KATRIN experiment: Measuring the mass of a “ghost particle”
December 14, 2018
To solve this riddle, the international KATRIN collaboration has collected the world’s foremost tritium-β-decay experts to steer this highly specialist experiment through an astro-particle maze.
The aim of the KATRIN experiment is to measure the mass of the neutrino - predicted at less than 10-10 of a hydrogen atom - so the margin of error is negligible.
But why is this all so important? Neutrinos are probably the most fascinating species of elementary particles, and indeed are referred to as the “ghost particles of the universe”. Although neutrinos are the lightest particles in our Universe, their tiny mass is a clear indication of elementary particle physics. They are the key to opening up a whole range of scientific issues linking the microcosm of elementary particles, to the largest structures in the Universe. On a grand scale neutrinos act as "cosmic architects" and take part in shaping the visible structures in the Universe, as they influence the formation and the distribution of galaxies. In many ways you can think of neutrinos as the “DNA of matter”.
Previous experiments have shown an upper limit to anti-neutrino mass of 2.3 eV/c2. Using the same measurement techniques, KATRIN will either improve this accuracy down to 0.2 eV/c2, or (if it is larger than 0.35 eV/c2) discover the actual mass. Either outcome requires an improvement by two orders of magnitude with respect to key experimental parameters.
Since neutrinos have no electrical charge, their energy will be measured against the shape of the electron spectrum generated by a tritium-β-decay, so the measurements are made using an electrostatic spectrometer. Due to the necessity for high sensitivity, this spectrometer unit has to operate in an absolute ultra-high vacuum (UHV) of nearly 10-11 mbar to avoid ‘false’ readings generated by residual atoms that have been ionized by cosmic radiation. This 200-ton spectrometer with a volume of 1,230 m3, is the world’s largest UHV vessel.
After 16 years of development and manufacturing, the final commissioning measurements on the KATRIN spectrometer were completed, and vacuum integrity was verified, allowing the transmission and background testing to commence in October 2016. Regular measurements started in July 2018, which are expected to continue for five years.
Like so much of the experimental work involving sub-atomic matter, KATRIN uses a wide array of pump technology and equipment to produce the UHV environment which is standard for this field of research. Super-conducting magnets will guide the electrons through a vacuum beam-line from the gaseous tritium source, through a differential pumping section to a high-resolution spectrometer, where the kinetic energy will be measured.
However, to achieve the ultra-high levels at which superconducting magnets work, they need to operate at UHV. An array of specialist pumps have been successfully employed to reduce the vacuum to the level required. These pumps include a set of fore vacuum and turbomolecular pumps (meeting special requirements to handle tritium throughputs of around 1 kg/a) and, as the main UHV pumping system, a setup of getter (NEG) strips with a pumping speed of 1x106 litres/sec. This array of pumps will provide the necessary pumping speed to achieve this UHV, in this case 10-11 mbar or better, in this large spectrometer vessel.
Discovering the weight of the “ghost particle” will be significant not only for particle physics, but also for cosmology, and will bring scientists a step closer to understanding the mysteries of the Universe. The KATRIN programme is one of the major neutrino experiments likely to yield such a significant result.