Conversion-electron detector

Weyer [50] developed a conversion electron detector based on a PPAC for Mossbauer spectroscopy. Conversion electrons are emitted from the surbce of a Mossbauer absorber after resonance absorption with a conversion coefficient a (a = 8.2 for 14.4 keV Fe). In principle, a conversion electron detector is very sensitive only for resonant Mossbauer 7-rays without interference of nonresonant 7-radiations. The PPAC is importantfor in-beam Mossbauer spectroscopy of implanted excited atoms and Rl nuclei in environments with high backgrounds. 

Fig. 1. A magnetic + Si(Li) combination conversion-electron spectrometer based on an "old" Siegbahn-SIStis magnet. 1) beam, 2) target,3) target-changing system, 4) collimator and current measurement, 5) Faraday-cup, 6) Pb shield, 7) anti-positron baffle, 8) detector, 10) cold fingers, 13) cylindrical plastic scintillator 14) light guide, 16) P.M. tube.

Ge detectors each provided with a cylindrical BGO + Nal Compton suppression shield. The conversion electrons were measured with two spectrometers each consisting of a mini orange filter and a Si(Li) detector. The transmission of the filter was optimized for detection of electrons with energies in the range 0.9- 1.3 MeV. For 196Pb the intensity of the E0 0t transition+in +... 

For spectrometry, semiconductor detectors with a thickness of the intrinsic region exceeding the maximum penetration distance of the electrons to be counted are suitable. Because of the continuous energy distribution of fi particles, the spectrum of a pure ji emitter is a curve with a maximum at medium energies and extending to a maximum energy max (Fig. 5.7), whereas conversion electrons give relatively sharp peaks. Some pure P emitters are listed in Table 7.5. 

FIGURE 1.9 The basic X-ray diffraction experiment is shown here schematically. X rays, produced by the impact of high-velocity electrons on a target of some pure metal, such as copper, are collimated so that a parallel beam is directed on a crystal. The electrons surrounding the nuclei of the atoms in the crystal scatter the X rays, which subsequently combine (interfere) with one another to produce the diffraction pattern on the film, or electronic detector face. Each atom in the crystal serves as a center for scattering of the waves, which then form the diffraction pattern. The magnitudes and phases of the waves contributed by each atom to the interference pattern (the diffraction pattern) is strictly a function of each atom s atomic number and its position x, y, z relative to all other atoms. Because atomic positions x, y, z determine the properties of the diffraction pattern, or Fourier transform, the diffraction pattern, conversely, must contain information specific to the relative atomic positions. The objective of an X-ray diffraction analysis is to extract that information and determine the relative atomic positions. 

Figure 3 demonstrates the electron spectrometer part of a depth-resolved conversion electron MOssbauer spectrometer specially designed for such measurements in our laboratory (10, 11). The electron spectrometer is of the cylindrical mirror type back-scattered K conversion electrons from resonantly excited Fe nuclei are resolved by the electrostatic field between the inner and outer cylinders (cylindrical mirror analyzer) and then detected by a ceramic semiconductor detector (ceratron). The electron energy spectra taken with this spectrometer indicate that peaks of 7.3-keV K conversion electrons, 6.3-keV KLM Auger electrons, 5.6-keV KLL Auger electrons, etc., can be resolved well, with energy resolution better than 4%. 

Detection of internal conversion electrons. Radioisotopes emitting internal conversion (IC) electrons also emit gammas and X-rays. The use of a single detector to count electrons will record not only IC electrons but also Compton electrons produced in the detector by the gammas. To eliminate the Compton electrons, one can utilize the X-rays that are emitted simultaneously with the IC electrons. Thus, a second detector is added for X-rays and the counting system... 

The characteristic X rays of the atomic electron energy levels of an excited daughter isomer are also measured conveniently by the Ge detector with gamma-ray spectrometer. These X rays are associated with conversion electrons emitted by the isomer, and their energies and emission fractions are listed in radionuclide compilations (see Section 9.2.3). Conventional detectors that measure gamma rays above about 30 keV are used to identify K X rays from elements heavier than Xenon. 

