PIXE chambers

UHV is not mandatory in PIXE, and the vacuum shared by the beam-line and specimen chamber is typically 10 6torr. The beam emerging from the accelerator has to be made uniform, while generating the minimum possible X-ray and y-ray background near the Si(Li) detector, and to this end graphite or tantalum collimators are to be preferred. PIXE chambers are often lined with graphite foil. 

Figure 3 (A) Schematic view of a PIXE chamber. Key 1, diaphragms to collimate the beam 2, carousel for the samples 3, lifting mechanism to position the sample in front of the beam 4, sample holders 5, X-ray detector 6, wheel with different absorbers to filter out low-energy X-rays 7, y-detector 8, protractor to position a surface barrier detector. (B) Photograph of a typical PIXE environment with a pelletron accelerator and micro- and macro-PIXE beam lines. (Reprinted with permission from Vis RD, Kramer JLAM, van Langevelde F, and Mars L (1993) The upgraded Amsterdam nuclear microprobe. Physics Review B Nuclear Instrument Methods BTI 41-44 Elsevier.)...

The bombarding particle beam is accelerated and transported in the high vacuum system of the accelerator. The X-ray detector unit is encapsulated in a separate clean vacuum system. The irradiation of the sample may be performed in vacuum or under atmospheric conditions, depending on the size, physical state, and other parameters of the sample. To define the physical conditions mentioned above, special PIXE chambers of internal beam and external beam configurations are used for in vacuo and atmospheric irradiations, respectively. 

Internal beam and external beam PIXE chambers (a and b, respectively). See text for details... 

By inserting a semiconductor x-ray detector into the analysis chamber, one can measure particle induced x-rays. The cross section for particle induced x-ray emission (PIXE) is much greater than that for Rutherford backscattering and PIXE is a fast and convenient method for measuring the identity of atomic species within... 

An RBS instrument can be divided into two basic components the parade accelerator and the analysis chamber or end stadon. PIXE and ERS analyses employ similar instrumentation, but use different incident ion beams or detectors. 

The analysis chamber is such that PIXE, RBS, PIGE, NRA and ERDA are routinely performed simultaneously with the microprobe. A 4-axis micron-level goniometer permits precise positioning. The radioactive beamline enters a shielded... 

The chamber may also be equipped at 180° to the beam with a (silicon surface barrier) detector for analysis of scattered protons, which provides the option of performing quantitative light element analysis by RBS (q.v.). In certain applications RBS can determine most of the matrix composition and PIXE the trace element contribution. 

By adding accessories to the sample chamber, or by changing the operating procedures, several other experiments can piggy-back on to the RBS analysis. For example HIBS, HFS, PIXE, NRA, CPAA and PIGE may all be accessible using a given particle accelerator. 

PIXE is a technique that uses a MeV proton beam to induce inner-shell electrons to be ejected from atoms in the sample. As outer-shell electrons fill the vacancies, characteristic X-rays are emitted and can be used to determine the elemental composition of a sample. Only elements heavier than fluorine can be detected due to absorption of lower-energy X-rays in the window between the sample chamber and the X-ray detector. An advantage of PIXE over electron beam techniques is that there is less charging of the sample from the incoming beam and less emission of secondary and auger electrons from the sample. Another is the speed of analysis and the fact that samples can be analyzed without special preparation. A disadvantage for cosmochemistry is that the technique is not as well quantified as electron beam techniques. PIXE has not been widely used in cosmochemistry. 

Figure 3. Diagram of the standard PIXE scattering chamber.

Figure 4. The electrostatic quadrupole triplet lens and target chamber used for micro PIXE showing the 30 sq mm Si(Li) X-ray detector and the absotber ladder at 135 the RBS detectors which are at 158 above and below the lens the Ge(Li) PIGE detector and collimator the electron flood gun and the movable zoom microscope used for focusing and alignment of the beam.

Our PIXE microprobe facility, the Laboratoire d Analyse par Reactions Nucliaires (LARN), allows the irradiation of small regions of a sample in a vacuum. The vacuum chamber in which the artifact is placed was designed... 

Figure 4 Schematic view of a specimen chamber used for micro-PIXE. Only two detectors are indicated - a Si(Li) for PIXE and a SBD for BBS.

In external beam or nonvacuum PIXE (in air or in a helium or nitrogen atmosphere), poorer detection limits are expected because of the background contribution from interactions in the beam exit foil material and in the air or chamber gas and, for the light elements, also because of the substantial attenuation of their soft X-rays by the same gases. However, practical detection limits in nonvacuum PIXE appear to be comparable to those in vacuum PIXE, at least for analyte elements with atomic number above 25. 

Methodology (PIXE evaluation software packages, detector systems [UTWSi(Li), PIN array], pPIXE chambers with precision goniometer and sample location setup, and nanobeam configurations). 

As NRA has grown from accelerator-based nuclear physics and expanded after the invention of solid-state detectors (the surface barrier Si detector for the detection of particles and Ge(Li) detectors for the detection of y rays), its instrumentation is very much similar to those used in particle and y-ray nuclear spectroscopy. The PIXE method also started to use an existing instrumentation, the Si(Li) X-ray detectors, nearly a decade later. Consequently, this review will refer to the previous O Sects. 33.1 and O 33.2 on PIXE and RBS concerning the acceleration and the formation of energetic ion beams, the internal and external sample chambers, scanning particle microprobe facilities, particle detection, and data acquisition. It will only deal with the characteristic features of the detection of ions and y rays produced in nuclear reactions. Neutrons are also produced in these reactions, but in practice they are rarely used for NRA. Because of space limitations, that technique (Bird and Williams 1989) will not be discussed. 

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