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Probing the charge structure of the neutron with a neutron polarimeter
Nuclear Instruments North-Holland
and Methods
in Physics
Research
B56/57
455
(1991) 455-458
Probing the charge structure of the neutron with a neutron polarimeter R. Madey Department
of Physics and Center for Nuclear Research, Kent State Uniuersity, Kent, OH 44242, USA
Knowledge of the distribution of electric charge within the neutron is fundamental to understanding both nucleon and nuclear structure. Little is known at the present time because the theoretical description of the deuteron is model dependent. Experiments are planned at the Bates electron accelerator facility and at CEBAF to probe the charge structure of the neutron in a way that should circumvent these large uncertainties by scattering longitudinally-polarized electrons from deuterium quasielastically and measuring the transverse polarization component Ps, of the recoil neutron, which lies in the scattering plane normal to the neutron momentum, Theory This polarization component Ps, is proportional to the electric form factor of the neutron, GE, in the impulse approximation. indicates that Ps, has almost no dependence on the deuteron model, and that it is insensitive to the influence of final-state interactions, meson-exchange currents, and isobar configurations.
1. Introduction form factor GE of the neutron is a quantity needed for the understanding of both nucleon and nuclear structure. The dependence of Gk on Q2, the four-momentum transfer squared, is determined by the distribution of charged quarks within the neutron. Because the net charge within the neutron is zero, the charge distribution is sensitive to models of the charged constituents. The Q2-dependence of GE tests quark models of the neutron wave function. The value of Gg is small and poorly known for all Q2 except for the slope at Q = 0, which was obtained to 2% accuracy by scattering neutrons from atomic electrons 111. Present models of the neutron predict different values of Gg at high momentum transfer; accordingly, good determinations of Gg will provide an important test of these models. Also, the influence of Gg is not negligible in the interpretation of electron scattering from nuclei at high momentum transfer. For these reasons, it is of great importance to determine Gi with smaller uncertainties than before. Our present knowledge of the electric and magnetic form factors G, and G, for protons and neutrons was obtained from measurements of the angular dependence of the cross section by elastic electron-proton scattering and quasielastic electron-deuteron scattering. Large systematic errors in past experiments arose because of uncertainties in the theoretical description of the deuteron, mostly from final-state interactions (FSI) and meson-exchange currents (MEC). For elastic electrondeuteron scattering, the deuteron structure introduces uncertainties that are magnified upon subtraction of the dominant proton contribution. For quasielastic e-d scattering, large uncertainties are introduced by the The
electric
fundamental
0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
(longitudinal/transverse) Rosenbluth separation for the small (charge) term and by subtraction of the dominant proton contribution. Platchkov et al. [2] reported measurements from Saclay of the deuteron structure function A(Q2) up to Q2 = 18 fmp2. They inferred GE up to 20 fme2 (= 0.78 (GeV/c)2) with the usual model dependences inherent in quasielastic electron-deuteron scattering. Prior to this work, the best published values were obtained by Bartel et al. [3] from quasielastic electron-deuteron scattering. The uncertainties or model dependences are too large to distinguish between different models. It is worth noting here that the (December 1989) Long Range Plan [4] by the Nuclear Science Advisory Committee to the Department of Energy and the National Science Foundation states that “the electromagnetic structure of the neutron . . . stands as a significant challenge for experimentalists”. Our knowledge of the electromagnetic structure of the neutron must come from measurements of the interaction of the currents with electroweak probes.
