Getting colder

[Earlier this year] we reported the bringing into operation of a 45 cm3 polarized proton target working at 0.55 K. It now looks as if the development can be carried further with the testing of a new refrigerator using a 10 cm3 sample, which can be maintained at a temperature of less than 0.05 K. This is a step not only towards higher rates of polarization but also towards "frozen spin" targets, which could allow particles emerging from collisions to be detected over almost 4π solid angle.

The refrigerator, built by T O Niinikoski of the Technical University of Helsinki in collaboration with the CERN Polarized Targets Group, is based on the principle of He3/He4 dilution. The use of high-speed pumps (250 m3/h) give a cooling power that is the greatest ever reached for such low temperatures, making it possible to maintain the sample at 0.03 K under normal operating conditions and to sustain temperatures as low as 0.022 K.

The final aim is to perfect 15 cm long targets for the large aperture magnet in the East Hall or the Omega spectrometer. To derive the maximum amount of information from these spectrometers it is desirable to analyse particles leaving the target at any angle. With targets built so far, very homogeneous and intense magnetic fields were necessary, with poles close to the target, reducing the solid angle for observations.

The lower the temperature, the longer the relaxation time for the polarized protons to revert to a completely disorientated state. At very low temperatures, the inertia of spins is such that prolonged polarization can be maintained even if the magnetic field ceases to have the homogeneity and intensity necessary for creating the effect. This is a "frozen spin" target. It is hoped that, in Omega’s 1.5 T field, the target will be able to maintain its polarization for several days at a temperature of 0.06 K.

• Compiled from texts on p353.


15-foot chamber

The major bubble-chamber facility at the National Accelerator Laboratory, a 15-foot chamber, is at an advanced stage of construction in the Neutrino Laboratory at the 200/500 GeV accelerator.

Initially it had been hoped to build a 25-foot chamber but, with no sign of money to construct a chamber of this size, the scale was trimmed down to a 15-foot version. An important factor has been the readiness of the NAL Group under W Fowler to bring in extensive help. Thus the chamber design incorporates ideas from Argonne on the magnet, Stanford on the expansion system, Brookhaven on the vessels, and CERN on the optics, piston and seal.

The volume of liquid is 30,000 litres, contained in an almost spherical vessel with a length along the beam direction of 15 foot. It is designed to operate with hydrogen, deuterium, neon or mixtures. The superconducting magnet will provide a field of 3 T at the centre of the chamber.

The expansion system will operate about once per second, giving the possibility of four expansions per accelerator cycle. The profligate use of six cameras, located at the top of the chamber in two triangular arrays, enables hadron pictures to be taken while neutrino experiments are running. Thus the neutrino beam to the chamber might absorb 70% of the beam early in the flat top, leaving the opportunity for a burst of charged particles at the end.

The project began in summer 1970. Since early this year, the globe of the 7 m diameter vacuum vessel has been a prominent feature of the NAL site. On 25 October the chamber body arrived at the laboratory, and was installed in its final position in the vacuum tank on 30 November. It is hoped that the first cool-down will take place in July 1972, and that the chamber will be ready for experiments by the beginning of 1973.

• Compiled from texts on pp358–359.


Compiler’s Note

Absolute zero, 0 K, thought of as the lowest temperature possible, is taken as −273.15°C. The LHC cryogenic system, the largest in the world, gets to within 1.9 K, making it one of the coldest places on Earth, colder than outer space at 2.7 K. Recently, a cubic metre of copper weighing 400 kg was cooled to 0.006 K (6m K) for the CUORE experiment in Gran Sasso (see "CUORE has coldest heart in the known universe"). The lowest temperature ever recorded, 0.0000000001 K (100 pK) was reached at the Helsinki University of Technology Low Temperature Lab by nuclear magnetic ordering – bbbrrrrr!

Those venerable bubble chambers surely had charisma, resembling iconic submersibles such as the Bathysphere of William Beebe and Otis Barton, who descended to a record-breaking 920 m in 1934, and the Deepsea Challenger of film director James Cameron, who, in 2012, hit the bottom of the Mariana Trench, the oceans’ deepest point, where the temperature is a comfortable 1 to 4°C but the pressure is 1000 times the value at sea level, 11 km above.