The UC Berkeley Paleomagnetism Lab features a superconducting rock magnetometer within a three-layer magnetostatic shield. The walk-in shield creates a low-field analytical and sample handling environment while the magnetometer is equipped with 3 very sensitive DC superconducting quantum interference devices (SQUIDs) to measure the remanent magnetization of earth and planetary materials.
A three-layer magnetostatic shield
In the lab, measurements are made of very weak remanent magnetizations necessitating shielding from Earth’s magnetic field as well magnetic fields resulting from the built environment (e.g. moving elevators). The necessary level of magnetic shielding is accomplished in the UC Berkeley Paleomagnetism Lab by a three-layer magnetostatic shield that was constructed by Gary Scott and his team at Lodestar Magnetics. Lodestar Magnetics constructs room-sized shields comprised of multiple layers of thin metal sheets that are nested within one another (like a Russian doll). The Berkeley shield is comprised of two layers of transformer steel and one layer of “Mu metal.”
Both magnetic remanence and magnetic permeability contribute to the overall shielding. The layers of transform steel in the Berkeley shield accomplish effective magnetic shielding through their permanent magnetization. This shielding is accomplished through remagnetizing the steel once installed such that the permanently magnetized domains realign into a low energy configuration respective to the magnetic field in the room (dominantly Earth’s geodynamo-created magnetic field). After this remagnetization of the steel, the magnetization outside of the sheet counteracts the static field in which it was magnetized (Scott, 1996). An effective shield can be created by using a single sheet, but the shielding is much better with two layers. The field prior to the installation of the field was an average of 34,480 nT. The first steel layer reduced the average field to 1,354 nT. The second steel layer reduced it further to 273 nT.
The third innermost layer is a Ni-Fe alloy known as mu-metal. Mu-metal has a high magnetic permeability and as a result magnetic flux lines are deflected into the mu-metal sheet thereby shielding the interior volume. After installation of the mu-metal shield, the average magnetic field inside of the 19 cubic meter (670 cubic foot) was 151 nT. Mechanical stress (e.g. denting) significantly reduces magnetic permeability of mu-metal so this inner layer was covered for protection prior to installation of the magnetometer.
Superconducting rock magnetometer system
The main instrument in the lab is a 2G Model 755 magnetometer that utilizes 3 highly sensitive DC-SQUID sensors in conjunction with pick-up coils to make measurements of the remanent magnetization of samples. A pulse-tube cryocooler cools the SQUID sensors to their operating temperature of ~4 K. Mu-metal shielding and superconducting lead shield reduces the field within the sensing region to single digit nT levels. The system has an open vertically-oriented bore that enables samples to be brought in and out of the measurement area.
The system is set-up with the ability to perform automated inline treatment steps and rock magnetic experiments involving: alternating field (AF) demagnetization, anhysteretic remanent magnetization (ARM) acquisition and isothermal remanent magnetization (IRM) acquisition. The electronics controlling these systems are located outside of the magnetostatic shield. Additionally, there is an inline Bartington system for measuring magnetic susceptibility.
The sample handler is a quartz-glass tube that holds samples using a strong vacuum. A distinct advantage of this type of sample holder is its low level of internal ferromagnetic contamination as well as the ability for it to be aggressively acid cleaned to remove surface contamination. The magnetization of the sample holder is typically around 3×10^-9 emu (3×10^-12 Am^2). An XY stage accommodates 99 samples and is automated to translates such that these samples can be picked up by the holder rod and introduced into the magnetometer facilitating high-throughput and high-resolution experimental protocols.
Do want to know more about how a SQUID actually works? Check out Julie Bowles‘s excellently written article in the IRM quarterly entitled “SQUID attack!”: http://www.irm.umn.edu/quarterly/irmq19-1.pdf
Getting to the lab
Coming by train and foot
If you are flying into SFO or OAK, the best way to get to UC Berkeley is on a BART train. From SFO or downtown San Francisco, take the Pittsburg/Bay Point train and then transfer at 19th Street Oakland Station to the Richmond train. This is a timed transfer so you just need to walk across the platform to the other train that is waiting or just about to arrive. You can then take this train to the Downtown Berkeley station.
Once you are at the Berkeley BART station it is a 0.6 mile (1 km) walk to McCone Hall. The lab is in Room 408 on McCone Hall’s 4th floor. See map below:
Here are directions if you are staying on campus at the faculty club.
Note the the faculty club is a bit hidden in a wooded glade. You can walk until you are at the iconic Campanile tower and then walk across South Drive and down stairs that bring you to faculty glade. Faculty Glade looks like this with the faculty club in the background:
Coming by car
The closest place to park to the lab is in the Lower Hearst Parking Structure on Hearst Ave on the north side of Berkeley’s campus near the intersection of Hearst Ave and Euclid Ave. If you are a guest of the lab, we can get you a daily parking permit to park there. Otherwise, there is also street parking nearby. The map below shows the location of the parking structure and the short walk from there to McCone Hall. The lab is on the 4th floor of McCone Hall in room 408. Prof. Swanson-Hysell’s office is on the third floor in room 393.
Making .sam header files from orientation data
Converting .sam files to MagIC format
Analyzing data using demag_gui.py
Tuning the magnetometer SQuIDs
Trapping the field in the magnetometer
Using the turbopump to establish magnetometer vacuum