2. Measurement
of GE with a neutron polarimeter
Arnold, Carlson, and Gross [5] suggested that GE might be determined more accurately by measuring the polarization of the recoil neutron after quasielastic scattering of a longitudinally-polarized electron from an unpolarized neutron. The components of the polarization of the recoil neutron lie in the scattering plane of the electron and the recoil neutron. The polarization component normal to the scattering plane vanishes in the one-photon-exchange approximation. The component of the neutron polarization parallel to the scatter-
B.V. (North-Holland)
IV. NUCLEAR
PHYSICS
456
R. Madey / Probing the charge structure of the neutron
ing plane but normal to the momentum transfer is proportional to Gg. According to the Madison convention [6], these nonzero components of the neutron polarization are Psr and P,,, where L’ denotes the direction of the path of the recoil neutron and, in a right-handed coordinate system, S’ lies in the scattering plane. The polarization transfer coefficient of special interest here is D,,, because it is related to Gz in the impulse approximation: IOD,,~ = - 2(G&G”,)[r(1+r)]“2
= -(GRG;)B(&,
tan(8J2)
Q’),
(1)
with I, = (GE)* + (Gb)2r[1
+ 2(1 + r) tan’(&/2)]
Q’)(Gk)“,
= (G;;)*+@,,
(2)
where r = Q2/4M2 and 0, is the electron scattering angle. For the case of a longitudinally-polarized electron beam, D,,, is determined from the relation Psf = P,D,,, by measuring the neutron polarization Psi for a known (measured) longitudinal electron polarization P,. Note that D,,, = Ps, for 100% polarization of the incident beam (i.e., for P, = 1). The yield 1, is proportional to the double-scattering cross section (in units of the Mott cross section) with unpolarized electrons. From eqs. (1) and (2) the polarization transfer coefficient D,,f is given by D,,,
-(G,/‘G,)B(&,
=
A( e,,
Q>
Q’) + (WGd2
(3)
’
with A(B,,
Q2)=r[1+2(1+r)
tan”(0,/2)]
(4)
and B( e,, Q’) = 2[ ~(1 + r)1’2 tan( 8,/2)].
(5)
Thus, a measurement of the polarization transfer coefficient D,,, (= PST/P,) yields the ratio GE/G,. Different models predict that this ratio GJG, increases with Q2; for example, for the dipole parameterization (viz., - G,/G, = r = Q2/4M2), eq. (3) may be written (for G, = --7GM):
B(4,
DLs = A(e,,
Q')T
Q”) +
(6)
T2.
when Q* < 1.0 (GeV/c)*, GE/G, can be neglected in comparison hence, in this approximation,
, A(& Q')
GE __-_D GM
Ls
B(e,,
-PSI
Q”) = P,
< a and (GE/Gh1)2 with A in eq. (3);
Q”) i3( e,, Q*)
A@,,
.
(7)
Arnold, Carlson, and Gross [5] calculated the polarization transfer coefficient D,,, at an electron scattering angle 0, = 50 o for five different form-factor models. All five models give plausible estimates for GE within the range covered by the large uncertainties at the present time. Arenhiivel [7] calculated the effect of the electric form factor of the neutron Gg on the polarization transfer in the d(Z, e’Ii) reaction in the quasifree region, where the deuteron serves as a neutron target while the proton acts mainly as a spectator. Using a nonrelativistic theory and a realistic nucleon-nucleon potential, he found that the polarization transfer coefficient D,,, (= Psi for P, = l), which vanishes for coplanar kinematics and unpolarized electrons, is most sensitive to Gg for neutron emission along the direction of the three-momentum transfer q in the quasifree case. Using the parametrization of GaIster et al. [8] for Gg # 0, he found that even away from the forward-emission direction with respect to the direction of the momentum transferq, the increase in D,,, by about 13% for GE # 0 prevails up to a neutron angle 0, of about 30 O, measured with respect to the direction of the momentum transferred to the neutron by the electron. In the forward direction with respect to q, ArenhGvel found also that the neutron polarization Ps, is insensitive to the influence of final-state interactions, meson-exchange currents, and isobar configurations, and that this lack of sensitivity holds again up to an angle 0, of 30” away from the forward direction with respect to q. Finally, Arenhiivel studied the influence of different deuteron wave functions on the polarization transfer coefficient D Ls~. His results for quasifree kinematics (i.e., for neutron emission along q) show almost no dependence on the deuteron model. The ArenhWel calculation shows that dynamical uncertainties are very small. Rekalo, Gakh, and Rekalo [9] used the relativistic impulse approximation to describe the polarization effects sensitive to the neutron electric form factor GE in deuteron electrodisintegration. In the neutron quasielastic peak, the neutron polarizations calculated in the relativistic approach agree with the results of Arenhijvel [7].
