Mica detectors

The whole apparatus was built to chemically identify an isotope of Rf decaying by SF with a half-life of 0.3 seconds, that had previously been synthesized and identified by a team of physicists at Dubna. In a number of experiments, I. Zvara and co-workers identified multiple SF tracks in the mica detectors when they used glass surfaces and temperatures of 300°C [104], They had shown in preparatory experiments with Hf, that indeed the transfer of Hf through the apparatus occurred within less than 0.3 s, and thus, that the experimental set-up was suited to study the short-lived Rf isotope [81]. A number of possible sources of SF tracks in the mica detectors other than the SF decay of an Rf isotope were discussed and ruled out. Further experiments with a slightly modified apparatus [106] were conducted immediately after the experiments described here. A total of 63 SF events were attributed to the decay of an Rf nuclide. 

Behind the column, mica solid-state detectors were positioned. They were kept at lower temperatures in order to adsorb the RfCl4 molecules. It was assumed that the produced isotopes of Rf decay at least partly by spontaneous fission. Mica is known to be well suited for identification of latent fission tracks. In a series of experiments that accumulated a total beam dose of 4 x 10 beam particles 65 fission tracks were detected along the mica detectors. These fission events were assigned to a spontaneously fissioning isotope of Rf, presumably Rf. Later, this assignment was questioned since additional measurements proved that this isotope has a half-life of only 20 ms, too short for a chemical study. It was therefore concluded that Rf with a half-life of 3 s and an assumed small fission branch was the isotope that labeled the separated molecule. 

The end window of the tube must be thin enough to permit the weaker radiations to enter the tube (aluminium, 6-8 mg cm-2 mica, 2 mg cm-2) but even so alpha particles and very weak beta emissions are either completely or partially absorbed. The emissions from the biologically important isotopes of tritium and carbon-14 fall into this category and alternative detectors should be used for these isotopes. 

Pyrometers -detectors for beam spectroscopy [SPECTROSCOPY, OPTICAL] (Vol 22) -micas m [MICA] (Vol 16)... 

Some practical designs for proportional counters are shown in Figure 18.5. In the cylindrical detector, a very thin window of split mica or Mylar plastic covers one end of the tube. It can be so thin (down to 150 p.g/cm3) that the absorption of a particles by the window is not extensive. An even more efficient arrangement is found with the hemispherical detector, where the radioactive sample can be introduced directly into the detector chamber. In the hemispherical detector, one detects 50% of all the... 

Table I contains our new values for the L2 and Ls absorption limits of Pt, Au and Bi. These measurements were taken with almost the same arrangement of apparatus as before. The X-ray tube was provided with an arm which reached nearly to the first slit of the spectrometer, thus reducing the absorption of X-rays by air. At the end of this arm and also on the front of the ionization chamber thin mica windows were employed which still further reduced the absorption of X-radiation. Finally the sensitivity of our electrometer had been increased many times. We were thus supplied with a much more sensitive detector of X-ray spectra than in our earlier measurements. This enabled us to use narrower slits and to obtain at the same time greater drops in our ionization currents for the absorption limits.

Fig. 12.1. a Schematic diagram of the experimental setup (1) the off-axis //3 parabolic mirror, (2) the laser beam, (3) the specially designed pulsed conical nozzle, (4) the cluster gas jet, (5) the focusing spectrometer with the spherically bent mica crystal, (6) the vacuum-compatible X-ray CCD camera, (7) the ion detector for TOF measurements, b Typical X-ray CCD image measured at an intensity of... 

Behind the column mica solid state detectors were positioned. They were kept at lower temperatures in order to adsorb the RfCl4 molecules. It was assumed that the produced isotopes of Rf decay at least partly by spontaneous fission. Mica is known to be well suited for identification of... 

The detection techniques applied in those early attempts were often surprisingly simple searches for spontaneous fission activities. The whole product mixture was collected on a catcher foil and exposed to mica, glass or polymer sheets to produce tracks of spontaneous fission events. By quickly rotating the catcher between detector foils during bombardment, this technique allows the detection of short-lived nuclides down to millisecond half-lives [88],... 

