Multi-Wire Proportional Chambers for Muography
Dr. Joffe is working with the Society of Physics Students (SPS) at Kennesaw State University to build a series of Multi-Wire Proportional Chambers as a tool to detect cosmic ray muons and measure muon flux. Muons are a charged subatomic particle, similar to electrons, but with a much higher mass (roughly 200 times greater). Although muons are unstable and eventually decay into electrons, cosmic ray muons are relatively long-lived. Energetic muons created by collisions between the solar wind and the upper atmosphere have long enough lifetimes to pass through the entire atmosphere to the earth’s surface where the density of their flow can be measured.
The ultimate goal of this project is to be able to use the proportional chambers to perform muon tomography; this is the study of the density variations in a fixed structure using the natural flow of cosmic ray muons from the upper atmosphere and measuring the rate of their absorption in the structure. Muon tomography essentially involves taking a three dimensional “x-ray” of a large structure using the existing natural cosmic rays as the radiating source; as such it is an ideal method for non-invasive archaeological investigations, and is currently being used by a collaboration of French and Japanese physicists to study the internal structure of the Great Pyramid at Giza. A recent issue of Nature features this muographic study and is an excellent introduction to the potential of muography for archeaology1. Here in North Georgia we have some excellent potential candidate structures for muographic investigation in the form of the approximately one thousand year old Etowah Mounds, the largest of which have never been excavated, and which are less than a half hour drive from the KSU campuses.
The Society of Physics Students is working with Dr. Joffe to design and build these detectors from the ground up using existing literature as a reference. The goal is to develop stable hands-free devices with electronic read outs that can detect precisely the trajectory of any muon which passes through the chamber and send the resulting information to a data collection system. The project currently has a set of prototype chambers built; prototype assembly occurred in the fall semester of 2017 and spring semester of 2018 using resources and materials provided by the Department of Physics. The prototypes are currently undergoing testing.
We have chosen a design allowing chamber construction to be simple and efficient, consisting of a wire array between two cathode planes. The wire array is under high voltage while the cathode plane connects to ground to create a potential gap inside the chamber. The array has alternating field-shaping and anode wires serving to isolate the anode wires from each other for better spatial resolution. In order for the multi-wire chamber to function properly, the anode wires must be at a high DC voltage (>1000 V). High-energy muons entering the chamber will ionize the gas between the wires; the ions themselves create secondary ions through an avalanche or cascade of secondary ionization. To operate the chamber as a muon detector it must contain an ionizing gas. We have chosen to use an Argon/CO2 mix in an 80:20 ratio; this mixture has a lower ionization threshold voltage required than dry air. The cascade of ions reaching the anode wires creates a current which then passes through the amplifier and finally the circuit passes the detection of the muon on to the data collection system. We will stack four chambers perpendicularly to each other allowing us to calculate muon trajectories for the purposes of muography, which is our intended final goal. Two of the students on the project have presented talks on their work in designing and building the prototype at the American Physical Society April meeting in Columbus, Ohio, one of which won a "Best Presentation Award", and one student, Michael Reynolds, has earned a Birla Carbon scholarship for work on the project this summer.