3. Experimental arrangement The experimental arrangement for the measurement of the charge form factor of the neutron from the d(Z, e’ii)p reaction requires a longitudinally-polarized electron beam to be incident on a liquid-deuterium target. A neutron polarimeter [lo] measures the transverse polarization Ps, of the recoil neutron at a laboratory emission angle en after quasielastic scattering of the longitudinally-polarized electron from an unpolarized neutron in deutrium. A magnetic spectrometer measures the momentum of the electron scattered at an
457
R. Madey / Probing the charge structure of the neutron angle 0,. The recoil neutron is measured in coincidence with the scattered electron. The kinetic energy of the neutron is obtained from a measurement of the neutron flight-time from the target to the front analyzing detectors in the polarimeter. A neutron pol~menter [lo] was designed and constructed at Kent State University (KSU) specifically for measurements of Gg. This polarimeter consists of twelve 10.16 cm-thick, scintillation counters - follr mineral oil (BC517L) primary scatterers (1 through 4) and two sets of four rear plastic (NE-102) analyzer detectors. The rear detectors are located at a polar angle 0 with respect to the direction of the incident neutrons. The mean flight path from the point midway between primary scatters No. 2 and No. 3 to the midpoint of each rear detector array is 2.0 m. The rear scintillators are 1.20 m long by 0.508 m wide; the front scintillators are 0.508 m long by 0.254 m wide. In front of each set of four detectors is a thin (0.95 cm) plastic scintillation counter to veto charged particles. The lucite plastic container for the front scintillators has a wall thickness of 0.95 cm. The design of the polarimeter is based on the properties of n-p scattering as a polarization analyzer. Elastic scattering of neutrons with a sideways polarization Psi from unpolarized hydrogen (and carbon) nuclei exhibits and up-down asymmetry. The asymmetry averaged over the angular acceptances of the polarimeter is the product of the polarization 9s~ and the analyzing power & of the scattering reaction averaged over the angular acceptances of the polarimeter. The conventional figure of merit is the product of the square of the analyzmg power and the scattering cross section. For neutron energies of 130-140 MeV, the optimum laboratory scattering angle B = 21”. Based on measurements of neutrons in test runs at the Bates electron accelerator, the neutron polarimeter must be contained in a shielding enclosure. The rear wall and the two side walls are concrete, 4 ft thick. The roof of the enclosure is covered with concrete roof beams, 2 ft thick. The interaction mean free path in concrete for 75 MeV neutrons is - 1 ft; therefore, the
transmission of 75 MeV neutrons through concrete is about 1.8% through 4 ft. The front wall consists of lead, 4 in. thick, supported by two steel plates, each 1; in. thick; in addition, steel blocks are used to collimate the front detectors of the polarimeter and to provide additional shielding for the rear detectors of the polarimeters. The transmission of neutrons (> 100 MeV) through this front wall shielding of 10.16 cm Pb plus 6.35 cm steel is 39%. The reduction in energy of a high-energy photon incident on this shielding is 2.5 X 10-lo. Steel shadow shields, 3 ft thick, block the direct path of neutrons from the target in order to obtain a measure of the room background. Because measurements of the neutron polarization need to be made at different values of Q2, the fourmomentum-transfer squared, it is necessary to move the neutron polarimeter to neutron scattering angles 9, that are matched ~~ematic~ly to electron scattering angles 0.; accordingly, the neutron pola~menter and its shielding need to be mounted on a movable platform (e.g., air pads). The set of kinematic conditions in table 1 illustrate the electron and neutron angles needed to make measurements of the neutron polarization for seven values of Qz in the range 0.15 I Q* (GeV/c)’ r: 1.5.
