Scientists usually need specific isotopes in order to perform their experiments. Many of the most interesting experiments require “rare” isotopes that are not found on earth and have to be made right before they are used. The A1900 is a highly selective and efficient separation device that uses superconducting magnets to select single isotopes from among the hundreds produced in nuclear reactions. The techniques used in the A1900 are so sensitive that even one nucleus out of 1,000,000,000,000,000,000 produced in the course of a week by collisions of the accelerated beam with a target can be selected and its properties studied.
How are rare isotopes produced at the NSCL? The answer lies in the combination of a production method—called projectile fragmentation—and a very sensitive filtering device—called the A1900. The filter is a device that can separate out the interesting isotope from among the many, many others produced by projectile fragmentation.
The name “A1900” comes from A for Analysis, and the number 1900 is the K-value or strength of the magnets. The K-value is also used in the names of the cyclotrons at the NSCL—the K500 and K1200—and is related to the speed of the ions that can be deflected or bent by the magnets. A higher K value corresponds to faster, and hence harder to bend, ions.
The A1900 is a highly selective, yet efficient separation device that passes individual isotopes—out of hundreds produced in a nuclear reactions—to the scientists for further study. These individual isotopes can be selected with nearly 100 percent efficiency. The A1900 has the best combination of power and efficiency in the world and was completed as part of the Coupled Cyclotron Facility in 2000.
The diagram/photo shows the layout of the A1900. The primary beam from the CCF strikes stationary nuclei in a target at the beginning of the device,. The targets are usually thin foils of carbon or beryllium that are a few millimeters thick. When the beam nucleus collides with a target nucleus, it is possible to knock out some of the neutrons and protons. If a large number of protons are removed while leaving most of the neutrons, then the product will be a “neutron-rich” rare isotope not normally found in nature. This process of proton and neutron removal is essentially “atom smashing” and is called projectile fragmentation. One of the main characteristics of this reaction is that the projectile fragments—which are sometimes rare isotopes—continue to fly straight out of the target and can be collected in the A1900.
The production target is followed by a series of superconducting quadrupole magnets that collect and focus the ions in the same way that a glass lens collects light. The quadrupole magnets are combined with dipole magnets that bend and filter the ions in the same way that prisms bend light. The whole system of magnets is approximately 22 meters long and able to select individual isotopes from among hundreds produced in the nuclear reactions. The end of the A1900, called the focal plane, contains specialized detectors that can identify and count each isotope as it arrives. Scientists use the detectors to “tune-up” the A1900 to select the isotope that they will study. The separated isotopes can also be transmitted to experiments many meters away from the A1900.
The A1900 is a third generation projectile fragment separator composed of 40 large diameter superconducting multipole magnets and four 45° dipoles with a maximum magnetic rigidity of 6 Tm. The A1900 has a solid angle of 8 msr, a momentum acceptance of 5.5% and can accept over 90% of a large range of projectile fragments produced at the NSCL. The A1900 is instrumented with position and timing detectors at the intermediate dispersive image and at the focal plane. The fragment yields can be modeled with the code LISE using configuration and option files available on the A1900 group’s web page.
Location: Transfer hall
Contact person: Tom Ginter
Commissioning the A1900 Projectile Fragment Separator; D.J. Morrissey, B.M. Sherrill, M. Steiner, A. Stolz, and I. Wiedenhöver, EMIS14, Victoria, Canada, 6-10 May 2002, D'Auria (ed.), Nucl. Instrum. Meth. B 204 (2003) 90.
A New High-resolution Separator for High Intensity Secondary Beams; D.J. Morrissey, and NSCL Staff, Nucl. Instrum. Meth. B 126 (1997) 316.