O. V. Miklukho

Multiwire proportional chambers in the HERMES experiment

The multiwire proportional chambers of the HERMES spectrometer are described. Each chamber consists of a series anode–cathode planes which measure position along axes at � 308; 08, and þ308 relative to horizontal. The anode wire spacing is 2 mm, while the anode–cathode plane spacing is 4 mm. The chambers are located in the spectrometer magnet gap which imposed strong constraints on their design. A nonflammable gas mixture of Ar(65%), CO2ð30%Þ and CF4ð5%Þ is used and an efficiency plateau 500 V wide is obtained. The readout is performed with the LeCroy PCOS4 data acquisition system modified according to the specific requirements of the HERMES experiment. A dedicated water cooling system has been implemented to control and stabilize the surface temperature of the electronic chips mounted on the on-chamber preamplifier boards. A spatial resolution of ’ 600 mm (rms) per plane has been measured. These chambers are essential for tracking low momentum particles which do not enter the drift chambers after the spectrometer magnet; these particles principally result from the decay of various unstable hadrons. The momentum and angular resolutions in tracking these low momentum particles has been studied and found to be similar to those obtained for high momentum tracks which traverse the entire tracking system of the HERMES spectrometer. # 2001 Elsevier Science B.V. All rights reserved. PACS: 29.40.Cs; 29.40.Gx; 07.50.� e

Polarization effects in the quasielastic (p, 2p) reaction with the nuclear S-Shell Protons at 1 GeV

The polarization of the secondary protons in the (p, 2p) reaction with the S-shell protons of nuclei 4He, 6Li, 12C, 28Si, 40Ca was measured at 1 GeV unpolarized proton beam. The spin correlation parameters Cij for the 4He and 12C targets also were for the first time obtained as well. The polarization measurements were performed by means of a two-arm magnetic spectrometer, each arm of which was equipped with the multiwire-proportional chamber polarimeter. This experiment was aimed to study a modification of the proton-proton scattering matrix in the nuclear medium.

Polarization in quasielastic (p, 2p) scattering on a 4He nucleus at 1 GeV

The polarization of secondary protons from the (p, 2p) reaction induced by 1-GeV protons incident to a 4He nucleus was measured in a kinematically complete experiment. By using a two-arm magnetic spectrometer, two secondary protons from this reaction were recorded in coincidence at unequal scattering angles of Θ1 = 18°−24.21° and Θ2 = 53.22° over a broad range of the recoil-nucleus momentum KB between 0 and 150 MeV/c. It was found that the measured polarization of either secondary proton is less than that observed in free elastic proton-proton scattering. The magnitude of this difference is determined by the mean binding energy of s-shell protons rather than by the effective nuclear density. The polarizations measured in inclusive quasielastic and elastic scattering of 1-GeV protons on 4He nuclei are presented for scattering angles in the range Θ1 = 18° − 24.21°.

Polarization in the quasielastic scattering of 1-GeV/c protons on light nuclei

Polarization measurements in the A(p, 2p)B reactions on 6Li, 7Li, and 28Si nuclei at a proton-beam energy of 1 GeV were performed in a kinematically complete experiment. By using a two-arm magnetic spectrometer, two secondary protons were recorded in coincidence at asymmetric scattering angles of θ1=15°−26° and θ2=58.6° for residual-nucleus momenta in the range KB=0–150 MeV/c. Either arm of the spectrometer was equipped with polarimeters based on proportional chambers. The data coming from this experiment are analyzed within the distorted-wave impulse approximation. It is shown that the polarization of recoil protons formed at angle θ2 in the interaction featuring a proton from the P shell of the 7Li nucleus can be described under the assumption of an effective intranuclear-proton polarization by using the single-particle shell-model wave function of the nucleus. Our data on the polarizations of the two protons from the reaction (p, 2p) on a 28Si nucleus also suggest the effective polarization of the protons in the D shell of the 28Si nucleus. It is found that, for high recoil-nucleus momenta of KB≥90 MeV/c, the effective polarization of the protons in the P shell of the 6Li nucleus—this polarization was discovered in studying the polarization of recoil protons in the reaction 6Li(p, 2p)5He—cannot be described within the shell model assuming LS coupling. As might have been expected, the polarization of recoil protons knocked out from the S shells of the 6Li and 7Li nuclei comply well with the predictions obtained in the impulse approximation with allowance for the depolarization effect alone.