4. Planned experiments
The use of a KSU neutron polarimeter [lo] to measure the polarization of neutrons from the d(e”, e’ii)p reaction underpins three exper~ents planned for probing the charge structure of the neutron. In a test run in June 1990, the Bates EXS-95 collaboration [ll] obtained an electron-neutron coincidence signal with the pulsed beam at the Bates electron accelerator. Bates E85-05 is designed to measure Gg at Q2 = 0.255 (GeV/c)2 with an incident beam energy of 868 MeV. A second experiment (Bates E89-04) is planned for the upgraded accelerator facility at Bates. The stretcher ring upgrade will improve the duty cycle to about 85%. which gives an advantage factor of about 85% over the present Bates
Table 1 Typical kinematic conditions for measuring Gg Beam energy
Four-momentumtransfer squared
Electron angle
Q2
fl,
:eVJ
[G&/cl2
PegI
kg1
4.0 4.0 4.0 3.2 2.4 2.4 1.6
2.0 1.5 1.1 0.75 0.50 0.30 0.15
23.9 19.8 16.5 16.7 18.1 13.7 14.4
42.0 47.5 52.8 57.2 60.5 66.8 71.0
Neutron angle
Electron momentum
Neutron momentum
Neutron energy
WV/cl
$kV,c,
$eV]
2935 3201 3408 2801 2132 2238 1518
1770 1462 1208 952 757 572 397
1063 798 590 398 267 160 80
kinetic
Pe’
IV. NUCLEAR
PHYSICS
458
R. Madey / Probing the charge structure of the neutron
facility with a duty of 1%. Bates E89-04 is designed to use 868 MeV electrons to measure Gg at two Q”-values of 0.30 and 0.60 (GeV/c)2. The third experiment (CEBAF E89-05) is planned for the Continuous Electron Beam Accelerator Facility (CEBAF) at Newport News, Virginia. The continuous beam provides an advantage factor of about 100 over the, present Bates facility. The preliminary design of CEBAF E89-05 includes several Q2-values in the range from about 0.2 to 2.0 (GeV/c)2. Another experiment is planned for the new electron accelerator facility at Mainz, Germany, with a duty cycle of 100%. The experiment at Mainz plans to use a neutron polarimeter with a different configuration than the KSU polarimeter. The polarimeter configuration in the Mainz experiment is of the so-called [lo] “parallelplanes” type.
5. Conclusion This paper describes briefly the basis for probing the charge structure of the neutron with a neutron polarimeter. Other techniques for probing the charge structure of the neutron include the scattering of polarized electrons from a polarized deuterium target, and the scattering of polarized electrons from a polarized 3He target. Experimentalists are rising to the challenge of probing the charge structure of the neutron.
References 111 V.E. Krohn and G.R. Ringo, Phys. Rev. 148 (1966) 1303. P-l S. Platchkov, A. Amroun, S. Auffret, J.M. Cavedon, P. Dreux, J. Duclos, B. Frois, D. Goutte, H. Hachemi, J. Martino, X.-H. Phan and I. Sick, Nucl. Phys. 508A (1990) 343c. [31 W. Bartel, F.W. Busser, W.R. Dix, R. Felst, D. Harms, H. Krehbiel, P. Kuhlmann, J. McElroy, J. Meyer and G. Weber, Nucl. Phys. B58 (1973) 429. [41 Nuclei, Nucleons, Quarks. Nuclear Science in the 1990s A Long Range Plan by the DOE/NSF Nuclear Science Advisory Committee (1989). [51 R.G. Arnold, C.E. Carlson and F. Gross, Phys. Rev. C23 (1981) 363. Convention, in: Polarization Phenomena in [61 Madison Nuclear Reactions, eds. H.H. Barschall and W. Haeberli (The University of Wisconsin Press, Madison, WI, 1970). [71 H. Arenhovel, Phys. Lett. B199 (1987) 13. PI S. Galster, H. Klein, J. Moritz, K.H. Schmidt, D. Wegener and J. Blechwenn, Nucl. Phys. B32 (1971) 221. 191 M.P. Rekalo, G.I. Gakh and A.P. Rekalo, J. Phys. G 15 (1989) 1223. and WI R. Madey, A.R. Baldwin, P.J. Pella, J. Schambach R.M. Sellers, IEEE Trans. Nucl. Sci. NS-36 (1989) 231. B.D. Anderson, A.R. BaldP11 Bates E85-05 Collaboration: win, T. Eden, D. Keane, R. Madey (Spokesman), D.M. Manley, J. Schambach, J.W. Watson. W.M. Zhang, Kent State University; A. Bernstein, W. Bertozzi, G. Dodson, M. Farkhondeh, S. Kowalski (Cospokesman), R. Milner, W. Turchinetz, L. Weinstein, Massachusetts Institute of Technology; J. Cameron, B. Ni, M. Spraker, Indiana University Cyclotron Facility; C.C. Chang, J.J. Kelly, University of Maryland; J.M. Finn, W. Herzog, P. Markowitz, R. Pourang, P. Rutt, William and Mary; R. Lourie, University of Virginia; P. Pella, Gettysburg College; B.S. Flanders, American University; F. Gross, J. Mougey, P. Ulmer, R. Whitney, CEBAF.
