| Massive magnet: VECC scientists in front of the K-500
How to produce and probe subatomic particles as well as various isotopes, or various avatars of the same elements, which exist only for a fleeting moment inside the fiery cauldrons of distant stars' Physicists at the Variable Energy Cyclotron Centre (VECC), Calcutta, know the answer. They have built a huge machine, K-500 Superconducting Cyclotron ' a particle accelerator which, besides unravelling many mysteries of the cosmos, may have down-to-earth applications like seeking out and destroying a vicious tumour.
“This machine, first of its kind in India and the largest in Asia, will start operating two years from now,” says Dr Bikash Sinha, director, VECC. “It will play a pivotal role in areas like magnetic resonance imaging (MRI), energy storage devices and rapid transit using magnetic levitation.”
To peer back in time and catch a glimpse of isotopes that existed in the early cosmos, scientists need to mimic conditions that prevailed then. At first, electrons are stripped off the atoms to make ions. The ions are sent down a beam pipe to be injected into the K-500 cyclotron.
“Inside the K-500 cyclotron, ions are exposed to a huge electromagnetic field generated by a massive magnet (50,000 times as powerful as the earth’s magnetic field). It consists of superconducting coil made of niobium-titanium ' not copper 'wire,” says Dr Rakesh Bhandari, associate director, VECC. Ordinary copper wire has resistance and it becomes hot when currents flow through it and melts when the current becomes too high.
“The coil used in the superconducting cyclotron is immersed in liquid helium at -269'C. At this temperature, the coil has virtually no electrical resistance and can carry a very high current,” says Dr Jayanta Chowdhury of the VECC. This produces a powerful magnetic field that holds the spinning ions inside the cyclotron as they accelerate. “The energetic ions speed through a vacuum pipe to the experimental vault where they hit a target element, triggering a nuclear reaction,” adds Chowdhury. For short-lived isotopes, the time from creation to investigation is very small ' shorter than even 0.00001 second. Scientists use a variety of experimental devices to learn what happens in this nuclear reaction.
Such reactions can cook up isotopes of various elements from lightweight oxygen to heavyweight uranium and even isotopes which don’t exist on earth anymore. Some exist only for fractions of a second. This provides a window to the world of isotopes, letting us know how much they weigh, what shapes they have, how big they are, and many more such parameters. The birth of such isotopes can also lead to useful technologies that may have a wide range of applications from diagnostics to therapies for many fatal diseases.
“For instance, a beam of protons (atoms without electrons) has many advantages for radiation treatment of cancer,” says Sinha. “Since protons have an electrical charge, they can be focused to a pencil-thin beam, if desired. They also have the property that most of the beam will stop at the same depth in the patient’s body.” This depth can be calculated, and the beam energy chosen to make the most of the beam stop at the cancer, destroying the cancer in the process with minimal damage to the surrounding tissue. “For this reason, proton therapy is the treatment of choice for inoperable tumours, such as tumours located in the eye, close to main arteries, or in regions of the brain that are inaccessible to surgery.”
According to Sinha, the cyclotron can accelerate deuterons, an isotope of hydrogen. Known as ‘heavy hydrogen,’ they differ from normal hydrogen in that they have a neutron as well as a proton in their nucleus. The cyclotron’s fast-moving deuterons are stopped in a target of beryllium just before their exit from the cyclotron. This produces a beam of high-energy neutrons, which is then directed against the cancer patient’s tumour. A new technique using a combination of neutrons and x-rays has been found to be particularly effective for the treatment of advanced prostate cancer.
Since the cyclotron is superconducting, its size is much smaller than a room-temperature cyclotron. “This miniaturisation allows the cyclotron to be mounted on the rings that rotate around the patient so that the cancer can be irradiated from several angles,” adds Sinha.