PR89-046

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PR89-046 Conditionally approved Should
demonstrate that the experiment can extract
information from states separated by 0.35 MeV
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Impulse Approximation limitations to the (e,ep)
reaction on 208Pb - Identifying correlations and
relativistic effects in the nuclear medium
K. Aniol , B. Reitz, A. Saha, J. M. Udias
Spokespersons Hall A Collaboration
Meeting June 23, 2005
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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1. What is being proposed

• Measure 208Pb(e,ep)207Tl cross sections in
  quasielastic kinematics
• Study low lying states in 207Tl
• g.s. 3s1/2
• 0.351 2d3/2
• 1.348 1h11/2
• 1.683 2d5/2
• 3.470 1g7/2
• Quasielastic kinematics xB 1, q 1 GeV/c , ?
  0.433 GeV/c
• Determine momentum distributions 0 lt pmiss lt
  500 MeV/c
• Determine ATL by measuring cross sections on
  either side of q.

K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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2. Physics Motivation
How well do we understand nuclear structure
? The shell model Basis upon which most model
calculations of nuclear structure rely.
The underlying physical picture Dense system of
fermions whose motions to first order can be
treated as independent particles moving in a mean
field. Electromagnetic interactions Best
probes for investigating the validity of the
independent particle picture because they are
sensitive to a much larger fraction of the
nuclear volume V.R. Pandharipande, I. Sick and
P.K.A. deWitt Huberts, Independentparticle
motion and correlations in fermions systems,
Reviews of Modern Physics 69 (981) 1997
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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2. Physics Motivation
Deviations from independent particle motion for
orbits near the Fermi surface are attributed to
various correlations which reveal their effects
in two ways (i) Changes in the occupation
probabilities of mean-field orbits. (ii) Changes
in the momentum distribution of particles,
especially above the Fermi momentum
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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(i) Changes in the occupation probabilities of
mean-field orbits
data 4, theory 5.
12C(e,ep) data over wide range of Q2. There
appears to be a Q2 dependence to the
spectroscopic factors from this reaction2. This
interpretation has been disputed and the Q2
dependence attributed to analysis methods of
SRC3. 208Pb(e,e'p) has been studied in the
past at low momentum transfers and reliable
spectroscopic factors in the range of 0.6 to 0.7
have been extracted from parallel kinematics
4,5,6 A measurement at high Q2 may help
address the question of a momentum transfer
dependence of the spectroscopic factors.
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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(ii) Momentum distributions gt 300 MeV/c
Excess strength at high pmiss
xB ? 1
E. Quint, thesis, 1988, NIKHEF
I. Bobeldijk et al., PRL 73 (2684)1994
J. M. Udias et al. PRC 48(2731) 1994
J.M. Udias et al. PRC 51(3246) 1996
This was explained via long-range correlations
in a nonrelativistic formalism Bobeldijk,6, but
also by relativistic effects in the mean field
model Udias,7.
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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2. Physics Motivation
Both spectroscopic factors and the strength at
high pmiss can be explained by two
approaches(1) Quasiparticle orbits plus
nonrelativistic DWIA. (2) RDWIA using
independent particle orbit solutions to the Dirac
equation. Are there other observables which can
help distinguish between these two explanations ?
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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Cross Section Asymmetry ATL
The cross section asymmetry is defined about q
The data taken at NIKHEF 6 were backward of q
where both theoretical procedures agree. But
forward of q the theories can disagree by two
orders of magnitudeLooking for unmistakable
signatures of correlations at high pmiss from
(e,ep) cross sections alone is a hard task
6 I. Bobeldijk et al., PRL 73 (2684)1994
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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ATL in 3He, 4He and 16O
If relativistic dynamical effects are the main
cause responsible for the extra strength, a
strong effect on ATL would be seen. There is a
notable difference in ATL between 3He and 4He due
to the density difference and in 16O.
16O ATL
p1/2
p3/2
M. Rvachev et al. PRL 9412320,2005
J. Gao et al. PRL843265, 2000
E04-107,2004
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ATL in 208Pb
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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ATL in 208Pb
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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Target Issues and Peak Extraction

• Two tested targets configurations capable of high
  current running
• Stacked lead foils in a 15 cm beer can LH2
  target
• Test run showed that the upstream foil can
  withstand at least 60 µA with only the horizontal
  sweep of the raster working at the time.
• (ii) Cold Lead in diamond sandwich
• E03-011 has demonstrated that a 0.5 mm lead foil
  sandwiched between two 0.15 mm diamond foils at
  cryogenic temperatures can withstand currents up
  to 80 uA and calculations indicate that they can
  operate at 100 µ A.

K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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Resolution for Various Target Configurations
The realistic spectrometer settings use the
published Hall A spectrometer characteristic. The
ideal spectrometer settings contribute no width.
NIKHEF-target refers to using the NIKHEF two foil
or one foil targets in our spectrometers and
kinematics. NIKHEF-paper refers to the results
from 6. In that experiment the scattered
electron energy was almost 7 times lower than in
the proposed kinematics. A comparison of the
widths for the same target between the realistic
and ideal spectrometers gives us a measure of the
spectrometers' contribution to the experimental
FWHM. Assuming the target effects and
spectrometer effects add in quadrature we
conclude that the spectrometers contribute about
0.5 Mev to the FWHM for our kinematics
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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GEANT Simulation and Peak Extraction
Excitation Energy (MeV), pm100MeV/c
A 0.2 mm thick lead target is a good compromise
between resolution and count rate.
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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Bismuth run to provide an experimental line shape.
209Bi(e,ep)208Pb, pm 200 MeV/c
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Beam Time Request
Cross Section Measurements 180 hrs
Spectrometer position changes, Optics and Bismuth run 32 hrs
Arc and EP Energy Measurements 4 hrs
Total 216 hrs(9 days)
The time estimates for the data runs have been
adjusted up by 30 to account for radiative
losses.Optics data taken with the carbon target
will help establish the missing mass calibration,
which is needed for the peak fitting. The bismuth
run will provide a line shape and also give a
missing mass calibration.
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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Summary
(1) The first measurements ever made in
quasielastic kinematics on the paradigmatic shell
model nucleus, 208Pb at high Q2 are
proposed.(2) Strength for pmiss gt 300 MeV/c
will give insight into nuclear structure issues
and confront theoretical models. (3) A new
observable ATL for the five low lying states of
207Tl will be measured.
K.Aniol, Hall A Collaboration Mtg., June 23, 2005
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Heavy Metal Collaboration