and Methods
in Physics
Research
B56/57
455
(1991) 455-458
Probing the charge structure of the neutron with a neutron polarimeter R. Madey Department
of Physics and Center for Nuclear Research, Kent State Uniuersity, Kent, OH 44242, USA
Knowledge of the distribution of electric charge within the neutron is fundamental to understanding both nucleon and nuclear structure. Little is known at the present time because the theoretical description of the deuteron is model dependent. Experiments are planned at the Bates electron accelerator facility and at CEBAF to probe the charge structure of the neutron in a way that should circumvent these large uncertainties by scattering longitudinally-polarized electrons from deuterium quasielastically and measuring the transverse polarization component Ps, of the recoil neutron, which lies in the scattering plane normal to the neutron momentum, Theory This polarization component Ps, is proportional to the electric form factor of the neutron, GE, in the impulse approximation. indicates that Ps, has almost no dependence on the deuteron model, and that it is insensitive to the influence of final-state interactions, meson-exchange currents, and isobar configurations.
1. Introduction form factor GE of the neutron is a quantity needed for the understanding of both nucleon and nuclear structure. The dependence of Gk on Q2, the four-momentum transfer squared, is determined by the distribution of charged quarks within the neutron. Because the net charge within the neutron is zero, the charge distribution is sensitive to models of the charged constituents. The Q2-dependence of GE tests quark models of the neutron wave function. The value of Gg is small and poorly known for all Q2 except for the slope at Q = 0, which was obtained to 2% accuracy by scattering neutrons from atomic electrons 111. Present models of the neutron predict different values of Gg at high momentum transfer; accordingly, good determinations of Gg will provide an important test of these models. Also, the influence of Gg is not negligible in the interpretation of electron scattering from nuclei at high momentum transfer. For these reasons, it is of great importance to determine Gi with smaller uncertainties than before. Our present knowledge of the electric and magnetic form factors G, and G, for protons and neutrons was obtained from measurements of the angular dependence of the cross section by elastic electron-proton scattering and quasielastic electron-deuteron scattering. Large systematic errors in past experiments arose because of uncertainties in the theoretical description of the deuteron, mostly from final-state interactions (FSI) and meson-exchange currents (MEC). For elastic electrondeuteron scattering, the deuteron structure introduces uncertainties that are magnified upon subtraction of the dominant proton contribution. For quasielastic e-d scattering, large uncertainties are introduced by the The
electric
fundamental
0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
(longitudinal/transverse) Rosenbluth separation for the small (charge) term and by subtraction of the dominant proton contribution. Platchkov et al. [2] reported measurements from Saclay of the deuteron structure function A(Q2) up to Q2 = 18 fmp2. They inferred GE up to 20 fme2 (= 0.78 (GeV/c)2) with the usual model dependences inherent in quasielastic electron-deuteron scattering. Prior to this work, the best published values were obtained by Bartel et al. [3] from quasielastic electron-deuteron scattering. The uncertainties or model dependences are too large to distinguish between different models. It is worth noting here that the (December 1989) Long Range Plan [4] by the Nuclear Science Advisory Committee to the Department of Energy and the National Science Foundation states that “the electromagnetic structure of the neutron . . . stands as a significant challenge for experimentalists”. Our knowledge of the electromagnetic structure of the neutron must come from measurements of the interaction of the currents with electroweak probes.
