December 10, 2009--Oncologists have a dream: they want to use highly energetic ion beams in accurately defined doses for a effective radiation treatment of tumors. Modern techniques based on intense laser pulses may play an important role in the future and replace expensive conventional particle accelerators.

Carbon beams are considered to be a very effective method of cancer therapy, as tumors are destroyed permanently with minimum trauma. Whereas conventional x-rays or electron beams cause significant damage to the surrounding healthy tissue on their pathway into the body, the high biological effectiveness of carbon beams can be precisely concentrated in the tumor, thus exclusively killing targeted cancer cells. Therefore, carbon ions are an outstanding tool for radiation therapy of deeply situated tumors located in highly sensitive regions like in the vicinity of the brain stem, where doctors would refuse to even contemplate surgical intervention.

Now, a team of physicists of the Cluster of Excellence Munich-Centre for Advanced Photonics (MAP) lead by Dietrich Habs (Ludwig-Maximilian University Munich) in cooperation with scientists of the Max-Born-Institute in Berlin have experimentally demonstrated a mechanism of laser-driven carbon-ion-beam generation that was predicted by theorists long ago.¹ (Figure is courtesy of Andreas Henig)

Ions from diomondlike carbon
The scientists generate the high-energy ions by irradiating diamondlike carbon foils with intense laser pulses. Atoms located within the foil are split into electrons and ions by the strong electric field of the laser focus, generating a plasma. The high laser intensity (about 10²⁰ times more intense than the sun) strongly heats the electrons and separates them in an expanding cloud from the heavier and therefore slower ions. A huge charge-separation field builds up, accelerating ions to velocities up to a tenth of light speed.

Previously, laser-accelerated ions exhibited a broad energy spectrum, whereas medical applications demand a well-defined particle energy to allow for a precise control of penetration depth and dose distribution in the body.

The group of Munich physicists is the first to experimentally demonstrate an acceleration process that gives all ions the same velocity. By changing the laser polarization from linear to circular and reducing the diamondlike carbon foil to only a few nanometers in thickness, uncontrolled heating of the particles and subsequent foil expansion was avoided. Instead, the laser light pushes the electrons collectively as a nanometer-thin layer in the forward direction, dragging carbon ions with it. The whole foil is driven like a sail by the light pressure of the laser.

The next challenge for the physicists is to further increase the energy of the laser-accelerated ion beam. At the moment it is not yet energetic enough to penetrate the human body far enough to reach deeply situated tumors.

REFERENCE:

1. DOI: 10.1103/PhysRevLett.103.245003
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--posted by John Wallace, johnw@pennwell.com

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