Particle-emitting radiation sources

Dose is related to the amount of radiation energy absorbed by people or equipment. If the radiation comes from a small volume compared with the exposure distance, it is idealized as a point source (Figure 8.3-4). Radiation source, S, emits particles at a constant rate equally in all directions (isotropic). The number of particles that impact the area is S t Tr where Tr is a geometric effect that corrects for the spreading of the radiation according to ratio of the area exposed to the area of a sphere at this distance i.e. the solid angle - subtended by the receptor (equation 8.3-4). 

An important reaction used quite widely for this purpose is irradiation by neutrons and measurement of die energies of radiations emitted. The source of the neutrons may be a nuclear reactor, a particle accelerator, or an isotopic source, that is, a sealed container in which neutrons are produced by alpha rays emitted by a source such as radium, sodium-24(24Na), yttrium-88f8sY), etc., and arranged so that the alpha rays react-with a substance such as beryllium which in turn emits neutrons. The neutrons react with stable nuclides in the sample to produce radioactive ones. Thus... 

Analytical techniques for the identification of the structure and chemistry of solid surfaces are based on detection of particles photons, electrons, and ions. The techniques are classified according to the particles or radiation used to excite the sample and detected particles (emission) to obtain information about the sample. Fig. 4.5 schematically shows the interaction of the excitation sources with the solid surfaces of materials various forms of energy are emitted from the solid surfaces and detected by energy analyzers. 

The different types of radiation sources used in the laboratory, such as radioactive elements emitting oc-particles, /i-particles and y-rays, X-ray sources, lasers and micro-wave radiation are to be treated with caution. Some radiation sources can cause damage to cells in the body. The health and safety guidelines listed in the manuals of the radiation sources should be followed. 

In contrast to the methods described in the preceding paragraphs, radiophotovoltaic conversion is a direet method. In a semiconductor the incident radiation generates free charge carriers, that are separated in the n,p-barrier layer of the semiconductor. Suitable radiation sources are radionuclides emitting p particles with energies below the limits of radiation damage in the semiconductors. These limits are about 145 keV for Si and about 350 keV for Ge. Therefore only Pm, Ni and T are suitable as radiation sources. By use of the combination Pm/Si, efficiencies of about 4% are obtained. 

The absorbed dose rate D (rad/h) imparted by beta particles at a certain distance from a part radiation source which emits beta-rays of a relatively high energy exceeding 0.8 MeV such as P and is represented as a rule-of-thumb by the expression given below ... 

Marie Curie (1867-1934) and her husband Pierre (1859-1906) took Becquerel s mineral sample (called pitchblende) and isolated the components emitting the rays. They concluded that the darkening of the photographic plates was due to rays emitted specifically from the uranium atoms present in the mineral sample. Marie Curie named the process by which materials give off such rays radioactivity the rays and particles emitted by a radioactive source are called radiation. 

Radioactive tracer techniques have long been used to study particle motion in solids fluidization systems. The advantage of this technique is that the flow field is not disturbed by the measurement facility and, therefore, the measurement of the motion of the tracers represents the actual movement of particles in the system. The tracer particles are usually made of gamma-emitting radioisotopes, and their gamma radiation is measured directly by scintillation detectors. Factors that affect gamma radiation measurement were identified as the characteristics of the radiation source, interactions of gamma rays with matter, the tracer s position relative to the detector, detector efficiency, and dead time of the measurement system. 

Consider a radioactive source emitting electrons and assume that one attempts to measure the number of electrons per unit time emitted by the source. For every atom of the source there is a probability, not a certainty, that an electron will be emitted during the next unit of time. One can never measure the exact number. The number of particles emitted per unit time is different for successive units of time. Therefore, one can only determine the average number of particles emitted. That average, like any average, carries with it an uncertainty, an error. The determination of this error is an integral part of any radiation measurement. 

Example 2.10 A radiation detector is used to count the particles emitted by a radioisotopic source. If it is known that the average counting rate is 20 counts/ min, what is the probability that the next trial will give 18 counts/min ... 

Spectroscopy is the aspect of radiation measurements that deals with measuring the energy distribution of particles emitted by a radioactive source or produced by a nuclear reaction. 

Plastic Bags Thin sheets of plastic are used to make items such as grocery bags. The sheets move under a source of promethium-147, emitting beta particles. The radiation intensity, measured under the plastic sheets, is used to monitor the thickness of the plastic. During this process, promethium changes into which element ... 

The practical counting efficiency e represents the probability that any particular photon or particle of radiation emitted by the sample source will be recorded by the detector. As explained in Section 8.2, its value may depend on many factors, including the detector, the type and energy of the radiation, the composition of the source, and the geometry of the source-detector configuration. It includes the loss factor in the pulse analysis system and attenuation and scattering fractions associated with the sample-detector system. All of these factors are discussed further in Section 8.2. 

A laboratory rat is exposed to an alpha-radiation source whose activity is 14.3 mfii. (a) What is the activity of the radiation in disintegrations per second In becquerels (b) The rat has a mass of 385 g and is exposed to the radiation for 14.0 s, absorbing 35% of the emitted alpha particles, each having an energy of 9.12 X 10 ). Calculate the absorbed dose in mil-lirads and grays, (c) If the RBE of the radiation is 9.5, calculate the effective absorbed dose in mrem and Sv. 

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