Design, Construction, and Performance of Cherenkov Detectors for PREx Experiment at Thomas Jefferson

1
Design, Construction, and Performance of
Cherenkov Detectors for PREx Experiment at Thomas
Jefferson National Accelerator Facility (JLab) in
Virginia (sponsored by the National Science
Foundation RUI Grant Parity-violating Electron
Scattering Experiments at JLab) Katie Su 11 and
Shorena Kalandarishvili 11 with Piotr Decowski
Introduction The goal of the 208Pb
Radius Experiment (PREx) is to measure the
neutron skin in the 208Pb nucleus with high
precision of 1. Due to the high ratio of
neutrons to protons (126 neutrons versus 82
protons), the radius of neutron distribution in
208Pb is slightly larger than the radius of its
proton distribution. This difference in radii is
very sensitive to model parameters, especially to
the symmetry term in nuclear potential good
knowledge of these parameters is required in the
broad range of calculations, from those used in
nuclear physics to those used in astrophysics
(neutron stars). Neutrons, unlike protons, are
electrically neutral, and therefore cannot be
probed by electromagnetic interactions. On the
other hand, neutrons respond to weak
interactions much more than protons, as neutrons
have a much larger weak charge than protons.
In the forthcoming PREx experiment at JLab, the
neutron distribution will be measured using weak
interactions extracted from the parity-violating
asymmetry in the scattering of longitudinally
polarized electrons. Two
different prototypes of detectors for this
experiment were designed and constructed at Smith
College in collaboration with UMass. One of them
consisted of a stack of quartz blocks interleaved
with tungsten plates, while another of a single
quartz block surrounded by a conical mirror.
Both versions were tested in January 2008 using
electron beam from the Continuous Electron Beam
Accelerator Facility (CEBAF) at JLab.
JLab
Overview of the CEBAF electron
accelerator at JLab

Setup of PREx

Layout of PREx Experiment at JLab
Computer Simulations of Thin Detector
Performance
Electrons scattered from lead target, pass
through the quartz blocks surrounded by a conical
mirror, and mounted in the detector placed in the
focal plane of the high resolution magnetic
spectrometer. While passing through the quartz,
the electrons release photons (Cherenkov
radiation), which propagate in the direction of a
photomultiplier (PM). Left panels in Fig.1
show the result of the computer simulations
describing relationship between the number of
photons reaching PM, and the length of the
conical mirror for two diameters of the PM
cathode (2 inch and 3 inch). The simulation was
run using different cone lengths (which resulted
in different numbers of photons), and the data is
shown on the curved plots. Different plots
illustrate changing distribution of photons
caused by changing the distance d between the
mouth of the cone and the quartz. Based on this
data, the cone length that maximizes the number
of photons passing through PM can be chosen.
These computer simulations show that a maximum
number of photons reaches the PM when the cone
has length of 6cm and the quartz is placed 2mm
away from the mouth of the cone.
39
d2mm
d10mm
d5mm
2 inch PM
d5mm
d2mm
d10mm
.162
6 cm
6 cm
Number of photons reaching PM
Relative width of distributions
Cone length (cm)
Cone length (cm)
50
3 inch PM
Side View
.14
7 cm
7 cm
Right panels in Fig.1 show the relationship
between the relative width of distribution of
number of photons reaching PM and the cone
length. The relative width of distribution
is calculated by dividing the normal distribution
width by its mean. The goal of the project is to
find the smallest relative width which provides
the best precision. The graph indicates that the
cone length of 6cm and distance of 2mm between
the quartz and the mouth of the cone yields the
best resolution of about 16.2.
Figure 1 Results of Simulations of Detector
Performance
Top View
Assembly of Thin Detector

Quartz Holder of Thin Detector

Quartz Holder Fully Mounted in Detector
Fig.2 presents the distribution of magnitude
of pulses from the PM measured during test run at
JLab in January 2008. The resolution of 27
represents the combined resolution of the
detector and of the photomultiplier, and agrees
with the simulation data.
Figure 2 Response of Thin Detector to 1 GeV
Electrons.
Conclusion Based on the test run performed at
Jlab this past January, the detectors were
designed appropriately. The detectors met the
parameters required for the PREx experiment.