The Polarized Deuteron Breakup
Experiment at COSY
1
F. Rathmann , S. Barsov , S. Dymov† , A. Kacharava, A. Khoukaz‡ ,
V. Komarov† , A. Kulikov† , A. Kurbatov† , N. Lang‡ , I. Lehmann ,
B. Lorentz , G. Macharashvili† , A. Mussgiller , H. Paetz gen. Schieck§ ,
R. Schleichert, H. Seyfarth , E. Steffens , H. Ströher , Yu. Uzikov†¶ ,
S. Yaschenko and B. Zalikhanov†
†
Institut für Kernphysik, Forschungszentrum Jülich, Germany
Laboratory of Nuclear Problems, Joint Institute for Nuclear Research, Dubna, Russia
Physikalisches Institut II, Universität Erlangen-Nürnberg, Germany
‡
Institut für Kernphysik, Universität Münster, Germany
§
Institut für Kernphysik, Universität zu Köln, Germany
¶
Kazakh State University, Almaty, Kazakhstan
ppn with forward emission of a
Abstract. A study of the deuteron breakup reaction pd
fast proton pair with small excitation energy E pp 3 MeV has been performed using the ANKE
spectrometer at COSY Jülich. The differential cross section of the breakup reaction, averaged up
to 8Æ over the cm polar angle of the total momentum of the pp pairs, has been obtained at six
proton beam energies T p 06, 0.7, 0.8, 0.95, 1.35, and 1.9 GeV. A first measurement of the vector
analyzing power A yp has been carried out, using a polarization normalization obtained with the
EDDA detector. In addition, for the first time asymmetries of pd elastic scattering at T p 500 MeV
have been recorded with the spectator setup at ANKE.
1. MEASUREMENT OF THE SPIN–AVERAGED CROSS SECTION
The breakup process p d pp n with forward emission of a fast proton pair with
small excitation energy E pp 3 MeV was studied at ANKE [1] at six beam energies
from 0.6 to 1.9 GeV. The process has never been explored under these conditions. The
experiment allows for a complete kinematical reconstruction of the events, whereby the
five–fold differential cross sections can be determined.
The study of the deuteron breakup pd ppn discussed here comprises two phases,
a) measurements with unpolarized deuterium target to obtain spin–averaged differential
cross sections, angular distributions, and using a polarized proton beam the vector analyzing power Ayp , and b) measurements with polarized deuterium target to obtain the
other polarization observables2 (tensor analyzing power, spin–spin and spin–tensor cor-
1 This work has been supported by the BMBF (contracts RUS 667-97, RUS 99/684, RUS 99/685, RUS
01/691, KAZ 99/001, 06 ER 831, and 06 ER 930), by the European Community (contract INTAS 933661), by the Forschungszentrum Jülich (FFE contract 41419786 [COSY–55]).
2 Details about the polarized target are described elsewhere in these proceedings[2, 3].
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
553
qNN (GeV/c)
dσ/dΩcm (µb/sr)
0.3
fast neutron
(not detected)
COSY internal
proton beam
0.4
ANKE
102
dp
pd
10
ANKE dipole D2
pd
(pp)+n
10-1
P1
exit window
50 cm
0.65
0.6
103
1
deuterium
cluster target
0.5
MWPC
P2
scintillation
hodoscope
∆
ONE
10-2
SS
0.5
1
1.5
2
2.5
3
Tp (GeV)
FIGURE 1. Left panel: Top view of the experimental setup with the forward detection system of the
ANKE spectrometer. Right Panel: Measured cross section of the process under study in the interval
E pp 3 MeV. The calculations with ONE+∆+SS model are performed using the NN potentials RSC
(dotted line) and Paris (solid). The individual contributions with the Paris potential are shown by thin full
lines. (Figure from ref. [5].)
relation parameters). The final goal of both phases is the measurement of a complete set
of observables. This goal cannot be achieved in one step. The well–defined event vertex
of the point–like unpolarized cluster–jet target in the first phase allows precise determination of the angular dependence of the differential cross section. In the second phase,
the storage cell target provides an extended vertex distribution along the proton beam direction. Therefore, in order to extract angular distributions of polarization observables,
the angular distribution of the differential cross section must be obtained first.
