Cryo Central - Nuclear Physics

Al Zeller
National Superconducting Cyclotron Lab (NSCL) at Michigan State University
zeller@nscl.msu.edu

Cryogenics has a long history in nuclear physics. The technology has its origins in the use of cold traps for maintaining a vacuum, which is required to prevent beam loss and for generating high voltages used in acceleration. These traps are still used today, although the cryopumps do not use liquid cryogens any longer.

Cryogenics is used in the following ways:

• Cryopumps
• Superconducting magnets
• Cryogenic targets
• Cooling electronics (Field Effect Transistors – FETs)
• Cooled detectors
• Superconducting Radio Frequency (SRF)
• Non-intercepting beam current monitors
• Bolometric measurements of X-ray and low-energy ions

Cryopumps are simply cryocoolers that are designed to provide a cold surface to condense out different gasses in the vacuum system. Water is pumped efficiently by liquid nitrogen-temperature surfaces and air is removed by a pump at about 10 Kelvin. An additional usage is to protect targets and detectors from contamination by pump-oil that has gotten into the vacuum system. A cold shroud is placed around the target to prevent buildup of pump oil.

The first superconducting accelerator magnet used in a particle accelerator was the K500 superconducting cyclotron magnet at National Superconducting Cyclotron Lab (NSCL) at Michigan State University. First operated in October 1981, it was producing accelerated beam 18 months before the Fermilab energy-doubler came online. Since then several superconducting cyclotrons have been built, culminating in the separated sector Superconducting Ring Cyclotron at RIKEN. Finished in 2006, is the RIKEN cyclotron is the world's largest at 8300 tons, 19 meters in diameter and 8 meters high.

In addition to accelerator magnets, superconducting dipoles and quadrupoles are used in the beam transport system at NSCL and RIKEN. Another widely used application of superconducting magnets is in superconducting Electron Cyclotron Resonance (ECR) ion sources. The ECR sources produce large currents of highly charged ions for acceleration in a variety of accelerators, including cyclotrons and linacs. Superconducting magnets and solenoids are also used as separation systems for charged particles produced when the beam hits the targets. These are typically large volume or high-field devices and act in the same way that prisms do for ordinary light.

Like the liquid hydrogen bubble chambers that were used in high-energy physics experiments during the last fifty years, liquid hydrogen (and other gasses) have been used as targets in nuclear physics experiments. In contrast to the bubble chamber, the target serves only as a source of nuclei and the particles are tracked or detected beyond the target. The Thomas Jefferson National Accelerator Facility (JLAB) uses both liquid hydrogen and deuterium targets. Using secondary beam, or beams of nuclei that are produced by reactions of the accelerated beams on target, the low intensity allows for solid hydrogen targets with and without a window. The hydrogen can simply be condensed onto a "cold finger" thereby reducing the amount of extra material the beam needs to penetrate.

One of the major sources of noise in nuclear electronics can be significantly reduced by operating at liquid nitrogen temperatures. Field-Effect Transistors (FET) are the main amplifying devices for most X-ray and gamma ray detection systems. Operating the FET at cryogenic temperatures reduces the noise width and increases the resolution.

Semiconductor photon detectors, like germanium crystals, are operated at liquid nitrogen temperatures. These types of detectors have been in use for about forty years and are still the mainstay for high-resolution detection of X-rays and gamma rays.

Superconducting Radio Frequency (SRF) technology has been in use in accelerators for nuclear physics since about 1984 when ATLAS at Argonne National Lab was built. While lead has been used as cavity material, very high-purity niobium is the material of choice. The big advantage of SRF is that it can be operated continuously. Conventional RF systems dissipate a large amount of power, in the form of heat, so are used in pulse modes. This limits the beam availability and causes problems in detectors because all of the "counts" (detected particles or photons) arrive at the same time. Depending on the frequency of the RF, the niobium cavities are cooled to 4.2 K or sometimes as low as 1.8 K (superfluid helium). Therefore, refrigeration capacity is an important part of any SRF operation.

While the current interest in SRF is driven by high-energy physics projects, such as the International Linear Collider, the majority of SRF applications are in nuclear physics. The Spallation Neutron Source is the newest facility to employ SRF, although it is not a nuclear physics facility. JLAB presently has the largest installed 1.8 K capacity in the world to operate their cavities. Other laboratories that do significant R&D on SRF in the United States are NSCL, ANL, Cornell University. Recently, the TRIUMF facility added an accelerator for secondary ions produced by a process called ISOLDE that uses SRF.

The standard way to measure beam currents without putting material in the beam's path is by measuring the inductance. This provides a measurement of the total beam current within some well-defined beam-currents. A more elegant way is to use the Meissner Effect. At low magnetic fields the magnetic field is excluded from the interior of a superconductor. The standard High Temperature Superconductor demonstration is to float a magnet above a superconducting disk. A non-intercepting beam current monitor, capable of determining the position of the beam, can be built using the same effect (T. Watanabe, et al., RIKEN Accel. Prog. Rep. 38 (2005) 273).

Low-energy deposition events, like electrons or low-velocity ions, can be measured with bolometers. These are superconducting materials at temperatures well below their transition temperature. For example, tin is cooled with a dilution refrigerator to about 70 mK. When a low-energy particle hits the detector the temperature rises and this increase is measured with a sensitive thermometer, resulting in both excellent resolution and high efficiency (A. Alessandrello, et al., Phy. Rev. Lett. 82 (1999) 513).