Charged-particle-induced reactions

There are several hundred radionuclides that have been used as radiotracers. A partial list of the properties of these nuclides and their production methods are shown in Table 4.1. The three common production mechanisms for the primary radionuclides are (n,y) or (n,p) or (n,a) reactions in a nuclear reactor (R), charged-particle-induced reactions usually involving the use of a cyclotron (C), and fission product nuclei (F), typically obtained by chemical separation from irradiated uranium. The neutron-rich nuclei are generally made using reactors or... 

Figure 10.9 Schematic diagram of a charged-particle-induced reaction.

Figure 10.10 Near threshold behavior of neutron and charged-particle-induced reactions. [From Ehmann and Vance (1991).]...

Aside on Barriers In our semiclassical treatment of the properties of charged-particle-induced reaction cross sections, we have equated the reaction barrier B to the Coulomb barrier. This is, in reality, a simplification that is applicable to many but not all charged-particle-induced reactions. 

The fact that neutrons can be absorbed by nuclei without overcoming a threshold (1 = 0 or s-wave reactions) makes neutrons extremely effective nuclear reactants. Neutron-induced reactions are the energy source for present-day commercial nuclear power (fission reactors) while charged-particle-induced reactions remain under study as power sources (fusion reactors). In this chapter we will consider the general features of nuclear fission reactors, following by the general features... 

With temperature, density and Ye as free parameters, many choices of initial NSE compositions may clearly be made, involving a dominance of light or heavy nuclides, as illustrated in Fig. 24. However, in view of its relevance to the supernova models, an initial NSE at temperatures of the order of 1010 K is generally considered. It favours the recombination of essentially all the available protons into a-particles (the region noted NSE n,o in Fig. 24). The evolution of this initial composition to the stage of charged-particle induced reaction freeze-out has been analyzed in detail by [60], and we just summarize here some of its most important features that are of relevance to a possible subsequent r-process ... 

Fig. 24. The likelihood of a DYR r-process for given combinations of the electron fraction Ye and the entropy per baryon s. A SoS-like r-process is expected for a suitable superposition of conditions between the black lines. The results inferred from an initial NSE phase at low s are smoothly connected to those of various nuclear network calculations for high s values. In the latter cases, the assumed expansion timescales imply that the freeze-out of the charged-particle induced reactions is reached after dynamical timescales Tdyn in excess of about 50 - 100 ms. The two dotted lines represent the contours of successful r-processing for Tdyn = 50 ms (left line) and 100 ms (right line) (see [59] for details)...

As the temperature decreases further, the QSE clusters fragment more and more into smaller clusters until total breakdown of the QSE approximation, at which point the abundances of all nuclides have to be calculated from a full nuclear reaction network. In the relevant a-particfe-rich environment, the reaction flows are dominated by (a, 7) and (a, n) reactions with the addition of radiative neutron captures. Nuclei as heavy as Fe or even beyond may result. For a low enough temperature, all charged-particle-induced reactions freeze-out, only neutron captures being still possible. This freeze-out is made even more efficient if the temperature decrease is accompanied with a drop of the density p, which is especialiy efficient in bringing the operation of the p2-dependent a - a + n reaction to an end. 

For most charged particle-induced reactions the atomic number of the radionucHde B is different from that of the analyte element A. This is the case for (p,n), (p,a), (d,n), (d,a), ( He,n), ( He,d), (o,n), and (a,d) reactions, but not for (p,d) and ( He,a) reactions. The radiochemical separation to be developed for CPAA is thus different from that for all non-nuclear analytical methods and for some other methods based on activation analysis, such as thermal- and fast-NAA (using the (n,y) and (n,2n) reactions, respectively) and PAA (using the ( y,n) reaction). Also, in principle, it is not necessary to separate the matrix, but rather the radionuclide(s) formed out of the matrix element(s). Again, the atomic number of the radionuclide(s) is usually different from that of the matrix element(s). Owing to the chemical separation involved, CPAA is considered to be an independent analytical method, not subject to the same systematic errors as other analytical methods. 

Charged particle-induced reactions, such as (p,n) and (p,a) reactions, yield radionuclides that are too rich in protons and consequently decay by positron (yS" ") emission or electron capture (EC), mostly followed by gamma (y) emission. Positrons are emitted with... 

On the other hand, charged-particle-induced reactions at low energies are suppressed by the Coulomb barrier. To account for this effect, a simple modification of the form... 

There are two kinds of methods for production of transuranium elements as indicated in the previous O Sect. 18.1.1 neutron capture reactions in nuclear reactors and charged-particle-induced reactions at accelerators. 

Some experimental production cross sections of transuranium nuclides including transactinides in charged-particle-induced reactions of the U isotopes and Cm targets are summarized in Table 18.3. 

Production cross sections in the charged-particle-induced reactions of U and targets... 

One important aspect for fast separations is the time needed to dissolve the target material. As long as solutions or very soluble salts can be irradiated the time will be rather small. However, in the case of charged particle induced reactions, metals are generally preferred as target materials due to their high heat conductivities as well as their mechani-... 

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