Figure 9.2. Conversion electron s measured whith a Si(Li) detector (from Knoll 1989, p. 464).

A thin solid-state detector with spectrometer is also useful for identifying and quantifying conversion electrons. Figure 9.2 shows the spectrum of conversion electrons from a thin source of For radionuclides that also emit beta par-... 

An important point in measurements is that conversion electrons are counted with the usual end-window beta-particle detectors and not with gamma-ray spectrometers. The decay fraction counted under these conditions with a beta particle detector is 1.094. The K Auger electrons may also be counted with beta particles and conversion electrons in an internal proportional counter. In a Ge detector and spectrometer, in addition to the gamma ray, the various K X rays are counted. 

Eq. (9.5) becomes complicated (NCRP 1985b) when some beta particles are counted in the gamma-ray detector or vice versa, or the decay scheme is more complex than for a beta particle followed by a single gamma ray. For example, conversion electrons may be in coincidence with beta particles and X rays, or... 

Because of the very limited escape depths of conversion electrons (about 1.8 pm in water, 0.25 pm in metallic iron), their detection is somewhat difficult. This seeming drawback provides a unique surface sensitivity. In a rotating disc electrode arrangement Kordesch et al. [539] have used a disc-shaped electrode that slowly rotates with part of the disc immersed in the electrolyte solution. As a thin electrolyte film thin enough to permit escape of conversion electrons adheres to the metal surface, potential control is always maintained. Conversion electrons were detected using a suitable gas-filled detector mounted close to the upper emersed part of the disc. In a study of passive oxide films on iron, the advantage of this approach was demonstrated beyond an unmatched surface sensitivity, the measurement time was reduced to a small fraction of that needed for transmission measurements [543]. An inherent drawback of the setup is the poor current distribution inside the very thin electrolyte film (its thickness is around 4 nm as reported by Gordon [540]). 

All detectors convert the X-ray energy into a secondary signal such as light in a phosphor or electronic charge in a direct conversion type detector. These processes also introduce statistical fluctuation, i.e., noise over and above that caused by the primary quantum noise. 

PIPS detectors have been designed to replace SSB (silicon surface barrier) and DJ (diffused junction) detectors in alpha and beta/(conversion-)electron spectroscopies. Passivation normally means a Si02 layer on the surface, a key to low noise and relative ruggedness. Due to ion-implanted junctions, PIPS s have a very thin (<50 nm) and well-defined dead... 

The avalanche photodiode detector (APD), which is able to detect conversion electrons with very high intensity can also be usefiil in special applications (Weyer 1976 Gruverman 1976). 

The most specific type of detectors used in Mfissbauer spectroscopy is the so-called resonance detector. In this type of detectors, mainly conversion electrons are detected in a gas-filled chamber in which an Fe-enriched standard target is placed. As conversion electrons can only form after the recoilless absorption of the y rays in the standard target, the nonresonant radiation is very effectively filtered off, and the resultant Mhssbauer spectrum has hardly any background (baseline). Therefore, the signal-to-noise ratio becomes excellent and the required measuring time (or the source activity that is needed) drastically decreases. 

For the calibration of charged-particle detectors, emission characteristics of some convenient conversion electron and alpha decay sources are presented in O Tables 56.5 and O 56.6, respectively. 

Gonversion Electron Mossbauer Spectroscopy (GEMS) is an alternative to normal Mossbauer spectroscopy. The y-rays emitted by the source, enter the electron detector, through a thin A1 window as shown in Fig. 4.11. The sample is mounted on the detector. Resonance absorption of y-rays in the sample is followed by de-excitation of Mossbauer nuclei, as a result of which conversion and corresponding Auger electrons enter the detector volume, and trigger an electronic impulse. The recoilless absorption is observed as a peak in the spectrum. 

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