

the old

nEl-

beam

[11],

^{where neutrons}

at

a production angle

of

3.40 were

used. In

the

NA2

beam, neutrons are collimated at 00

with

^respect

to the primary beam. This

^means

for

a quasielastic process on a nucleon

in

the target nucleus, that the proton remains

with

small momentum

in

the nucleus after the interaction, and thus one may expect a reduction of the 0o quasielastic peak due to Pauli blocking.

We have measured the neutron energy spectrum with the symmetric two arm time-of-flight spectrometer, installed at the center

of

experiment

II.

Details

of

the set-up have already been

given in a

^previous

report[lO]. Two

more detectors were added on the small angle side

of

each arm,

in

order to increase the solid angle. The measurement was done with a CH2 target, and elastic events were selected by requiring coplanarity and the correct opening angle

for

the detected neutron proton

pair.

The energy

of

the incoming neutron was measured by time-of-flight between the signal

in

one of the start counters (recoil proton) and the rf-signal

of

the accelerator, which was operated

in

the 17

MHz

(60 ns) mode.

From the reconstructed elastic events, the energy distri- bution

of

the incoming neutrons was obtained [12], ^taking

into

account the elastic scattering cross section on hydro- gen. The result is shown in

fig.

1. Compared to the energy spectrum

of

the

nEl

(3.4o) beam there

is

a clear suppres- sion of the quasielastic peak with respect to the low energy continuum.

Neutron Polarization in NA2

The same elastic event sample could also be used

to

ex-

tract the neutron beam

polarization. As

described

in

^out previous report

[0],

^the neutrons are produced from longi- tudinally polarized protons. For the present measurement, the neutron polarization was turned to the vertical direction with two dipole magnets behind the collimator. The settings were chosen

for

550

MeV

neutrons, leaving a longitudinal component

at

smaller energies. The polarization was in- verted every second at the

ion

source. The polarization of the neutron beam was determined

from

the asymmetry €

given by

,E,E_\ffi

^/il ^{.nt + \/R1 .LI}

(1)

where

L

and R means neutron detected

in left

or right arm, and the index arrow means spin of the incoming neutron up

or down. With

this method, taking advantage

of

the sym-

metric set up, detector efficiencies and acceptances ^cancel to a large extent. The resulting polarization

[2]

^{as a func-}

tion of

the energy

is

shown

in fig. 2. As

compared with our earlier results

[1]

^{at 3.4o} production, there

is

an ex- cellent agreement of the energy dependence. However, the absolute value

of

the neutron polarization

is

smaller. This

is

due to a smaller

initial

proton polarization (0.75)

in

this experiment, as compared to the earlier value [11] of ^0.85,

The beam polarization P6 was also checked at the en- ergy of the quasielastic peak using the setup of the polarized target station (experiment

I) but with a

CH2

target.

Fig- ure

3

shows the measured analyzing power Aoo,.o

of

^the

ôp

scattering, normalized

with

P6

=

^0.36

+

0.02 to phase

shift analysis predictions (PSA). This value of P6 is in good agreement

with

the above results. The angular dependence

of

Aoono also reproduces

well

the PSA predictions.

-0.1

200 300 600

E. tMeM

KIN '

Figure

2:

Neutron polarization as a function

of

the en-

ergy.

The agreement

with

our previous result

is

excel- lent,

if

^{the present} proton polarization

of

0.75

is

taken

into

account.

T b o 6

100

²⁰⁰

300

⁴⁰⁰

E61,IMeV]

500

⁶⁰⁰

Ao

.9 €

H

ip

o lrd - o ^-03

È

Fil

Ao

-j

Th

izit in,

S1

At

lari

difl

ics, dih cial ope

imt

are the targ tere scin

oft

usin spe( syst loca cess histr the {,4

4,5

400

't 00

*lr+l

J

l.l

'i

o

Ic

Kosko Kokho o

n Aoono Konno

P(= A)

o I Korro Kokro n

k

K

orko

Norkn n

n Aoonn Konno Donon

P(: A\

n

6

Korro Norrr.

