Fission track detectors

Examinations of the same and of other lead-bearing samples for spontaneous fission events with large proportional counters in Dubna seemed to confirm these findings, but further measurements [37] of thin samples sandwiched between two plastic fission-track detectors showed that the events were background caused by cosmic-ray induced reactions of lead. Other groups [38] found no evidence for spontaneous fission activities in lead and other samples at a lower detection limit of 10" 3 g/g achieved with the sandwich technique. Even lower limits down to 10"17 g/g can be reached by etching... 

Another method of measuring fission rates is by using fission track detectors, as discussed in Sec. 16.9.3. 

Chapter 17 deals with special detectors and spectrometers that have found applications in many different fields but do not fit in any of the previous chapters. Examples are the self-powered detectors, which may be gamma or neutron detectors, fission track detectors, thermoluminescent dosimeters, photographic emulsions, and others. 

The extreme case of the search for element 114 is flue dust collected during the industrial processing of lead from galena [50]. Eka-lead should be more volatile than lead, and hence enriched in flue dust. Samples collected from 10 to lO" tons of galena were concentrated further by chemical and mass separations. They were finally exposed to reactor neutrons but no induced fission events were found with fission track detectors. The deduced concentration limit of 10 -10 g/g is by far the lowest achieved in searches for superheavy elements in Nature. 

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). 

The largest collector surface for elements impinging on the Globe is the sea, of course. Heavy elements deposited in seawater are enriched in certain sediments such as manganese nodules, iron-manganese hydroxides. Fission tracks were found [83] in feldspar inclusions in such nodules, but no evidence was obtained [40,45] for spontaneous fission activities by counting nodules with neutron detectors. 

Direct searches for superheavy elements in the U+ U reaction were undertaken at the unilac by several groups. All these efforts remained without positive evidence. The data are summarized in Figure 13. The curve labeled chem [106] was obtained with off-line chemical separations [107] and an assay for a-and spontaneous fission activities here, the 10 picobam level was reached for half-lives between several days and years. Attempts to detect short-lived nuclides were less sensitive. The curve labeled gas holds for an on-line search [108] for components volatile at room temperature. wheel [106] refers to fission track detection in the unseparated product mixture deposited on a rotating catcher, rec [109] to implantation of recoil atoms in a surface barrier detector, and JET to on-line transport from target to detector with a gas jet [91,110],... 

Solid-track detectors have found application in the investigation of spontaneous fission of transuraiuum nuclides, of cosmic radiation at high altitudes of the order of 20 km and in dating of minerals by counting the number of tracks. 

Another application of track detectors is dosimetry of a particles and neutrons. For neutron dosimetry the track detectors may be covered with uranium foils in which the neutrons induce fission. Alternatively, the detectors may be covered with a foil containing B or Li, and the a particles produced by (n, a) reactions are recorded. 

Since tracks caused by radiation damage are very stable, they can be investigated after very long periods of time. Many minerals contain a record of damage by fission products or cosmic rays that has been conserved over millions of years. This makes track detectors very valuable for geochemistry and space science. 

Fission tracks are observed in solids due to spontaneous or neutron-induced fission of heavy nuclei. The primary tracks can be made visible under an optical microscope by treatment with chemicals, by which track diameters of the order of 0.1 to 0.5 pm are obtained. The method is the same as that used with track detectors (section 7.12). The length of the fission tracks depends on the nature of the minerals and varies between about 10 and 20 pm. With respect to dating, the only important source of fission tracks is spontaneous fission of Spontaneous fission of other naturally occurring heavy nuclides gives no measurable density of fission tracks neutron-induced fission is, in general, negligible and the tracks of recoiling atoms due to a decay are very short (of the order of 0.01 pm). 

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. 

Apatite fission track ages and thermal histories (Boettcher McBride, 1993) were obtained using the external detector method (Naeser, 1979). 

This chapter discusses in detail all the neutron detection methods mentioned above, as well as the Bragg crystal spectrometer, the time-of-flight method, compensated ion chambers, and self-powered neutron detectors (SPND). Other specialized neutron detectors, such as fission track recorders and thermoluminescent dosimeters, are described in Chap. 16. 

Figure 6. Schematic representation of different fission track dating procedures (after Hurford and Green 1982). Of these, only population and external detector methods have gained wide currency.

Figure 7. The sequence of steps involved in the external detector method of fission track dating. This method is now the dominant procedure used in most fission track dating laboratories for apatite because of its ease of handling, suitability for automation and its provision of single grain age information.

Crowley KD, Yoimg J (1988) Automated mirror-image stage (AMIS) for external-detector fission-track analysis. Nucl Tracks Radiat Meas 17 410... 

Gleadow AJW, Covering JF (1977) Geometry factor for external detectors in fission track dating. Nucl Track Detection 1 99-106... 

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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