2. Measurement
of GE with a neutron polarimeter
Arnold, Carlson, and Gross [5] suggested that GE might be determined more accurately by measuring the polarization of the recoil neutron after quasielastic scattering of a longitudinally-polarized electron from an unpolarized neutron. The components of the polarization of the recoil neutron lie in the scattering plane of the electron and the recoil neutron. The polarization component normal to the scattering plane vanishes in the one-photon-exchange approximation. The component of the neutron polarization parallel to the scatter-
B.V. (North-Holland)
IV. NUCLEAR
PHYSICS
456
R. Madey / Probing the charge structure of the neutron
ing plane but normal to the momentum transfer is proportional to Gg. According to the Madison convention [6], these nonzero components of the neutron polarization are Psr and P,,, where L’ denotes the direction of the path of the recoil neutron and, in a right-handed coordinate system, S’ lies in the scattering plane. The polarization transfer coefficient of special interest here is D,,, because it is related to Gz in the impulse approximation: IOD,,~ = - 2(G&G”,)[r(1+r)]“2
= -(GRG;)B(&,
tan(8J2)
Q’),
(1)
with I, = (GE)* + (Gb)2r[1
+ 2(1 + r) tan’(&/2)]
Q’)(Gk)“,
= (G;;)*+@,,
(2)
where r = Q2/4M2 and 0, is the electron scattering angle. For the case of a longitudinally-polarized electron beam, D,,, is determined from the relation Psf = P,D,,, by measuring the neutron polarization Psi for a known (measured) longitudinal electron polarization P,. Note that D,,, = Ps, for 100% polarization of the incident beam (i.e., for P, = 1). The yield 1, is proportional to the double-scattering cross section (in units of the Mott cross section) with unpolarized electrons. From eqs. (1) and (2) the polarization transfer coefficient D,,f is given by D,,,
-(G,/‘G,)B(&,
=
A( e,,
Q>
Q’) + (WGd2
(3)
’
with A(B,,
Q2)=r[1+2(1+r)
tan”(0,/2)]
(4)
and B( e,, Q’) = 2[ ~(1 + r)1’2 tan( 8,/2)].
(5)
Thus, a measurement of the polarization transfer coefficient D,,, (= PST/P,) yields the ratio GE/G,. Different models predict that this ratio GJG, increases with Q2; for example, for the dipole parameterization (viz., - G,/G, = r = Q2/4M2), eq. (3) may be written (for G, = --7GM):
B(4,
DLs = A(e,,
Q')T
Q”) +
(6)
T2.
when Q* < 1.0 (GeV/c)*, GE/G, can be neglected in comparison hence, in this approximation,
, A(& Q')
GE __-_D GM
Ls
B(e,,
-PSI
Q”) = P,
< a and (GE/Gh1)2 with A in eq. (3);
Q”) i3( e,, Q*)
A@,,
.
(7)
Arnold, Carlson, and Gross [5] calculated the polarization transfer coefficient D,,, at an electron scattering angle 0, = 50 o for five different form-factor models. All five models give plausible estimates for GE within the range covered by the large uncertainties at the present time. Arenhiivel [7] calculated the effect of the electric form factor of the neutron Gg on the polarization transfer in the d(Z, e’Ii) reaction in the quasifree region, where the deuteron serves as a neutron target while the proton acts mainly as a spectator. Using a nonrelativistic theory and a realistic nucleon-nucleon potential, he found that the polarization transfer coefficient D,,, (= Psi for P, = l), which vanishes for coplanar kinematics and unpolarized electrons, is most sensitive to Gg for neutron emission along the direction of the three-momentum transfer q in the quasifree case. Using the parametrization of GaIster et al. [8] for Gg # 0, he found that even away from the forward-emission direction with respect to the direction of the momentum transferq, the increase in D,,, by about 13% for GE # 0 prevails up to a neutron angle 0, of about 30 O, measured with respect to the direction of the momentum transferred to the neutron by the electron. In the forward direction with respect to q, ArenhGvel found also that the neutron polarization Ps, is insensitive to the influence of final-state interactions, meson-exchange currents, and isobar configurations, and that this lack of sensitivity holds again up to an angle 0, of 30” away from the forward direction with respect to q. Finally, Arenhiivel studied the influence of different deuteron wave functions on the polarization transfer coefficient D Ls~. His results for quasifree kinematics (i.e., for neutron emission along q) show almost no dependence on the deuteron model. The ArenhWel calculation shows that dynamical uncertainties are very small. Rekalo, Gakh, and Rekalo [9] used the relativistic impulse approximation to describe the polarization effects sensitive to the neutron electric form factor GE in deuteron electrodisintegration. In the neutron quasielastic peak, the neutron polarizations calculated in the relativistic approach agree with the results of Arenhijvel [7].
