IV 4 "A clean measurement of the neutron skin of 208Pb through parity=
violating electron scattering", JLAB exp 99-012 (P. Souder and R. Mich=
aels, spokesmen)
This is an exciting extension of electron scattering to measure nuclear
p=
arameters with unprecedented accuracy. Nuclear structure calculations
pre=
dict the existence of a neutron skin in nuclei which changes the distribu=
tion of protons and neutrons by several per cent. FOR EXAMPLE
IN PB IT IS GENERALLY ASSUMED THAT THE NEUTRON RADIUS EXCEEDS THE PROTON
RADIUS BY ABOUT 0.25 FM WITH THE PROTON RADIUS AT 5.5MFM. THIS IS A DIFFERENCE
OF ABOUT 5%. Attempts to measure thi=
s difference in neutron and proton rms radii with hadronic probes are
fra=
ught with theoretical uncertaintiesI, LEADING TO UNCERTAINITIES
IN THE NEUTRON RADIUS OF 5% OR MORE.
IN ADDITION TO PROVIDING PRECISE INFORMATON ABOUT A NUCLEAR QUANTITY
THAT SHOULD BE CALCULABLE WITH MODERN HARTREE FOCK CALCULATIONS, THE DETERMINATION
OF THE NEUTRON RADIUS IS ALSO OF INTEREST IN ASTROPHYSICAL COMPUTATIONS
OF UNSTABLE ASYMMETRIC NUCLEI(1) AND IS NEEDED IN THE ATOMIC CALCULATIONS
THAT COMPUTE PARITY VIOLATIONS IN HEAVY ATOMS.
. This experiment proposes to take adv=
antage of a recent calculation1 which demonstrates that parity violating
=
electron scattering is mainly sensitive to the distribution of neutrons
i=
n the nucleus. This is so because the Z0 couples mainly to the neutrons.
=
The interaction between an electron and a nucleus depends on the ordinar=
y electromagnetic vector potential and an axial vector potential due
to Z=
0 exchange. The axial vector potential, in turn, is proportional to
the n=
eutron density distribution. By using polarized electrons one can measure=
the asymmetry in the elastic scattering between positive helicity and
ne=
gative helicity electrons. The asymmetry arises from the interference
of=
the electromagnetic and axial vector potentials. The proposal seeks
to d=
etermine the neutron rms radius in 208Pb to within 1%.
The proposal has a deferred status because of the issue of a suitable
lea=
d target which could withstand the beam without melting. Our group
has of=
fered to undertake the design and construction of the target in collabora=
tion with R. Michaels. A novel approach using thin diamond layers in
good=
thermal contact with the lead has been suggested by. R. Michaels. Our
fi=
rst task is to construct a cooled target ladder which could support
this =
lead /diamond sandwich. We will use our machine shop facilities for
the f=
abrication of the target and our Van de Graaff accelerator for trial
runs=
to test the design. In the event this sandwich design does not work,
we=
will work on developing other schemes for high-powered lead targets.
(1) Parity violating elastic scattering and Coulomb distortions, Physica=
l Review C57(3430)1999
(2) "A clean measurement of the neutron skin of 208Pb through
parity=
violating electron scattering", JLAB exp 99-012 (P. Souder and R.
Mich=
aels, spokesmen)