The Sweeper Magnet

The properties of neutron-rich matter, such as neutron stars, can be studied with exotic beams of nuclei consisting of many more neutrons than protons. These nuclei bombard a target where some of these neutrons are stripped off. NSCL has sophisticated detectors that measure the nuclear remnants (S800) and the also the neutrons (MoNA, Neutron Walls). The sweeper magnet separates the neutrons and the remnants so that they can be detected in these devices. The magnet generates a strong magnetic field using superconducting coils. As neutral particles, the neutrons are not affected by the magnetic field and fly straight after the reaction. However, the charged remnants are “swept” away in a different direction towards, for example, the S800 or another detection system. The sweeper thus acts as an auxiliary device that serves the actual detectors.

Expanded Description

The sweeper magnet weighs about 50,000 pounds and generates a magnetic field of 40,000 Gauss which is about 400 times stronger than a typical refrigerator magnet. The superconducting wire is held at a temperature of –452 °F and can carry a current of 500 Amperes.

The magnet is placed immediately behind a target where the exotic neutron-rich nuclei react and break up into a charged nuclear fragment and one, two or more neutrons. The magnet has a large gap of 14 cm which allows the neutrons to fly straight towards the existing Neutron Wall detectors or MoNA.

The charged fragments typically have velocities of about 40 percent of the speed of light, or 55 million miles per hour. The magnetic field of the magnet is strong enough to bend these particles by 40° over a distance of only 1 meter. The magnet can be placed directly in front of the S800, which can detect and analyze the properties of these fragments with high resolution.

Alternatively, the sweeper also has its own lower-resolution detection system. This system can determine all the detailed properties of the fragments following the breakup—the charge, mass, angle, velocity, momentum and energy. By combining this information with the corresponding information about the neutrons, it is possible to reconstruct the properties of the original neutron-rich exotic nucleus.

Importance of the Sweeper magnet

Neutron rich matter plays an important role in the formation of the elements on the earth, the solar system and the whole universe. Many elements are created by forming nuclei which contain many more neutrons than protons. These nuclei decayed a long time ago and no longer exist on earth. Exotic beams are able to recreate these nuclei, and many experiments at NSCL are designed to study these nuclei. The sweeper magnet is an essentially tool in these studies because it is able to separate the neutrons from charged particles created in the reactions. The sweeper is really an auxiliary device that serves the actual detectors.

Technical Information

The sweeper magnet is built at the National High Magnetic Field Laboratory (NHMFL) at Florida State University. It is a superconducting dipole magnet with a maximum field of 4 T. The bend radius is 1 m with a bend angle of 400. It has a vertical gap of 14 cm which allows for neutron coincidence experiments (with the neutron walls or MoNA) covering about ±70. The total weight of the magnet is more than 50,000 lbs.

Status: Operational

Location: N2 vault

Contact person: Michael Thoennessen

Funding acknowledgement: The construction of the sweeper magnet was funded by the National Science Foundation through Major Research Instrumentation grant PHY-9871462.

References:

A Compact Sweeper Magnet for Nuclear Physics. A. F. Zeller, J. C. DeKamp, M. Thoennessen, B. M. Sherrill, P. G. Hansen, M. Bird, Y. Eyassa, S. W. Van Sciver, and K. W. Kemper, Advances in Cryogenic Engineering 45 (2000) 643.

Structral Design and Analysis of Compact Sweeper Magnet for Nuclear Physics; S. Prestemon, M. D. Bird, D. G. Crook, Y. M. Eyssa, J. C. DeKamp, L. Morris, M. Thoennessen, and A. F. Zeller, IEEE Transactions on Applied Superconductivity 11 (2001) 1721.