Counting systems

Following a decay period of at least 15 hrs, the 1,524 MeV 7-photopeak of 42K is counted for 40 min with a Ge-U solid state detector and a 4096-channel analyzer. This more sophisticated counting system is necessary to obtain the required resolution, since with a Na iodide detector, the 1,524 MeV peak of 42 K overlaps with the 1.369 MeV 7-peak of the 24Na decay spectrum... 

This section will deal briefly with some aspects of expls safety peculiar to neutron activation analysis expts. We are concerned here with a) the possible effect of the ionizing radiation dose on the energetic material which will cause it to be more sensitive or hazardous to normal handling as an expl, and b) the potential direct expl hazards involved in the physical and mechanical transportation of samples to and horn the irradiation source and in a nuclear counting system... 

In this chapter, we present the principles of conventional Mossbauer spectrometers with radioactive isotopes as the light source Mossbauer experiments with synchrotron radiation are discussed in Chap. 9 including technical principles. Since complete spectrometers, suitable for virtually all the common isotopes, have been commercially available for many years, we refrain from presenting technical details like electronic circuits. We are concerned here with the functional components of a spectrometer, their interaction and synchronization, the different operation modes and proper tuning of the instrument. We discuss the properties of radioactive y-sources to understand the requirements of an efficient y-counting system, and finally we deal with sample preparation and the optimization of Mossbauer absorbers. For further reading on spectrometers and their technical details, we refer to the review articles [1-3]. 

The complexity of the Co emission spectrum and the low fraction of the desired 14.4 keV radiation require an efficient Mossbauer counting system that is able to discriminate photons of different energies and reject the unwanted events. Otherwise a huge nonresonant background would add to the counting statistics of the spectra and fatally increase the noise of the spectrometer. 

The sample was analysed with a primary ion beam of extracted from an oxygen cold cathode discharge type source. A well-focussed 200 nA spot was attained at 4keV per molecular ion. The beam was scanned over a square of side 400 pm to produce a uniform primary beam current density and thus a flat-bottomed crater. In order to eliminate crater-edge effects, the counting system was only enabled when the centre of deflection of the beam was in a central area 125 x 125 pm. 

The usual whole numbers, integers such as 1,2,3,4..., are usually referred to as Arabic numerals. It seems, however, that the basic decimal counting system was first developed in India, as it was demonstrated in an Indian astronomic calendar which dates from the third century AD. This system, which was composed of nine figures and the zero, was employed by the Arabs in the ninth century. The notation is basically that of the Arabic language and it was the Arabs who introduced the system in Europe at the beginning of the eleventh century. 

Figure 2. Schematic drawing of the AEl IM-20 ion microprobe. The magnetic analyzer and ion counting system are linked to an on-line computer for automated...

For those who are concerned with the preparation of samples for the direct 14C counting systems, this summary vividly illustrates the effects on the final age value as a result of the addition of ppm levels of modern carbon for samples in different age ranges. For a sample with an actual age of 75,000 years, for example, a 100 ppm addition of modern carbon results in approximately a 5000-year error in the final measured age. Hopefully, contamination of a sample with modern carbon would be a relatively rare occurrence. A more probable situation would involve the addition of carbon differing in age from the original samples from several hundred up to several tens of thousands of years. 

In anticipation of the development to operational status of the ion or direct counting systems, it would be helpful if we could compare these values with projected counting errors for the two types of direct counting systems being developed. Table 4 lists projections for the Rochester Van de Graaff facility [49] and the University of California Lawrence Berkeley cyclotron system employing an external ion source [31,50]. Table 4 also lists the sample sizes and approximate measurement periods for both systems. This data illustrates the potential extention in dating... 

For fluorescence decay curves of the J-aggregate LB films of [CI-MC] mixed with various matrix agents, measured with a picosecond time-resolved single photon counting system, three components of the the lifetimes fitting to exponential terms in the following equation ... 

Our results demonstrated clearly that the lifetime data are more sensitive to subtleties of the micromechanistic photophysics. In this case we were able to establish inadequacies of the two-component model that were not detected by intensity quenching measurements alone. It is also clear that resolution of the detailed mechanism in these complex polymer systems will require even better lifetime data than we are able to obtain with a conventional flash lamp-based time-correlated photon counting system. 

The apparatus is an electronic, liquid-bome particle-counting system that uses a light-obscuration sensor with a suitable sample feeding device. It is the responsibility of those performing the test to ensure that the operating parameters of the instrumentation are appropriate to the required accuracy and precision of the test result. 

Another limit source of uncertainty in isotope ratio measurements by mass spectrometry is the dead time of the ion detector for counting rates higher than 106cps, because a lower number of counts are usually registered than actually occur. Dead time correction of the detector is required if extreme isotope ratios are measured by channel electron multipliers and pulsed counting systems.86... 

Figure 17.3—Counting system, a) Device used to measure the activity of a low-energy radioisotope using the method of two coincident detectors. A single ft emission can produce hundreds of photons. It is thus possible to measure photons in opposite directions using two photomultiplier tubes (PMT). Counting only occurs if both PMTs produce a signal that is not offset by more than a few nanoseconds b) device involving a PMT in a counting well used to measure luminescence produced by a sample that has been mixed with a scintillation cocktail (in aqueous or non-aqueous media).

Instrumentation. The over-all counting system is diagrammed in Figure 12. Maximum energy resolution was achieved by installing an internally cooled FET preamplifier adjacent to the Ge(Li) detector. 

In recent years, several developments have generated new possibilities in liquid scintillation counting. The most important event was the development of the bialkali photomultipliers with considerably improved quantum efficiency. In addition, several new scintillators and solvents of higher purity have become available. With the resulting counting systems it has been possible to achieve sensitivities comparable with gas counters, with much less time and effort involved. 

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