Doppler-broadening spectroscopy

A broad overview of traditional methods and recent developments in experimental positron spectroscopy is presented. A discussion of the generation and detection of positrons and their annihilation radiation is followed by a survey of techniques used for positron lifetime measurement, Doppler broadening spectroscopy and angular correlation of annihilation radiation, and the opportunities presented by combining these methods (e.g. in age-momentum correlation) and/or extending their capabilities by the use of monoenergetic positron beams. Novel spectroscopic and microscopic techniques using positron beams are also described. 

Samples for Doppler broadening spectroscopy should be mounted in an evacuated chamber to avoid sample contamination on... 

Djourelov, N., Suzuki, T., Ito, Y, Shantarovich, V., and Kondo, K., Gamma and positron irradiation effects on polypropylene studied by coincidence Doppler broadening spectroscopy. Radial. Phys. Chem., 72, 687-694 (2005). 

The limited application of positron beams is explained by the fact that Doppler-broadening spectroscopy is the only detection method that works with beams without any difficulty. Although angular correlation measurements are possible with positron beams, it requires a very high efficiency beamline. Only very few beams fulfill the requirements of ACAR measurements, but their results are very impressive. The combination of low-energy positron beams and 2D-ACAR measurements is the only possibility to gain direct information on the momentum distribution of electrons at the surface of metals and semiconductors (Chen et al. 1987). 

Positron annihilation spectroscopy can provide essential information about the deterioration in the mechanical properties of RPV steels (microstructural defects and precipitates) during their irradiation, which is known as neutron embrittlement. Currently, there are three main techniques based on annihilation phenomena positron lifetime spectroscopy, Doppler-broadening spectroscopy and angular correlation measurements. 

Beam Spectroscopy. Both specificity and sensitivity can be gready enhanced by suppressing coUisional and Doppler broadening. This is accompHshed in supersonic atomic and molecular beams (296) by probing the beam transversely to its direction of dow in a near-coUisionless regime. 

The use of CW tunable semiconductor lasers as a source in IR spectroscopy research makes possible a very great increase in resolving power over traditional IR grating spectrometers. IR studies with laser sources have been done on several gases (e.g., H20,NH3,SF6,N0). The laser line width is typically 1/100th the width of the Doppler-broadened absorption lines of the gases, so the fine details of the IR line shapes are... 

The measurements of 2s — Is transitions in magnetically trapped hydrogen have achieved a relative accuracy of one part in 1012 [21] by means of two-photon spectroscopy which eliminates the first-order Doppler broadening. It is hoped that this technique will allow the measurement of the Is — 2s transition with the accuracy limited only by the shape of the transition line dictated by quantum electrodynamics, i.e. to a few parts in 1015. Further, if the center of the Is — 2s line could be determined with the accuracy of a few parts in 103 of its width, the relative accuracy for this transition would increase to a few parts in 1018. 

Fig. 1.5. Schematic of a gamma-ray energy spectrometer for Doppler broadening studies. The signal from the detector pre-amp is processed by spectroscopy and biassed amplifiers (SA and BA respectively) before being recorded in the multichannel analyser (MCA).

Such beams have many uses, including some important applications in spectroscopy. In particular, pressure broadening of spectral lines is removed in an effusive beam and, if observations are made perpendicular to the direction of the beam, Doppler broadening is considerably reduced because the velocity component in the direction of observation is very small. 

The still necessary increase in accuracy requires an additional effort. A simultaneous spectroscopy of pionic and muonic hydrogen atoms is planned as the muonic X-rays do not show any strong interaction broadening, but exhibit Doppler broadening similar to pionic atoms. A method was found to measure pionic and muonic X-rays simultaneously. The reduced masses of pionic and muonic hydrogen exhibit almost the same ratio as two lattice plane differences of quartz. With a two crystal set up the pionic and muonic X-rays can be Bragg reflected to the same CCD detector. 

This Doppler width can be avoided by typical sub-Doppler laser spectroscopy techniques. Laser saturation spectroscopy with a resolution close to the natural line width was used for a test of Special Relativity at the ESR. For such sub-Doppler resolution one must also take into account the small additional broadening and shift arising from the angle 0 between laser beam and ion beam in the Doppler formula. At an interaction length of 10 meters and more, angles are easily controlled to be better than 1 mrad. This limits a possible shift, which enters by... 

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