Measurement of the polarization correlation parameter Cnn in the pp elastic scattering with an unpolarized proton target and an unpolarized 1-GeV proton beam

The polarization correlation parameter Cnn in the elastic proton-proton scattering was for the first time measured with a high statistical accuracy at 1 GeV using an unpolarized proton beam incident on an unpolarized proton target. Both outgoing protons from the reaction were detected in coincidence by means of a two-arm focusing magnetic spectrometer installed at nonsymmetric angles corresponding to the center-of-mass scattering angle Θcm = 62.25°. The correlation parameter Cnn, as well as polarizations P1 and P2 of the secondary protons, were measured by means of multiwire proportional chamber polarimeters placed at the focal planes of each spectrometer arm. The obtained data are compared with the predictions of the phase-shift analysis. The connection with the experiment on Cnn and polarization measurements in the (p, 2p) reaction on 4He and other light nuclei in similar kinematics is discussed

Study of polarization in quasi‐elastic break‐up reaction 6Li(p,2p)5He in complete kinematics at 1 GeV

The polarization of recoil protons from the reaction 6Li(p,2p)5He at 1 GeV of incident proton beam energy was measured with a new two‐arm magnetic spectrometer with a carbon polarimeter. The effective polarization of the P‐shell protons of 6Li nucleus was calculated and found to be in a qualitative agreement with the predictions of the cluster model rather than with those of the shell model.

Polarization parameters of elastic pp scattering in the angular range 22.5°–42.5° at an energy of 1 GeV and partial-wave analysis of data

The polarization properties of elastic scattering are studied experimentally for pp interaction at 1 GeV. The respective experimental results are presented for the spin-transfer parameter Kn00n and the polarization Pn000 at c.m. angles of 22.5°, 27.5°, 32.5°, and 42.5° and for the analyzing power A000n at c.m. angles of 22.5° and 27.5°. The experiment employed an unpolarized proton beam from the accelerator of the Petersburg Nuclear Physics Institute, a polarized proton target, and a high-resolution magnetic spectrometer. The data from this experiment are analyzed and compared with the predictions of partial-wave analyses.

Polarizations for 12C(p, 2p) Reactions at 1 GeV

We have measured polarizations P of outgoing protons in (p, 2p) reactions at an incident energy of 1 GeV for three kinds of targets. The experimental result shows a distinct reduction from IA calculation values using NN interaction in free space and the reduction is found to be monotonic to the effective mean density estimated with DWIA. This is consistent with the previous result obtained at 392 MeV, though the incident energy is quite different. We have also measured an angular distribution of the polarization for 12C target. All of the data, including both for forward nd backward outgoing protons, show similar reduction from the IA calculation, though, again, outgoing energies are quite different from 130 MeV to 890 MeV. From these results, it is concluded that these reductions are nuclear structure or interaction originated and not caused by the reaction mechanism such as multi‐step processes or distortions.

Study of nuclear medium effect on hadrons in nuclei by nucleon quasifree scattering

Polarizations of outgoing two protons in (p,2p) reactions leading to either 1s1/2 or 2s1/2 deep hole state have been measured at an incident energy of 1 GeV. The result shows distinct reduction from calculated values using NN interaction in free space and the reduction is found to be monotonic function of the mean density estimated with DWIA. Those are consistent with results at 392 MeV. From these result, it is concluded that these reductions are nuclear structure or interaction originated and not caused by reaction mechanism such as contribution of two step processes.