Background Symmetry Energy Project (SEP) is one of the current projects at NSCL. Its physics goals include the determination the equation of state of nuclear matter, density dependence of symmetry energy, etc. Heavy ion collisions (Ca, Sn, etc.) are studied experimentally and with computer simulations. The project is an international collaboration.
Equation of State Energy in nuclei: Symmetry Energy Term Image from http://www.nscl.msu.edu/~tsang/iso_Texas_11.pdf
Detector Design A Time Projection Chamber (TPC) is designed to detect pions and charged particles emitted in heavy ion collisions. The charged particles produced in heavy ions collision will ionize the gas in the chamber. The ionized gas is drifted towards the pad plane by electric and magnetic field. The drift time and the position of the ionized gas can be used to generate the tracks of primary charged particles. It is designed and made in US and will be installed in RIKEN, Japan. Image from http://www-rnc.lbl.gov/EOS/
Lid and electronics Field cage Enclosure Voltage step down
Contributions in TPC design Use of Computer-Aided Design (CAD) software Design modification Model construction Rotation structure design Stress calculations
CAD Software used Autodesk Inventor, a Computer-Aided Design (CAD) software is used for the 3D design of TPC.
Design Modification Examples of my contributions: Changed the color of the cooling rod. Added the copper strips on the corners of the field cage. Modified the position of the standoff in voltage step down. Modified the dimension of the enclosure.
Foam Model Making The foam model of TPC is made and shipped to Japan to ensure it can be placed inside the magnet. Made together with Jon Barney and Justin Estee. MSU RIKEN
Rotation Structure Design The TPC will be assembled upside down since there are wires to be attached to the bottom of top plate. It has to stand on its side to move down the hallways at NSCL. One idea is to rotate the TPC around its center of mass:
Stress analysis The frame structure should be able to support the TPC(~520kg). The condition of the TPC on its side sitting on a cart is simulated by inventor. Less than 2mm deformation is observed. Simulation to rotate the TPC to different orientation is still in progress.
Analysis of Computer Simulated Collision Data Simulations are done by Hang Liu using the supercomputer in Austin, Texas Improved Quantum Molecular Dynamics Model (ImQMD) is currently used, the results would be compared to transport theory(BUU) and real collision.s More than 60000 collision events are generated for each reaction. The collision under different initial conditions at different energies and impact parameters are simulated: Examples: - Sn124+Sn124 (sn124s) - Sn124+Sn112 (sn112m) - Sn112+Sn124 (sn124m) - Sn112+Sn112 (sn112s) Visualization of collisions in computer simulation Photo from Y.X. Zhang www.imqmd.com/income/zhang1.pdf
Contributions in Data Analysis Computation knowledge of Fortran was used Some observables were analyzed Neutron-to-proton (n/p) ratio Tritium-to-helium3 (t/ 3 He) ratio Ri value
n/p ratio Example: E70b7x0.7 - beam energy = 70MeV/A - impact parameter = 7fm - stiffness of equation of state of nuclear matter (gamma) = 0.7
After colliding, fragments with lower energy have a higher neutron content, while that with higher energy have a higher proton content. The graphs of n/p ratio for other reactions and graphs of double ratio were also plotted.
t/ 3 He ratio t/ 3 He ratio is interesting because neutron is hard to detect in experiment and hence the error in experimental value of n/p ratio is high. More tritium(t) are produced in lower energy while more 3 He are produced in higher energy in general.
The error in this result is larger than that of n/p ratio. The count number at high energy is small, which produces a relatively high statistical error. More events will be simulated to reduce the error.
Ri value Ri is the isospin transport ratio, which is a measure of isospin diffusion. X AA refers to the neutron-rich system (sn124+sn124), X BB refers to the proton-rich system (sn112+sn112). If no diffusion, Ri(X AA ) = 1; Ri(X BB ) = -1. If isospin equilibrium is reached, Ri(X AB ) = Ri(X BA ) = 0. In theory, X is the asymmetry of the fragments. Two types of Ri: Ri(n,frag) and Ri(zmax>20)
In general, the isospin diffuse more at lower beam energy and lower gamma. Current plan is to compare Ri at more beam energies. Graphs for comparing different incident energies at fixed impact parameters were made. The next step is to compare for different impact parameters.
Acknowledgement Thanks to Betty Tsang, Bill Lynch, Fei Lu, Rebecca Shane, Jon Barney and Justin Estee for all their help! Thanks to Department of Physics, CUHK for the opportunity of SURE!