Recoil nuclei

Forward recoil spectrometry (FRS) [33], also known as elastic recoil detection analysis (ERDA), is fiindamentally the same as RBS with the incident ion hitting the nucleus of one of the atoms in the sample in an elastic collision. In this case, however, the recoiling nucleus is detected, not the scattered incident ion. RBS and FRS are near-perfect complementary teclmiques, with RBS sensitive to high-Z elements, especially in the presence of low-Z elements. In contrast, FRS is sensitive to light elements and is used routinely in the detection of Ft at sensitivities not attainable with other techniques [M]- As the teclmique is also based on an incoming ion that is slowed down on its inward path and an outgoing nucleus that is slowed down in a similar fashion, depth infonuation is obtained for the elements detected. 

Let us consider stripping reactions first and, in particular, the most commonly encountered stripping reaction, the (d, p) reaction. Formally, the result of a (d, p) reaction is to introduce a neutron into the target nucleus, and thus this reaction should bear some resemblance to the simple neutron capture reaction. But because of the generally higher angular momenta associated with the (d, p) reaction, there can be differences between the two reactions. Consider the A (d, p) B reaction where the recoil nucleus B is produced in an excited state B. We sketch out a simple picture of this reaction and the momentum relations in Figure 10.16. 

When these heavy recoil nuclei are the result of a complete fusion of the projectile and target nuclei, they are usually called evaporation residues because they result from a deexcitation of the primary complete fusion product by particle evaporation (emission). In intermediate energy and relativistic nuclear collisions, the momentum transfer to the target nucleus is much less, and the energy of the recoiling nucleus is 5-100 keV/nucleon. Such recoils are usually called heavy residues ... 

The irradiation of a medium by neutrons also leads to formation of charged particles via the secondary processes. The most important among the latter is the elastic scattering of a neutron with formation of a recoil nucleus. This process is most efficient in media consisting of light elements. Besides this, the slow and thermal neutrons are efficiently captured by certain types of nuclei with ejection of either a proton (for... 

PROBLEM 11.26.1 Compute the Doppler recoil energy ER for 26pe57 and a linear speed vr needed to counteract the Doppler shift. Calculate also the speed of a free recoiling nucleus vr, and comment on why this calculated speed is too big. 

All a particles originating from a certain decay process are monoenergetic, i.e. they have the same energy. The energy of the decay process is spht into two parts, the kinetic energy of the a particle, E, and the kinetic energy of the recoiling nucleus, Fn ... 

An important characteristic of radioactive decay is that the momentum of the emitted particle must be balanced by the momentum of the product nucleus. In suitable cases it should be possible to detect the recoil nucleus. Thorium-228, which decays to radium-224 having an energy of 97 keV, has been used to study the oxidation of a number of metals. Its advantages are its low volatility (as thorium oxide) and its relatively low rate of diffusion in lighter metals. The maximum range of recoils in solids is of the order of 300-500 A and for thinner oxide layers, its distance from the surface can be measured. One difSculty with quantitative work is that the radium undergoes a sequence of further decay, which complicates calculation of recoil ranges, and calibration may be necessary. 

Derive the expression for the laboratory energy of the recoil nucleus. 

It can be shown that the energy of the recoiling nucleus E, eqn. (4.36)) is much smaller than the kinetic energy of the ejected electron E and may be ignored. The mathematical expression to use is (4.32). 

Because the momentum and energy evolved in the decay must be conserved, they are distributed between the product nucleus (sometimes called the recoil nucleus) and the emittedg alpha particle. The unit of energy commonly used in describing nuclear decay and radiation is the electron volt and its multiples. One electronvolt equals 1.6 x 10 J, which is numerically equal to the electron charge e in coulombs. 

To calculate Er, consider an isolated nucleus of mass M that had been at rest before the decay took place and the photon got emitted. It follows from momentum conservation that pN = Py where pN is the magnitude of the momentum of the recoiled nucleus and py = Eyic is that of the photon. The recoil energy can therefore be given as r = ps) l 2M). Since r is small compared with Eq, one can set = Eq — Eq and thus the recoil energy can be... 

The phenomenon of the recoil-free resonant absorption of gamma rays is the result of the special dynamics of nuclei in solids. This fact was recognised by Mossbauer, who interpreted his apparently anomalous observations in terms of lattice dynamics of a similar nature to those associated with neutron scattering (Mossbauer, 1938). He found that the recoil momentum of the recoiling nucleus is shared by many nuclei of the crystal and thus the recoil energy is extremely small. This cooperative... 

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