The three-spectrometer facility at the Mainz microtron MAMI Blomqvist, K.I; Boeglin, W.U; Böhm, R ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
02/1998, Volume:
403, Issue:
2
Journal Article
Peer reviewed
A set-up of three high-resolution magnetic spectrometers, for simplicity named A, B and C, has been built as the central facility for the precise determination of double and triple coincidence cross ...sections of hadron knock-out and meson production through the scattering of electrons at the Mainz microtron MAMI. The spectrometers A and C with point-to-point optics in the dispersive plane and parallel-to-point optics in the non-dispersive plane have a solid angle of 28 msr and a momentum acceptance of 20 and 25%, respectively. They each consist of a quadrupole, a sextupole and two dipole magnets, reaching maximum momenta of 735 and 550 MeV/c, respectively. The spectrometer B has a solid angle of 5.6 msr and a somewhat reduced momentum acceptance of 15%, but it reaches a maximum momentum larger than that of the MAMI electron beam (855 MeV/c). It consists of a single-clamshell dipole magnet with point-to-point optics in both planes. Each spectrometer is equipped with a position-sensitive detector system consisting of four planes of vertical drift chambers, two planes of plastic scintillators and a threshold gas Cherenkov detector. The operational experiences demonstrate that all three spectrometers exceed the specifications. Selected results of double (e, e′
x) and triple (e, e′
x
1
x
2) coincidence experiments,
x
1 and
x
2 standing for charged hadrons, are presented, which demonstrate the performance of the whole set-up.
Precise data on the neutron magnetic form factor Gmn have been obtained with measurements of the ratio of cross sections of D(e,e′n) and D(e,e′p) up to momentum transfers of Q2=0.9 (GeV/c)2. Data ...with typical uncertainties of 1.5% are presented. These data allow for the first time to extract a precise value of the magnetic radius of the neutron.
The neutron magnetic form factor
G
mn
has been determined via a measurement of the ratio of cross sections
D(
e,
e′
n) and
D(
e,
e′
p). The absolute detection efficiency of the neutron detector was ...measured with high accuracy using tagged neutrons produced from
H(
n,
p)
n elastic scattering by means of a high intensity neutron beam. This approach minimizes the model dependence and improves upon the weakest points of previous experiments. Data in the range
q
2=0.2–0.8 (GeV/c)
2 with uncertainties of <2% are presented.
Absolute differential cross sections for the reaction ep-->epgamma have been measured at a four-momentum transfer with virtuality Q2 = 0.33 GeV2 and polarization epsilon = 0.62 in the range 33.6 to ...111.5 MeV/c for the momentum of the outgoing photon in the photon-proton center of mass frame. The experiment has been performed with the high-resolution spectrometers at the Mainz Microtron MAMI. From the photon angular distributions, two structure functions which are a linear combination of the generalized polarizabilities have been determined for the first time.
The differential cross-sections σ
0
= σ
T
+ εσ
L
, σ
LT
, and σ
TT
of
electroproduction from the proton were measured from threshold up to an additional center-of-mass energy of 40MeV, at a value of ...the photon four-momentum transfer of
Q
2
= 0.05 GeV
2
/c
2
and a center-of-mass angle of θ = 90
°
. By an additional out-of-plane measurement with polarized electrons σ
LT'
was determined. This showed for the first time the presence of an imaginary part of the
s
-wave above the
threshold, which is usually interpreted as a unitary cusp. The predictions of the Heavy Baryon Chiral Perturbation Theory (HBChPT) are in disagreement with these data. On the other hand, the data are somewhat better predicted by the MAID phenomenological model and are in good agreement with the dynamical model DMT.
The focal plane proton-polarimeter for the 3-spectrometer setup at MAMI Pospischil, Th; Bartsch, P.; Baumann, D. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
05/2002, Volume:
483, Issue:
3
Journal Article
Peer reviewed
Open access
For
(
e
→
,
e′
p
→
)
experiments the 3-spectrometer setup of the A1 collaboration at MAMI has been supplemented by a focal plane proton-polarimeter. To this end, a carbon analyzer of variable ...thickness and two double-planes of horizontal drift chambers have been added to the standard detector system of Spectrometer A. Due to the spin precession in the spectrometer magnets, all three polarization components at the target can be measured simultaneously. The performance of the polarimeter has been studied using elastic
p(
e
→
,
e′
p
→
)
scattering.
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