Scientific English (Tuesday, 13 February, 2012)

1. Scientific English (Tuesday, 13 February, 2012) PIXE (Proton-induced X-ray emission; Particle-induced X-ray emission) From Wikipedia, the free encyclopedia Particle-induced X-ray emission or proton-induced X-ray emission (PIXE) is a technique used in the determining of the elemental make-up of a material or sample. When a material is exposed to an ion beam, atomic interactions occur that give off EM radiation of wavelengths in the x-ray part of the electromagnetic spectrum specific to an element. PIXE is a powerful yet non-destructive elemental analysis technique now used routinely by geologists, archaeologists, art conservators and others to help answer questions of provenance, dating and authenticity. The technique was first proposed in 1970 by Sven Johansson of Lund University, Sweden, and developed over the next few years with his colleagues Roland Akselsson and Thomas B Johansson. Recent extensions of PIXE using tightly focused beams (down to 1 μm) give the additional capability of microscopic analysis. This technique, called microPIXE, can be used to determine the distribution of trace elements in a wide range of samples. A related technique, particle-induced gamma-ray emission (PIGE) can be used to detect some light elements.

2. Theory Three types of spectra can be collected from a PIXE experiment: X-ray emission spectrum. Rutherford backscattering spectrum. Proton transmission spectrum. X-ray emission Quantum theory states that orbiting electrons of an atom must occupy discrete energy levels in order to be stable. Bombardment with ions of sufficient energy (usually MeV protons) produced by an ion accelerator, will cause inner shell ionization of atoms in a specimen. Outer shell electrons drop down to replace inner shell vacancies, however only certain transitions are allowed. X-rays of a characteristic energy of the element are emitted. An energy dispersive detector is used to record and measure these X-rays. Only elements heavier than fluorine can be detected. The lower detection limit for a PIXE beam is given by the ability of the X-rays to pass through the window between the chamber and the X-ray detector. The upper limit is given by the ionisation cross section, the probability of the K electron shell ionisation, this is maximal when the velocity of the proton matches the velocity of the electron (10% of the speed of light), therefore 3 MeV proton beams are optimal.

3. Proton backscattering Protons can also interact with the nucleus of the atoms in the sample through elastic collisions, Rutherford backscattering, often repelling the proton at angles close to 180 degrees. The backscatter give information on the sample thickness and composition. The bulk sample properties allow for the correction of X-ray photon loss within the sample. Proton transmission The transmission of protons through a sample can also be used to get information about the sample. Protein analysis Protein analysis using microPIXE allow for the determination of the elemental composition of liquid and crystalline proteins. MicroPIXE can quantify the metal content of protein molecules with a relative accuracy of between 10% and 20%. The advantage of microPIXE is that given a protein of known sequence, The X-ray emission from sulfur can be used as an internal standard to calculate the number of metal atom per protein monomer. Because only relative concentrations are calculated there are only minimal systematic errors, and the results are totally internally consistent. The relative concentrations of DNA to protein (and metals) can also be measured using the phosphate groups of the bases as an internal calibration. Artifact analysis MicroPIXE is a useful technique for the non-destructive analysis of paintings and antiques. Although it provides only an elemental analysis, it can be used to distinguish and measure layers within the thickness of an artifact.

4. AMS (Accelerator Mass Spectrometer or Spectrometry) Accelerator mass spectrometry (AMS) differs from other forms of masspectrometry in that it accelerates ions to extraordinarily high kinetic energies before mass analysis. The special strength of AMS among the mass spectrometric methods is its power to separate a rare isotope from an abundant neighboring mass ("abundance sensitivity", e.g. 14C from 12C).The method suppresses molecular isobars completely and in many cases can separate atomic isobars (e.g. 14N from 14C) also. This makes possible the detection of naturally occurring, long-lived radio-isotopes such as 10Be, 36Cl, 26Al and 14C. Their typical isotopic abundance ranges from 10−12 to 10−18. AMS can outperform the competing technique of decay counting for all isotopes where the half-life is long.

