Radiation counting systems

This section is addressed to the person who has not seen or used radiation instruments. Its purpose is to present a general description of the physical appearance and operation of the basic components of a radiation counting system. Every component is treated like a black box, i.e., input and output are discussed without any details about how the output is obtained. Details about the construction and operation of individual units are given in later chapters. 

CHI-SQUARED TEST, A procedure for determining the probability that two different distributions are actually samples of the same population. It can be used to check whether a radiation counting system has an appropriate degree of randomness. 

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. 

Still, it is easy to show that the counting rate in one s radiation detector, C, is equal to the rate of disintegration of the radioactive nuclei present in a sample, A, multiplied by a constant related to the efficiency of the radiation measuring system. Thus... 

If the applied voltage is not too high, the size of the output pulse is proportional to the amount of energy deposited in the detector by the incoming radiation particle/ photon. (This is why the detector is called a proportional counter.) It is called a counter because the number of output pulses are counted by a counting system. The read-out can be either a total number of counts or a count rate (in cpm or cps). 

In the time resolved Raman measurements on radiation-chemical systems, optical multichannel detection offers some distinct advantages over the photon counting techniques. The intense Cerenkov pulse associated with the electron pulse is intense enough to saturate a photomultiplier tube (PMT). In an optical multichannel detector, the Cerenkov pulse can be effectively gated off by turning the detector on within a few nanoseconds after the electron pulse is over. Apart from this, such spectra are free from the variation in electron or laser pulse intensity unlike the spectra obtained by single channel devices. 

The most important elements affecting the energy resolution of a radiation detection system are the three statistical factors mentioned above in relation to the width F. It is worth repeating that in energy measurements it is the energy resolution of the counting system (detector-preamplifier-amplifier) that is the important quantity and not the energy resolution of just the detector. 

This chapter presents a brief and general description of electronic units used in radiation measurements. The subject is approached from the viewpoint of input-output —i.e., the input and output signals of every component unit or instrument are presented with a minimum of discussion on circuitry. The objective is to make the reader aware of the capabilities and limitations of the different types of units and, at the same time, create the capacity to choose the right component for a specific counting system. 

The pulse produced at the output of a radiation detector has to be modified or shaped for better performance of the counting system. There are three reasons that necessitate pulse shaping ... 

The counting system depends on the radiation detected. Modem activation analysis systems depend on the detection of gamma rays and X-rays and very... 

The first chapter defines the energy range of the different types of radiation for which instruments and methods of measurement are considered it gives a brief discussion of errors that emphasizes their importance and, finally, it presents a very general description of the components of a counting system. This last part of the chapter is necessary because a course on radiation measurements involves laboratory work, and for this reason the students should be familiar from the very beginning with the general features and functions of radiation instruments. 

Instrumental neutron activation lends itself to on-stream applications. Figure 4.1 shows a schematic representation of on-stream and discrete sample instrumental activation analysis. In the on-stream example the sample flows through an irradiation coil and then through a counting coil. In the discrete sample application, the sample, in a suitable container, is first irradiated and then taken close to the radiation- detector. The radiation detectionanalyzing system shown here is similar to the one described for energy dispersive X-ray spectrometry. 

---