s s

Aoo*

I k Aooks k

k

Aookk Thrget orientation

Beam orientation No re-scattering

With

re-scattering of recoil proton

in

the polarimeter

Table ¹

: List

^{of spin} observables to measure. We used a four-index notation, X d, ! ", ^{which refers to scattered} (d), recoil (r), beam

(f)

and târget (c) spin orientations. Each index can take either value

k, n,

s or o according to the particle polarization orientation. The direction  is defined as being along the particle trajectory,

â

along the normal to the scattering plane, and ,î orthogonal to the other two axes

(â x

/c). o means either unpolarized or that the particle polarization

is

not observed.

Aoono

have registered

-

^{15.106 double} scattering raw np-events during the March period to measure the parameters Aoono, Aonoo and Konno,

-

30'106 single scattering raw np-events

in

July

to

measure the spin correlation panrmeters Aoonn, Asnlrp and Aoork, and

-

^{30. 106 double} scattering raw np- events

in

October to measure the parameters Aoono, Aonoo, Konno,Korro

àîd Koso.

^The data analysis

is

^performed at Geneva University on an Appoll??o-network (consisting

of

^{1 Appoll??o}

^DN

^10000,

⁴ DN

2300 stations, exabyte carhidges) and at Prague University.

500 0r,

SA

Spectrometer 2

At

the second experimental target staton, the

nEl

liquid hy- drogen target was installed at the center ofexperiment

II

and put into operation. For the run periods

in

July and Septem-

ber

^{1992, an}

ASL

magnet was installed

on

the turntable.

In

front of

it,

two drift-chambers

with

horizontal and verti- cal readout were installed,

in

order to measure position and angle

of

the recoiling protons. Behind the magnet, a drift- chamber equipped carbon polarimeter was installed, which allowed to measure the bending and the polarization of the

protons.

Due

to the

spin rotation caused

by

the magnet, also longitudinal polarization

of

the protons may be anal- ysed. The spectrometer was set at about 8o, and no neuton detector was used

for

this small proton angle. Data for the measurement

of

Konno and

Ko}lo

have been taken and the analysis is

in

progress.

In

the forthcoming runs, also larger angle settings

will

be used,

with

neutron detectors

for

n-p coincidences.

.2

600

I

2

Figure

3:

Calibration

of the

beam polarization P6 at

-540 ^MeV

^and measurement

of the

analyzing power Aooro using the setup at experiment

I

and a CH2 target.

The parameter P6

=

^Q.JS

+

0.02 was found by normal- izing these data to the PSA. The angular dependence is in good agreement

with

the PSA predictions.

Spectrometer L

At

the first experimental target stâtion, the frozen spin po- larized target, constructed

at

PSI,

is

an instrument where different high technologies are mastered: vacuum, cryogen- ics, superconductivity, ultra-low temperature using an

3/Ie

dilution refrigerator cryostat, hyperfrequences,

RMN,

spe- cial chemical materials, automatic control for a stand-alone operation. The target volume of 4 x 4

x

4 cm3 is relatively important, and the orientations

k

and

î: of

the polarization are possible. However this target was working only during the summer

period. It

was replaced

with a CH2 or aC

target during the March and the October periods. The scat- tered neutrons are detected

by a

^hodoscope

of

I

I

^plastic

scintillations bars

of

^8x20x130 cm3

ll3l.

The polarization of the scattered proton

is

measured

in

a polarimeter,

built

using Charpak chambers.

A

high data rate

is

needed and special features have been implemented

in

the acquisition system:

1) a

fast

trigger with two

hardware levels,

2)

a local readout

of

each detector using a CES-starburst pro- cessor

for

the compacting

of

the data and

for

the control histogrammings, 3) the data transfer on exabyte unit using the PSI-TANDEM software and a VAX4000

station.

We

This work has been funded by the Federal Minister for Research and Technology

(BMFT)

under the contract No 06FR263I, and the Fonds National Suisse pour la Recherche Scientifique.

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