Surface analysis barrier detectors

W. K. Chu, J. W. Mayer, and M. -A. Nicolet. Backscattering Spectrometry. Academic Press, New York, 1978, brief section on nuclear reaction analysis, discussions on energy loss of ions in materials, energy resolution, surface barrier detectors, and accelerators also applicable to NRA ... 

Eor analysis of emitted particles, solid state surface barrier detectors (SBD) are used inside the scattering chamber to measure the number and energy of the reaction products. Stopper foils are used to prevent scattered projectiles from reaching the detector. Depth profiles can be obtained from the energy spectra, because reaction products emitted in deeper layers have less energy than reaction products emitted from the surface. The concentration in the corresponding layer can be determined from the intensity of reaction products with a certain energy. 

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. 

The difficulties include the inconvenience of handling radioactivity and the necessity for obtaining an accurate radiochemical analysis of two phases containing several elements (which often involves complicated spectra). Highly sensitive instrumentation is required for the analysis e.g. a Li-Si surface barrier detector for a particles, a 2 r gas counter for (3-radiation and a Li-Ge detector for 7-radiation. Great care is required during source preparation, which is best done by electrodeposition. 

Radioactivity of uranium can be measured by alpha counters. The metal is digested in nitric acid. Alpha activity is measured by a counting instrument, such as an alpha scintillation counter or gas-flow proportional counter. Uranium may be separated from the other radioactive substances by radiochemical methods. The metal or its compound(s) is first dissolved. Uranium is coprecipitated with ferric hydroxide. Precipitate is dissolved in an acid and the solution passed through an anion exchange column. Uranium is eluted with dilute hydrochloric acid. The solution is evaporated to near dryness. Uranium is converted to its nitrate and alpha activity is counted. Alternatively, uranium is separated and electrodeposited onto a stainless steel disk and alpha particles counted by alpha pulse height analysis using a silicon surface barrier detector, a semiconductor particle-type detector. 

At 1 MeV/amu energies, the dE/dx and total energy measurements are made with either gas ionization detectors or silicon surface-barrier detectors or a combination of these. The time-of-flight detector serves as an additional positive-ion mass analysis stage. It is most useful for the heaviest (slowest) ion such as I and consists of two time-pickoff detectors with time resolution of a few hundred picoseconds. 

Spectra. The energy spectrum is collected from the particles emitted from all depths simultaneously using a silicon surface barrier detector, electronic amplifiers, an analog-to-digital converter and a multichannel analyzer. A reference pulse is fed into the electronics to monitor the stability of the system thus allowing corrections to be made should electronic drift occur during the course of the measurement. Specific systems are described in the references (1 -4,6,7,12-17). By using computer-based data acquisition systems, the depth profile can be displayed at the time of analysis. 

Spin analysis was carried out by a Mott-scattering detector with a spherically symmetric acceleration field operated at typically 70 keV without retarding potentials [3]. Surface barrier detectors were used as electron detectors. The figure of merit x ///q amounts to about 2.4 x 10 ". The advantages of this type of... 

Most modern alpha-particle spectral analysis is performed with a semiconductor such as a surface barrier detector with a very thin dead region in front of the active... 

Figure 5 Components (not to scale) of a typical nuclear microprobe system (A) electrostatic particle accelerator (B) primary object aperture (C) secondary collimator (D) focusing system (E) scanning system (F) video camera and microscope (G) surface barrier detector for scattered particles (H) X-ray detector (I) specimen (J) surface barrier detector for transmitted particles (STIM) (K) front-end CAMAC with data bus (L) main computer and display with elemental map. (Reprinted with permission from Maenhaut W and Malmqvist KG (2001) Particle-induced X-ray emission analysis. In Van Grieken RE and Markowicz AA (eds.) Handbook of X-Ray Spectrometry, 2nd edn. Ch. 12, pp. 719-809. New York Dekker Marcel Dekker Inc.)...

A large number of radiometric techniques have been developed for Pu analysis on tracer, biochemical, and environmental samples (119,120). In general the a-particles of most Pu isotopes are detected by gas-proportional, surface-barrier, or scintillation detectors. When the level of Pu is lower than 10 g/g sample, radiometric techniques must be enhanced by preliminary extraction of the Pu to concentrate the Pu and separate it from other radioisotopes (121,122). Alternatively, fission—fragment track detection can detect Pu at a level of 10 g/g sample or better (123). Chemical concentration of Pu from urine, neutron irradiation in a research reactor, followed by fission track detection, can achieve a sensitivity for Pu of better than 1 mBq/L (4 X 10 g/g sample) (124). 

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