The experimental setup is shown in Fig. 1 (left panel). The protons stored in the
COSY ring ( 3 1010) impinged on a deuterium cluster jet target [4], which provided
a target thickness of about 13 1013 atoms/cm2. The produced charged particles, after
passing the magnetic field of the dipole D2, were registered by a set of three multiwire
proportional chambers (MWPC) and a scintillation counter hodoscope. Each wire chamber contains a horizontal and a vertical anode wire plane (1 mm wire spacing), and two
planes of inclined strips, that allowed us to obtain the required resolution of 08 to
1.2% (rms) in the momentum range 0.6 to 2.7 GeV/c. The hodoscope consists of two
layers, containing 8 and 9 vertically oriented scintillators (4 to 8 cm width, 1.5 to 2 cm
thickness). It provided a trigger signal, an energy loss measurement, and allowed for
the determination of the differences in arrival times for particle pairs hitting different
counters.
The statistics taken at Tp 1 GeV is insufficient even to determine the three–fold
differential cross sections. Therefore, in order to compare the energy dependence of the
cross section to the theoretical prediction (ONE+∆+SS model [5]), the cross section was
integrated over the interval 0 E pp 3 MeV and averaged over the pair emission angle
cm 8Æ . The results are shown in Fig. 1 (right panel).
in the range 0 θ pp
554
2. BEAM POLARIMETRY AT ANKE AND FIRST GLANCE AT AYP
A first short run with polarized beam was mainly utilized to develop a method to determine the beam polarization at ANKE. The EDDA detector [6] was employed to obtain
an unambiguous reference value for the beam polarization. A measurement of the beam
polarization at EDDA is possible only at energies above 07 GeV (1343 MeV/c). In
addition, the employed polyethylene targets at EDDA do not tolerate beam intensities
exceeding about 5 108 stored protons. According to the previous beam time request, we
wanted to perform a measurement of Ayp at 0.5 and at 1.0 GeV, therefore a macro cycle
was realized, which consisted of two flattops at 0.5 and 1.0 GeV (Fig. 2, left panel).
The beam polarization was measured in separate cycles with appropriately reduced in-
Asymmetry & Polarization
1
0.8
0.6
EDDA
0.4
ANKE
0.2
0
21/09 22/09 23/09 24/09 25/09 26/09 27/09 28/09
Date of measurement
FIGURE 2. Left panel: Schematic picture of the cycle with two flattops, at T p 0510 GeV
(10901696 GeV/c). Right panel: EDDA beam polarization and asymmetry measured at ANKE. Filled
circles denote the measured beam polarization at 0.7 GeV, filled squares the 1 GeV flattop polarization
measured at EDDA. Stars indicate the asymmetry of small–angle scattered protons off the ANKE deuteron
target, and the open symbols correspond to the measured false asymmetries.
tensity (micro–pulsing), but otherwise identical to the data–taking cycles. At 0.5 GeV
data were taken for 10 min, subsequently the beam was ramped to a short 1 GeV flattop. The beam spin was alternated from cycle to cycle. Polarization data with EDDA
were recorded once or twice per day, a) during flattop at 1 GeV to monitor the polarization and b) during the ramp at 0.7 GeV, with modified flattop durations. Since between
0.5 and 0.7 GeV the depolarizing resonances in COSY are understood and compensated, this procedure provides the beam polarization at 0.5 and at 1.0 GeV. Since the
polarization data recorded with EDDA did not provide a continuous monitoring of the
beam polarization, the up/down ( , ) beam spin asymmetry of protons scattered off the
deuterium target was measured with ANKE. Elastically and quasi-elastically scattered
protons between θlab 5Æ to 11Æ were registered in the forward detection system (FD).
At the same time proton pairs from the breakup process under study were recorded.