3. Experimental arrangement The experimental arrangement for the measurement of the charge form factor of the neutron from the d(Z, e’ii)p reaction requires a longitudinally-polarized electron beam to be incident on a liquid-deuterium target. A neutron polarimeter [lo] measures the transverse polarization Ps, of the recoil neutron at a laboratory emission angle en after quasielastic scattering of the longitudinally-polarized electron from an unpolarized neutron in deutrium. A magnetic spectrometer measures the momentum of the electron scattered at an
457
R. Madey / Probing the charge structure of the neutron angle 0,. The recoil neutron is measured in coincidence with the scattered electron. The kinetic energy of the neutron is obtained from a measurement of the neutron flight-time from the target to the front analyzing detectors in the polarimeter. A neutron pol~menter [lo] was designed and constructed at Kent State University (KSU) specifically for measurements of Gg. This polarimeter consists of twelve 10.16 cm-thick, scintillation counters - follr mineral oil (BC517L) primary scatterers (1 through 4) and two sets of four rear plastic (NE-102) analyzer detectors. The rear detectors are located at a polar angle 0 with respect to the direction of the incident neutrons. The mean flight path from the point midway between primary scatters No. 2 and No. 3 to the midpoint of each rear detector array is 2.0 m. The rear scintillators are 1.20 m long by 0.508 m wide; the front scintillators are 0.508 m long by 0.254 m wide. In front of each set of four detectors is a thin (0.95 cm) plastic scintillation counter to veto charged particles. The lucite plastic container for the front scintillators has a wall thickness of 0.95 cm. The design of the polarimeter is based on the properties of n-p scattering as a polarization analyzer. Elastic scattering of neutrons with a sideways polarization Psi from unpolarized hydrogen (and carbon) nuclei exhibits and up-down asymmetry. The asymmetry averaged over the angular acceptances of the polarimeter is the product of the polarization 9s~ and the analyzing power & of the scattering reaction averaged over the angular acceptances of the polarimeter. The conventional figure of merit is the product of the square of the analyzmg power and the scattering cross section. For neutron energies of 130-140 MeV, the optimum laboratory scattering angle B = 21”. Based on measurements of neutrons in test runs at the Bates electron accelerator, the neutron polarimeter must be contained in a shielding enclosure. The rear wall and the two side walls are concrete, 4 ft thick. The roof of the enclosure is covered with concrete roof beams, 2 ft thick. The interaction mean free path in concrete for 75 MeV neutrons is - 1 ft; therefore, the
transmission of 75 MeV neutrons through concrete is about 1.8% through 4 ft. The front wall consists of lead, 4 in. thick, supported by two steel plates, each 1; in. thick; in addition, steel blocks are used to collimate the front detectors of the polarimeter and to provide additional shielding for the rear detectors of the polarimeters. The transmission of neutrons (> 100 MeV) through this front wall shielding of 10.16 cm Pb plus 6.35 cm steel is 39%. The reduction in energy of a high-energy photon incident on this shielding is 2.5 X 10-lo. Steel shadow shields, 3 ft thick, block the direct path of neutrons from the target in order to obtain a measure of the room background. Because measurements of the neutron polarization need to be made at different values of Q2, the fourmomentum-transfer squared, it is necessary to move the neutron polarimeter to neutron scattering angles 9, that are matched ~~ematic~ly to electron scattering angles 0.; accordingly, the neutron pola~menter and its shielding need to be mounted on a movable platform (e.g., air pads). The set of kinematic conditions in table 1 illustrate the electron and neutron angles needed to make measurements of the neutron polarization for seven values of Qz in the range 0.15 I Q* (GeV/c)’ r: 1.5.
