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Physics News Update
Number 797, October 16, 2006 by Phil Schewe, Ben Stein, and Davide Castelvecchi

Elements 116 and 118 Are Discovered

At the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, physicists (including collaborators from Lawrence Livermore National Lab in the United States) have sent a beam of calcium-48 ions into a target of californium-249 atoms to create temporarily a handful of atoms representing element 118. The nucleus for these atoms have a total atomic mass of 294 units.

In fact, only three of these atoms, the heaviest ever produced in a controlled experiment, were observed. After sending 2 x 1019 calcium projectiles into the target, one atom of element 118 was discovered in the year 2002 and two more atoms in 2005. The researchers held up publication after seeing their first specimen in order to find more events. According to Livermore physicist Ken Moody, speaking at a press conference today from Livermore, the three events have been well studied and the odds of a statistical fluke at work here are less than a part in 100 thousand.

Caution would naturally be on the minds of anyone announcing a new element; Evidence for element 118 was offered once before, by a team at the Lawrence Berkeley National Laboratory (see PNU 432), but this claim was later retracted (PNU 550) when it was discovered that some of the data had been falsified.

In searching through 1019 collision events, how do you know you have found a new element? Because of the clear and unique decay sequence involving the offloading of alpha particles, nuclear parcels consisting of two protons and two neutrons. In this case, nuclei of element 118 decay to become element 116 (hereby itself discovered for the first time), and then element 114, and then element 112 by emitting detectable alphas. The 112 nucleus subsequently fissions into roughly equal-sized daughter particles.

The average lifetime observed for the three examples of element 118 was about one millisecond, not long enough to perform any kind of chemical tests (you'd need an hour's time for that). Element 118 lies just beneath radon in the periodic table and is therefore a kind of noble gas.

The Dubna-Livermore team previously announced the discovery of elements 113 and 115 (see PNU 672) and next hope to produce element 120 by crashing a beam of iron atoms into a plutonium target. To build nuclei much heavier than this you would need a beam of neutron-rich radioactive nuclei; the proposal to build an accelerator in the United States for doing just this has been stalled.

Oganessian et al., Physical Review C, October 2006
Livermore press release

A Single Pixel Is Worth a Thousand Words

A single-pixel digital camera, scientists at Rice University believe, will reduce power consumption and storage space without sacrificing spatial resolution. The new approach aims to confront one of the basic dilemmas of digital imaging, namely the huge waste factor.

Consider that a megapixel camera will, when you take the picture, capture and momentarily store a million numbers (the light levels from the pixels). No camera can store that much information for hundreds of pictures, so an immediate data compression takes place right there inside the camera. A tiny microprocessor performs a Fourier transform; that is, it converts the digital image into a weighted sum of many sinusoid waves. Instead of a million numbers, the representation of the image can now been compressed into something like 10,000 numbers, corresponding to the most important coefficients from the mathematical transformation. These are the numbers actually retained for later processing into pictures.

The Rice camera saves space and energy by eliminating the first step. It gets rid of the million pixels. Instead, it goes right to a transformed version (about 10,000 numbers rather than a million) by viewing the scene prismatically with a single pixel. No, the light from the object doesn't go through a prism, but it is viewed about 10,000 different ways. The light, in a quick succession of glances, bounces off the myriad individually driven facets of a digital micromirror device, or DMD (see Wikipedia entry). The mirrors of a DMD (only a micron or so in size) do not image an object or record data but merely steer light; they can be individually angled in such a way that the light strikes a photo detector or not, depending on whether the light is representing a digital 1 or 0 at that moment.

The main idea is that the DMD is acting as a sort of analog optical computer. Each time the pixel views the object, a different set of orientations is imposed on the array of micromirrors. And, in an interesting twist, the Rice camera uses random orientations. Looking like the haphazard splotch of black and white squares of a crossword puzzle, the DMD's surface is reflective here and dark there; some of the mirrors will faithfully reflect light from the object to the pixel while others will, in effect, appear black. Then the object is viewed again with a different micromirror activation pattern; again the pixel will record an overall light level. This process recurs about 10,000 times.

Later, offline on a computer, the single pixel light levels, along with the micromirror patterns are processed using new algorithms to reconstruct a sharp image. This isn't quite the old type of imaging process, the kind used in X-ray crystallography or CAT scans (which also convert pinpoints of data into images), but a new kind of imaging called compressive sensing that is only about two years old.

To summarize, the acquisition of imaging data is reduced many-fold (saving on data storage), only a single pixel is needed (freeing up valuable space in the primary detector), and the bulk of the processing can be offloaded to a remote computer rather than a chip inside the camera, thus greatly reducing power needs and extending the usefulness of batteries.

Rice researchers Richard Baraniuk (richb@rice.edu) and Kevin Kelly (kkelly@rice.edu) say that an additional virtue of the camera is that with only a single pixel, the detector (a photodiode) can be as fancy as you want. It can even accommodate wavelengths currently unavailable to digital photography, such as X-rays, terrahertz waves, even radar.

A working camera prototype has been built. One of the main tasks is to reduce the time it takes to record an image; the price for compressing space, pixels, and power is to spread everything out in time since the cyclops-like pixel must blink ten thousand or more times to capture the image. As Baraniuk says, the Rice form of photography is multiplexed in time. The Rice results were reported last week at the Frontiers in Optics Meeting of the Optical Society of America (OSA) held in Rochester, N.Y.

Pictures of the setup and imaging results on the Rice Web site and in the research paper (PDF)
Contact Richard Baraniuk (richb@rice.edu) or
Kevin Kelly (kkelly@rice.edu)
Rice University

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