The relative luminosity was determined from inelastically scattered protons
near zero degree, simultaneously detected in the FD. Online track reconstruction provided information (angle and momentum) to select the scattered protons. Both proton
count rates and observed asymmetry were sufficiently large to monitor the beam polarization during the run. The resulting polarizations from EDDA and asymmetries from
ANKE ε N N N N are shown in Fig. 2 (right panel). The asymmetries from
555
ANKE were obtained for 2 h runs carried out right after the EDDA polarization measurements. The data are quite stable in time, thus averaging was justified. At T p 07 GeV,
EDDA 0645 0009, while at T 10 GeV, PEDDA 0577 0001. The measured
Pbeam
p
beam
asymmetries at ANKE at Tp 05 GeV correspond to ε = 0294 0006. The
false asymmetries ε and ε were obtained by analyzing cycles with the same
polarization direction, yielding ε ε 2 0002 0009. The effective anEDDA , is thus Aeff 0456 0011.
alyzing power of ANKE, given by the ratio ε Pbeam
y
A clean separation of the pd elastic scattering events at small angles was achieved
by detection of the scattered deuterons using the vertex spectator detector [11] in coincidence with a fast proton detected in the FD (see Fig. 3 (panel a). A comparison of
T p = 500 MeV
T p =500 MeV
0.6
Ayp
9000
8000
0.55
7000
0.5
6000
0.45
5000
0.4
4000
0.35
3000
0.3
2000
0.25
1000
0
0.8
0.9
1
1.1
1.2
1.3
o
o
p, GeV/c (7.0 < Θ<7.5 )
a)
0.2
4
6
8
10
12
14
16
18 20 22
Θlab , degree
b)
FIGURE 3. Beam polarimetry utilizing pd elastic scattering: a) Momentum of protons registered in the
FD in coincidence with deuterons stopped in the second layer of spectator detector. b) Comparison of the
angular dependence of the proton vector analyzing power measured at ANKE at 500 MeV (circles) and
the data obtained at 544 MeV [7] (squares), 796 MeV [8] (rhombuses) and 800 MeV [9] (triangles).
the obtained ANKE data at 500 MeV with other data available from literature is shown
in Fig. 3 (panel b). Detection of pd elastic scattering events with the spectator counter
can be directly applied to measure the beam polarization at ANKE at 796 MeV, utilizing
the existing precise data from Irom et al. [8]. It is possible to export a calibrated measurement of the beam polarization from 796 MeV to other energies, higher or lower, as
shown in Ref. [10].
A first measurement of the vector analyzing power Ayp for the breakup process could
be obtained. The analyzing power Ayp is expected to follow a linear function of the
neutron emission angle θn in the range from 180Æ to 165Æ : Ayp αyp 180Æ θn (Fig. 4,
left panel). In Fig. 4 (right panel) the obtained angular dependence of the observed
asymmetry for the breakup process is shown. The dependence exhibits a linear behaviour
and the obtained value of the slope parameter αyp 0041 0011 deg1 . Thus, the
measured preliminary value differs by about two standard deviations from the theoretical
value of 0020 deg1 , given by the solid curve in Fig. 4 (left panel) for Tp 05 GeV.
556
Asymmetry
0.6
Deuteron break-up asymmetry (Epp < 3 MeV)
Luminocity corr: 0.966, dead-time corr: 0.993, bkg level: 5 %
Pr
0.5
0.4
0.3
0.2
eli
mi
na
ry
0.1
0
-0.1
-0.2
-0.3
164
N+ = 238, N- = 171
166
168
170
172
174
176
178
180
Recoil neutron θCM angle, degree
Thu Oct 24 11:36:26
11:33:32 2002
11:35:22
FIGURE 4. Left panel: Vector analyzing power A yp of the proton versus the neutron scattering angle θ cm
in the p d pps n reaction at E pp 3 MeV for the different mechanisms at kinetic energies T 05,
0.65, 0.85, and 1 GeV: ONE (DWBA) (dashed–dotted), ∆ (dotted), ONE ∆ SS (dashed) and ONE
(DWBA) ∆ SS (full) [12]. Right panel: Measured asymmetry of the neutron emission at backward θ cm
angle in the p d
pp n reaction at T p 05 GeV with excitation energy E pp 3 MeV. The solid
line shows a linear fit to the data.
ACKNOWLEDGMENTS
We would like to thank the EDDA collaboration for their support during the polarized
measurements, in particular the help by Heiko Rohdjeß and Dieter Prasuhn is gratefully
acknowledged.
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2.
3.
4.
5.
6.
7.
8.
9.
10.
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12.
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Details about the spectator setup of ANKE can be found at www.fz-juelich/ikp/anke/vertex.
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