4. Planned experiments
The use of a KSU neutron polarimeter [lo] to measure the polarization of neutrons from the d(e”, e’ii)p reaction underpins three exper~ents planned for probing the charge structure of the neutron. In a test run in June 1990, the Bates EXS-95 collaboration [ll] obtained an electron-neutron coincidence signal with the pulsed beam at the Bates electron accelerator. Bates E85-05 is designed to measure Gg at Q2 = 0.255 (GeV/c)2 with an incident beam energy of 868 MeV. A second experiment (Bates E89-04) is planned for the upgraded accelerator facility at Bates. The stretcher ring upgrade will improve the duty cycle to about 85%. which gives an advantage factor of about 85% over the present Bates
Table 1 Typical kinematic conditions for measuring Gg Beam energy
Four-momentumtransfer squared
Electron angle
Q2
fl,
:eVJ
[G&/cl2
PegI
kg1
4.0 4.0 4.0 3.2 2.4 2.4 1.6
2.0 1.5 1.1 0.75 0.50 0.30 0.15
23.9 19.8 16.5 16.7 18.1 13.7 14.4
42.0 47.5 52.8 57.2 60.5 66.8 71.0
Neutron angle
Electron momentum
Neutron momentum
Neutron energy
WV/cl
$kV,c,
$eV]
2935 3201 3408 2801 2132 2238 1518
1770 1462 1208 952 757 572 397
1063 798 590 398 267 160 80
kinetic
Pe’
IV. NUCLEAR
PHYSICS
458
R. Madey / Probing the charge structure of the neutron
facility with a duty of 1%. Bates E89-04 is designed to use 868 MeV electrons to measure Gg at two Q”-values of 0.30 and 0.60 (GeV/c)2. The third experiment (CEBAF E89-05) is planned for the Continuous Electron Beam Accelerator Facility (CEBAF) at Newport News, Virginia. The continuous beam provides an advantage factor of about 100 over the, present Bates facility. The preliminary design of CEBAF E89-05 includes several Q2-values in the range from about 0.2 to 2.0 (GeV/c)2. Another experiment is planned for the new electron accelerator facility at Mainz, Germany, with a duty cycle of 100%. The experiment at Mainz plans to use a neutron polarimeter with a different configuration than the KSU polarimeter. The polarimeter configuration in the Mainz experiment is of the so-called [lo] “parallelplanes” type.
5. Conclusion This paper describes briefly the basis for probing the charge structure of the neutron with a neutron polarimeter. Other techniques for probing the charge structure of the neutron include the scattering of polarized electrons from a polarized deuterium target, and the scattering of polarized electrons from a polarized 3He target. Experimentalists are rising to the challenge of probing the charge structure of the neutron.
References 111 V.E. Krohn and G.R. Ringo, Phys. Rev. 148 (1966) 1303. P-l S. Platchkov, A. Amroun, S. Auffret, J.M. Cavedon, P. Dreux, J. Duclos, B. Frois, D. Goutte, H. Hachemi, J. Martino, X.-H. Phan and I. Sick, Nucl. Phys. 508A (1990) 343c. [31 W. Bartel, F.W. Busser, W.R. Dix, R. Felst, D. Harms, H. Krehbiel, P. Kuhlmann, J. McElroy, J. Meyer and G. Weber, Nucl. Phys. B58 (1973) 429. [41 Nuclei, Nucleons, Quarks. Nuclear Science in the 1990s A Long Range Plan by the DOE/NSF Nuclear Science Advisory Committee (1989). [51 R.G. Arnold, C.E. Carlson and F. Gross, Phys. Rev. C23 (1981) 363. Convention, in: Polarization Phenomena in [61 Madison Nuclear Reactions, eds. H.H. Barschall and W. Haeberli (The University of Wisconsin Press, Madison, WI, 1970). [71 H. Arenhovel, Phys. Lett. B199 (1987) 13. PI S. Galster, H. Klein, J. Moritz, K.H. Schmidt, D. Wegener and J. Blechwenn, Nucl. Phys. B32 (1971) 221. 191 M.P. Rekalo, G.I. Gakh and A.P. Rekalo, J. Phys. G 15 (1989) 1223. and WI R. Madey, A.R. Baldwin, P.J. Pella, J. Schambach R.M. Sellers, IEEE Trans. Nucl. Sci. NS-36 (1989) 231. B.D. Anderson, A.R. BaldP11 Bates E85-05 Collaboration: win, T. Eden, D. Keane, R. Madey (Spokesman), D.M. Manley, J. Schambach, J.W. Watson. W.M. Zhang, Kent State University; A. Bernstein, W. Bertozzi, G. Dodson, M. Farkhondeh, S. Kowalski (Cospokesman), R. Milner, W. Turchinetz, L. Weinstein, Massachusetts Institute of Technology; J. Cameron, B. Ni, M. Spraker, Indiana University Cyclotron Facility; C.C. Chang, J.J. Kelly, University of Maryland; J.M. Finn, W. Herzog, P. Markowitz, R. Pourang, P. Rutt, William and Mary; R. Lourie, University of Virginia; P. Pella, Gettysburg College; B.S. Flanders, American University; F. Gross, J. Mougey, P. Ulmer, R. Whitney, CEBAF.