The S800 Spectrograph

The top of the S800 spectrograph's second 75-ton dipole magnet. more

Scientists use the S800 spectrograph to study the nature of the atomic nucleus and its behavior at the very small scales. One can think of the S800 as a magnetic prism which can deflect charged particles of different energies similar to the way that a prism deflects light of different colors. The goal is to acquire knowledge on atomic nuclei in conditions that are not found in nature except in stars or supernovae.

Expanded Description

The S800 combines two key factors to achieve its performance.

  • High resolution, meaning it is capable of distinguishing between two particles of slightly different energies.
  • Large acceptance, meaning it collects and measures a large number of the particles after they have reacted with the target.

These two features are what make this instrument unique and allow precise measurements of rare phenomena.

The S800 is equipped with sensitive detectors that measure the positions and angles of particles deflected by the magnetic fields. Sophisticated software is then used to deduce the characteristics of the particles before and after the reaction. Various types of experiments are performed using this technique, sometimes in combination with other types of detectors located around the target to get a more complete picture of each reaction. For example, strange modes of vibration of nuclei can be studied, as well as exchange of nucleons (protons or neutrons) during the split moment of a nuclear reaction between an accelerated nucleus and a target nucleus.

But the most attractive domain of experiments lies in the study of unstable nuclei that decay after some period of time, where the very structure itself is unknown and new phenomena can arise.

The Importance of the S800

The ultimate goal of experiments performed on the S800 spectrograph is to acquire knowledge on atomic nuclei in conditions that are not found in nature except in stars or supernovae. Used in combination with accelerated beams of heavy ions, this instrument allows experimentalists to explore the unknown properties of nuclei involved in the stellar synthesis of all elements found on earth. In a more fundamental aim, it is essential to the understanding of the world of quantum mechanics, for which the system of protons and neutrons that constitute a nucleus is a perfect site.

The laws of quantum mechanics are only accessible experimentally at a microscopic scale and are mostly hidden in our everyday macroscopic world. Being able to find a suitable medium to experimentally study these fundamental laws of nature is essential and basic to our understanding of our universe. The atomic nucleus is one of the best laboratories for studying these laws because it has the required scale and is composed of a different number of interacting particles—neutrons and protons. The experimental results obtained from the S800 are directly compared to theoretical calculations produced by models governed by quantum mechanics and are paramount to the progress of our understanding. Although it seems unreasonable to use such a big instrument to study such tiny particles, the size of the S800 spectrograph is dictated by the energy of the particles accelerated by the coupled cyclotrons. The more energetic they are, the more difficult they are to deflect ,and hence the bigger the spectrograph has to be. In other words, the S800 is an essential tool in the cyclotron laboratory. Without it, all the benefits of a high quality particle accelerator, such as the coupled cyclotrons, would be wasted.

Technical Information

The S800 spectrograph combines both high resolution and high acceptance in a single device and is specially designed for reaction studies with radioactive beams. Its large acceptances both in solid angle (20 msr) and momentum (5%) are well adapted to the large emittances of secondary beams produced by projectile fragmentation. The high resolution is achieved via an analytical reconstruction method in which aberrations are calculated a priori from the magnetic field maps and used directly to correct the raw data. The spectrograph is installed vertically on a carriage that can rotate from 0° to 60°. Its maximum rigidity is limited to 4 Teslameter (Tm). The S800 is preceded by an analysis line that allows for different optical modes of operations, either focussing or dispersion matched. The maximum rigidity of the analysis line is limited to 4.9 Tm.

Status: Operational

Location: S3 vault

Contact person: Daniel Bazin

Funding acknowledgement: The S800 construction was initiated under the NSCL Phase II construction project (NSF PHY-8215585) and completed under the NSCL Cooperative Operative Agreement (NSF PHY-9214992).


    The S800 spectrograph; D. Bazin, J. A. Caggiano, B.M. Sherrill, J. Yurkon, A. Zeller, EMIS-14 conference proceedings, Victoria, BC, Canada, May 6-10, 2002, Nucl. Instr. Meth. B 204 (2003) 629.
    doi: 10.1016/S0168-583X(02)02142-0

    Focal plane detector for the S800 high-resolution spectrometer; J. Yurkon*, D. Bazin, W. Benenson, D.J. Morrissey, B.M. Sherrill, D. Swan, R. Swanson, Nucl. Instr. Meth. A 422 (1999) 291.
    doi: 10.1016/S0168-9002(98)00960-7