A peek at cosmic fugitives
doi:10.1038/nindia.2011.56 Published online 25 April 2011
Researchers have used a low-cost polymer detector film to capture signs of charged particles and rare exotic particles in cosmic rays. The detector film could be useful for monitoring radioactivity in both nuclear reactors and natural environments.
Detector films are used in devices called solid-state nuclear track detectors (SSNTDs). When charged particles pass through an SSNTD, they leave behind narrow damage trails on the detector film. The damage trails are etched out using chemical reagents, with etch pit geometry revealing the identity of the particle that carved the trail.
Astrophysicists have long predicted the existence of matter and stars made from strangelets, a hypothetical particle that has equal numbers of up, down and strange quarks. After a star explodes, it eventually becomes a neutron star, which is composed of neutrons and other subatomic particles. The neutron star then cools and emits neutrinos (chargeless particles) to become a strange star. However, detectors have yet to catch a glimpse of strangelets.
To capture trails of strangelets and other cosmic particles, the researchers designed an SSNTD whose detector film was composed of polyethylene terephthalate (PET). They used the REX-ISOLDE particle accelerator facility at CERN to generate and accelerate ions beams of xenon, krypton and titanium. They exposed their SSNTD to these ion beams to study the charge response of PET. Each exposure lasted for 30 seconds, with 1 cm2 of PET receiving around 50,000 charged particles.
The exposed PET samples were then etched in sodium hydroxide solution, studied under an optical microscope and analysed using image analysis software. The damage trails of krypton and titanium ions showed different track lengths.
"The advantage of PET is that it is sensitive to charged particles with very low velocities, which would otherwise escape detection," says Sibaji Raha, one of the researchers. "We plan to set up a large array of PET at Sandakphu in the Eastern Himalayas to look for strangelets in cosmic rays."
The authors of this work are from: Centre for Astroparticle Physics and Space Science and Department of Physics, Bose Institute, and Department of Physics, Barasat Government College, Kolkata, India, and CERN, Geneve, Switzerland.