Beta-particle energies

The counting efficiency for the shown system approaches 52% for radionuclides with high maximum beta-particle energies (see Fig. 2A.2). This value exceeds the 39.6% based on the geometry of a 2.2-cm-dia. sample on a filter relative to the detector window. The counting efficiency exceeds the... 

Curves for this equation have been obtained by measuring various beta-particle emitters in several geometric arrangements of source and detector. One of the empirical relations found between p and the maximum beta-particle energy, E, in MeV, is shown by Equation 4.2 ... 

The appropriateness of the value of e0 can be checked by comparison with the counting efficiency values obtained for very thin samples - where f approaches 1.0 - in Experiment 2. Interpolation is needed for the K maximum beta-particle energy of 1.311 MeV, and the lesser value of e0 at the larger sample diameter in the present experiment must be taken into account. An estimate based on calculating the detector-sample geometry underestimates the counting efficiency because additional beta particles are back-scattered into the detector from the planchet and its support. 

In contrast to alpha emission, beta emission is characterized by production of particles with a continuous spectrum of energies ranging from nearly zero to some maximum that is characteristic of each decay process. The jS particle is not nearly as effective as the alpha particle in producing ion pairs in matte r because of its small mass (about /7(XK) that of an alpha particle), At the same time, its penetrating power is substantially greater than that of the alpha particle. Beta-particle energies are frequently related to the thickness of an absorber, ordinarily aluminum, required to stop the particle. 

The typical curve of beta-particle attenuation in aluminum absorbers, shown in Fig. 2.6, at lower energies resembles the exponential attenuation observed for gamma rays (see Section 2.4.4). The final part of the line curves downward to reach the distinct range associated with Fmax- Attenuation curves for the various beta-decay radionuclides differ because of the different beta-particle energy spectra, but both the characteristic range and the approximately exponential attenuation have been used to estimate maximum beta-particle energies (Evans 1955). 

Two or more radioisotopes of the same element that cannot be measured by spectral analysis require integration of effective separation of impurities and radiation detection of the selected distinguishing decay characteristics. To determine the amounts of Sr and °Sr in a sample, for example, interfering radionuclides such as Ra and must be removed only then can the two strontium radioisotopes be distinguished in terms of the radioactive decay of Sr, ingrowth of the daughter, and detector response to beta-particle energies, as discussed in Section 6.4.1. 

The typical purification method for rare earths is coprecipitation with ferric hydroxide, dissolution in dilute acid, precipitation as fluoride in strong mineral acid solution, dissolution in strong nitric acid with boric acid to complex fluoride, and precipitation for counting as the oxalate in dilute acid solution (Stevenson and Nervik 1961). Because Pm has no stable isotope, another rare earth (such as lanthanum) is added as carrier. The " Pm precipitate can be counted with a proportional counter, or can be dissolved and measured with an LS counter because of the low beta-particle energy. If small amounts of the other rare earth radionuclides are detected by gamma-ray spectrometric analysis, the beta-particle count rate of Pm can be calculated by difference. 

An aqueous sample may be added to the cocktail directly, after minor prior processing, or at the end of a radiochemical separation procedure. Direct addition is the equivalent of gross activity counting discussed in Section 7.2.4 except that some spectral analysis may be possible. Alpha particles can be differentiated from beta particles by deposited energy, pulse shape, and decay time. Self-absorption is of no concern. Quenching and luminescence, discussed in Section 8.3.2, often occur. Identification by maximum beta-particle energy is approximate, and requires comparison to radionuclide standards. 

Note maximum beta-particle energy is given... 

The counting efficiency for charged parficles wifh an LS counfer can approach 4 r geomefry, as shown in Table 8.1. If decreases fo abouf 25% for because of low beta-particle energies. 

Figure 8.6. Example of quenching in a beta-particle energy deposition spectrum in an LS counter.

At the same time, other aliquots are counted with gas proportional and LS counters to determine the presence of alpha and beta particles. The detectors are operated in a mode that distinguishes between alpha and beta particles. The LS counter can suggest maximum beta-particle energies and show alpha-particle energies from... 

Emax- The maximum beta-particle energy for a beta-particle group. 

Sodium-24. This quite short-lived, extremely active element (half-life 15 hour, 1.4 MeV beta-particle energy + two gamma rays of energy (1.4 and 2.8 MeV) is readily available. It can be used wherever a study of sodium is needed. Since sodium readily ionizes it exchanges very rapidly. Thus, studies of longer duration (such as a determination of pathway of a particular metabolite) Eire not normally possible with this element. It has been used to study sodium tTEmsport across plasma membrEmes. 

The average beta particle energy of a beta emitter is approximately one-third of the maximum energy. 

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