Dual Target Design for CLAS12
Omair Alam and Gerard Gilfoyle
Department of Physics, University of Richmond
Introduction
One of the fundamental goals of nuclear physics is to
understand the structure and behavior of strongly
interacting nuclei in terms of its basic constituents,
quarks and gluons. An important step towards this goal
is the characterization of the internal structure of the
nucleon; the elastic electric and magnetic form factors
of the proton and neutron are key ingredients of this
characterization. The elastic electromagnetic form
factors are directly related to the charge and current
distributions inside the nucleon and are among the
basic observables of the nucleon.
The Laboratory
Jefferson Lab (JLab), located in Newport News Virginia,
focuses on understanding the nature of the quarkgluon interaction that binds protons, neutrons, and
nuclei together. The central scientific instrument at
JLab is the Continuous Electron Beam Accelerator
Facility (CEBAF) (See Figure 1). CEBAF creates a precise,
continuous, beam of electrons that allows exclusive
measurements (detect multiple particles from each
event) to be made. CEBAF now runs at energies up to
12 GeV. Hall B currently houses the CEBAF Large
Acceptance Spectrometer (CLAS12) (See Figure 2).
CLAS12 consists of eight detector subsystems for the
base equipment with more than 60,000 channels.
There are two major parts, the forward detector and
the central detector. It will detect and measure the
properties of charged and neutral particles produced
in collisions with the electron beam.
Measuring the Neutron Magnetic Form
Factor
An experiment to measure the neutron magnetic form factor(G ) is planned for the new CLAS12 (JLab
n
m
Experiment E12-07-104). This form factor is extracted from the ratio of quasielastic electron-neutron to
electron-proton scattering off a liquid deuterium target. A collinear liquid hydrogen target will be used to
measure efficiencies at the same time as production data is collected from the liquid deuterium target. To test
target designs, we have simulated the dual-target geometry, support structures and cryogenic transport
systems for CLAS12.
Dual Target Simulation
We wrote a Perl script that utilized the CLAS12 Monte Carlo code gemc and Geant4 API (Application Program
Interface) to define specific geometries and materials for the target. Figure 3 shows the positions and
geometries of the two target cells as well as the structures holding them. We made the following design
decisions in order to make the target structures easy to modify and the Perl script easy to reuse and read:
1. Made separate procedures for each target component to make replacement or modification of individual
parts of the target easy.
2. Established proper dependencies between all the target components to ensure that changes made to any
substructure would reflect across the whole target therefore making changes easy to implement. Some of
the geometries such as hemispherical caps with conical craters were made using advanced features of
Geant4 such as addition and subtraction of solids.
3. Extensively documented and commented the Perl script to increase code readability.
The target cells are surrounded by support structures and tubing for transporting cryogenic liquids for cooling
the target system. There are three sets of two tubes arranged around the target cells at intervals of 120
degrees as shown in Figure 4. We added these ancillary systems to the target definition script because:
4. These structures could potentially alter the properties of the particles scattered from the targets upon
interaction.
5. They might produce background particles when the primary reaction products pass through them.
The output of the Perl script is a set of database entries read by gemc at runtime.
Figure 5 shows the complete target structure consisting of all the aforementioned components along with the
scattering chamber and the aluminum outer casing.
Testing the Program
Figure 1: CEBAF site and end station
Figure 2: Design Drawing of CLAS12
Electron-nucleon events are produced first with the QUasiElastic Event Generator (QUEEG) which models the
internal motion of the nucleons in deuterium[1]. These results are used as input to the CLAS12, physics-based
simulation Monte Carlo, GEMC. This Geant4-based program simulates the particles interaction with each
component of CLAS12, including the target material, the supporting systems and the cryogenic transport systems
and is used as a tool to study the response of the detector. We used straight tracks with no magnetic field to match
the gemc geometry with the assumed target geometry. For more complex analysis, Clara, a data analysis
framework developed by Jefferson Lab scientists, was used for event reconstruction and to study the effect of the
target material on large angle electron scattering in CLAS12.
Figure 3: Dual Target
Figure 4: Cryogenic Transport System
Figure 5: Dual Target with Support
Structures and Cryogenic transport
Results
Figure 6 shows the distributions of the percent momentum
difference, , where is the reconstructed momentum and is
the generated momentum, for an electron scattering angle
for 25 < < 35 and vertex position along the beam axis v z
for -2.5 cm < vz < -0.5 cm. There is little difference between
the two distributions.
Figure 7 represents the difference between the distributions
for target in and target out. There is no statistically
significant difference between the two distributions within
uncertainties.
Figure 6: Momentum difference distribution with Target
In and Out
Figure 7: Difference in count between that for target
in and target out
Summary and
We simulated the
CLAS12 Dual Target geometry including
Conclusion
supporting structures and cryogenic transport systems. An
initial study of the impact of this dual-target structure
revealed limited effects on the electron momentum and
angular resolutions.
References
[1] G.P. Gilfoyle and O. Alam et. al. CLAS-NOTE 2014-007,
Jefferson Lab., 2014