Fig. 1.3 Setup for first chemical experiments with element 104 - now Rf Dubna, the mid-1960s [10]. The broken frames outline the placement of resistive heaters, paraffin and cadmium shielded the detectors from neutrons to prevent induced fission of uranium impurities in mica. Thermal decomposition of NaNbClg was the source of NbC E vapor. A Faraday cup was placed inside the target chamber (not shown).

The fact that mica (and fused silica) can serve as solid state track detector (SSTD) of fission fragments was reported shortly before the final stage of development of the method for element 104 [12-14], In the dielectric solids, fission fragments produce tiny tracks visible by electron microscopy. Mica and silica are very resistant to active chemical reagents and elevated temperatures. The tracks proved to stay in hostile conditions of real experiments for a reasonably long time. Thanks to this, after the end of bombardment (EOB), the mica sheets could be etched with hydrofluoric acid to enlarge the tracks to micrometer size they were distinct in appearance and were searched out by scanning the surface of the detectors with an... 

For long-lived y-active nuclides obtained with sufficient yield, which is the case for lighter homologs of TAEs, the profiles can be revealed by spectrometric measurements from outside the column. The profiles of short-lived a-activities can be measured only when the column is a channel formed by surfaces of electronic particle detectors. Such measurements are done in real time of course, they also work well for s.f. nuclides. The latter can also be registered in open columns made of the materials which can serve as solid-state track detectors of fission fragments (fused silica, mica) see Sect. 1.2.2. 

Freshly cleaved mica substrates (Spruce Pine Mica Co., Spruce, NC) were vacuum-coated with an approximately 500 A thick layer of gold at one end this served as the reference/counter electrode contact. In all experiments the tip was laterally positioned approximately 1 mm from the Au contact and the imaging process was operated in the constant current mode. In ac-admittance measurements, a small ac voltage (10 mV peak-to-peak amplitude, 10 kHz) was superimposed on the dc bias and the admittance was measured with a phase-sensitive detector. If not otherwise mentioned, the mica substrate was heated with one small drop (—10 /xL) of a phosphate... 

Sealed gas-proportional counter. This detector is a sealed or closed system with a fixed volume of filler gas (Figure 3.38b). The filler gas used in a sealed proportional counter may be Ne, Kr, or Xe. The detector is sealed with thicker windows than those used in the FC and therefore do not leak. Window materials include polymers, mica, aluminum, and beryllium. The window thickness generally prevents the sealed proportional counter from being used for the measurement of light elements from Be to Na. It is used for analyzing elements from A1 to Ti. 

In addition to NAA, neutrons are widely used in prompt radiation analysis for the determination of concentration and spatial distribution of elements in different matrices. For example, a track-etched detector (LR-115, Makrofol KG, CR 39, mica) placed on the polished surface of a sample is irradiated with fast or thermal neutrons then etched with a suitable chemical to deduce the concentration profiles from the track distributions. This method can also be used for the detection of suspended and dissolved U, Th, and Pu in water by (n,f) reactions N in polymers by the N(n,p) C reaction B and Li in semiconductors or glasses by the B(n,a) Li and Li(n,ot) H reactions, respectively. For the detection of fission fragments the use of mica is recommended. 

The situation becomes much more complicated when the radionuclides used undergo a decay or spontaneous fission, i.e., if non-penetrating radiation needs to be detected. If these radionuclides are long-lived, the colunm is cut into small sections, the substances adsorbed on its inner surface are washed off with appropriate solutions, and the resulting solutions are analyzed. In the case of short-lived radionuclides, e.g., transactinide elements, semiconductor or track detectors (mica, quartz) placed inside the column are used. They allow single decays to be determined at each point of the column. In principle, other methods may be used to determine the distribution of substances in the column, e.g., photospectrometry and X-ray fluorescence. 

During the experimental studies of chemical properties of transactinides early approaches exploited the possibility to detect spontaneous fission in thermochromatographic columns by mica or quartz track detectors at temperatures below 400°C, while more modern setups employ Si detectors to register a decay or spontaneous fission at room temperature and below (liquid nitrogen). Then, the technique, however, is limited to the study of highly volatile atoms or compounds. 

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