5. The method Generally, negative ions are created (atoms are ionized) in an ion source. In fortunate cases this already allows the suppression of an unwanted isobar, which does not form negative ions (as 14N in the case of 14C measurements). The pre-accelerated ions are usually separated by a first mass spectrometer of sector-field type and enter an electrostatic "tandem accelerator". This is a large nuclear particle accelerator based on the principle of a Tandem van de Graaff Accelerator operating at 0.2 to many million volts with two stages operating in tandem to accelerate the particles. At the connecting point between the two stages, the ions change charge from negative to positive by passing through a thin layer of matter ("stripping", either gas or a thin carbon foil). Molecules will break apart in this stripping stage. The complete suppression of molecular isobars (e.g. 13CH− in the case of 14C measurements) is one reason for the exceptional abundance sensitivity of AMS. Additionally, the impact strips off several of the ion's electrons, converting it into a positively charged ion.

6. In the second half of the accelerator the now positively charged ion is accelerated away from the highly positive center of the electrostatic accelerator which previously attracted the negative ion. When the ions leave the accelerator they are positively charged and are moving at several percent of the speed of light. In a second stage of mass spectrometer, the fragments from the molecules are separated from the ions of interest. This spectrometer may exist of magnetic or electric sectors, and so called velocity selectors, which utilizes both electric fields and magnetic fields. After this state, no background is left, unless a stable (atomic) isobar forming negative ions exists (e.g. 36S if measuring 36Cl), which is not suppressed at all by the setup described so far. Thanks to the high energy of the ions, these can be separated by methods borrowed from nuclear physics, like degrader foils and gas-filled magnets. Individual ions are finally detected by single-ion counting (with silicon surface-barrier detectors, ionization chambers, and/or time-of-flight telescopes). Thanks to the high energy of the ions, these detectors can provide additional identification of background isobars by nuclear-charge determination.

7. Applications The applications are many. AMS is most often employed to determine the concentration of 14C, e.g. by archaeologists for radiocarbon dating. An accelerator mass spectrometer is required, over other forms of mass spectrometry, because of their insufficient abundance sensitivity, and to resolve stable nitrogen-14 from radiocarbon. Due to the long half-life of 14C, decay counting requires significantly larger samples. 10Be, 26Al, and 36Cl are used for surface exposure dating in geology. 3H, 14C, 36Cl, and 129I are used as hydrological tracer. Accelerator mass spectrometry is widely used in biomedical research. In particular 41Ca has been used to measure bone resorption in postmenopausal women.

8. 美鑑識中心 命名李昌鈺（Henry C. Lee Institute of Forensic Science） 以國際鑑識權威李昌鈺為名的鑑識中心，前天在美國康乃狄克州正式啟用。李昌鈺在台灣成長、求學，留美時專攻刑事鑑定，後因以科學證據屢破別人破不了的「冷案」，揚名國際。他服務多年的康州紐海芬大學 (University of New Haven），特以其名為鑑識中心新大樓命名。 李昌鈺期許這個研究中心成為鑑識科學重鎮，「不僅是美國第一，也是全球第一。」這座鑑識科學研究中心耗資約4.29億元台幣，兼顧實務及學習用途。走進一樓，參觀者立刻看到地面透視雷達、電訊鑑識網路衛星系統，及高強度雷射等先進器材。 1998年時任康州警政廳長的李昌鈺，與原本就有鑑識課程的紐海芬大學聯手，創立鑑識科學研究中心，近年該校成為有志學習鑑識者嚮往的知識殿堂，目前有500名學生在此攻讀鑑識。新大樓啟用當晚，李昌鈺就在那上了第一堂課。一位學生說，自從高二解剖過一個豬胚胎後，就迷上鑑識。學校董事長卡普蘭更盛讚李昌鈺對學校的貢獻，「終於有個能襯托大師的設備了。」